Stability Problems Associated With Channel Relocation (cont.)

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If S1 is of such magnitude that Equation 5.1 cannot be satisfied with still larger S2, the possibility

of the river changing to a braided channel because of steeper slope should be carefully

evaluated. With steeper slope, there could be an increase in sediment transport which could

cause degradation, and the effect would be extended both upstream and downstream of the

relocated reach. The meander patterns could change. Considerable bank protection might be

necessary to contain lateral migration which is characteristic of a braided channel, and if the

slope is too steep, head cuts could develop which migrate upstream with attendant effects on

the geometry of the channel. Even when changes in slope are not very large, a short-term

adjustment of the average river slope occurs, consistent with the sediment transport rate, flow

velocities and roughnesses, beyond the upstream and downstream limits of channel

improvement. For small changes in slope, the proportionality (Equation 5.16), QS ~ Qs D50

tends toward equilibrium with slight increases in bed sediment size, D50, and adjustment in the

sediment transport rate, Qs.

A small increase in the new channel width could be considered which tends to maintain the

same stream power, τo V, in the old and new channels. That is,

(τ o V)1 = (τ o V)2

(5.31)

_ A/W, Equation 5.42 leads to

With substitution of τo = γRS, V = Q/A and R = A/P

W2 = S2 W1 / S1

(5.32)

Any designed increase in width should be limited to about 10 to 15 percent. Wider channels

would be ineffective. Deposition would occur along one bank and the effort of extra excavation

would be wasted. Furthermore, bar formation would be encouraged, with resultant tendencies

for changes in the meander pattern leading to greater maintenance costs for bank stabilization

and removal of the bars to hold the desired river alignment.

The depth of flow in the channel is dependent on discharge, effective channel width, sediment

transport rate (because it affects bed form and channel roughness) and channel slope.

Methods for evaluating flow depth were discussed earlier in this chapter.

The foregoing discussion pertains to alluvial channels with silt and sand sized bed materials.

For streams with gravel and cobble beds, the usual concern is to provide adequate channel

cross-sectional dimensions to convey flood flows. If the realigned channels are made too

steep, there is an increased stream power with a consequent increase in transport rate of the

bed material. The deposition of material in the reaches downstream of the crossing tends to

form gravel bars and encourages changes in the planform of the channel. Short-term changes

in channel slope can be expected until equilibrium is reestablished over extended reaches both

upstream and downstream of the rechannelized reach. Bank stabilization may be necessary to

prevent lateral migration, and periodic removal of gravel bars may also be necessary.

5.58