Stormwater detention systems are easy to underestimate until a major storm reveals what happens without them. How does a development site manage runoff without giving up land above ground? Underground detention is where most engineers land first – and for good reason. When impervious surfaces replace permeable land, runoff volume and velocity increase sharply. Municipalities require developers to manage that increase before it reaches downstream pipes and waterways. Underground detention does that work out of sight, beneath parking lots and roadways, without consuming land that has value. Foley Products, a CMC Precast Company, supplies underground stormwater detention systems including our proprietary StormPrism product, precast concrete box culverts, reinforced concrete pipe, and outlet control structures – every component a detention system needs to function.
Detention vs. Retention: Why the Distinction Matters for Your Project
Detention and retention describe fundamentally different engineering approaches, and that difference drives the system design from the beginning.
Detention holds stormwater temporarily and releases it at a controlled rate matched to pre-development runoff conditions. The goal is not to stop discharge but to slow it – to cut the peak so downstream pipes and channels receive water at a manageable rate rather than all at once. Most municipal stormwater ordinances require detention for developed sites because it protects downstream infrastructure without demanding permanent water storage on site.
Retention holds water permanently or allows it to infiltrate into the ground over time. It is used where groundwater recharge is a specific project goal, or where soil permeability makes infiltration viable. Not every site has soil conditions that support infiltration, and many municipalities do not accept it as the primary management strategy for commercial or high-impervious developments.
For most urban development projects – commercial, mixed-use, residential, and municipal – detention is the requirement. The system stores runoff underground and releases it through a calibrated outlet at a rate that mimics what the site would have produced before it was developed. Our StormPrism system is built specifically for detention and can incorporate water quality treatment components when local regulations require both volume management and pollutant removal.
How Underground Stormwater Detention Systems Work
The mechanics are clear. The engineering in each component is where it gets precise.
During a storm, runoff from parking lots, rooftops, and roadways reaches catch basins, curb inlets, or direct pipe connections and enters the underground storage structure. The structure fills from the bottom up as the storm intensity builds. The outlet control structure – a calibrated assembly of orifices, weirs, or both – governs the rate of release. Engineers size that outlet to match the pre-development peak flow for the design storm, so the downstream network sees no greater discharge than it would have before the site was built.
Foley Products, a CMC Precast Company, has supplied underground stormwater detention components at significant project scale. One project involves our precast concrete conduits forming the primary infrastructure network for a 1.3-square-mile drainage area, designed to manage flow through 100-year storm events. Flooding issues on that site predate the city’s 1969 incorporation. These systems are not demonstrations. They are infrastructure, and they are working.
We supply all the core components: StormPrism units, precast concrete box culverts, reinforced concrete pipe, outlet control structures, and the connecting infrastructure that ties it together. Managing those components through a single supplier matters for project managers and contractors who need accountability across a system where everything has to fit and function together.
What Are the Best Materials for Underground Stormwater Detention?
Three primary options make up most underground concrete detention systems: reinforced concrete pipe, precast box culverts, and purpose-built systems like StormPrism. The right choice depends on required storage volume, available footprint, and the site’s load and access conditions.
Reinforced concrete pipe and box culverts have backed underground detention for decades. They deliver proven structural performance under traffic and soil loading, a service life that frequently reaches 100 years, and no structural degradation from sustained hydrostatic pressure. Unlike flexible plastic chamber systems, concrete holds its shape and load-carrying capacity regardless of the water level inside – which matters for structures that may stay full for extended periods after major events.
StormPrism is our proprietary precast concrete underground detention system. Foley Products, a CMC Precast Company, holds all U.S. and Canadian patents and trademarks outside California, acquired from Pre-Con Products in 2023. StormPrism is designed specifically for detention applications and delivers high storage efficiency within a compact plan area – more stored volume in less footprint than traditional pipe or box culvert layouts. It can serve as a standalone system or integrate with stormwater quality treatment components when a treatment train approach is required.
Plastic and HDPE chamber systems are used in underground detention as well. They are lighter and faster to install, which can reduce initial construction cost. Under sustained traffic loading, high hydrostatic pressure, or aggressive bedding conditions, plastic systems are more structurally sensitive than concrete alternatives. In applications where traffic-rated performance, long service life, and structural reliability under water load are the governing requirements, concrete holds a performance margin that plastic chambers do not.
How Engineers Size an Underground Detention System
Sizing starts with hydrology. Engineers use TR-55 – the Urban Hydrology for Small Watersheds method developed by the USDA Natural Resources Conservation Service – to calculate pre- and post-development peak runoff rates for design storm events. Required design frequencies typically range from the 2-year through 100-year storm, depending on local regulatory requirements.
