Presented in this chapter are the technical criteria and design
standards for hydraulic evaluation and design of open channels (natural
and artificial). Open channel flow can be extremely complex, and often
entire textbooks are devoted to the subject. Discussions and hydraulic
standards are provided for different channel types that are most likely
to be found in Mesa County.
The ultimate responsibility for the design of a safe and stable
channel rests with the professional design engineer. A good understanding
of the site conditions is vital to the production of a stable channel
design. The information presented in this chapter must be considered
to be the minimum standards upon which channel evaluation and design
shall be based. Additional analysis that goes beyond the scope of
this manual may be necessary for unique or unusual channel conditions.
For additional information, the users of this manual are encouraged
to refer to other textbooks and technical publications addressing
this subject.
(Res. 40-08 (§ 801), 3-19-08)
Several terms and parameters are used and must be understood
when analyzing open channel flows. These are described below.
Area (A). The area always means the cross-sectional
area of the flow, and is measured perpendicular to the direction of
flow.
Critical depth (dc). This refers to the depth
of flow under critical flow conditions.
Critical flow. This refers to flow at critical
depth or velocity, where the specific energy is a minimum for a given
discharge. Critical flow is very unstable.
Critical slope. This refers to the slope which,
for a given cross-section and flow rate, results in critical flow.
Critical velocity. This refers to the velocity
of flow under critical flow conditions.
Depth (d). If not specified otherwise, depth
of flow refers to the maximum depth of water in the cross-section.
Energy grade line (EGL). The grade line of
the water surface profile plus the velocity head, or the specific
energy line.
Froude number (Fr). This is a dimensionless
number, equal to the ratio of the velocity of flow to the velocity
of very small gravity waves, the latter being equal to the square
root of the product of the acceleration of gravity and the flow depth,
or:
Where:
Fr<1.0, flow is subcritical;
|
Fr = 1.0, flow is critical; and
|
Fr>1.0, flow is supercritical
|
V = velocity (fps)
|
A = cross-sectional area of flow (sf)
|
T = top width of flow (ft.)
|
Dh = hydraulic depth (ft.)
|
Gradually varied flow. Varied flow in which
the depth does not change abruptly over a comparatively short distance.
Hydraulic depth (Dh). The hydraulic depth is
the ratio of area in flow to the width of the channel at the fluid
surface, or Dh=A/T.
Hydraulic grade line (HGL). In an open channel,
the hydraulic grade line is the profile of the free water surface.
Hydraulic gradient (Hg). The slope of the hydraulic
grade line is the profile of the free water surface.
Hydraulic radius (Rh). The hydraulic radius
is the cross-sectional area of flow divided by the wetted perimeter,
or Rh=A/Pw.
Normal depth. When the flow depth is constant
along a channel reach; that is, when neither the flow depth nor velocity
is changing, the depth is said to be normal.
Slope (S). Slope may refer to the channel bed,
the hydraulic grade line, or energy grade line.
Surface spread (T). The surface spread is the
width at the top of the flow, measured perpendicular to the flow direction.
Uniform flow. Uniform flow occurs when flow
has a constant water area, depth, discharge, and average velocity
through a reach of channel.
Wetted perimeter (Pw). The wetted perimeter
is the portion of the perimeter of a flow conveyance facility that
is in contact with the flowing water.
(Res. 40-08 (§ 802), 3-19-08)
Any water flow that is conveyed in such a manner that top surface
is exposed to the atmosphere is defined as open channel flow. This
type of flow occurs in all channel types described in GJCM 28.32.100
including canals, ditches, drainage channels, culverts, and pipes
under partially full flow conditions. The hydraulics of an open channel
can be very complex, encompassing many different flow conditions from
steady-state uniform flow to unsteady, rapidly varying flow. Most
of the problems in stormwater drainage involve uniform, gradually
varying or rapidly varying flow states. Examples of these flow conditions
are illustrated in Figure 28.32.030. Steady uniform flow is most commonly
treated flow in open channel hydraulics, in which the depth of flow
remains constant over the time interval studied. The calculations
for uniform and gradually varying flow are relatively straightforward
and are based upon similar assumptions (e.g., parallel streamlines).
Rapidly varying flow computations (e.g., hydraulic jumps and flow
over spillways), however, can be very complex, and the solutions are
generally empirical in nature.
Presented in this chapter are the basic equations and computational
procedures for uniform, gradually varying and rapidly varying flow.
The user is encouraged to review the many hydraulics textbooks written
on this subject.
(Res. 40-08 (§ 803), 3-19-08)
Open-channel flow is uniform if the depth of flow is the same
at every section of the channel. For a given channel geometry, roughness,
discharge and slope, there is only one possible depth for maintaining
uniform flow. This depth is referred to as the “normal depth.”
For uniform flow within a prismatic channel (i.e., uniform cross-section),
the water surface will be parallel to the channel bottom. While uniform
flow rarely occurs in nature and is difficult to achieve in a laboratory,
a uniform-flow approximation is generally adequate for planning and
design purposes.
The computation of uniform flow and normal depth shall be based
upon the Manning or uniform flow equation:
Where:
Q
|
=
|
flow rate (ft.3/s)
|
n
|
=
|
Manning roughness coefficient
|
A
|
=
|
area (ft.2)
|
P
|
=
|
wetted perimeter (ft.)
|
R
|
=
|
hydraulic radius, R = A/P (ft.)
|
S
|
=
|
slope of the energy grade line (ft./ft.)
|
For prismatic channels, the energy grade line (EGL), hydraulic
grade line (HGL), and the bottom can be assumed parallel for uniform,
normal depth flow conditions.
The variables dependent on channel cross-section geometry (i.e.,
area and hydraulic radius) can be lumped together as the conveyance
(K) of the channel. This simplifies the uniform flow equation to the
following expression:
Table 28.32.040(a) presents equations for calculating many of
the parameters required for hydraulic analysis of different uniform
channel sections.
Tables 28.32.040(b), 28.32.040(c), 28.32.040(d), and 28.32.040(e)
provide a list of Manning roughness coefficient values for many types
of conditions that may occur in Mesa County. The uniform flow equation
and its constituent parameters are readily computed using handheld
calculators and personal computers.
(a) Gradually Varying Flow.
The most common occurrence of
gradually varying flow in storm drainage is the backwater created
by culverts, storm drain inlets, or channel constrictions. For these
conditions, the flow depth will be greater than normal depth in the
channel, and the water surface profile (a.k.a. “backwater curve”)
is computed using either the direct-step or standard step method.
(1) Direct-Step Method.
The direct-step method is best suited
to the analysis of open prismatic channels. Water surface profiles
in simple prismatic channels can be computed manually. Chow (1959)
presents the basic method for applying the direct-step analysis. The
direct-step method is also available in many handheld and personal
computer programs. The most general and widely used programs are the
U.S. Army Corps of Engineers’ HEC-2 Water Surface Profiles and HEC-RAS River Analysis System. The design engineer
may use these programs or proprietary computer software to compute
water surface profiles for channel and floodplain analyses.
(2) Standard-Step Method.
The standard-step method is required
for the analysis of irregular or nonuniform cross-sections. Because
the standard-step method involves a more tedious iterative process,
this manual recommends that design engineers use computer programs
such as HEC-RAS to accomplish these calculations.
Figure 28.32.040: Specific Energy Curve
|
(b) Rapidly Varying Flow.
Rapidly varying flow is characterized
by very pronounced curvature of the water surface profile. The change
in water surface profile may become so abrupt as to result in a state
of high turbulence. Calculation methods for gradually varying flow
(e.g., direct-step and standard-step methods) do not apply for rapidly
varying flow. There are mathematical solutions to some specific cases
of rapidly varying flow, but the solutions to most rapidly varying
flow problems rely on empirical data.
The most common occurrence of rapidly varying flow in storm
drainage applications involves weirs, orifices, hydraulic jumps, nonprismatic
channel sections (transitions, culverts and bridges), and nonlinear
channel alignments (bends). Each of these flow conditions requires
detailed calculations to properly identify the flow capacities and
depths of flow in the given section. The design engineer must be cognizant
of the design requirements for rapidly varying flow conditions and
shall include all necessary calculations as part of the design submittal
documents. The design engineer is referred to the hydraulic references
for the proper calculation methods to use in the design of drainage
facilities with rapidly varying flow facilities.
(Res. 40-08 (§ 803.1), 3-19-08)
(a) The critical flow through a channel is characterized by several important
conditions regarding the relationship between the flow, specific energy,
and slope of a particular hydraulic cross-section (Figure 28.32.040).
Critical state is characterized by the following conditions:
(1) The specific energy (E=y+v2/2g) is at
a minimum for a given discharge (Q).
(2) The discharge (Q) is a maximum for a given specific energy (E).
(3) The specific force is a minimum for a given discharge (Q).
(4) The velocity head (v2/2g) is equal to
half the hydraulic depth (D/2) in a channel of small slope.
(5) The Froude number (Fr) is equal to 1.0.
(b) Typically, channels must not be designed to flow at or near critical
state (0.80<Fr<1.2). If the critical state of uniform flow exists
throughout an entire reach, the channel flow is critical and the channel
slope is at critical slope (Sc). A slope less
than Sc will cause subcritical flow. A slope
steeper than Sc will cause supercritical flow.
A flow at or near the critical state is unstable. Factors creating
minor changes in specific energy, such as channel debris or minor
variation in roughness, will cause a major change in depth.
(c) The criteria of minimum specific energy for critical flow results
in the definition of the Froude number (Fr) as follows:
Where:
Fr
|
=
|
Froude number (dimensionless)
|
v
|
=
|
velocity (ft./s)
|
g
|
=
|
gravitational acceleration (32.2 ft./s2)
|
A
|
=
|
channel flow area (ft.2)
|
T
|
=
|
top width of flow area (ft.)
|
D
|
=
|
hydraulic depth, D=A/T (ft.)
|
(d) The critical depth in a given trapezoidal channel section with a
known flow rate can be determined using the following method:
(1) Step 1.
Compute the section factor (Z).
Where:
|
Z
|
=
|
section factor
|
Q
|
=
|
flow rate (cfs)
|
g
|
=
|
gravitational acceleration (32.2 ft./s2)
|
(2) Step 2.
Determine the critical depth in the channel
(dc) from Figure 28.32.050, using appropriate
values for the section factor (Z), the channel bottom width (b), and
the channel side slope (z).
For other prismatic channel shapes, Equation 28.32-5 determines
the critical depth using the section factors provided in Table 28.32.040(a).
|
(Res. 40-08 (§ 803.2), 3-19-08)
All open channels shall be designed with the limits as stated in GJMC §
28.32.140 through
28.32.330. The following design procedures shall be used when the design runoff in the channel is flowing in a subcritical condition (Fr<1.0).
(a) Transitions. Subcritical transitions occur when transitioning one
subcritical channel section to another subcritical channel section
(expansion or contraction), or when a subcritical channel section
is steepened to create a supercritical flow condition downstream (e.g.,
a sloping spillway entrance).
Figure 28.32.060 presents a number of typical subcritical transition
sections. The warped transition section, although most efficient,
shall only be used in extreme cases where minimum loss of energy is
required since the section is very difficult and costly to construct.
Conversely, the square-ended transition shall only be used when either
a straight-line transition or a cylinder-quadrant transition cannot
be used due to topographic constraints or utility conflicts.
(b) Contractions.
The energy loss created by a contracting
section may be calculated using the following equation:
Where:
Ht
|
=
|
energy loss (ft.);
|
Ktc
|
=
|
contraction transition coefficient
|
V1
|
=
|
upstream velocity (ft./s); and
|
V2
|
=
|
downstream velocity (ft./s); and
|
g
|
=
|
gravitational acceleration (32.2 ft./s2)
|
Figure 28.32.060 shows contraction loss coefficient
(Ktc) values for the typical open-channel transition
section.
(c) Expansions.
The energy loss created by an expanding
transition section may be calculated using the following equation:
Figure 28.32.060 also shows expansion loss coefficients
(Ktc) values for typical open-channel transition
sections.
(d) Transition Length.
The length of the transition section
shall be long enough to keep the streamlines smooth and nearly parallel
throughout the expanding (contracting) section. Experimental data
and performance of existing structures have been used to estimate
the minimum transition length necessary to maintain the stated flow
conditions. Based on this information, guidelines for the minimum
length of the transition section are as follows:
Lt ≥ 0.5Lc (ΔTw)
|
(28.32-8)
|
Where:
Lt
|
=
|
minimum transition length (feet);
|
Lc
|
=
|
length coefficient (dimensionless); and
|
ΔTw
|
=
|
difference in the top width of the normal water surface upstream
and downstream of the transition (feet).
|
Table 28.32.060 below summarizes the transition length
coefficients for subcritical flow conditions. These transition length
guidelines are not applicable to cylinder-quadrant or square-ended
transitions.
