Stormwater Showdown: Underground Detention vs. Dry Basins for Commercial Development in the Kansas City Metro

By: Luke McIntosh, Partner, Civil Engineer at Davidson Architecture and Engineering

KANSAS CITY — As commercial development continues to boom across the Kansas City metro, developers and engineers face a critical decision in their site planning: how to manage stormwater runoff. Local regulations mandate that new developments handle stormwater to prevent downstream flooding and pollution. Two of the most common solutions are traditional dry detention basins and modern underground detention systems. While both serve the same fundamental purpose, their differences in cost, land use, and maintenance can significantly impact a project's bottom line and long-term viability.

The Dry Detention Basin: A Traditional Approach

A dry detention basin, often referred to as a detention pond, is a familiar sight in suburban and commercial areas. It is an open, excavated area designed to temporarily hold and slowly release stormwater runoff after a rain event. Between storms, the basin remains dry.

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Pros:

·         Lower Upfront Cost: Dry detention basins are typically less expensive to construct than underground systems. The primary costs are for excavation, grading, and the installation of the outlet structure.

·         Simple Function: The design is straightforward and relies on gravity and a controlled outlet to manage water flow.

·         Visual Amenity (with design): While often a simple, grassy depression, some municipalities in the metro area, such as Olathe, encourage basins visible to the public to be designed as landscaped amenities with native grasses and other plantings.

Cons:

·         Significant Land Footprint: This is the most significant drawback. A dry detention basin requires a large portion of a development site that cannot be used for buildings, parking, or other amenities. This can be a major disadvantage on a high-value commercial lot.

·         Regular Maintenance: Dry basins require consistent maintenance to function properly. This includes regular mowing, removing debris from the outlet structure, and periodic sediment removal. Failure to maintain these areas can lead to clogging, system failure, and potential code violations from jurisdictions like Kansas City, Kansas, which require annual certification by a licensed engineer.

·         Aesthetic Challenges: While some dry basins are designed with landscaping, many are not and can be perceived as an eyesore or a wasted space, especially if they are poorly maintained.

The Rise of Underground Detention: Maximizing Land Value

Underground detention systems, also known as detention vaults or chambers, are large, subsurface structures designed to store stormwater. They are often made of modular plastic chambers or large-diameter pipes and are installed beneath parking lots, roadways, or other paved areas.

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Pros:

·         Maximizes Usable Land: This is the key benefit. By placing the detention system underground, developers can utilize the surface space for parking, building expansion, or landscaping, which is especially valuable in high-density urban areas and on small or unusually shaped parcels. The increase in usable land can translate to higher property value and a greater return on investment.

·         Aesthetic and Safety Benefits: Underground systems are out of sight and do not take up surface space, improving the overall aesthetics of a commercial property. They also eliminate the safety concerns associated with open water, such as drowning hazards.

·         Protection from Freezing: Because they are buried, these systems are insulated from freezing, ensuring year-round functionality in the Kansas City climate.

·         Improved Water Quality (with Pre-treatment): While the primary function is flow control, many underground systems can be coupled with pre-treatment features to remove pollutants, oils, and sediment, further enhancing their environmental benefits.

Cons:

·         Higher Initial Cost: The initial capital cost for an underground system is generally higher than for a dry basin. This is due to the cost of the materials and the specialized installation process.

·         Maintenance Complexity: While they require less frequent maintenance, the process is more complex. It typically involves specialized equipment to access and clean the chambers, requiring a professional stormwater management service.

·         Vulnerability to Clogging: Underground systems can be susceptible to clogging from sediment and debris if pre-treatment measures are not in place and maintained. A clogged system can be expensive to repair and lead to system failure.

Making the Right Choice for Your Kansas City Project

For commercial developers in the Kansas City metro, the choice between a dry detention basin and an underground system is not always simple. It involves careful cost-benefit analysis.

·         For large, suburban developments with ample land, a dry detention basin may be the most economical and straightforward solution.

·         For high-density urban infill projects, retail centers, or properties with limited space, the higher upfront cost of an underground system is often justified by the ability to maximize the usable surface area and increase the property's long-term value.

Ultimately, the right choice depends on the specific site conditions, project goals, and local regulations. Consulting with a civil engineer or a stormwater management professional who is familiar with Kansas City's regulations is crucial to ensure a compliant, cost-effective, and sustainable solution.