Managing Flood Control Systems Through Storms and Floods

Thursday - July 27 2023, 21:14 UTC - 1 year ago

The US has over 50,000 reservoirs used for water supply, drought relief, and flood protection. During extreme storms, reservoir managers must make difficult decisions about how to use the reservoir to balance releasing water to avoid flooding downstream and limiting water to help downstream communities. Improving forecasting could reduce flood damage.

The arduous task of cleaning up from catastrophic flooding is underway across the Northeast after storms stretched the region’s flood control systems nearly to the breaking point. As rising global temperatures make extreme storms more common, the nation’s dams and reservoirs – crucial to keeping communities dry – are being tested. California and states along the Mississippi River have faced similar flood control challenges in 2023. Managing these flood control systems is a careful balancing act. Do managers release water to make room for the storm’s runoff, increasing the risk of flooding downstream, or hold as much as possible to protect downstream farms and communities, which could increase the chance of larger floods if another storm comes through? The earlier decisions can be made, the better the chance of avoiding downstream damage. But forecasts aren’t always reliable, and waiting for the rain to fall may mean acting too late.

I managed flood control reservoirs in Iowa and locks and dams along the Mississippi and Illinois rivers for a decade, and I now research the operation of large systems of reservoirs for flood control at the University of Iowa’s Iowa Flood Center. Here’s what reservoir managers think about during storms, and how efforts to improve forecasting may soon be able to reduce flood damage: .

The many roles of dams .

The United States is home to over 50,000 operable reservoirs that are overseen by dozens of state and federal agencies. Cumulatively, these dams store more water than Lakes Erie and Tahoe combined. Thousands of square miles of rainfall may run off the landscape into rivers and streams and ultimately drain into a single reservoir. Using a gated outlet, reservoirs smooth streamflow throughout the year by storing water during heavy rains and releasing it to offset the effects of drought. This helps ensure a reliable water supply for agriculture, power generation and residential use. Importantly, the reservoirs also provide flood protection for downstream communities.

Extreme storms can mean difficult trade-offs .

Reservoir management can be drastically complicated when rainfall occurs in concentrated bursts. Reservoir operators are ready around the clock to respond to heavy rain. By adjusting gates within a reservoir’s outlet, water can be stored behind the dam, just like a bathtub with the drain partially blocked. That allows operators to release water slowly, in a controlled manner, to avoid flooding downstream communities. Operators can also help downstream communities at risk of flash flooding by limiting the amount of water they release from the reservoir. That decision has to be made quickly, though – water takes time to move downstream. If the flow is cut too late, the manager may squander the opportunity to help.

It’s when the entire region is getting heavy rain – both upstream and downstream from the reservoir – that reservoir operators face the greatest stress. When rainfall is heavy or multiple storms occur in a short period, there often is not enough time to release the accumulated water from one event to make room for the next storm. If a reservoir is full, an overflowed dam could cause catastrophic downstream flooding.