Laserfiche WebLink
378-4 Pond <br />Principal Spillway. A pipe <br />conduit, with Headed <br />appurtenances, shall be placed <br />under or through the dam, except <br />where rock, concrete, or other <br />types of mechanical spillways are <br />used, or where the rate and <br />duration of flow can be safely <br />handled by a vegetated or earth <br />spillway, unless otherwise <br />specified by the State Engineer of <br />Colorado. <br />The crest elevation shall be no <br />less than 0.5 ft. below the crest <br />of the emergency spillway for dams <br />having a drainage area of 20 acres <br />or less, and no fees than 1 ft. . <br />for those having a drainage area <br />of more than 20 acres. <br />For erosion control dams, there <br />shall ba installed an ungated <br />outlet pipe no less-than twelve <br /><12) inches in diameter and large <br />enough to drain within thirty-six <br />hours any impoundments in excess <br />of two acre-feet. Dams that store <br />less than two acre-feet must <1) <br />be large enough to store the <br />runoff from a 5 year 24 hour <br />frequency storm where the <br />emergency spillway i=_ located on <br />erosive soils or (2) be large <br />enough to contain the runoff from <br />a 2 year 24 hour frequency storm <br />if the spillway is on non-erosive <br />soils and (3) must store at least <br />the ten year sediment yield from <br />the watershed. Otherwise an <br />ungated principal spillway shall <br />be constructed. <br />Erosive soils a-e considered to be <br />those with an e-osiveness index <br />"K" greater than 0.3~; this <br />includes most SCs, SPs, SP-SMs, <br />SMs, and some CHs. <br />Whan there is a possibility of • <br />injury to vested water rights <br />downstream, the Colorado Division <br />Engineer may require that any dams <br />built on the wate-• course above <br />must have a pipe conduit. <br />Where it is desirable to reduce <br />the frequencv of operation of the <br />emergency spillway the minimum <br />capacity of the principal spillway <br />shall be the discharge from a 2 <br />year frequency, 24-hour storm. <br />The minimum pipe discharge shall <br />be increased to that of.a 5 year <br />frequency, 24-hour duration event <br />when the spillway is located on <br />erosive soil. <br />When the discharge of the <br />principal spillway is considered <br />in calculating peak outflow <br />through the emergency spillway, <br />the crest elevation of the inlet <br />shall be such that full flow wil • <br />be generated in the conduit before <br />there is discharge through the <br />_ emergency spillway. The inlets <br />and outlets shall be designed to <br />function satisfactorily for the <br />full ranoe of flow and hydraulic <br />head anticipated. <br />The capacity of the pipe conduit <br />shall be adequate to discharge <br />long-dur=tion, continuous, or <br />frequent flows without flow <br />through the emergency spillways. <br />Pioe conduits under or through the <br />dam shall meet the following <br />recuirements: 'the pipe shall be <br />capable of withstanding extarnal <br />loading without yielding, <br />buckling, or cracking. Flexible <br />pipe strength shall not be less <br />tnan that necessary to support the <br />design load with a maximum of 5 <br />percent deflection. The inlets <br />and outlets shall be strvcturall~ <br />sound and made of materials <br />compatible with those of the pipe. <br />SCS, Colorado, January 1983 <br />