Chapter 28.32 Open Channels

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.

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.

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

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)

§ 28.32.050 Critical flow computation.

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:

The specific energy (E=y+v²/2g) is at a minimum for a given discharge (Q).

The discharge (Q) is a maximum for a given specific energy (E).

The specific force is a minimum for a given discharge (Q).

The velocity head (v²/2g) is equal to half the hydraulic depth (D/2) in a channel of small slope.

The Froude number (Fr) is equal to 1.0.

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 (S_c). A slope less than S_c will cause subcritical flow. A slope steeper than S_c 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.

The criteria of minimum specific energy for critical flow results in the definition of the Froude number (Fr) as follows:

(28.32-4)

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.)

The critical depth in a given trapezoidal channel section with a known flow rate can be determined using the following method:

Step 1.

Compute the section factor (Z).

(28.32-5)

Where:
Z	=	section factor
Q	=	flow rate (cfs)
g	=	gravitational acceleration (32.2 ft./s²)

§ 28.32.060 Design procedures – Subcritical flow.

Step 2.

Determine the critical depth in the channel (d_c) 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).

FIGURE 28.32.050

(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).

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.

Contractions.

The energy loss created by a contracting section may be calculated using the following equation:

(28.32-6)

Where:

H_t	=	energy loss (ft.);
K_tc	=	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 (K_tc) values for the typical open-channel transition section.

Expansions.

The energy loss created by an expanding transition section may be calculated using the following equation:

(28.32-7)

Figure 28.32.060 also shows expansion loss coefficients (K_tc) values for typical open-channel transition sections.

FIGURE 28.32.060

§ 28.32.070 Design procedures – Supercritical flow.

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:

L_t ≥ 0.5L_c (ΔT_w)

(28.32-8)

Where:

L_t	=	minimum transition length (feet);
L_c	=	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 (L_c) 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 (L_c) 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 (L_c)
= 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.

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.

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:

L_T	=	b₁– b₃	(28.32-9)
		2tanθ

Where:

L_T	=	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,

(28.32-10)

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:

(28.32-11)

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

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:

L_t ≥ 1.5 (ΔW) Fr

(28.32-12)

Where:

L_t	=	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

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:

(28.32-13)

Where:

L_c	=	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.

§ 28.32.080 Design procedures – Superelevation.

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:

S ≤	12	(28.32-14)
	R_E

Where:

S	=	channel slope (feet per feet);
R_E	=	Reynolds Number, R_E =	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).

FIGURE 28.32.070(b)

(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:

Δy =	CV²W	(28.32-15)
	rg

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)

§ 28.32.090 Transitions.

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).

Transitions in open channels are generally designed for the following four flow conditions.

Subcritical flow to subcritical flow.

Subcritical flow to supercritical flow.

Supercritical flow to subcritical flow (hydraulic jump).

Supercritical flow to supercritical flow.

§ 28.32.100 Types of open channels and their selection.

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:

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.

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.

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.

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.

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.

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.

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)

§ 28.32.110 Natural channel systems.

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.

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:

Geologic factors, including soil conditions;

Hydrologic factors, including possible changes in flow and runoff, and the hydrologic effects of changes in land use;

Geometric characteristics of the stream, including the probable geometric alterations that developments will impose on the channel;

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

Ecological/biological changes that will result from physical changes that may in turn induce or modify physical changes.

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:

Stream discharge;

Sediment load;

Longitudinal slope;

Characteristics of bed and bank material;

Bank and bed resistance to flow;

Vegetation or lack thereof;

Geology, including type of sediment; and

(8)

Constructed improvements.

(Res. 40-08 (§ 805), 3-19-08)

§ 28.32.120 Channel morphology.

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.

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)

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.

Braided Channel

Meandering Channel

Straight Channel

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)

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:

(28.32-16)

and

(28.32-17)

Where:

Q	=	Average discharge
Qs	=	Sediment supply
b	=	Channel width
d	=	Channel depth
λ	=	Meander wavelength
S	=	Bed slope
P	=	Sinuosity

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.

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 SQ^1/4 is less than or equal to 0.0017, it tends to be meandering. If SQ^1/4 is greater than or equal to 0.01, systems tend toward a braided pattern. Streams that have SQ^1/4 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.

Some specific examples of man-induced changes to the natural stream/river systems that could cause undesired responses by channel morphology are as follows:

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.

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.

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.

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.

§ 28.32.140 General design criteria for improved open channels.

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)

§ 28.32.130 Channel restoration.

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)

§ 28.32.150 Channel type selection.

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:

Hydraulic Factors.

Slope of thalweg.

Right-of-way.

Capacity needed.

Basin sediment yield.

Topography.

Ability to drain adjacent lands.

Structural Factors.

Cost.

Availability of material.

Areas for wasting excess excavated material.

Seepage and uplift forces.

Shear stresses.

Pressures and pressure fluctuations.

Momentum transfer.

Environmental Factors.

Neighborhood character.

Neighborhood aesthetic requirements.

Need for new green areas.

Street and traffic patterns.

