Understanding Stormwater Retention and Detention

Managing stormwater is one of the main motivations behind installing green roofs. However, there’s often confusion surrounding the terms retention and detention in stormwater management. These two concepts are distinct despite being interrelated, and understanding their differences is crucial for designing effective green roofs and other water management systems.

Retention vs. Detention in Stormwater

To put it simply, retention refers to holding onto water permanently. With retention, the water doesn’t drain away. Instead, it’s gradually absorbed into the soil, evaporates, or is taken up by plants and released back into the atmosphere through a process called evapotranspiration. Retained water doesn’t contribute to runoff at all—it stays within the natural water cycle.

On the other hand, detention is all about slowing down the flow of water. Excess rainwater is temporarily stored and then released at a controlled rate, ensuring the drainage system isn’t overwhelmed. Detained water does eventually flow out, but the process reduces the risk of flooding during heavy rain.

Think of retention as a sponge that soaks up water indefinitely, while detention is more like a reservoir that holds water temporarily before letting it drain away.

The Evolution of Stormwater Management

Traditionally, urban stormwater management heavily relied on detention methods. Engineers designed systems to channel rainwater into basins or drainage pipes, controlling how quickly excess water was released. This approach worked to an extent but often failed to address the root cause of runoff—hard surfaces like roads and rooftops that prevent water absorption.

With the rise of green infrastructure, retention strategies gained more attention. Green roofs, rain gardens, and permeable surfaces were introduced to mimic natural systems, allowing water to be absorbed and evaporated rather than rushing into drains. By the late 1990s and early 2000s, retention had become an integral concept in urban planning.

Why These Concepts Matter for Green Roofs

Green roofs often use a combination of retention and detention to manage stormwater.

• Retention in Green Roofs: The vegetation, soil, and substrate layers soak up rainfall, preventing runoff entirely. Over time, this water is returned to the atmosphere, helping to reduce the overall volume of rainwater entering drainage systems.
• Detention in Green Roofs: For large downpours, some green roofs incorporate additional systems that temporarily store excess water. This could include special retention units or design features that slow down how quickly water flows off the roof, buying time for drainage systems to cope.

Both methods work together to minimise the impact of storms, reduce flooding, and support a healthier urban water cycle.

Why Clarity Is Important

Despite its importance, the use of clear definitions for retention and detention has often been inconsistent in the green roof industry. Misunderstanding these concepts can lead to poorly designed systems that either fail to address runoff or overburden drainage networks.

By focusing on a clear, consistent understanding of retention and detention, the industry can better communicate the benefits of green roofs and create stronger, more efficient systems. For example, while retention is excellent for day-to-day rainfall, combining it with detention systems ensures better performance during extreme weather.

Adopting Smarter Stormwater Solutions

Urban areas worldwide are facing increasing problems from flooding and stormwater due to climate change and dense development. Better stormwater management that incorporates both retention and detention can help cities become more resilient. Green roofs are a prime example of how these concepts can be applied effectively.

By fully understanding the differences between retention and detention, we can create more functional and reliable green solutions. Investing in these strategies not only protects urban areas from floods but also promotes sustainable design and healthier environments.

Moving forward, adopting a more consistent use of terminology in the industry will be vital. By untangling these concepts, we can pave the way for smarter infrastructure and greener cities that are better equipped to handle the challenges of tomorrow.