(A) Development standards for ultra low-density and low-density projects. Ultra low-density projects and low-density projects shall comply with each of the following standards, in addition to the general standards found in § 151.36:
(1) Stormwater runoff from the development shall be transported from the development by vegetated conveyances to the maximum extent practicable.
(2) All development and redevelopment shall be located outside the riparian buffer zone and the flood protection zone. These zones shall be in accordance with the following provisions:
(a) Except where other applicable buffer standards are more restrictive, the riparian buffer zone shall extend a minimum of 50 feet landward of all perennial and intermittent surface waters. The most restrictive standards shall apply.
(b) The riparian buffer zone shall remain undisturbed unless otherwise permitted by this section.
(c) The flood protection zone shall extend throughout the FEMA 100-year floodplain as identified on the current Flood Insurance Rate Map (FIRM) published by FEMA. The flood protection zone shall remain undisturbed unless otherwise permitted by this section.
(d) No development or redevelopment is permitted within the riparian buffer zone or the flood protection zone except for stream bank or shoreline restoration or stabilization, water dependent structures, and public or private projects such as road crossings and installations, utility crossings and installations, and greenways, where no practical alternatives exist.
(e) Permitted activities within the riparian buffer zone and the flood protection zone shall minimize impervious coverage, direct runoff away from surface waters to achieve diffuse flow, and maximize the utilization of non-structural BMPs.
(f) Where the riparian buffer zone and the flood protection zone both are present adjacent to surface waters, the more restrictive shall apply.
(3) The approval of the stormwater permit shall require an enforceable restriction on property usage that runs with the land, such as a recorded deed restriction or protective covenants, to ensure that future owners maintain the site consistent with the approved project plans.
(4) All development and redevelopment projects shall provide permanent on-site BMPs to lower the nitrogen export amounts as part of the stormwater management plan and accompany the land-disturbing plan submittal. BMPs are to be in accordance with and as specified in the Design Manual.
(5) For low-density projects only, structural and non-structural BMPs shall be used to ensure there is no net increase in peak flow leaving the site from the pre-development conditions for the one-year, 24-hour storm. Runoff volume drawdown time shall be a minimum of 48 hours, but not more than 120 hours.
(6) General engineering design criteria for all projects shall be in accordance with 15A NCAC 2H .1008(c), as explained in the Design Manual.
(7) Developers must manage runoff so that after development the site will not exceed the target curve numbers in the table in division (B) below.
(8) Ultra low-density projects and low-density projects may be eligible for target curve number credits, as described in division (B) below.
(B) Maximum curve number after development. Developers must manage runoff so that after development the site will not exceed the following composite curve numbers, in accordance with procedures specified in the United States Department of Agriculture, Natural Resource Conservation Service, Technical Release 55, Urban Hydrology for Small Watersheds.
Project Density | Maximum Composite Curve Number, by Soil Group | |||
A | B | C | D | |
Ultra-low | 43 | 63 | 76 | 81 |
Low | 48 | 66 | 78 | 83 |
High | N/A | N/A | N/A | N/A |
(C) Target curve number credits.
(1) Purpose. The purpose of estabhshing a stormwater credit system is to provide incentives to implement better site design and locate new development in a manner that causes less impact to aquatic resources. Certain development practices reduce the generation of stormwater from the site; thereby reducing the size and cost of stormwater storage. In addition, these practices can provide partial removal of many pollutants. The credit system directly translates into cost savings and better protection of water resources.
(2) Disconnected impervious surfaces. Disconnected impervious surfaces, including permeable pavers, are encouraged. Runoff from these disconnected surfaces must be spread over pervious areas as sheet flow. As a credit, these disconnected impervious surfaces will be assigned the lower curve number specified by procedures of the United States Department of Agriculture, Natural Resources Conservation Service, Technical Release 55, Urban Hydrology for Small Watersheds.
