Jonathan Allen, CPESC, TN-QHP (Tennessee) presented “Sinkhole Policy and Analysis, Knox County, Tennessee,” and Darrell Nicholas, PE and Dillon O’Donnell, PE (Tennessee) presented “Implementation of Flood Mitigation Measures in Karst Areas of Knox County, Tennessee” at the Tennessee Stormwater Association (TNSA) Conference at Montgomery Bell State Park in Burns, Tennessee on October 21, 2021.

Jonathan Allen is a Senior Scientist based in Tennessee with more than 15 years of experience in performing and managing water and natural resource projects. He has supported multiple client sectors, including local and federal governments, departments of transportation, railroad entities, private industry, and power utilities.

Darrell Nicholas is a Principal Engineer based in Tennessee with more than 35 years of experience focused on construction management and quality assurance for a wide range of complex, multiple stakeholder projects. His experience spans the project life cycle, from project planning, conceptual design, and cost estimating to detailed design, field inspection, and construction quality assurance and compliance monitoring.

Dillon O’Donnell is an Engineer based in Tennessee who supports a variety of project and service areas, including stormwater modeling and flood mitigation services, contaminated sediment remediation and power utility construction projects, industrial permitting, and field support for site characterization and low-flow groundwater sampling.

With speakers and participants from across Tennessee focusing on MS4 Permit Compliance and MS4 Programs, the TNSA Annual Conference informs, instructs, and motivates stormwater, erosion control, and watershed management practitioners, professionals, and government officials. The conference organizers anticipate a range of participating entities such as local and state government stormwater managers; environmental, engineering, and legal consulting firms; research organizations; and watershed groups.

TNSA’s mission is to assist members with their local efforts to comply with state and federal clean water laws, including stormwater regulations through the U.S. Environmental Protection Agency and the Tennessee Department of Environment and Conservation. TNSA’s goal is to protect and improve the quality of the waters of Tennessee through the exchange of information and knowledge regarding the design, construction, maintenance, administration, and operation of stormwater facilities.

Abstracts

Sinkhole Policy Comparison and Analysis, Knox County, Tennessee
Presenter: Jonathan Allen, Geosyntec
Date/Time: October 21, 2021 at 8:00 – 8:30 a.m. EST
Much of Knox County is drained by karst features subject to flooding during sustained rainfall, including closed surface depressions known as sinkholes, and underground drainage networks of openings ranging from enlarged cracks to large caves. Because the potential risks of developing near karst terrain are complex and multifaceted, Knox County engaged Geosyntec Consultants, Inc. (Geosyntec) to evaluate potential improvements in their policy-based approaches to be protective of karst environments and nearby structures and people while balancing potential restrictions on development. The first step was to evaluate other guidance documents from areas with a similar abundance of karst terrain as Knox County to understand their requirements for development near sinkholes. The study of karst terrain in Kentucky is well-established and recognized nationally. In 2012, the Kentucky Geological Survey published guidance for policymakers to use in developing ordinance language for the protection of property and humans from karst-related hazards, which was incorporated in the Kentucky Model Ordinance. Geosyntec provided Knox County with a report that provided a comparison of their policies to the Kentucky Model Ordinance (Model Ordinance)—considered a baseline standard—and the ordinances of three “peer” municipalities in Tennessee with similar karst terrain: (i) Clarksville, (ii) Cookeville, and (iii) Johnson City.

The comparison put Knox County’s requirements into a larger context to better understand potential areas of improvement and/or change. Ultimately, the goal of Knox County is to protect public safety and minimize damage to the extent practical from future flooding events. While Knox County’s sinkhole policy largely conforms to the Model Ordinance, Knox County requirements are more complex and stringent in some areas. Likewise, Knox County ordinances, regulations, and guidance are sometimes more detailed or more restrictive than their peers. Many areas in Knox County are drained by karst features subject to flooding during large, sustained rainfall events. The complexity of the karst terrain combined with the more extensive urbanization in Knox County provides justification for use of a thorough, more detailed approach with greater restrictions compared to the selected peer communities, which was highlighted by the widespread flooding of sinkholes in February 2019.

In reviewing the Model Ordinance and peer community ordinances, areas identified for Knox County evaluation included the following: (i) enhanced clarifications with respect to sinkhole flood risk determination, particularly in areas that may be part of a larger sinkhole or valley of sinkholes; (ii) requirements for sinkhole discovery during construction; (iii) requirements for sinkhole maintenance to prioritize safety concerns and mitigate potential damages to property and habitable structures; (iv) best management practices for protection of groundwater; and (v) integration of additional information into Knoxville, Knox County, Knoxville Utilities Board Geographic Information System (KGIS).

Implementation of Flood Mitigation Measures in Karst Areas, Knox County, Tennessee
Presenter: Darrell Nicholas and Dillon O’Donnell, Geosyntec
Date/Time: October 21, 2021 at 8:30 – 9:00 a.m. EST
On February 23, 2019, approximately 5.1 inches of rainfall were measured at Knoxville’s McGhee Tyson Airport comprising nearly 40% of February’s monthly rainfall total of 13.1 inches. Flooding related to the February 2019 flood event resulted in nearly $32.5 million in estimated property damage to businesses and homes with an additional $11 million in estimated damage to public sector facilities. Four of the worst impacted areas were selected for further study. The four areas of concern represent a variety of reported flood damages (e.g., flooded basements, inundated roadways) across a range of development densities. While the four areas have a long history of flooding, the increasing urbanization of Knox County has made the consequences of flooding more severe in recent years.

Site visits were conducted to make detailed observations of watershed and infrastructure characteristics to support surface water modeling and root cause analyses. A review of existing data (e.g., past flood study reports, Federal Emergency Management Agency (FEMA) maps, topographical information, stormwater infrastructure maps, and high-water marks) was conducted to supplement information collected during site visits. The study revealed that the hydrologic characteristics associated with the karst geology played an important role in the flooding of each of the four areas. The analysis of the root causes for flooding in each study area subsequently provided a framework for developing mitigative measures. The advantages and disadvantages of the potential solutions for flooding were evaluated by Knox County, and rough order of magnitude cost estimates were developed for a short list of options.

Combining the results of the hydrologic study and root cause analyses with the evaluation of costs and benefits allowed Engineering and Public Works to enter informed discussions with local stakeholders and county leadership. Understanding and identifying targeted engineering and policy-based solutions with the highest benefit to cost ratio allowed the selection of solutions and the subsequent development of a Knox County budget for 2020 to help mitigate flood damage. This presentation discusses the implementation of engineering measures to mitigate flooding and improve public safety in the unique set of hydrogeologic and development characteristics present in Knox County.

More Information

About the event: TNSA Annual Conference
About TNSA: https://www.tnstormwater.org/
For consultation regarding watershed planning, contact Darrell Nicholas at dnicholas@geosyntec.com or Jonathan Allen at jallen@geosyntec.com
Learn more about Jonathan: Jonathan Allen – LinkedIn
Learn more about Darrell: Darrell Nicholas – Geosyntec Profile