When you pave over a site, you accelerate the speed at which water runs off. During a heavy storm, this accelerated water rushes into the municipal storm sewer all at once, overwhelming the city’s infrastructure and causing flash floods. To prevent this, municipalities strictly enforce a “Zero Net Runoff” policy: the post-development peak flow of a site cannot exceed its pre-development peak flow.
To comply with this law, landscape architects and civil engineers must design systems that temporarily trap the excess water on-site and release it slowly over time. This is achieved through the construction of stormwater basins. However, confusing the terminology and the mathematical function of these basins can lead to catastrophic design failures.
Detention vs. Retention: The Critical Difference
While often used interchangeably by novices, these two systems serve entirely different hydrological functions:
- Detention Basins (Dry Ponds): These are designed to temporarily detain water. They fill up during a storm to “shave the peak” off the floodwave, and then slowly drain out completely through a restricted outlet pipe over 24 to 48 hours. Between storms, they are dry, usable landscapes (like a sunken soccer field).
- Retention Basins (Wet Ponds): These are designed to permanently retain water. They look like natural lakes. They hold a permanent pool of water, and during a storm, the water level simply rises and slowly spills over a weir. They are used when groundwater tables are high or soil infiltration is impossible.
Pro Tip: The Multi-Stage Outlet Structure
A detention basin cannot simply have a single hole at the bottom for drainage. It must be engineered to handle different intensities of storms. Professionals use a concrete Multi-Stage Riser. A small orifice at the bottom restricts flow during a common 2-year storm. A larger grate higher up allows faster flow during a 10-year storm to prevent the basin from overflowing. And an emergency spillway at the very top prevents the structural berm from washing out during a 100-year flood event.
Key Metrics for Basin Sizing
Designing a safe and legally compliant detention basin requires precise routing calculations (Hydrographs) that balance three variables:
| Hydrological Variable | Definition | Design Impact |
|---|---|---|
| Inflow Hydrograph (Peak Q) | The massive, rapid spike of runoff entering the basin from the paved site. | Dictates the total overall volume the basin must be dug to hold. |
| Available Storage Volume | The total cubic feet of space inside the basin below the emergency spillway. | Must be large enough to hold the difference between Inflow and Outflow. |
| Restricted Outflow Rate | The speed at which water is allowed to leave the basin via the outlet pipe. | Must be mathematically restricted to match the historical pre-development flow rate. |
Calculate Your Storage Requirements
Estimating the required volume for a detention basin using the Modified Rational Method involves complex routing calculations to match the inflow curve against the restricted outflow curve.
Ensure your site development meets all municipal runoff restrictions by utilizing our Stormwater & Hydrology Calculators. Accurately calculate the difference between pre- and post-development flows, determine the exact cubic footage required for your dry basins, and engineer safe, compliant stormwater infrastructure that protects downstream communities.