Town of Chapel Hill
Stormwater Impact Statement and Stormwater Management Plan Guidelines
(Effective January 1, 1997)
Introduction
Pursuant to the Town of Chapel Hill Land Use Management Ordinance Section 5.4, Stormwater Management, all applications for developments or subdivisions and any building (other than single-family or two-family dwellings resulting in less than or equal to 5,000 square feet of land disturbance) within the Town of Chapel Hill Planning Jurisdiction must include a Stormwater Impact Statement and a Stormwater Management Plan As authorized by the Chapel Hill Land Use Management Ordinance, affirmative exemption to all or part of the requirements of the Stormwater Impact Statement may be granted by the Town Manager (Manager).
For single-family or 2-family dwellings, see Item J. below.
Site Analysis and Narrative
A site analysis and narrative shall be provided as a part of the Stormwater Impact Statement or Stormwater Management Plan submittal. Characteristics of the existing site and proposed development shall include the following:
A. Land use, density, and impervious surface area;
B. Location, topography, on-site and off-site drainage conditions;
C. Phasing and timing of project; and
D. Existing delineation of the Resource Conservation District (RCD – perennial and intermittent streams as determined by the Town), jurisdictional wetlands, soil series, Federal/State wildlife lands, and regulatory FEMA Special Flood Hazard Areas. If an intermittent or perennial stream may be on or within 150 feet of the property to be developed, the applicant must submit a request for an RCD determination/confirmation to be performed by Town staff for each identified stream.
Stormwater Management Plan (for all Applicable development)
A Stormwater Management Plan shall be submitted with the Stormwater Impact Statement detailing site location, existing and proposed stormdrainage system(s), stormwater detention structure(s), BMP’s/IMP’s, grading, landscaping, erosion control features, and Resource Conservation District location(s). The plan must present the existing and proposed conditions and features at a scale and quality suitable to include all impacted areas (on-site and off-site).For certain applications, a pre-application meeting with the Town Stormwater Management Engineer and other staff may be prudent to discuss alternatives.
Impact Statement Analyses
A. SCOPE OF IMPACTED AREA
A pre-application meeting with the Town Stormwater Management Engineer is encouraged to determine the limits of the study to be included in the Stormwater Impact Statement, and to define study elements required. Possible study elements of concern may include, but are not limited to:
1. Backwater impacts;
2. Effects on existing upstream and/or downstream drainage conveyance facilities;
3. Upstream and/or downstream volumes, discharges and velocities;
4. Ability of natural drainage channel to convey additional volume, discharges and velocities;
5. Potential mitigation measures; and
6. Site-specific considerations.
B. RELEASE RATE MANAGEMENT: (Applicable for all development except one-family and two-family less than or equal to 5,000 sf of disturbed land area)
Accepted engineering practices will be required determining both pre-development and post-development stormwater peak discharge data. Hydrographs may be required on a site-specific basis.
The peak discharge rate* for the post-development conditions shall be no greater than the peak discharge rate for the pre-development conditions for the local 1-year, 24-hour duration, 2-year, 24-hour duration and the 25-year, 24-hour duration return period storms. Acceptable methodologies for computing peak flow rates include: the Rational Method, HEC-1 or HEC-HMS methods, USDA TR-55, or other methods subject to approval by the Town Stormwater Management Engineer.
*Depending on the development site location, size/area and the condition of the existing conveyance system and associated lands, the Manager may waive or change the peak discharge rate criteria in part or in whole if, based on an approved Stormwater Impact Statement, it is demonstrated that detention would intensify existing peak discharges or might otherwise create problems on abutting and/or downstream properties.
C. VOLUME MANAGEMENT: (Not applicable for single-family and two-family lots existing prior to January 27, 2003 or single-family and two-family lots or lots platted after January 27, 2003 with less than or equal to 5,000 sf disturbed land area)
Using USDA Soil Conservations Service Curve Number (CN) method for total run-off volume (or other approve method) the increase in volume from pre-development conditions to post-development conditions for the 2-year/24-hour storm, (where P=3.6”) shall be managed on-site using infiltration, re-use or other approved BMP/IMP methods.
