2918-25 STORMWATER final 4_1_25.doc.pdf

25 i. Demonstrate through hydrologic and hydraulic analysis that for stormwater leaving the site, post-construction runoff hydrographs for the current and projected 2-, 10-, and 100-year storm events, as defined and determined in § 25-29.5c and 5d respectively, of this ordinance, do not exceed, at any point in time, the pre-construction runoff hydrographs for the same storm events; ii. Demonstrate through hydrologic and hydraulic analysis that there is no increase, as compared to the pre-construction condition, in the peak runoff rates of stormwater leaving the site for the current and projected 2-, 10-, and 100-year storm events, as defined and determined pursuant to § 25-29.5c and 5d respectively, of this ordinance, and that the increased volume or change in timing of stormwater runoff will not increase flood damage at or downstream of the site. This analysis shall include the analysis of impacts of existing land uses and projected land uses assuming full development under existing zoning and land use ordinances in the drainage area; iii. Design stormwater management measures so that the post-construction peak runoff rates for the current and projected 2-, 10-, and 100-year storm events, as defined and determined in § 25-29.5c and 5d, respectively, of this ordinance, are 50, 75 and 80 percent, respectively, of the pre-construction peak runoff rates. The percentages apply only to the post-construction stormwater runoff that is attributable to the portion of the site on which the proposed development or project is to be constructed; or iv. In tidal flood hazard areas, stormwater runoff quantity analysis in accordance with § 25-29.4S3(i), (ii), and (iii) above is required unless the design engineer demonstrates through hydrologic and hydraulic analysis that the increased volume, change in timing, or increased rate of the stormwater runoff, or any combination of the three will not result in additional flood damage below the point of discharge of the major development. No analysis is required if the stormwater is discharged directly into any ocean, bay, inlet, or the reach of any watercourse between its confluence with an ocean, bay, or inlet and downstream of the first water control structure. 3. 4. The stormwater runoff quantity standards shall be applied at the site’s boundary to each abutting lot, roadway, watercourse, or receiving storm sewer system. §

the first water control structure. 3. 4. The stormwater runoff quantity standards shall be applied at the site’s boundary to each abutting lot, roadway, watercourse, or receiving storm sewer system. § 25-29.4—. Calculation of Stormwater Runoff and Groundwater Recharge: A. Stormwater runoff shall be calculated in accordance with the following: 1. The design engineer shall calculate runoff using the following method: (a) The USDA Natural Resources Conservation Service (NRCS) methodology, including the NRCS Runoff Equation and Dimensionless Unit Hydrograph, as described in Chapters 7, 9, 10, 15 and 16 Part 630, Hydrology National Engineering Handbook, incorporated herein by reference as amended and supplemented. This methodology is additionally described in Technical Release 55 - Urban Hydrology for Small Watersheds (TR-55), dated June 1986, incorporated herein by reference as amended and supplemented. Information