Sinkhole Protection

In regions with karst geology, characterized by soluble bedrock like limestone and dolomite, sinkholes are a common feature of the landscape. These depressions often serve as direct conduits for surface runoff to enter underground aquifers, making them critical points for both stormwater management and groundwater protection. Development in and around these features poses risks of localized flooding, groundwater contamination, and geotechnical instability.

The Lexington-Fayette Urban-County Government (LFUCG) Sinkhole Ordinance, adopted in 1985, is an early and focused example of a local regulation designed to address these challenges. It establishes a framework for managing development in a karst environment by controlling land use and stormwater runoff in and around sinkholes. The ordinance prioritizes site-specific analysis over prescriptive standards, creating a flexible system for protecting sensitive geologic features while allowing for responsible land development. It serves as a useful reference for communities seeking to integrate hydrogeologic considerations into their land use and Stormwater Management programs.

Key Provisions

The LFUCG ordinance outlines a process-driven approach that relies on technical review and site-specific analysis by qualified professionals. Its core components define where and how development can occur in relation to sinkholes and their drainage areas.

Applicability and Mapping

The ordinance applies to any proposed development or subdivision plan on land containing a sinkhole, its immediate drainage area, or a sinkhole cluster area. The presence of a sinkhole is initially determined by its depiction on the official Unified Mapping Program topographic maps, though its precise limits may be refined by field survey with the concurrence of the Urban County Engineer. All development and subdivision plans must clearly delineate these features, including any associated nonbuildable areas or restricted fill zones.

Sinkhole Nonbuildable Areas

A central feature of the ordinance is the establishment of “sinkhole-related nonbuildable areas.” These areas function as variable-width buffers where no buildings, parking areas, or other structures are permitted. Unlike a fixed-setback ordinance, the boundary of the nonbuildable area is determined by the Planning Commission on a case-by-case basis. The decision is informed by a review of the site’s topography, geology, soils, and known history, as well as stormwater analysis provided by the developer’s engineer. While the nonbuildable area typically follows the topographical limits of the sinkhole, the Commission can expand or contract it based on site-specific conditions and risk factors.

Stormwater Management Standards

The ordinance provides developers with several pathways for managing stormwater runoff in sinkhole drainage areas, reflecting a performance-based approach. The primary goal is to prevent increased flooding or adverse impacts on the subsurface drainage system.

The options for development within a sinkhole’s immediate drainage area include:

1. Divert Runoff: A developer may proceed with development by designing an alternative surface drainage system that directs runoff away from the sinkhole. This diverted flow must remain within the same surface drainage basin and cannot be directed into another sinkhole drainage area or a stream with known flooding problems. Land that cannot be provided with an alternative drainage system can be set aside as open space, and the ordinance allows for density to be transferred to the developable portions of the site.
2. Use Sinkhole with No Adverse Impact (Alternative 1): A developer can propose to use the sinkhole for surface runoff if substantive engineering and hydrogeological reports demonstrate that the increased quantity of runoff will not aggravate flooding. This analysis must be based on “state-of-the-art field studies” and must show no negative impacts on the proposed development, adjacent properties, or connected subsurface sinkhole systems. This option requires the concurrence of the Division of Engineering.
3. Use Sinkhole with Runoff Control (Alternative 2): A developer can also use the sinkhole for drainage if runoff from the developed area is managed through retention or detention basins. The outflow from these basins must be regulated so that the post-development flow rate is no greater than the pre-development rate for a set of specific design storms.

Restrictions on Filling

The ordinance permits some filling within a sinkhole or its drainage area with approval from the Urban County Engineer. However, it explicitly prohibits the construction of any principal or accessory buildings with soil-bearing foundations on this fill. Any structure built in a restricted fill area must be founded entirely on solid rock, a provision that addresses the significant geotechnical risks associated with building on unconsolidated material in a karst environment.

Permitting and Plan Requirements

Compliance is integrated into the standard subdivision and development plan review process. The developer is responsible for submitting plans that identify all relevant sinkhole features and for providing any required engineering or hydrogeological studies. The municipal Division of Engineering serves as the technical review authority, making recommendations to the Planning Commission, which holds the final approval authority. As a condition of approval, the developer must add specific notes to the final plan, disclosing the presence of nonbuildable areas, foundation restrictions, and a disclaimer that plan approval does not guarantee freedom from future sinkhole problems.

