This chapter presents criteria regulating the design and construction
of certain hydraulic structures commonly encountered in Mesa County
at storm drain and culvert outlets as well as in open channels that
are intended to convey storm runoff. Many of the structures discussed
herein vary widely in physical and hydraulic parameters, and are thus
presented with general design criteria only. The designer is referred
to the applicable references for further detail on the hydraulic theory
and design processes for these structures.
Many of the structures discussed in this chapter are highly
visible and easily accessible to the general public. In some cases,
these structures may attract onlookers who are unaware of the inherent
dangers associated with their operation. Therefore, it is imperative
that the design of such structures incorporate posted warnings, pedestrian
barriers, fences, and/or other safety apparatuses. It is recommended
that the designer consult with Mesa County and/or other local jurisdictions
to coordinate the planning and design of the structures in this chapter.
(Res. 40-08 (§ 901), 3-19-08)
The design of open channels for conveyance of stormwater runoff is governed by the maximum permissible velocities for a given channel type. These velocities, presented in Chapter
28.32 GJMC, Open Channels, are primarily related to the erosive potential of nonclear stormwater flow. In some locations, such as those adjacent to schools or parks, it may be advisable to further reduce design velocities to diminish safety risks to children. Drop structures are used at locations where the use of channel lining materials is undesirable or does not sufficiently reduce design velocity in the channel (see Chapter
28.32 GJMC for the design of lined channels). A properly designed drop structure will effectively reduce the design slope in a channel segment and dissipate the energy produced by the drop without adverse erosive effects to the channel bed.
Control sill grade control structures, or low-flow check structures, are used for velocity and grade control in wide, relatively stable floodplains and wetland areas. These structures are addressed in GJMC §
28.32.340(d)(3).
The first three structures addressed in this section are typically selected for drops of 5.0 feet or less, but may be used in series (stepped drops). The vertical riprap drop structure (hard drop basin) is limited to 3.0 feet per drop for safety reasons. For larger single drops, the straight drop spillway, the baffled apron, or one of the structures found in GJMC §
28.36.070 through
28.36.100 may be employed.
(Res. 40-08 (§ 902), 3-19-08)
The GSB drop structure has recently become one of the most commonly
installed drop structures in both new construction and channel retrofit
situations. Relatively good hydraulic effectiveness and generally
pleasing aesthetics likely contribute to this trend. However, the
local availability of rock that meets the size and quality requirements
for this structure weighs heavily on the economic viability of GSB
drops. Much of the design data presented herein is attributed to the Urban Drainage Criteria Manual for the Denver-area UDFCD.
The excess energy created by the invert drop is dissipated in
two ways in a GSB structure. The additional channel roughness of the
grouted boulders themselves is secondary in energy dissipation to
the hydraulic jump formed in the drop basin downstream. However, improper
design or construction of the grouted boulders, including faulty rock
or grout selection and placement, may result in sweepout of the hydraulic
jump due to excessive velocity in the drop basin.
The GSB structure is intended for use only in grass-lined channels with upstream velocities within the limits set forth in Chapter
28.32 GJMC. With some variation in design (as outlined in this chapter), GSB structures may be used with channels containing or not containing a trickle channel or a low-flow channel.
Plans, profiles, sections, and details of typical GSB structures
are found in Figures 28.36.030(a), 28.36.030(b), 28.36.030(c), 28.36.030(d),
28.36.030(e), and 28.36.030(f). Design of these structures involves
five components: Rock and grout, upstream channel and approach apron,
the drop face, the drop basin, and the exit apron. Seepage concerns
are addressed in the design of the approach apron and associated cutoff
wall.
(a) Rock and Grout.
Grouted boulders must be placed upstream
of, and along, the crest of the drop, in the drop face and basin,
and along the sill at the end of the basin.
Boulder sizing is based upon the critical velocity, Vc1, in the channel upstream from the drop structure.
If a trickle channel or low-flow channel is present, the maximum critical
velocity of that channel and the main channel is used to find the
rock-sizing parameter:
Where:
Rp
|
=
|
Rock-Sizing Parameter
|
Vc1
|
=
|
Maximum Upstream Critical Velocity (fps)
|
S
|
=
|
Longitudinal Slope (ft./ft.)
|
Gs,rock
|
=
|
Specific Gravity of Rock = 2.55 unless otherwise certified by
quarry
|
This parameter is used in Table 28.36.030(a) to find
minimum boulder dimension, Dr.
Table 28.36.030(a): Grouted Sloping Boulder (GSB) Drop Structure
Rock Sizing
|
---|
Rock Sizing Parameter, Rp
|
Minimum Boulder Dimension, Dr
|
---|
Less than 4.50
|
18 inches
|
4.50 to 4.99
|
24 inches
|
5.00 to 5.59
|
30 inches
|
5.60 to 6.39
|
36 inches
|
6.40 to 6.99
|
42 inches
|
7.00 to 7.50
|
48 inches
|
Adapted from USDCM (UDFCD) Table HS-5
|
Note that standard riprap rock gradation is not utilized in GSB structures. Instead, the boulders are placed in one layer directly on the graded and compacted subgrade (compaction per GJMC §
28.32.310(g)(6)), as close together as is possible, and in such a manner so as not to adversely disturb the subgrade. The flattest surface of each boulder shall be oriented upward and shall be as horizontal as possible. The boulders shall be cleaned with water before grouting to improve grout-rock adherence. The boulders shall be placed by such methods that are less likely to cause breakage or significant blemishes and shall be checked for significant cracking before grouting. Damaged boulders shall be replaced before grouting.
Grout is used to fill the voids between the boulders from the
subgrade to one-half of the boulder height from the subgrade. In the
drop basin only, this is increased to three-quarters of the boulder
height from the subgrade to promote draining. Excessive grouting leads
to a reduction in hydraulic capacity and energy dissipation, and may
endanger the structural stability of the drop. Selection, mixing,
placement, and finishing of grout shall comply with the specifications
set forth in Table 28.36.030.
(b) Upstream Channel and Approach Apron.
