(A) Downstream impact analysis.
(1) The downstream impact analysis must be performed in accordance with the “10% rule,” and a copy of the analysis must be provided with the permit application. The purpose of the downstream impact analysis is to determine if the project will cause any impacts on flooding or channel degradation downstream of the project site. The analysis must include the assumptions, results and supporting calculations to show safe passage of post-development design flows downstream. This analysis shall be performed at the outlet(s) of the site, and downstream at each tributary junction to the point(s) in the conveyance system where the area of the portion of the site draining into the system is less than or equal to 10% of the total drainage area above that point.
(2) The typical steps in the application of the 10% rule are:
(a) Using a topographic map, determine the point downstream where the proposed site equals 10% of the total drainage area, called the 10% point. Identify all tributary junctions between the downstream site boundary and the 10% point. All points identified, as well as the outlet of the site, are known as 10% rule comparison points.
(b) Using a hydrologic model with existing land uses, determine the pre-development peak runoff rate (cfs) for the ten-year design storm event at each comparison point.
(c) Insert the proposed site design and proposed BMPs into the land uses and determine the post-development peak runoff rate for the ten-year design storm at each comparison point.
(d) If the post-development peak discharge rate is equal to or less than pre-development conditions at all comparison points, no further analysis is required.
(e) If the ten-year post-development peak discharge rate is greater than the pre-development peak discharge rate at any comparison point, then one of the following actions must be taken:
1. Revise the site plan for the proposed site to incorporate better use of natural features, design additional structural control facilities, reduce impervious cover, or alter timing of peak flows to lower post-development flows at each comparison point to pre-development levels.
2. Obtain a flow easement from downstream property owners through the 10% point where the post-development peak discharge rate is higher than the pre-development peak discharge rate.
3. Work with the town to determine other acceptable approaches to reduce the peak discharge rate for the ten-year storm.
(3) For further information on the 10% rule, refer to the Stormwater Manual, available online.
(B) Standards for stormwater control measures.
(1) Evaluation according to contents of Design Manual. All stormwater control measures and stormwater treatment practices (or BMPs) required under this ordinance shall be evaluated by the Stormwater Administrator according to the policies, criteria, and information, including technical specifications and standards and the specific design criteria for each stormwater practice, in the Design Manual. The Stormwater Administrator shall determine whether proposed BMPs will be adequate to meet the requirements of this chapter.
(2) Determination of adequacy; presumptions and alternatives. Stormwater treatment practices that are designed, constructed, and maintained in accordance with the criteria and specifications in the Design Manual will be presumed to meet the minimum water quality and quantity performance standards of this chapter. Whenever an applicant proposes to utilize a practice or practices not designed and constructed in accordance with the criteria and specifications in the Design Manual, the applicant shall have the burden of demonstrating that the practice(s) will satisfy the minimum water quality and quantity performance standards of this chapter. The Stormwater Administrator may require the applicant to provide the documentation, calculations, and examples necessary for the Stormwater Administrator to determine whether such an affirmative showing is made.
(3) Separation from seasonal high water table. For BMPs that require a separation from the seasonal high-water table, the separation shall be provided by at least 12 inches of naturally occurring soil above the seasonal high-water table.
(4) Changes to stormwater plan based on emerging technologies. Subject to the standards of this chapter and other applicable law, a developer, in conjunction with the development’s lot owners, may submit an application to revise the approved stormwater plan so as to use new technologies or best management practices not available when the original stormwater plan was approved. Innovative technologies may be used on a demonstration basis for a period of one year while simultaneously collecting data on the effectiveness of the technology according to its design. If at the end of the demonstration period the technology is not performing according to its intended design functions as judged by the Stormwater Administrator, the developer must retrofit the site with a standard technology.
(C) Additional stormwater management measures. In some cases, the Stormwater Administrator may require more stringent stormwater management measures where it is determined that additional measures are required to protect water quality and maintain existing and anticipated uses of these waters or to minimize off-site damage from stormwater runoff in accordance with the purpose of this chapter as defined in § 151.04.
(D) Dedication of BMPS, facilities and improvements. The town may elect to accept dedication of any existing or future stormwater management facility for maintenance, provided such facility meets all the requirements of this chapter and includes adequate and perpetual access and sufficient area, by easement or otherwise, for inspection and regular maintenance.
(E) Low impact development project classifications.
(1) All development or redevelopment to which this chapter applies may be submitted for classification as a low impact development (LID) project.
(2) Classification as a low impact development project requires that the development mimic the pre-developed hydrologic conditions defined as woods in good condition for the two-year, 24-hour storm, within 10%. The pre-developed conditions shall include: its hydrologic balance, frequency distribution of high flows; magnitude, frequency, and duration of low flows; groundwater recharge (or infiltration), and flow length and pattern. The conditions shall be measured through the comparison between measures of the pre-developed and developed conditions including: total runoff volume, time of concentration, curve number, and peak discharge. Mimicry of the pre-developed hydrologic conditions may be achieved through such techniques as the minimization of disturbed areas and the use of on-lot distributed retention storage as described in more detail on Wake County’s Stormwater web site under low impact development.
(3) The following techniques must be used to achieve LID classification:
(a) Natural site design in consultation with the town;
(b) Site buildings, roads, and other land disturbance in the least environmentally-sensitive areas, preserving steep slopes, naturally well-draining soils, and other hydrologically valuable features undisturbed;
(4) In addition, one of the following two techniques must be used to achieve LID classification:
(a) Bio-retention systems;
(b) On-site infiltration;
(5) In addition, at least two of the following techniques must be used to achieve LID classification:
(a) Retention of 50% of vegetated area, including open space, landscaping, or forests;
(b) Use of permeable pavement for all private driveways, private roads, sidewalks, and parking areas in accordance with the North Carolina Stormwater Best Management Practices Design Manual;
(c) Installation of one rain cistern per lot or three rain barrels per lot;
(d) Installation of vegetated roofs;
(f) Use of reclaimed water for all buildings in accordance with state and local laws.
(g) Use of innovative LID techniques subject to the approval of the town.
(6) For development and redevelopment projects achieving classification as LID projects, the Stormwater Administrator shall reduce or waive the stormwater permit fee if stipulated in the fee schedule duly adopted by the applicable governing board.
(7) Upon collaboration with the Planning Board, the Stormwater Administrator may develop and apply an expedited review schedule for development or redevelopment projects achieving classification as LID projects. Such a review schedule will depend upon the continued availability of local government resources to conduct expedited reviews.
(Ord. 2010-27, passed 1-6-2010)