This "quick and dirty" streambank survey method will provide quick numbers for comparing the potential tons of soil loss between various eroding banks, as well as predicting the total tons of sediment contribution to the river each year. Figures like this can be used to set priorities for planting vegetation. Note: This method makes the simplifying assumption that the maximum loss is 1 foot per year. The method is really designed to give comparative amounts for diffent stretches of the same river. It is not meant to give the absolute values that you might imply from the way the results are presented.
A streambank erosion study such as this was conducted on the Boquet River as the New York State Department of Environmental Conservation planned to install a salmon fish ladder at the dam in Willsboro. It was feared that widespread and uncontrolled bank erosion along the river could result in salmon nesting and spawning areas filled by sediment which would cause a reduction or loss of the fishery. Therefore a Department of Environmental Conservation Fishery Technician and a Soil Conservation Service Geologist inventoried the river.
It was determined that extensive erosion occurred in areas where: 1) sandy or sandy/gravelly noncohesive soils existed; 2) where banks had been cleared of trees and/or brush; 3) where livestock had free access to the river; and 4) where extensive meandering occurred. Data collected on significant erosion areas was entered in a formula to predict tons of sediment loss per year from eroding banks.
For a field study, data should be gathered on field sheets (see below) which are then used to calculate the tons of sediment loss per year according to this formula. The formula first multiplies a number of factors (soil type, stream alignment, amount of bank vegetation, stream gradient, bank slope, and slope of any depositional bar across the river) in order to arrrive at a "recession rate" - these factors are discussed more completely later on the page (below the field sheet). The recession rate is an estimate of the number of feet the bank is likely to recede in a year.
The recession rate is then multiplied by the height and length of the eroding streambank in order to get volume. To turn volume into weight, the volume is multiplied by how many pounds particular soils weigh per foot, then divided by 2000 in order to arrive at tons of erosional loss per year. These calculations are laid out on the field sheet which follows.
Field Sheet for Calculating Streambank ErosionRiver: __________ Date: __________ Name of Observer: ____________________Right or Left Bank (circle one) - right or left is determined when looking downstream Describe Location:________________________________________________________________ Length of Eroding Bank (ft) L = __________ Average Height of Eroding Bank (ft) H = __________
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Soil Texture: Soils with a high percentage of clay (high plasticity) are generally less susceptible to high rates of streambank erosion than noncohesive silts, sands, and gravels. A streambank with high clay content may give the appearance of being in an eroding condition, however, because little to no vegetation cover will grow there. In general, susceptibility to erosion increases as soil texture becomes more coarse and cohesiveness decreases.
Stream Alignment: As the curvature of an eroding bank becomes greater, so does the rate of erosion. Erosion will usually be confined to the outside edge of the curved section. Significant erosion does not generally occur along straight sections of streams. The highest erosion rates are found in stream sections with a high percentage of curvature.
Vegetation at Top of Bank: Large trees and/or thick woody vegetation tightly bordering streambanks usually prevents erosion. Some bank protection is provided by the tree root systems. Shallow rooted vegetation or complete lack of vegetation provides no protection and therefore indicates higher erosion rates.
Stream Gradient: The higher or larger the stream gradient, the greater the rate of flow and the greater the potential for streambank erosion.
Slope of the Eroding Bank: The slope of an eroding bank is an indictor of erosion rate. A vertical slope or undercut bank generally means a high rate of erosion. This will usually be combined with overhanging vegetation at the top of the bank, held there only by the root structure, or in wooded areas, by leaning or fallen trees. The lesser the slope of the eroding bank, the lower the erosion rate.
Slope of Inside Depositional Bar: The width of a stream generally remains the same. As erosion occurs on the outside edge of a bend in a stream, deposition occurs on the inside. The slope of this depositional bar is an indicator of the rate of erosion. A slight slope is indicative of a rapid rate of erosion. An additional indicator is the presence or absence of vegetation on the depositional bar. A lack of vegetation signifies a rapid erosional rate.