For flow approach velocities of 12 ft./sec. or less, the transition
length coefficient (Lc) shall be 4.5. This
represents a 4.5L:1W expansion or contraction, or about a 12.5-degree
divergence from the channel centerline. For flow approach velocities
of more than 12 feet/second, the transition length coefficient (Lc) shall be 10. This represents a 10L:1W expansion or
contraction, or about a 5.75-degree divergence from the channel centerline.
Table 28.32.060: Transition Length Coefficients for Subcritical
Open Channels
|
---|
Flow Approach Velocity (v) (ft/s)
|
Transition Length Coefficient (Lc)
|
---|
= 12
|
4.5
|
> 12
|
10
|
(Res. 40-08 (§ 803.3), 3-19-08)
Mesa County and the City of Grand Junction do not encourage supercritical channels, which typically are concrete-lined. The information presented herein is for completeness and in anticipation that analysis of a supercritical channel may be necessary in the future. All supercritical channels shall be designed within the limits as stated in GJMC §
28.32.140 through
28.32.330. The following design procedures shall be used when channels are designed to flow in a supercritical condition (Fr>1.0).
Supercritical flow can become unstable in response to relatively
minor disturbances to the channel cross-section; even small obstruction
can sometimes cause a hydraulic jump. Good design practice is to test
supercritical flow stability during events smaller than the design
flow by evaluating flows of specific more frequent storm events (e.g.,
10-year, two-year, etc.), or testing successive fractions (e.g., one-half,
one-quarter, and further if necessary) of the design flow. Also, the
designer shall test for small variations in n-value as well. However,
only calculations for the full design flow are required to be submitted
for review.
(a) Transitions.
The design of supercritical flow transitions
is more complicated than subcritical transition design due to the
potential damaging effects of the oblique jump created by the transition.
The oblique jump results in cross waves and higher flow depths that
can cause damage if not properly accounted for in the design. Supercritical
transitions can be avoided by designing a hydraulic jump, which must
also be carefully designed to assure the jump will remain where the
jump is designed to occur. Hydraulic jumps shall be designed to take
place only within concrete-lined portions of the channel, such as
energy dissipation or drop structures.
(b) Contractions.
igure 28.32.070(a) presents an example
of a supercritical contracting transition, with upstream flow contracted
from width b1 to b3 and
a wall diffraction angle of θ. The oblique jump occurs at the
points A and B where the diffraction angles start. Wave fronts generated
by the oblique jumps on both sides propagate toward the centerline
with a wave angle ß1. Since the flow pattern is symmetric, the
centerline acts as if there was a solid wall that causes a subsequent
oblique jump and generates a backward wave front toward the wall with
another angle ß2. These continuous oblique jumps result in turbulent
fluctuations in the water surface.
To minimize the turbulence, the first two wave fronts are designed
to meet at the center and then end at the exit of the contraction.
Using the contraction geometry, the length of the transition shall
be as follows:
LT
|
=
|
b1 – b3
|
(28.32-9)
|
2tanθ
|
Where:
LT
|
=
|
transition length (ft.);
|
b1
|
=
|
upstream top width of flow (ft.);
|
b3
|
=
|
downstream top width of flow (ft.);
|
θ
|
=
|
wall angle as related to the channel centerline (degrees).
|
Using the continuity principle,
Where:
y1
|
=
|
upstream depth of flow (ft.)
|
y3
|
=
|
downstream depth of flow (ft.)
|
Fr1
|
=
|
upstream Froude number
|
Fr3
|
=
|
downstream Froude number
|
Also, by the continuity and momentum principles, the
following relationship between the Froude number, wave angle, and
wall angle is:
Where:
ß1
|
=
|
Initial wave angle (degrees)
|
By trial and error, this design procedure can be used
to determine the transition length and wall angle. Figure 28.32.070(b)
offers a faster solution than trial and error using Equation 28.32-10
and Equation 28.32-11 (above). Figure 28.32.070(b) can also be used
to determine the wave angle (ß), or may be used with the equations
to determine the required downstream depth or width parameter if a
certain transition length is desired or required.
To minimize the length of the transition section, the ratio
of downstream and upstream flow depths should generally be greater
than 2.0 and less than 3.0 (2.0<y3/y1<3.0). The downstream Froude number should generally
be greater than 1.7 to help avoid undulating hydraulic jumps downstream.
For further discussion on oblique jumps and supercritical contractions,
refer to Chow (1959).
Figure 28.32.070(a): Supercritical Contraction Transition
and Angle Definitions
|
(c) Expansions.
A properly designed expansion transition
expands the flow boundaries at approximately the same rate as the
natural flow expansion. Based on experimental and analytical data
results, the minimum length of a supercritical expansion shall be
as follows:
Lt ≥ 1.5 (ΔW) Fr
|
(28.32-12)
|
Where:
Lt
|
=
|
minimum transition length (ft.);
|
ΔW
|
=
|
difference in the top width of the normal water surface upstream
and downstream of the transition
|
Fr
|
=
|
Upstream Froude number
|
(d) Transition Curves.
A transition curve may be used to
reduce the required amount of freeboard or radius of curvature in
a rectangular channel. The length of the transition curve measured
along the channel centerline shall be determined as follows:
Where:
Lc
|
=
|
length of transition curve (ft.);
|
D
|
=
|
distance from the start of curve to point of first maximum superelevation
(ft.). Typically D=3L w; see description of how to apply superelevation allowance in GJMC § 28.32.080;
|
W
|
=
|
top width of design water surface (ft.);
|
V
|
=
|
mean design velocity (ft./s); and
|
y
|
=
|
depth of design flow (ft.).
|
The radius of the transition curves shall be twice the
radius of the main bend. Transition curves shall be located both upstream
and downstream of the main bend.
(e) Slug Flow and Roll Waves.
Steep channels with significantly
rapid flows (Fr>2.0) are prone to developing pulsating flow profiles,
often called slug flows or roll waves. These standing waves can cause
flow to exceed freeboard limits and possible damage to the channel
lining. The design engineer may resolve pulsating flow issues either
by adjusting the channel slope to prevent the development of these
waves or providing additional freeboard to account for the height
of the standing waves.
Theoretically, slug flow will not occur when the Froude number
is less than 2.0. To avoid slug flow when the Froude number is greater
than 2.0, the channel slope shall be as follows:
Where:
S
|
=
|
channel slope (feet per feet);
|
RE
|
=
|
Reynolds Number, RE =
|
uR
|
|
v
|
u
|
=
|
mean design velocity (feet per second);
|
R
|
=
|
hydraulic radius (feet); and
|
v
|
=
|
kinematic viscosity of water (ft2/s).
|
More detailed discussion of pulsating flow is beyond
the scope of this manual. Several references, including Chow (1959)
and Clark County (2000), provide further discussion of this topic.
The Los Angeles County Flood Control District (1982) has developed
nomographs for determining the appropriate freeboard allowance for
roll wave height based on empirical research at the California Institute
of Technology (Brock, 1967).
(Res. 40-08 (§ 803.4), 3-19-08)
Superelevation is the transverse rise in water surface that
occurs around a channel bend, measured between the theoretical water
surface at the centerline of a channel and the water surface elevation
on the outside of the bend. Superelevation in bends shall be estimated
from the following equation:
Where:
r
|
=
|
radius of curvature at centerline of channel (ft.);
|
C
|
=
|
curvature coefficient (see Table 28.32.080);
|
Δy
|
=
|
rise in water surface between design water surface at centerline
of channel and outside water surface elevation (ft.);
|
W
|
=
|
top width at the design water surface at channel centerline
(ft.);
|
V
|
=
|
mean channel velocity (ft./s); and
|
g
|
=
|
gravitational acceleration (ft./s2).
|
The curvature coefficient C shall be 0.5 for subcritical flow
conditions. For supercritical flow conditions, the curvature coefficient
shall be 1.0 for all trapezoidal channels and for rectangular channels
without transition curves, and 0.5 for rectangular channels with transition
curves. Table 28.32.080 provides superelevation curvature coefficients
for various flow regimes, cross-section shapes, and types of curves.
Bends in supercritical channels create cross-waves and superelevated
flow in the bend section as well as further downstream from the bend.
In order to minimize these disturbances, best design practice is to
design the channel radius of curvature to limit the superelevation
of the water surface to 2.0 feet or less. This can be accomplished
by modifying Equation 28.32-15 to determine the allowable radius of
curvature of a channel for a given superelevation value.
Table 28.32.080: Superelevation Curvature Coefficients
|
---|
Flow Type
|
Cross-Section
|
Type of Curve
|
Curvature C
|
---|
Subcritical
|
Rectangular
|
No Transition
|
0.5
|
Subcritical
|
Trapezoidal
|
No Transition
|
0.5
|
Supercritical
|
Rectangular
|
No Transition
|
1
|
Supercritical
|
Trapezoidal
|
No Transition
|
1
|
Supercritical
|
Rectangular
|
with Spiral Transition
|
0.5
|
Supercritical
|
Trapezoidal
|
with Spiral Transition
|
1
|
Supercritical
|
Rectangular
|
with Spiral Banked Transition
|
0.5
|
Source: Corps EM 1110-2-1601 (July 1991)
|
(Res. 40-08 (§ 803.5), 3-19-08)
(a) Channel transitions occur in open channel design whenever there is a change in channel slope or shape and at junctions with other open channels or storm sewers. The goal of a good transition design is to minimize the loss of energy as well as minimize surface disturbances from cross-waves and turbulence. Special cases of transitions where excess energy is dissipated by design are drop structures and hydraulic jumps. Channel drop structures are discussed in GJMC §
28.32.340(d)(2).
(b) Transitions in open channels are generally designed for the following
four flow conditions.
(1) Subcritical flow to subcritical flow.
(2) Subcritical flow to supercritical flow.
(3) Supercritical flow to subcritical flow (hydraulic jump).
(4) Supercritical flow to supercritical flow.
(c) For definition purposes, the conditions in subsections
(b)(1) and
(2) of this section will be considered as subcritical transitions and are later discussed in GJMC §
28.32.110 through
28.32.130. The conditions in subsections
(b)(3) and
(4) of this section will be considered as supercritical transitions and are later discussed in GJMC §
28.32.110 through
28.32.130.
(Res. 40-08 (§ 803.6), 3-19-08)
Open channels can be categorized as either natural or engineered
(artificial). Natural channels include all watercourses that are carved
and shaped by the erosion and sediment transport process. Engineered
channels are those constructed by human efforts. Open channels can
be separated into six different types:
(a) Natural Channels.
Watercourses are carved and shaped
by natural erosion processes before urbanization occurs. As the channel’s
tributary watershed urbanizes, natural channels often experience erosion
and may need grade control checks and localized bank protection to
stabilize.
Natural channels are also strongly influenced by urbanization
in the watershed, which significantly alters the hydrology and therefore,
the geometry of the natural channels. If the watershed imperviousness
exceeds around 10 percent, it is likely that channel geometry will
be altered such that a natural channel is no longer viable and mitigation
measures will be required, such as bank and bed stabilization measures.
(b) Grass-Lined Channels.
Among various types of constructed
or modified drainageways, grass-lined channels are most desirable.
They provide channel storage, lower velocities, groundwater recharge,
and various multiple-use benefits. Low-flow areas may need to be concrete,
rock-lined, or otherwise reinforced with vegetation to minimize erosion
and maintenance problems.
(c) Wetland Vegetation Bottom Channels.
A subset of grass-lined
channels that are designed to encourage the development of wetlands
or certain types of riparian vegetation in the channel bottom. These
channels offer potential benefits that may include wildlife habitat,
groundwater recharge and water quality enhancement. In low-flow areas,
the banks may need supplemental reinforcement to protect against undermining.
(d) Concrete-Lined Channels.
Concrete-lined channels are
high velocity artificial drainageways that are not encouraged. However,
in retrofit situations where existing flooding problems need to be
solved and where right-of-way is limited, concrete channels may offer
advantages over other types of open drainageways. Special attention
shall be taken to provide safety measures (i.e., fence) around the
concrete-lined channels.
(e) Riprap-Lined Channels.
Riprap-lined channels offer a
compromise between a grass-lined channel and a concrete-lined channel.
They can reduce right-of-way needs as compared to grass-lined channels
and avoid the higher costs of concrete-lined channels. Riprap-lined
channels are not encouraged.
(f) Other Lined Channels.
A variety of artificial channel
liners are on the market, all intended to protect the channel walls
and bottom from erosion at higher velocities. These include gabion,
interlocked concrete blocks, concrete revetment mats formed by injecting
concrete into double layer fabric forms, and various types of synthetic
fiber liners. As with rock and concrete liners, all of these types
are best considered for helping to solve existing urban flooding problems
and are not recommended for new developments. Each type of liner has
to be scrutinized for its merits, applicability, how it meets other
community needs, its long-term integrity, and maintenance needs and
costs. Channels lined with artificial materials are not permitted
in new development areas of Mesa County, including the City of Grand
Junction, except by variance to or deviation from these criteria.
