Selection and Design of Channel Rehabilitation Methods
Watson et al. (1997) found that the skewness of the flow distribution was strongly related to
sediment yield. Skewness, as defined by Chow (1964) is a measure of the lack of symmetry of a
distribution. For example, with the coefficient of skewness, Cs, at zero (Cs = 0), the distribution is
symmetric; with Cs > 0, the distribution is skewed to the right with a long tail on the right side; and with Cs
< 0 the distribution is skewed to the left. Figure 6.19 depicts the relationship developed by Watson et al.
(1997) between the skewness of the flow distribution for the 15-minute data, and the annual sand yield per
unit area for nine streams within the DEC watersheds in Mississippi. A trend of increasing sediment yield
with increasing skewness is evident. The outlier, Hickahala Creek, has an anomalously high, and
unexplained, sand yield. Figure 6.19 indicates that within the channelized DEC streams, the sand yield
(tons per acre) increased by a factor of almost ten for a three-fold increase in skewness. As the peak
discharge of a watershed increases due to land use change, channelization, or channel incision, the
skewness increases. Conversely, the decrease in the peak discharges associated with flow control will
reduce the skewness of the flow distribution, and result in a reduction in the downstream delivery of
sediment.
2.0
1.8
1.5
1.3
1.0
0.8
0.5
0.3
0.0
6
8
10
12
14
16
18
Skewness Using 15-Minute Data
15 - Minute Data
Skew*0.094 - 0.48, R^2 = 0.82
Figure 6.19 Annual Sediment Load as a Function of Flow Distribution
Skewness
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