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Channel Processes: Gully Erosion

Local base level shifts in ephemeral drainageways can accelerate incision processes and excess erosion of both channel bed and banks of the ephemeral gully. The Rosgen classification system is used to classify the ephemeral gully and to obtain the dimension, pattern, profile, and channel materials as a function of drainage area and width/depth ratio associated with the entrenched A, F or G stream types. Rates of incision and bank erosion (enlargement) will utilize the same computational methods used for stream bank erosion and channel stability (i.e. aggradation, degradation, confinement). An example of gully erosion is shown in Figure 39.

Figure 39

Figure 39. Example of a G5 gully - Florida.

Mitigation in gully systems is generally associated with grade control structures to control the head-cut advancement, back-sloping and vegetating exposed streambanks to decrease streambank erodibility. In descending priority order, other restoration/stabilization methods for entrenched rivers involve re-establishing the incised channel back up to the original surface, constructing a stable stream type in place, and creating a confined, but stable stream type, such as converting a G stream type to a B stream type or an F stream type to a Bc- stream type (Rosgen 1997).

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