(g) Selection of Channel Type.
Mesa County and the City
of Grand Junction do not have a preference for any particular channel-lining
system, as long as it is properly evaluated on its merits. Each type
of channel must be evaluated for its longevity, integrity, maintenance
requirements and costs, and general suitability for community needs,
among other factors. Selection of a channel type that is most appropriate
for the conditions that exist at a project site shall be based on
a multi-disciplinary evaluation, which may include hydraulic, structural,
environmental, sociological, maintenance, economic, and regulatory
factors.
(Res. 40-08 (§ 804), 3-19-08)
(a) In general, a natural channel system continually changes its position
and shape as a result of hydraulic forces acting on its bed and banks.
These changes may be slow or rapid and may result from natural environmental
changes or from changes caused by human activities. When a natural
channel is modified locally, the change frequently causes alteration
in channel characteristics both upstream and downstream. The response
of a natural channel to human-induced changes often occurs in spite
of attempts to control the natural channel environment.
(b) Natural and human-induced changes in natural channels frequently
set in motion responses that can be propagated for long distances.
In spite of the complexity of these responses, all natural channels
are governed by the same basic forces but to varying degrees. It is
necessary that a natural channel system design be based on adequate
knowledge of:
(1) Geologic factors, including soil conditions;
(2) Hydrologic factors, including possible changes in flow and runoff,
and the hydrologic effects of changes in land use;
(3) Geometric characteristics of the stream, including the probable geometric
alterations that developments will impose on the channel;
(4) Hydraulic characteristics such as depth, slope, velocity of streams,
sediment transport, and the changes that may be expected in these
characteristics over space and time; and
(5) Ecological/biological changes that will result from physical changes
that may in turn induce or modify physical changes.
(c) Effects of development in natural channels, flood control measures,
and constructed channel structures have proven the need for considering
the immediate, delayed, and far-reaching effects of alterations imposed
on natural channel systems. Variables affecting natural channels are
numerous and interrelated. Their nature is such that, unlike rigid-boundary
hydraulic problems, it is not possible to isolate and study the role
of each individual variable. Because of the complexity of the processes
occurring in natural flows that influence the erosion and deposition
of material, a detached analytical approach to the problem may be
difficult and time consuming. Most relationships describing natural
channel processes have been derived empirically. The major factors
affecting natural channel geometry are:
(4) Characteristics of bed and bank material;
(5) Bank and bed resistance to flow;
(6) Vegetation or lack thereof;
(7) Geology, including type of sediment; and
(8) Constructed improvements.
(Res. 40-08 (§ 805), 3-19-08)
(a) When seeking to utilize or modify a natural channel, an understanding
of the mechanism of its morphology is important. Without incorporating
a thorough understanding of the geomorphic conditions of the stream
and the watershed, alterations to channels or to their watersheds
can lead to unexpected instabilities, bring about unwanted erosion
or aggradation, and cause significant damage to fluvial systems.
The morphology of a stream is a result of the variables that
determine the quantity of water and sediment it carries, including
the geology, soils and vegetation of the stream and watershed, the
hydrology and dominant discharge of the system, and the slope of the
stream. The following is a short discussion of some fundamentals of
fluvial geomorphology. The users of this manual are encouraged to
review the related textbooks and other technical literatures on the
subject for more detailed discussions. The following is a partial
list of some of the related publications:
•
|
Dave Rosgen, illustrated by Hilton Lee Silvey, Applied River
Morphology, 1996.
|
•
|
Lane, E.W., 14957. A study of the shape of channels formed by
natural streams flossing in erodible material: M.R., D. Sediment Series
No. 9, U.S. Army Engineer Division, Missouri River, Corps of Engineers,
Omaha, NE.
|
•
|
Ritter, Dale F., 1986. Process Geomorphology. Wm C. Brown Publishers,
Dubuque, Iowa.
|
•
|
Simons Li and Associates, 1982. Engineering Analysis of Fluvial
Systems.
|
(b) There are three general principles governing the geomorphology of
a natural stream system. First, riverine systems are dynamic. Erosion
and aggradation can occur over a relatively short period of time (as
sudden as one storm event) and can result from unstable conditions
brought about by changing hydrologic or sediment-supply conditions
(either natural or human induced). However, because all systems are
dynamic, normal progression of a stream is not always a result or
a symptom of instability. Second, the responses resulting from changes
to a channel or its watershed are complex. Morphologic responses can
be anticipated but cannot always be quantitatively predicted, even
by the most experienced engineers. Additionally, short reaches of
streams cannot be looked at individually; a change to a short stretch
or even to a single area of the stream can cause unwanted or unexpected
alterations upstream or downstream of the change. Third, most geomorphic
boundaries within a riverine system can be classified as thresholds.
Gradual changes to a channel or its watershed will not always bring
about gradual responses. Instead, gradual changes may build up to
a threshold so that a small-scale occurrence, such as a moderately
large flood, will seemingly cause a catastrophic result. (Simons Li
and Associates, 1982)
(c) Natural streams can be classified generally into three prevailing
patterns. These patterns, straight, meandering and braided, are characteristics
of the responses of a system to its prevailing discharge and load.
(d) Straight and meandering streams are two manifestations of similar
dynamics. The thalwegs in both shift from bank to bank and sediment
deposition and erosion within the channel bottom establish a series
of riffles and pools. Straight channels have relatively straight banks;
meandering streams have sinuous banks. Straight channels are fairly
rare; most natural channels have some degree of sinuosity. Although
meandering and straight streams can be in quasi-equilibrium, their
thalwegs, meanders and riffle-pool sequences migrate in predictable
patterns if left untouched. Braided systems, unlike meandering and
straight, do not have a single trunk; they have a network of branches
and series of islands. The single branches usually meander to some
degree. Braided channels convey low to medium flows in the series
of branches; large flows intermingle into a single floodplain. Meandering
and straight systems are generally more stable than braided. Braided
channels tend to carve new channels and deposit islands at a relatively
fast pace and be horizontally unstable. The divisions between the
three classifications are imprecise and relatively indistinct. A given
stream can have reaches of each classification, and given reaches
can include characteristics of one or more pattern. (Ritter, 1986)
(e) Any change to a variable of a natural stream system, such as the
slope or dominant discharge, can change the morphology and/or the
existing stream pattern according to the three principles outlined
above. These changes can be somewhat predicted; much work has been
done to establish relationships between the variables and characteristics
of natural streams. Two general relationships for predicting morphological
responses to changes in riverine variables are as follows:
and
Where:
Q
|
=
|
Average discharge
|
Qs
|
=
|
Sediment supply
|
b
|
=
|
Channel width
|
d
|
=
|
Channel depth
|
λ
|
=
|
Meander wavelength
|
S
|
=
|
Bed slope
|
P
|
=
|
Sinuosity
|
(f) An increase in mean annual discharge will generally cause an increase
in channel depth, width, and meander wavelength and a decrease in
bed slope. An increase in sediment supply will generally cause an
increase in channel width, meander wavelength and bed slope and a
decrease in sinuosity and channel width. Because the average flow
rate is usually directly related to sediment supply, these relationships
can become complex when both flow and sediment supply increase or
flow increases and sediment supply decreases, or vice versa. Additionally,
changes to one or more channel morphology characteristics can cause
changes to other characteristics. An increase in slope can cause a
decrease in channel depth or a decrease in meander wavelength. Further
complicating these relationships are variables such as the average
grain-size and type of sediment, the percentage of sediment carried
as bed load, and the geology of the valley, all which can affect the
responses of the stream and contribute to unexpected or seemingly
counterintuitive results.
(g) A general relationship between slope, mean annual discharge and the
tendency of a system to be meandering or braided has been established
by Lane (1957). They found that if a stream’s SQ1/4 is less than or equal to 0.0017, it tends to be
meandering. If SQ1/4 is greater than or
equal to 0.01, systems tend toward a braided pattern. Streams that
have SQ1/4 between 0.0017 and 0.01 are
in an intermediate zone and can be either braided or meandering with
a greater tendency to respond to flow and slope alterations with a
change in river pattern. These relationships are complicated and not
absolute.
(h) Some specific examples of man-induced changes to the natural stream/river
systems that could cause undesired responses by channel morphology
are as follows:
(1) Change in Flow.
As demonstrated in the above relationships,
a decrease in flow due to diversion or reservoir routing change can
cause a decrease in channel width, depth, and sinuosity and an increase
in slope; an increase in flow due to development can have the opposite
effect. In addition to these changes, the corresponding decrease or
increase of average stage of the main stem of a river can have significant
effects on the streams’ tributaries. If the average stage decreases,
the tributaries’ energy slopes will increase, increasing the
ability of the tributary to transport sediment, which can cause degradation
of its channel, commonly referred to as headcutting. Similarly, an
increase in stage in the main stem can lead to aggradation within
its tributaries. Both of these scenarios can do serious damage to
the tributary channel and increase its horizontal instability. Headcutting
can cause bank destabilization and failure. Aggradation can cause
increased flooding potential and rerouting of the channel.
(2) Channelization.
The channelization of a natural stream
to allow increased conveyance often straightens channels and cuts
off meanders, causing an increase in slope through the improved stretch.
This can increase velocities and degradation through the stretch and
then decrease slopes and increase aggradation downstream of the stretch.
The increase in slope and average discharge can also cause a meandering
system to tend toward a braided configuration that can lead to further
horizontal and vertical instabilities. In addition, by lowering the
average stage, channelization will affect the stream’s tributary
channels in the same manner as the first example.
(3) Construction of Dams.
The construction of both large-
and small-scale dams can have far-reaching effects on a stream system.
Without a design approach that will allow frequent flows to travel
through the dam unadulterated, some suspended sediment and most bed
load will be deposited upstream of the dam. This will decrease slopes
and change channel configuration upstream and release clear water
and potentially cause scour and degradation in the downstream reach.
(4) Construction of Bridges and Culverts.
The construction
of bridges and culverts, in addition to the well-documented local
scour issues, can cause more regional channel morphology problems.
An undersized bridge or culvert can decrease velocity and increase
average stage upstream of the bridge, causing deposition and affecting
the tributaries’ channels. Scour around the bridges can cause
an increase in sediment supply in the channel, leading to deposition
downstream.
(i) There are many additional examples of morphological problems that
can be caused by manmade changes on a natural stream system. Any substantial
modification to a natural channel system shall be evaluated carefully
to determine the potential adverse impacts on the stream system both
upstream and downstream of the proposed modification.
(Res. 40-08 (§ 805.1), 3-19-08)
The practice of channel restoration is becoming more common
in the United States as the negative effects of urbanization, channelization,
and other hydraulic “improvements” have taken their toll
on the sediment balance, channel stability, biological habitats, and
the aesthetic and recreational benefits of the impacted rivers and
streams.
Although it may not be feasible to restore a disturbed stream/river
system back to its original condition, channel restoration projects
can help expedite the natural channel recovering process and help
to recreate an environment that closely resembles the original configuration
of the stream system. Channel/river restoration projects typically
involve reconnection of the floodplain back to its channel, establishment
of wetland areas around the channel, restoration of meanders, point-bars
and riffle-pool sequences, and recreation of the chemical and biological
complexity that exists in the natural channel system. Benching, allowing
for a low-flow meandering channel with terraced banks above the low-flow
channel, is a popular technique that allows for expansive riparian
plant and wildlife habitat, recreation opportunities, and unique flood
control options. Channel restoration usually involves a significant
degree of both planting and seeding native, wetland, and self-sustainable
vegetations within the channel and along the banks.
A design team comprised of hydraulic engineers, fluvial geomorphologists,
biologists and botanists who are highly knowledgeable of the system
should be involved in the channel restoration design process. Furthermore,
due to the advantage of irregular alignments and channel cross-sections,
the construction phase shall be carefully managed and overseen to
ensure that the design is fully incorporated into the final improvement.
(Res. 40-08 (§ 805.2), 3-19-08)
GJMC §
28.32.140 through
28.32.260 present general design standards that apply to all improved channels. GJMC §
28.32.270 provides specific design criteria for natural and alluvial bed channels. GJMC §
28.32.280 through
28.32.320 provide specific design criteria for fixed-bed type channels that include: grass-lined channels, wetland bottom channels, riprap-lined channels, concrete-lined channels, and channels with other types of linings.
Depending on the local conditions, the specific requirements
for a particular type channel may be more strict than the general
criteria outlined in this chapter. In addition, unique and unusual
site conditions may require additional design analysis be performed
to verify the suitability of the proposed channel design for the project
site.
(Res. 40-08 (§ 806), 3-19-08)
Six general different types of open channels were presented in GJMC §
28.32.100. In general, the use of concrete-lined and riprap-lined channels is discouraged. The selection of a channel type was presented in general terms in GJMC §
28.32.100(g). The following multi-disciplinary factors shall be used if selecting the channel that is most suitable for a specific site:
(a) Hydraulic Factors.
