The lateral stability of different stream reaches can be compared by means of a dimensionless
erosion index. The erosion index is the product of its median bank erosion rate expressed in
channel widths per year, multiplied by the percent of reach along which erosion occurred,
multiplied by 1,000. Erosion indexes for 41 streams in the United States are plotted against
sinuosity in Figure 5.28. The length of most of these reaches is 25 to 100 times the channel
width. The highest erosion index values are for reaches with sinuosity ranging between 1.2
and 2. Erosion indexes are large for sinuous braided and sinuous point bar (wide bend)
streams. Equiwidth streams tended to be relatively stable. The erosion index value of 5, in
Figure 5.28, is suggested as a boundary between stable and unstable reaches. Brice (1984)
considers that reaches having erosion indexes values less than 5 are unlikely to cause lateral
erosion problems at bridges. An example on the use of Figure 5.28 is presented in Section
5.9, Problem 4.
A general assessment of bank stability can be made considering the following aspects.
Bank Erosion Rates. It is theoretically possible to determine bank erosion rates from factors
such as water velocity and resistance of the banks to erosion. See HEC-18 (Richardson and
Davis 2001) for a discussion of recent advances in measuring erosion rates. The results in
Figure 5.26 provide a first approximation of migration rate of a bend regardless of the hydraulic
conditions and sediment characteristics. Past rates of erosion at a particular site provide the
best estimate of future rates. In projecting past rates into the future, consideration must be
given to the following factors: (1) the past flow history of the site during the period of
measurement, in comparison with the probable future flow history during the life span of the
highway crossing. The duration of floods, or of flows near bankfull stages, is probably more
important than the magnitude of floods; and (2) human-induced factors that are likely to affect
bank erosion rates. Among the most important of these are urbanization and the clearing of
floodplain forests.
Figure 5.28. Erosion index in relation to sinuosity (Brice 1984).
5.61
Previous Page