D. WATER QUALITY MANAGEMENT: (Applicable for all development except single-family and two-family less than or equal to 5,000 sf of disturbed land area)
The applicant shall utilize stormwater Integrated Management Practices (IMP’s) and/or Best Management Practices (BMP’s), as approved by the Manager, to treat stormwater runoff from all disturbed, built-upon, and impervious areas associated with the development. The IMP/BMP shall be minimally designed to remove 85% average annual total suspended solids (TSS) from post-development stormwater runoff. The IMP/BMP may also be used to provide for volume management and release rate management as approved by the Manager. Further information regarding the IMP/BMP practices will be made available in the Town Design Manual.
Submit proposal(s) for IMP’s/BMP’s to the Town Stormwater Management Engineer for review. The assumed TSS removal efficiencies for properly designed IMP’s/BMP’s are as follows:
|
Wet Detention Ponds |
85% |
|
Extended Detention Wetlands |
85% |
|
Pocket Wetlands |
35% |
|
Sand Filters |
85% |
|
Bioretention Areas |
85% |
|
Grassed Swales (100 linear ft. per drained acre) |
35% |
|
Extended Dry Detention Areas |
50% |
|
Filter Strips |
35% |
|
Infiltration Devices |
85% |
|
Level Spreaders & Buffers |
85% |
Single devices may be used or devices may be used in combination to achieve the required pollutant removal of 85% average annual TSS. As experience grows in the use and effectiveness of the devices and methods, other IMP’s/BMP’s and/or other specifications may be considered for use in Chapel Hill. The NCDWQ and the Town of Chapel Hill will continue to review and modify both design and removal efficiencies as appropriate.
E. NUTRIENT LOADING CALCULATIONS: (Not Applicable for development less than 1 acre of disturbance)
Nutrient loading calculations shall be included with the Stormwater Impact Statement. Yearly loads are calculated by multiplying the area of each land use by the appropriate loading coefficient as provided in Table 1. Required calculations shall include the pollutant loading of total nitrogen and total phosphorus. Treating stormwater for nutrient loads is not currently required by the Town.
TABLE 1
Chapel Hill Pollutant Loading Coefficients (lbs/acre/yr)*
|
LAND USE TYPE |
TOTAL N |
TOTAL P |
|
Low Density Residential (< 12% impervious or < 1 unit/acre) |
5.2 |
0.7 |
|
Moderate/High Density Residential (> 12% impervious or > 1 unit/acre) |
7.4 |
1.2 |
|
Office/Institutional |
8.8 |
1.6 |
|
Commercial |
13.2 |
1.6 |
|
Industrial |
11.2 |
1.4 |
|
Undeveloped/Forest/Open |
0.6 |
0.08 |
|
Pasture |
2.6 |
0.5 |
* Values calibrated for the Town of Chapel Hill and based on data from Hartigan (1983), CDM (1989), Haith (1992), and Schueler (1987).
In order to perform these loading calculations, the land use types and relative acreage must first be determined. For example, a proposed development contains 100 residential half acre lots (moderate density). A 20 acre commercial district is also planned. The remaining tracts (15 acres) will remain undeveloped open space. The total nitrogen loading calculations for this proposed development would be as follows:
Moderate Residential - 100 lots x ˝ acre............................................................ 50 acres
- 50 acres x 7.4 lbs/acre/yr............................................ 370 lbs/yr
Commercial - 20 acres x 13.2lbs/acre/yr.............................................. 264 lbs/yr
Undeveloped - 15 acres x 0.6 lbs/acre/yr.................................................. 9 lbs/yr
Total Nitrogen Load........................................................................................... 643 lb/yr
Although nutrient runoff is currently not regulated in Chapel Hill, this nutrient analysis is required as part of the Stormwater Impact Statement data submittal.
