Channel Design

Most Efficient Hydraulic Section

Most efficient Hydraulic Section

• A section that gives maximum section factor, for a specified flow area, is called the most efficient hydraulic section or best hydraulic section.

Most efficient Hydraulic Section• The best hydraulic section minimizes the area required to convey a specified

flow, however, the area which must be excavated to achieve the flow area required by the best hydraulic section may be significantly larger if the overburden which must be removed is considered.

• It may not be possible to construct a stable best hydraulic section in the available natural material. If the channel must be lined, the cost of the lining may be comparable with the cost of excavation.

• The cost of excavation depends not only on the amount of material which must be removed, but also on the ease of access to the site and the cost of disposing of the material removed.

• The slope of the channel in many cases must also be considered a variable since it is not necessarily completely defined by topographic considerations. For example, while a reduced channel slope may require a larger channel flow area to convey the specified flow, the cost of excavating the overburden may be reduced.

Geometric Elements of best hydraulic sections

• The most efficient hydraulic section is the one that yields the minimum wetted perimeter for a given area.

Erodible Channels

• If the channel bottom or sides are erodible, then the design requires that the channel size and bottom slope are selected so that channel is not eroded.

• Methods used to design such channels are:– Permissible Velocity Method– Tractive Force Method

Permissible Velocity Method• The channel size is selected

such that the mean flow velocity for the design discharge under uniform flow conditions is less than the permissible flow velocity.

• The permissible velocity is defined as the mean velocity at or below which the channel bottom and sides are not eroded.

(Choudhry, 2008)

Permissible Velocity Method

1. For the specified material, select value of Manning , side slope (Table 9-2, Chaudhry), and the permissible velocity .

2. Determine the required hydraulic radius, , form Manning formula, and the required flow area, , from the continuity equation,

3. Compute the wetted perimeter, 4. Determine the channel bottom width, , and the flow depth,

for which the flow area is equal to that computed in step 2 and the wetted perimeter, , is equal to that computed in step 3

5. Add a suitable value for the freeboard using Table 9-1.

Example

• Design a channel to carry a flow of 6.91 . The channel will be excavated through stiff clay () at a channel bottom slope of 0.00318.