§ 26-175. Stormwater system criteria.

(a)

General.

(1)

The provisions set forth in this section are applicable to all developments and redevelopments not exempted from submittal of a stormwater management plan, as defined in section 26-171.

(2)

The provisions set forth in this section are not applicable to developments that have an approved stormwater management plan prior to the date of adoption of this article.

(3)

Where such criteria exist, all stormwater facilities and systems, including those designed and constructed for water quality treatment, channel protection, overbank flood protection, and extreme flood protection, shall be designed and constructed in accordance with the criteria, standards, and specifications presented in this article and in the county stormwater management manual, as amended.

(4)

The portions of the new development or redevelopment on which stormwater management facilities and systems are located shall be shown on the plat and recorded with the deed as permanent drainage or water quality easements.

(5)

Banks of all streams, channels, ditches and other earthen stormwater conveyances shall be left in a stabilized condition upon completion of the project. No actively eroding, bare or unstable vertical banks shall remain. Placement of riprap and other hard armor as the sole bank protection method on blue line streams is only allowed when vegetative bank stabilization alternatives are not technologically feasible.

(6)

The director has the authority to require additional water quantity standards, including restrictions on peak velocity and/or runoff volumes or less frequent design events, in areas where the director has determined, through stormwater master plans, engineering studies, and/or other regulatory water quality requirements, a history of existing or documented flooding or erosion problems, or engineering judgment, that additional restrictions are needed to limit adverse impacts of the proposed development downstream or upstream of the site.

(7)

All hydrologic and hydraulic computations utilized in the design of stormwater treatment and control facilities must be performed using the calculation methods presented in the county stormwater management manual, as amended, unless equivalent methods are pre-approved by the director.

(8)

All hydrologic and hydraulic computations utilized in the design of stormwater facilities must be prepared by a registered engineer proficient in the field of hydrology and hydraulics and licensed to practice engineering in the state.

(9)

The director may waive or modify any of the stormwater system criteria provided in this section if adequate water quality treatment, and/or channel protection, and/or overbank flood protection, and/or extreme flood protection is suitably provided by a downstream or shared off-site stormwater facility.

(b)

Water quality treatment.

(1)

Stormwater runoff from the development or redevelopment site must be treated to remove pollutants prior to discharge from the development or redevelopment site in accordance with the stormwater treatment standards and criteria provided in the county stormwater management manual, as amended.

(2)

Stormwater quality treatment must be achieved through the use of one or more structural and/or non-structural best management practices, that are designed and constructed in accordance with the design criteria, guidance, and specifications provided in the county stormwater management manual, as amended.

(3)

Best management practices or technologies that are not included in the county stormwater management manual, as amended, may be approved for the treatment of stormwater quality on a case-by-case basis provided that the following conditions are met:

a.

The best management practice or technology, as applied to the site, meets the water quality goals published in the county stormwater management manual, as amended. The performance ability of the best management practice must be verified by an independent third party.

b.

BMPs that have unacceptably high maintenance requirements may not be installed within public rights-of way or on public property. Such judgments shall be made by the director after review of applicable information submitted by the designer.

(4)

Additional watershed or site-specific stormwater quality requirements may be required by the director, in order to satisfy local or state NPDES, TMDL or other regulatory water quality requirements.

(c)

Downstream channel protection.

(1)

The channel protection volume (the runoff volume from the one-year frequency, 24-hour storm) shall be captured and discharged over no less than a 24-hour period using acceptable hydrologic methods.

(2)

Downstream channel protection can be provided by an alternative approach in lieu of controlling the channel protection volume subject to prior approval by the director. Sufficient hydrologic and hydraulic analysis that shows that the alternative approach will offer adequate channel protection from erosion must be presented.

(d)

Overbank and extreme flood protection.

(1)

Overbank flood protection shall be provided such that the calculated peak discharge of stormwater runoff resulting from the two-year, ten-year, and 25-year return frequency, 24-hour duration storm events shall be no greater after development or redevelopment of the site than that which would result from the same two-year, ten-year, and 25-year return frequency, 24-hour duration storms on the same site prior to development or redevelopment.