Grouted boulders
shall be placed on grade with the upstream channel for a minimum of
8.0 feet upstream from the drop crest (this shall be referenced as
the “approach apron”). Buried riprap shall be installed
from the upstream end of the approach apron to a point at least 8.0
feet upstream along the channel flowline. The riprap shall be D
50 = 12 inches (UDFCD Type M) and shall be installed per the criteria described in GJMC §
28.32.300. The grouted boulder approach apron shall be continuous across the width of the channel (except as described in the following paragraph) and up each bank to the elevation of the normal depth for the design flow at that location. The buried riprap shall be installed across the channel bottom and up each bank to the elevation of one-half the normal depth for the design flow at that location.
For grass-lined channels with a concrete or rock-lined trickle channel (see GJMC §
28.32.280(c)(1)), the approach apron and upstream riprap protection are discontinuous across the channel cross-section to allow the trickle channel flowline to continue unimpeded to the drop crest (see Figure 28.36.030(a)). While this is necessary to retain the effectiveness of the trickle channel in conveying base or nuisance flows, it tends to create a concentrated jet at the location of the trickle channel during higher flow periods. The additional energy introduced to the basin in these cases may be partially dissipated by the installation of large boulders or baffles in the trickle channel and/or a meandering trickle channel through the drop basin itself. These options are not shown in the GSB details (Figures28.36.030(a), 28.36.030(b), 28.36.030(c), 28.36.030(d), 28.36.030(e) and 28.36.030(f)), but are similar to the trickle channel/drop basin controls used for the vertical riprap drop structure (see Figure 28.36.030(g)).
Grouted rock is particularly susceptible to failure from undermining
and the subsequent loss of the supporting bank material. (HEC-11)
This refers to the high potential for seepage and piping under and
around the drop structure. Since the GSB structure is rigid and essentially
monolithic, seepage under the grouted boulders and the resultant transport
of subgrade particles will eventually lead to structural failure.
Therefore, a seepage cutoff section is required as shown in Figures
28.36.030(a), 28.36.030(b), 28.36.030(c), 28.36.030(d), 28.36.030(e)
and 28.36.030(f). As noted in the details, the dimensions of the vertical
cutoff shall be determined based on geotechnical investigations and
seepage analysis or shall comply with the minimum cutoff criteria
set forth in the appropriate figures. The seepage cutoff shall be
installed prior to the placement of the grouted boulders at the drop
crest, and shall include a keyway for the grout/cutoff interface as
shown in the details.
(c) Drop Face.
The drop face shall consist of grouted boulder
“steps” of vertical dimension no greater than one-half
of the minimum boulder dimension, Dr, from
Table 28.36.030(a). The overall drop face slope must not exceed 4H:1V;
flatter slopes are permissible and encouraged due to improved aesthetics
and energy dissipation. Slopes steeper than 4H:1V may reduce structural
stability.
The grouted boulders are continuous across the entire bottom
width on the drop face – the trickle channel flowline equals
the main channel flowline in the drop section. The grouted boulders
also continue up each bank to the elevation equivalent to the downstream
channel normal depth (sequent subcritical depth) plus freeboard or
the channel critical depth plus 1.0 foot, whichever is greater.
A weep drain system shall be installed behind the drop face
to relieve hydrostatic pressure in drops exceeding 5.0 vertical feet.
See details in Figures 28.36.030(a), 28.36.030(b), 28.36.030(c), 28.36.030(d),
28.36.030(e) and 28.36.030(f).
(d) Drop Basin.
The basin area shall be constructed of continuous
grouted boulders of the same dimensions as the drop face section (boulder
size, crest and basin width, height of bank protection). However,
the grout level is increased to three-quarters of the boulder height
in the basin, and shall be sloped to drain to the centerline of the
channel (or trickle channel if applicable).
The basin is depressed below the downstream channel invert by
2.0 feet for drops of 5.0 feet or less. This helps to stabilize the
hydraulic jump. For drops exceeding 5.0 feet, a sequential depth analysis
is necessary to determine basin depression depth (a minimum of 2.0
feet applies). Sequential depth analysis is not presented in this
title; refer to a hydraulics text such as Open-Channel Hydraulics (Chow, 1959) for explanation.
Basin length shall be a minimum of 15 feet for nonflexible downstream channel lining (concrete, grouted riprap, geosynthetic linings) and a minimum of 20 feet for downstream channels with flexible linings. A row of 36-inch or larger grouted boulders shall be placed at the downstream end of the basin. The top of this sill shall be equal to the invert of the downstream channel. For channels with a concrete or rock-lined trickle channel, there shall be a break in the end sill of width equal to that of the trickle channel. The trickle channel shall continue downstream through the sill and exit apron with scour protection as specified in Chapter
28.32 GJMC.
(e) Exit Apron and Downstream Channel.
The exit apron shall
consist of buried riprap of size D
50 = 12 inches (UDFCD Type M) and shall be installed per the criteria described in GJMC §
28.32.300. The riprap shall extend across the channel (except in the trickle channel as applicable) and up the banks to an elevation equal to the top of the adjacent grouted boulders. This riprap protection shall extend downstream from the end sill a minimum distance of twice the drop height or 10 feet, whichever is greater.
(Res. 40-08 (§ 902.1), 3-19-08)
This type of drop structure consists of an approach apron (grouted
rock), a vertical concrete crest wall, a jump basin with end sill
(grouted rock or concrete), and downstream channel scour protection.
While an effective method for drop design, these structures shall
be avoided if possible in areas of significant public use or in highly
visible locations due to safety concerns and low aesthetic appeal.
Vertical drop structures shall be avoided in channel reaches which
may be utilized for boating or other recreation activities in or adjacent
to the water. The maximum allowable drop for a vertical drop structure
of this type shall be 3.0 feet.
(a) Rock and Grout.
Rock used upstream of the crest wall shall have a minimum dimension of 12 inches in any direction. Rock used downstream of the drop shall have a minimum dimension of 18 inches in any direction. Grouting requirements are identical to those presented in GJMC §
28.36.030 for the GSB drop structure.
(b) Approach Apron.
