S 2 Q1/ 4 ≤ .0007
S2 Q1/4 ≤ 0.0017 (English)
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
_ A/W, Equation 5.42 leads to
With substitution of τo = γRS, V = Q/A and R = A/P
W2 = S2 W1 / S1
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.