(6) Ability to drain adjacent lands.
(b) Structural Factors.
(2) Availability of material.
(3) Areas for wasting excess excavated material.
(4) Seepage and uplift forces.
(6) Pressures and pressure fluctuations.
(c) Environmental Factors.
(2) Neighborhood aesthetic requirements.
(3) Need for new green areas.
(4) Street and traffic patterns.
(5) Municipal or county policies.
(8) Water quality enhancement.
(d) Sociological Factors.
(1) Neighborhood social patterns.
(2) Neighborhood children population.
(e) Maintenance Factors.
(2) Repair and reconstruction needs.
(Res. 40-08 (§ 806.1), 3-19-08)
All new open channels shall be designed, at a minimum, to safely
confine and convey the runoff from the 100-year design event.
(Res. 40-08 (§ 806.2), 3-19-08)
Selection of an appropriate channel roughness value for a given
channel section is important for the hydraulic capacity analysis and
design of open channel. The roughness value can vary significantly
depending on the channel type and configuration, density and type
of vegetation, depth of flows, and other hydraulic properties.
Tables 28.32.040(b), 28.32.040(c), 28.32.040(d) and 28.32.040(e) show recommended values for the Manning roughness coefficient for various channel types and conditions. Manning roughness coefficients for riprap channels shall be computed based on the criteria outlined in GJMC §
28.32.040.
(Res. 40-08 (§ 806.3), 3-19-08)
Open channel drainage systems shall be designed assuming uniform flow conditions. GJMC §
28.32.040 presents the uniform flow equation and methods for calculating uniform flow.
(Res. 40-08 (§ 806.4), 3-19-08)
Open channels shall have a minimum longitudinal gradient of
0.5 percent whenever practical. Flatter grades may be approved with
prior consultation with Mesa County or applicable governing agencies.
Open channels with grades flatter than 0.5 percent shall have provisions
for the drainage of nuisance low flows.
Horizontal alignment changes of two degrees or less may be accomplished without the use of a circular curve for subcritical flow designs (Fr<1.0, see GJMC §
28.32.060). Curves must be used for supercritical flow designs (Fr>1.0), no matter the degree of change in horizontal alignment. Curved channel alignments shall have superelevated banks in accordance with GJMC §
28.32.080.
Spiral transition curves shall be used upstream and downstream
of curves for supercritical channel designs with reverse curves or
horizontal alignments with consecutive circular curves. Spiral curves
may also be used to reduce required superelevation allowances and
cross-wave disturbances.
(Res. 40-08 (§ 806.5), 3-19-08)
The design of open channels shall be governed by maximum permissible
velocity. This design method assumes that a given channel section
will remain stable up to a maximum permissible velocity; provided,
that the channel is designed in accordance with the standards presented
in this manual. Table 28.32.200 presents the maximum permissible velocities
for several types of natural, improved, unlined, and lined channels.
Regardless of these maximum permissible velocities, the channel
section shall be designed to remain stable at the final design flow
rate and velocity. The design flow may not always be based on the
highest flow velocity. Therefore, best practice is to confirm channel
section stability during events smaller than the design flow. This
may be accomplished by evaluating flows of specific more frequent
storm events (e.g., 10-year, two-year, etc.), or testing successive
fractions (e.g., one-half, one-quarter, and further if necessary)
of the design flow. However, only calculations for the full design
flow are required to be submitted for review.
Additional geotechnical and geomorphologic investigation and
analyses may be required for natural channels or improved unlined
channels to verify that the channel will remain stable based on the
maximum design velocities.
(Res. 40-08 (§ 806.6), 3-19-08)
Flow can be classified as critical, subcritical, or supercritical
according to the level of energy in the flow. This energy is commonly
expressed in terms of a Froude number (Fr) and critical depth (d
c). GJMC §
28.32.050 discusses the characteristics of critical flow and describes methods for determining Froude number and critical depth. All channel design submittals shall include the calculated Froude number (Fr) and critical depth (d
c) for each unique reach of channel to identify the flow state and
verify compliance with these criteria.
Flow at or near the critical state (Fr=1.0 or d=dc) is unstable. As a result, minor factors such as channel
debris have the potential to cause severe and acute changes in flow
depth. Whenever practicable, channels shall be designed to convey
their design flow following the flow energy limitations described
in Table 28.32.210. When necessary to convey flows at or near critical
state (0.86<Fr<1.13), flow instabilities may be accommodated
by providing additional freeboard.
Table 28.32.210: Limitations on Flow Energy for Rectangular
and Trapezoidal Channels
|
---|
Design Flow Condition
|
Froude Number
|
---|
Subcritical
|
Fr<0.86
|
Supercritical
|
Fr>1.13
|
In rare cases, the specific energy relationship of a cross-section might result in a situation where flows less than the design flow may have a greater depth than the depth calculated for the design flow. The design engineer shall check supercritical channel designs to evaluate whether the channel will maintain freeboard requirements (GJMC §
28.32.220) during flows less than the design flow (see suggested method in GJMC §
28.32.200).
(Res. 40-08 (§ 806.7), 3-19-08)
(a) In the context of this manual, freeboard is the additional height
of a flood control facility (e.g., channel, levee, or embankment)
measured above the design water surface elevation. In this way, the
freeboard will provide a factor of safety when designing open channels.
(b) Open channel facilities conveying a design flow of less than 10 cfs shall have a minimum freeboard of 0.5 feet. Open-channel facilities conveying a design flow of 10 cfs or more shall have a design freeboard based on a minimum freeboard of 1.0 foot, with allowances for velocity, superelevation, standing waves, and/or other water surface disturbances such as slug flow. GJMC §
28.32.060 and
28.32.070 provide design methods for calculating these allowances. Equation 28.32-18 and Equation 28.32-19 describe the minimum design freeboard for subcritical and supercritical flow design, respectively:
Where:
hfr
|
=
|
minimum required freeboard (ft.);
|
v
|
=
|
flow velocity (ft./s);
|
g
|
=
|
gravitational acceleration (32.2 ft./s2);
|
Cv2W
|
=
|
superelevation allowance (ft.), see GJMC § 28.32.080; and
|
rg
|
Δy
|
=
|
allowances for other hydraulic phenomenon (ft.), (e.g., standing waves, slug flow – see GJMC § 28.32.070(e)).
|
(c) Superelevation allowance is a function of flow velocity, channel geometry, and channel alignment. Applying transition curves to the alignment may reduce the required superelevation allowance. GJMC §
28.32.080 discusses the calculations of superelevation allowance in more detail. The superelevation allowance shall be applied to both banks of the channel. The superelevation allowance shall be applied to channel bends in the following manner:
(1) Begin at a point 5.0 times the characteristic wave length of the
design flow (5Lw), measured from the downstream
tangent point of the curve, with no superelevation allowance.
(2) Taper uniformly to the full superelevation allowance at a point 3.0
times the characteristic wave length of the design flow (3.0Lw), measured from the downstream tangent point of the
curve.
(3) Maintain the full superelevation allowance through the curve.
(4) Continue the top of bank elevation level from the upstream tangent
point of the curve to its intersection with the normal top of bank.
(d) Equation 28.32-20 and Equation 28.32-21 describe the characteristic
wavelengths for subcritical and supercritical flow, respectively.
The freeboard under the lowest chord of bridge deck
(i.e., the soffit elevation) shall be a minimum of 1.0 foot during
the 100-year design event. In cases where the bridge has been designed
to withstand hydraulic forces of floodwaters and impact from large
floating debris, the water surface elevation upstream of the bridge
shall maintain a freeboard of at least 1.0 foot below the roadway
crest and the finished floors of structures within the zone influenced
by the bridge headwater.
This manual only describes Mesa County’s and the City
of Grand Junction’s minimum freeboard requirements for open
channel design. Major drainageways involving road crossings or other
types of crossings, streams that the Federal Emergency Management
Agency (FEMA) has mapped as special flood hazard areas, might have
significantly different freeboard requirements.
(Res. 40-08 (§ 806.8), 3-19-08)
Channel transitions occur in open channel design whenever there
is a change in channel slope or shape and at junctions with other
open channels or storm drains. Properly designed flow transitions
mimic the expansion or contraction of natural flow boundaries as best
as possible, as well as minimize surface disturbances from cross-waves
and turbulence. Drop structures and hydraulic jumps are special transitions
where excess energy is dissipated by design. Transitions in open channels
are generally designed for either subcritical or supercritical flow
transitions.
Hydraulic jumps shall be designed to take place only within energy dissipation or drop structures, and not within an erodible channel. Subcritical transitions shall satisfy the minimum transition lengths described in GJMC §
28.32.060. Supercritical transitions shall satisfy the minimum transition lengths described in GJMC §
28.32.070. Special transitions such as drop structures and hydraulic jumps shall satisfy the specifications described in GJMC §
28.32.340.
(Res. 40-08 (§ 806.9), 3-19-08)
(a) Access.
Any easement encompassing a channel shall be
wide enough to provide for the channel structure and adequate maintenance
access. Easements shall be placed on one side of lot or ownership
lines in new developments and in existing developments where conditions
permit.
(1) The minimum width of any channel easement shall be the top width
of channel plus four feet on each side of channel.
(2) Channels with a top width of less than 40 feet require a minimum
12-foot-wide service road parallel to one side of the channel and
a four-foot-wide access on the opposite side, whenever practicable.
(3) Channels 40 feet or more in top width require service roads on both
sides of the channel that are a minimum of 12 feet wide, whenever
practicable.
In all cases, vehicular access to the channel facility must
be provided at intervals of 1,000 feet or less, whenever practical.
Access easements must be at least 12 feet wide, with a maximum grade
of 10 percent.
|
Access ramps shall slope down in the down-gradient direction
whenever practicable. Access ramps designed for personnel access shall
have a maximum slope of 10 percent. When designed to accommodate vehicular
traffic, maintenance access ramps shall be designed to Mesa County
or the local jurisdiction private road standards, whichever is applicable.
|
(b) Safety.
Specific safety requirements shall be determined
on a case-by-case basis in consultation with Mesa County or the applicable
local jurisdiction. As a minimum, guardrails or other approved traffic
barriers as described in the Mesa County Road and Bridge Specifications
shall be provided when a channel is located next to traffic and according
to the AASHTO official guidance.
Fencing or access barriers, as required by the governing agency,
are required for channels abutting residential developments, schools,
parks, and pedestrian walkways as follows:
(1) Fencing is required for all concrete-lined or riprapped channels
where the design frequency storm produces a velocity that exceeds
5.0 feet per second or 2.0 feet in depth. Fencing is also required
when velocity times the depth exceeds 10.0 within 5.0 feet of the
flow water’s edge.
(2) Fencing or access barriers required for all unlined alluvial-bed,
grass-lined, and wetland-bottom channels with side slopes steeper
than 4H:1V where the design frequency storm produces a velocity that
exceeds 5.0 feet per second or 2.0 feet in depth. Fencing is also
required when velocity times the depth exceeds 10.0 within 5.0 feet
of the flow water’s edge.
Gates shall be provided for maintenance and emergency access
at regular intervals, with 20-foot-wide gates placed 1,000 feet on
center and four-foot gates placed 500 feet on center or portion thereof.
Fencing or access barriers shall be located at a minimum of six inches
inside the easement boundary lines unless otherwise approved.
|
(Res. 40-08 (§ 806.10), 3-19-08)
Open channel facilities are often located within or adjacent to sensitive environmental areas. The design engineer must investigate which permits might be necessary from various agencies, such as the U.S. Army Corps of Engineers (e.g., GJMC §
28.16.140 through
28.16.190, wetland permit). It is important that the final permits and/or permit conditions allow for the future and perpetual maintenance of a channel facility without the necessity of returning to a permitting agency for regular maintenance activities.
(Res. 40-08 (§ 806.11), 3-19-08)
Where failure of an open-channel facility might cause flooding
of a public road or structure, the facility shall have an operation
and maintenance plan. These operation and maintenance plans shall
specify regular inspection and maintenance at specific time intervals
(e.g., annually before the wet season) and/or maintenance indicators
when maintenance will be triggered (e.g., vegetation more than six
inches in height). Operation and maintenance plans shall ensure that
vegetation is removed or maintained on a regular basis to maintain
the function of the facility.
Flood control channels require lifetime maintenance. The project
owner and design engineer shall consult with Mesa County and the local
jurisdiction to determine which maintenance mechanism is required
for a particular project. At a minimum, privately owned and maintained
detention facilities shall have a recorded easement agreement with
a covenant binding on successors or other mechanism acceptable to
Mesa County and the local jurisdiction.
(Res. 40-08 (§ 806.12), 3-19-08)
Natural open channels are important drainage elements that contribute
to the image and livability in an urban environment. The areas around
open channels may have other uses that facilitate trails, open space
areas, and wildlife habitat.
