(A) Generally.
(1) Water quality (SCM) and water quantity (SWM) control measures shall be designed and installed for development in watersheds as defined in the city’s Zoning Ordinance to achieve 85% removal of Total Suspended Solids (TSS) and the required flow reduction, if required.
(2) All new development, redevelopment and post construction measures shall meet or exceed the standards as stated in the North Carolina Division of Water Quality Storm water Best Management Practices Manual (NCDENR DWQ SCM Design Manual), latest edition, or as modified herein.
(3) Storm drainage easements must encircle the water quality facility/SCM and/or SWM facility. Easement must be shown ten feet from the top of the berm of the facility or ten feet from the toe of slope for the facility’s embankment, whichever is greater. A storm drainage easement must be shown for any pipe system or channel leading to or draining from the facility. SCM/SWM facilities must be enclosed within easements, privately owned and facilities maintained by a HOA or other approved entity.
(4) A minimum 15-foot wide SCM maintenance access easement must be shown from the street right-of-way to the SCM facility.
(5) High density projects shall control and treat the difference in storm water runoff volume leaving the project site between pre- and post-development conditions for, at a minimum, a one-year, 24-hour storm event. Runoff volume drawdown time shall be a minimum of 24 hours, but not more than 120 hours.
(6) All built upon area shall be a minimum of 30 feet landward of perennial and intermittent surface waters.
(7) The submission of the storm water management plan, storm drainage system plan, and the best management practices design plan shall include all calculations. The storm water detention information sheets shall be prepared, signed and sealed by a professional engineer licensed to practice engineering in the state. The SCM maintenance plan should be included in construction drawing.
(8) As-built certifications from a registered professional engineer or professional land surveyor are required after SCM and/or SWM construction is complete. Certifications shall be considered proof that the registered professional has inspected the SCM and/or SWM construction, and has verified all dimensions, widths, elevations and overall construction to assure the facility was constructed as per the approved plans.
(9) All SCM and/or SWM devices shall be maintained by the property owner or homeowner’s association. The devices shall be inspected and maintained by the developer/owner/HOA in accordance with the approved design plans. Costs for maintenance and repairs shall be borne by the property owner or homeowner’s association.
(10) High density projects shall control the peak storm water runoff discharged from the site for those areas discharging directly to a city storm drainage system or directly to a property not owned by the applicant of the project. The discharge shall not be any greater than the pre-development discharge based on a ten-year, 24-hour storm event. The emergency overflow shall be designed to discharge a 50-year, 24-hour storm event.
(11) The storm water management facilities shall be designed for ease of maintenance, access and aesthetic appearance. All SWM facilities are privately owned, operated and maintained by the property owner or HOA. The construction plans, plat and HOA documents shall include a detailed maintenance and inspection schedule for each proposed SCM and SWM facility.
(B) Drainage generally.
(1) (a) Drainage structures shall be sized using rational method for drainage areas less than 200 acres. Drainage area delineation maps, calculations used for drainage design, and spread calculations used for inlet spacing shall be submitted for review with construction plans.
(b) Detailed design requirements may be found in the manual NCDOT - Guidelines for Drainage Studies and Hydraulic Design. Drainage areas greater than 200 acres shall be designed by SCS Method, or other methods as approved by the city. Runoff coefficients for rational design shall be as listed below.
Recommended Runoff Coefficient Values
| |
Description of Area
|
Runoff Coefficient (C)
|
Drives, walks, roofs | 0.95 |
Gravel areas | 0.55 |
Lawns | 0.30 |
Parks and cemeteries | 0.30 |
Wooded | 0.25 |
Streets | 0.95 |
Commercial: | |
Office parks | 0.75 |
Shopping centers | 0.80 |
Industrial: | |
Heavy | 0.80 |
Light | 0.70 |
Residential (Including Streets): | |
Multi-family, attached | 0.70 |
Single-family (lot <20,000 sf) | 0.60 |
Single-family (lot >20,000 sf) | 0.50 |
(2) Storm drainage pipe and structure information shall be listed on each sheet of plan and profile drawings. This information shall include pipe diameter, material, grade, inverts, structure type and grate type. A drainage schedule shall be provided in a table format with corresponding pipe or structure numbers shown in the plan view (See sample in Appendix). Structure numbers shall be labeled in the plan view adjacent the structure, enclosed in a hexagon. Numbering shall begin at the outlet of the pipe system, increasing upstream, with no repeating numbers.
(3) Pipe systems or channels located on private property shall be the responsibility of the property owner or the homeowner’s association, not the city.
(4) RCP (minimum Class III) or HDPE N12 soil tight dual wall bell end pipe or equal is required for all drainage pipe. Pre-cast concrete pipe and joints shall be in accordance with ASTM C 443 using rubber (elastomeric) gasket seals which are in compliance with ASTM C 1619, Class C (unless otherwise noted). Joints shall be single offset bell and spigot type with profiled gaskets.
(5) Drainage pipes are to be placed along the curb line of curb and gutter streets. Pipes shall cross streets perpendicular to the street centerline. Off-street drainage systems shall intersect catch basins perpendicular to the street, not cut diagonally across lots, and generally follow lot lines. All structures shall have lids or grates and be accessible for maintenance (no blind boxes).
