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Home My WebLink About20081551 Ver 2_Stormwater Info_20090102 -::.n..cl (,1 / %wn of :Mooresvi{fe NORTH CAROLINA JOHN C. VEST, P.E. PUBLIC SERVICES POST OFFICE Box 878 MOORESVILLE, NORlH CAROLINA 28115 704-663-7282 December 31, 2008 [ii) r~ I'-~' r.:::;i nM r;a 1M ~LS~L5U\:lI.5~ JAN ~ 2009 Ms. Cyndi Karoly NC Department of Natural Resources Division of Water Quality 1650 Mail Service Center Raleigh, NC 27699 DENk.. WAl t:K QUAi..i 1'1 WEn.ANDS AND STORMWATER BRANCH Reference: DWQ Project #: 08-1551 Town of Mooresville Presbyterian Road Pump Station and Pipelines Dear Ms, Karoly, The Town of Mooresville commits to require on-site stormwater management measures (such as level spreaders, bioretention or constructed wetlands) for additional development on parcels in the service area for the proposed Presbyterian Road Pump Station and Pipelines project. These requirements are described in the Town's Land Development Standards as follows: The Town of Mooresville requires that development and re-development activities properly manage and control stormwater runoff rate, volume, pollutants, and erosion/sedimentation as necessary to protect and safeguard the environment, property, health, safety and welfare of citizens within the Town's jurisdiction, The Chapter 6: Drainage Design Standards of the Land Development Standards is included as an attachment for your reference, If you have any questions or require additional information, please contact me at (704) 799-4290, Sincerely, ~e.~ John C, Vest, P,E. Public Services Director Town of Mooresville cc: Kent Lackey, Black & Veatch DRAINAGE Drainage Design Standards General Storm Water Manaaement Reauirements 1. The Town of Mooresville requires that development and re-development activities properly manage and control stormwater flUloff rate, volume, pollutants, and erosion/sedimentation as necessary to protect and safeguard the environment, property, health, safety and welfare of citizens within the Town's jurisdiction. 2. The following are required stormwater management performance criteria for all land disturbing activity in the Town ofMooresville: a Stormwater treatment shall be designed to achieve average annual 85% Total Suspended Solids (TSS) removal and must apply to the volume of post-development runoff resulting from the first one-inch of rainfall. b, The stormwater runoff volume leaving the site post-development shall not exceed the stormwater flUloff volume leaving the site pre-development (existing conditions) for the 10ca12-year frequency, 24-hour duration storm event for all development. Runoff volume drawdown time shall be a minimum of 24 hours, but not more than 120 hours. Volume reduction may be achieved by detention, hydrologic abstraction, recycling and/or reuse, or any other accepted scientific method. c. The runoff rate leaving the site post-development shall not exceed the stormwater flUloff rate leaving the site pre-development (existing conditions) for the local I-year, 2-year, and 10-year, 24-hour duration storm events. d. Land disturbance within the stream channel of any perennial and intermittent stream shall be prohibited. e. Deed restrictions, protective covenants, restrictive language, or other measures shall be incorporated by the development to ensure that subsequent development activities maintain the development (or redevelopment) consistent with the approved plans. Restrictions shall also ensure that all water quality measures, such as vegetative buffers remain undisturbed. 3. Considerations in selecting and using storm water management means and measures for a specific development will include, but are not limited to: site applicability, public safety, spatial requirements, soil characteristics, hydrologic benefits, slope, existing land use conditions, maintenance requirements, location within the watershed, overlay districts, buffer requirements, tree protection, easements, etc. 4. Stormwater flUlofffrom the development shall be transported from the development by vegetated conveyances to the maximum extent practicable. 5. All built-upon area shall be at a minimum of35 feet landward of all perennial and intermittent surface waters (i.e., the buffer zone). This zone shall begin at the most landward limit of the top of bank or the rooted herbaceous vegetation and extend landward a distance of 35 feet on all sides of 1 DRAINAGE the surface water, measured horizontally on a line perpendicular to the surface water. For ponds, lakes and reservoirs located within a natural drainage way, this zone shall begin at the most landward limit of the normal water level or the rooted herbaceous vegetation and extend landward a distance of 35 feet, measured horizontally on a line perpendicular to the surface water. A perennial or intermittent surface water shall be present if the feature is approximately shown on either the most recent version of the soil survey map prepared by the Natural Resources Conservation Service of the United States Department of Agriculture or the most recent version of the 1:24,000 scale (7.5 minute) quadrangle topographic maps prepared by the United States Geologic Survey (USGS). An exception to this requirement may be allowed when surface waters are not present in accordance with the provisions of 15A NCAC 2B .0233 (3) (a) (see http://ncrules.state.nc.us/ncacl). 