2020-09-21_REVISION - C1981044 (9)

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4VO COLORADO Division of Water Resources Department of Natural Resources State Engineer Standard Plans and Specifications for Construction of Livestock Water Tank and Erosion Control Dams • Site Selection Foundation soils should be firm to provide adequate support for the embankment and should have low permeability to allow for water retention. Site selection should consider potential downstream property damage in the event of a dam failure. Construction of dams in boggy areas, areas with non -uniform fractured rock, or sands/gravels is not recommended and an engineer should be hired to evaluate the site conditions. Any part of the reservoir basin excavated below grade cannot expose groundwater. Embankment Design Backfill material to be used for construction of the cutoff trench and embankment should be a suitable clay material and contain no material larger than 6 inches in diameter. The upstream slope should be constructed with a slope no steeper than 3:1, and the downstream slope should be no steeper than 2:1 (see cross section below). The dam crest should have a minimum width of 10 feet and the surface should be graded with positive drainage toward the reservoir basin. It is recommended that rock rip rap or other suitable material be placed on the upstream slope of the embankment to protect it from wave action. A suitable gravel or geosynthetic material should be placed under the rip rap to prevent fine material from washing out from behind the larger rock. The embankment should be fenced to restrict livestock from accessing the dam since they damage the protective vegetation and increase erosion. Embankment Construction The topsoil and all organic material should be removed from the foundation of the proposed dam site. Organic soil should only be reused for placement on the completed embankment to promote the re -growth of vegetation. A cutoff trench should be excavated under the full length of the centerline of the dam with sloping sides (1:1 min.), a minimum bottom width of 3 feet and a depth of 3 feet. The foundation of the dam should be scarified/ripped to a depth of 6-inches to provide proper contact between the native foundation and embankment. This surface should then be moisture treated before placement of fill. Fill material should be placed in layers not exceeding 12 inches in thickness prior to compaction. Suitable backfill material should have enough clay and moisture content to roll a small ball by hand. If this cannot be done, the soil is likely too dry or does not have adequate clay content. Each lift should be thoroughly compacted using a sheeps foot compactor. Care should be taken not to allow the top layers of the soil to dry out between placement of lifts. Fill should be placed in uniform lifts that cover the entire embankment length and width. • Outlet - A minimum diameter of 12 inches is recommended and should be protected at the upstream end by a trash rack. • Emergency Spillway The spillway should have sufficient width to provide capacity to route the runoff from the drainage basin above the dam during rainfall/runoff events. A minimum of 3 feet of freeboard is required from the bottom of the emergency spillway to the top of thedam. • Example Plan View and Cross Section eft Abu Mot Nrservolr 3 Feet Freeboard Crest Width Minimum from Dam (10 Foot Min) Downstream Stope Reservoir Water Crest to Spillway Crest to Drain Surface ---------- -------------------+--- Spillway Crest ------- ------ Outlet cordu6 Thrall Dam C 1 Q TOO Upstreslope V Natural GrdxN1 am Surface Outlet Conduit Pula Abutment i sp'.' Cutoff Trench Centerline of dam Plan View Cross Section /2019)