(A) Applicable regulations and standards. Stormwater improvements shall meet all applicable county, state and federal regulations. Those regulations require that the stormwater system meet water quality and water quantity standards and that no property be adversely affected by the stormwater runoff from the development. In addition to the standards of this section, other applicable stormwater regulations include the following:
(1) Town and/or county regulations:
(a) Stormwater management and sediment control ordinance;
(b) Standard specifications for road and drainage systems; and
(c) County Transportation Committee’s standard specifications for roadway construction.
(2) State regulations:
(a) State Stormwater Management and Sediment Reduction Act, being S.C. Code §§ 48-14-10 et seq.; and
(b) State Coastal Zone Management Act, being S.C. Code §§ 48-39-10 et seq.
(3) Federal regulations: EPA stormwater regulations and the Clean Water Act, § 404, being 33 U.S.C. §§ 1344. Of particular interest to most developers is the requirement to obtain a permit for land disturbance.
(B) Wetlands. It is the responsibility of the developer to locate and to protect wetlands (saltwater and freshwater). No development affecting any wetland area can be approved by town without prior approval by appropriate wetlands protection agencies, including the U.S. Army Corps of Engineers.
(C) Drainage basin consideration.
(1) Careful consideration of the entire drainage basin must be made when designing stormwater facilities for a given development.
(2) Runoff quantities from any land area depend on many factors including land area, slopes, soil types, vegetation, channelization and existing development, all of which must be considered by the stormwater designer.
(3) Even though the development may be a minor segment of the entire basin, the designer must consider the runoff from the development itself, any upstream property whose runoff enters the development and also any downstream property may be affected by the runoff from the development.
(4) Runoff from upstream properties can be determined by the existing conditions. Any future development/redevelopment of those properties will be addressed at the time that development/ redevelopment occurs. Runoff onto downstream properties include the total runoff quantity from the development including any runoff from upstream properties.
(D) Stormwater runoff.
(1) Stormwater runoff quantities are determined primarily by the amount of rain that falls during any rainfall event. For the purposes of design, the design storm shall be the ten-year, 24-hour rainfall event, SCS Type III distribution and a rainfall intensity of 6.8” in a 24-hour period. For existing channels passing through the development from upstream, the design storm is a 25-year, 24-hour rainfall event, SCS Type III distribution and a rainfall intensity of 8.0” in a 24-hour period.
(2) Unless special detention facilities are provided, developers are required to provide facilities designed for peak rate flow. That is, the pipe channels must be large enough to handle the maximum rate of flow so that there is no overflow at any time.
(3) Developers shall provide stormwater detention facilities in accordance with all application federal, state and county regulations. The rate of flow shall equal pre-development rates.
(E) Types of drainageways. The following types of drainageways shall be allowed:
(1) Pipe structures. Pipe structures may be round, rectangular or oval. Pipe material shall be appropriate for the situation and comply with the County Transportation Commission standard specifications and the town’s standard specifications for road and drainage systems.
(2) Earth channels. Earth channels are usually trapezoidal shaped, must be 2:1 minimum side slopes. All disturbed areas shall be vegetated.
(3) Swales. Swales must be 5:1 minimum slope, with all disturbed areas vegetated.
(4) Lined channels. Channels shall be lined with rock or other approved material.
(5) Sheet flow. Sheet flow shall be acceptable only where routine maintenance is not required, such as discharge into a designated wetland area.
(F) Drainage easements.
(1) All new developments shall have adequate drainage outfalls. Drainage easements must be dedicated to use of the public, if maintenance will be conducted by a public agency or private entity, such as an approved Homeowner’s Association.
(2) One purpose of required drainage easements is to allow access to drainage facilities by maintenance crews and equipment. The width and length of the easements must be of sufficient dimension to allow space for appropriate construction personnel and equipment to maneuver and to inspect, mow, regrade, reshape and clean out the drainage easement. Although the land area designated as easement is the property of the landowner, the landowner may not use it in any way that adversely impacts or restricts access or maintenance by the maintenance entity.