6. The approval of the plans shall require an enforceable restriction on property usage that runs with the land, such as a recorded deed restriction or protective covenants, to ensure that future development and redevelopment maintains the site consistent with the approved project plans, Desifln Basis I. The design standards addressed in this Section shall apply to all drainage facilities designed and constructed in the Town of Mooresville. Where these standards conflict with those of the NCDOT, Iredell COlDlty, or NCDENR that applies to a given facility, the more stringent shall apply, 2. Hydrologic Method: Drainage structures shall be sized using the Rational Method for areas under two hundred (200) acres. For areas over two hundred (200) acres, the SCS method shall be used. The time of concentration shall not be less than 5 minutes. Calculations used for drainage design shall be submitted along with the Plans for review and with the final approved Plans. 3. The following Table swnmarizes recommend fWloff coefficients for use in the Rational Method: Lawns Wooded Streets Gravel Areas Drives, Walks, Roofs Parks and Cemeteries Residential (including streets): Single-Family (lot <20,000 sf) Single-Family (lot >20,000 sf) Multi-Famil , Attached Industrial: Light Hea Commercial: Office Parks Sho in Centers R~ommeDded Runoff Coefficient Values Description Runoff of Area Coefficient C 0.30 0.25 0.95 0.55 0.95 0.30 0.60 0.50 0.70 0.70 0.80 0.75 0.80 2 DRAINAGE 4. The attached Samvle Storm Drainage Chart (Figure 1) shall be shown on all plans that rely on the Rational Method to size drainage facilities. 5. Design Freauencv Policv: Minimwn design frequency shall be 25 years for storm sewers carrying off-site drainage and 10 years for incidental drainage. Rainfall Intensity-Duration-Frequency data for hydrologic analysis shall be based on that published for Charlotte, NC. 6. Channel Design: Channel side slopes shall be stable throughout the entire length and side slope shall be a maximwn of 3: 1 (H: V) for ease of maintenance. Open channel drainage systems shall be designed to handle the lO-year design storm. The channel protection required to prevent erosion is determined by computing the velocity in the channel at the design discharge and comparing that velocity with the permissible value for the type of channel lining used. 7. All streams that drain more than one square-mile in Iredell County are regulated by FEMA, therefore development in those flood plains is restricted. Please refer to Iredell County's Floodplain Management Policy (see http://www.co.iredell.nc.uslDepartments/Planning). 8. Hydraulics: Pipe inlets and culvert situations are to be checked for inlet and outlet headwater control so as to insure that headwater will not encroach on uphill adjacent property or create a hazard to existing and future development. Twelve-inches of freeboard is required for all culverts up to 3 feet in diameter. I8-inches of freeboard is required for larger culverts. 9. Water SupDlv Watersheds: If the proposed development is located in a water supply watershed then it is subject to land development restrictions as detailed in Article 14 of the Town ofMooresville's Zoning Ordinance. Article 14.6 of the Town ofMooresville's Zoning Ordinance specifies the maintenance of undisturbed vegetative buffers along streams under certain circumstances. Drainage Lines 1. Storm drainage lines that are located on private property and that do not receive runoff from public rights-of-way are not under the jurisdiction of the Town and are not subject to the requirements herein. However they will be inspected by the Town. 2. All storm drainage lines located in the right-of-way or that receive runoff from the right-of-way are under the jurisdiction of the Town and must consist of reinforced concrete pipe (RCP), minimum Class lll. Corrugated metal pipe (CMP), or HOPE shall not be used in such instances, 3. Permanent Utility Easement (PUE) for storm drainage lines shall be a minimwn width of IS-feet. Larger pipe, deeper drainage lines or close proximity to existing or planned buildings may require additional PUE as required by the Director of Engineering and as summarized in the following: 3 DRAINAGE PIPE SIZE MIN. PERMANENT UTILITY EASEMENT (PUE) WIDTH 15" 15' 18" 15' 24" 15' 30" 20' 36" 20' 42" 25' 48" 25' 54" + 30'+ 4. The minimum pipe diameter for storm drainage lines shall be 15 inches. 5. The minimum slope on all storm drainage lines shall be 1.0% or the slope that will produce a velocity of2 feet per second (fps) when the pipe is flowing full, whichever is greater. The maximum velocity shall not exceed 10 fps, 6. Storm drainage lines shall cross under streets at right angles. Storm drainage lines greater than 18" are not to cross diagonally under the radii at intersections as illustrated below in Figure 2. Engineer must show that lines under 18" crossing radii diagonally can be fully maintained within the right-of- way or dedicated easement and the easement shall not encroach on a building envelope. Pipes Greater than 18" storm 0rII1n Mlnhole w Not Accept_ble Accept_ble Figure 2 7. When Rep is used at a stream, then an 8-footjoint must be used at the lower end (nearest the stream) to prevent undermining. 8. Storm drainage lines shall generally be located below the curb and gutter of streets or along lot lines. 9. Storm drainage lines shall have a minimum 18 inches of cover. Pipe diameters 30" and larger shall be deep enough to accommodate drainage structures. 