07/25/2007 Env Bd Packet - Laserfiche WebLink

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(g) A permit is not required for mill and overlay of a public roadway, sidewalk or trail that does not create additional impervious surface. 3. STORMWATER MANAGEMENT PLAN REQUIREMENTS. (a) Modeling. (i) A hydrograph method or computer program based on Natural Resources Conservation Service Technical Release #20 (TR -20) and subsequent guidance must be used to analyze stormwater runoff for the design or analysis of flows and water levels within and off the project site. Composite Curve Numbers shall not include directly connected impervious surfaces. (ii) In determining Curve Numbers to model runoff in the post - development condition, the Hydrologic Soil Type of areas within construction limits is to be shifted down one classification to account for compaction and soil smearing. (iii) The 100 -year critical event analysis of flood levels, storage volumes, and flow rates for waterbodies and stormwater management basins must include both the 24 -hour rainfall and the 10-day snowmelt events. (b) Standards. (i) Management Plan Framework. (a) When a stormwater basin is necessary, regional siting is preferred when regional management would not divert supply away from a local recharge area or groundwater - dependent natural resource. In evaluating the appropriateness of peak flow and water quality management in an existing regional basin, the District will consider whether it previously approved the basin and whether the basin was designed for build -out of the site to the extent proposed. (b) A stormwater management plan must conform to the District - approved local water management plan. (c) The proposed project must not adversely affect water level off the site during or after construction. (d) A landlocked basin may be provided an outlet only if it: (1) Retains a hydrologic regime that complies with District Wetland Alteration Rule F; (2) Provides sufficient dead storage volume to retain back-to- back 100 -year, 24 -hour rainfalls and runoff; and (3) Does not create adverse downstream flooding or water quality conditions as a result of increased discharge rate or volume, or other factors. (ii) Water Quality and Volume Control. DRAFT DOCUMENT 10 • •