(6) All storm drainage easements beyond the public road right-of-way shall be shown on the plan sheet and labeled, “Private Storm Drainage Easement” (PSDE). Easement width shall be 20 feet minimum unless otherwise approved by the city and considered a Public Storm Drain Easement (SDE). Larger pipes may require wider easements.
(7) Storm drain outfalls shall be piped to the rear of any possible buildings and released in a natural drainage ditch, stream or positive outlet. It may be necessary to cross off-site properties and to obtain private drainage easements to ensure discharge into the natural water course. All costs shall be borne by the developer.
(8) Street catch basins shall be NCDOT 840.01 or city standard double catch basin with type E, F or G grates as appropriate (NCDOT 840.03). Solid wall, pre-cast concrete structures conforming to NCDOT 840.45 are acceptable (no waffle walls). All pipe openings in pre-cast structures must be cast or cored. Shop drawings must be submitted for approval for pre-cast structures prior to their installation.
(9) Yard inlet catch basins shall be NCDOT 840.05. Yard inlets shall be designed to have a maximum ponding depth of one foot at the ten-year storm event. Ten-year storm event shall be contained within a PSDE and be owned and maintained by the property owners sharing said easement.
(10) Base flood protection elevations on the affected lots and flood limit lines shall be shown on construction site plans. Flood elevations shall be obtained from FEMA data, or from calculations signed and sealed by a registered professional engineer if FEMA maps are unavailable.
(11) All drainage structures shall be accessible by removal of grates or manhole covers. Blind boxes are not permitted. Steps shall be installed in accordance with NCDOT standards.
(12) Low impact development design criteria shall be used including: conserving natural areas, impose disconnected impervious surfaces and promote infiltration practices.
(C) Storm drainage design.
(1) Minimum design frequency shall be 50-year for storm drainage systems carrying off-site drainage (cross drainage) under a thoroughfare street, 25-year design frequency for cross drainage on residential streets, and ten-year design frequency for incidental drainage. Flows shall be estimated assuming fully developed land use conditions. Estimated design flows shall be increased by a factor of 1.2 for 50-year designs, and 1.1 for 25-year designs.
(2) The minimum pipe diameter shall be 12 inches. A minimum velocity of 2.5 fps shall be maintained in all drainage lines.
(3) When RCP is used to discharge to a stream, an eight-foot joint must be used at the lower end (nearest the stream) because of possible undermining.
(4) Minimum cover for RCP CL III pipe or HDPE N12 is two-foot. Pipe diameters 30-inches and larger shall be deep enough to accommodate drainage structures. Class IV and Class V RCP may be considered for a reduction in cover.
(5) Drainage lines shall have at least a one-foot vertical distance when crossing water and sanitary lines. A conflict crossing chart shall be provided on all plan sheets.
(6) The grade of the street and flow will determine acceptable catch basin placement and spacing. Spread shall not exceed eight feet measured from the face of the curb, based on a rainfall intensity of four inches per hour. Inlet spread calculations shall be submitted in a format similar to the schedule as shown in the Appendix.
(7) Adequate drainage control structures shall be provided at all street intersections, usually upstream of the intersection. Drainage shall be captured prior to curb returns. Inlets shall not be placed in the curb radius.
(8) Flared-end-sections will be used on the inlet/outlet end of any pipe receiving or discharging surface water (i.e., to or from a ditch, channel or creek). Head walls are required for pipes larger than 36 inches in diameter. Rip rap aprons with a fabric liner will be used at outlets as necessary to dissipate flows. Rip rap shall be placed on the stream or channel to a height of one foot above the top of the culvert tapering down to the apron. Rip rap shall be designed per NCDENR standards using the ten-year storm event.
(9) Pipe inlets shall be checked for inlet and outlet headwater control. Hw/D ratio shall not exceed 1.2 for pipe inlets.
(10) Drop in elevation at structures (invert in elevation minus invert out elevation) shall be a minimum of 0.20 feet. At change in pipe sizes, pipe crowns shall be matched.
(11) Minimum cover for Class III concrete pipe shall be two feet within the public right-of-way.
(12) Storm drainage outfalls shall be designed to dissipate concentrated flows, limit discharge velocity and discharge to a positive outlet.
(D) Construction.
(1) Storm drainage materials, installation and construction standards shall meet State Department of Transportation Standard Specifications for Roads and Structures, and Roadway Standard Drawings as revised July 2006, or as subsequently amended, except where standards in these guidelines are more stringent.
(2) Contractors will be responsible for a one-year warranty on the storm drainage installed beginning the date final acceptance is given by the city.
(3) The city will not maintain any newly installed infrastructure until final acceptance of the project is granted by City Council. The one-year warranty period will not begin until final acceptance. The developer may not request final acceptance until all deficiencies have been satisfactorily addressed as determined by the city and a set of as-built drawings certified by the engineer of record have been submitted, reviewed and accepted by the city. It shall be the responsibility of the developer to request the final acceptance.
(Ord. passed 9-27-2021) Penalty, see § 153.999