Selected Provisions, Annotated

The nonbuilding area shall follow the limits of the sinkhole in most cases. However, the nonbuilding area may be expanded or contracted by action of the planning Commission where warranted due to the nature of the specific sinkhole, the underlying geology, soils, drainage, and any related information…

Lexington-Fayette Urban-County Government Sinkhole Ordinance, SRA 85-2, Sec. 2

This provision establishes a flexible, performance-based buffer system. Instead of a uniform setback, the protected area is tailored to the specific risks of each sinkhole, allowing regulators to adapt to site-specific hydrogeologic conditions.

…any increase in the quantity of surface runoff due to development…will not aggravate flooding on the proposed development, adjacent existing development, or connected/adjacent sinkhole sub-surface systems. Such engineering and geologist reports must be substantive and based on state-of the-art field studies…

Lexington-Fayette Urban-County Government Sinkhole Ordinance, SRA 85-2, Sec. 4.A

This clause underscores the ordinance’s reliance on detailed, professional analysis. It places the burden of proof on the developer to demonstrate through rigorous study that the proposed stormwater management approach is protective of the complex and often poorly understood subsurface drainage networks typical of karst terrain.

Any sinkhole or restricted fill area identified here has been determined to be unsuitable for soil bearing foundation, and the entire structure of any building constructed therein must be founded on solid rock.

Lexington-Fayette Urban-County Government Sinkhole Ordinance, SRA 85-2, Sec. 5

Here, the ordinance extends beyond water management to address structural and geotechnical safety. This provision directly mitigates the risk of structural failure due to subsidence or instability of fill material placed in a sinkhole, a critical consideration for safe development.

What Makes it a Useful Model

The LFUCG Sinkhole Ordinance provides a valuable framework for communities grappling with development pressures in geologically sensitive areas. Its primary strength lies in its flexible, performance-oriented approach. By avoiding rigid, one-size-fits-all standards for setbacks and stormwater design, it empowers technical staff and decision-making bodies to require protections tailored to the specific conditions of each site. This approach acknowledges the high variability of karst landscapes and encourages detailed hydrogeologic investigation as a prerequisite for development.

The ordinance effectively integrates multiple regulatory concerns, including Groundwater Protection, stormwater control, flood prevention, and geotechnical safety. This holistic view ensures that development proposals are evaluated for a wide range of potential impacts. The clear delineation of roles—with the Division of Engineering providing technical review and the Planning Commission making land use decisions—creates a transparent and logical administrative process. The requirement for detailed plan notes also serves as an important risk communication and long-term disclosure tool for future property owners.

While modern ordinances might include more detailed criteria for water quality treatment or specify a wider range of design storms, the fundamental structure of the Lexington ordinance remains a sound model. It establishes a defensible, science-based process for reviewing development in areas where surface water and groundwater are directly connected, a key principle in any Source Water Protection strategy.

Adaptation Checklist for Municipalities

Communities can use the LFUCG ordinance as a starting point for developing their own regulations for karst terrain. Key steps in the adaptation process include the following:

• Identify and formally adopt the official topographic, geologic, or sinkhole inventory maps that will serve as the basis for identifying regulated features.
• Define the specific roles and review authority of the planning department, engineering division, and planning commission or other governing body.
• Establish clear and detailed submission requirements for hydrogeologic and engineering studies, including required professional qualifications for those who prepare them.
• Review and update the specified design storms to reflect current local precipitation data and standards found in the jurisdiction’s Design Manual or stormwater regulations.
• Determine if density transfer provisions are appropriate and consistent with local zoning codes and comprehensive plans.
• Develop specific criteria for what constitutes a “substantive and state-of-the-art” study to ensure consistent and high-quality technical submissions from applicants.
• Integrate the ordinance with other relevant local codes, including those for general Stormwater Management, Floodplain Management, and erosion and sediment control.
• Draft standard legal language for required plan notes, deed restrictions, or covenants to ensure their enforceability and long-term effectiveness.
• Incorporate explicit enforcement procedures and a penalty schedule by cross-referencing the municipality’s general code enforcement authority.
• Consider establishing a fee structure to cover the municipal cost of reviewing complex technical reports, which may require hiring third-party expert consultants.
• Define the review and approval process for “alternative surface drainage” proposals to ensure they do not cause unintended flooding or erosion impacts downstream.