Lakshmi: Downside to SC’s heavy rainfall: drainage problems

September 4, 2013 

Lakshmi

Columbia, the Midlands, the state and much of the Southeast are drowning in above-average rainfall — 10-12 inches above normal in Columbia.

Even as the West suffers a severe drought and deadly wildfires, such as the recent one in Yosemite National Park, South Carolinians have been surprised with the deluge after nearly a decade of lower-than-normal rainfall.

Some of the effects are pleasant: Lawns are green even when the sprinklers have been turned off; streams, rivers and lakes are flowing full. We made it through what most people consider summer without the temperature in Columbia reaching triple digits — a first in years — resulting in lower cooling costs.

However there is one definite drawback. Drainage has become a bigger problem, especially in and around the city.

Rainwater generally is carried away as runoff, infiltrates into the ground or evaporates into the atmosphere. When the intensity of rainfall is lower than the infiltration rate, the water usually disappears into the subsurface. When the intensity is greater than the infiltration rate or the capacity of the subsurface soil to it, the excess water ponds on the surface or may gush, washing away soil and vegetation in its path. The less the water filters into the ground, the greater the runoff, and the water flows faster the steeper the slope.

Soil erosion results in instability and undermines structures, creating cracks in floors and walls, potholes in the pavement and collapsed hill slopes. Vegetation helps to slow down the flow of water and improves infiltration. In fact, good roots hold on to soil and slow down the erosion.

Drainage is an integral part of any building plan. We plant lawns around houses and other buildings to divert rainwater away from the walls and slow down the flow of water to reduce its erosive capacity. We design roadways so the water flows away from the pavement and does not stand on it or infiltrate underneath it, reducing the incidence of potholes. Other drainage techniques include placing French drains around low-lying structures to prevent water from ponding or eroding the soil.

One has to be prepared to withstand the vagaries of weather. The long-term forecast calls for a snowy, icy winter in the Midlands, and ice can cause even more problems than excess water.

Venkat Lakshmi

Department of Earth and Ocean Sciences

USC

Columbia

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