Residential architects who specialize in the hot, humid, southern climate

Best practices to mitigate stormwater runoff

Here is an example of a rain garden that Thomas Angell, of Verdant Enterprises, designed for the Cole Residence.

Here is an example of a rain garden that Thomas Angell, of Verdant Enterprises, designed for the Cole Residence.

In David Lauderdale’s interview with Al Segars in last Sunday’s paper, Al said that we need to “go above and beyond. See that your homeowners’ association maintains it storm-water retention ponds so they function as promised.” You can read Lauderdale's article here.

Beaufort County has a storm-water runoff management ordinance for new single family houses in unincorporated areas of the county that are not in an approved community storm-water runoff system and single family houses that are renovated in excess of 50% of the appraised value of the building. The ordinance requires the mitigation of the storm-water within the property limits.

For the rest of us, we should all go above and beyond by reducing the amount of storm-water on our properties that needs to be mitigated. The easiest way to reduce runoff is by reducing the amount of impervious surfaces on the property. This includes: using gravel or pervious pavers for your drive; and limiting the amount of patios and terraces or paving them with pervious pavers.

The best management practice is to collect and store the rainwater for reuse or slow infiltration. There are two options for collecting rainwater; either a rain barrel or a cistern. Both are connected to your gutters and downspouts. A rainbarrel is used to collect water for use in your garden. Be sure that the rainbarrel has a cover so that it is not a mosquito nursery. A cistern is larger and is the storage tank portion in a complete rainwater harvesting system that filters and stores water for any normal household use. If the water is to be used for potable needs it must go through additional filtration and water purification.

Another option is a raingarden which is a shallow bowl shaped depression of loose absorbent soils that is planted with deep-rooted native perennials and grasses. The runoff slowly soaks into the ground and reduces the amount of runoff entering our marshes and rivers. The design of the rain garden should be incorporated with the entire garden design. According to Garden Design Magazine rain gardens can help reduce storm-water waste by up to 99 percent.

The county has a very easy to use on-line worksheet that calculates how much runoff needs to be mitigated.  To determine the total excess runoff to be mitigated you will need to know the following information before you begin: the square footage of your roof; the square footage of other impervious areas; the square footage of your lot; your soil type, sandy or clayey; and the area of your lot that is irrigated. You next enter the number and size of storage and reuse systems want to use. The worksheet then computes the natural infiltration capacity of the lot to control runoff. If the first two practices do not control all of the rainwater, the worksheet determines the size of a raingarden to capture all of the runoff.