When we develop land, we change the natural system. Often times, we increase both the peak runoff rate as well as the volume of runoff. The increase in both developed runoff rate and volume can be harmful to downstream channels, resulting in degradation. This degradation has effects on habitat as well as water quality by increasing sediment loads.
Detention Ponds are not typically designed to reduce the volume of storm runoff (typically measured in cubic feet or acre feet); rather, their intent is to reduce the runoff rate (typically measured in cubic feet per second, CFS).
In order to reduce the volume of storm runoff, it is necessary to implement design features which enhance infiltration.
A good example of a design feature that enhances infiltration is the porous paver system. Porous pavers work to enhance stormwater infiltration.
Modular block pavers are one example of a porous paver product. These are designed to create spaces through which water can infiltrate into an underlying gravel layer.
Runoff is stored in this underlying gravel layer and allowed to slowly infiltrate into the soil below. Seeing the installation steps for a porous paver system can be illustrative.
The installation steps for a porous paver system are outlined below.
Step 1-Subgrade Preparation
Subgrade is exposed and leveled in preparation for the application of the first layer of gravel.
Step 2—First Layer of Gravel Applied
The first layer of gravel, typically referred to as the “base course” is applied. The thickness of this layer varies, with the intent being to provide a certain amount of holding volume for storm runoff. Runoff will fill the voids of the gravel and slowly infiltrate into the underlying subgrade.
The black fabric seen in the picture is a geotextile filter fabric, intended to prevent the intermingling of the gravel and the underlying soil.
Step 3—First Layer of Gravel Leveled
The first layer of gravel is roughly leveled in preparation for the second layer of gravel.
Step 4—Second Layer of Gravel Applied
The second layer of gravel, typically referred to as the “leveling course” is applied, and leveled.
Step 5 —Sand Layer Applied and Pavers Laid in Place
In some cases, a layer of sand is applied to further enhance leveling. Once final leveling is completed, the pavers are laid in place. Depending on the paver product, sand is sometimes swept over the top of the finished surface to fill the gaps between the pavers.
At the end of the process, we have an attractive and durable surface that is capable of allowing stormwater runoff to infiltrate to the soil below.
Photos: A. Cvar