(A) Design methods and criteria. The design professional may use generally accepted design procedures to determine runoff quantities. The Modified Rational Method or the Modified Soil Conservation Service Method is typically used by the Town of James Island Public Works Department to review submitted designs. The design professional shall submit data showing the drainage basin, the location of areas of differing imperviousness, the runoff factors for each zone of imperviousness, and the data for rainfall and time factors used in the determination of peak runoff rates. The design should consider seasonal high water table elevations. The design must take into consideration the channeled runoff. The design shall comply with water quality standards established by federal, state, and local regulations.
(B) Approved outfalls.
(1) All street and development drainage shall be discharged into either:
(a) A tidal stream of adequate size;
(b) A running stream with continuous flow (freshwater creek or river) of adequate size;
(c) An existing drainage way (pipe, ditch, or canal) of adequate size for which there is an adequate easement, and which is maintained by the Town of James Island or another responsible public agency; or
(d) A wetland or critical area capable of receiving the discharge without it negatively impacting any property adjacent thereto.
(2) The outlet or receiving area must be of sufficient size and grade, etc. to receive the anticipated quantity of runoff from each contributing drainage basin along the route of the outlet in addition to the anticipated increase in quantity of runoff from the subject development. Where the proposed outlet ends, or near the South Carolina DHEC-OCRM Critical Line, the construction plans shall clearly show that there is an outlet of adequate size and slope to the final point of discharge. The entire length of the outlet, except approved portions within the South Carolina DHEC-OCRM Critical Area, shall be constructed on dedicated drainage easements with cleared, stable maintenance shelves for continued maintenance of the drainage way. The entire development's outlet, from sources of collection of runoff to final point of discharge, shall be evaluated by the design professional and upgraded by the developer as deemed necessary by the Public Works Director at no cost to the town.
(C) Drainage and other work involving another public agency. When drainage is discharged into a drainage way maintained by, or intended for maintenance by, a public agency other than Town of James Island Public Works, or if work is to be done within the road right-of-way of a public agency other than the Town of James Island, written approval must be obtained from that public agency. A copy of this approval must be furnished to the Public Works Director prior to commencing work.
(D) Rainfall determination.
(1) The peak runoff rates shall be determined based on the storm time/rainfall rate following a pattern Type-III Rainfall Hydrograph as defined in the Soil Conservation Service Manual TR-55.
(2) The design recurrence interval shall be taken to be 10 years for the collector system within the subject development, 25 years for any channeled drainage flowing through the development, and 25 years for any primary outfall drainage way from the development.
(3) Average return frequencies (24 hour) are defined as:
(a) 1-year: 3.8 inches;
(b) 2-year: 4.6 inches;
(c) 5-year: 5.9 inches;
(d) 10-year: 7.0 inches;
(e) 25-year: 8.0 inches;
(f) 50-year: 8.9 inches;
(g) 100-year: 10.2 inches; or
(h) As revised by the South Carolina State Climatology Office.
(E) Detention pond design criteria.
(1) The peak release rate of stormwater from all developments where detention is utilized shall not exceed the peak stormwater runoff rate from the area in its pre-developed state for all intensities, up to and including the 25-year frequency storm.
(2) The design professional shall provide information required to support the pond design shown. Calculations shall be signed and sealed by the design professional and shall include stage-storage volumes, areas, depths, summary information (to include pond crest information, outfall flow rates, and computer program information), etc. Pond design shall include the proposed static water level and the two-year, 10-year, and 25-year frequency storm crest elevations. These elevations shall be shown on the drainage plans, as well as the proposed 1' contour lines from the pond bottom to 1' above the 25-year frequency storm crest elevation.
(3) Static water elevation in ponds shall not be less than 3' below the finished centerline elevation of the road in the general area and ponds shall not encroach on the road right-of-way at any time.
(4) The design should provide outfall structures that are low maintenance in nature as approved by the Public Works Director.
(5) The design shall provide an emergency spillway. The location, structure, invert elevations, and outlet of the emergency spillway shall be shown on the construction plans. The design shall address the 50-year and 100-year frequency storm flow rates through the emergency spillway to an approved outfall (drainage easements will be required for the emergency outfall route). Projected flood limits on downstream properties for these storm flow rates are also to be shown on the drainage plan.
