By Brandon Jones, Catawba Riverkeeper
On November 12th, heavy rainfall across the basin resulted in record flooding, property damage, and fatalities. Parts of the basin experienced similar flooding in February of this year and in June of 2019. Is this normal? Are floods becoming more common? Here are a few things to consider.
The Earth’s climate is warming. According to the latest National Climate Assessment (a congressionally mandated report written by experts from 13 federal agencies), warming is expected to increase the frequency of intense storms across the south east. This trend is most observable in Charlotte which has the longest and most complete rainfall data. This is a gradual increase of frequency which will not be felt each year or in every area.
The population of the Catawba basin has risen dramatically. Cities like Charlotte and Rock Hill are some of the fastest growing in the country. Developments are springing up in every county, particularly near the river and her reservoirs. This increases the amount of impervious or runoff generating area. Every inch of rain that lands on pavement passes about 27,000 G/acre of water the streams immediately. That same inch of rain landing on forest will sink in the soil and gradually move to streams over weeks or months. Because of this, flash flooding is more common in cities. At the same time there are more people living in the floodplain than ever before. While there have been some buyouts in the most flood prone areas, waterfront property remains extremely desirable.
Finally, it is important to remember that the Catawba does not function like a normal river. It has 11 dammed reservoirs whose storage capacity and releases are managed by Duke Energy. Unlike the South Fork or smaller tributaries, the same amount of rainfall may generate vastly different flooding on our main stem lakes. With accurate forecasting and enough time, they could theoretically prevent all flooding. However, rain forecasting is notoriously inaccurate and several of the dams can only pass water slowly. These chokepoints (James, Rhodhiss, Lookout Shoals, Mountain Island, and Wateree), prevent the reservoirs levels from being rapidly dropped and are where flooding occurs. As part of their most recent permit renewal, Duke is currently upgrading the Wateree dam to increase the release capacity. The largest floods occur when storm systems develop quickly and deliver more rainfall than forecasted.
The recent rain event dropped more than 4” in 24hrs. For much of the area this was a 50 year event, meaning each year there is a 2% chance. Human activities are making these events more likely and the impacts worse. By the same token, individual actions and smarter building ordinances may increase resiliency. To minimize your property’s contribution to runoff, we recommend disconnecting your downspouts from underground stormwater pipes and installing a raingarden or barrel. To prepare for future storms, ask your state and local representatives to support floodplain buyouts and require low impact development across the basin.