CLEMSON — During 2015’s super storm in early October, 51 regulated dams either failed or breached in South Carolina with devastating consequences. Violent deluges washed away roads, bridges and other structures, resulting in millions of dollars in damage.
With still-powerful Hurricane Matthew expected to threaten the coast of South Carolina this weekend, the potential for heavy rains and flood-like conditions once again looms over much of the state.
As of 1 p.m. Wednesday, about 60-70 hours remain before the storm might descend on Beaufort County in the southeast corner of the state. But it’s not just coastal counties that are at risk. Heavy rains could extend many miles inland, creating flood conditions over a massive area that contains thousands of dams, most of which were built to create ponds for recreation, irrigation, livestock, stormwater management and a variety of other purposes.
Is it too late for dam owners who have not already evacuated to take precautions to ensure the integrity of their dams and help avoid what happened last year?
Cal Sawyer, water resources specialist for Clemson Cooperative Extension, says the most important thing dam owners can do during the time remaining is to inspect their emergency (also called auxiliary) spillways, which are structures designed to control the release of flood waters into a downstream area. Dam owners should make certain that their emergency spillways are clear of excess vegetation, brush, debris and other hindrances that could clog them up or otherwise impede elevated flow.
“During last year’s storm, most of the earthen dams that survived were the ones that had functioning emergency spillways,” said Sawyer, an associate professor in the agricultural sciences department. “Dams typically fail when too much water rushes over the tops of them, resulting in concentrated flows that can eventually cause a complete collapse. But emergency spillways act as runarounds, funneling the excess water around the dam. If these spillways are poorly maintained, they can back up and fail to perform their function. As a result, the dam takes a severe pounding that could — and should — have been avoided.”
Pond dams are designed to function properly even during heavy rains. An L-shaped pipe that rises vertically on the upstream side of the dam and then exits through the wall acts as a primary spillway that drains water into the downstream area. This creates an equilibrium that maintains water levels below flood stage. But during periods of extraordinary rainfall, the riser pipe can’t drain the water fast enough. That’s when the emergency spillway comes into play. In a well-maintained system, the excess water diverts into the spillway located to the left or right of the dam and flows along its length to the downstream area.
“I would recommend that anyone who has a pond on their property — whether it’s an agricultural operation or even an urban setting — to go out now and do a visual inspection of your emergency spillway,” Sawyer said. “Does it look stable? Is it clear and free of debris? If it is, you’re in the best possible shape. If it isn’t, then you need to evaluate how much time you have between now and when the storm hits and then do as much as you can, within reason, to get rid of anything that might impede water flow through the spillway. This includes chopping down small trees, shrubs and anything else that might have grown up there. Also, inspect your primary outlet. Make sure that if there is a trash rack, it is free and clear of debris or objects of any kind that might affect water flowing through it.”
Sawyer cautions not to overdo it. Rather than tearing small trees and bushes up by the roots, it’s better to chop then down at the base and leave the roots, which will help prevent soil erosion.
“You want to cut out what’s in there as low to the ground as you can and then move the debris out of any area that might be subject to flooding,” Sawyer said. “If you do this, you’ll be performing a valuable service not just for yourself and your own property, but for others as well. Under certain conditions, even just a few hours of work could make the difference between your dam surviving or failing.”