10. Engineer of Record must ensure that proposed storm drains will not conflict with existing sanitary sewer service laterals. 4 DRAINAGE 11. Storm drainage lines shall have at least a one-foot vertical separation from all water and sanitary lines. 12. Storm drain outfalls shall be piped to the rear of any possible building envelopes. 13. Flared-End-Sections or end walls plus an appropriate energy dissipater shall 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). Flared end sections may be used with pipes 36-inches or less in diameter. Drainage Structures 1. Street catch basins shall be NCDOT 840.01 with type E grate as appropriate (NCDOT 840.03). Solid wall, square precast concrete structures conforming to NCDOT 840.45 are acceptable. No round structures shall be allowed. No waffle wall boxes shall be allowed. All pipe openings in precast structures must be cast or cored. Unless the structure is stamped by the manufacturer as NCDOT approved, shop drawings must be submitted for approval prior to their installation. 2. Junction boxes shall be NCDOT 840.31,840.32 or 840.34. No waffle boxes shall be allowed. 3. Catch basins for curb and gutter must conform to NCDOT 840.01 or 840.02. Frames, Grates and Hoods shall conform to NCDOT 840.03. No catch basins will be allowed in driveways. 4. Yard inlets shall be NCDOT 840.05. 5. Curb and gutter is required in all cases. The minimum longitudinal grade for curb and gutter is 2%. 6. Inlets should be placed at (upstream of) intersections and all low points in the gutter grade to prevent gutter flow from crossing traffic and pedestrian lanes of the intersecting road. There shall be no boxes in curb radii. Inlets are normally placed upstream of pedestrian crossings to intercept the gutter flow before it reaches the crosswalk. Where pavement surfaces are warped, as at cross streets, ramps, or transitions between super elevated and normal sections, gutter flow should be diverted into the storm drainage system to prevent water flow across the roadway. Where a curbed roadway crosses a bridge, gutter flow should be intercepted before it reaches the bridge. Runoff from areas adjacent to streets should be intercepted before reaching the pavement. This applies to water that would normally nm onto the street from side streets or from cut slopes and areas along the edge of pavement. A minimum of two inlets (double catch basin) should be placed in the sag of vertical curves. 7. Inlets on a continuous grade should be spaced to limit the spread of stormwater onto the pavement. The spacing of inlets is based on the allowable spread, grade of the street, flow and the inlet capacity. Maximum spread shall be 8 feet from the face of the curb based on a rainfall intensity of 4 inches/hour. Manning's n-value for concrete curb and gutter calculations shall be 0.016, The flow bypassing an inlet must be included in the flow arriving at the next inlet. 8. The maximum length of pipe without installation of a catch basin, curb inlet or junction box is 400 feet. 9. Spot grades or profiles shall be provided in cul-de-sacs to ensure positive drainage. 5 DRAINAGE 10. 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, grate type, and the drainage area and flow into the pipe structure. This information should be written in a table format with corresponding pipe or structure numbers shown in plan view. Drainage structures shall be numbered (with hexagonal labels) in increasing order as one moves from downstream to upstream. 11. All manholes shall be clearly marked with the words "storm sewer." 12. If a proposed structure exceeds 12'-0" vertical height, a structural design will be required for approval. Best ManaGement Practices 1. Article 14.13 of the Town of Mooresville's Zoning Ordinance specifies the use of storm water best management practices (BMP's) under various land use scenarios and watershed locations. All Stormwater BMP's shall be designed in accordance with the "Stormwater Best Management Practices" manual, latest edition, prepared by the NCDENR Developers are encouraged to maintain or provide vegetative buffers to meet water quality requirements. 2. All BMP riser type structures shall be precast concrete and be fitted with galvanized steel trash racks. Corrugated metal is not allowed. All emergency overflow weirs shall be stabilized with east- in-place concrete, 4-inch minimum thickness. All BMP's that permanently impound water shall be fitted with ductile iron drain piping with a valve located in the riser structure. The valve shall have a ri~er stem such that it can be accessed from above without entering the structure, 3. Rules in the North Carolina Administrative Code (NCAC), specifically 15A NCAC 2H.I 008, "Design of Stormwater Management Measures," provides information on the appropriate minimum engineering design criteria for BMP's (see http://ncru1es.state.nc.us/ncac.asp). Earthen BMP's shall be designed with the following minimum requirements: 1. Exposed side slopes shall be 3: 1 or flatter 2. Maintenance access shall be provided; typically 10 feet at the top of dam 3, Design shall include a spoil area for dredged material 4. Detention pond worksheets are available on the Engineering Department website for use during the reVIew process. 6 , . . ~ ~ ~ Q r--- i I "" 00 ....... f-o I !is ~ 0.. v ~-< Cl E-<> ~~ r:: 'C O~ (I) (I) s:::: g:~ 0:: ~ . 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