Stormwater Retention vs. Detention

Understanding the Differences and the Role of Modular Systems

Stormwater management is a big deal when it comes to urban development, protecting the environment, and dealing with extreme weather.  As cities grow and storms get more intense, it’s important to have smart ways to handle all that water, prevent floods, and keep water sources clean. Two of the main strategies used are retention and detention systems. While they work differently, both help manage stormwater effectively.  There’s also a third option — modular stormwater systems — which bring the best of both worlds and save valuable space.

Let’s break down how these modular systems work, the pros and cons of each, and when they are the best choice.

What Is Stormwater and Why Does It Matter?

Stormwater is the rain and melted snow that doesn’t soak into the ground. In natural areas, this water gets absorbed by soil and plants or evaporates. But in cities, all the pavement, buildings, and parking lots stop it from soaking in, leading to fast-moving runoff that can flood streets and carry pollutants into rivers, lakes, and oceans.

That’s why stormwater management is so important, especially in urban centers.  Stormwater Retention and Detention systems help slow down and control this water, preventing flooding and damaging storm drainage systems and reducing pollution while very importantly, returning water to the earth - restoring the natural water cycle. 

Stormwater Retention Systems: Holding Water for the Long(er) Haul

What Are Stormwater Retention Systems?

A stormwater retention system is an engineered solution designed to capture, store, and gradually release stormwater runoff to prevent flooding, erosion, and water pollution. These systems play a crucial role in managing stormwater in urban and industrial areas by reducing the strain on municipal drainage infrastructure and improving water quality.

Common Above-Ground Types of Stormwater Retention Systems:

1. Retention Basins (Wet Ponds): These ponds hold stormwater and help remove pollutants before the water slowly drains out, but have a number of disadvantages.
2. Rain Gardens (Bioretention Cells): These are small, landscaped areas within neighborhoods with plants and soil that green up the area and absorb and filter rainwater.
3. Green Roofs: Covered with vegetation, these roofs soak up rain, produce significant evaporation, and reduce runoff while also cooling buildings.
4. Constructed Wetlands: Designed to look and work like natural wetlands, these systems help clean and store stormwater.
5. Permeable Pavements: These special surfaces let rainwater seep through instead of pooling or running off into drains.
   

Pros and Cons of Above-Ground Retention Systems:

✅ Help recharge groundwater by letting water soak into the soil. 
✅ Improve water quality by filtering out pollutants naturally. 
✅ Support wildlife since ponds and wetlands attract birds, fish, and other creatures. 
✅ Can look great in public spaces, making parks and neighborhoods more appealing. 

❌ Takes up a lot of space, which can be tough in crowded neighborhoods and cities. 
❌ Needs regular upkeep, especially for things like algae control and sediment removal. 
❌ Can be a safety risk, especially for young kids, since these systems always hold water.
❌ Creates a breeding ground for mosquitos and other insects.

Stormwater Detention Systems: Temporary Storage to Prevent Flooding

What Are Stormwater Detention Systems?

Detention systems hold stormwater temporarily and slowly release it to a drainage system to prevent sudden floods. Unlike retention systems, they don’t keep water for extended periods — they drain out within a few days or so after a storm.

 

Common Types of Stormwater Detention Systems:

1. Dry Detention Basins (Dry Ponds): These basins collect water during storms and drain it out gradually. They’re empty most of the time.
2. Underground Detention Systems: These hidden storage areas hold water temporarily and work well in places where space is tight.
3. Blue Roofs: Specially designed flat roofs that store rainwater temporarily and slowly release it.
4. Constructed Wetlands: Sometimes used for detention, these wetlands hold excess water and let it drain slowly.
   

Pros and Cons of Detention Systems:

✅ Prevents flooding by significantly slowing down stormwater runoff. 
✅ Takes up less space than retention ponds, making it better for urban areas. 
✅ Safer for public spaces since they don’t hold water all the time. 
✅ More affordable than retention systems since they don’t require as much maintenance. 

❌ Doesn’t improve water quality much, since water drains out too fast for filtering unless costly cleaning devices are installed with the system.   
❌ Not very attractive, especially dry basins that sit empty most of the time. 
❌ Needs upkeep, like clearing out debris and making sure drains don’t clog.

Modular Stormwater Systems: The Best of Both Worlds

What Are Modular Stormwater Systems?

GRAF EcoBloc is a modular underground stormwater system that works for both retention and detention.  It’s designed to efficiently manage runoff, prevent flooding, and improve water conservation while saving space and reducing water pollution.  Unlike traditional above-ground retention or detention ponds, EcoBloc is installed underground, making it ideal for urban environments where land is costly and limited.

How GRAF EcoBloc Works

1. Stormwater Collection: Captures runoff from roofs, streets, and parking lots.
   Storage:  Holds the water underground, reducing surface runoff.
2. Controlled Discharge: Slowly releases excess water into drainage systems to prevent flooding (Detention).
3. Infiltration Option: Can allow water to soak back into the ground to purify and recharge groundwater (Retention).
4. Reuse Possibilities: Can be connected to irrigation or other non-drinking water systems (Conservation).
   

How Does the EcoBloc System Work? – A breakdown of how EcoBloc captures, stores, and manages stormwater effectively.

 

Advantages of Modular Stormwater Systems

✅ Saves space, since they go underground and don’t take up valuable land - ie:  be placed under parking lots and laneways. 
✅ Customizable and versatile, so they can be designed to fit different needs and locations.
✅ Can be both a permeable (on-site retention/infiltration) or impermeable (detention) below-ground tank. 
✅ Long-lasting, made with durable materials that resist damage. 
✅ Reduces flood risks, while also improving water management. 
✅ Can store water for reuse, which helps cut down on demand for city water. 

❌ Can be more expensive upfront, since installation requires digging and materials. 
❌ Needs occasional maintenance, like clearing sediment to keep the system working well.

Retention vs. Detention: What’s the Difference?

 

Which One Should You Choose?

Picking the right system depends on your space, budget, goals, and of course location regulations and guidelines.

Need flood control in a small space?  Go with a detention basin or modular system.
Want to improve water quality and recharge groundwater?  Retention systems are the better choice.
Looking for a smart, space-saving solution?  Modular systems give you the best of both worlds.

Stormwater Management – Learn about different solutions for handling stormwater, including retention, detention, and infiltration systems.

 

Final Thoughts

Stormwater management is key to preventing floods, protecting water quality, and keeping cities livable.  Retention systems hold water longer term to help clean it and refill groundwater, while detention systems focus on controlling peak runoff to prevent sudden floods.  Modular stormwater systems offer a smart, space-saving solution that can do both,  all with a high loading capability to install them under heavy vehicle loading areas like parking lots.

No matter which system is used, planning ahead and choosing the right setup makes all the difference in managing stormwater effectively and sustainably.