Duluth UDC § 1021

Subject to the applicability criteria in Section 1020.02 and exemptions in Section 1020.03, the following stormwater management standards apply. Additional details for each standard can be found in the GSMM Section 2.2.2.2: 1. Design of Stormwater Management System: The design of the stormwater management system shall be in accordance with the applicable sections of the GSMM as directed by the Administrator. Any design which proposes a dam shall comply with the Georgia Safe Dams Act and Rules for Dam Safety as applicable. 2. Natural Resources Inventory: Site reconnaissance and surveying techniques shall be used to complete a thorough assessment of existing natural resources, both terrestrial and aquatic, found on the site. Resources to be identified, mapped, and shown on the Stormwater Management Plan, shall include, at a minimum (as applicable): a. Topography (minimum of 2-foot contours) and Steep Slopes (i.e., Areas with Slopes Greater Than 15%), b. Natural Drainage Divides and Patterns, c. Natural Drainage Features (e.g., swales, basins, depressional areas), d. Natural feature protection and conservation areas such as wetlands, lakes, ponds, floodplains, stream buffers, drinking water wellhead protection areas and river corridors, e. Predominant soils (including erodible soils and karst areas), and f. Existing predominant vegetation including trees, high quality habitat and other existing vegetation. 3. Better Site Design Practices for Stormwater Management: Stormwater management plans shall preserve the natural drainage and natural treatment systems and reduce the generation of additional stormwater runoff and pollutants to the maximum extent practicable. Additional details can be found in the GSMM Section 2.3. 4. Stormwater Runoff Quality/Reduction: Stormwater Runoff Quality/Reduction shall be provided by using the following: 283 | P a g e Article Ten. Erosion Control and Stormwater Management a. For development with a stormwater management plan, the applicant shall choose (A) Runoff Reduction and additional water quality shall not be required. To the extent (iA) Runoff Reduction has been determined to be infeasible for all or a portion of the site using the Practicability Policy, then (iiB) Water Quality shall apply for the remaining runoff from a 1.2 inch rainfall event and must be treated to remove at least 80% of the calculated average annual post-development total suspended solids (TSS) load or equivalent as defined in the GSMM. i. Runoff Reduction - The stormwater management system shall be designed to retain the first 1.0 inch of rainfall on the site using runoff reduction methods, to the maximum extent practicable. ii. Water Quality – The stormwater management system shall be designed to remove at least 80% of the calculated average annual post-development total suspended solids (TSS) load or equivalent as defined in the GSMM for runoff from a 1.2 inch rainfall event. b. If a site is determined to be a hotspot as detailed in Section 1020.02, the City of Duluth may require the use of specific or additional components for the stormwater management system to address pollutants of concern generated by that site. 5. Stream Channel Protection: Stream channel protection shall be provided by using all of the following three approaches: a. 24-hour extended detention storage of the 1-year, 24-hour return frequency storm event; b. Erosion prevention measures, such as energy dissipation and velocity control; and c. Preservation of any applicable stream buffer. 6. Overbank Flood Protection: Downstream overbank flood protection shall be provided by controlling the post-development peak discharge rate to the pre-development rate for the 25-year, 24-hour storm event. 7. Extreme Flood Protection: Extreme flood protection shall be provided by controlling the 100-year, 24- hour storm event such that flooding is not exacerbated. 8. Downstream Analysis: Due to peak flow timing and runoff volume effects, some structural components of the stormwater management system fail to reduce discharge peaks to pre-development levels downstream from the site. A downstream peak flow analysis shall be provided to the point in the watershed downstream of the site or the stormwater management system where the area of the site comprises 10% of the total drainage area in accordance with Section 3.1.9 of the GSMM. This is to help ensure that there are minimal downstream impacts from development on the site. The downstream analysis may result in the need to resize structural components of the stormwater management system. 9. Stormwater Management System Inspection and Maintenance: The components of the stormwater management system that will not be dedicated to and accepted by the City of Duluth, including all drainage facilities, best management practices, credited conservation spaces, and conveyance systems, shall have an inspection and maintenance agreement to ensure that they continue to function as designed. All new development and redevelopment sites are to prepare a comprehensive inspection and maintenance agreement for the on-site stormwater management system. This plan shall be written in accordance with the requirements in Section 1024.02.