A grouted rock apron shall be installed
across the entire bottom width (including trickle and low-flow channels)
and up each bank to the elevation equal to upstream channel normal
depth plus 1.0 foot. The rock shall be buried to a depth such that
the top of the grout is equal to the invert of the upstream channel
at every point across the channel. This approach apron shall extend
upstream from the crest wall a minimum of 10 feet.
(c) Vertical Crest Wall.
The concrete crest wall conforms
to the upstream inverts for the trickle or low-flow channel and the
main channel across the bottom width. The wall shall extend a minimum
of 5.0 feet into the undisturbed banks. However, all design dimensions
including minimum structural width, wall thickness, footer size and
geometry, and reinforcement shall be determined using accepted structural
analysis methods and determination of potential creep, heave, buoyancy
and uplift due to seepage pressures, and all other considerations
associated with the design of a retaining wall.
An impervious backfill material is recommended both upstream
and downstream adjacent to the crest wall and footers to act as a
horizontal seepage cutoff. In lieu of this material, other appliances
may be employed to ensure minimized seepage around/under the crest
wall. Piping, the transport of structural supporting material away
from its intended location, is a common cause of structure instability
and failure.
(d) Basin.
The basin is a depressed, hard-surface area which
redirects the plunging flow from the crest horizontally. At lower
flows, the energy dissipated by this redirection may be sufficient
to return the flow to a subcritical state. However, the primary energy
dissipation method for this structure is a hydraulic jump formed in
the basin. When the upstream channel is composite (utilizing a trickle
or low-flow channel), the approach velocity tends to be higher in
the smaller sub-channel zone than in the main channel zone. Therefore,
for the design flow, the basin length and downstream protection requirements
may differ for the two zones. By placing large boulders (60 percent
to 80 percent of critical depth in height) between the location of
nappe impingement on the basin floor and a point at least 10 feet
from the end sill, the required basin length for the sub-channel zone
may be reduced to that of the main channel zone. Otherwise, the following
calculations must be applied to both zones independently.
The drop will be treated hydraulically as a straight-drop spillway
and analyzed per Chow’s (1959) method:
A “drop number,” DN, must
first be calculated in order to relate other associated lengths and
depths:
Where:
q
|
=
|
Discharge per Unit Width for the Subject Zone (cfs/ft.)
|
g
|
=
|
Gravitational Constant = 32.2 ft.2/s
|
h
|
=
|
Effective Height of Drop (ft.)
|
Note that the effective drop height must include the
basin depression depth. Using the drop number, the following relationships
can be solved:
|
(28.36-3)
|
|
(28.36-4)
|
|
(28.36-5)
|
|
(28.36-6)
|
Where:
Ld
|
=
|
Drop Length (ft.)
|
yp
|
=
|
Pool Depth Under Nappe (ft.)
|
y1
|
=
|
Depth Upstream of the Hydraulic Jump (ft.)
|
y2
|
=
|
Subcritical Sequent Depth (ft.)
|
See Figure 28.36.030(g) for illustration of these variables.
These values assume that atmospheric pressure is maintained under
the nappe, thus the designer is responsible for incorporation of aeration
devices as necessary. Drop length, Ld, refers
to the horizontal distance from the crest wall to the location of
depth y1, upstream of the hydraulic jump.
The basin design length, for the subject zone, is given by Equation
28.36-7:
Where:
Lb
|
=
|
Basin Design Length (ft.)
|
Dj
|
=
|
Distance from Location of Depth y1 to
Jump (ft.)
|
Lj
|
=
|
Length of Jump ≅ 6 · y2
|
The distance from the point of nappe impingement on
the basin floor to the upstream end of the hydraulic jump is determined
by a water surface profile analysis as presented in most hydraulic
design texts.
Basin depression depth below the downstream channel invert is
determined by comparing the subcritical sequent depth, y2, with the tailwater depth in the downstream channel,
yTW. If y2 exceeds yTW, the jump will be swept downstream and possibly out
of the basin. This situation is to be avoided since significant erosion
may take place if the jump occurs in an unarmored location in the
channel. If yTW exceeds y2, the jump is pushed upstream toward the wall, potentially submerging
jump. Hydraulically, this is not problematic, but the structural design
of the crest wall may be affected by the additional forces. Basin
depression effectively adds to the tailwater depth in the downstream
channel, controlling the location of jump formation. Therefore, the
minimum basin depression depth, B, is the maximum of the following:
This is the height of the end sill and downstream invert
above the downstream end of the depressed basin. The end sill shall
be constructed of reinforced concrete or grouted boulders of a minimum
36-inch dimension. This acts as a protected transition back to the
channel invert.
(e) Downstream Channel Protection.
The channel directly
downstream from the end sill shall be protected for a minimum of 10
feet in the direction of flow with buried riprap of size D50 = 12 inches (UDFCD Type M) or grouted rock with a
minimum dimension of 12 inches.
In cases where the sub-channel zone basin length is longer than
the main channel zone (no additional boulders or baffles placed in
the basin to dissipate the center jet), the additional protection
shall extend a lateral distance equal to the bottom width of the trickle
channel from each edge of the trickle channel. This results in an
extended protection zone with a width equal to three times the trickle
channel bottom width.
(Res. 40-08 (§ 902.2), 3-19-08)
The straight drop spillway is very similar hydraulically to the vertical hard drop basin presented in GJMC §
28.36.040. The primary difference exists in the shaping of the spillway downstream from the crest to closely resemble the shape of the lower nappe, i.e., the bottom of the jet formed by the flow suddenly departing the crest. This results in a “classic” spillway shape, as used for major reservoir spillways and channel drops alike. The straight drop spillway itself is not a significant energy dissipation structure, and must be paired with an induced hydraulic jump basin as presented in GJMC §
28.36.090.
The shape of a straight drop spillway is dependent on the shape
of the nappe, which varies with head over the crest and the shape
of the approach to the spillway. The reader is referred to Open-Channel Hydraulics (Chow, 1959), Hydraulic
Design of Spillways (USACOE, 1992), or other texts for design
of these structures.
Figure 28.36.050 shows a typical straight drop spillway configuration.