(3) Reforestation. The planting of trees and shrubs is encouraged as a means of reducing runoff. As a credit for such practices, reforested areas in dedicated open space will be assigned the curve number for woods in good condition per procedures in the United States Department of Agriculture, Natural Resources Conservation Service (NRCS), Technical Release 55, Urban Hydrology for Small Watersheds. Areas planted with trees or shrubs must meet the following standards to qualify for the credit.
(a) Tree/shrub density and spacing. Planted trees or shrubs must meet the minimum density and spacing standards of the NRCS, as specified in the Field Office Technical Guide. Existing trees or shrubs may be used toward meeting the planting standard.
(b) Mulching. An initial application of mulch is required for the area, designated for reforestation. Mulching must meet applicable standards of the NRCS, as specified in the Field Office Technical Guide. Existing groundcover may be used toward meeting the mulching standard.
(4) Cluster, conservation and open space subdivisions. Cluster, conservation and open space subdivisions are encouraged. In applying curve number calculations to such developments, calculations must take into account the lots’ proportionate share of right-of-way and permanent open space.
(5) Calculations regarding ponds, lakes, and streams. Surface water bodies may not be assigned a curve number for impervious surfaces. Instead, such water bodies will be removed from calculations so that developments are not penalized for their presence. Surface water bodies shall be included in calculations of project density.
(D) Development standards for high-density projects. High-density projects shall implement stormwater control measures that comply with each of the following standards, in addition to the general standards found in § 151.36.
(1) The measures shall control and treat runoff from the first inch of rain. Runoff volume drawdown time shall be a minimum of 48 hours, but not more than 120 hours.
(2) All structural stormwater treatment systems used to meet these requirements shall be designed to have a minimum of 85% average annual removal for total suspended solids (TSS).
(3) All development and redevelopment projects shall provide permanent on-site BMPs to lower the nitrogen export amounts as part of the stormwater management plan and accompany the land-disturbing plan submittal. BMPs are to be in accordance with and as specified in the Design Manual.
(4) Structural and non-structural BMPs shall be used to ensure there is no net increase in peak flow leaving the site from the pre-development conditions for the one-year, 24-hour storm. Runoff volume drawdown time shall be a minimum of 48 hours, but not more than 120 hours.
(5) General engineering design criteria for all projects shall be in accordance with 15A NCAC 2H .1008(c), as explained in the Design Manual.
(6) All development and redevelopment shall be located outside the riparian buffer zone and the flood protection zone. These zones shall be in accordance with the following provisions:
(a) Except where other applicable buffer standards are more restrictive, the riparian buffer zone shall extend a minimum of 50 feet landward of all perennial and intermittent surface waters. The most restrictive standards shall apply.
(b) The riparian buffer zone shall remain undisturbed unless otherwise permitted by this section.
(c) The flood protection zone shall extend throughout the FEMA 100-year floodplain as identified on the current Flood Insurance Rate Map (FIRM) published by FEMA. The flood protection zone shall remain undisturbed unless otherwise permitted by this section.
(d) No development or redevelopment is permitted within the riparian buffer zone or the flood protection zone except for stream bank or shoreline restoration or stabilization, water dependent structures, and public or private projects such as road crossings and installations, utility crossings and installations, and greenways, where no practical alternatives exist.
(e) Permitted activities within the riparian buffer zone and the flood protection zone shall minimize impervious coverage, direct runoff away from surface waters to achieve diffuse flow, and maximize the utilization of non-structural BMPs.
(f) Where the riparian buffer zone and the flood protection zone both are present adjacent to surface waters, the more restrictive shall apply.
(7) The approval of the stormwater permit shall require an enforceable restriction on property usage that runs with the land, such as recorded deed restrictions or protective covenants, to ensure that future development and redevelopment maintains the site consistent with the approved project plans. Buffer widths and locations shall be clearly delineated on all plans, final plat, and as-builts.
(Ord. 2010-27, passed 1-6-2010; Ord. 2012-05, passed 7-20-2011)