Local inflows from tributaries, field drains, road side ditches, or other sources often
play an important part in the siting of grade control structures. Failure to provide protection
from local drainage can result in severe damage to a structure (U.S. Army Corps of
Engineers, 1981). During the initial siting of the structure, all local drainage should be
identified. Ideally, the structure should be located to avoid local drainage problems.
However, there may be some situations where this is not possible. In these instances, the
local drainage should either be re-directed away from the structure or incorporated into the
structure design in such a manner that there will be no damage to the structure.
12.2.7 DOWNSTREAM CHANNEL RESPONSE
Since grade control structures affect the sediment delivery to downstream reaches, it
is necessary to consider the potential impacts to the downstream channel when grade control
structures are planned. Bed control structures reduce the downstream sediment loading by
preventing the erosion of the bed and banks, while hydraulic control structures have the added
effect of trapping sediments. The ultimate response of the channel to the reduction in
sediment supply will vary from site to site. In some instances the effects of grade control
structures on sediment loading may be so small that downstream degradational problems may
not be encountered. However, in some situations such as when a series of hydraulic control
structures is planned, the cumulative effects of sediment trapping may become significant.
In these instances, it may be necessary to modify the plan to reduce the amount of sediment
being trapped or to consider placing additional grade control structures in the downstream
reach to protect against the induced degradation.
12.2.8 GEOLOGIC CONTROLS
Geologic controls often provide grade control in a similar manner to a bed control
structure. In some cases a grade control structure can actually be eliminated from the plan
if an existing geologic control can be utilized to provide a similar level of bed stability.
However, caution must always be used when relying on geologic outcrops to provide long-
term grade control. In situations where geologic controls are to be used as permanent grade
control structures, a detailed geotechnical investigation of the outcrop is needed to determine
its vertical and lateral extent. This is necessary to ensure that the outcrop will neither be
eroded, undermined or flanked during the project life.