(a) Typical Open Channel Design Sections for Natural Channels.
Typical open channel design sections for natural channels are
presented in Figure 28.32.270. The selection of a design section for
a natural channel is generally dependent on the value of developable
land versus the cost to remove the land from a floodplain. The costs
for removal depend on the rate of flow, slope, alignment and depth
of the channel as well as material and fill costs for construction
of the encroachment. The design section discussed herein varies from
no encroachment to the level of encroachment at which point an improved
channel (unlined or lined) becomes more economical or is required
to adequately protect the proposed development. The design standards
of natural channels are the same for both major and minor drainageways.
For natural channel sections, the engineer shall identify, through
stable channel (normal depth) calculations, the stability or instability
of the channel to contain the major storm flows. If this analysis
demonstrates that either bank erosion outside of the designated flow
path (easement and/or right-of-way) or channel degradation is likely
to occur, then an analysis of the magnitude and extent of the erosion
may be necessary. In such a condition, the design engineer shall meet
with the local official to determine:
(1) What additional analysis will be prepared to estimate the potential
extent of lateral and vertical channel movement;
(2) What is the estimate of the potential risk to the proposed development
from channel degradation and/or bank failure;
(3) What solutions and/or remedies are available which can mitigate the
potential risk to the proposed development; and
(4) What improvements and/or reduction in encroachment in or adjacent
to the subject channel will be required to allow approval of the subject
development.
(b) General Design Considerations and Evaluation Techniques for Natural
Channels.
(1) The channel and overbank areas shall have adequate conveyance capacity
for the major storm runoff.
(2) Natural channel segments with a calculated flow velocity greater
than the allowable flow velocity shall be analyzed for erosion potential
with a suitable methodology using standard engineering practice. Additional
erosion protection may be required.
(3) The water surface profiles shall be defined so that the 100-year
floodplain can be delineated.
(4) Filling of the floodplain fringe may reduce valuable storage capacity
and may increase downstream runoff peaks, and therefore shall be avoided.
(5) Erosion control structures, such as drop structures or check dams,
may be required to control flow velocities for both the minor storm
and major storm events.
(6) Plan and profile information (i.e., HEC-2 or HEC-RAS output) for
both existing and proposed floodplain site conditions shall be prepared.
(7) The engineer shall verify, through stable channel (normal depth)
calculations, the suitability of the floodplain to contain the flows.
If this analysis demonstrates erosion outside of the designated flow
path (easement and/or right-of-way), an analysis of the equilibrium
slope and degradation or aggregation depths is required and suitable
improvements identified.
With many natural channels, erosion control structures may need
to be constructed at regular intervals to decrease the thalweg slope
and to minimize erosion. However, these channels shall be left in
as near a natural state as possible. For that reason, extensive modifications
shall not be pursued unless they are found to be necessary to avoid
excessive erosion with substantial deposition downstream.
|
The usual rules of freeboard depth, curvature, and other rules,
which are applicable to artificial channels, do not apply for natural
channels. There are significant advantages that occur if the designer
incorporates into his planning the overtopping of the channel and
localized flooding of adjacent areas, which remain undeveloped for
the purpose of being inundated during the major runoff peak.
|
If a natural channel is to be modified or encroached upon for
a development, then the applicant shall meet with the agencies with
jurisdiction over the channel to discuss the design concept and to
obtain the requirements for planning, design analysis, and documentation.
Channel stability analysis must be based on peak runoff rates from
the long-term projected, urbanized watershed tributary to the channel.
|
(c) Natural Unencroached Channels.
Natural encroached channels
are defined as channels where overlot grading from the development
process does not encroach into the 100-year floodplain of a given
channel. Although the development does not alter the flow carrying
capacity of the floodplain, the development must be protected from
movement of the floodplain boundaries due to erosion and scour. Therefore,
the designer needs to identify locations susceptible to erosion and
scour and provide a design that reinforces these locations to minimize
potential damage to the proposed development. For natural channels
with velocities that exceed stable velocities, erosion protection
may include the construction of buried grade control/check structures
to minimize head-cutting and subsequent bank failures.
(d) Natural Encroached Channels.
Natural encroached channels
are defined as channels where the development process has encroached
into the 100-year floodplain fringe. This definition includes both
excavation and/or fill in the floodplain fringe. The designer shall
prepare a design that will minimize damage to the development from
movement of the floodplain boundaries due to erosion and scour. Consideration
of erosion protection is similar to that for unencroached channels
with emphasis on protection of the fill embankment.
(e) Bank-Lined Channels.
Bank-lined channels are channels
where the banks will be lined but the channel bottom will remain in
a natural state with minimal regrading. The concerns with bank-lined
channels are to minimize scour of the channel bottom at the bank lining
interface as well as maintaining a stable natural channel. The designer
shall prepare a design that addresses scour depths at the lining interface
to assure that the lining extends below this depth to avoid undermining
of the lining.
(f) Partially Lined Channels.
Partially lined channels are
defined as channels in which half of the channel is lined and the
other half is left in a natural or unimproved condition. The concerns
with partially lined channels are twofold. First, the improvement
and lining of one side of the channel will cause changes to the hydraulic
parameters of the unlined section which could increase erosion and
scour in the unlined section. Second, floods which occur during the
temporary condition may damage the improved channel section and require
avoidable costly repairs.
Partially lined channels will only be allowed if:
(1) The bottom paving is bonded, or there is another mechanism in place
to pay for the bottom paving once the channel is completed.
(2) Erosion in the unlined section is addressed to the satisfaction of
the local official.
(3) Scour below the lining is addressed to the satisfaction of the local
official.
The analysis and design shall show that the proposed temporary
channel does not adversely impact the hydraulic parameters and stability
of the unlined section in a significant way.
|
(g) Bio-Engineered Channels.
Bio-engineering is an applied science that integrates structural, biological and ecological principles to construct living structures (plant communities) for erosion, sediment, and flood control purposes. In many instances, bio-engineered channel stabilization measures can be safely utilized to supplement other stabilization measures. Successful application of bio-engineered stabilization measures depends upon accurate diagnosis of the causes of channel stability problems, rather than just treating visible problem areas. The references section of this manual (Chapter
28.72 GJMC) provides useful resources on bio-engineered solutions for natural channel stabilization.
(Res. 40-08 (§ 807), 3-19-08)
Grass-lined channels are desirable artificial channels from
an aesthetic point of view. The engineer shall design grass-lined
channels in accordance with the criteria presented herein, and any
special considerations due to project site specific requirements.
Applicable design parameters include:
(a) Longitudinal Channel Slopes.
In a grass-lined channel,
slopes are determined by the maximum permissible velocity requirements.
A minimum longitudinal gradient of 0.5 percent shall be provided whenever
practical.
(b) Roughness Coefficients.
ables 28.32.040(d) and 28.32.040(e)
present roughness coefficients for grass-lined channels. The design
engineer is to assume a mature channel that has substantial vegetation
with minimal maintenance.
(c) Trickle and Low-Flow Channels.
After urbanization, because
of lawn irrigation return flows and runoff from directly connected
impervious areas, it is not uncommon to see dry waterways having a
continuous flow. A trickle channel is required on all urban grass-lined
channels. Properly designed earth trickle channels are acceptable.
Because of low maintenance and limited infiltration, concrete trickle
channels are also acceptable. In the case of larger streams and rivers,
a low-flow channel may be more appropriate than a trickle channel.
(1) Trickle Channels.
Trickle channels are recommended for
grass-lined channels with a 100-year design flow less than or equal
to 200 cfs. The trickle channel capacity shall convey five percent
of the 100-year design flow rate or five cfs, whichever is greater.
There is no freeboard requirement for trickle channels. The flow capacity
of the main channel shall be determined without considering the flow
capacity of the trickle channel. Care shall be taken to ensure that
low flows enter the trickle channel without flow paralleling the trickle
channel or bypassing inlets to the channel.
Trickle channels are not typically required for swales and other
minor drainageways and grass-lined channels conveying a 100-year peak
runoff of 20 cfs or less. For these smaller channels, the design engineer
shall evaluate the factors such as drainage slope, flow velocity,
soil type, and upstream impervious area, and specify a trickle channel
when needed based on their engineering judgment.
(i)
Concrete Trickle Channel.
Concrete trickle channels
help minimize erosion, silting, and excessive plant growth. Figure
28.32.280 illustrates a typical concrete trickle channel. The concrete
trickle channel shall have a minimum depth of six inches. A Manning
roughness coefficient value of n equals 0.015 will be used to design
the concrete trickle channel.
(ii)
Rock-Lined Trickle Channel.
Rock-lined trickle channels shall have a minimum depth of 12 inches, with the Manning roughness coefficient determined as described in GJMC §
28.32.170. The minimum stone size for rock-lined trickle channels shall be six inches.
(2) Low-Flow Channels.
Low-flow channels are used to contain
relatively frequent flows within a recognizable channel section. Low-flow
channels are recommended for channels with a 100-year flow greater
than 200 cfs, and at a minimum have the capacity to convey the two-year
flow event with no freeboard. The overall flow capacity of the channel
shall include the capacity of the low-flow channel.
Low-flow channels shall have a minimum depth of 12 inches. The maximum side slopes of the low-flow channel shall be 2.5H:1V. The main channel depth limitations (subsection
(e) of this section) do not apply to the low-flow channel area of the overall channel cross-section.
(d) Bottom Width.
The bottom width selected shall be based
on factors such as possible wetland mitigation requirements, constructability,
channel stability, maintenance requirements, and multi-use purposes.
The minimum channel bottom width shall be 5.0 feet for channels
with a concrete trickle channel, 20 feet in channels with a riprap
trickle channel, and 30 feet in channels with a low-flow channel.
(e) Flow Depth and Freeboard.
Swales and grass-lined channels conveying a 100-year flow less than or equal to 10 cfs shall have a minimum freeboard of six inches. Grass-lined channels conveying larger discharges shall meet the minimum freeboard requirements outlined in GJMC §
28.32.220.
The recommended design depth of flow for a grass-lined channel (outside the low-flow channel area) is 5.0 feet for a 100-year flow of 1,500 cfs or less whenever practical. Excessive depths shall also be avoided in channels with greater design flows to the maximum extent practicable. GJMC §
28.32.220 discusses access and safety for open channels, including thresholds for flow depth and velocity.
(f) Side Slopes.
Side slopes of a grass-lined channel shall
be not steeper than 3H:1V.
(g) Grass Lining.
Satisfactory performance of a grass-lined
channel depends on constructing the channel with the proper shape
and preparing the area in a manner to provide conditions favorable
to vegetative growth. Between the time of seeding and the actual establishment
of the grass, the channel is unprotected and subject to considerable
damage unless interim erosion protection is provided. See Chapter
28.6 GJMC, Construction Site Stormwater Runoff Control, for requirements.
The grass lining for channels may be seeded or sodded with a
grass species that is adapted to the local climate and will flourish
with minimal irrigation. Channel vegetation is usually established
by seeding. In the more critical sections of some channels, it may
be required to provide immediate protection by transplanting a complete
sod cover. All seeding, planting, and sodding shall conform to local
landscape recommendations.
(h) Horizontal Channel Alignment and Bend Protection.
The
potential for erosion increases along the outside bank of a channel
bend due to the acceleration of flow velocities on the outside part
of the bend. Thus, it is often necessary to provide supplemental erosion
protection at bends in natural or grass-lined channels.
The minimum radius for channels with a 100-year runoff of 20
cfs or less shall be 25 feet. For channels carrying larger flows,
horizontal channel alignment shall be limited based on the presence
of erosion protection.
No channel bend protection is required along bends where the
radius is greater than two times the top width of the 100-year water
surface. Channels without bend protection shall have a radius of curvature
greater than two times the 100-year flow top width or 100 feet, whichever
is greater.
When erosion protection is provided, the minimum radius of curvature
shall be 1.2 times the 100-year flow top width, or 50 feet, whichever
is greater.
Erosion protection shall extend downstream from the end of the
bend a distance that is equal to the length of the bend measured along
the channel centerline.
(i) Maintenance.
Grass-lined channels shall be maintained
to ensure that vegetation is removed or maintained on a regular basis
to maintain the function of the facility. The project owner shall
ensure that appropriate mechanism is in place to provide maintenance
for the lifetime of the facility.
(Res. 40-08 (§ 808), 3-19-08)
This section presents minimum design criteria for wetland-bottom
channels. The design engineer shall design wetland-bottom channels
in accordance with the criteria presented herein, and any special
considerations for a particular design situation.
Under certain circumstances, such as when existing wetland areas
are affected or natural channels are modified, the Corps of Engineers
Section 404 permitting process may mandate the use of channels with
wetland vegetation. In other cases, a wetland bottom channel may better
suit individual site needs if used to mitigate wetland damages somewhere
else or if used to enhance urban runoff quality. Wetland-bottom channels
are similar to grass-lined channels, except that wetland vegetation
growth is encouraged by eliminating the concrete-lined trickle channel
and flattening longitudinal slope so that low flows have low velocities.
