Choosing the right stormwater management system starts with a question most project teams ask too late: what does this specific site need? Not what worked on the last project, and not what a neighboring municipality specified. Every site carries its own combination of drainage pressures, soil behavior, regulatory requirements, and spatial constraints – and the system that performs well on one parcel can be the wrong answer entirely on the next one. At Foley Products, a CMC Precast Business, we’ve worked across enough diverse sites and regulatory environments to know that getting this decision right early changes a project’s trajectory. Here’s how to think through it.
What Determines the Right Stormwater Management System for Your Site
The selection of a stormwater management system is not a catalog exercise. It starts with a clear-eyed look at what the site is telling you.
Drainage area and watershed characteristics set the floor for everything. How large is the contributing basin? What are the slope, land cover, and impervious surface conditions that determine how quickly and in what volume runoff arrives at your system during a storm event? Those inputs drive pipe sizing, storage volume, and outlet capacity before any product discussion begins.
Soil infiltration capacity shapes what is physically possible. Well-drained, sandy soils may support a retention or infiltration-based approach that allows water to move back into the ground. Dense clay or low-permeability soils typically require a system designed to store runoff and release it at a controlled rate because the soil will not absorb it on any useful schedule.
Available footprint changes the conversation on constrained sites. Dense commercial parcels, tight suburban infill projects, and mixed-use developments often cannot accommodate an above-grade detention pond or open basin. That physical reality frequently pushes the decision toward underground stormwater management systems that store water below grade and return the surface to paving, structures, or landscaping.
Regulatory and permitting requirements set the boundaries. The National Pollutant Discharge Elimination System (NPDES) establishes the federal baseline for stormwater discharge management, but state agencies and local municipalities add their own layers. Some jurisdictions require that post-development peak discharge rates match pre-development conditions. Others require water quality treatment before any runoff leaves the site. Understanding what your permit requires belongs at the beginning of system selection, not the end.
For underground systems, traffic and structural loading deserve specific attention. A detention structure installed beneath a parking lot, roadway, or any vehicle-bearing surface needs to be designed and rated for those loads. Specifications should confirm this well before the project reaches the bid stage.
Detention vs. Retention: What the Difference Means for Your Project
These two terms are often used interchangeably, creating real confusion during planning. They are meaningfully different approaches, and which one you need is typically determined by your permit, not your preference.
A detention system temporarily stores stormwater and releases it slowly through a controlled outlet – a weir, an orifice plate, or a similar structure. The system empties between storms. Its purpose is to reduce peak discharge rates so that downstream infrastructure, pipes, channels, and receiving water bodies are not overwhelmed. Published guidance from Contech Engineered Solutions offers a useful field indicator: if there is an outlet pipe or weir on the structure, it is detention. These systems are typically required when permits specify that post-development runoff rates must match pre-development conditions.
A retention system holds water on a more permanent basis. There is no positive outlet for the design storm. Instead, stored water infiltrates the surrounding soil, evaporates or transpires, or is captured for reuse, such as irrigation or dust control. Retention is most often specified when the project goal is groundwater recharge or water quality improvement, and it depends on soils being sufficiently permeable to support infiltration at the required rate.
Hybrid configurations are common in practice. In Florida, civil engineers frequently design systems that pair retention for water-quality treatment with detention for volume control because state and regional Water Management District standards require both. As RSP Engineers notes in published stormwater guidance, modern projects in regulated coastal states often use hybrid systems that combine retention and detention within a single site design. The permit determines which approach applies. Site conditions determine whether it is physically achievable.
Underground Systems and the Space-Efficiency Advantage
When a site cannot support an above-ground pond or open basin, underground stormwater management relocates that storage function below grade, returning the surface to productive use. We offer three primary approaches: reinforced concrete pipe, box culverts, and StormPrism.
Reinforced concrete pipe and box culverts bring decades of documented performance in underground detention. Both are durable, structurally reliable, and accepted by state Departments of Transportation – which matters for any project on or adjacent to public right-of-way.
StormPrism is our modular underground stormwater detention system, engineered to address the maintenance and safety challenges that have historically made underground stormwater management harder than it needs to be. The architecture is open-air with flat, unobstructed floors. Sightlines inside the structure extend more than 300 feet in any direction, including diagonally, which means a clear path to the exit is always visible during inspection or maintenance. That is a deliberate design decision, one that eliminates the confined-space conditions created by compartmentalized interiors in older underground systems.
