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SEDIMENT BASIN (No.) Standard 350 NATURAL RESOURCES CONSERVATION SERVICE CONSERVATION PRACTICE STANDARD ARIZONA SEDIMENT BASIN (No.) CODE 350 DEFINITION Where the Hazard Class of the dam is Low. Where site conditions shall be such that A basin constructed with an engineered outlet, runoff from the design storm can be safely formed by an embankment or excavation or a passed through (1) a natural or constructed combination of the two. auxiliary spillway, (2) the combination of a principal spillway and auxiliary spillway, or (3) PURPOSE a principal spillway. To capture and detain sediment laden runoff, or This practice is also serves to: other debris for a sufficient length of time to Preserve the capacity of reservoirs, wetlands, allow it to settle out in the basin. ditches, canals, diversion, waterways, and streams CONDITIONS WHERE PRACTICE Prevent undesirable deposition on bottom APPLIES lands and developed areas or trap sediment originating from construction sites or other This practice applies to urban land, construction disturbed areas sites, agricultural land, and other disturbed Reduce or abate pollution by providing lands: basins for deposition and storage of silt, sand, Where physical conditions or land ownership gravel, stone, agricultural waste solids, and preclude treatment of a sediment source by the other detritus installation of erosion-control measures to keep soil and other material in place or where a CRITERIA sediment basin offers the most practical solution to the problem. Sediment Basins shall be designed on an Where failure of the basin will not result in individual basis to meet site conditions and loss of life, damage to homes, commercial or functional requirements. They shall be part of an industrial buildings, main highways or railroads; approved and overall engineering plan for or in the interruption, service or use of public irrigation, drainage, wildlife, recreation, utilities. channel improvement, or similar purposes. Where the product of the storage times the effective height of the dam or embankment is Design and implementation of subsidiary less than 3,000. Storage is the volume, in acre- components and/or structures shall meet all feet, in the reservoir below the elevation of the applicable Natural Resource Conservation crest of the auxiliary spillway. Service (NRCS) conservation practice Where the effective height of the dam is 35 standards. The criteria for the design of any feet or less. The effective height of the dam is components not specifically addressed in NRCS the difference in elevation, in feet, between the practice standards or specifications shall be auxiliary spillway crest and the lowest point in consistent with sound engineering principles the cross section taken along the centerline of and/or manufacturer recommendations. the dam. If there is no auxiliary spillway, the top of the embankment is the upper limit. NRCS Field Office Tech Guide, Section IV NRCS-AZ Page 1 of 14 October 2010 Standard 350 All pertinent safety codes (OSHA, ADOSH) and Location. Sediment basins are the last line of manufacturer’s recommendations shall be defense for capturing sediment when erosion has followed in the installation and operation of the already occurred. When possible construct equipment. basins prior to soil disturbance in the watershed. Choose the location of the sediment basin so that Laws and Regulations. Sediment basin design, it intercepts as much of the runoff as possible installation, and construction must comply with from the disturbed area of the watershed. all applicable federal, state, Tribal and local Choose a location that minimizes the number of laws, rules, and regulations. Laws and entry points for runoff into the basin and regulations of particular concern include those interference with construction or farming involving water rights, land use, pollution activities. Do not locate sediment basins in control, property easements, wetlands, perennial streams. preservation of cultural resources, and endangered species. Basin Capacity. The sediment basin must have sediment storage capacity, detention storage and The owner is responsible for securing necessary temporary flood storage capacities. For permits and water rights, complying with all maximum sediment retention, design the basin laws and regulations, and meeting legal so that the detention storage remains full of requirements applicable to the installation, water between storm events. However, if site operation, and maintenance of this practice and conditions, safety concerns, or local laws associated structures. preclude a permanent pool of water, design all or a portion of the detention and sediment storages Cultural Resources and Wildlife to be dewatered between storm events. If it is Habitat. Impact to cultural