(A) Storm drainage. A drainage system shall be designed and constructed by the subdivider to provide for the proper drainage of the surface water of the subdivision, and the drainage area of which it is a part, to permit the unimpeded flow of natural watercourses.
(1) Design and construction standards.
(a) Drainage systems shall be designed and constructed by the Subdivider/developer consistent with the design principals and standards contained herein and established by SCDHEC- OCRM.
(b) Drainage systems in all cases shall conform to cross sections, dimensions, erosion control measures and grades as shown on the approved construction plans.
(2) The off-street drainage system shall include the watershed affecting the subdivision, and shall be extended to a natural watercourse or publicly maintained drainage facility that is adequate to receive the storm drainage.
(3) Where adequate existing public storm sewers are reasonably accessible as determined by the Director of the Department of Engineering and Development Services, or his or her designee, require that the system proposed for the land being subdivided be connected thereto.
(4) Detention/retention ponds, lakes, and the like.
(a) Where detention/retention ponds, lakes, and the like are incorporated as part of the drainage system, the designing engineer shall designate a minimum floor elevation.
(b) The minimum elevation shall be such that flooding shall not result from a 100-year flood and in accordance with the town's Flood Damage Prevention Chapter.
(5) Ditches.
(a) Open drainage ditches (a ditch is a waterway with side slopes 3:1 or steeper) shall not be permitted in the town.
(b) Existing ditches above the OCRM critical line shall be piped, filled, and regraded, when required by the Planning Commission based upon staff's recommendation.
(6) The Subdivider/developer shall be responsible for providing positive drainage for each lot individually as a part of the project development.
(a) Individual lot drainage shall be detailed on the construction plans and shall include, but not be limited to, yard drains for lawn areas, side and rear yard features to include swales and a piped system of inlets and other items necessary for the removal of water from the lot.
(b) It is not necessary to provide pad- ready lots during subdivision development.
(c) However, positive drainage is required for each lot prior to plat approval.
(d) Rear yard pipe systems are required and shall be designed to accommodate ten- year storm events.
(e) Inlets shall be designed to minimize debris clogging.
(f) To promote responsible land development practices, it shall be unlawful to:
1. Place fill along the outer perimeter of any subdivided property;
2. Alter the ground above the existing elevation in a manner that solely benefits the stormwater management of the subdivision and directly impacts adjacent properties by way of casting surface water onto or impeding the flow of surface runoff from another property; or
3. Place fill or impediments to surface flow within ten feet of any pre-existing perimeter lot line abutting adjacent properties.
4. The provisions of this division shall not apply to the following:
a. Existing subdivided lots recorded with the Office of the Register of Deeds for Charleston County prior to the date of the adoption of the ordinance enacting this division (Ord. 240004);
b. Internal lot lines to the subdivision which conform to a unified grading plan;
c. Continuation of a planned development with the same ownership as in a phased development;
d. When fill is used to ensure positive flow through a low-lying areas without redirection of the pre-determined flow path; and
e. When fill is added to meet minimum road elevation when the design incorporates stormwater management improvements to capture roadway runoff and all offsite runoff to prevent impacts to adjacent properties.
(7) Upon application for Preliminary Plat, the applicant shall submit such inspection fee for drainage systems based upon a per linear foot of roadway centerline as may be established by Town Council.
(8) Filling, grading, permanent erosion control features, and installation of drainage systems for lots and streets shall be provided prior to Final Plat approval.
(9) Prior to the termination of the two-year warranty period, an inspection of the piped drainage systems may be performed by a Town inspection official.
(a) This inspection will be to ensure that all piped drainage systems are free and clear of silt, sediment, and trash associated with construction activities prior to acceptance by the Town.
(b) Under the Two-Year Warranty included in § 7 of Appendix B, the developer is noted as responsible for maintaining free and clear drainage systems during the warranty period.
(c) Therefore, the developer shall be responsible for:
1. Maintaining clean drainage systems during the two-year warranty period; and
2. All costs associated with cleaning the piped drainage systems.
(10) Failure to respond to cleaning requests shall result in the cessation of future phase work until such a time as the drainage system is cleared and accepted by the inspection official.
(B) Flood areas. If the area being subdivided, or any part thereof, is located within the boundary of a designated flood hazard area or coastal high hazard area, as delineated by the Flood Insurance Rate Maps of the town, adequate plans and specifications for protection from flooding shall be provided as herein required and as may be specified by the Planning Commission.
(1) Land subject to flooding.
(a) Any plat of a subdivision that contains land subject to flooding shall be accompanied by evidence that no appreciable expansion of the area subject to flooding would result from the proposed development of the land being subdivided, and that the proposed development will be adequately protected from inundation without appreciable interference with the flow of any watercourse or into an impounding basin.
(b) All such evidence, including surveys and specifications, shall be submitted with the preliminary plat and no plat shall be approved in the absence thereof.
