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(A) Statement of compliance required with drainage plan submittal. A determination that the development is in compliance with the Town Flood Damage Prevention Ordinance (Chapter 152) and the design standards listed in this chapter must be included with the drainage plan submittal.
(B) Drainage design guidelines.
(1) Piped drainage systems and culverts.
(a) Piped collection systems for public streets (catch basins, inlets, cross drains, and longitudinal piping) shall be designed for the ten-year frequency storm event.
1. Catch basins and inlets shall be spaced so that the spread in the street for the design flow shall not exceed eight feet.
2. A check should be made at all low points to determine and minimize the impacts from less frequent storms.
3. All inlets shall be of the curb-opening-type and low-point basins shall have double wings to provide for drainage flow in both directions entering the inlet.
(b) Road elevations in A and V zones shall allow passage during a five-year event as a minimum.
(c) Design of road culverts.
1. Road culverts (e.g., pipes carrying stormwater under a street or road from one side of the right-of-way to the other) shall be designed for a 100-year frequency flood event.
2. The area inundated under these conditions shall be contained within a defined drainage easement.
(d) The design flow, acres drained, and the hydraulic grade line for the design flow shall be shown on the plans.
(e) Design of pipes and culverts.
1. Pipes and culverts should be designed using the methods given in the hydraulic engineering circulars published by the Federal Highway Administration.
2. The Burkli-Ziegler and Talbot formulas shall not be used since they have questionable applicability to conditions in the town area.
3. All pipe joints shall be wrapped with filter fabric in 18-inch wide sections, utilizing, at a minimum, fabric with a mass per unit area of (English measure) 5.0 ounces per square yard, and a thickness of 60 mils; or (metric measure) 170.0 grams per square meter, and a thickness of 1.5 millimeters.
(f) Energy dissipaters shall be provided as needed, where excessive velocities occur.
(g) Use of reinforced concrete pipes.
1. Reinforced concrete pipes shall be used at public street locations and easements designated for maintenance by the town.
2. Exceptions may be made by the Director of Planning and Development for those not at street locations, provided complete data is submitted.
(h) Non-corrosive pipes shall be used at all other locations, subject to the request of and approval by the Director of Planning and Development.
(C) Minimum design standards.
(1) A combination of storage and controlled release of stormwater runoff shall be required for all development and construction that creates the peak rate of runoff from the site, unless the designer certifies that no downstream damage will occur due to the increase in runoff.
(2) Certifications are required at the time of approval.
(3) The peak release rate of stormwater from all developments where detention is required shall not exceed the peak stormwater runoff rate from the area in its natural undeveloped state for all intensities up to and including the 100-year frequency.
(4) The same methodology must be used for calculating the pre-development and post-development rates of runoff from a site.
(5) The “bowstring” method may be used for designing detention facilities for which the drainage area is less than three acres and a factor of two applied to the estimated volume.
(6) Reservoir routing techniques shall be used for designing all detention facilities for which the drainage area is three acres or larger.
(7) For mosquito control, ponds are to be designed to dry in three days or to contain a minimum permanent depth of 24 inches of water.
(8) All stormwater storage facilities and water quality BMPs in the town are to be privately owned and maintained.
(9) Stormwater management facilities may include both structural and nonstructural elements. Natural swales and other natural runoff conduits shall be retained where practicable.
(10) Where additional stormwater management facilities are required to satisfy the minimum control requirements, the following measures are examples of what may be used:
(a) Stormwater detention structures (dry basins);
(b) Stormwater retention structures (wet ponds);
(c) Facilities designed to encourage overland flow, slow velocities of flow, and flow through buffer zones;
(d) Infiltration practices.
(11) Where detention and retention structures are used, designs that consolidate these facilities into a limited number of large structures will be preferred over designs that utilize a large number of small structures.
(12) A waiver shall only be granted after the applicant has submitted a certification of no impact, stamped by a professional engineer and containing descriptions, drawings, and any other information necessary to evaluate the proposed land-disturbing activity. A separate written waiver request shall be required if there are subsequent additions, extensions or modifications that would alter the approved stormwater runoff characteristics to a land-disturbing activity receiving a waiver.
(13) Discharge velocities shall be reduced to provide a nonerosive velocity flow from a structure, channel or other control measure, or the velocity of the two-year, 24-hour-design storm runoff in the receiving waterway prior to the land-disturbing activity, whichever is greater.
(14) For all stormwater management facilities, a hydrologic-hydraulic study shall be done showing how the drainage system will function with and without the proposed facilities. For such studies, the following land use conditions shall be used. Existing land use data shall be taken from the most recent aerial photograph, field checked and updated.
(a) For the design of the facility outlet structure, use developed land use conditions for the area within the proposed development and existing land use conditions for upstream areas draining to the facility.
(b) For any analysis of flood flows downstream from the proposed facility, use existing land use conditions for all downstream areas.
(c) The effects of existing upstream detention facilities can be considered in the hydrologic-hydraulic study.
(d) Drainage report format.
1. All basin maps submitted as part of the study shall clearly indicate node numbers linked with any model output.
2. The drainage report must also have tabbed sections linked to a table of contents.
(15) Drainage plans can be rejected by the Town Engineer if they incorporate structures and facilities that will demand considerable maintenance, will be difficult to maintain, or utilize numerous small structures, if other alternatives are physically possible.
(16) The drainage system and all stormwater management structures within the town (including both public and private portions) will be designed to the same engineering and technical criteria and standards. The Department of Planning and Development's review will be the same whether the portion of the drainage system will be under public or private control or ownership.
(17) All stormwater management measures shall be designed in accordance with the design criteria contained in the SCDHEC or Town stormwater BMP manual and/or stormwater design manual, using procedures contained in this manual or procedures approved by the Town Engineer.
(Ord. 07045, passed 8-14-07)