(Res. 40-08 (§ 902.3), 3-19-08)
The fixed costs associated with the construction of a baffled
apron structure (hereafter referred to as a baffle chute drop) typically
limit their use to larger drops from an economic standpoint, although
the actual minimum size is limited to that length required to incorporate
the minimum number of baffle rows. These drop structures are most
effective at unit discharge rates between 35 and 60 cfs per foot.
However, a value in this range can often be attained by altering the
width of the chute. Most often, transition walls are employed to direct
wider upstream channel flow to a narrower chute, decreasing the cost
of the drop structure. When designed and built correctly, these structures
are effective and last for many years with minimal maintenance requirements.
While the baffle chute drop can pass most sediment and debris,
larger debris may become caught behind the baffles or in the narrowed
chute, disabling the structure’s ability to dissipate energy.
This can lead to an effectively higher invert in the chute and overtopping,
and can also allow the nearly unimpeded flow in the chute to exit
the structure at erosive velocities. Therefore, debris-control structures
are recommended upstream of the drop, and regular inspection and maintenance
may be necessary.
The baffle chute drop structure does not rely on the formation
of a hydraulic jump as its primary energy dissipation process. Instead,
excess energy in the chute flow is dissipated by redirection over
and around baffle blocks, which are arranged in offset rows to avoid
the passing of high-velocity jets between the blocks. Since a hydraulic
jump is not part of the design, there are no tailwater requirements
for this structure. However, potential scour due to relatively high
velocities at the end of the chute and in the downstream transition
section necessitate a protected exit apron and/or scour hole.
Figure 28.36.060 presents an isometric view of a baffle chute
drop with typical dimensional requirements. Note that this figure
does not indicate structural requirements such as concrete thickness
or reinforcement, footer depths and dimensions, or seepage control.
These factors shall be assessed and approved by qualified professionals.
(a) Upstream Channel Transition.
Typically, the design width
of the baffle chute drop is less than the upstream channel width for
economic and sizing reasons as well as to attain unit discharge rates
in the desired range. The headwalls and/or wingwalls associated with
this transition are subject to design constraints set forth in this
manual and shall be designed using proper structural analysis techniques.
The designer should note that the effective width of a conduit or
channel is often considerably smaller than the physical width due
to the separation of flow from the abutment/conduit interface.
The approach section downstream from the transition is designed
to maintain an approach velocity of less than the critical velocity
at the crest. Recommended approach velocities are presented in Figure
28.36.060. The concrete flow alignment apron, reaching from the abutment/conduit
interface to the chute crest, shall be a minimum of 5.0 feet in length
and shall be equal to the chute in width along its entire length.
In certain cases, the transition section may not sufficiently reduce
the specific energy of the flow to achieve the proper approach (alignment)
apron velocity. In these situations, the crest may be raised by up
to 12 inches above the approach apron invert.
If a trickle channel is present in the upstream channel, it
shall continue through the transition section and apron, and shall
maintain a continuous flowline through any raised crest.
Transition and apron wall heights are determined by backwater
analysis at peak flow, with a freeboard equal to or greater than that
of the upstream channel.
(b) Baffled Chute.
The chute floor, walls, and baffles shall
be constructed of reinforced concrete and shall be structurally designed
to withstand all geotechnical, hydrostatic, and hydrodynamic (impact
and frictional) forces imposed by the specific site conditions, including
a reasonably conservative factor of safety for all loading. The chute
floor shall have a slope no steeper than 2H:1V (Z:1, ZMAX = 2). The chute walls shall be vertical and shall be tied to the
floor, upstream wall or abutments, and downstream abutments with properly
sized and installed steel reinforcement.
The baffle blocks shall be reinforced concrete of the
dimensions shown in Figure 28.36.060. Baffle blocks shall be adequately
reinforced and tied with steel reinforcement to the chute floor. A
key-in interface is recommended to stabilize the blocks on the chute
floor. The block height normal to the chute floor is defined in Equation
28.36-9:
Where:
H
|
=
|
Block Height Normal to Chute Floor
|
yc
|
=
|
Critical Depth at Peak Flow
|
There shall be at least four rows of baffle blocks.
Baffle block rows shall be spaced at Z · H along the direction
of flow, and shall be staggered such that jets of water not directly
impinging on a baffle block within a two-row distance are minimized.
The blocks and the spaces between the blocks shall be equal to 1.5H
except where the width is limited by the chute wall. All baffle rows
shall be symmetrical along the centerline of the chute. When a trickle
channel exists, the top row of baffles shall be aligned such that
the maximum percentage of the trickle channel flow width is not impingent
upon any baffles in the first row.
Chute walls shall be at least 3H in height normal to the chute
floor. Other dimensional requirements may be found in Figure 28.36.030.
Where a hard-surface exit apron is not employed, at least 1.5
rows of baffles shall be buried in riprap. This allows for the exposure
of additional baffle blocks as loose rock is displaced to form a scour
hole or to adapt to a lower downstream channel invert.
Downstream transition walls (headwalls and/or wingwalls) shall
be of a height equal to the design normal depth in the downstream
channel plus 1.0 foot of freeboard. They shall extend from the chute
walls at an angle of 45 to 90 degrees for a distance necessary to
contain any eddies that may form in this area.
(c) Basin/Exit Apron.
There exist two primary design options
for the basin downstream from the baffle chute. The first, a hard-surface
basin, is used if the invert of the downstream channel is expected
to remain approximately constant over the life of the drop structure.
This basin is constructed of either reinforced concrete tied to the
downstream end of the chute floor or grouted rock, the latter of which
further dissipates energy in the flow and protects the downstream
channel from excessive degradation.
Even more energy dissipation is often achieved with the installation of a preformed or nonpreformed scour hole at the chute exit. The former is a riprap-lined depressed basin that approximately imitates the dimensions of the scour hole that would form if loose rock was placed as backfill. The riprap and basin sizing requirements are found in GJMC §
28.32.340(c) and Figure 28.32.340(a). The designer may substitute the downstream design flow normal depth for D
o in the relevant equations and figures. W
o is equal to the width of the chute for the purpose
of this design.