There are potential benefits associated with a wetland bottom
channel, such as wildlife and water quality enhancement as the base
flows move through the marshy vegetation.
(a) Longitudinal Channel Slope.
The longitudinal channel
slope shall be set to the maximum permissible velocity criteria provided
in Table 28.32.200 is not violated. To prevent channel degradation,
the channel slope shall be determined assuming there is not wetland
vegetation on the bottom (i.e., “new channel”). In addition
to the velocity requirements, the Froude number for the new channel
condition shall be less than 0.7.
(b) Roughness Coefficients.
The channel shall be designed
for two flow roughness conditions. As previously mentioned, a Manning’s
roughness coefficient assuming there is no growth in the channel bottom
is used to set the channel slope. This is referred to as the new channel
condition. The mature channel condition assumes that wetland vegetation
in the channel bottom has been established. The required channel depth
including freeboard is determined assuming mature channel conditions.
A composite Manning’s roughness coefficient shall be used
for the new channel condition design and the mature channel condition
design. The composite Manning’s roughness coefficient is determined
by the following equation (Chow, 1959):
Where:
nc
|
=
|
Manning’s roughness coefficient for the composite channel
(Dimensionless)
|
n0
|
=
|
Manning’s roughness coefficient for areas above the wetland
area (Dimensionless)
|
nw
|
=
|
Manning’s roughness coefficient for the wetland area (Dimensionless)
|
Po
|
=
|
Wetland perimeter of channel cross-section above the wetland
area (feet)
|
Pw
|
=
|
Wetland perimeter of the wetland channel bottom (feet)
|
For grass-lined areas above the wetland area, use a
Manning’s roughness coefficient n0 equals
0.035. Manning’s roughness coefficients for the wetland area
(nw) can be obtained from Figure 28.32.290.
(c) Low-Flow and Trickle Channel.
Concrete trickle channels are not permitted in wetland bottom channels. Low-flow channels may be used when the 100-year flow exceeds 1,000 cfs. The design of the low-flow channel shall be according to GJMC §
28.32.280(c)(2).
(d) Bottom Width.
The following design factors shall be
considered in selecting an appropriate channel bottom width:
(1) Wetland mitigation requirements.
(3) Channel stability and maintenance.
(e) Flow Depth and Freeboard.
Typically, the maximum design
depth of flow (outside of the low-flow channel area) should not exceed
5.0 feet for a 100-year flow of 1,500 cfs or less. For greater flows,
excessive depths shall be avoided to minimize high velocities and
for public safety considerations.
Wetland bottom channels shall meet the minimum freeboard requirements outlined in GJMC §
28.32.220.
(f) Side Slopes.
Side slopes shall not be designed steeper
than three horizontal to one vertical.
(g) Grass-Lining.
The side slopes may be grass-lined according to the guidelines provided previously in GJMC §
28.32.280(g).
(h) Channel Bend Protection.
Channel bends shall be designed according to the criteria discussed previously in GJMC §
28.32.280(h).
(i) Channel Crossings.
Whenever a wetland bottom channel
is crossed by a road, railroad or a trail requiring a culvert or a
bridge, a drop structure shall be provided immediately downstream
of such a crossing. This will help reduce the silting-in of the crossing
with sediments. A one-foot to two-foot drop is recommended. The designer
shall determine the hydraulics of the crossing and the drop structure
and design the structures to ensure the stability of the channel.
(j) Life Expectancy.
Wetland bottom channels are expected
to fill with sediment over time. This occurs because the bottom vegetation
traps some of the sediments carried by the flow. The life expectancy
of such a channel will depend primarily on the land use of the tributary
watershed and could range anywhere from 20 to 40 years before major
channel dredging is needed. However, life expectancy can be dramatically
reduced, to as little as two to five years, if land erosion in the
tributary watershed is not controlled. Therefore, land erosion practices
need to be strictly controlled during new construction within the
watershed and all facilities need to be built to minimize soil erosion
in the watershed to maintain a reasonable economic life of a wetland
bottom channel.
(Res. 40-08 (§ 809), 3-19-08)
This section presents minimum design criteria for riprap-lined
channels. The engineer shall design riprap-lined channels in accordance
with the criteria presented herein, and any special considerations
arising out of a particular design situation.
Riprap-lined channels are defined as channels in which riprap
is used for lining of the channel banks and the channel bottom to
control erosion. Design standards for riprap-lined channels shall
also be used for transitions and bends when lined with riprap. Riprap
is used to control erosion in both channel banks and beds, transition
sections upstream and downstream of hydraulic structures, at bends,
and bridges.
Loose or grouted riprap is cost effective for short channel
reaches. Riprap lining might be appropriate for (1) flows that produce
channel velocities in excess of allowable values; (2) channels where
side slopes need to be steeper than 3:1; (3) at channel bends and
transitions; and (4) low-flow channels.
Specific design parameters for riprap-lined channels include:
(a) Longitudinal Channel Slope.
The maximum permissible
velocity for riprap-lined channels is presented in Table 28.32.200.
In steeper terrain, drop structures may be used to achieve the desired
design velocities.
(b) Roughness Coefficients.
The Manning’s roughness
coefficient, n, for hydraulic computations may be estimated for loose
riprap using the following equation.
n = .0395(d50)1/6
|
(28.32-23)
|
Where:
d50
|
=
|
mean stone size (feet)
|
This equation (Anderson, 1968) does not apply to grouted
riprap (n equals 0.023 to 0.030) or to very shallow flow (hydraulic
radius is less than or equal to two times the maximum rock size) where
the roughness coefficient will be greater than indicated by the formula.
(c) Low-Flow Channel.
The design criteria for the low-flow channel are discussed in GJMC §
28.32.280(c)(2).
(d) Bottom Width.
The following design factors shall be
considered in selecting an appropriate channel bottom width.
(2) Channel stability and maintenance;
(4) Trickle/low-flow channel width.
(e) Flow Depth.
As preliminary criteria, the design depth
of flow for the major storm runoff flow shall not exceed 7.0 feet
in areas of the channel cross-section outside the low-flow or trickle
channel.
(f) Side Slopes.
Due to stability, safety, and maintenance
considerations, riprap-lined side slopes shall be two horizontal to
one vertical or flatter.
(g) Toe Protection.
Where only the channel sides are to
be lined, additional riprap is needed to provide for long-term stability
of the lining. In this case, the riprap blanket shall extend a minimum
of three feet below the proposed channel bed, and the thickness of
the blanket below the proposed channel bed shall be increased to a
minimum of three times d50 to accommodate possible
channel scour during floods. If the velocity exceeds the permissible
velocity requirements of the soil comprising the channel bottom, a
scour analysis shall be performed to determine if the toe requires
additional protection.
(h) Beginning and End of Riprap-Lined Channel.
At the upstream
and downstream termination of a riprap lining, the thickness shall
be increased 50 percent for at least three feet to prevent undercutting.
Depending on the site-specific conditions, concrete cutoff walls at
both ends may be necessary.
(i) Loose Riprap Lining.
Loose riprap, or simply riprap,
refers to a protective blanket of large loose angular stones that
are usually placed by machine to achieve a desired configuration.
The term loose riprap has been introduced to differentiate loose stones
from grouted riprap.
Many factors govern the size of the rock necessary to resist
the forces tending to move the riprap. For the riprap itself, this
includes the size and weight of the individual rock, the shape of
stones, the gradation of the particles, the blanket thickness, the
type of bedding under the riprap, and slope of the riprap layer. Hydraulic
factors affecting riprap include the velocity, current direction,
eddy action, and waves. Figure 28.32.300(a) provides typical cross-sections
for riprap-lined channels.
Experience has shown that riprap failures generally result from
undersized individual rocks in the maximum size range, improper gradation
of the rock which reduces the interlocking of individual particles
and improper bedding for the riprap which allows leaching of channel
particles through the riprap blanket.
(1) Riprap Material.
Rock used for loose riprap, grouted
riprap, or wire enclosed riprap shall be hard, durable, angular in
shape, and free from cracks, overburden, shale and organic matter.
Neither breadth nor thickness of a single stone shall be less than
one-third of its length and rounded stone shall be avoided. Rock having
a minimum specific gravity of 2.65 is preferred; however, in no case
shall the specific gravity of the individual stones be less than 2.50.
Classification and gradation for riprap are shown in Table 28.32.300(a)
and are based on a minimum specific gravity of 2.50 for the rock.
Because of its relatively small size and weight, riprap Class 150
must be buried with native topsoil and revegetated to protect the
rock from vandalism.
Riprap lining requirements for a stable channel lining are based
on the following relationship which resulted from model studies by
Smith and Murray (Smith, 1965)
Where:
d50
|
=
|
Rock size for which 50 percent of riprap by weight is smaller
(feet)
|
V
|
=
|
Mean channel velocity (fps)
|
S
|
=
|
Longitudinal channel slope (feet/feet)
|
Ss
|
=
|
Specific gravity of rock (minimum Ss =
2.50) (dimensionless)
|
The riprap blanket thickness shall be at least 2.0 times
d50 and shall extend up the side slopes to
an elevation of the design water surface plus the calculated freeboard.
(2) Bedding Requirements.
Long-term stability of riprap
erosion protection is strongly influenced by proper bedding conditions.
A large percentage of all riprap failures is directly attributable
to bedding failures. Gradations for granular riprap bedding are shown
in Table 28.32.300(b).
Properly designed bedding provides a buffer of intermediate
sized material between the channel bed and the riprap to prevent movement
of soil particles through the voids in the riprap. Three types of
bedding are in common use: generic single layer granular bedding,
granular bedding based on the T-V (Terzughi-Vicksburg) methodology,
and filter fabric.
(i)
Granular Bedding – Generic Design.
The gradation of a single layer bedding specification is based on
the assumption that said bedding will generally protect the underlying
soil from displacement during a flood event. The single layer bedding
design does not require any soil information, but in order to be effective
covering a wide range of soil types and sizes, this method requires
a greater thickness than the T-V method.
A single 12-inch layer of said granular bedding can be used
except at drop structures. At drop structures, filter fabric must
be added below the 12-inch layer of granular bedding.
(ii)
Granular Bedding – T-V Design.
The
T-V design establishes an optimum granular bedding gradation for a
specific channel soil. Since this method designs the granular bedding
for a particular soil, the allowable granular bedding thickness may
be much less than the generic design.
The specifications for the T-V reverse filter method relate
the gradation of the protective layer (filter) to that of the bed
material (base) by the following inequalities:
|
(28.32-25)
|
|
(28.32-26)
|
|
(28.32-27)
|
Where the capital “D” refers to the filter
grain size and the lower case “d” to the base grain size.
The subscripts refer to the percent by weight which is finer than
the grain size denoted by either “D” or “d.”
For example, 15 percent of the filter material is finer than D15(filter) and 85 percent of the base material is finer
than d85(base).
When the T-V method is used, the thickness of the resulting
layer of granular bedding may be reduced to six inches. However, if
a gradation analysis of the existing soils shows that a single layer
of T-V method designed granular bedding cannot bridge the gap between
the riprap specification and the existing soils, then two or more
layers of granular bedding shall be used. The design of the bedding
layer closest to the existing soils shall be based on the existing
soil gradation. The design of the upper bedding layer shall be based
on the gradation of the lower bedding layer. The thickness of each
of the two or more layers shall be four inches.
(iii)
Filter Fabrics.
Filter fabric is not a complete
substitute for granular bedding. Filter fabric provides filtering
action only perpendicular to the fabric and has only a single equivalent
pore opening between the channel bed and the riprap. Filter fabric
has a relatively smooth surface which provides less resistance to
stone movement. As a result, it is recommended that the use of filter
fabric in place of granular bedding be restricted to slopes no steeper
than 2.5 horizontal to one vertical, and that such filter fabric only
replace the bottom layer in a multi-layer T-V method granular bedding
design. The granular bedding shall be placed on top of the filter
fabric to act as a cushion when placing the riprap. Tears in the fabric
greatly reduce its effectiveness so that direct dumping of riprap
on the filter fabric is not allowed and due care must be exercised
during construction. Nonetheless, filter fabric has proven to be an
adequate replacement for granular bedding in many instances. Filter
fabric provides adequate bedding for channel linings along uniform
mild sloping channels where leaching forces are primarily perpendicular
to the fabric.
At drop structures and sloped channel drops, where seepage forces
may run parallel with the fabric and cause piping along the bottom
surface of the fabric, special care is required in the use of filter
fabric. Seepage parallel with the fabric may be reduced by folding
the edge of the fabric vertically downward about two feet (similar
to a cutoff wall) at 12-foot intervals along the installation, particularly
at the entrance and exit of the channel reach. Filter fabric has to
be lapped a minimum of 12 inches at roll edges with upstream fabric
being placed on top of downstream fabric at the lap.