Municipal or county policies.

Wetland mitigation.

Wildlife habitat.

(8)

Water quality enhancement.

Sociological Factors.

Neighborhood social patterns.

§ 28.32.170 Manning roughness coefficient.

Neighborhood children population.

Pedestrian traffic.

Recreational needs.

Maintenance Factors.

Life expectancy.

Repair and reconstruction needs.

Maintainability.

Proven performance.

Accessibility.

Regulatory Factors.

Federal regulations.

State regulations.

Local regulations.

(Res. 40-08 (§ 806.1), 3-19-08)

§ 28.32.160 Hydraulic capacity.

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)

§ 28.32.180 Uniform flow.

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)

§ 28.32.190 Vertical and horizontal alignment.

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)

§ 28.32.200 Maximum permissible velocity.

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.

TABLE 28.32.200

(Res. 40-08 (§ 806.6), 3-19-08)

§ 28.32.210 Subcritical and supercritical flow.

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=d_c) 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)

§ 28.32.220 Freeboard.

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.

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:

	(28.32-18)
	(28.32-19)

Where:

h_fr	=	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	=	superelevation allowance (ft.), see GJMC § 28.32.080; and
Δy	=	allowances for other hydraulic phenomenon (ft.), (e.g., standing waves, slug flow – see GJMC § 28.32.070(e)).

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:

Begin at a point 5.0 times the characteristic wave length of the design flow (5L_w), measured from the downstream tangent point of the curve, with no superelevation allowance.

Taper uniformly to the full superelevation allowance at a point 3.0 times the characteristic wave length of the design flow (3.0L_w), measured from the downstream tangent point of the curve.

Maintain the full superelevation allowance through the curve.

Continue the top of bank elevation level from the upstream tangent point of the curve to its intersection with the normal top of bank.

Equation 28.32-20 and Equation 28.32-21 describe the characteristic wavelengths for subcritical and supercritical flow, respectively.

	(28.32-20)
	(28.32-21)

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)

§ 28.32.230 Flow transitions.

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)

§ 28.32.240 Access and safety.

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.

The minimum width of any channel easement shall be the top width of channel plus four feet on each side of channel.

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.

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.

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:

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.

§ 28.32.270 Design criteria for natural and alluvial-bed (erodible) channels.

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)

§ 28.32.250 Environmental permitting.

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)

§ 28.32.260 Maintenance.

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.

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:

What additional analysis will be prepared to estimate the potential extent of lateral and vertical channel movement;

What is the estimate of the potential risk to the proposed development from channel degradation and/or bank failure;

What solutions and/or remedies are available which can mitigate the potential risk to the proposed development; and

What improvements and/or reduction in encroachment in or adjacent to the subject channel will be required to allow approval of the subject development.

General Design Considerations and Evaluation Techniques for Natural Channels.

The channel and overbank areas shall have adequate conveyance capacity for the major storm runoff.

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.

The water surface profiles shall be defined so that the 100-year floodplain can be delineated.

Filling of the floodplain fringe may reduce valuable storage capacity and may increase downstream runoff peaks, and therefore shall be avoided.

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.

Plan and profile information (i.e., HEC-2 or HEC-RAS output) for both existing and proposed floodplain site conditions shall be prepared.

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.

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.

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.

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.

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:

The bottom paving is bonded, or there is another mechanism in place to pay for the bottom paving once the channel is completed.

Erosion in the unlined section is addressed to the satisfaction of the local official.

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.

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.

FIGURE 28.32.270

(Res. 40-08 (§ 807), 3-19-08)

§ 28.32.280 Grass-lined channels.

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:

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.

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.

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.

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.

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.

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.

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.

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.

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.

Side Slopes.

Side slopes of a grass-lined channel shall be not steeper than 3H:1V.

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.

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.

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.

FIGURE 28.32.280

(Res. 40-08 (§ 808), 3-19-08)

§ 28.32.290 Wetland-bottom channel.

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.

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.

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):

(28.32-22)

Where:

n_c	=	Manning’s roughness coefficient for the composite channel (Dimensionless)
n₀	=	Manning’s roughness coefficient for areas above the wetland area (Dimensionless)
n_w	=	Manning’s roughness coefficient for the wetland area (Dimensionless)
P_o	=	Wetland perimeter of channel cross-section above the wetland area (feet)
P_w	=	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 (n_w) can be obtained from Figure 28.32.290.

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).

Bottom Width.

The following design factors shall be considered in selecting an appropriate channel bottom width:

Wetland mitigation requirements.

Constructability.

Channel stability and maintenance.

Multi-use purpose.

Low-flow channel width.

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.

Side Slopes.

Side slopes shall not be designed steeper than three horizontal to one vertical.

Grass-Lining.

The side slopes may be grass-lined according to the guidelines provided previously in GJMC § 28.32.280(g).

Channel Bend Protection.

Channel bends shall be designed according to the criteria discussed previously in GJMC § 28.32.280(h).

§ 28.32.300 Design criteria – Riprap-lined channels.

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.

FIGURE 28.32.290

(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:

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.