A nonpreformed scour hole is constructed by backfilling up to the existing downstream channel invert with loose rock. The loose rock shall be at least 2.0 feet deep and shall extend a minimum of 4H feet horizontally parallel to the chute. The rock backfill area shall be of such a width to reach the ends of the downstream abutment walls. Rock size is based on the riprap selection criteria set forth in Chapter
28.32 GJMC. Placement of the rock must not damage the buried baffle blocks. With sufficient operation time, the force of the flow from the baffle chute will displace the loose rock in such a way so as to form a stable scour hole.
The scour hole options, especially the latter, tend to adapt
somewhat automatically to changing conditions in the downstream channel,
including a gradually lowered invert elevation. However, it is still
recommended that a protective channel lining be installed in the downstream
channel for an appropriate distance to allow flow to return to a nearly
steady state.
(Res. 40-08 (§ 902.4), 3-19-08)
(a) The structures described in this section are similar in many ways to the channel drop structures of GJMC §
28.36.020 through
28.36.060. However, while the drop structures’ primary purpose is to allow a channel to quickly change elevation without excessively increasing the specific energy of the flow in the downstream channel, these structures are designed to dissipate excess energy already present in the upstream channel. These energy dissipation structures are often employed at transitions from nonflexible channels or conduits to channels with flexible linings or other velocity restrictions. This includes culvert and storm drain outlets to open channels. They are also occasionally used at locations where the energy produced by a channel drop exceeds the limitations of the channel drop structures. As mentioned in GJMC §
28.36.050, straight-drop spillways must be paired with one of the structures in this section to dissipate the energy associated with the high-velocity flows.
(b) The structures in this section are divided into three categories:
(1) Increased roughness basins.
(2) Induced hydraulic jump basins.
(Res. 40-08 (§ 903), 3-19-08)
Increased roughness basins are designated for use in locations where the upstream Froude number does not exceed 3.0. Further restrictions apply to each type, including maximum velocities and maximum cross-sectional flow areas. These basins include the riprap basin (preformed scour hole) and the array of drop structures introduced in GJMC §
28.36.020 through
28.36.060.
The FHWA’s Hydraulic Design of Energy Dissipators
for Culverts and Channels (HEC-14) also presents methods
for the design of increased resistance devices for pipes, box culverts,
and channels. These devices are intended to create a tumbling flow
pattern along steep reaches of conduits and channels, thereby maintaining
an allowable average velocity. However, due to the economic advantages
of other options and to the relatively flat terrain in developed portions
of Mesa County, these structures are not included in this title.
(a) Riprap Basin.
The riprap basin/preformed scour hole
is effective for the dissipation of excess energy from upstream conduits
and channels complying with the following:
(1) Maximum allowable upstream flow area must be equal to or less than
the equivalent full-flow area of a 36-inch pipe.
(2) Maximum upstream flow velocity must be equal to or less than 15 feet
per second at any flow depth.
The design procedure for this structure is presented in GJMC § 28.32.340(c) and Figure 28.32.340(a).
|
(b) Drop Structure as an Energy Dissipator.
While the first two drop structures listed in GJMC §
28.36.020 through
28.36.060 are intended to dissipate only energy produced by the drop itself, they can in certain cases be used as dissipators of upstream energy. The most significant restriction is that flow in the upstream channel must be in a subcritical state before reaching the structure. The combination of a small channel drop and local energy dissipation can effectively reduce the velocity in the downstream channel.
(Res. 40-08 (§ 903.1), 3-19-08)
Induced hydraulic jump basins are commonly used for large and
small projects alike. They are highly effective at utilizing the hydraulic
jump phenomenon to dissipate excess energy and return the flow to
a subcritical depth. While the space required for these structures
is relatively large, they are typically less expensive than impact-type
basins on a unit-discharge basis. Five distinct induced hydraulic
jump basin designs are presented herein. The designer shall incorporate
adequate seepage controls as part of the design of all structures.
Riprap protection shall be provided for an appropriate distance downstream
of all structures in this section where the receiving channel has
a flexible lining.
(a) CSU Rigid Boundary Basin.
The Colorado State University
Rigid Boundary Basin (CSU RBB) utilizes offset rows of baffles (roughness
elements) to force supercritical flow from a conduit into a hydraulic
jump. The only basin in this category to be designed as entirely on-grade,
the CSU RBB is useful for locations with restrictive vertical alignment
criteria. However, the upstream Froude number is restricted to a value
of 3.0. Figures 28.36.090(a) and 28.36.090(b) present sketches and
data for the design of this structure.
The design procedure for the CSU RBB is presented in HEC-14,
Chapter VII-A.
(b) USBR Type II Basin.
This basin utilizes chute blocks
and a dentated sill to induce a hydraulic jump in the basin. An isometric
sketch of this basin is presented in Figure 28.36.090(c). Unlike the
CSU rigid boundary basin, the USBR Type II basin does not allow for
subcritical upstream flow by forcing the flow through critical depth
prior to the jump basin. Therefore, this basin requires an upstream
Froude number between 4.0 and 14.0. The required tailwater depth for
this basin varies with the Froude number per Figure 28.36.090(d),
in which the solid “design curves” incorporate the required
10 percent factor of safety. This basin is intended for rectangular
sections only, thus transitions may be required upstream of the structure.
The design procedure herein is intended for unit discharge rates of
up to 500 cfs per foot width. The incoming chute to the basin can
be of any slope, but slopes greater than 2:1 shall incorporate a radius
curve to allow for a smooth transition to the basin floor. Sequent
depths for a free hydraulic jump, USBR Type II basin, and USBR Type
III basin are shown in Figure 28.36.090(e).
The design procedure for the USBR Type II Basin is presented
in HEC-14, Chapter VII-D.
(c) USBR Type III Basin.
This basin utilizes chute blocks,
baffle piers, and a solid end sill (no dentates) to induce a hydraulic
jump in the basin. An isometric sketch of this basin is presented
in Figure 28.36.090(f). This basin requires an upstream Froude number
between 4.5 and 17.0. This can in part be controlled by the height
and slope of the upstream chute, but the designer shall be aware that
lower basin elevations can cause the jump to move upstream and submerge
the chute, negating its ability to increase the influent Froude number.
The required tailwater depth for this basin is at least full conjugate
depth as indicated in Figure 28.36.090(d). This basin is intended
for rectangular sections only, thus transitions may be required upstream
of the structure.
