Selection and Design of Channel Rehabilitation Methods
6.2.2.2 Geotechnical Considerations
The above discussion focused only on the hydraulic aspects of design and siting of grade control
structures. However, in some cases, the geotechnical stability of the reach may be an important or even
the primary factor to consider when siting grade control structures. This is often the case where channel
degradation has caused, or is anticipated to cause, severe bank instability due to exceedance of the critical
bank height (Thorne and Osman, 1988). When this occurs, bank instability may be widespread throughout
the system rather than restricted to the concave banks in bendways. Traditional bank stabilization measures
may not be feasible in situations where system-wide bank instabilities exist. In these instances, grade
control may be the more appropriate solution.
Grade control structures can enhance the bank stability of a channel in several ways. Bed control
structures indirectly affect the bank stability by stabilizing the bed, thereby reducing the length of bankline
that achieves an unstable height. With hydraulic control structures, two additional advantages are apparent
with respect to bank stability: (1) bank heights are reduced due to sediment deposition, which increases
the stability of the banks with regard to mass failure; and (2) by creating a backwater situation, velocities
and scouring potential are reduced, which reduces or eliminates the severity and extent of basal cleanout
of the failed bank material, thereby promoting self-healing of the banks.
An analysis of all cross sections surveyed each year during the period 1993 through 1996 with a
bank angle greater than 50 degrees was made using the Darby and Thorne (1996a,b) method. Table 6.2
summarizes the results of the analyses.
Table 6.2 Bank Stability Analysis Summary
1993
1994
1995
1996
Number of Cross Sections
Total
28
25
26
41
With Grade Control
15
14
13
20
Without Grade Control
13
11
13
21
Failing Reach Length (m)
With Grade Control
457
0
0
305
Without Grade Control
2,403
2,904
4,169
3,342
Total Volume Failing (cu. m)
With Grade Control
1,463
0
0
1,527
Without Grade Control
6,488
10,456
15,424
22,059
These data show a significant increase in the number of cross sections greater than 50 degrees in
1996 over the previous three years, with an approximately equal number of the steeper banks occurring
in reaches with and without grade control. The increase in steep banks may be due to several factors,
however, no reason is adequately known to offer an opinion.
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