F. EROSION AND SEDIMENT MANAGEMENT
Any development causing the disturbance of more than 5000 sf of land area requires erosion control measures. For disturbance greater than 20,000 sf of land area, an Erosion Control Permit is required by Orange County. For development causing one (1) acre or more of land disturbance, the applicant must also submit an erosion control performance guarantee or bond with the Town. Contact the Engineering Department for the value requirements of this guarantee
The Stormwater Impact Statement must include a general description of the proposed erosion and sediment control measures and these measures shall be indicated on the Stormwater Management Plan. If applicable, a phasing schedule for construction and/or removal of proposed control devices will also be required to ensure adequate protection for all phases of the development. Inspection and approval of the installed devices by the Town or Orange County is required.
G. BEST MANAGEMENT PRACTICES AND INTEGRATED MANAGEMENT PRACTICES (BMP/IMP’S)
If applicable, where the post-development peak discharge rate or volume of runoff exceeds the pre-development rate or volume and to achieve average annual 85% total suspended solids removal, BMP/IMP’s will be required. These practices may include structural or non-structural measures. Structural measures may include but are not limited to level-spreaders, curb cuts or diffuse flow, grassed swales, wet or dry detention basins, wet retention basins, extended dry detention basins, bioretention areas, pervious pavement and other infiltration practices. Non-structural measures may include but are not limited to utilizing natural buffers, limiting impervious surfaces and limiting disturbed areas.
H. MAINTENANCE AND OPERATIONS PLAN
The Stormwater Impact Statement for developments other than single-family, two-family, and duplex proposing stormwater control facilities shall include a Maintenance and Operations Plan. This plan shall detail the types and frequency of inspection and maintenance operations (major and minor), equipment necessary to perform maintenance activities, access to the stormwater control facility, disposal methods for uncontaminated and contaminated materials, and information regarding the facility owner(s) and party or parties responsible for facility operation and maintenance. The Town may require that a perpetual maintenance bond be posted.
I. Reserved Storm Drainageway Easement
All engineered stormwater facilities intended for mitigation of peak discharges, volume or to provide water quality treatment shall be located within an easement entitled: “Reserved Storm Drainage Easement”. Unless specifically designated as being “Public”, these easements and the facilities/functions they serve are considered by the Town to be private, and the Town assumes no responsibility for necessary inspection, operation, and/or maintenance duties.
J. STORMWATER MANAGEMENT PLAN FOR SINGLE AND TWO-FAMILY RESIDENTIAL DEVELOPMENT (resulting in more than 5000 sf of land disturbance) :
A P.E. Certification and Stormwater Management Plan (SWMP) is required for all Zoning Compliance Permit/Building Permit Applications for single or two-family development more than 5,000 sf disturbance. The P.E. certification and SWMP shall indicate the BMP’s/IMP’s to manage peak discharge rate and provide 85% TSS removal and if applicable provide volume management to the extent practicable. The SWMP may include all stormwater impact calculations in lieu of a separate Stormwater Impact Statement. See Item F. above for erosion control measures required. Prior to issuance of a Certificate of Occupancy (CO), the PE certification must be submitted certifying that the stormwater management measures were installed as shown on the approved Stormwater Management Plan.
If you have any questions regarding these Guidelines, contact the Town Stormwater Management Engineer at (919) 968-2833.
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REFERENCES CITED |
CDM. 1989. Watershed Management Study: Lake Michie and Little River Reservoir Studies.
Field, Richard, M.L. O’Shea and K. K. Chin. 1993. Integrated Stormwater Management. Lewis Publishers. Boca Raton, FL.
Haith, D.A., et al. 1992. Generalized Watershed Loading Functions: User’s Manual. NY.
Hartigan, J.P., et al. 1983. Calibration of NPS Model Loading Factors. Journal of Environmental Engineering Division. 109: no. 6 pp. 1259-1272.
Schueler, Thomas R. 1987. Controlling Urban Runoff: A Practice Manual for Planning and Designing Urban BMPs. Metropolitan Washington Council of Governments, Washington, DC.
Stormwater Best Management Practices 1999. North Carolina Department of Environment and Natural Resources, Division of Water Quality, Water Quality Section.
Urbonas, Ben, and P. Stahre. 1993. Stormwater Best Management Practices and Detention. Prentice Hall. Englewood Cliffs, NJ.