As the name implies, open-channel flow is concerned with the conveyance of water with a free surface. This article primarily concerns lined and unlined channels such as encountered along roadways in highway design.
The parameters to consider in choice of channel cross-section include hydraulics, safety, maintenance, economics, and the environment. These considerations are usually so interdependent that optimizing one can have detrimental effects on the others. The hydraulic engineer’s objective is to achieve a reasonable balance among the competing criteria.
Safety is always of primary concern to the highway engineer. If the channel is located far enough away from the traveled way, an adequate recovery zone may be available for vehicles accidentally leaving the roadway. Additionally, with regard to safety, a channel with flattened sideslopes and a curved transition to the bottom is preferred to allow time for recovery for the errant vehicle. (See Chap. 6, Safety Systems.)
Periodic maintenance is required for hydraulic channels regardless of the cross-sectional design chosen. Access should be planned and provided for maintenance personnel and equipment. The proliferation of sediment and debris and the growth of vegetation can cause erosion or reduction of the capacity of the channel. The channel design should balance the cost of preventing these restrictions against the anticipated increased costs of removing them as they accumulate.
The proposed channel location and shape affect the economics of the project. A channel located away from the traveled way may be safer for the traveling public and more aesthetically pleasing; however, these considerations must be balanced against the potential increase in right-of-way costs as well as other associated costs. The shape also affects the cost of the channel. A channel with vertical sidewalls will typically be more expensive than one with sloping sides; the vertical walls must not only maintain flow within the channel but must also be designed to retain the earth outside the channel.
Proposed channel improvements must take into account the possible effects the project will have with regard to erosion, sedimentation, water quality, aesthetics, and fish and wildlife. Local, state, and federal resources and flood control agencies have an interest in drainage improvements and environmental impacts and should be contacted early in the planning process for input, cooperation, and assistance. A partial list of these agencies may be found in the AASHTO Highway Drainage Guidelines.
The necessary hydraulic parameters should be determined early in the design phase. As previously mentioned, the scope of the hydrologic study should be proportional to the importance of the hydraulic structure involved, the type of highway, the impacts on the local property, and potential risks involved. The hydraulic design of the channel involves selecting the cross-section and lining to maintain the flow predicted from the hydrologic study. The capacity of the channel is affected by its size, shape, roughness, and slope.
The slope is generally controlled by the existing terrain, and the engineer has little control over this. As much as is practical, however, the engineer should avoid sudden changes in the slope as well as the alignment of the channel. Abrupt changes in channel alignment can lead to unintentional channel changes by aggradation and avulsion. Abrupt changes in slope can cause either erosion, if the grade is steepened, or an accumulation of buildup, if it is flattened.
Erosion and deposition may also be limited by controlling the velocity of the flow. The velocity of the water is dependent upon the size, shape, roughness, and slope of the channel as well as the quantity of flow. Recommended flow velocities for unlined
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TABLE 5.5 Recommended Permissible Velocities for Unlined Channels
Source: From Highway Design Manual, California Department of Transportation, with permission. |
channels are shown in Table 5.5. Velocities in lined channels can generally be much greater. To minimize deposition of sediment, the minimum gradient should be about 0.5 percent for earth-lined and grass-lined channels and 0.35 percent for paved channels. Also, decreasing gradients should be avoided.
