Over the past week, rivers in North Carolina have broken previous flood records, many of which were set by Hurricane Matthew just two years earlier. Hurricane Florence dropped two to three feet of rain, causing major flooding along the Cape Fear, Lumberton, and Neuse rivers.
The Cape Fear River alone carried enough water to fill an Olympic size swimming pool every two seconds—destroying property and highlighting the limits of our country’s infrastructure.
Upstream of the flooding on the Cape Fear and Neuse rivers are large dams operated by the U.S. Army Corps of Engineers, an agency that came under scrutiny for how it operated dams during the 2011 Mississippi and Missouri river floods. The Army Corps must balance keeping reservoirs full enough to provide drinking water, hydropower, navigation, and recreation with keeping reservoirs empty enough to assist with flood control.
We spent the past few years building a database to understand how their reservoirs balanced this two-fold task across the nation.
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We learned that these competing purposes become particularly problematic in the Southeast in the late summer as the need for stored water is highest and the risk for hurricanes is greatest. To balance these needs, the Army Corps develops “water control plans” that lay out how empty or full reservoirs should be.
The Army Corps records reservoir water levels, inflow, and outflow to show how dams operate in real-time. These data let us see that reservoirs operated according to their water control plans up until Florence flooding began. As Florence began dumping rain, these reservoirs captured the flow of rivers upstream of the dam and stopped releasing water downstream to reduce flooding.
At the peak of flooding, both Jordan and Falls dams had more than doubled the amount of water stored prior to Florence. Water levels continue to rise in both reservoirs, storing water that would otherwise add to the deluge downstream. Although simple in concept, this infrastructure represents a staggering achievement of engineering and planning.
But it’s important to recognize a few key aspects of Army Corps dams. In times of drought, the Army Corps can be criticized for not diverting from its water control plans to store more water for water supply. Events like Florence are a good reminder that these reservoirs have multiple purposes, and shifting off their plans could create incredible flood risk.
Although these reservoirs are performing as intended, there are limits to infrastructure. There have been and will be floods that these dams will not be able to store. Even the gargantuan Glen Canyon Dam risked flooding in 1983. All infrastructure has limits and at some point there will be a flood that is simply beyond the capacity of any dam, including Jordan and Falls lakes.
Climate change matters and it will push the limits of our infrastructure. A warming climate allows more precipitation to be held in the atmosphere, increasing the potential for high intensity precipitation, and appears to create conditions that make ‘stalling’ hurricanes like Florence, and what Texas saw with Harvey, more likely.
The Army Corps’ infrastructure operated and performed incredibly well during Florence. However, the efficacy of this, and all water infrastructure, will be challenged in the future. We can rely on it only so much, and likely need to begin expecting floods for which this infrastructure is ineffective.