(A) All storm water control structures shall be designed by a state registered professional engineer. Other storm water systems shall be designed by a state registered professional with qualifications appropriate for the type of system required; these registered professionals are defined as professional engineers or landscape architects, to the extent that the G.S. Ch. 89A allow and land surveyors, to the extent that the design represents incidental drainage within a subdivision, as provided in G.S. § 89C-3(7).
(B) All storm water controls shall use wet detention ponds as a primary treatment system, unless alternative storm water management measures, as outlined in division (C) below, are used. Wet detention ponds shall be designed for specific pollutant removal according to modeling techniques approved by the State Division of Environmental Management. Specific requirements for these systems shall be in accordance with the following design criteria:
(1) Wet detention ponds shall be designed to remove 85% of total suspended solids in the permanent pool and storage runoff from a one-inch rainfall from the site above the permanent pool;
(2) The designed runoff storage volume shall be above the permanent pool;
(3) The discharge rate from these systems following the one-inch rainfall design storm shall be such that the runoff does not draw down to the permanent pool level in less than two days and that the pond is drawn down to the permanent pool level within at least five days;
(4) The mean permanent pool depth shall be a minimum of three feet;
(5) The inlet structure shall be designed to minimize turbulence using baffles or other appropriate design features;
(6) Vegetative filters shall be constructed for the overflow and discharge of all storm water wet detention ponds and shall be at least 30 feet in length. The slope and width of the vegetative filter shall be determined so as to provide a non-erosive velocity of flow through the filter for a ten-year, 24-hour storm with a ten-year, one-hour intensity with a slope of 5% or less. Vegetation in the filter shall be natural vegetation, grasses or artificially planted wetland vegetation appropriate for the site characteristics; and
(7) All storm water control structures shall be enclosed by a fence with a minimum height of six feet.
(C) Alternative storm water management systems, consisting of one treatment option or a combination of treatment options, may be used, The design criteria for approval shall be 85% average annual removal of total suspended solids. Also, the discharge rate shall meet one of the following criteria:
(1) The discharge rate following the one-inch design storm shall be such that the runoff draws down to the pre-storm design stage within five days, but not less than two days; or
(2) The post development peak discharge rate shall equal the pre-development rate for the one-year, 24-hour storm.
(D) In addition to the vegetative filters required in division (B)(6) above, all land areas outside of the pond shall be provided with a ground cover sufficient to restrain erosion within 30 days after any land disturbance. Upon completion of the storm water control structure, a permanent ground cover shall be established and maintained as part of the maintenance agreement described in § 152.123(C).
(E) A description of the area containing the storm water control structure shall be prepared and filed consistent with § 152.126(A) and (B), as a separate deed with the County Register of Deeds along with any easements necessary for general access to the storm water control structure. The deeded area shall include the storm water control structure, vegetative filters, all pipes and water control structures, berms, dikes and the like, and sufficient area to perform inspections, maintenance, repairs and reconstruction.
(F) Qualifying areas of the storm water control structure may be considered pervious when computing total built-upon area; however, if the structure is used to compute the percentage built-upon area for one site, it shall not be used to compute the built-upon area for any other site or area.
(Ord. passed 4-5-2010; Ord. passed 5-3-2021)