Fine silt and clay has been found to clog the openings in filter
fabric. This prevents free drainage which increases failure potential
due to uplift. For this reason, a granular filter is often more appropriate
bedding for fine silt and clay channel beds.
(j) Grouted Riprap Lining.
Grouted riprap provides a relatively
impervious channel lining which is less subject to vandalism than
loose riprap. Grouted riprap requires less routine maintenance by
reducing silt and trash accumulation and is particularly useful for
lining low-flow channels and steep banks. The appearance of grouted
riprap is enhanced by exposing the tops of individual stones and by
cleaning excess grout from the projecting rock with a wet broom prior
to curing. Figure 28.32.300(b) provides a typical cross-section for
a grouted riprap lining.
(1) Riprap Material.
The rock used for grouted riprap is
different from the standard gradation of riprap in that the smaller
rock has been removed from the groundwater to allow larger void spaces
and, therefore greater penetration by the grout. The riprap specifications
are shown on Table 28.32.300(c). Riprap smaller than Class 400 shall
not be grouted.
(2) Bedding Material.
The bedding material requirements
are the same as for loose riprap.
(3) Cutoff Trench.
As the riprap layer is placed, a cutoff
trench shall be excavated around the rock section at the top of the
slope and at the upstream and downstream edges. The trench shall be,
at a minimum, the full depth of the riprap and bedding layer and at
least one foot wide. This trench is filled with grout to prevent water
from undermining the grouted rock mass.
(4) Grout.
After the riprap has been placed to the required
thickness and the trench excavated, the rock is sprayed with clean
water which cleans the rock and allows better adherence by the grout.
The rock is then grouted using a low pressure (less than 10 psi) grout
pump with a two-inch maximum diameter hose. Using a low pressure grout
pump allows the work crew time to move the hose and vibrate the grout.
Vibrating the grout with a pencil vibrator assures complete penetration
and filling of the voids. After the grout has been placed and vibrated,
a small hand broom or gloved hand is used to smooth the grout and
remove any excess grout from the rock. The finished surface is sealed
with a curing compound.
The grout shall consist of six sacks (564 pounds) of cement
per cubic yard, and the aggregate shall consist of 30 percent of 3/8-inch
coarse gravel and 70 percent natural sand. The grout shall contain
7.5 percent plus or minus 1.5 percent air entrainment, have a 28-day
compressive strength of at least 2,000 psi, and have a slump of seven
inches plus or minus two inches. Fiber reinforcement shall be used
such as 1.5 pounds per cubic yard of Fibermesh or an approved equivalent
amount. A maximum of 25 percent fly ash may be substituted for the
cementations material.
(k) Channel Bend Protection.
When riprap protection is required
for a straight channel, increase the rock size by one category (e.g.,
Class 300 to Class 400) through bends. The minimum radius for a riprap-lined
bend is 1.2 times the top width and in no case less than 50 feet.
Riprap protection shall extend downstream from the end of the bend
a distance that is equal to the length of the bend measured along
the channel centerline.
(l) Transition Protection.
Scour potential is amplified
by turbulent eddies in the vicinity of rapid changes in channel geometry
such at transitions and bridges. For these locations, the riprap lining
thickness shall be increased by one size category.
Protection shall extend upstream from the transition entrance at least 5.0 feet and extend downstream from the transition exit at least 10 feet. See GJMC §
28.32.060 and
28.32.070 for further discussions on transitions.
(m) Concrete Cutoff Walls.
Transverse concrete cutoff walls
may be required for riprap-lined channels where a resulting failure
of the riprap lining may seriously affect the health and safety of
the public. The designer shall consult with the local officials prior
to design of riprap-lined channels to determine if concrete cutoff
walls are required as well as their sizing and spacing, if required.
(n) Riprap-Lined Channels on Steep Slopes.
Achieving channel
stability on steep slopes usually requires some type of channel lining.
The only exception is a channel constructed in durable bedrock.
On mild slopes, the water velocity is slow enough and the depth
of flow is large enough (relative to the riprap size) that a reasonable
estimate of the resistance to flow can be made. On steep channels,
the riprap size required to stabilize the channel is on the same order
of magnitude or greater than the flow depth, which invalidates the
Manning’s relation. Since the resistance to flow is unknown,
an estimate of the velocity needed for the design of the riprap cannot
be accurately estimated.
A graphically based methodology was developed for the U.S. Department
of the Interior, Office of Surface Mining Reclamation and Enforcement
(SIMONS, 1989) to design riprap-lined channels on steep slopes (supercritical
flow). This methodology was based on a study by Bathurst, 1979, that
analyzed the hydraulics of mountain rivers where roughness elements
are on the same order of magnitude as the depth of flow. Using the
resistance equation developed by Bathurst, the velocity can be estimated
for a given riprap size. The velocity is then used to predict the
stability of the riprap.
This procedure shall be used for all riprap-lined channels whose
depth of flow is equal to or less than d50 as
computed initially using Equation 28.32-24.
(1) Rock Size.
Five sets of design curves (Figures 28.32.300(c)
through 28.32.300(g)) have been developed from Bathurst’s relationship
to simplify riprap design for steep channels. The design curves were
developed for channels with two to one side slopes and bottom widths
of zero feet, six feet, 10 feet, 14 feet, and 20 feet. The curves
were terminated at the point where flow velocity exceeded 15 fps.
A median rock diameter could be determined that would be stable at
higher flows and velocities; however, rock durability at velocities
greater than 15 fps becomes of greater concern.
For a given flow, channel slope, and channel width, Figures
28.32.300(c) through 28.32.300(g) will provide the median riprap size.
When the channel slope is not provided by one of the design curves,
linear interpolation is used to determine the riprap size. This is
done by extending a horizontal line at the given flow through the
curves with slopes bracketing the design slope. A curve at the design
slope is then estimated by visual interpolation. The design D50 size is then chosen at the point that the flow intercepts
the estimated design curve. Linear interpolation can also be used
to estimate the D50 size for bottom widths
other than those supplied in the figures.
For practical engineering purposes, the D50 size specified for the design shall be given in 0.25-foot increments.
The final minimum design size is determined using Table 28.32.300(d).
(2) Riprap Gradation for Steep Slopes. Table 28.32.300(e) provides ratios
used to determine the D
10, D
20, and D
max rock sizes from the D
50 rock size determined in subsection
(n)(1) of this section. It is important to establish a smooth gradation from the largest to the smallest sizes to prevent large voids between rocks.
(3) Riprap Thickness for Steep Slopes.
For riprap linings
on steep slopes, a thickness of 1.25 times the median rock size is
recommended. The maximum resistance to the erosive forces of flowing
water occurs when all rock is contained within the riprap layer thickness.
Oversize rocks that protrude above the riprap layer reduce channel
capacity and reduce riprap stability.
(4) Riprap Placement on Steep Slopes.
Improper placement
is another major cause of failure in riprap-lined channels. To prevent
segregation of rock sizes, riprap shall be dumped directly from trucks
from the top of the embankment, and draglines with orange peel buckets,
backhoes, and other power equipment to place riprap with minimal handwork.
(5) Freeboard.
Figures 28.32.300(c) through 28.32.300(g)
also provide the depth of flow for a given flow rate, channel slope,
and channel dimensions. The minimum required freeboard is given by
Equation 28.32-18 for subcritical flow or 28.32-19 for supercritical
flow. The velocity can be estimated by dividing the flow rate by the
area of flow.
(6) Bedding Requirements on Steep Slopes.
Either a granular bedding material or filter fabric may be used on steep slopes according to the requirements previously specified in subsection
(i) of this section.
(Res. 40-08 (§ 810), 3-19-08)
This section presents minimum design criteria for concrete-lined channels. The design engineer is responsible for confirming that a channel design meets these criteria, the general open channel criteria outlined in GJMC §
28.32.140 through
28.32.260, and any specific considerations due to project specific site requirements.
Applicable design parameters include:
(a) Longitudinal Channel Slope.
Concrete-lined channels
have the ability to accommodate supercritical flow conditions and
thus can be constructed almost on any naturally occurring slope. The
maximum slope is determined by the maximum permissible velocity shown
in Table 28.32.200.
(b) Roughness Coefficients.
See figures presented in Tables
28.32.040(b) through 28.32.040(d) for appropriate Manning roughness
coefficient for concrete-lined channels. For subcritical flow for
concrete-lined channels, check the Froude number using a Manning roughness
coefficient of 0.011.
(c) Low-Flow Channel.
The bottom of the concrete-lined channel shall be sloped to confine the low flows to the middle or one side of the channel. Low-flow channels are defined in GJMC §
28.32.280(c)(2).
(d) Bottom Width.
The bottom width of the concrete-lined
channel shall be a minimum of 4.0 feet.
(e) Flow Depth.
There are no flow depth requirements for
concrete-lined channels.
(f) Side Slopes.
Vertical or flatter side slopes may be
designed for concrete-lined channels.
(g) Concrete-Lining Section.
(1) Thickness.
Concrete-lined channels shall have a minimum
thickness of six inches for flow velocities less than 30 feet per
second (fps). For flow velocities of 30 fps and greater, the minimum
thickness shall be seven inches.
(2) Concrete Joints.
The following design standards, found
to work in similar conditions, are suggested. Alternatives will be
considered on a case-by-case basis.
(i) Channels shall be continuously reinforced without transverse joints.
Expansion/contraction joints (without continuous reinforcement) shall
only be installed where the new concrete lining is connected to a
rigid structure or to an existing concrete lining which is not continuously
reinforced. The design of the expansion joint shall be coordinated
with local officials.
(ii)
Longitudinal joints, where required, shall be constructed on
the sidewalls at least 1.0 foot vertically above the channel invert.
(iii)
All joints shall be designed to prevent differential movement.
(iv)
Construction joints are required for all cold joints and where
the lining thickness changes. Reinforcement shall be continuous through
the joint and the concrete-lining shall be thickened at the joint.
(3) Concrete Finish.
The surface of the concrete lining
shall be provided with a wood float finish unless the design requires
additional finishing treatment.
Excessive working or wetting of the finish shall be avoided
if additional finishing is required.
(4) Concrete Curing.
It is suggested that concrete-lined
channels be cured by the application of a liquid membrane-forming
curing compound (white pigmented) upon completion of the concrete
finish. All curing shall be completed in accordance with the standard
specifications of the local government agency.
(5) Reinforcement Steel.
(i) Steel reinforcement shall be at a minimum grade 40 deformed bars.
Wire mesh shall not be used.
(ii)
Ratio of longitudinal steel area to concrete cross-sectional
area shall be greater than 0.0905 but not less than a No. 4 rebar
placed at 12-inch spacing. The longitudinal steel shall be placed
on top of the transverse steel.
(iii)
Ratio of transverse steel area to concrete cross-sectional area
shall be greater than 0.0025 but not less than a No. 4 rebar placed
at 12-inch spacing.
(iv)
Reinforcing steel shall be placed near the center of the section
with a minimum clear cover of three inches adjacent to the earth.
(v) Additional steel shall be added as needed. If a retaining wall structure
is used, the structure shall be designed by a registered structural
engineer with structural design calculations submitted for review
and approval.
(6) Earthwork.
As a minimum, the following areas shall be
compacted to at least 90 percent of maximum density as determined
by ASTM 1557 (Modified Proctor). Additional requirements may be required
by the geotechnical report.
(i) The 12 inches of subgrade immediately beneath concrete lining (both
channel bottom and side slopes).
(ii)
Top 12 inches of maintenance road.
(iii)
Top 12 inches of earth surface within 10 feet of concrete channel
lip.
(7) Bedding.
A geotechnical report shall be submitted which
addresses the required bedding necessary for the specific concrete
section under consideration.
(8) Underdrain and Weepholes.
A transverse concrete cutoff
shall be installed at the beginning and end of the concrete-lined
section of channel and at a maximum spacing of 90 feet. The concrete
cutoffs shall extend a minimum of 3.0 feet below the bottom of the
concrete slab and across the entire width of the channel lining. Longitudinal
cutoffs, a minimum of 3.0 feet in depth, at top lining are required
to ensure integrity of the concrete lining.
If the channel is continuously reinforced without transverse
joints then a concrete cutoff is required to be incorporated into
the expansion/concrete joint.
(h) Special Consideration for Supercritical Flow.
Supercritical
flow in an open channel in an urbanized area creates hazards which
the designer shall take into consideration. Careful attention shall
be taken to ensure against excessive waves which may extend down the
entire length of the channel from only minor obstructions. Imperfections
at joints may rapidly cause a deterioration of the joints, in which
case a complete failure of the channel can readily occur. In addition,
high velocity flow entering cracks or joints creates an uplift force
by the conversion of velocity head to pressure head which can damage
the channel lining.
