Quantcast Bioengineering Treatments (cont.)

 
  
 
Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
from a combination of plastic and wood. They are constructed by nailing planks to the top
and bottom of 15- to 20-cm spacer logs. These planks form stringers, which tie into the
streambank at right angles. Planks are nailed to the top and bottom stringer boards and run
parallel to the streambank. The entire structure forms a crib, which can be constructed
onshore and moved by a loader or backhoe to the installation site. Once in the stream, the
LUNKERS is placed in position and anchored by driving 1.5-m lengths of steel-reinforcing
rod through predrilled holes in the structures and then into the streambed. These structures
are set in a line that simulates the outside bend of a meander. After the structures are in place,
the area behind them is filled with rock , which also is used to cover the structure, and then
the entire area is covered with soil (Hunter, 1991). Often, the soil is planted with various
kinds of vegetation, either woody or herbaceous. Care must be taken to tie the ends
into the bank with a transition of rock or into a hardpoint to prevent flanking.
Another hard structure placed in the toe zone to stabilize the toe is a "Bank Crib with
Cover Log" (Figure 15). This is described by the USDA Forest Service (1985). Like the
LUNKERS, it is used to protect unstable streambanks at the toe while at the same time
providing excellent overhead cover for fish. The design is a simple crib with abutment logs
extending as far back into the bank as necessary to assure structural stability (1.3 to 1.8 m in
stable soils and 3 m or more in unstable soils). The lower abutment logs should be near water
level and should extend 45 to 60 cm from the bank. The cover log can then be pinned to the
crib log and the lower abutment. The structure can be from one to several logs high,
depending upon bank height. The only materials required are logs on site and 1.6 cm rebar
to join the logs. Installing structures is fairly time consuming, due to the amount of digging
required. One crew should be able to install 6 to 9 m of crib (two crib logs high) per day if
logs are reasonably close to the site. Water adjacent to some eroding banks requiring
abutment work is sometimes too shallow to make effective use of cover logs. It has been
noted by some that rocks need to be added below the crib log and upstream and downstream
from the structure to avoid scour and flanking respectively.
Log revetments are similar to bank cribs with cover logs except these are used to harden
the toe and continue up the bank by lining the bank with logs (Figure 16). Then, flood-
tolerant plants are placed at the top of and shoreward to the revetment. Depending on the
height of the revetment, this may be in the splash, bank, or terrace zones. They are placed
with butt ends facing upstream and are overlapped in a shingle fashion. They are secured with
cables that are looped around the logs and then are fastened to dead men in the bank. Care
must be taken to ensure their longevity by placing rock on both the upstream and downstream
ends to prevent flanking of the structure. Rock should also be placed at the toe of the
structure to prevent scour.
B-31

 


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