The USBR Type III basin is limited to a unit discharge rate
of 200 cfs per foot width, but can handle velocities up to 50 or 60
feet per second. The design is intended to effectively initiate and
shorten the hydraulic jump, thereby reducing the space requirements
for the structure. However, the baffle piers, which are essential
for controlling the jump, must be carefully designed to comply with
the procedure outlined below. The incoming chute to the basin can
be of any slope, but slopes greater than 2:1 shall incorporate a radius
curve to allow for a smooth transition to the basin floor.
The design procedure for the USBR Type III Basin is presented
in HEC-14, Chapter VII-E.
(d) USBR Type IV Basin.
At locations where the upstream
flow is supercritical but still in the relatively low range of Froude
numbers, the USBR Type IV basin can be employed. Designated for Froude
numbers between 2.5 and 4.5, the jump is defined by Chow (1959) as
an “oscillating jump.” This type of hydraulic jump can
produce potentially destructive downstream wave action, so the recommended
tailwater depth for this structure is higher than that for the Type
III basin.
Like the Type II basin, this structure utilizes chute blocks
and an end sill. However, the end sill in this case is solid, not
dentated. This basin is intended for rectangular sections only, thus
transitions may be required upstream of the structure. An isometric
sketch with general dimensions is presented in Figure 28.36.090(g).
The design procedure for the USBR Type IV basin is presented
in HEC-14, Chapter VII-F.
(e) SAF Stilling Basin.
The Saint Anthony Falls (SAF) stilling
basin is similar to the USBR Type III Basin in that it utilizes chute
blocks, baffle piers (floor blocks), and an end sill to induce and
maintain a steady hydraulic jump in the basin. Also similar to the
Type III, it produces a jump that is significantly shorter than a
natural hydraulic jump (approximately 80 percent of the length), thereby
reducing the required length of the basin and downstream protection.
Plan and profile views of the SAF basin are provided in Figure
28.36.090(h). Note that the basin itself may be laterally flared to
better fit the downstream channel. This flare is labeled as z (longitudinal):1
(lateral), wherein the variable z is limited to values equal to or
greater than 2.0. However, all side walls, headwalls, and wingwalls
shall be vertical.
The SAF Basin may be used at the base of straight-drop spillways,
at culvert and storm drain outlets, and in canals. It is required
that flow entering the basin be supercritical, but this can usually
be achieved by proper upstream chute design. The allowable range of
Froude numbers for this structure is 1.7 to 17.0.
The design procedure for the SAF stilling basin is presented
in HEC-14, Chapter VII-G.
(Res. 40-08 (§ 903.2), 3-19-08)
Impact basins dissipate energy by causing the high-velocity
flow to encounter an obstruction, redirecting the flow in directions
other than the influent path. This action effectively negates a large
percentage of the velocity head that would otherwise potentially cause
damage to the downstream channel. While these structures tend to be
costly on a unit-discharge basis, they require far less space than
many other dissipation options. Three types of impact basins are presented
in this section.
The designer of the energy dissipators discussed herein is responsible
for ensuring adequate structural design, including the analysis of
all forces incident on the structure, calculation of creep and overturning
potential, and design and installation of seepage controls. Necessary
seepage controls may include cutoff walls, liners, weep drains, and/or
other devices. The designer is referenced to applicable texts concerning
subgrade compaction, concrete mixing, steel reinforcement, calculation
of external forces, and retaining wall design.
(a) Contra Costa Energy Dissipator.
This structure is intended
for use with small to medium culverts with medium to high velocity
flows. It is also designed to operate with minimal tailwater, although
some tailwater improves the dissipator’s performance. Tailwater
depth is limited to one-half of the culvert height. The Contra Costa
dissipator is best for locations where the design flow depth at the
culvert outlet is less than the culvert height. Therefore, culvert
effluent depth is limited to one-half of the culvert height. The Froude
number of the culvert outlet flow is limited to a maximum of 3.0.
The Contra Costa energy dissipator is a concrete structure designed
to be placed in a trapezoidal channel with side slopes of 1:1 and
a bottom width between one and three times the culvert height (D ≤
W ≤ 3D). If a natural channel exists at the structure location,
the structure width shall conform to that channel, with a maximum
width of 3D. The structure consists of two continuous baffles of different
heights across the basin floor as well as a vertical end sill. All
parts of the structure shall be reinforced concrete and shall be tied
to the downstream end of the culvert with steel reinforcement bars
if possible. Profile and section views with dimensional definitions
are provided in Figure 28.36.100(a).
The design procedure for the Contra Costa energy dissipator
is presented in HEC-14, Chapter VIII-A.
(b) Hook-Type Energy Dissipator.
The hook-type dissipator,
also called aero-type, is used at culvert outlets with Froude numbers
in the range of 1.8 to 3.0. Each dissipator utilizes three hook structures
in the basin that redirect a portion of the high-velocity flow up
and back into the basin flow. This action creates a large amount of
turbulence, thereby dissipating some of the excess energy in the flow.
At Froude numbers exceeding about 3.0, the dissipation effects are
greatly diminished.
This energy dissipation structure is designed to use either
of two basin configurations. The first type contains vertical wingwalls
at the culvert exit which are warped smoothly to side slopes of 1.5:1
at the end sill (see Figure 28.36.100(b)). The second configuration
is a trapezoidal channel with a constant cross-section throughout
the basin (see Figure 28.36.100(d)). Hook details for the two configurations
are found in Figures 28.36.100(c) and 28.36.100(e), respectively.
The design procedure for the Hook-Type Energy Dissipator is
presented in HEC-14, Chapter VIII-B.
(c) Impact-Type Energy Dissipator (USBR Type VI).
Also called
the baffle-wall energy dissipator or baffled outlet, this structure
is compact and highly effective for the control of high-energy flows
exiting a conduit or rectangular channel section. Consisting of a
vertical-walled basin with a single large vertical hanging baffle,
energy is dissipated by impact with the baffle and secondarily by
eddies formed in the basin. At the design flow, this structure dissipates
energy more effectively than a hydraulic jump (See Figure 28.36.100(g)),
and has no minimum tailwater depth. However, its debris-handling capability
and maximum tailwater depth limit (discussed later) limit the locations
at which the structure can be used. Further limitations include a
maximum discharge of 400 cfs per structure and a maximum upstream
velocity of 50 feet per second. This latter value is intended to minimize
damage to the baffle due to cavitation. Where these limits are exceeded,
two or more structures may be built adjacent to one another to accommodate
the excess flow.
