Title 15 - Public Improvements

15.10.400 Stormwater Detention.

Stormwater detention facilities shall be constructed with all proposed developments that result in an increase in peak discharge from the site whenever downstream homes or habitable buildings are flooded in a 1% or more frequent storm, or when such homes and buildings would be flooded as a result of future development, either individually or cumulatively in the watershed, or whenever streets are subjected to flooding in excess of the Standards, except that detention is not required under any of the following circumstances:

1. A.
   When such flooding occurs within the Special Flood Hazard Area, as defined in 18.360 and the watershed is greater than 5000 acres.
2. B.
   If the project does not increase impervious area.
3. C.
   When such development meets both of the following: 

   1. 1.
      The development is one or more contiguous lots or parcels under common ownership and not part of a larger common plan of development, or redevelopment, that in total includes one (1) acre or less of land. 
   2. 2.
      When such development is residential in nature and the allowed density under the zoning district is less than or equal to 12.5 dwelling units per acre. 
4. D.
   For developments larger than one (1) acre, where stormwater detention has not been previously provided on the site, a cumulative 8,000 square feet of additional impervious surface may be permitted without providing detention. Cumulative impervious area shall be measured from September 1984, the effective date of the first detention ordinance. For purposes of this ordinance, pervious payment shall be considered fully pervious. 
5. E.
   Problems are identified in a watershed that had not previously required a detention study, and if the remaining increase in flooding caused by the cumulative remaining development in the watershed is less than 0.05 feet for building flooding or 0.2 feet for roadway flooding, or if the increase in peak flow rate from the cumulative remaining development in a basin is less than 0.5%.
6. F.
   If the City Engineer determines that detention would be ineffective to prevent flooding or would aggravate existing flooding conditions.
7. G.
   When the City Engineer determines that reasonably anticipated future problems can be solved by alternate storm drainage improvements constructed through the use of escrowed funds in accordance with this Section or other funding methods, or when alternative storm drainage improvements are constructed by the developer or when such downstream problems are due to a unique characteristic of a downstream building or buildings that could be corrected at reasonable cost and effort by their owner or owners.
8. H.
   Either a current or previous owner of a development site tributary to the Kingston Lake Regional Detention Facility has contributed funds to 151st Street Associates for the construction of that facility, and there is no flooding of homes and habitable structures or flooding of streets in excess of the Standards between the development site and Kingston Lake.
9. I.
   "Flooding" of homes and habitable buildings, for the purposes of stormwater detention, shall be defined as when the structure’s lowest adjacent grade is less than one foot above the energy grade line of the 1% storm for fully developed conditions upstream AND the property owner is unable to fill adjacent to the structure to meet this criteria.

(History: Ord. SWM-3426 §3, 2024; ​​​​​​​Ord. SWM-2853 §5, 2010; SWM-2668 §5, 2008; SWM-2505 §1, 2004; SD-1365 §1, 86; SD-1276 §1, 84; SD-1234 §3, 83)

Effective on: 1/1/1901