(A) The Town adopts as its MS4 stormwater design and best management practices (BMPs) manuals for stormwater management, construction and permanent, the following publications, which are incorporated by reference in this chapter as if fully set out herein:
(1) TDEC Erosion Prevention and Sediment Control Handbook, most current edition.
(2) Town of Collierville Stormwater Drainage Manual, most current edition.
(3) A collection of Town approved BMPs developed or collected by the Town that comply with the goals of the Town's MS4 permit and/or the CGP.
(4) This chapter.
(B) The Town's BMP manual(s) include a list of acceptable BMPs including the specific design performance criteria and operation and maintenance requirements for each stormwater practice. These include Town approved BMPs for permanent stormwater management, including green infrastructure BMPs.
(C) The Town manual(s) may be updated and expanded from time to time, at the discretion of the Town's Engineering/Development Department, based on improvements in engineering, science, monitoring and local maintenance experience, or changes in federal or state law or regulation. Stormwater facilities that are designed, constructed and maintained in accordance with these BMP criteria will be presumed to meet the minimum water quality performance standards.
(D) Stormwater quantity control is an integral component of overall stormwater management. Quantity control is effectively flood control, reducing potential damages and health risks, but because uncontrolled runoff can cause erosion, it can also be a form of water quality control. All projects meeting the applicability requirements of this chapter shall provide a permanent stormwater storage facility, unless certain criteria are met as determined by the Town Engineer, or his or her designee, which shall be based on qualified studies submitted by the engineer of record. Exceptions to stormwater quantity requirements shall be made based on criteria, including but not limited to the following:
(1) Redevelopment projects where post-redevelopment conditions reduces the flow rate from existing conditions.
(2) The site is located in the lower portion of the drainage basin (generally lower one-fourth) and requiring a storage facility would have a negative effect on the stormwater drainage system.
(3) There is a regional detention facility which provides for stormwater management of the site.
(4) There is excess capacity of the existing stormwater drainage system to convey the 100-year storm event for the drainage basin in a fully developed condition.
(5) Additional criteria as set by the Town's Engineering/Development Department.
(E) The following design criteria are established for stormwater quantity control. Stormwater quantity designs shall meet the multi-stage storm frequency storage requirements as identified below. In the case of a discrepancy between this chapter and the Town of Collierville Drainage Manual, most current edition, the most stringent requirement(s) will apply.
(1) All stormwater storage facilities shall be designed such that the maximum allowable release rate of stormwater runoff generated on-site shall not exceed the pre-development peak rate of runoff as calculated by the engineer of record for the two-year, ten-year and 25-year storm event using methods outlined in the Town of Collierville Drainage Manual or other standard engineering methods as approved by the Town's Engineering/Development Department.
(2) Earthen stormwater storage facilities shall be designed with an emergency spillway sized to carry the 100-year storm event while maintaining a one foot freeboard between the top of the embankment and water surface elevation.
(F) All calculations shall be performed by an engineer registered in the state.
(G) Urban areas have two separate and distinct drainage systems that function to remove excess precipitation with minimum inconvenience and property damage. This dual system is comprised of a minor (active) system and a major (passive) system. The minor system includes the curb and gutter area of the street, roadside ditches, inlets, storm sewers, culverts, channels, or other conveyance facilities designed to convey the design storm runoff. The major system comes into operation once the minor system's capacity is exceeded. The major system consists of excess capacity in storm sewers, culverts, streets, property line drainage swales, and other natural and lined channels. The capacity of the major system shall have capacity to convey the major storm runoff without life hazard or property damage.
(1) Design storm frequency for land use/development type for the minor (active) system shall be as follows:
(a) Residential - 25 years;
(b) Commercial/business/industrial - 25 years;
(c) Critical areas (hospitals, interstates) - 50 years.
(2) Design storm frequency per street classification for the minor (active) system shall be as follows:
(a) Locals - 25 years;
(b) Collectors - 25 years;
(c) Arterials - 50 years.
(3) Design storm frequency for the major (passive) system shall be 100 years.
(H) The methodology for hydrologic and hydraulic design calculations for the pre-development and post-development conditions for the design storms is specified in the BMP Manual. These calculations must show that the proposed stormwater management measures are capable of controlling runoff from the site in compliance with this chapter and the guidelines of the Town of Collierville Drainage Manual. Required calculations include:
(1) A description of the design storm frequency, duration, and intensity where applicable;
(2) Time of concentration;
(3) Soil curve numbers or runoff coefficients;
(4) Peak runoff rates and total runoff volumes for each watershed area;
(5) Infiltration rates, where applicable;
(6) Culvert, stormwater sewer, ditch and/or other stormwater conveyance capacities;
(7) Flow velocities;
(8) Data on the increase in rate and volume of runoff for the required design storms; and
(9) Documentation of sources for all computation methods and field test results.
(I) If hydrologic or topographic conditions warrant greater control than that provided by the minimum control requirements, the Town's Engineering/Development Department may impose any and all additional requirements deemed necessary to control the volume, timing, and rate of runoff.
(Ord. 2012-18, passed 11-12-12)