1. Determine water depth at toe of bank slope under normal water level conditions:
A. If more than 3 feet, then revetments may not stabilize bank.
B. If 3 feet or less, then revetments should work.
2. Determine streambank soil type and anchoring method by using 3/4" steel rod (scrap rebar)
to probe at least 4 feet into channel bottom and streambank:
A. If bedrock is less than 4 feet below streambank, anchors won't hold.
B. If bedrock is more than 4 feet below surface, and soils are rocky, use T-post or duckbill anchor.
C. If bedrock more than 4 feet below surface, and soils are sand, silt, or
clay, use T-post, duckbill, or screw-in anchors.
3. Determine size of channel constriction the revetment will cause, and optional approaches:
A. Difference in elevation (in feet) from top of eroded bank to toe of bank slope = ("3A").
B. Tree crown diameter (in feet) needed = answer to 3A x 0.67 = (3B).
C. Tree crown radius = answer to 3B x 0.5 = (3C).
D. Measure stream width (average) in feet at revetment site = feet (3D).
E. Percent constriction caused by revetment = (3C ÷ 3D) x 100 = % (3E).
F. If answer to 3E is less than 15%, then one row of anchored trees should work.
Use of at least two rows is recommended if bank height will accommodate it.
G. If 3E is greater than 15%, a second row is needed, but with smaller diameter tree crowns.
Also, the narrower the stream width, the greater the likelihood water will overtop one row.
4. Determine total length (in feet) of trees needed:
A. Length of bank needing coverage by the revetment (measure beyond area of exposed soils) = feet (4A).
B. Total length of trees needed, accounting for tree overlap during construction = 4A x 1.2 = ______ feet (4B).
5.Determine number of trees needed:
A.Number of trees needed for one row = total length of trees needed (4B) ÷ estimated
average height of trees available = trees for one row (5A).
B.Height of revetment = height of area needing protection, measured vertically from the toe of the slope
up to a point 1-2 feet above where flows will spread out across the low bank into the floodplain = feet (5B).
C.Number of rows up the bank = 5B ÷ estimated average compressed tree crown diameter
[cedar & arbor vitae compress more than fir trees] = rows (5C).
D.The number of rows up the bank is also the number of trees in a "set"; the number of trees needed for
one row = the number of sets needed. 5A = ets (5D).
E.Estimated total number of trees needed for the revetment = 5A x 5C = trees (5E).
F.If height or diameter of trees found available is different than the estimate, re-calculate number of
trees, anchors, clamps and cable length needed, and adjust purchases.
6.Determine the number of anchors needed
A. For a one-row revetment, the number of anchors = 1 + number of trees in one row = _______ anchors.
B. For a multiple-row revetment, the number of anchors = 2 x the number of sets + 2 = _______ anchors.
7. Determine the number of cable clamps (sized for cable) needed = 2 x number of anchors. Bring extra clamps to replace any lost in the water. Some lost clamps can be retrieved with a large magnet.
8. Estimating the amount of cable or wire needed, especially for multiple row revetments, is difficult considering the difference between tree species on how much they compact when wrapped with cable. The following are principles to follow:
A. When using cable, 1/4" steel aircraft cable is flexible and adequate in most circumstances.
If in real doubt, double it, but washouts rarely happen due to a 1/4" cable breaking.
C. When the revetment will have one row of trees, the amount of cable/wire needed to wrap
around the trees = [ 4 ft. x the number of anchors (6A) ] + 20% = feet of cable or wire (8C).
D When the revetment design is for multiple rows, the amount of cable
needed = [ 2 x revetment height (5B) ] x (the number of sets + 1) + 20% = feet of cable or wire (8D).
E. If you are using anchors without pre-attached cable for sinking into the soil, you must
estimate the depth of your anchor installations, add 0.5 - 1.0 ft. more for a loop to clamp the
cable onto the anchor, multiply that sum times the number of anchors, and add that total to
your results for 8C or 8D, whichever is applicable.
A revetment covering 200 ft. of eroded bank needs 240 ft. of trees to cover the first row [200 x 1.2 = 240] (see worksheet items 4A&B). If using 8 ft. trees, 30 trees are needed for a one-row revetment [240 ÷ 8 = 30] or 30 sets of trees are needed for a multiple row revetment (item 5D). If the revetment needs to cover an eroded area 5 ft. high, and the average tree crown diameter is 2 ft., then the revetment needs to have 3 rows [5 ÷ 2 = 2.5 ' 3] (item 5C). Three rows x 30 sets = 90 trees are needed (item 5E). Sixty-two anchors are needed [2 x 30 sets + 2 = 62] (item 6B) and 124 clamps (item 7) are needed. Each set needs 10 ft. of cable for wrapping [2 x 5 ft. eroded area height], so 10 ft./ set x (30 sets + 1) = minimum 310 ft of cable needed (item 8D). Purchase an additional 20% for a margin of error. Cut the cable in lengths of 10 ft. per set at first. If you find less cable is needed for wrapping, reduce that length, saving cable for its next use.
If using anchors without pre-attached cable, and they are to be sunk 3 ft. into the stream-bank, add [3 ft. + 0.5-1.0 ft. for the loop ] x [30 sets +1] to the 310 ft. minimum before calculating the 20% extra and cut the cable in 13.5-14 foot lengths at first (item 8E).
Back to Guide No 12
This Guide is one of a series of Ohio Stream Management Guides covering a variety of watershed and stream management issues and methods of addressing stream related problems. The first several guides in the series are overview guides intended to give the reader an understanding of the functions and values of streams. For more information about stream management programs, issues and methodologies, see Guide No. 05 Index of Titles or call the ODNR Division of Water at 614/265-6739. Paper copies of the Guides are available from the Ohio Department of Natural Resources. Single copies are available free of charge and may be reproduced. Please contact one of the below for paper originals:
The Ohio Department of Natural Resources
Division of Water
Phone: (614) 265-6740
Fax: (614) 447-9503
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