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Water: WARSSS

Streambank Erosion Risk

Streambank erosion can be a large contributor to total sediment supply and is associated with river instability, land loss, fish habitat loss, flood issues and many other related problems. For appropriate mitigation, identification of the sources of the sediment and stability problems is essential. Since streambanks can be directly observed and measured, inference from observed relations are used to provide a risk assessment. Data required for this level are as follows:

a) Stream types
b) Aerial photographs, drainage area maps
c) Regional curves
(bankfull dimensions vs drainage area)
d) Bankfull width
e) Radius of curvature
f) Riparian species composition
g) Bank height
h) Bankfull depth

The relation developed for riparian vegetation composition represents the susceptibility of streambanks to erosion. The rooting depth and density associated with the riparian plant community is very important to reduce the risk of bank failures. The internal strength afforded by plants with dense and deep root structure reduces mass wasting due to shear and cantilever bank failures. Plants can also reduce fluvial entrainment by protecting exposed soil from detachment. The relations in Figure 93 are used to obtain the risk rating for vegetation influence to bank erosion risk.

The bank height ratio is the height of the study bank divided by the bankfull depth at the toe of the bank. The greater the value is above 1.0, the greater likelihood of increased erosion due to various erosional processes, including mass wasting, dry ravel, freeze-thaw and rill erosion. The relation of bank height ratio to risk rating is shown in Figure 94.

The tighter the radius of curvature, the greater is the boundary shear stress and velocity gradient directed to the bank, causing accelerated erosion. Radius of curvature can be obtained from aerial photography or rapidly measured in the field. The radius of curvature is divided by the bankfull width to obtain the risk rating in Figure 95. The overall risk ratings are then adjusted by stream type for the adjective rating of a given reach as shown in Figure 78. Ratings for specific reaches adjusted by stream type are then recorded in


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