(3) In situations where the developer is required to provide drainage easements, but is not required to construct the drainageway (such as when a tract already having road frontage is subdivided), the easement area provided shall:
(a) Be of sufficient width to allow future construction of the drainageway (a dedication of a temporary construction easement in addition to the permanent drainage easement is acceptable and may be required);
(b) Be of sufficient width to provide a channel with bottom width to carry the stormwater (three-foot minimum), 2:1 side slopes, plus 20-foot shoulder on one side, plus five-foot shoulder on the other side (both sides must have 20-foot shoulders where top bank to top bank channel width exceeds 15 feet);
(c) Be free of obstructions (including trees) that would prevent the construction or maintenance of the drainageway with heavy equipment; and
(d) Be located so that there is continuity with the drainageway/existing drainage easements upstream and downstream. While it is not required that the easement be located on or adjacent to a property, that method is preferred since maintenance activities can be disruptive and somewhat unsightly.
(G) Drainage rights-of-way. Drainage rights-of-way are similar to street rights-of-way. The land area is not included in any tax lot and the developer/lot owner does not pay taxes on the designated area nor do they have any rights to use the land except as approved by the town and/or county. The developer designs and constructs drainage facilities within the designated area just as or he or she would normally do within a drainage easement area.
(H) Obstacles in drainageways. Existing obstacles (including trees, buildings, utility poles or transformer structures, items of historical significance) immediately adjacent to drainageways may be left in place provided that all of the following criteria are met:
(1) The drainage system or the obstacle cannot be feasiblely relocated;
(2) Adequate and safe easement space for maintenance by motorized equipment is provided;
(3) The drainage is not obstructed; and
(4) Drainage easement width is increased to accommodate obstructions.
(I) Detention storage. Detention is the temporary holding back of stormwater and releasing it at a controlled rate. The entity responsible for maintaining detention basins shall be identified at the time of preliminary plat.
(J) Dry basin. An excavated area, within the stormwater system, that is relatively dry at all times excepting during and immediately after a rainfall event. All slopes must be gentle (maximum 3:1) with vegetation cover for erosion control. Publicly dedicated basins shall not be used as a picnic or recreation area. Trees may be located within the basin except where the location would interfere with pond maintenance. Concrete or other lining is acceptable. The basin and outlets must be properly designed to meet computed quantity requirements. An emergency overflow spillway must be provided. Private parking lots, for example, may be designed to function in this capacity upon acknowledgment from the design engineer and applicant that flooding will occur.
(K) Wet basin (pond). Extending the detention basin down to groundwater level will provide maximum detention storage. The pond must be excavated to provide a minimum pond depth of six feet at time of lowest groundwater elevation (drought conditions); be populated by fish to ensure mosquito control; and meet all requirements of SCDHEC-Division of Mining and Solid Waste Permitting. The developer shall be liable for any claims resulting from ponded water, even if the town and/or county accepts the developer’s design.
(L) Soil information. Considerable information regarding town soils is available in a manual entitled Soil Survey of Charleston County, South Carolina prepared by the Soil Conservation Service (now called the Natural Resources Conservation Service), U.S. Department of Agriculture, current edition.
(M) Soil erosion control. Considerable information regarding soil erosion control is available in a manual entitled Erosion and Sediment Control Practices for Developing Areas prepared by the State Land Resources Conservation Commission. A certain acreage of land disturbing activities must be approved by the State Land Resource Commission.
(N) Maintenance.
(1) All developments whose streets and drainageways are maintained by the town and/or county must first be constructed (or upgraded) to meet the current standards outlined in this guideline.
(2) Any development required to meet town standards but intended to be maintained privately must be designed to meet all standards outlined in this guideline and further described in the county’s standard specifications for road and drainage systems. With approval of the decision-making body, construction of a portion of the facilities, such as removal of trees, can be deferred until such time as the town and/or county is asked to maintain the facilities. In all such situations, the developer is required to submit a maintenance plan showing specifically what work is to be accomplished, schedules, who is to accomplish that work and how payment for that work will be made. The potential owners of the development or portion of the development must be made aware of those factors through Homeowner’s Association bylaws and/or deed restrictions. The town and/or county will not perform any maintenance services until the roadway and/or drainageways have been brought up to current standards by others.
(Ord. 20-8, passed 12-7-2000)