Stormwater Management Plan

Introduction

The Knox County Stormwater Management Plan (SWMP) was prepared to document the activities and functions that comprise the Knox County Stormwater Management Program. This Stormwater Management Plan describes that program and any known future changes or enhancements. Secondarily, the SWMP was written in order to comply with State water quality permit requirements.

Background: Knox County and Stormwater

In order to understand Knox County’s local stormwater management program and the activities included in this SWMP, it is necessary to be aware of the water resource characteristics of the County, a history of the County’s stormwater management program and the regulations and policies that form the basis for stormwater management activities.

Knox County is rich in water resources, with local watersheds draining into two major rivers: the Tennessee River and the Clinch River. The county’s watersheds have a diverse topography, ranging from broad, flat floodplains to gradually rolling hills surrounded by steep, rocky ridges. The soils in these watersheds are mostly composed of loams and clays, which vary in their ability to soak up rainfall and in their propensity to erode when exposed to rainfall. Streams in the wide floodplains are typically slow-moving and can provide a significant amount of floodwater storage, while many of the streams in hilly areas can dry completely during the late summer and fall. These hillside streams can also be “flashy” during storm events, filling rapidly and moving water quickly to downstream areas.

Knox County's population has grown from 319,694 in 1980 to 475,609 in 2020. As more people have moved into the county, the corresponding development of buildings, roads, and other areas has changed the natural hydrologic characteristics of county watersheds and streams. As areas urbanize, roofs and pavement now cover areas that once allowed rainwater to infiltrate into the ground, thus increasing the volume of stormwater runoff that is delivered to local streams. The result is that local streams must carry more water, at a faster rate and for a longer amount of time. Consequently, stream beds and banks that cannot tolerate these changes begin to erode, often significantly widening the stream and sometimes moving its location altogether.

Stormwater runoff from development can impact Knox County streams in other ways. Rainwater falling on buildings, roadways and construction activities can become contaminated with sediments, suspended solids, nutrients phosphorous and nitrogen, metals, pesticides, organic material, and floating trash. These pollutants are then carried into local streams. Unlike sanitary wastewater and industrial wastewater, most stormwater is not treated prior to entering streams. Pollution of stormwater runoff must be prevented at the source. Clearly, development and its potential impact on local streams, human safety, and health lead to the need for formalized stormwater management at the local level. Prior to the late 1990s, the Knox County stormwater management program focused almost exclusively on street and lot drainage control (e.g., ditches, culverts, and detention ponds). This was the case in most small and medium‐sized communities in the United States, where the conveyance of stormwater runoff quickly and safely away from developed areas and roadways was the only stormwater‐related priority. Across the nation, Federal, state, and local government priorities have expanded over time with the recognition that this narrow focus contributes to the deterioration and loss of natural drainage ways, floodplains, riparian areas, and other water resources, all of which are valuable for drainage control and water quality management. Knox County’s program now has a more comprehensive charge, which is the management of stormwater quantity and quality.

Water Quality

The overarching goal of the program is to improve surface water quality to a level where they meet or exceed minimum state and national standards. Working towards this goal Knox County performs activities designed to improve or protect water quality though NPDES permit compliance and watershed initiatives programs. By performing activities described in the subsequent section the type and quantity of pollutants in stormwater are reduced. Some impairments to surface water quality are caused by pollution sources that are outside of the purview of Knox County and are not addressed in this plan.

NPDES Permit and the Six Minimum Control Measures

Knox County operates a Municipal Separate Storm Sewer System (MS4) which is a system of conveyances which collect and conveys stormwater runoff from roads, sidewalks, and parking lots and discharged into local waterbodies. Stormwater discharging from the MS4 often carries pollutants with it which can be harmful to local waterbodies. This stormwater is regulated by the EPA’s National Pollution Discharge Elimination System (NPDES) through a permit program administered by the Tennessee Department of Environment and Conservation. Additional information about Knox County’s NPDES permit can be found here: https://www.knoxcounty.org/stormwater/npdes_permit.php

Information about TDEC’s NPDES Permit System and a copy of the current and previous permits can be found on the following websites:

TDEC NPDES Municipal Separate Storm Sewer System (MS4) Program

TDEC Small MS4 General Permit TNS000000

The permit issued on September 1^st of 2022 required some changes to the stormwater management program which Knox County is expected to complete by September 1^st, 2024.