(6) All detention facilities that require public maintenance shall be provided with access for maintenance via a constructed 30' wide drainage right-of-way from the nearest road to the facility. Plans shall include a barrier structure at the primary road right-of-way to limit vehicular access. The detention facility or pond shall include a 20' wide cleared shelf with easement around the top perimeter of the facility for maintenance access.
(7) For all publicly maintained detention facilities, a public drainage easement shall be dedicated over the entire facility, including the outfall and emergency spillway. For privately-maintained detention facilities, a private drainage right-of-way must provide access to the pond, the pond's outfall, and all areas defined by the pond maintenance plan.
(8) Dry ponds shall be designed with 3:1 (horizontal: vertical) side slopes and shall drain dry within 72 hours.
(9) Retention or wet ponds shall be designed with 3:1 (horizontal: vertical) side slopes above the static water level and 2:1 (horizontal: vertical) side slopes below the static water level; shall have a minimum depth of 6'; and shall be stocked with mosquito larvae-eating fish. Town of James Island Mosquito Control should be consulted regarding fish stocking information.
(F) Infiltration/exfiltration drainage systems.
(1) The Town of James Island will discourage any drainage system that is dependent solely upon infiltration/exfiltration of stormwater runoff for the proper functioning of the system.
(2) Any such system shall be approved only by specific authority of the Public Works Director shall approve any such system.
(3) The Town of James Island will not maintain or be responsible for any infiltration/exfiltration facilities. Any such facility shall be maintained by a responsible entity of, or for, the development, and arrangements for proper and perpetual maintenance shall be guaranteed to the Town of James Island prior to the approval of the development.
(G) Additional road drainage requirements.
(1) The Public Works Director will independently evaluate any storm drainage piping (other than cross-line pipes) designed to be placed at a depth or location such that the repair of that culvert would adversely impact the flow of vehicular traffic. Culvert piping crossing under the street shall be at an angle of not more than 30 degrees from the perpendicular of the street.
(2) Pipeline discharge capacities shall exceed maximum peak runoff rates. Ponding or head pressure shall not be considered in pipeline size determination. Computations for all drainage way size determinations shall be provided to the Public Works Director. Drainage ways located laterally off of the street should be piped to the back lot line or for the first 150' from the street right-of-way line, whichever comes first.
(3) Where a drainage outlet pipe extends into a lake or other similar outlet, riprap shall be placed under and around the end joint or joints of pipe as needed and on slopes at the end of the pipe.
(4) Minimum longitudinal slope shall be 0.4% except where specifically approved in writing by the Public Works Director.
(5) Submerged piping or partially submerged piping shall not be used unless where specifically approved in writing by the Public Works Director. Submerged pipe systems shall require a means of accessing the submerged pipe for maintenance purposes.
(6) Catch basins or junction boxes shall not be located within the radius portion of street intersections. No manhole covers or water valves will be located within the curb or gutter area or within the paved area of the roadway.
(7) To allow for backfill and compaction operations, 2' minimum horizontal clearance between pipelines or structures shall be provided. Reinforced concrete or riprap of a material and gradation approved by the Public Works Director shall be placed at the ends of all culverts, bends, or junction points in drainage ways and/or other locations as determined by the Public Works Director (see details in Article A.6.).
(H) Additional drainageway requirements.
(1) Where drainage is directed into an existing ditch, canal, or tidal stream by use of an open ditch or pipeline, the elevation at the bottom of the existing ditch, canal, or tidal stream at the point of entry, and approximately 100' upstream and downstream, shall be shown on the drainage plans. In addition, the elevation of the bottom of the inlet ditch or invert elevation of inlet pipe at the outlet end shall be shown along with the bottom width of the existing canal or tidal stream.
(2) Where drainage ways are piped, catch basins shall be provided as required to appropriately receive and discharge incoming drainage. In no case shall the catch basins be more than 300' apart.
(3) Junction boxes with stubs shall be constructed at both ends of cross-line pipes for cross-ditches, at the outlet end of cross line pipes at outlet ditches, and at other locations as appropriate. Reinforced concrete pipes of the required sizes shall be used for all inlet and outlet stubs. Stub pipes of the required sizes shall be used to convey the street ditch drainage into the junction boxes and the inlet invert of the stub shall normally be installed approximately 0.3' below the street ditch grade. Stub pipes shall be extended a minimum of 20' from the junction box to provide maintenance vehicle access to maintenance shelves along outfall ditches.