Generally, there should not be a drastic reduction in cross-section
shape, and diligent care shall be taken to minimize the change in
wetted area of the cross-section at bridges and culverts. Bridges
and other structures crossing the channel shall be anchored satisfactorily
to withstand the full dynamic load which might be imposed upon the
structure in the event of major debris plugging.
The concrete lining shall be protected from hydrostatic uplift
forces, which are often created by a high-water table or momentary
inflow behind the lining from localized flooding. Generally, an underdrain
will be required under and/or adjacent to the lining. The underdrain
shall be designed to be free draining. With supercritical flows, minor
downstream obstructions do not create any backwater effect. Backwater
computation methods are applicable for computing the water-surface
profile or the energy gradient in channels having a supercritical
flow; however, the computations shall proceed in a downstream direction.
The designer shall take care to ensure against the possibility of
unanticipated hydraulic jumps forming in the channel.
(Res. 40-08 (§ 811), 3-19-08)
Other channel linings include all channel linings that are not
discussed in the previous sections. These include composite-lined
channels, which are channels in which two or more different lining
materials are used (i.e., riprap bottom with concrete side slope lining).
They also include gabions, soil cement linings, synthetic fabric and
geotextile linings, preformed block linings, reinforced soil linings,
and flood walls (vertical walls constructed on both sides of an existing
floodplain). The wide range of composite combinations and other lining
types does not allow a discussion of all potential linings in this
manual. For these linings not discussed in this manual, supporting
documentation will be required to support the use of the desired lining.
A guideline of some of the items which should be addressed in the
supporting documentation is as follows:
(a) Structural integrity of the proposed lining.
(b) Interfacing between different linings.
(c) The maximum velocity under which the lining will remain stable.
(d) Potential erosion and scour problems.
(e) Access for operations and maintenance.
(f) Long-term durability of the product under extreme meteorological
and soil conditions.
(g) Ease of repair of damaged section.
(h) Past case history (if available) of the lining system in other arid
areas.
(i) Potential groundwater mitigation issues (i.e., weepholes, underdrains,
etc.).
These linings will be allowed on a case-by-case basis. Mesa
County and/or other jurisdictions may reject the proposed lining system
in the interest of operation, maintenance, and protecting the public
safety.
|
(Res. 40-08 (§ 812), 3-19-08)
A minor drainageway is defined as a channel/drainageway with
a contributing tributary area of less than 160 acres. Additional flexibility
and less stringent standards may be allowed for minor drainageways.
Only the differences in a channel type’s design as a minor drainageway
versus that of a major drainageway are presented in this section.
(a) Freeboard.
For grass, concrete and riprap-lined swales
and drainageways with a 100-year flow of equal to or less than 10
cfs, the minimum freeboard requirement is six inches.
(b) Curvature (Horizontal).
For grass, wetland bottom, concrete
and riprap-lined swales and drainageways, the minimum radius with
a 100-year runoff of 20 cfs or less shall be 25 feet.
(c) Trickle Channel.
For grass, concrete and riprap-lined
swales and drainageways, with 100-year runoff peaks of 20 cfs or less,
trickle channel requirements will be evaluated for each case. Trickle
channels help preserve swales crossing residential property. Factors
to be considered when establishing the need for trickle channels are:
drainage slope, flow velocity, soil type, and upstream impervious
area.
(Res. 40-08 (§ 813), 3-19-08)
Presented in this section are the design standards for appurtenances
to improved channels. All improved channels shall be designed to include
these appurtenances.
(a) Maintenance Access Road.
A maintenance access road with
a minimum passage width of 12 feet shall be provided along the entire
length of all improved channels with 100-year design capacity equal
to or greater than 50 cfs. For such channels with less than 50 feet
in top width, one maintenance access shall be provided as part of
the channel improvements. For channels with greater than 50 feet in
top width, the maintenance road shall be located in or within 10 feet
horizontal distance from the bottom of the channel or on both sides
of the channel top.
For channels with the maintenance access road at or near the
channel bottom, ramps to said road shall be provided at a maximum
10 percent slope. Said ramps shall slope down in the down gradient
direction of the channel.
(b) Safety Requirements.
The following safety requirements
are required for concrete-lined channels. Similar safety requirements
may be required for all other channels.
(1) A six-foot-high galvanized-coated chain link or comparable fence
shall be installed to prevent unauthorized access. The fence shall
be located at the edge of the right-of-way or on the top of the channel
lining. Gates, with top latch, shall be placed at major access points
or 1,320-foot intervals, whichever is less.
(2) Ladder-type steps shall be installed not more than 1,200 feet apart
and shall be staggered on alternating sides of the channel to provide
a ladder every 600 feet. The bottom rung shall be placed approximately
12 inches vertically above the channel invert.
(c) Culvert Outlet Protection.
If the flow velocity at a
culvert or storm sewer outlet exceeds the maximum permissible velocity
for the local soil or channel lining, channel protection is required.
This protection usually consists of an erosion resistant reach, such
as riprap and stilling basin, to provide a stable reach at the outlet
in which the exit velocity is reduced to a velocity allowable in the
downstream channel.
The following basin sizing procedure shall be used for culvert
sizes less than or equal to 36 inches in diameter or equivalent open
area and outlet velocities less than 15 fps. For larger culverts or
outlet velocities greater than 15 fps, the outlet protection design
provided for in USDOT, 1983 shall be used.
(1) Basin Configuration.
The length of the outlet protection
(La) is determined using the following empirical
relationships that were developed for the U.S. Environmental Protection
Agency (USEPA, 1976):
and
Where:
Do
|
=
|
Maximum inside culvert width (feet);
|
Q
|
=
|
Pipe discharge (cfs);
|
TW
|
=
|
Tailwater depth (feet)
|
Where there is no well defined channel downstream of
the apron, the width, W, of the outlet and of the apron (as shown
in Figure 28.48.150(a)) shall be as follows:
and
The width of the apron at the culvert outlet shall be
at least three times the culvert width.
Where there is a well-defined channel downstream of the apron,
the bottom width of the apron shall be at least equal to the bottom
width of the channel and the lining shall extend at least 1.0 foot
above the tailwater elevation and at least two-thirds of the vertical
conduit dimension above the invert.
The apron side slopes shall be 2:1 or flatter, and the bottom
grade shall be level.
(2) Rock Size.
The median stone diameter, d50, is determined from the following equation:
Existing scour holes may be used where flat aprons are
impractical. Figure 28.32.340(a) shows a general design of a scour
hole. The stone diameter is determined using the following equations.
Also,
Where
y
|
=
|
depth of scour hole below culvert invert
|
The other riprap requirements are as indicated in the
previous sections for channel lining.
(d) Low-Flow Grade Control Structures.
(1) Introduction.
With the advent of floodplain management
programs, developers and local governments frequently decided to preserve
the floodplain. Since urbanization causes more frequent and sustained
flows, the trickle/low-flow channel becomes more susceptible to erosion
even though the overall floodplain may remain stable and able to resist
major flood events.
Erosion of the low-flow channel, if left uncontrolled, can cause degradation and destabilization of the entire floodplain. Low-flow check structures are designed to provide control points and establish stable bed slopes within the base flow channel. The check structures can be small versions of the drop structures described in GJMC §
28.36.030 through
28.36.060 or in many instances simply control sills across the floodplain. Low-flow check structures are not appropriate in instances such as completely incised floodplains or very steep channels.
In addition, low-flow structures inherently permit a considerable
amount of sediment to be transported downstream as urbanization takes
place, which negatively impacts stormwater quality. Therefore, complete
channel stabilization using drop structures in conjunction with channel
bed and bank stabilization at the time of development may be required.
(2) Drop Structure Grade Control Structures.
The grouted
sloping boulder drop structure and the vertical riprap drop structure
designs can be adapted for use as check structures. The analysis steps
are the same with the additional consideration of (i) stable bed slope
for the unlined trickle or low-flow channel and (ii) potential overflow
erosion during submergence of the check structure and where flow converges
back from the main channel sides or below the check structure.
The basic design steps for this type of structure include the
following:
(i) Determine a stable slope and configuration for the low-flow zone.
For unlined channels, discharges from full floodplain flow to the
dominant discharge shall first be considered. The dominant discharge
is more fully explained in sediment transport texts such as Simons,
Li and Associates (1982).
(ii)
The configuration of the low-flow zone, and number and placement
of the check structures, has to be reviewed. Typically, the floodplain
slope is steeper, often on the order of critical conditions. If the
checks are widely spaced, the trickle channel depth can be quite deep
downstream of the check, leading to concentration of higher flows
into the trickle channel and the check. A good rule of thumb is to
not have the trickle channel more than two feet deep at the crest
of the check, or more than four feet deep below the check structure
(relative to the overbank).
(iii)
A hydraulic analysis shall be performed using the discharge
that completely fills the check structure at its crest (the primary
design flow).
(iv)
The secondary design flow is that flow which causes the worst
condition for lateral overflow around the abutments and back into
the basin or trickle channel below. The goal is to have the check
structure survive such an event with minimal or reasonable damage
to the floodplain below. The best approach is to estimate unit discharges,
velocities and depths along overflow paths. The unit discharges can
be estimated at the crest or critical section for the given total
flow. Estimating the overflow path around the check abutment is difficult
and requires practical judgment. Slopes can be derived for the anticipated
overflow routes and protective measures devised such as grouted rock.
(v) Seepage control is also important, as piping and erosion through
or around these structures is a frequent problem. It is advisable
to provide a cutoff which extends laterally at least 5.0 to 10 feet
into undisturbed bank at minimum and has cutoff depth appropriate
to the profile dimensions of the check.
Additional information related to design of drop structures can be found in GJMC § 28.36.030 through 28.36.060.
|
(3) Control Sill Grade Control Structures.
Another type
of check structure that can be used to stabilize low-flow channels
within wide, relatively stable floodplains is the control sill shown
in Figure 28.32.340(b). The sill can be constructed by filling an
excavated trench with concrete, if soil conditions are acceptable
for trenching, or forming a simple wall if a trench will not work.
The sill crosses the low-flow channel and shall extend a significant
distance into the adjacent floodplain on both sides. The top of the
sill conforms to the top of the ground at all points along its length.
Riprap or other erosion control methods can then be added as erosion
occurs.
The basic design steps are:
(i) Determine a stable slope as described above.
(ii)
Determine spacing of the sills based on the difference in slope
between the natural and projected stable slope and the amount of future
drop to be allowed (not to exceed three feet).
(Res. 40-08 (§ 814), 3-19-08)
(a) An open channel is to be constructed for Doe Creek downstream of
John Boulevard and north of Rose Subdivision.
Assume the following conditions for this problem.
Q100 = 191 cfs
|
Invert elevation downstream of John Boulevard = 4,918
|
Invert elevation downstream of Rose Subdivision = 4,917
|
Channel improvement length = 900 feet
|
Due to aesthetics and sufficient right-of-way, a grass-lined
channel should be constructed.
Side slope = z = 3
|
Bottom width = b = 10 feet
|
n = 0.035 for grass-lined channel
|
Since the 100-year, 24-hour flow is less than 200 cfs,
a trickle channel should be constructed in the proposed channel bottom.
(b) Solution.
(1) Step 1.
Determine the depth of water during a 100-year
flow event.
Slope
|
=
|
4,918 – 4,917
|
= 0.0011 feet/foot
|
900
|
The Manning equation can be rewritten so that the depth
of flow, y, in a trapezoidal channel is on one side of the equation.
(by = zy2)
|
5/3
|
=
|
(Q)
|
(n)
|
|
(b = 2y(1 = z2)1/2)2/3
|
|
(S1/2)
|
(1.49)
|
|
Solving by trial and error,
Y = 3.7 feet
(2) Step 2.
Calculate the water velocity in the proposed
channel during a 100-year flow event using the Manning equation.
= 2.5 fps
Since the water velocity of the proposed channel (2.5 fps) is
less than the maximum permissible water velocity in a grass-lined
channel, a grass-lined channel can be used at this location.
(3) Step 3.
Design the trickle channel.
Assume dimensions for a concrete trickle channel:
Bottom width = 5 feet
|
Depth = 1 foot
|
Side slopes = vertical
|
The capacity of the trickle channel is:
Q = 13.16 cfs
(4) Step 4.
Verify that trickle channel has sufficient capacity.
The minimum capacity of the trickle channels is:
Since the capacity of the proposed trickle channel (13.2
cfs) is greater than the required capacity (9.6 cfs), the proposed
trickle channel is adequate.
(5) Step 5.
Determine the freeboard required for the proposed
channel.
Fb = 0.5 +
|
(.5)2
|
= 0.6 feet,
|
2 * 32.2
|
but minimum = 1.0 foot.
Therefore use Fb = 1.0 foot.
(6) Step 6.
The cross-section of the proposed channel is
shown in Figure 28.32.350.
(Res. 40-08 (§ 815), 3-19-08)