Selection and Design of Channel Rehabilitation Methods
For reaches without grade control, the total volume of failure has increased by a factor of 3.4 for
reaches without grade control as bank heights increase due to incision. By comparison, the total volume
of failure for reaches with grade control has not significantly increased and has been zero for two of the four
years. In 1996, the volume of steep bank cross section failure with grade control was only 7% of similar
cross sections without grade control. The data appear to confirm the value of grade control in reducing
bank height for bank stabilization in incising streams.
6.2.2.3 Flood Control Impacts
Channel improvements for flood control and channel stability often appear to be mutually exclusive
objectives. For this reason, it is important to ensure that any increased post-project flood potential is
identified. This is particularly important when hydraulic control structures are considered. In these
instances the potential for causing overbank flooding may be the limiting factor with respect to the height
and amount of constriction at the structure. Grade control structures are often designed to be hydraulically
submerged at flows less than bankfull so that the frequency of overbank flooding is not affected. However,
if the structure exerts control through a wider range of flows including overbank, then the frequency and
duration of overbank flows may be impacted. When this occurs, the impacts must be quantified and
appropriate provisions such as acquiring flowage easements or modifying structure plans should be
implemented.
Another factor that must be considered when designing grade control structures is the safe return
of overbank flows back into the channel. This is particularly a problem when the flows are out of bank
upstream of the structure but still within bank downstream. The resulting head differential can cause
damage to the structure as well as severe erosion of the channel banks depending upon where the flow
re-enters the channel. Some means of controlling the overbank return flows must be incorporated into the
structure design. One method is simply to design the structure to be submerged below the top bank
elevation, thereby reducing the potential for a head differential to develop across the structure during
overbank flows. If the structure exerts hydraulic control throughout a wider range of flows including
overbank, then a more direct means of controlling the overbank return flows must be provided. One
method is to ensure that all flows pass only through the structure. This may be accomplished by building
an earthen dike or berm extending from the structure to the valley walls which prevents any overbank flows
from passing around the structure (Forsythe, 1985). Another means of controlling overbank flows is to
provide an auxiliary high flow structure which will pass the overbank flows to a specified downstream
location where the flows can re-enter the channel without causing significant damage (Hite and Pickering,
1982).
6.2.2.4 Environmental Considerations
Projects must work in harmony with the natural system to meet the needs of the present, without
compromising the ability of future generations to meet their needs. Engineers and geomorphologist are
responding to this challenge by trying to develop new and innovative methods for incorporating
environmental features into channel projects. The final siting of a grade control structure is often modified
to minimize adverse environmental impacts to the system.
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