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Rain Management Guide FAQ

Frequently Asked Questions

This site is being regularly updated with questions. 
Remember that RMG=Rain Management Guide.

Q: What are the new regulations and what do they mean for developers in Chattanooga?
A: See the Land Disturbance Requirements chart brief for specifics and the ordinance language for details.
See RMG Chapter 1 above for an overview.  See the ordinance for details of the law.
Basically, the new regulations require new developments and redevelopments to capture runoff on site more than ever before.  Sites larger than an acre have to design their site to capture and treat runoff for up to a one inch rainstorm (which is approximately 86% of Chattanooga storms) throughout most of Chattanooga and up to 1.6 inches for new development in the South Chickamauga watershed.  This design has to be permitted before construction can begin.  The development and redevelopment are responsible for the rain water that lands on their site in a one inch or less storm.  The stay on volume requirement is determined by site impervious area and the design must manage water for volume, water quality and peak flow rate.

Q: Why the new regulations?
A: The City's stormwater permit (NPDES-National Pollutant Discharge Elimination System) required that the city establish a new stromwater management program that implements and enforces practices that reduce stormwater runoff and pollution leaving new development and redevelopment sites.   This program had to be implemented by December 1, 2014 for the city to avoid penalties and fines from the Tennessee Department of Environment and Conservation.
The new regulations may be the first introduction to low impact development and green infrastructure for some developers in Tennessee, but the trend towards low impact development is sweeping the country both by force of regulation and social activism to efficiently manage stormwater and the pollutants carried by runoff.  The increased development that couples our increase in population means an increase in impervious surfaces.  Higher volumes of stormwater and stormwater pollutants are hitting our streams and stormwater systems.  We cannot afford to maintain our stormwater infrastructure and continue increasing the volume of water it manages without incurring large costs by way of floods, poor water quality and cumbersome drainage problems.
Low impact development and green infrastructure are methods of stormwater management that work to replicate the natural hydrologic processes.  This means more infiltration and in general holding the water where it lands.  This lessens the burden of volume hitting our streams during rain events.

Q: When do I have to comply?
A: See the Land Disturbance Requirements chart brief below for specifics and the ordinance language linked above for details.

Land Disturbance Requirements Chart:

RMG SOV Applicability Chart

Q: What is the process for getting a permit?
A: Below and here are the basic process.  See the RMG for details.  


Q: How much more is this going to cost me?
A: It may not cost you more at all.  Cost is completely site dependent.  Some sites will cost less and some sites will cost more.  It completely depends on the site existing conditions and the BMPs chosen for complying with requirements.

Q: Who is responsible for maintenance?
A: The property owner is responsible for maintenance.  See RMG chapter 5 and ordinance for details (BMPs now have to get approved by a PE every five years).

Q: How can I or my engineer calculate the stay-on-volume requirement?
A: See RMG Chapter 7 and Appendix K both linked above.

Q: What about South Chickamauga creek watershed, what are the requirements there?
A: The SOV for South Chickamauga Creek watershed is 1.6 inches as opposed to the one inch because it has both impaired waterways and the Chickamauga crayfish, an endangered species.

Q: Why do we have these regulations again?  Why are the requirements higher in South Chick?
A: These regulations are mandated by the state.
South Chickamauga Creek is an EPA impaired stream that also inhabits an endangered species, the Chickamauga crayfish.

Q: I believe my site has a hardship and cannot meet requirement, what are my options?
A: 1-Apply for hardship thorough the Land Development Office (LDO).
2-If granted, then the SOV requirement can be met with Credit Coupons, offsite mitigation or in-lieu fee.  The 80% TSS capture is still required.

Q: I want credit coupons to sell, what are the requirements and process?
A: If your site is GI favorable and you can exceed the base SOV requirement, you can both receive an annual WQ fee discount and earn credit coupons that can be sold on market.  See the Credits & Incentives Manual for more information.

Q: What is offsite mitigation?
A: Offsite mitigation within the same watershed can meet requirement with 1.5 times SOV when there is a hardship.  See the Credits & Incentives Manual.

