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Channel Roughening Fact Sheet
What is Channel Roughening?
Channel Roughening is accomplished by constructing very low and frequently
spaced "V" shaped weirs constructed from logs or rock across swales
and drainage ways.
When is Channel Roughening Used?
Channel Roughening is used on small drainage ways in burned areas where the
natural erosion protection from vegetation and woody debris has been lost. It is
used where sufficient numbers of logs are available, or rock can be substituted
for the logs. Channel Roughening increases infiltration, adds roughness, reduces
erosion, and helps retain small amounts of sediment within the channel. Channel
Roughening should be effective for a period of one to three years, providing
short term protection in areas where permanent vegetation will eventually return
to provide long term erosion control. Channel Roughening should not be used on
channels with drainage areas greater than 50 acres. Straw wattles or similar
light weight material should not be used for channel roughening where the area
drained by the channel exceeds 1 acre. Channel roughening will have limited
effectiveness for channels that were unstable and eroding prior to the wildfire
event.
How is Channel Roughening Installed?
Using Logs. 6" to 12"
diameter logs are cut to a length approximately equal to the channel width, up
to about 30 feet. (longer logs are too hard to handle). Limbs are removed to the
extent necessary for the log to lie flat on the ground. Beginning from each side
of the channel, a shallow trench (about 3 to 5 inches deep) is dug at a 60
degree angle upstream towards the center of the channel, creating a
"V" configuration as shown in Figure 1. A log is placed in each side
of the trench with its butt end towards the side of the channel, and seated with
tamped backfill such that water flowing down the channel will not run under it.
Where the soil in the channel is deep enough, the logs should be braced with
wooden stakes. 2" x 2" x 24" long wooden stakes spaced four feet
apart should be driven on the downstream side of the logs, until the top of the
stake is even with the top of the log. When complete, the upstream point of the
"V" must be the lowest point along the constructed weir, the top of
the weir should be no more than 6 to 9 inches above the channel at the center,
and the weir must extend the full width of the channel on either side.
Using Rock Riprap. Graded angular rocks
ranging in size from 4 inches to 16 inches can be substituted for logs. The
trench is constructed in the same configuration as before, except that the
trench must be at least 12 inches deep and twice as wide as the largest rock.
The trench is filled with the rock such that the upstream point of the
"V" is the lowest point along the constructed weir, the top of the
weir should be no more than 6 to 9 inches above the channel at the center, and
it must extend the full width of the channel on either side.
How Much Channel Roughening is Required?
The log or rock weirs used for channel roughening should be uniformly spaced
along the channel beginning near the upper end of the drainage way. The maximum
recommended spacing between "V" shaped weirs is shown below:
Channel Slope
(%) |
"V" Weir Spacing
(feet) |
< 2 |
200 |
2 - 5 |
100 |
5 - 10 |
50 |
10 - 20 |
25 |
>20 |
Not recommended |
FIGURE 1 - Log "V" Weir for Channel Roughening
FIGURE 2 - Rock "V" Weir for Channel Roughening
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