Geomorphic Assessment of Channel Systems
Evidence of scour at bridge pilings and culverts is particularly important as an indicator of the
amount of degradation that has occurred since the construction of the structure.
3.1.3 CHANNEL STABILITY ASSESSMENT
The third phase of a geomorphic assessment involves an analysis of the channel
stability. This is accomplished by the refinement and detailed analysis of all the
historical and archive data previously collected, interpretation of the field reconnaissance
observations, and the integration of these data to provide an overall assessment of the system.
3.1.3.1 Identification of Geomorphically Similar Reaches
One of the first steps in the channel stability assessment is to divide the channel into
geomorphically similar reaches. When establishing reach limits, consideration should be given
to: changes in channel slope, tributary locations, presence of geologic controls, planform
changes, location of channel control structures (grade control structures, dams, culverts, etc.),
changes in bed material size, major sediment sources (gravel mines, sediment laden tributaries,
etc.), changes in channel evolution type, or other significant hydrologic or geomorphic
changes. Initial reach limits may be made early during the field investigation, but may be
refined following more detailed analysis.
3.1.3.2 Specific Gage Analysis
Perhaps one of the most useful tools available to the river engineer or
geomorphologist for assessing the historical stability of a river system is the specific gage
record. A specific gage record is a graph of stage for a specific discharge at a particular
gaging location plotted against time (Blench, 1969). A channel is considered to be in
equilibrium if the specific gage record shows no consistent increasing or decreasing trends
over time, while an increasing or decreasing trend is indicative of an aggradational or
degradational condition, respectively. An example of a specific gage record is shown in Figure
3.3.
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