The NPDES permit requires that Knox County’s Stormwater Management Program includes six minimum control measures. Those measures are listed here and further expanded upon in the following sections.

Public Education and Outreach on Stormwater Impacts
Public Involvement/ Participation
Illicit Discharge Detection and Elimination (IDDE)
Construction Site Stormwater Runoff Control
Post-Construction/Permanent Stormwater Management in New Development and Redevelopment
Pollution Prevention/Good Housekeeping

1.&2. Public Information and Education and Public Involvement and Participation

The Public Information/Education and Involvement/Participation programs are a requirement of the State of Tennessee NPDES General Permit for Discharges from Small Municipal Separate Storm Sewer Systems (MS4). As described in sections 4.2.1 and 4.2.2 of the permit, Knox County is required to develop and implement a plan to educate targeted audiences on stormwater impacts and facilitate public participation in the stormwater management program; Public Education/Outreach and Public Participation/Involvement are commonly referred to as Minimum Control Measure (MCM) 1 & 2 respectively. Knox County has developed a Public Information and Education (PIE) plan that describes our strategy for implementing these two interconnected MCMs that is most likely to have significant positive stormwater impacts.

You can view the PIE Plan here.

Knox County began implementing a public education program in 2003 with the issuance of the first NPDES permit. Since then, the County has developed a robust education and outreach program that incorporates County initiatives outside the NPDES permit scope as well as permit-required activities.

You can find more information about Knox County’s education and participation activities here.

Illicit Discharge Detection and Elimination (IDDE)

The purpose of the IDDE program is to meet permit compliance of Section 4.2.3 of the State of Tennessee NPDES General Permit for Discharges from Small Municipal Separate Storm Sewer Systems (MS4) by detecting, identifying, and eliminating all non-stormwater discharges into the Knox County storm sewer system in the unincorporated areas of the County. This is accomplished in many ways including, but not limited to dry weather screening field inspections, complaint and spill response, and general and targeted education and outreach. See the Illicit Discharge Detection and Elimination webpage and plan listed below for more details.

Knox County began implementing this program in 2003 when the first NPDES Permit was issued to Knox County. Since that time, the County has met all permit requirements which includes, but not limited to: (1) mapping all of the County’s storm sewer system on public properties and right-of-way (almost 40,000 assets surveyed); (2) implementing a dry weather screening program which is actively looking for illicit discharges during dry weather (over 1200 inspections in the last 6 yrs.); (3) responding to spills and illicit discharge complaints (30-50 complaints/yr.); (4) issuing Special Pollutant Abatement Permits (SPAPs) to business and land uses that have high pollutant potential (5) and doing general and targeted public education/outreach and involvement through special events, brochures, presentations, and campaigns around Knox County (you can find more information on Education & Public Participation here.

To see the current IDDE plan for this permit cycle, please click the link below.

Illicit Discharge Detection and Elimination Webpage

Knox Co Illicit Discharge Detection and Elimination plan

Construction Site Stormwater Runoff Control

Construction site runoff control has been overseen by Knox County EPW staff on developments within the unincorporated parts of the County since the first small MS4 permit was issued in Tennessee. The County is responsible under this permit for ensuring operators who hold a TN Construction General Permit are fulfilling their permit obligations, and that any deficiencies in their operations are corrected.

Water quality degradation in urbanizing watersheds starts when development begins. Erosion from

construction sites and other disturbed areas contribute large amounts of sediment to streams. As

construction and development proceed, impervious surfaces replace the natural land cover and

pollutants from human activities begin to accumulate on these surfaces. During storm events, these

pollutants are then washed off into the streams. There are a number of other causes of nonpoint source

pollution in urban areas that are not specifically related to wet weather events including leaking

sewer pipes, sanitary sewage spills, and illicit discharge of commercial/industrial wastewater and

wash waters to storm drains.