Q: What is the in-lieu fee?
A: When there is a hardship, a fee can be paid to the city at $45/ft3 for the amount of development.

Q: Is the Science behind or application of Green Infrastructure new?
A: No.  The science behind infiltration, water reuse and the screening and dissipation of pollutants through nutrient uptake, soil and other medium filtering has been widely known and used for decades.  Municipal waste water treatment facilities have used forms of biological, screening and settling treatment for 30+ years.

Q: If stormwater pollutants are allowed to enter subsurface facilities and infiltrate into soil, won't this just pollute the groundwater?
A: Studies have shown that with the exception of runoff from highly polluted urban hotspots, infiltration does not cause groundwater contamination.  Pollutants are broken down naturally in the underground soil, stone or sand medium.  What remains is insufficient to cause groundwater pollution.  Conversely, the former treatment method of attenuation and extended detention results in urban contaminants (metals, oil/grease, bacteria, temperature, sediment) being conveyed directly, through large increases in volumes, to receiving streams and downstream floodplains.

Q: Are Runoff Reduction practices a preferable method of managing flood control as opposed to detention ponds?
A: Yes.  RMG style practices can replace the need for extended detention and flood control for most of the smaller storm events and even eliminate the need for detention ponds, entirely.  Former technology, extended detention ponds resulted in lower base flow rates by starving underlying soils of rainwater from small frequent storms that would naturally infiltrate and simultaneous vastly increase runoff to streams and rivers thus damaging embankments and destroying habitat.  Because of the increased volume the ponds flow at a predevelopment rate for a longer period of time. This increased time results in increased shear stresses on the soils. Natural banks exhibit increased erosion downstream from properly design extended detention because they can naturally handle the higher stresses, but only for short durations and at lower frequencies when minor damage is repaired by vegetation over a long period of time. RMG style practices that do not discharge runoff from a 1" rain event will not contribute to this phenomenon. Over time these practices will be retrofitted to the existing public and private drainage system as redevelopment occurs, thus reducing the causes of such erosion making man-made and natural repairs both more effective and longer lasting.

Q: Runoff reduction is supposed to "mimic nature". What does this mean?
A: While no man-made system or process will ever exactly match nature, RMG practices are a much closer representation of both flow and pollution filtration/reduction than current extended detention practices. Infiltration, evapotranspiration, soil storage and screening more accurately mimics natural, undeveloped hydrology.  Detention ponds and pipe that merely convey flows do not mitigate the abrupt shift in the water balance caused by land development.  The new mandatory standards are more beneficial to water quality, erosion and sediment control, and channel protection.  Separation from the public drainage system reduces the City's stormwater infrastructure and maintenance costs.  Volume reduction also reduces expenditures for offsite flood control and flood prevention programs and measures.

Q: How will "fees-in-lieu" work, and be calculated?
A: TDEC requires that if the City chooses to implement a "fee in lieu of" payment policy, fees are to be based on 150% of the actual estimated cost of SOV measures on that site if limiting site restrictions (such as karst, foundry sand, high permanent water table, or bedrock) did not exist.  To save the developers from preparing and the City from reviewing detailed cost estimates for every site, ARCADIS Engineering was asked to propose an average benchmark.  After researching local and regional cost estimates, the City and ARCADIS proposed $30/cf to cover land acquisition, permitting, design, installation, and recurring maintenance. This was the low end of the price range found during our research. Therefore, since $30/cf is the cost (i.e., 100%) and TDEC's requirement is 150%, this equates to a $45/cf mitigation fee. With payment of this fee the City is choosing to take permanent responsibility for a private development's SOV responsibility.  A site's mitigation fee will be calculated by multiplying the unachieved Stay on volume by $45 per cubic foot.

Q:  Do these new regulations apply to new residential sites?
A: Single family homes on individual lots that are not part of a larger plan of common development are exempt from runoff reduction standards. Single family homes that are part of a larger plan of common development, and disturb at least an acre of land, will have to install RMG practices in accordance with the approved subdivision plan.

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Chattanooga TN 37402 (map)
(423) 643-6311  


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