Replicating Natural Runoff Through Retention and Dissipation

A simulation model for estimating retention volumes
By Randel Lemoine, Stormwater E-magazine September 2008

Natural watersheds retain and dissipate most rainwater. This water is retained on the surfaces of vegetation and in ground depressions, such as puddles, wetlands, and marshes. Natural processes such as transpiration by plants, infiltration into the soil, and evaporation dissipate this water. A natural watershed’s retention and dissipation capacity is sufficient to prevent any runoff from occurring during most rainfalls. Occasionally, when there is a heavy rainfall, a small amount of the rainwater becomes surface runoff that enters nearby creeks, rivers, and lakes.
The natural processes that retain and dissipate the rainwater are diminished when land is developed, whether for agriculture or for urban use. Land development removes vegetative cover, fills in low areas, compacts the soil, and creates impervious areas. The result is increased water runoff flowing more frequently across the land and discharging into the watershed’s rivers, streams, and lakes. This increased runoff causes downstream flooding, accelerated soil loss from erosion, unstable stream banks, and pollution of water resources.

Problems in Mitigating Increased Runoff
Detention basins temporarily hold collected runoff and slowly release the water. They are constructed in an attempt to mitigate the downstream flooding problems by limiting the peak discharge rate of the runoff. However, they do not reduce the volume of runoff discharged into the nearby creeks, rivers, and lakes. Consequently, the runoff volume discharged remains greater than when the land was in its natural condition. Therefore, detention basins fail to match the natural runoff pattern that occurred prior to the land being developed. Streambank erosion, stream channel instability, and occasionally even downstream flooding continue to be problems.
Retention basins hold a certain volume of water. There are two types of retention basins: water-quality basins and water-volume basins. Water-quality retention basins remove pollutants collected by the runoff. These basins allow the runoff to pass through after holding it long enough to give natural processes time to remove a percentage of the pollutants. They do not reduce the volume of runoff discharged. Water-volume basins capture and dissipate the runoff, thereby reducing the volume and frequency of discharges from a site. A discharge of runoff occurs only when the runoff volume exceeds the basin’s maximum retention volume. However, the actual volume available for retaining the runoff from the next rainfall depends upon the dissipation of the water held from the previous rainfall. Therefore, a key factor in determining the effectiveness of a water-volume basin is the dissipation rate.
Two commonly used methods for estimating the maximum retention volume for a water-volume retention basin are the “90% Rule” and the “Two-Year-Difference Rule.” The 90% Rule requires the capture of 90% of the runoff coming from a developed site. The Two-Year-Difference Rule requires that the maximum retention volume should be equal to the difference between the two-year runoff from the developed site and the two-year runoff from the site in a natural undeveloped condition. Neither rule addresses the necessary dissipation rate relative to the storage volume. Therefore, it is uncertain that the maximum retention volume derived by these rules will adequately address the adverse effects caused by the increased runoff coming from developed land.

An Alternative Method for Determining Retention Volume and Dissipation
An alternative to these methods is to use a simulation model. This model is set up on a Microsoft Excel spreadsheet and uses local historical precipitation data. The runoff volume for each day of the simulation is estimated using the TR-55 runoff equations (USDA 1986). The retained water volume for each day is calculated by taking the difference between the precipitation volume and the runoff volume, then subtracting the daily dissipation volume. This retained water volume is added to the precipitation of the next day, which is valid because the effect of the retained water on the next day’s runoff volume has the same effect as if it were part of the precipitation for the next day. Adding the previous day’s retained water to the precipitation provides the continuity needed for determining the appropriate combination of retention and dissipation to replicate the natural runoff.

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