The objectives of Knox County’s construction site stormwater management regulations are:

To protect streams within Knox County from sedimentation and other pollutants that may result from construction activities.
To rely first on erosion controls and phasing to reduce the potential for off-site sedimentation.
To prevent off-site sedimentation from land-disturbances of any size.
To mirror, to the extent practical, the requirements of the State of Tennessee Construction General Permit; and
To comply with the requirements of the State of Tennessee Municipal Separate Storm Sewer System (MS4) permit.

Knox County’s erosion prevention and sediment control program guidance is based upon the Tennessee Erosion and Sediment Control Handbook.

Erosion prevention is the first line of defense to prevent off-site sedimentation. In the past, erosion within a construction site has been considered acceptable and part of the overall construction process, and emphasis was placed on the control of eroded sediments. However, erosion increases the potential for off-site sedimentation and increases site grading costs. Relying first on erosion prevention measures reduces the potential for enforcement actions resulting from off-site discharges of sediment, the maintenance requirements for sediment control measures, and overall grading costs.

Minimize the area that is disturbed. Developers and contractors should disturb only building envelopes, leaving the surrounding areas undisturbed thereby maintaining pre-development infiltration rates and runoff coefficients. If water quality buffers and natural areas will be used as post-construction water quality controls, limiting the disturbed area will be required in those areas at a minimum.

Sequence land disturbing activities to minimize the amount of time that such areas are exposed to storm events. For example, if a development will ultimately disturb 75 acres, the land disturbing activities should be phased or sequenced into smaller, more manageable sections with EPSC measures prescribed for each section.

Sediment must be retained on-site. Construction and land-disturbing activities inherently cause sediment migration. However, the Knox County Stormwater Management Ordinance prohibits off-site sediment discharges. Sediment controls must be designed to retain sediment on the development site and prevent sediment from discharging onto adjacent property, into the storm drain system, or into the street.

All disturbed areas must be permanently stabilized after construction has ceased. To prevent the potential for ongoing erosion and sedimentation, permanent ground cover must be provided on all areas that were disturbed during construction. The permanent ground cover can consist of any of the following: permanent grasses or other permanent vegetative cover; asphalt or concrete pavement; rip rap or other hard armor for channels and slopes; or buildings.

The following documents and webpages provide additional information regarding this program.

Knox County Development Services Webpage
Knox County Stormwater Management Manual

Post-Construction/Permanent Stormwater Management in New Development and Redevelopment

Knox County has a post construction permanent stormwater management program which aims to ensure that Stormwater Control Measures (SCMs) on new developments and re-developments are designed, operated, and maintained in accordance with the previsions of the section 4.2.5 of the NPDES Small MS4 General Permit and to reduce pollutants in stormwater discharges to the maximum extent practicable. The program includes plans review, site inspections, and a measure to ensure that SCMs are adequately operated and maintained. The 2022 permit contains requirements to fully implement the Permanent Stormwater Management program by September 1^st 2024. Knox County is working to develop and modify the program accordingly as described in the Knox County Permanent Stormwater Implementation Plan.

The following documents and webpages provide additional information regarding this program.

Permanent Stormwater Webpage
Knox County Development Services Webpage
Knox County Stormwater Management Manual
Homeowner Guides for SCMs

Municipal Operations; Pollution Prevention Good Housekeeping

The Pollution Prevention/ Good Housekeeping minimum control measure is comprised of set of processes, actions, and procedures which reduces the impact of municipal operations on stormwater quality. The intent is to ensure that Knox County’s municipal operations are performed in a manner which reduce stormwater pollution. Details and federal rational can be found here: EPA 2018 PPGH Factsheet.

Knox County’s Pollution Prevention Plan can be found here

Monitoring

Knox County Stormwater has a monitoring program that provides data to identify pollution sources in stormwater and surface water, and aids in determining the effectiveness of the stormwater management program. Knox County is implementing a jurisdiction-specific monitoring plan which was approved by TDEC on July 31, 2024.

You can view Knox County’s Monitoring Plan here