(2) In no case shall any fill, levee, or other protective works be approved unless sufficient compensating adjustments of waterways, ditches, or impounding basins are made to prevent any appreciable expansion of flood hazard areas.
(3) Street elevations.
(a) No street shall be approved that would be subject to frequent inundation of flooding based on a five-year storm event.
(b) The minimum street elevation shall be 6½ feet.
(c) Low points in streets shall be designed to accommodate a 50-year flood frequency without stormwater over-topping the curb.
(4) Floor and mechanical system elevations.
(a) Minimum floor and mechanical system elevations shall be shown on the construction plans and plats for all lots.
(b) These elevations shall be either the elevations established by the Flood Damage Prevention Chapter in special flood hazard areas, or the elevations determined by the designer in conjunction with the project development.
(C) Erosion, runoff, and sedimentation control.
(1) Every subdivision shall be served by storm drainage facilities, including drain sewers, catch basins, culverts, impoundment ponds, and other facilities required to insure that the waters draining from the developed property, including that drainage from the property in its natural condition and that increased or diverted by reason of the development, shall not be delivered to the area drainage facilities in such volume or rate as to overtax the capacity of the facilities and cause flooding conditions in the event of a 100-year precipitation condition. All such drainage facilities shall be designed and installed in accordance with the regulations of the town.
(2) Engineering and construction on any land within the town shall be carried out in such a manner as to protect neighboring persons and property from damage or loss resulting from excessive stormwater runoff, soil erosion, or deposition upon private property or public streets of water-transported silt and debris.
(3) Drainage plans and studies shall be submitted for review by the Department of Engineering and Development Services.
(a) These plans and studies shall be prepared by a design professional currently registered to practice in the state, with his or her stamp affixed.
(b) Profile for drainage pipes shall be included and will show existing ground line, finished grade, drainage structures, intersecting sanitary sewers and other intersecting underground utilities.
(c) Existing and proposed utility lines crossing the storm drain pipe shall be shown in the profile on their proper elevation.
(d) Pipes that conflict at crossings shall be adjusted with adequate details included.
(e) Grate inlets are not permitted in the right-of way of public streets.
(D) Coordination required.
(1) All land development activity must comply with the applicable sections of the latest edition of the South Carolina Stormwater Management and Sediment Control Handbook for Land Disturbance Activities.
(2) A stormwater approval letter from SCDHEC-OCRM shall be submitted to the town before construction approval or a Clearing and Grading Permit will be issued.
(3) No project shall receive final approval or acceptance by the town nor will a certificate of occupancy be issued for commercial projects, until such time as a statement is received from the project engineer certifying that construction is complete and in accordance with approved plans.
(E) 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, for example, pipes carrying storm water 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, or his or her designee, for those not at street locations, provided complete data is submitted.
(h) Non-corrosive pipes shall be used at all other locations, subject to a request with complete data as approved/disapproved by the Director of Planning and Development, or his or her designee.
(2) Stormwater control facilities.
(a) A combination of storage and controlled release of storm water runoff shall be required for all development and construction that create the peak rate of runoff from the site, unless the designer certifies that no downstream damage will occur due to the increase runoff.
(b) Certifications are required at the time of approval.
(c) The peak release rate of storm water from all developments where detention is required shall not exceed the peak storm water runoff rate from the area in its natural undeveloped state for all intensities up to and including the 100-year frequency.
(d) The same methodology must be used for calculating the pre-development and post- development rates of runoff from a site.
(e) 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.
(f) Reservoir routing techniques shall be used for designing all detention facilities for which the drainage area is three acres or larger.
(g) 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.
(h) All stormwater storage facilities within single-family residential subdivisions in the town are to be privately owned and maintained; provided, however, the owner(s) thereof shall grant to the town a perpetual non-exclusive easement that allows for public inspection and emergency repair, in accordance with the terms of a pond maintenance agreement, which is required hereunder and found in § 15 of Appendix B.
(F) Drainage certifications. Design certifications, as set forth in § 14 of Appendix B, shall be furnished with all proposed construction.
('81 Code, §§ 154.36, 154.43, 156.005, 156.011, 156.012) (Ord. passed 6-9-92; Am. Ord. passed 2-9-93; Am. Ord. 93066, passed 2-8-94; Am. Ord. 99012, passed 4-21-99; Am. Ord. 99016, passed 6-8-99; Am. Ord. 00035, passed 7-11-00; Am. Ord. 00063, passed 1-9-01; Am. Ord. 02045, passed 7-9-02; Am. Ord. 06016, passed 4-12-06; Am. Ord. 07015, passed 6-12-07; Am. Ord. 13027, passed 6-11-13; Am. Ord. 16082, passed 10-11-16; Am. Ord. 21041, passed 5-11-21; Am. Ord. 24004, passed 2-15-24)