§ 153.042  DRAINAGE PLAN REQUIREMENTS.
   (A)   Storm drainage.  A drainage system shall be designed and constructed to provide for the proper drainage of the surface water and the drainage area of which it is a part, to permit the unimpeded flow of natural watercourses, to provide for water quality control, and where necessary, water quality improvements. The information, plans, specifications, certifications and design evaluations described in the drainage master plan, preliminary plat checklist, or site plan checklist, whichever is appropriate, shall be submitted to the Town Engineer. Checklists shall be provided by the Department of Engineering and Development Services, and may be amended from time to time.
      (1)   Design and construction standards.
         (a)   Drainage systems shall be designed by a licensed engineer and constructed by the subdivider/developer consistent with the design principals and standards contained herein, described in the Town BMP manual and established by SCDHEC-OCRM.
         (b)   In all cases, drainage systems shall conform to cross sections, dimensions, erosion control measures and grades as shown on the approved construction plans.
      (2)   Watershed.  The watershed containing the site shall be shown, and shall be extended to a natural watercourse or publicly maintained drainage facility that is adequate to receive the storm drainage.
      (3)   Connection to public storm sewers.  Where adequate existing public storm sewers are reasonably accessible, as determined by the Director of Engineering and Development Services or his or her designee, the system proposed for the land being subdivided is required to be connected thereto.
      (4)   Ditches and swales.
         (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 SCDHEC-OCRM critical line shall be piped, filled and regraded, based upon staff recommendations.
         (c)   SWALES are grass-lined drainage ways with 10 to 1 side slopes, and are encouraged as a means to manage stormwater in a manner that is protective of water quality.
      (5)   Positive drainage for each lot.  The subdivider/developer shall be responsible for including as part of the drainage plan provisions for 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 standard 6490 modified grate or an equivalent designed to minimize debris clogging.
      (6)   Inspection fee.  Upon application for preliminary subdivision 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.
      (7)   Permanent erosion control features and drainage systems for streets and lots.  Filling, grading, permanent erosion control features, and installation of drainage systems for lots and streets shall be provided prior to final plat approval.
   (B)   Erosion, runoff and sedimentation control.  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.
   (C)   Plan requirements.  Drainage plans shall include:
      (1)   The information, plans, specifications, certifications and design evaluations described in the drainage master plan, preliminary plat checklist, or site plan checklist, whichever is appropriate.  Checklists shall be provided by the Engineering and Development Services Department, and may be amended from time to time;
      (2)   No grate inlets in the right-of way of public streets;
      (3)   A stormwater pollution prevention plan as described in § 153.043;
      (4)   A permanent maintenance plan for all stormwater management facilities; and
      (5)   Design components that meet the following special situations, as appropriate and as described in S.C. Regulation 72-304, Appendix B, Chapter III, C.3.XIII.
         (a)   All projects must detain the greater of one inch of runoff from the entire site, or one-half inch of runoff from the developed portion of the site; and must demonstrate the rate of release extends for 24 hours and does not exceed the pre-development flow rate.
         (b)   For projects located within 1,000 feet of shellfish beds, 1-1/2 inches of runoff from the developed portion of the site must be retained onsite.
         (c)   For bridges that traverse saltwater and/or critical areas.
            1.   No treatment is necessary for runoff from bridge surfaces spanning SA waters. This runoff can be discharged through scupper drains directly into surface waters. However, the use of scupper drains should be limited as much as feasibly possible.
            2.   If the receiving water is either ORW or SFH, then the stormwater management requirements shall be based on projected traffic volumes and the presence of any nearby shellfish beds. The following matrix lists the necessary treatment practices over the different classes of receiving waters.
            3.   The average daily traffic volume (ADT) is based upon the design carrying capacity of the bridge.
 
Water Quality Class
Average Daily Traffic Volume
0-30,000
Greater than 30,000
ORW (within 1,000 feet of shellfish beds)
***
***
ORW (not within 1,000 feet of shellfish beds)
**
**
SFH (within 1,000 feet of shellfish beds)
**
***
SFH (not within 1,000 feet of shellfish beds)
**
**
SA
*
*
 
*** The first inch of runoff from the bridge surface must be collected and routed to an appropriate stormwater management system, or routed so that maximum overland flow occurs encouraging infiltration before reaching the receiving body. Periodic vacuuming of the bridge surface should be considered.
** A stormwater management plan must be implemented, which may require the over-treatment of runoff from associated roadways to compensate for the lack of direct treatment of runoff from the bridge surface itself. Periodic vacuuming should be considered. The use of scupper drains should be limited as much as feasibly possible.
* No treatment is required. The use of scupper drains should be limited as much as feasibly possible.
         (d)   For golf courses adjacent to receiving water bodies.
            1.   Minimum setbacks from the receiving water body of 20 feet for all manicured portions of the golf course (fairways, greens and tees) are required, unless other acceptable management techniques are approved and implemented to mitigate any adverse impacts.
            2.   All drainage from greens and tees must be routed to interior lagoons or an equivalent stormwater management system.
            3.   To prevent the conversion of the stormwater system to critical area and to maintain positive drainage at high tides, all outfalls from the lagoon system must be located at an elevation above the critical area (if the discharge is to critical area),  and above the normal water elevation a distance to allow for storage of the first inch of runoff. The volume that must be stored shall be calculated by multiplying the area of all the greens and tees by one inch.
            4.   No greens or tees shall be located on marsh hummocks or islands, unless all drainage can be conveyed to the interior lagoon system or to an equivalent onsite stormwater management system.
            5. Stormwater impacts to freshwater wetlands shall be limited by providing minimum-20-foot buffers, or an accepted alternative, between manicured areas (fairways, greens and tees) and the wetlands. This minimum buffer must be increased if land application of treated effluent is utilized in the area.
            6.   An integrated pest management system designed in accordance with current best technology practices must be employed on the course to limit the application of chemicals that, if over-applied, may leach into the ground and adjacent surface waters.
            7.   In accordance with South Carolina Department of Health and Environmental Control requirements, a two-foot separation must be maintained between the surface of the golf course and the ground water table where spray effluent is applied.
            8.   The normal ground water elevation must be established by a registered engineer or soil scientist.
            9.   All projects within 1,000 feet of shellfish beds must retain the first 1-1/2 inches of runoff.
            10.   If spray effluent or chemicals are applied to the turf via the irrigation system, all spray heads must be located and set so as to prevent any aerosols from reaching adjacent critical areas.
         (e)   For mines and landfills.
            1.   When mining or landfill projects are located within one-half mile of receiving water bodies, pumping of ground water from sediment basins must be done with floating intakes only. Pumping of these basins must cease whenever the water levels come to within two feet of the pond bottom.
            2. In addition, landfill planning must be designed on a comprehensive site basis for stormwater management and sediment/erosion control to include management practices for each separate cell as it is phased into the landfill.
(Ord. 07045, passed 8-14-07; Am. Ord. 13027, passed 6-11-13; Am. Ord. 21039, passed 5-11-21)