Selection and Design of Channel Rehabilitation Methods
The above discussion illustrates that the design of grade control structures is not simply a hydraulic
exercise. Rather, there are many other factors that must be included in the design process. For any
specific situation, some or all of the factors discussed in this section may be critical elements in the final
siting of grade control structures. It is recognized that this does not represent an all inclusive list since there
may be other factors not discussed here that may be locally important. For example, in some cases,
maintenance requirements, debris passage, ice conditions, or safety considerations may be controlling
factors. Consequently, there is no definitive cookbook procedure for designing grade control structures
that can be applied universally. However, consideration of each factor in an analytic and balanced fashion,
and avoiding reliance on empircal procedures, can lead to effective and intelligent use of grade control
structures.
6.3
FLOW CONTROL
Although bank stabilization and grade control are the primary structural methods utilized in channel
rehabilitation, project goals often require the implementation of flow control. Unfortunately, predicting the
channel response to flow control is extremely difficult. The complexity of channel response is illustrated
in Lane's relationship (Figure 3.19), which indicates that a reduction in discharge, would allow a steeper
slope to exist in the channel. Therefore, the degradational potential in a channel due to excessive slope
would be minimized or even eliminated by the reduced discharge. However, this scenario is based on the
assumption that the sediment load (Qs) and the bed material size (D50) remain unchanged. This is seldom
the case since the trapping of sediment in the reservoir generally results in a reduced sediment supply
downstream of the dam. According to Lane's relationship, a reduced sediment load downstream of a dam
would result in a flatter slope that would result from bed degradation. Thus, the reduction in discharge and
sediment load downstream of a dam tends to produce counteracting results. The ultimate channel response
depends on the relative magnitude of the changes in discharge and sediment load, and on the downstream
watershed characteristics such as tributary inputs, geologic controls, bed and bank materials, and existing
channel morphology.
Because of these complexities, it is extremely difficult to predict the anticipated channel response
to the construction of a dam. Therefore, it is difficult to develop definitive design criteria or guidance for
the use of flow control as a means of accomplishing project goals. There are also numerous environmental
consequences that must be considered. Generally, where flow control has been used for channel
stabilization, it has been based primarily on past experiences with similar projects in the area. For example,
studies by the Natural Resources Conservation Service in Mississippi showed that when over 60% of the
Abiaca Creek watershed area in Mississippi was controlled by flood water retarding structures, the channel
instability problems were significantly reduced (Water Engineering & Technology, 1989). Obviously, each
watershed will behave differently, and the Abiaca results can not be applied regionally without further
investigations. However, the results do provide some indication that channel stability can be improved by
flow control.
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