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, ~ toO .... -.oJ As mentioned previously, field measurement is the most reliable and accurate source of channel erosion data. However, the time involved in monitoring your sites, in wet years and in dry years, often precludes this method of data collection for project sedimentation studies. The remainder of this discussion will be on field procedures for estimating channel erosion. FIELD ESTIMATE PROCEDURE (DIRECT VOLUME METHOD) The direct volume method is summarized in Equation 6. '. lateral recession 2 s/ton (6) = E E = erosion in tons/year 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 arE 1S are channel banks or bottoms that are bare, rilled or gullied. They genera lly have sloughed soi 1 at thei r bases. A grassed bank or rock bank is considered to be noneroding. The actual eroding area is defined by the height or width and the length to obtain squ'are feet of eroding area. The height is not vertical; it is measured on the bank surface as a slope height. '....'IICfl ~:,.:.;jnal erosicr. is irwc~t::i~~~ in a pr',:,j~~: ~P"'?3 t~e pt?rce.,tage of eroding versus noneroding areas must be establ ished so that sample data can be expanded over the entire watershed. As an example, for streambanks, you may divide the streams in a watershed into geomorphic provinces (mountain streams, foothhill streams, flood plain streams), flow regimes (ephemeral, intermittent, or perennial), or use Horton's stream ordering (Figure 5). The total miles of streambanks must then be measured. Based on time and study intensity, a sample size must be selected. Five to ten percent is generally an adequate sample but to be statistically accurate a sample size of up to 25 percent may be required. This dependS on the variability of the streams. Areas to be sampled are then ,marked on a map and the direct volume method is used to qu~ntify erosion from the eroding areas in the samples. If only 30 percent of the banks in the sample are eroding, this is interpreted to mean only 30 rorcent of the banks in the watershed are eroding. By knowing the erosion rate in tons per bank mile and knowing the total bank miles of stream you can calculate the total tons of erosion from streambanks in that watershed. For example: --10 miles of 1st order streams in the Doodad Watershed --2 miles are sampled (4 bank miles) . --5,280 feet are eroding --52BO T 4 bank miles = 25% eroding --100 tons erosion from sample --tons erosion from sample divided by bank miles eroding is erosion rate --100 tonsIl bank mile eroding. 100 tons/yr/bank mile --total bank erosion is 25% x 10 miles of streams x 2 (banks/mile) . 5 bank miles eroding and 5 bank miles eroding x 100 tons/yr/bank mile. 500 tons benk erosion 17