Laserfiche WebLink
Chapter IV <br /> sound. In all cases, the minimum elevation of the top of the settled embankment is 1.0 <br /> feet above the crest of the designed flow emergency spillway elevation. The upstream <br /> and downstream slopes have been stabilized with rip rap,when necessary, or were <br /> revegetated. <br /> Excavated sediment ponds have been constructed by excavating a pit or "dug out." <br /> The dug out design consists of an embankment less than 3 feet above the original <br /> ground. The design characteristics regarding water capacity and sediment storage are <br /> the same as for the embankment impoundment structures. <br /> All sedimentation ponds are designed to control the runoff from the 10 year/24 hour <br /> storm event as determined from the peak inflow and outflow hydrographs. Currently <br /> the impoundments are not designed with baffles. However, the impoundments are <br /> located to maximize sediment trap efficiency without causing short circuiting. <br /> Aluminum Sulfate (Alum) is used as necessary for settling of sediment in the rock <br /> tunnel ponds above outfall #016. <br /> Sediment storage volumes of the impoundments have been determined using the <br /> universal soil loss equation, and to accommmodate at least .1 acre foot for each acre of <br /> disturbed area. The soil loss equation was used to calculate sediment storage volumes <br /> by evaluating Coal Basin's vegetative, soil and geologic setting. <br /> Sedimentation ponds will be maintained by periodically removing sediment when the <br /> sediment storage exceeds 60% of the designed volume. Th sediment will be removed <br /> by using either a small dragline or a backhoe. The sediment, if geochemical tests <br /> prove its suitability, will be used as topsoil. <br /> Sedimentation ponds have been designed to be dewatered after each major runoff <br /> event. The ponds will be dewatered by natural seepage and infiltration, and through <br /> the use of an automatic or manual dewatering device. The automatic dewatering device <br /> is designed to be non-clogging and to have a discharge rate that will achieve and <br /> maintain the required detention time. The devices have been located at the lower <br /> elevation of the 10 year storm capacity volume; however, not below the maximum <br /> elevation of the sediment storage volume. <br /> The impoundments have been designed with an appropriate combination of principal <br /> and/or emergency spillways. The design includes, where necessary, the use of energy <br /> dissipators, antiseep collars and rip rap in accordance with standard engineering <br /> practices. The spillways hve been designed so that the peak runoff from a 25 year/24 <br /> hour storm will be conveyed around or through the structure without compromising the <br /> stability of the structure. <br /> All sedimentation ponds, whether individual or in series, have been designed, <br /> constructed and certified under the direction or supervision of a professional engineer. <br /> 18 <br />