Required storage volume is the difference between what the developed site produces and what it is permitted to release, accumulated across the design storm duration. That number drives the size and configuration of the underground structure. The outlet control structure is then designed to release stored water at the permitted rate through a calibrated set of orifices and weirs that manage discharge across multiple storm sizes simultaneously.
For sites larger than approximately five acres, TR-55 typically gives way to full hydrograph routing methods that better capture how storm runoff volume evolves over time. Local stormwater management manuals specify which method applies and which storm frequencies must be managed.
Our engineering team works directly with project engineers on StormPrism sizing and box culvert detention configurations – so the system design meets both hydraulic performance requirements and manufacturing feasibility before a single order is placed. Every project also gets an assigned project manager, giving your team a single point of contact from specification through delivery.
Maintenance That Keeps Underground Detention Systems Performing
A detention system that is not maintained will not perform to spec. Sediment accumulates in storage chambers and reduces effective volume over time. A clogged outlet control structure detains more water than designed, creating upstream flooding or overloading conditions that were never part of the analysis.
Concrete systems carry a real maintenance advantage. Concrete does not degrade from sediment weight or sustained water contact. Accumulated material can be removed during routine maintenance without damaging the structure – which is not reliably true for plastic chamber systems, where sediment loading and pressure washing can compromise structural integrity.
Routine maintenance for underground concrete detention includes sediment removal from storage chambers, joint integrity inspection, outlet control structure cleaning, and inlet structure maintenance. After storm events that exceed the design frequency, a post-storm inspection confirms the system performed within intended parameters and identifies sediment accumulation that needs addressing.
Precast concrete detention systems designed with manholes, inspection ports, and proper access structures make that maintenance achievable without excavation. Specifying those access features upfront is worth it – systems that are hard to maintain get maintained less often, and deferred maintenance costs more than the access hardware would have.
Underground and Out of Sight – but Never Out of Mind
Underground stormwater detention is a long-term infrastructure commitment. The systems going in the ground today will manage runoff for decades, and the choices made during design and material selection determine how well they do that job – and what it costs to keep them doing it.
Foley Products, a CMC Precast Company, brings full-system capability to underground detention: StormPrism for storage efficiency in a tight footprint, precast concrete box culverts and reinforced concrete pipe for proven large-volume detention, outlet control structures, and the engineering support to help project teams get the design right from the beginning. With 18 facilities across 9 states, we have the manufacturing capacity and delivery reliability to keep complex, multi-phase stormwater projects on schedule.
If you are an engineer working through a stormwater submittal, a contractor managing a multi-phase development schedule, or a developer who needs the detention system approved and off the critical path – reach out to Foley Products, a CMC Precast Company. We can walk through system design, product selection, and project logistics from first inquiry through final delivery.
Summary
Underground stormwater detention systems resolve a core urban development challenge: managing the increased runoff from impervious surfaces without sacrificing usable land above ground. The system captures runoff in underground storage – built from precast concrete pipe, box culverts, or purpose-built systems like StormPrism – and releases it at a controlled rate through a calibrated outlet structure. Sizing relies on hydrologic modeling methods including TR-55, which calculates required storage volume based on pre- and post-development peak flows across multiple design storm frequencies. Precast concrete is the material of choice for demanding detention applications because it holds structural integrity under traffic loading, sustained hydrostatic pressure, and sediment accumulation – conditions that can compromise plastic chamber systems over time. Foley Products, a CMC Precast Company, supplies every component a detention system requires, along with the engineering team to help project engineers get the specification right.
Frequently Asked Questions
Does Foley Products, a CMC Precast Company, supply outlet control structures along with underground detention systems?
Yes. Foley Products, a CMC Precast Company, supplies outlet control structures as part of our stormwater management product line. We provide full-system capability for underground detention projects – StormPrism units, precast concrete box culverts, reinforced concrete pipe, and outlet control structures – so project managers have a single accountable supplier across all primary components.
Can underground concrete detention systems be installed under parking lots or roadways?
Yes. Precast concrete detention systems – including StormPrism, box culverts, and reinforced concrete pipe – are traffic-rated and routinely installed beneath parking lots and roadways. Concrete’s structural self-sufficiency under load makes it the right material for applications where vehicles will travel over or near the detention structure.
What is the StormPrism system and how does it differ from standard box culvert detention?
StormPrism is a proprietary precast concrete underground stormwater detention system. Foley Products, a CMC Precast Company, holds all U.S. and Canadian patents and trademarks outside California, acquired from Pre-Con Products in 2023. StormPrism is engineered specifically for detention applications and provides high storage efficiency within a compact plan area – typically storing more volume in less footprint than standard box culvert or pipe arrangements. It can integrate with stormwater quality treatment components when both volume management and water quality goals must be addressed.