resources, wetlands determined that periodic removal of sediment and Federal and state protected species shall be will be practicable, the capacity may be evaluated and avoided or minimized to the proportionately reduced. extent practicable during planning, design and implementation of this conservation practice in Design the sediment storage for a minimum of accordance with established National policy, 900 ft3/acre of disturbed area. The sediment General Manual (GM) Title 420-Part 401; Title storage volume is calculated from the bottom of 450-Part 401, Title 190-Parts 410.22 and the basin. Design the detention storage for a 410.26, National Planning Procedures minimum of 3,600 ft3/acre of drainage area. The Handbook (NPPH), National Cultural detention volume is calculated from the top of Resources Procedures Handbook (NCRPH), the sediment storage to the crest of the principal National Food Security Act Manual (NFSAM), spillway. and the National Environmental Compliance Handbook (NECH). Sediment yield shall be computed and the sediment basin sized using one or more of the General Criteria Applicable to All Purposes following references: National Engineering The design of dams, spillways, and drainage Handbook, Section 3 – Sedimentation; facilities shall be according to Arizona Engineering Field Manual, Chapter 10 – Gully Conservation Practice Standard 378 (Pond), Treatment; Engineering Field Manual, Chapter Conservation Practice Standard 410 (Grade 15 – Irrigation; Actual field studies; or other Stabilization Structure) or according to the approved NRCS references. requirements in NRCS TR-60 (Earth Dams and Reservoirs), as appropriate, for the class and Where field data for sediment bulk density is not kind of structure being considered. available, the designer may assume that one ton of sediment occupies one cubic yard. NRCS Field Office Tech Guide, Section IV NRCS-AZ Page 2 of 14 October 2010 Standard 350 Flood storage is based on the required design An auxiliary spillway must be provided for each storm for the auxiliary spillways. Flood storage embankment, unless the principal spillway is is calculated between the crest of the principal large enough to pass the peak discharge from spillway and the crest of the auxiliary spillway. the routed design hydrograph and anticipated A minimum of 1 foot in elevation is required trash without over topping the embankment. between the principal and auxiliary spillways. The minimum criteria for a closed conduit principal spillway without an auxiliary spillway Required Sediment Basin Storage Capacities are a conduit with a cross-sectional area of 3 ft2 or more, an inlet that will not clog, and an Flood Storage Auxiliary Spillway Based on aux. spwy. elbow designed to facilitate the passage of trash. flow reqmt. 1.0' min. The minimum capacity of a natural or Detention Principal Spillway constructed auxiliary spillway shall be that Storage 3600 cu. ft./acre required to pass the peak flow expected from a Sediment Storage 900 cu. ft./acre design storm of the frequency and duration shown in the following tables, less any reduction creditable to conduit discharge and detention Principal and Auxiliary Spillway Design storage. Minimum Spillway Capacity for Design the principal spillway to carry Drainage Area of 20 acres or less (discharge) long-duration, continuous, or Effective Storage Minimum Design frequent flows without discharge through the Height of Storm auxiliary spillway. The diameter of the principal Embankment spillway pipe must be 6 inches or greater. (feet) (acre-feet) 10 or less Less than 9.2 10-year, 24-hour The principal spillway can be designed to 10 or less More than 9.2 25-year, 24-hour remove only water from the temporary flood More than 10 Less than 9.2 50-year, 24-hour storage or it can be designed to dewater all or part of the detention storage. Design the Minimum Spillway Capacity for principal spillway to drawdown the temporary Drainage Area of 20 acres or greater Height of Storage Minimum flood storage within 24 hours. Drawdown times Embankment1 Design Storm2 for the detention storage can be longer to (feet) (acre-feet) improve sediment trapping. 10 or less Less than 9.2 25-year, 24-hour 10 or less More than 9.2 50-year, 24-hour When design discharge of the principal spillway More than 10 Less than 9.2 100-year, 24-hour is considered in calculating peak outflow 1 As defined under criteria through the auxiliary spillway, the crest 2 Select rain distribution based on climatological elevation of the inlet shall be such that the full region flow will be generated in the conduit before there is discharge through the auxiliary The minimum capacity of a closed auxiliary spillway. The inlets and outlets shall be designed spillway shall be sufficient to pass the peak
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