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Streambank Erosion
NRCS, WI
Field Office Technical Guide
11/03
Streambank Erosion
Field Measurement Procedure
The best way to quantify streambank erosion is to
measure it directly in the field. The basic procedure
in measuring streambank erosion is to survey, flag, or
in some way fix a before" image of the channel you
are evaluating. This establishes the baseline
condition. Changes due to erosion can then be
monitored over time by going back to the study area
and re-measuring from your fixed reference points.
Channel cross-sections can be surveyed and plotted
on a periodic basis to monitor change. Stakes or pins
can be driven into channel banks flush with the
surface. The amount of stake or pin exposed due to
erosion is the amount of change at the streambank
erosion site between your times of observation.
Field Estimate Procedure (Direct Volume
Method)
The field measurement procedure is the most
accurate way to measure streambank erosion.
However, the time involved in monitoring your site,
in wet years and dry years, often precludes this
method of data collection. The Direct Volume
Method can be used to estimate streambank erosion
at your site. The Direct Volume Method is
summarized in the following equation:
(eroding area) (lateral recession rate) (density) = erosion in tons/year
2000 lbs/ton
The eroding area is in square feet, the lateral
recession rate is in feet/year, and density is in
pounds/cubic feet (pcf).
Determining Eroding Area
Eroding areas are channel banks that are bare, rilled
or gullied. They generally have sloughed soil at their
bases. A grassed bank or rock bank is considered to
be non-eroding. The actual eroding area is defined
by multiplying the height and the length to obtain
square feet of eroding area. The height is measured
on the bank surface as the slope height; not the
vertical height.
Average Annual Lateral Recession Rate
The average annual recession rate is the thickness of
soil eroded from a bank surface (perpendicular to the
face) in an average year. Recession rates are
measured in feet per year. Channel erosion often
occurs as chunk or blowout type erosion. A channel
bank may not erode for a period of years when no
major runoff events occur. When a major storm does
occur, the bank may be cut back tens of feet for short
distances. It is necessary to assign recession rates to
banks with such a process in mind. When a bank is
observed after a flood and ten feet of bank has been
eroded, that ten feet must be averaged with the years
when no erosion occurred. This will result in a much
lower average annual recession rate than a recession
rate for one storm.
Selecting the average annual lateral recession rate is
the most critical step in estimating channel erosion
using the direct volume method. A historical
perspective is needed in many instances. Old
photographs, old survey records, and any other
information that helps to determine the bank
condition at known times in the past are very useful
data. In most instances, such information is lacking
and field observations and judgement are needed to
estimate recession rates.
Cultural features are often helpful in determining
recession rates. Exposed bridge piers, suspended
outfalls or culverts, suspended fence lines are all
possible indicators of lateral recession. Discoloration
on the bridge piers may show the original channel
bottom elevation. Given the date of the bridge
installation, a recession rate can be calculated for that
reach of stream. Culverts are generally installed
flush with a bank surface. The amount of culvert
exposed and age of the culvert allows for the
calculation of a recession rate.
Exposed tree root is probably the most common field
evidence of lateral recession. Roots will not grow
towards a well-drained, exposed, eroding channel
bank. The amount of root exposed should be
increased by at least a factor of two to account for NRCS, WI
Field Office Technical Guide
11/03
soil that was in the bank and that the root was
growing in. By dividing the length of root exposed
and the thickness of soil around the root by the age of
the tree, a recession rate can be estimated.
Much experience and professional judgement are
required to estimate channel recession rates. It is
often not possible to directly measure recession rates
in the field. Therefore, the following table has been
included which relates recession rates to narrative
descriptions of banks eroding at different rates.
Lateral
Recession
Rate
Category
Description
(ft/yr)
0.01-0.05
Slight
Some bare bank but active erosion not readily apparent. Some rills but
no vegetative overhang. No exposed tree roots.
0.06-0.2
Moderate
Bank is predominantly bare with some rills and vegetative overhang.
Some exposed tree roots but no slumps or slips.
0.3-0.5
Severe
Bank is bare with rills and severe vegetative overhang. Many exposed
tree roots and some fallen trees and slumps or slips. Some changes in
cultural features such as fence corners missing and realignment of
roads or trails. Channel cross section becomes U-shaped as opposed to
V-shaped.
0.5+
Very Severe
Bank is bare with gullies and severe vegetative overhang. Many fallen
trees, drains and culverts eroding out and changes in cultural features as
above. Massive slips or washouts common. Channel cross section is
U-shaped and stream course may be meandering.
Volume Weight Conversions
The volume (cubic feet) of eroded material is
obtained by multiplying eroding areas by a lateral
recession rate. To convert this volume of eroded
material to a weight, the dry density of the soil must
be known. The following table lists soil textures with
corresponding volume weights.
Soil Texture
Volume-Weight
Clay
60-70 pcf
Silt
75-90
Sand
90-110
Gravel
110-120
Loam
80-100
Sandy loam
90-110
Gravelly loam
110-120 NRCS, WI
Field Office Technical Guide
11/03
Example
Farmer Browns cattle have access to the stream
running through his pasture. On the south side of the
stream, 700 feet of bank is bare with rills and
overhanging vegetation. Exposed tree roots are
evident with many fallen trees and slumps. Bank
height is 8 feet measured along the bank. Soil type is
predominantly sandy loam. On the north side of the
stream, 300 feet of bank is predominantly bare with
some rills and vegetative overhang. There are some
exposed tree roots but no slumps are evident. Bank
height is 10 feet and the soil texture is a loam.
Annual erosion at the site using the Direct Volume
Method:
(eroding area) (lateral recession rate) (density) = erosion in tons/year
2000 lbs/ton
South bank:
700 ft x 8 ft x 0.4 ft/yr x 100 pcf = 112 t/yr
2000 lbs/ton
North bank:
300 ft x 10 ft x 0.1 ft/yr x 80 pcf = 12 t/yr
2000 lbs/ton
112 t/yr + 12 t/yr = 124 tons/year eroding at the site.