Channelization and Channel Modification Activities and Impacts
where structures are impassable, openings, fish ladders or other passageways must be incorporated into
the structure design to allow fish migration.
4.2.9.3 Remedial Measures
When designing hydraulic control structures, overbank flooding concerns must be addressed. The
potential for causing overbank flooding may be the limiting factor with respect to the height and amount of
constriction at the structure. If the structure exerts control through a wider range of flows including
overbank, then the frequency and duration of overbank flows may be impacted. The impacts must be
quantified and appropriate provisions such as acquiring flowage easements or modifying structure plans
should be implemented. The safe return of overbank flows must be considered when siting the structure.
One method is to design the structure to be submerged below the top bank elevation, thereby reducing the
potential for a head differential to develop over the structure during overbank flows. Direct means of
controlling overbank flows include constructing an earthen dike or berm extending from the structure to the
valley walls to prevent flows from passing around the structure and constructing an auxiliary high flow
structure that will pass overbank flows to a specified downstream location.
4.2.9.4 Operation and Maintenance of Grade Control Structures
Monitoring and maintenance of grade control structures is essential to ensure adequate
performance. Because of the dynamic nature of streams, lack of monitoring and required maintenance can
result in complete failure of expensive installations. Monitoring should include upstream and downstream
conditions that may have future impacts upon the project. Examples are: 1) changes in upstream channel
alignment may threaten bank stabilization works downstream 2) channelization work may induce
degradation upstream and may change hydraulic and geomorphic conditions downstream and 3) significant
changes in operating procedures of reservoirs upstream of the project site or significant land use changes
may change hydraulic and geotechnical parameters at the site. A monitoring program should consist of site
inspections, site surveys, geomorphic observations, hydrologic and hydraulic data, geotechnical data, and
environmental aspects.
4.2.10 BANK STABILIZATION
As discussed in Chapter 3 of this manual, the instability and subsequent failure of stream banks
commonly result from a combination of hydraulic, geomorphic, and geotechnical factors. Scour occuring
on the outside of channel bends increases bank heights and subsequently leads to bank failures. The terms
streambank erosion and streambank failure are often used to describe the removal of bank material
(Biedenharn et al., 1997). Erosion generally refers to the hydraulic process where individual soil particles
at the banks surface are carried away by the tractive force of the flowing water. Therefore, the erosive
forces are generally greater at higher flows. The primary erosion processes are parallel flow, impinging
flow, piping, freeze thaw, sheet erosion, rilling and gullying, wind waves, and vessel forces. Streambank
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