For upstream conduits with a slope greater than 15 percent and
for all open channels, it is recommended that there be a horizontal
section from the outlet brink to a point at least four conduit widths
upstream. Rectangular upstream channels shall have sidewalls of a
height equal to or greater than the walls of the dissipator basin
and shall always have a zero longitudinal slope for a minimum of three
channel widths upstream from the entrance to the basin.
Figure 28.36.100(f) presents the configuration and necessary
dimensions for the design of the USBR Type VI structure. Note that
the optional notches near the edges of the basin are included to create
concentrated jets for self-cleaning purposes.
One of the most important design features of this structure
is its ability to pass the entire design discharge over the top of
the baffle. This is important to prevent upstream flooding in the
case of complete clogging of the area under the baffle. However, this
flow configuration is not nearly as effective and shall not be relied
upon as an alternative energy dissipation method. Therefore, the debris
and ice buildup potential at a given location shall be analyzed prior
to selection of this structure as the energy dissipator for that outlet.
While some tailwater (up to h3+h2/2) improves the performance of the dissipator, depths
over this height shall be avoided. Significant degradation of performance
occurs with tailwater depths greater than h3+h2, thus the USBR Type VI structure shall
not be installed in these conditions.
The design procedure for the impact-type energy dissipator is
presented in HEC-14, Chapter VIII-C.
(Res. 40-08 (§ 903.3), 3-19-08)
Every channel has a maximum allowable flow depth which, when
exceeded, may cause damage to the banks and eventually failure of
the channel. Occasionally, overflow from a storm drainage system enters
an irrigation canal (this shall be avoided unless specific consent
is granted by the owner/operator of the canal). In these situations,
it is typically necessary to remove the overflow from the canal at
some downstream location. The structures in this section are intended
to remove excess water from a channel to maintain a specified water
surface elevation or to allow the water in a channel section to be
drained. The latter may be necessary to inspect, maintain, or repair
the channel, or in the event of an embankment failure, to redirect
some of the escaping flow to an acceptable location.
Wasteway is the term commonly applied to the channel to which
the main channel excess flow is diverted. A wasteway shall have the
capacity to convey the maximum flow that can be diverted through all
diversion structures located upstream, and shall deliver the excess
flow to an acceptable disposal point.
Two types of diversion structures which can act as overbank
prevention structures are presented herein: the side-channel spillway
and the gated turnout.
(Res. 40-08 (§ 904), 3-19-08)
A side-channel spillway is the most effective structure for
automatic removal of excess flow in a channel since its capacity increases
with depth over its crest. The spillway crest is usually parallel
to the channel alignment except at terminal wasteways (at the end
of a canal). Typically, the spillway crest is set approximately 0.2
feet above the normal design depth for the channel to allow for normal
wave action. The length of the spillway is then controlled by the
required overflow discharge capacity and the maximum allowable water
surface elevation in the channel. A standard rule of thumb is to ensure
no more than 50 percent encroachment on the freeboard of the channel
banks in the vicinity of the spillway. A detailed procedure for the
design of a side-channel spillway turnout and wasteway is not presented
in this manual due to the infrequent application of such a structure
in stormwater runoff designs. However, Equation 28.36-10 is the basic
design equation for the side-channel spillway (suppressed rectangular
weir):
Where:
Q
|
=
|
Design Flow over the Spillway (cfs)
|
Lc
|
=
|
Crest Length (ft.)
|
H
|
=
|
Height of Channel Water Surface over Crest (ft.)
|
(Res. 40-08 (§ 904.1), 3-19-08)
To allow for manual release of water from a channel for the
purpose of water level control, maintenance access, et cetera, gated
turnouts are often installed at wasteways. It is common practice to
include at least one gated turnout at any side-channel spillway location
for flushing and additional water level control. Again, specific design
procedures are not presented, but the general orifice equation is
given:
Where:
Q
|
=
|
Design Flow through Gate (cfs)
|
C
|
=
|
Orifice Coefficient ≅ 0.6
|
h
|
=
|
Height of Water Surface Over Gate Centerline (ft.)
|
A
|
=
|
Area of Orifice (ft.2)
|
g
|
=
|
Gravitational Constant (32.2 ft./s2)
|
(Res. 40-08 (§ 904.2), 3-19-08)
GJMC §
28.36.150 through
28.36.170 present appurtenances for use in conjunction with pipe systems, specifically those designed for the transport of stormwater.
(Res. 40-08 (§ 905), 3-19-08)
Pipe collars are transverse fins that extend from the pipe into
the surrounding earth and function as barriers to percolating water
and burrowing rodents (USBR 1974). Due to the relative smoothness
and impermeability of pipe, percolated water tends to collect and
move along the soil adjacent to a pipe’s outer wall. This action,
typically called piping, tends to transport soil particles away from
the pipe, potentially causing the pipe to experience structural problems.
Failure of the backfill and ultimately the pipe itself can lead to
hydraulic failure of the pipe system as well as the failure of surface
structures such as roadways and buildings.
