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• Should the rills continue to develop to an unacceptable level, appropriate <br />corrective measures or methods will be undertaken. <br />For concentrated flow features or structures, such visual <br />observations tend to be inadequate. More involved methods are normally <br />required to assess the rate of the erosion process which is occurring. The <br />evaluation process for concentrated flow structures may include, either <br />singly or in combination, one or more of the following methods: <br />1) Determining the rate of degradation or agradation within a given <br />reach using a static reference point, with feature monitoring <br />being conducted after a minimum of two significant flows which <br />generate runoff in the feature (i.e., a significant flow is <br />defined as a precipitation event equal to or greater than the 2- <br />year, 6-hour storm); <br />2) Adequacy of armoring using the procedure contained in SCS <br />Technical Note No. 25, Appendix A (see Appendix 9.3-A); or <br />3) Ability to pass a given or designated storm event's runoff volume <br />by calculating required design minimum and actual existing <br />• capacity, including adequacy of structure lining materials. <br />Since drought is cyclic in the western United States, evaluation of a <br />feature during a single season most frequently will not give adequate <br />information as to whether or not significant degradation or agradation is <br />occurring. Ideally, monitoring channel dynamics should be conducted <br />through at least one full drought cycle (i.e., seven to eight years). <br />However, this length of time is probably not acceptable regulatorily. So, <br />for Method 1 a minimum two year period is proposed to evaluate the dynamics <br />of features which appear or have the potential to exhibit unacceptable <br />levels of erosion. <br />Static reference points may be established in several ways for Method <br />1. A common and easily applied technique is the use of erosion pins (fence <br />posts perform well in smaller drainages with low flow volumes) . One or <br />more pins are driven into the ground with a precise reference line <br />established above ground level. The depth of the feature is recorded over <br />time using this reference line. Pins may also be driven at the top and <br />bottom of the feature to evaluate length dynamics. For larger features <br />which carry significant flow volumes, a series of permanent cross-sections <br />• may be established which utilize a relative or absolute control point. The <br />wash is then surveyed periodically and cross-sections are developed. <br />Renewal-3 9.3-20 June 13, 1997 <br />