a specific situation. The decision to use one method or another depends upon several factors
project objectives, and time and cost constraints.
12.2.2 GEOTECHNICAL CONSIDERATIONS
The above discussion focused only on the hydraulic aspects of siting grade control
structures. However, in some cases, the geotechnical stability of the reach may be an
important or even the primary factor to consider when siting grade control structures. This
is often the case where channel degradation has caused, or is anticipated to cause, severe bank
instability due to exceedance of the critical bank height (Thorne and Osman, 1988). When
this occurs, bank instability may be widespread throughout the system rather than restricted
to the concave banks in bendways. Traditional bank stabilization measures may not be
feasible in situations where system-wide bank instabilities exist. In these instances, grade
control may be the more appropriate solution.
Grade control structures can enhance the bank stability of a channel in several ways.
Bed control structures indirectly affect the bank stability by stabilizing the bed, thereby
reducing the length of bankline that achieves an unstable height. With hydraulic control
structures, two additional advantages with respect to bank stability: (1) bank heights are
reduced due to sediment deposition, which increases the stability of the banks with regard to
mass failure; and (2) by creating a backwater situation, velocities and scouring potential are
reduced, which reduces or eliminates the severity and extent of basal cleanout of the failed
bank material, thereby promoting self-healing of the banks.
12.2.3 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
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 siting 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