Unified Development Ordinance, 4th supplement, September 2017
Part 10A: Unified Development Ordinance City of Raleigh, North Carolina 9 – 14 Effective Date: September 01, 2013 Supp. No. 4 CHAPTER 9. NATURAL RESOURCE PROTECTION | Article 9.2. Stormwater Management Sec. 9.2.2. Active Stormwater Control Measures discharge for post-development conditions than pre-development conditions. The same methodologies used to calculate stormwater runoff must be used for both pre-development and post-development conditions. b. For any denuded area on sites between 5 and 15 acres in size the peak stormwater runoff leaving the site at each discharge point for the two- year storm and 10-year storm shall be no greater during construction than for pre-development conditions. For any land disturbing activity on sites, greater than 15 acres in size the peak stormwater runoff leaving the site at each discharge point for the two-year storm, 10-year storm and 25-year storm shall be no greater during construction than for pre- development conditions. However, this regulation shall not be applicable when the development site conforms to all of the following: i. The disturbed acreage is less than 5 acres; and ii. The two-year peak discharge for the disturbed condition, for all points of discharge, is less than 10% of the peak discharge from the contributing watershed as measured at the nearest receiving watercourse. 2. Exemptions The stormwater runoff control requirements do not apply to sites with any of the following conditions. a. The development complies with the stormwater control master plan approved for its drainage basin. b. The increase in peak stormwater runoff between pre-development and post-development conditions for the two-year and 10-year and 25-year storms is 10% or less at each point of discharge. c. The maximum impervious surface coverage of the lot, including any existing impervious surfaces, is no more than 15% and the remaining pervious portions of the lot are utilized to convey and control the stormwater runoff of the lot to the maximum extent practical. In the event that the site is subsequently subdivided, reduced by recombination or the impervious surface is equal to or exceeds 15% the site may no longer be exempt. d. Compliance with Sec. 9.2.2.E.1. above, would result in greater adverse downstream impact, such as local flooding, as determined by City- approved engineering studies. e. Compliance with the 10-year storm and 25-year storm runoff limitations in Sec. 9.2.2.E.1. above results in no benefit to current and future downstream development, as determined by City-approved engineering studies. 3. Additional Runoff Controls The City may require the installation of stormwater runoff control measures for projects without any stormwater measures present when the benchmarks contained in the subsections a. through d. below are applicable. The City reserves the right to require additional stormwater runoff control measures for projects which are compliant with Sec. 9.2.2.A or Sec. 9.2.2.E.1. above, if stormwater runoff from the site could cause adverse effects on other properties including, without limitation, public streets, greenways and utility easements. a. As part of an application for rezoning, subdivision or site plan for sites at or upstream of documented structural flooding cases, the applicant shall submit a stormwater impact analysis to the PublicWorks Director. b. This requirement does not extend to sites initially zoned and added to the territorial coverage of as a result of annexation, extraterritorial jurisdictional expansion or otherwise or application of any overlay district. c. The stormwater impact analysis shall look at the flood level differences between pre-development and post-development conditions for the 25-, 50- and 100-year storm events. If the analysis shows an increase greater than 0.04 feet between pre-development and post-development flood levels at the site of structural flooding then mitigation to pre- development flood conditions will be required to prevent further damage to the affected property. d. In the case where the area of the subject property is less than 5% of the drainage area, measured to the location of the documented structural flooding, then this analysis shall not be required. e. In the event flood levels are increased, then the affected property owners will be notified in writing of any increase by the applicant.