Design of Open-Channel Waterways

Open channels can be designed using the concept of maximum permissible depthof flow. The fundamental premise of this approach is that for any selected channelthere exist a limiting depth of flow above which scour will occur. Any depth lessthan the limiting depth is non-eroding. This limiting depth is termed the maximumpermissible depth of flow. For a particular lining material, the maximum permissibledepth of flow is determined by the channel gradient and the erodibility of theunderlying soil. Alternately, the design could also be based on the concept of themaximum permissible velocity. Both concepts are connected through the ManningEquation.

Design Steps

1. Perform hydrologic computations and select design flows.

2. Estimate soil erodibility.

3. Define type of channel lining material desired.

4. Define channel slope and any restrictions on channel geometry.

5. Determine maximum permissible depth of flow, or maximum permissiblevelocity of flow, for lining material.

6. Select channel geometry and channel lining suitable for the design flowsbeing considered.

7. Consider other possible factors.

Hydrologic Computations

Waterways are normally sized to carry the runoff from the 24-hour rainfall with a10-year return period. If a vegetative lining is feasible, and a temporary lining is tobe used during the establishment period, a lower return period might be consideredfor the design of the temporary lining. Temporary linings include bare soil, or strawwith erosionet. The materials for temporary linings are usually biodegradable.

Soil Erodibility

Use of the design charts for maximum permissible depth of flow requiresspecification of the soil erodibility class characteristics of the underlying soil. A soilmay be identified as being highly erodible, very erodible, moderately erodible,slightly erodible, or erosion resistant.

In general, sandy non-cohesive soils tend to be highly erodible, large grainedgravel-silt-clay mixtures are erosion resistant, and colloids are moderately erodible.Estimates of erodibility class may be based on soil erodibility determinations used inconjunction with the Universal Soil Loss Equation.

Channel Lining Material

The lining material determines factors such as the hydraulic and scour resistance ofthe waterway. Choice should be based on economic considerations, such as initialcapital outlay, and the cost of labor and machinery required for maintenance.

Rock riprap specifications should include not only rock size, thickness of riprap, toetrench dimensions, but also durability, hardness, angularity, and resistance toweathering.

Restrictions on Channel Geometry

It is important to identify restrictions or constraints that must be placed on channelgeometry. The constraints may have the effect of limiting or increasing the size(depth, width, or both) of the channel.

On the one hand, the presence of roads, buildings, or established property lines maylimit the available space, and hence, restrict the waterway width and/or side slopes.A stable channel design of relatively narrow width and steep side slopes can oftenbe achieved with the use of more rigid lining materials that incorporate some"retaining wall" features (gabion baskets, sheet piling, concrete).

On the other hand, for purposes of safety, erosion resistance, construction ormaintenance ease, channel side slopes involving flexible linings should be keptrelatively flat. For erosion resistance, it is suggested that side slopes be no greaterthan 3:2 for medium-textured soils, 1:1 for cohesive well-drained clay, and 4:1 fornon-cohesive sands. Ideally, side slopes should be 2:1 or flatter for erosion

resistance , and still flatter slopes may be necessary for construction or otherreasons.

Maximum Permissible Depth of Flow, dmax.

The maximum permissible depth of flow can be determined for selected liningmaterial(s), channel slope(s), and soil erodibility classes (or rock D50) from the tablebelow.

Maximum permissible depth of flow as a function of channel slope, channel lining, and soil erodibility

3.281 s-1.0Rock riprap

D50 = 0.125 m

6.562 s-1.0Rock riprap D50 = 0.25 m

1.0158 s-0.5911.1228 s-0.5911.2300 s-0.591Grass mixture-

Low Retardance

1.7530 s-0.5031.9040 s-0.5032.0552 s-0.503Grass mixture-

High Retardance

1.0778 s-0.6471.3672 s-0.6471.6569 s-0.647Straw mulch and

erosionet

0.0732 s-0.6820.1358 s-0.6820.1988 s-0.682Bare soil

Highly ErodibleSoil

ModeratelyErodible Soil

ErosionResistant Soil

Maximum permissible depth of flow (ft)Channel Lining

If the dmax so determined for particular conditions is not compatible with geometryconstraints that have been established, a new lining material should be selected.

Maximum non-scouring velocities for open channels

Good grass cover

Medium grass cover

Bare Soil

Maximum permissible velocities on establishedcover (ft/s)

Material

--8.0 Hard cementedcomglomerates

-7.0 6.0 Shale, hardpan, soft rock

-6.0 5.0 Coarse gravels

8.0 6.0 4.5 Stiff clay or stiff gravelly soil

7.5 5.5 3.5 Firm clay loam

6.5 4.5 2.5 Sandy soil

5.5 4.0 2.5 Coarse sand

5.0 3.0 1.5 Light loose sand

4.5 2.5 1.0 Light silty sand

Channel Geometry

The channel geometry can now be determined for selected channel lining materialson the basis of factors such as channel slope, water flow rate, and soil erodibility.

Parabolic cross-sections most closely approximates natural channels. Trapezoidaland triangular cross-sections tend to become parabolic over time.

This stage of the design involves one step for unlined channels, channels lined withstraw mulch, or channels lined with rock riprap. Two steps are necessary forchannels lined with grass mixtures.

Except for grass-lined waterways, cross-sectional characteristics can be determinedby using the Manning equation. The hydraulic radius, wetted perimeter, and area areall functions of depth. The channel is sized so as to ensure that the flow depth is lessthan or equal to dmax.

For channels lined with grass mixtures, waterway dimension should first bedetermined for conditions when the channel is most susceptible to erosion, that iswhen the established vegetation is sparse or dormant. This design step provides astable waterway cross section for the maximum permissible depth of flow occurringon the established vegetative lining in its most vulnerable condition.

The second step is then required for the vegetated channel design to determine thedepth of flow required to transmit the design flow rate when the grass mixture is

mature and dense. In this condition the channel is less susceptible to erosion, but is providing maximum retardance to the water flow. The depth required to carry thedesign flow over the increased retardance conditions has been termed the depth fordesign capacity, dcap, and is based on dmax.

dcap = dmax + (dcap-dmax)

Comparisons of channel geometries for various lining materials allow confirmationof a suitable lining for the design flows considered.

Other Design Considerations

^ Freeboard. Most lining materials should extend to the top of the bank or atleast 3 ft above the design water level (measured along the slope).

^ Protection in bends. Extra protection from erosion is often required at bendsand corners of channels with flexible linings. Where possible, circular curvesshould be used.

^ Tile outlets. Where drainage tiles outlet into the waterway, both the outletportion of the tile and the bank surrounding the outlet pipe require specialconsideration.

^ Construction and maintenance of equipment. A decision on the final sizeand shape of the waterway should take into account the type of equipment tobe used. The channel design may be widened or curved, and the side slopesmay be flattened to facilitate construction, maintenance, or both.