(2)

Extreme flood protection shall be provided such that the calculated peak discharge of stormwater runoff resulting from a 100-year frequency, 24-hour duration storm shall be no greater after development or redevelopment of the site than that which would result from a 100-year frequency, 24-hour duration storm on the same site prior to development or redevelopment.

(3)

A downstream hydrologic analysis shall be performed to determine if the proposed development or redevelopment causes an increase in peak discharge as compared to pre-development runoff rates for the same site, or has the potential to cause downstream channel and streambank erosion. This analysis must be done for the two-year, ten-year, 25-year and the 100-year return frequency, 24-hour duration storm events, at the outfall(s) of the site, and at each downstream tributary junction and each public or major private downstream stormwater conveyance structure to the point(s) in the stormwater system where the area of the portion of the site draining into the system is less than or equal to ten percent of the total drainage area above that point.

(4)

If peak discharge increases are identified in the ten percent downstream analysis area, as defined in subsection (d)(3), downstream flood protection shall be provided such that calculated peak discharges for the two-year, ten-year, 25-year and 100-year return frequency, 24-hour duration storm events after development or redevelopment are no greater after development or redevelopment of the site than that which would result from the same duration storms in the same downstream analysis area prior to development or redevelopment. These criteria must be applied throughout the ten percent downstream analysis area.

(5)

Downstream flood protection can be provided by downstream conveyance improvements and/or purchase of flow easements in lieu of peak discharge controls subject to prior approval by the director and satisfaction of the following requirements:

a.

Sufficient hydrologic and hydraulic analysis must be presented that shows that the alternative approach will offer adequate protection from downstream flooding for all potentially affected downstream property owners.

b.

The applicant is responsible for submittal and approval of any necessary CLOMR prior to construction, and a LOMR upon completion of construction.

c.

The applicant is responsible for all state and federal permits that may be applicable to the site including TDEC NPDES and ARAP permits, US Army Corps of Engineers section 404 permits, and TVA section 26A permits.

d.

Developments and redevelopments that do not cause an increase in peak discharges are not exempt from conformance with water quality treatment and downstream channel protection requirements stated in subsections (b) and (c), respectively.

(e)

Pipes, channels, and other stormwater components.

(1)

The design of the stormwater system, excluding stormwater facilities for water treatment, channel protection, and overbank, extreme and downstream flood protection shall be based on the 25-year frequency storm. This criterion shall be applied to both closed conduit and open channel components. Minor systems that discharge to sinkholes must be designed to safely carry the 100-year frequency storm event.

(2)

All drainage systems shall be designed to insure that no habitable finished floor elevations are flooded for the 500-year frequency storm, and that no structures are located within the vertical projection of the ten-year floodplain line (i.e., located within the ten-year floodplain).

(3)

Pipe material specifications are as follows:

a.

Reinforced concrete pipe is required for pass through drainage.

b.

Reinforced concrete pipe is required for locations within stormwater detention or retention ponds and their outlet structures.

c.

Reinforced concrete pipe is required for location in public rights of way for all streets classified as "minor collectors" and above. For other pipe installations within public rights of way (i.e., local, residential streets), reinforced concrete, dual wall high density polyethylene, or aluminized corrugated metal pipe may be used. Other metal pipe materials, including corrugated metal, are not permitted. At no less than 30 days after the time of installation, visual inspection and deflection testing will be performed on all pipes and certified in accordance with TDOT Standard Specifications for Road and Bridge Construction including Supplemental Specifications - Section 607.09 of the Standard Specifications for Road and Bridge Construction dated March 1, 2006, to ensure structural integrity. All pipes with deflections greater than 5 percent of the nominal pipe diameter, undue misalignment, or poor joint construction shall be replaced by the contractor at his expense.

d.

It shall be the responsibility of the property owner to provide all necessary design, date, and installation details for construction to ensure failure of cross drains, pipes, culvers or drainage systems will not occur, and prevent flooding or potential property damage on adjacent properties or right-of-ways.

e.

Final pipe design specification must be indicated in the stormwater management plan and approved by the director.

(Ord. No. O-07-12-101, § 2(§ 4.5), 1-28-08)