StormPrism supports detention, infiltration, and rainwater harvesting, and the modular format allows engineers to configure the layout around irregular footprints or unusual site geometry that would make a conventional rigid structure difficult to accommodate. For projects that also require water quality treatment, our separator and filtration systems integrate with StormPrism in a treatment-train approach, or can be specified as standalone systems, depending on what the permit requires.
Do Underground Stormwater Systems Require Regular Maintenance?
Yes – and those maintenance requirements belong in the system selection conversation well before the permit application is filed.
Most jurisdictions require a post-construction maintenance agreement as a condition of stormwater permit approval for underground systems. That agreement typically establishes inspection intervals, sediment removal procedures, and documentation requirements. Annual inspection is a common baseline, per guidance referenced in the American Society of Civil Engineers’ analysis of underground detention system performance, with additional checks recommended following significant storm events.
What varies considerably between systems is how that maintenance is carried out in practice. Underground systems with internal walls, columns, or compartments create confined-space conditions that complicate access, restrict equipment movement, and introduce occupational safety risks. Sediment removal in those environments requires more time, more personnel, and sometimes specialized equipment to navigate around obstructions.
StormPrism’s open-floor design changes that dynamic. The flat, unobstructed interior allows a vacuum truck to move through the structure and remove sediment without having to work around internal components. There are no trip hazards, and the open sightlines address the confined-space concerns that would otherwise require additional safety protocols under OSHA guidelines. Over a 20 or 30-year service life, the difference in maintenance efficiency between an open system and a compartmentalized one adds up – both in cost and in the safety conditions for the crews doing the work.
Building Stormwater Infrastructure That Holds Up Over Time
A stormwater management system is not a short-horizon investment. The system installed today needs to perform through future storms, regulatory inspections, and maintenance cycles that extend well past project closeout. Giving drainage area, soil conditions, available footprint, permit requirements, and long-term maintainability the same engineering attention as any other element of the site plan is not over-engineering the decision. It is how you avoid a system that performs on paper but creates ongoing problems in the field.
At Foley Products, A CMC Precast Business, our team works through that process with civil engineers, contractors, and municipal clients across 18 manufacturing facilities in nine states. Whether a project calls for underground detention with StormPrism, a reinforced concrete pipe system, box culverts, or an integrated treatment train, we can help identify the configuration that fits the site, meets the permit, and holds up over the long term. Contact Foley Products to talk through what your project needs.
Summary
Selecting a stormwater management system requires matching the right approach to what the site and permit call for. Watershed characteristics, soil infiltration capacity, available footprint, regulatory requirements, and structural loading all factor into the decision before a single product gets specified. Detention systems control peak discharge by holding and releasing runoff at a controlled rate through a restricted outlet, while retention systems store water for infiltration, reuse, or evapotranspiration, and hybrid configurations handle permits that require both. For constrained sites, underground options, including reinforced concrete pipe, box culverts, and StormPrism’s modular detention system, deliver full stormwater management below grade without sacrificing surface land. When water quality treatment is also required, separators and filtration units integrate into the system as part of a treatment train. Maintenance access and long-term serviceability belong in the selection criteria from the start, not as an afterthought once the system is already in the ground.
Frequently Asked Questions
What Is the Difference Between a Detention Pond and an Underground Detention System?
A detention pond stores runoff in an open above-grade basin that requires ongoing surface maintenance – mowing, bank erosion control, and periodic dredging. An underground detention system like StormPrism stores the same volume below grade, freeing the surface for parking, buildings, or landscaping while achieving the same hydraulic function. The core engineering difference is land use and maintenance access, not storage performance. Both systems control peak discharge through a restricted outlet; where they differ is in where the storage volume is located and what it costs to maintain over a multi-decade service life.
How Do I Know If My Site Needs a Water Quality Treatment System in Addition to Detention?
If your state or local stormwater permit requires pollutant removal before discharge leaves the site, treatment is a permit condition, not an option. Separator systems remove settleable solids and floatables from the runoff stream. Filtration systems address finer particulates and some dissolved pollutants. When paired with an underground detention system in a treatment-train configuration, these components can satisfy both quantity control and water-quality requirements within a single integrated system.
Can a Stormwater Management System Be Designed for an Irregular or Constrained Site Footprint?
Yes. Modular systems like StormPrism are built around configurable layouts that adapt to sites where a standard rectangular basin is not an option. That design flexibility is useful on commercial, industrial, and mixed-use sites where the underground geometry must accommodate existing utilities, easements, or structural footings. Our engineering team can develop a complete StormPrism system design for permitting, budgeting, bid documents, and final submittal.
