FLOOD06421

• County: Statewide
• Basin: Statewide
• Title: Highways in the River Environment
• Date: 2/1/1990
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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not convey the sediment from the small systems through it and aggradation was
initiated. Within a short period of time sufficient aggradation had occurred so as to
jeopardize water intakes, sewage out falls and so forth. As a consequence of the
adverse action in the vicinity of the cutoff and within the bendway itself, it was
finally decided that it would be more beneficial to restore the river to its natural
form throUlJh the bend. This actiorl was taken and the serious problems were
alleviated.
In such a haphazard program of river development, the highway engineer would
be hard pressed to maintain and plarl for his highway system along and over this
reach of river.
Another common case occurs with the development of reservoirs for storage
and flood control. These reservoirs serve as traps for the sediment normally flowing
throu9h the river system. With sediment trapped in the reservoir, essentially clear
water is released downstream of the dam site. This clear water has the capacity to
transport more sediment than is immediately available. Consequently the channel
begins to supply this deficit with resulting degradation of the bed or banks. This
degradation may significantly affect the safety of bridges in the immediate
vicinity. Again, the degraded or widened main channel causes steeper gradients on
tributary streams in the vicinity of the main channel. The result is degradation in
the tributary streams. It is entirely possible, however that the additional sediments
supplied by the tributary streams would ultimately offset the degradation in the
main channel. Thus, it must be recognized that downstream of ~torage structures
the channel may either aggrade or degrade and the tributaries will be affected in
either case.
There are important responses induced upstream of reservoirs as well as
downstream. When the stream flowing. into a reservoir encounters the ponded
water, its sediment load is deposited forming a delta. This deposition in the
reservoir flattens the gradient of the channel upstream. The flattenin9 of the
upstream channel induces alJ9radation causing the bed of the river to rise,
threatening highway installations and other facilities. F or example, Elephant Butte
Reservoir. built on the Rio Grande. has caused the Rio Grande to aqqrade many
miles upstream of the reservoir site. This change in bed level can have very
significant effects upon bridges, other hydraulic structures and all types of training
and stabilization works. Ultimately the river may be subjected to a flow of
magnitude sufficient to overflow existing banks, causing the water to seek an

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