(1) General Provisions.
(a) These guidelines will outline the way storm water detention and green infrastructure will be calculated. Storm water detention is a necessary part of most storm water treatment BMPs and is required for most developments.
(b) The Storm Water Ordinance of the City of Cookeville is posted at the City of Cookeville Public Works Department's website and must be read by anyone attempting to perform storm water calculations with the City of Cookeville. It contains provisions for fees, right-of-entry, definitions, easements and penalties. A major purpose of the Storm Water Ordinance is to improve water quality, storm water quantity as well as comply with the city's NPDES Permit for Discharges from Small Municipal Separate Storm Sewer Systems (MS4).
(c) Due to the MS4 permit requirements, projects over one (1) acre in disturbed area require additional controls; therefore the guidelines are separated into two (2) categories: one for projects less than one (1) disturbed acre and another for projects with more than one (1) disturbed area.
(d) Storm water detention is required for any new development or re-development containing 5,000 square feet or more of impervious area. (Cookeville Municipal Code § 14-608, Cookeville Zoning Code, Cookeville Subdivision Regulations). Where additions to the existing impervious area of the site bring the total area of impervious surface to 5,000 square feet or more, only the additional impervious area is required to provide detention.
(e) Storm water detention is defined as limiting the peak discharge rate for the post developed conditions to be no greater than the peak discharge rate for the predevelopment conditions. This must be accomplished using the two (2)-year, five (5)-year, and ten (10)-year storms. The estimated design rainfall amounts for either twenty-four (24)-hour depths or intensities for any duration, can be found by accessing the following NOAA National Weather Service Atlas 14 data for Tennessee:
(f) Redevelopment of property shall be evaluated using one of the following methods unless otherwise determined by the Public Works Department:
i. Rational method – evaluate the pre-development conditions using the appropriate meadow/pastureland runoff coefficient or as existing conditions; the more conservative condition shall be used.
ii. USDA Technical Release Number 55 (TR-55) – evaluate the pre-development conditions as either grassland conditions or existing conditions; the more conservative conditions shall be used.
(2) Projects Disturbing Less Than One Acre.
(a) The first flush volume is defined as the first one (1) inch of direct runoff from the impervious areas of the contributing drainage basin. The first flush volume must be captured and released over a minimum of twenty-four (24) hours and a maximum of seventy-two (72) hours. No first flush runoff of impervious surfaces shall bypass the treatment facility. Storm water recharge by infiltrating the first flush volume into the soil on site is the preferable method of first flush treatment. Infiltration may be accomplished by the use of bioretention basins, rain gardens, bio swales, green parking for overflow lots, or other methods pre-approved by the Director of Public Works or his designee. Infiltration of the first flush may not be possible if the site is located on unsuitable soils. Where infiltration of storm water is not possible or ill advised, treatment of the first flush is defined as a practice or structure that removes eighty percent (80%) total suspended solids (TSS) and other anticipated pollutants such as oil and grease.
(b) A dry detention basin must have a minimum of two percent (2%) slope in the bottom of the basin in order to drain properly. Side slopes should generally be three to one (3:1) (H:V) or flatter, unless traversable access has been designed.
(c) All hydrological and hydraulic computations for storm water detention facilities must be prepared and stamped by a registered engineer (licensed in the state of Tennessee) and proficient in this field. Plans must show sufficient information to enable the builder to construct the facility as required.
(d) Underground detention is the use of large underground structures to provide necessary volumes for attenuating storm water peak flows. Underground structures generally provide little or no storm water quality benefits. The following minimum requirements must be met before an underground storage facility will be considered for approval:
i. The underground detention structure must provide adequate access for inspection from the surface. Public safety must be considered.
ii. The underground detention structure must be constructed of durable materials with a typical one hundred (100)-year lifetime. Detention storage volume shall not include the porous space within a stone or gravel bed without soils reports stamped by a geotechnical engineer stating that infiltration into the soils should exceed one-half inch (1/2") per hour infiltration rate.
iii. The underground detention structure shall be designed to have positive drainage into the receiving channel or storm water sewer assuming there is a ten (10)-year flood in the receiving facility.
iv. The underground detention structure shall not receive surface runoff directly from parking lots. Surface water shall be directed to a BMP that improves water quality such as an oil/water separator, grass filter strips (of sufficient size to filter the water quantity), or a proprietary water quality devise approved by the Public Works Department.
v. Design measures must be taken to trap and store sediments in locations where cleanout and maintenance can be easily performed. This generally requires that some type of water quality inlet or other storm water treatment BMP must be installed upstream from the underground detention facility. Proprietary water quality devices must be approved by the Public Works Department.
vi. A detailed maintenance and inspection plan must be submitted and approved (including inspection schedules and guidelines). Evidence of responsibility and financial budgeting must be presented.
