A. Purpose. This article is adopted to improve public safety, to control the rate of flow of storm water, to minimize increases in the peak flow rates of storm water runoff caused by development, to minimize any detrimental effect on water quality by the completed facility, and to avoid such adverse effects during construction.
B. Applicability: All new development (new construction, expansion, modification, or alteration), which results in an increase in storm water runoff of 10% or more, shall comply with this Storm Water Management Ordinance, excluding any single residential lot or minor residential subdivision. All major residential subdivisions, commercial and/or industrial developments shall comply with this ordinance.
C. Critical Areas and No Increase Zones. The Planning Commission hereby establishes four Critical Areas and four No Increase Zones, the boundaries of which are depicted on the City of Paris, Storm Water Management, Critical Areas and No Increase Zones map.
A critical area shall be defined as any area of the city that, after careful review and research, the city manager, city planner, and city planning commission agree shall be regarded as extremely sensitive to increases in storm water runoff.
A no increase zone shall be defined as an area designated by the city planning commission, within which no increase in storm water runoff shall be allowed, except as expressly authorized in Section B of this ordinance.
D. Waivers. The Planning Commission shall reserve the right to waive the detention requirements for any development which is physically located entirely outside of the boundaries of a No Increase Zone, as shown on the City of Paris, Storm Water Management, Critical Areas and No Increase Zones map, and meets the conditions of one of the following:
1. Results in an increase in storm water flow of 10% or less, as calculated by comparing the post-development runoff to the predevelopment runoff, or
2. Results in an increase in storm water flow of 5 cfs or less, as calculated by comparing the post-development runoff to the predevelopment runoff.
E. Design Criteria. The following designs standards shall be adopted:
1. Storm drains and culverts shall be designed to pass the storm water flow associated with a 25-year storm event.
2. Bridges shall be designed to pass the storm water flow associated with a 100-year storm event.
3. Detention facilities and outfall structures shall be designed to provide a minimum of 0.5 feet of freeboard above the maximum water surface level and retain the increase(s) in storm water flow associated with the following storm events:
5. Detention facility overflow structures shall be designed to pass the storm water flow associated with a 50-year storm event.
6. The detention facility shall be analyzed based on the storm water flow associated with a 100-year storm event and shall be designed such that the facilities shall not encounter damage due to the 100-year flow.
7. Detention facilities may be provided by use of detention ponds, underground detention piping, parking area depressions, or other methods that meet the approval of the City Manager, City Planner and Regional Planning Commission. Such facilities shall be clearly delineated on the grading plan.
F. Design Computations. The following computations, if applicable, shall be submitted to the Planning Department as required for inclusion on the agenda of the monthly Planning Commission meeting:
1. Pre- and Post-development runoff calculations.
For drainage areas less than 100 acres, storm water runoff shall be computed using the Rational Equation method as follows: Q=ciA, where "i" is the rainfall intensity for the applicable storm event. Rainfall intensity shall be determined from the IDF curve for the City of Memphis, Tennessee. The IDF curve can be found in either of the following documents:
1. Local Government Public Works Standards & Specifications, Drawing Number RIDFC- 1; Rainfall Intensity-Duration-Frequency Curves.
2. Tennessee Department of Transportation, Design Division, Roadway Design Guidelines, First Edition, October 1987, Chapter 2, Figure 2-10.
The rainfall intensity shall be computed based on the time of concentration for the drainage area. Time of concentration shall be determined from an acceptable nomograph or other method. In no case shall the time of concentration be less than 5 minutes. An acceptable nomograph for computing time of concentration can be found in:
Tennessee Department of Transportation, Design Division, Roadway Design Guidelines, First Edition, October 1987, Chapter 2, Figure 2-11.
For drainage areas exceeding 100 acres, storm water runoff shall be computed using the Regional Regression Equations for Hydrologic Area 4 of Tennessee. The Regression equations can be found in:
Tennessee Department of Transportation, Design Division, Roadway Design Guidelines, First Edition, October 1987, Chapter 2, Table 2-4.
If computer software is used to complete the design, the drainage submittal shall include all inputs, outputs, warnings, and errors produced by the software.