(4) For minor swale ditches of 1' depth or less along side or rear lot lines that only drain a small interior area and where street drainage is not involved, a drainage easement not less than 15' in width may be utilized.
(I) Drainage and erosion control structures and materials.
(1) All work, materials, methods, and equipment, unless otherwise specified herein, shall be in accordance with the requirements of the CTC (SS).
(2) All concrete pipes shall conform to ASTM Specifications C-76, Class III, Wall B. Joints shall conform to ASTM, C-443. Jointing materials shall be all-weather preformed joint sealant.
(3) Metal pipes shall be approved by the Public Works Director and shall be installed as per the recommendations of the manufacturer for their intended use. For metal piping subject to saltwater exposure, only approved aluminum piping may be used.
(4) Other piping materials shall be considered. Specifications should be submitted to the Public Works Director along with the design data and construction plans showing the specific intended use. Materials and methods approved by the SCDOT will be considered upon submittal of SCDOT documentation and recommendations pertaining to the use of such materials.
(5) Ample cover shall be provided to properly protect pipelines during construction as well as for designed usage. Minimum allowable cover for pipe at subgrade shall be not less than 6" for paved areas, and 12" for unpaved areas and in no case less than that recommended by the manufacturer.
(6) Minimum cover for other materials and usages shall be considered at the time of submittal of construction plans to the Public Works Director.
(7) All structures shall be shown clearly on the construction plans with details to show all lines, grades, elevations, joints, reinforcing, materials of construction, etc. All appropriate specification data shall be shown on the construction plans.
(8) Junction boxes, curb inlets, outfall boxes, or any other enclosed drainage structure exceeding 4'6" in depth shall be constructed with interior step fixtures.
(9) Precast concrete structures will be considered. Design and specification data should be submitted to the Public Works Director along with the design and construction plans showing the specific intended use.
(10) A performance bond guaranteeing restoration work must be posted with the Town of James Island and remain in effect for a period of two years from the date of acceptance of the restoration work (date of release by the Director of the Department of Public Works). This bond must be in a format approved by the Town Attorney and in accordance with terms and conditions of §§ 153.300 et seq.
(J) Open channel baffles. Baffles of an approved design shall be constructed in open channels where the gradient is 0.70% or greater and shall be spaced as shown:
Gradient (percent) | Spacing (feet) |
0.70 to 0.99 | 100 (max) |
1.00 to 1.49 | 75 (max) |
1.50 to 1.99 | 50 (max) |
2.00 to 3.00* | 50 (max) |
* Where the gradient is 2% or greater for an open channel, additional easement width equal to the maximum depth of the channel shall be provided. For a gradient greater than 3%, the channel shall be piped unless otherwise approved by the Public Works Director. | |
(K) Easements for storm drainage facilities.
(1) Drainage easements shall be provided for all drainage facilities intended to be included in the maintenance program of the Town of James Island. The easement shall provide adequate space for access to the facility; adequate space for the operations involved in cleaning, repairing, reconstructing, material storage and dewatering, and hauling materials to or from the area; adequate space for turning and maneuvering of the equipment; and adequate space for the sloshing and splashing of the materials being handled.
(2) The portions of the easement intended for equipment operations shall have suitable foundations to support the maintenance equipment and shall be graded to drain the working area but not graded to slopes or elevations causing difficulty in the operation of the maintenance equipment. Maximum lateral grade for areas in which draglines may be used in the maintenance operations shall be 5%.
(3) Minimum drainage easement widths shall be no less than the following:
Required Drainage Easement Widths for Piped Drainageways | ||
Pipe Size (Maximum) | Minimum Depth to Invert | Width of Drainage Easement |
18" > and < =24" | 5.0' | 12'-16' |
24" < and < 42" | 5.0'-7.0' | 16'-20' |
42" < and < 54" | 7.0' | 20'-24' |
54" < and < 72" | 7.0'-9.0' | 24'-30' |
(4) For depths greater than shown, larger pipe sizes, or multiple lines of pipe, additional easement width, as required by the Public Works Director, shall be provided. Required drainage easement widths for open ditches shall be as shown on details in Article A.6. The side slope of an open ditch shall not exceed 2:1, and, subject to soil stability, may be required to be flatter.