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' 1 , <br />above the pre-mining channel invert should be extended downward. This <br />slope will generally be between 2,5 and 4 feet horizontal for every <br />one foot vertical. The actual slope will depend on the method of bank <br />stabilization proposed at the site. <br />3.5 HF.ADC[TITING CON'T'ROL <br />Ffien mining in the river bed is permitted, gravel operators must gre- <br />vent long term headcutting from propagating in the upstream direction. <br />The removal of material from the riverbed results in excessive head- <br />cutting which can and does endanger structures upstream crossing the <br />river or a stream or located in the adjacent banks of same. It is im- <br />perative that the operators install positive controls as specified <br />below to prevent headcutting of any type from occurring upstream as a <br />result of gravel extraction from the river bottom. <br />3.5.1 Control Structures <br />The impacts of the instream gravel extraction must be mitigated <br />with a positive structural control at the upstream end of the <br />instream pit. The structure must be protected from failure <br />through a wide variety of flow rates, gravel pit depths and <br />inconsistent soil properties. <br />Instream control structures will generally require the con- <br />struction of a structural control incorporating a double row of <br />sheet piling such as that shown on Figure 3.1. Site specific <br />designs incorporating specific soils and geometric information <br />shall be prepared and submitted to District for approval. <br />UD6FCD GRAVEL GUIDELINES 12/17/B7 <br />3-3 <br />