(3) Projects Disturbing More Than One Acre. Projects disturbing more than one (1) acre for new and redevelopment will be required to meet the site design standards set out by the Tennessee Department of Environment and Conservation (TDEC) for compliance with the city's National Pollutant Discharge Elimination System (NPDES) permit as well as minimum volume controls.
(a) Site design standards for all new construction and redevelopment disturbing one (1) acre or more must use, in combination or alone, storm water management measures that are designed, built and maintained to infiltrate, evapotranspire, harvest and/or use the first inch of every rainfall event preceded by seventy-two (72) hours of no measurable precipitation. This first inch of rainfall must be one hundred percent (100%) managed with no discharge to surface waters.
(b) Storm water management measures that are often referred to as green infrastructure that can infiltrate the first inch of rainfall are the goal that TDEC is aiming for that they term runoff reduction. Green infrastructure includes many practices including: rain gardens, bioretention, infiltration swales, etc.
(c) Some sites are not suitable for infiltration methods and infiltration requirements may be set aside on such sites, with prior agreement by the Public Works Department. Limitations to using runoff reduction methods include, but are not limited to:
i. Where the potential for introducing pollutants into the ground water exists, unless pretreatment is provided;
ii. Where pre-existing soil contamination is present in areas subject to contact with infiltrated runoff;
iii. The presence of sinkholes or other karst features on the site or in close proximity;
iv. Insufficient infiltration capacity of soils;
v. An extensive presence of shallow ground water table, shallow bedrock or other restrictive layers;
vi. Presence of contractive or expansive soils in close proximity to structures.
(d) Pre-development infiltration capacity of the soils at the site must be taken into account in selection of runoff reduction management measures.
(e) Incentive standards for redevelopment sites: a ten percent (10%) reduction in volume of rainfall to be managed for any of the following types of development. Such credits are additive such that a maximum reduction of fifty percent (50%) of the standard one (1) inch is possible for a project that meets all five (5) criteria:
i. Redevelopment.
ii. Brownfield redevelopment.
iii. High density (> 7 units per acre).
iv. Vertical density (> 18 units per acre).
v. Mixed use.
(f) For all projects that cannot meet one hundred percent (100%) of the one (1) inch runoff reduction requirement unless subject to the incentive standards, the remainder of the one (1) inch rainfall must be treated prior to discharge with a practice or technology documented to remove eighty percent (80%) total suspended solids (TSS). The treatment practice or technology must be designed, installed and maintained to continue to meet this performance standard.
(g) In addition to the runoff reduction requirement for the one (1) inch rainfall minimum volume controls are required to detain those storms greater than one (1) inch in depth.
(h) A dry detention basin must have a minimum of two percent (2%) slope in the bottom of the basin in order to drain properly. Side slopes should generally be three to one (3:1) (H:V) or flatter, unless traversable access has been designed.
(i) All hydrological and hydraulic computations for storm water detention facilities must be prepared and stamped by a registered engineer (licensed in the state of Tennessee) and proficient in this field. Plans must show sufficient information to enable the builder to construct the facility as required.
(j) Underground detention is the use of large underground structures to provide necessary volumes for attenuating storm water peak flows. Underground structures generally provide little or no storm water quality benefits. The following minimum requirements must be met before an underground storage facility will be considered for approval:
i. The underground detention structure must provide adequate access for inspection from the surface. Public safety must be considered.
ii. The underground detention structure must be constructed of durable materials with a typical one hundred (100)-year lifetime. Detention storage volume shall not include the porous space within a stone or gravel bed without soils reports stamped by a geotechnical engineer stating that infiltration into the soils should exceed one-half inch (1/2") per hour infiltration rate.
iii. The underground detention structure shall be designed to have positive drainage into the receiving channel or storm water sewer assuming there is a ten (10)-year flood in the receiving facility.
iv. The underground detention structure shall not receive surface runoff directly from parking lots. Surface water shall be directed to a BMP that improves water quality such as an oil/water separator, grass filter strips (of sufficient size to filter the water quantity), or a proprietary water quality devise approved by the Public Works Department.
v. Design measures must be taken to trap and store sediments in locations were cleanout and maintenance can be easily performed. This generally requires that some type of water quality inlet or other storm water treatment BMP must be installed upstream from the underground detention facility. Proprietary water quality devices must be approved by the Public Works Department.
vi. A detailed maintenance and inspection plan must be submitted and approved (including inspection schedules and guidelines).
(as added by Ord. #O16-06-16, August 2016, and amended by Ord. #O16-09-21, November 2016, and Ord. #O17-06-13, August 2017)