While percolation is expected around many storm drains and culverts,
especially near pipe inlets, those with higher (5H:1V or greater)
percolation gradients are often candidates for pipe collars. The percolation
gradient is the slope of a line from an inlet water surface to a point
of relief for the percolated water. The difference in water surface
elevations between the upstream end of the percolation path and the
point of relief is ΔHperc. Lane’s
weighted creep method is used to determine a percolation factor (weighted-creep
ratio), which is compared to the allowable ratio for the soil type
at a given site. First, determine the weighted-creep length:
Where:
Lwc
|
=
|
Weighted creep length (ft.)
|
ysteep
|
=
|
Vertical path distance along the structure (steeper than 45°)
(ft.)
|
xmild
|
=
|
Horizontal path distance along the structure (flatter than 45°)
(ft.)
|
Lsc
|
=
|
Percolation path distance that shortcuts the soil (ft.)
|
Then determine the percolation factor, Rwc to 1:
Table 28.36.150 presents minimum recommended weighted-creep
ratios for a range of soil types:
Table 28.36.150: Lane’s Minimum Recommended Weighted-Creep
Ratios
|
---|
Material
|
Minimum Ratio
|
---|
Very fine sand or silt
|
8.5:1
|
Fine sand
|
7.0:1
|
Medium sand
|
6.0:1
|
Course sand
|
5.0:1
|
Fine gravel
|
4.0:1
|
Medium gravel
|
3.5:1
|
Course gravel w/cobbles
|
3.0:1
|
Boulders w/some cobbles and gravel
|
2.5:1
|
Soft clay
|
3.0:1
|
Medium clay
|
2.0:1
|
Hard clay
|
1.8:1
|
Very hard clay/Hardpan
|
1.6:1
|
Adapted from USBR 1974, Untitled Table, Page 364
|
Where the weighted-creep ratio calculated in Equation 28.36-13
does not exceed the applicable recommended ratio from Table 28.36.150,
or does not exceed 2.5:1, pipe collars shall be installed.
Figure 28.36.150 presents basic dimensions for pipe collar fittings
on reinforced concrete pipe (RCP) and corrugated metal pipe (CMP).
(Res. 40-08 (§ 905.1), 3-19-08)
Every horizontal or vertical pipe bend in a storm drain, culvert,
inverted siphon or other pipe structure shall be analyzed for stability.
As the momentum of flow changes around a bend, forces are exerted
on the bend that must be countered by the pipe walls, soil pressure,
pipe joints, and friction. When the dynamic thrust exceeds the allowable
force on any of these resistance devices, a thrust block is installed
at the bend. A thrust block typically consists of a rough block of
concrete poured around the outside of a pipe bend in direct contact
with the outer wall of the pipe.
The thrust force on a pipe bend is calculated by vector components
(x, y, and z) to simplify the process. In the equations below, “x”
represents the horizontal direction of flow upstream of the bend,
“y” represents the horizontal direction of flow normal
to “x,” and “z” represents the vertical direction
along which gravity acts. Equations 28.36-14 through 28.36-16 are
adapted from Roberson et al., 1998, using conservation of momentum
to find reaction forces. Pipe cross-sectional area and internal pressure
are assumed to be constant through the bend, with pressure assumed
to equal the surcharge depth above the pipe crown, if applicable.
|
(28.36-14)
|
|
(28.36-15)
|
|
(28.36-16)
|
Where:
FR
|
=
|
Reaction force required to hold bend in place, lbf
|
ρ
|
=
|
Density of water ≅ 62.4 lbs/ft3
|
Q
|
=
|
Flow rate in pipe, cfs
|
V1x
|
=
|
Average pipe velocity upstream of the bend, fps
|
V2x
|
=
|
V1x cos θ, V2y = V1x sin θ, V2z = V1x sin θ
|
p
|
=
|
Internal pipe pressure, psf
|
g
|
=
|
Gravitational constant, 32.17 ft./s2
|
A
|
=
|
Cross-sectional flow area of pipe, sf
|
θ
|
=
|
Total bend angle (vertical or horizontal)
|
Wbend
|
=
|
Weight of the pipe in the bend, lbs.
|
Wwater
|
=
|
Weight of the water in the bend, lbs.
|
Subscripts 1 and 2 indicate conditions just upstream and just
downstream of the bend.
|
In addition to soil bearing pressure, force on a bend is resisted
by friction between the pipe and the soil. A sliding coefficient of
0.35 is recommended for purposes of calculating the friction force
(USBR 1974).
Where calculations indicate that sliding or displacement of
a horizontal bend may occur, a thrust block is installed to increase
the effective bearing area on the soil such that the load is adequately
dispersed. Vertical bends may require an anchor block to provide additional
weight to resist the resultant vertical force. Calculation of resisting
forces for a vertical bend may include full pipe weight and anchor
block weight, but shall not include the weight of earth cover on the
bend. This allows for safe operation of the pipe even with reduction
or removal of cover material (USBR 1974).
(Res. 40-08 (§ 905.2), 3-19-08)
References for this section include USBR, 1974 and Linsley and
Franzini, 1964.
(a) Drain (Blow-Off) Valves.
A blow-off valve is intended
to allow for the draining of a structure that typically will otherwise
not fully drain. Most commonly used in long inverted siphons, blow-off
valves may be gravity-fed, pumped, or a combination of both, depending
on the invert of the discharge pipe. The design and installation of
blow-off valves and related pipes shall incorporate pressure-rated
joints and provisions for operation and maintenance access.
(b) Pressure Relief Valves.
Pressure relief valves are used
to exhaust excess air pressure from a pipeline to protect the pipe
from bursting and to remove large volumes of entrapped air that may
significantly impact the hydraulic capacity of the pipe. The valves
are set to open at a predetermined pressure so as to allow for a sealed
pipeline under normal operating pressures.
These valves are commonly utilized in smaller pressure pipelines such as water supply lines to limit the effects of hydraulic transients (water hammers), but are occasionally used in stormwater systems. Inverted siphons (GJMC §
28.52.030) often require a venting system to prevent blowback of air entrained in the water, although an open air vent (no valve) is usually an acceptable solution given an exhaust point that is well above the hydraulic grade line.
An air venting system of some type is required at all locations
where the crown of a pipe is higher than the crown elevations upstream
and downstream from that point.
(c) Air Inlet Valves.
Air inlet valves operate in a similar
fashion to pressure relief valves, but instead allow air into a pipeline
to avoid internal pressure to drop too far below atmospheric pressure.
As water drains from a sealed pipeline, a partial vacuum is created
that can collapse or severely damage the pipe. Air inlet valves operate
either by a float (water level) control or by opening at a set pressure
difference like a pressure relief valve.
High points in a pipeline shall always be designed with an air
venting system to avoid extreme positive or negative internal pressures
as compared to atmospheric pressure.
(Res. 40-08 (§ 905.3), 3-19-08)