Why Paving a Site Accelerates Its Water
Paving over a site accelerates the speed at which water runs off, and during a heavy storm, that accelerated water rushes into the municipal storm sewer all at once, overwhelming city infrastructure and causing flash floods. To prevent this, municipalities enforce zero net runoff, meaning the post-development peak flow of a site cannot exceed its pre-development peak flow, and complying with this means designing systems that temporarily trap excess water on site and release it slowly over time through stormwater basins.
Detention Versus Retention, the Critical Difference
Detention basins, or dry ponds, temporarily detain water, filling during a storm to shave the peak off the floodwave before slowly draining out completely through a restricted outlet pipe over roughly twenty-four to forty-eight hours, remaining dry, usable landscapes, such as a sunken soccer field, between storms. Retention basins, or wet ponds, permanently retain water instead, looking like natural lakes with a permanent pool whose level simply rises during a storm and slowly spills over a weir, used specifically where groundwater tables run high or soil infiltration isn’t feasible.
The Multi-Stage Outlet Structure
A detention basin can’t simply have a single hole at the bottom for drainage, since it has to handle storms of very different intensities. Professionals use a concrete multi-stage riser instead, where a small orifice at the bottom restricts flow during a common, frequent storm, a larger grate positioned higher up allows faster flow during a less frequent but more intense storm, and an emergency spillway at the very top prevents the structural berm from washing out during a genuinely extreme flood event.
The Three Variables That Size a Basin
Designing a safe, compliant detention basin requires routing calculations that balance three variables together. The inflow hydrograph, the massive, rapid spike of runoff entering the basin from the paved site, dictates the total overall volume the basin has to be dug to hold. Available storage volume, the total cubic feet of space inside the basin below the emergency spillway, has to be large enough to absorb the difference between inflow and outflow. Restricted outflow rate, the speed at which water is allowed to leave through the outlet pipe, has to be mathematically restricted to match the historical pre-development flow rate exactly.
Where the Calculation Work Goes
Estimating the required volume for a detention basin using the Modified Rational Method involves complex routing calculations matching the inflow curve against the restricted outflow curve. CalcType’s Stormwater & Hydrology Calculators calculate the difference between pre- and post-development flows and determine the exact cubic footage required for dry basins to meet municipal runoff restrictions.