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.

Low-Flow Channel.

The design criteria for the low-flow channel are discussed in GJMC § 28.32.280(c)(2).

Bottom Width.

The following design factors shall be considered in selecting an appropriate channel bottom width.

Constructability;

Channel stability and maintenance;

Multi-use purpose;

Trickle/low-flow channel width.

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.

Side Slopes.

Due to stability, safety, and maintenance considerations, riprap-lined side slopes shall be two horizontal to one vertical or flatter.

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.

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.

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.

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)

(28.32-24)

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)
S_s	=	Specific gravity of rock (minimum S_s = 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.

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).

TABLE 28.32.300(a)

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.

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.

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 D_15(filter) and 85 percent of the base material is finer than d_85(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.

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.

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.

Bedding Material.

The bedding material requirements are the same as for loose riprap.

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.

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.

TABLE 28.32.300(c)

(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.

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).

Riprap Gradation for Steep Slopes. Table 28.32.300(e) provides ratios used to determine the D₁₀, D₂₀, and D_max rock sizes from the D₅₀ 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.

TABLE 28.32.300(d)

TABLE 28.32.300(e)

FIGURE 28.32.300(a)

FIGURE 28.32.300(b)

FIGURE 28.32.300(c)

FIGURE 28.32.300(d)

FIGURE 28.32.300(e)

FIGURE 28.32.300(f)

FIGURE 28.32.300(g)

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.

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.

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.

§ 28.32.310 Design criteria – Concrete-lined channels.

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:

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.

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.

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).

Bottom Width.

The bottom width of the concrete-lined channel shall be a minimum of 4.0 feet.

Flow Depth.

There are no flow depth requirements for concrete-lined channels.

Side Slopes.

Vertical or flatter side slopes may be designed for concrete-lined channels.

Concrete-Lining Section.

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.

Concrete Joints.

The following design standards, found to work in similar conditions, are suggested. Alternatives will be considered on a case-by-case basis.

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.

Longitudinal joints, where required, shall be constructed on the sidewalls at least 1.0 foot vertically above the channel invert.

All joints shall be designed to prevent differential movement.

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.

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.

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.

Reinforcement Steel.

Steel reinforcement shall be at a minimum grade 40 deformed bars. Wire mesh shall not be used.

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.

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.

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.

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.

The 12 inches of subgrade immediately beneath concrete lining (both channel bottom and side slopes).

Top 12 inches of maintenance road.

Top 12 inches of earth surface within 10 feet of concrete channel lip.

All fill material.

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.

§ 28.32.320 Design criteria – Other channel linings.

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:

Structural integrity of the proposed lining.

Interfacing between different linings.

The maximum velocity under which the lining will remain stable.

Potential erosion and scour problems.

Access for operations and maintenance.

Long-term durability of the product under extreme meteorological and soil conditions.

Ease of repair of damaged section.

Past case history (if available) of the lining system in other arid areas.

§ 28.32.330 Design standards for minor artificial drainageways.

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.

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.

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.

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)

§ 28.32.340 Channel appurtenances.

Presented in this section are the design standards for appurtenances to improved channels. All improved channels shall be designed to include these appurtenances.

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.

Safety Requirements.

The following safety requirements are required for concrete-lined channels. Similar safety requirements may be required for all other channels.

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.

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.

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.

Basin Configuration.

The length of the outlet protection (L_a) is determined using the following empirical relationships that were developed for the U.S. Environmental Protection Agency (USEPA, 1976):

(28.32-28)

and

(28.32-29)

Where:

D_o	=	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:

(28.32-30)

and

(28.32-31)

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.

Rock Size.

The median stone diameter, d₅₀, is determined from the following equation:

(28.32-32)

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.

(28.32-33)

Also,

(28.32-34)

Where

depth of scour hole below culvert invert

The other riprap requirements are as indicated in the previous sections for channel lining.

Low-Flow Grade Control Structures.

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.

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:

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).

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).

A hydraulic analysis shall be performed using the discharge that completely fills the check structure at its crest (the primary design flow).

FIGURE 28.32.340(a)

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.

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:

Determine a stable slope as described above.

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).

FIGURE 28.32.340(b)

(Res. 40-08 (§ 814), 3-19-08)

§ 28.32.350 Example application.

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.

Solution.

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		=	(S^1/2)	(1.49)

Solving by trial and error,

Y = 3.7 feet

Step 2.

Calculate the water velocity in the proposed channel during a 100-year flow event using the Manning equation.

V	=	1.49	S^1/2 R^2/3
		n

= 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.

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

Step 4.

Verify that trickle channel has sufficient capacity.

The minimum capacity of the trickle channels is:

Min. QT_c = 0.05 * Q₁₀₀

Min. Q_c = 9.6cfs

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.

Step 5.

Determine the freeboard required for the proposed channel.

F_b = 0.5 +	V²
F_b = 0.5 +	2g

F_b = 0.5 +	(.5)²	= 0.6 feet,
	2 * 32.2

but minimum = 1.0 foot.

Therefore use Fb = 1.0 foot.