Channelization and Channel Modification Activities and Impacts
4.2.6
DIVERSION CHANNELS
Diversion channels are constructed to divert waters from the main channel for purposes such as
flood control, municipal water supply, and irrigation. A type of diversion channel used for flood control
is a flood bypass channel or floodway. It is a separate channel into which flood waters are directed to
lessen the impact of flooding on the main river system. Diversion channels on large river systems such as
the Mississippi River can consist of adjacent low-lying areas or old river courses. Control structures may
be located at the head of the diversion channel to divert flows during periods of high water and return flows
during low water. Some diversion channels bypass the flood flows into an adjacent waterway, while others
return the flows back into the same stream a distance downstream from the point of the diversion.
Diversion channels are often used in urban areas where it is not possible to widen the existing channel due
to development. Diversion channels may be used to provide a means of diverting floodwater across the
neck of a meander or series of meanders (Acheson, 1968). Major design considerations for diversion
channels include: 1) determining if the channel should convey partial or all flows 2) design of appropriate
controls 3) sizing of the channel to convey the design discharge and 4) design to reduce maintenance
(Nunnally, 1985). To be effective in reducing the flood stage, the distance between the point of diversion
and point of return to the main channel must be of sufficient length to prevent backwater effects.
Additionally, it is essential to consider potential morphologic effects on both the main channel and receiving
channel.
4.2.6.1 Hydraulic Effects
According to Nunnally and Shields (1985), diversion channels generally have steeper slopes than
the main channel. This can lead to stability problems such as erosion of the channel bed and banks. The
bed of tributary channels may be higher than that of the floodway channel, and bed degradation may
migrate upstream of the tributary, resulting in excessive sediment transport and deposition in the floodway.
Methods to mitigate channel instability such as grade control, channel lining, and bank stabilization may be
required on diversion projects.
Additionally, diversion flows can have an adverse impact on the main channel. From Lane's
Balance, it can be seen that reducing the river flow in the main channel due to a diversion, with the slope
and particle size remaining constant, will result in a decrease in sediment transport capability, thus
aggradation could occur in the channel between the point of the diversion and the point of re-entry. If too
much bed material is diverted, the sediment transport capability of the stream may increase, thus
accelerating channel instability. Flow returning to the main channel from a diversion can also result in
accelerated erosion of the channel and banks. Vanoni (1977) reported that in Alkali Creek in Wyoming,
flow returning to the main channel from a diversion resulted in bed erosion. The channel eroded down to
an armored layer of large gravel and cobbles, after which the banks began to erode, resulting in the
implementation of bank stabilization measures. It is essential that a detailed geomorphic and sediment
transport analysis be conducted at the design stage of a diversion project to plan for potential problems.
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