As communities face urban development combined with increased rainfall, stormwater solutions are becoming a necessary priority. People are asking important questions like “How do you control storm water?” and “What is the best solution for stormwater mitigation?” One of the most effective approaches involves stormwater storage systems that capture and manage runoff before it becomes a problem.
When exploring stormwater management, two commonly mentioned methods are underground water detention systems and underground water retention systems. Although the terms are often used interchangeably, they have distinctly different functions. Understanding these differences is key to choosing the right system for your project.
What is an underground water detention system?
A water detention system works by temporarily holding stormwater runoff from developed surfaces such as parking lots and buildings. When integrated underground, they allow the stored water to gradually infiltrate into the soil, reducing both the volume and velocity of runoff. This helps control downstream flooding and erosion on the surface. Additionally, the infiltration process recharges the groundwater table, which in turn helps counteract the long-term challenges of land subsidence and saltwater intrusion into freshwater aquifers.
Porous pavers by Invisible Structures, Gravelpave2 and Grasspave2, extend these benefits at the surface level. Their unique configuration offers up to 92% void spaces, allowing them to store about 35 inches of rainwater per hour per square foot in its base course. Because this captured water is held in place before slowly infiltrating into the ground, the pavers act as a detention system in their own right. When combined with an underground detention system, they create a comprehensive stormwater management strategy that maximizes storage capacity while supporting groundwater recharge and long-term environmental resilience.
What is a water retention system?
Unlike a water detention system, which temporarily holds runoff, an underground water retention system is designed to store stormwater for a longer period of time. This “retained” water should then be pumped out and can be repurposed for irrigation or flushing. Such a practice helps projects maximize gray water use while easing demand on municipal water infrastructure.
To ensure consistent performance year after year, underground water retention systems need maintenance to remove any sediment buildup over the years. Professionals can integrate a filtration system or install porous pavers like Grasspave2 above the retention system to address this need. Because the stormwater management system utilizes vegetation, it can filter larger particles through natural bioremediation processes. This helps keep the underground retention system cleaner and extending the overall effectiveness over time.
Why opt for underground stormwater storage systems?
Retention and detention systems can also be on-ground solutions like ponds and swales. But these surface stormwater management solutions, while effective, are not always feasible in dense urban environments where space is limited and expensive. In any case, underground stormwater storage systems prove to be a smarter, space-efficient approach.
By placing detention or retention systems below the surface, functional areas such as parking lots, parks and walkways can be preserved above them. This will help municipalities, developers and designers save space, improve aesthetics and create reliable stormwater storage solutions that also support environmental goals like groundwater recharge or reduced reliance on municipal water supplies.
Rainstore3 by Invisible Structures is engineered to serve as an efficient underground retention system as well as an underground detention system. This modular, stackable underground containment system can be installed beneath porous pavers to deliver performance and sustainability. With an impressive 94% void space, Rainstore3 delivers nearly 25 gallons of water storage per unit—or up to 600 gallons at maximum stack height—without the need for gravel fill. Its modular design allows for flexible layouts and depths, making installation efficient and cost-effective, particularly in high-value urban settings where every square foot matters.
Choosing the best stormwater storage system for your project
So, what is the best solution for stormwater mitigation? The truth is it depends on site conditions, available space and project goals. For example, New York's South Battery Park City Resiliency Project, where Rainstore3 was installed as both a water retention and graywater system. This approach enhanced the landscape and provided critical protection for Lower Manhattan against storms like Hurricane Sandy.
In contrast, some projects with limited open space may benefit more from detention systems. For instance, a corporate campus in Massachusetts used Rainstore3 as an underground detention basin beneath a large parking lot. This helped the team effectively managing peak runoff flows and replenish the groundwater without sacrificing surface space.
No matter what the application, pairing porous paving systems with underground detention or retention solutions often delivers the most effective, long-term results. To see how Rainstore3 works in real-world projects, explore our case studies.