Quantcast Velocities for Design Purposes

 
  
 
Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
Sometimes surface drainage water intercepts the terrace zone from inland areas and can
cause gullying not only in the terrace zone, but in the other zones on the bank. This water
should be diverted or controlled with a small furrow or trench at the top of the bank. This
trench should be sodded to prevent erosion.
Velocities for Design Purposes
The purpose of this section is to provide some velocity information that bioengineering
systems have been noted to sustain so that planners and designers have a basis for choosing
bioengineering systems and the particular kind of system. Some of the velocity information
was derived from the literature while other information was measured at local points at case
study locations where bioengineering treatments were installed. Velocities vary so much
within a stream that local velocities near the treated section are the most valuable.
Admittedly, the measured velocities are much lower than considered maximum threshold
values that could be sustained by the installed structures. This is because when measurements
were made, they were made with current meters in the local vicinity of the bioengineering
treatment on the fall of the hydrograph when water levels and currents during flood events
were not a safety hazard. Remote current meters exist, but would have been silted in or
damaged by debris flow during these flood events.
Most of the velocity information in the literature concerns itself with turf grass cover that
was designed for erosion control ditches or waterways. Little information exists on
combinations of systems, i.e., bioengineering treatments, containing both herbaceous and
woody species. Engineer Manual, EM 1110-2-1205 (US Army Corps of Engineers, 1989),
states that herbaceous or woody vegetation may be used to protect channel side slope areas
(depending on the frequency of inundation, velocity, and geotechnical constraints to
infrequent flooding) and other bank areas where velocities are not expected to exceed 6 to
8 feet per second (fps). Information concerning influence of vegetation (bermuda grass) or
variation of velocity with depth below water surface is shown in Henderson and Shields
(1984) who cites Parsons (1963).
The splash and bank zones will be the principal focus for bioengineering applications. It
is in these zones that the designer must tailor vegetation types and bioengineering structures
to be commensurate with velocities that they can sustain. Hoag (1993) suggests that
maximum flow velocities should not exceed 3 fps for herbaceous plantings, 3-5 fps for woody
and herbaceous mixed plantings, 5-8 fps for woody plantings alone, and that maximum flows
above 8 fps require soil-bioengineering approaches.
For the case studies examined and monitored for this report, measured velocities for local
flow conditions around the bioengineering treatment never exceeded 10 fps. Maximum
velocities sustained and recorded by bioengineering treatment types are shown in Table 2.
As previously mentioned, these may not represent the maximum velocities encountered, as
they were usually taken on the fall of the hydrograph. Also, local roughness imparted by the
bioengineering treatment would have slowed velocities in its vicinity.
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