River Levees and Flood Control
By Joan Whetzel
It is the nature of river systems to overflow their banks during rainy seasons or heavy snow melts. Those people choosing to build communities along rivers frequently find their homes, farms, and businesses flooded as a result. But those same people will forever be drawn to the water, not only for the commerce, but for the love of the water. Since river flooding is embedded in Mother Nature's very nature, the solution to our flooding concerns has to be designed by people. Manmade levees help control the flooding, thus preventing much of the financial burden that these natural disasters leave in their wake.
The Intended Purpose of Levees
Earthen levees consist of artificially steep river canyons that are intended to contain the extra water produced during heavy rain and snow melt events. The raised water levels, in most cases, will not escape the earthen embankment which would normally flood out the nearby communities. However, weather conditions don’t always cooperate with mankind’s grand plans. Heavier than normal precipitation and levee breaks can defeat even the best levee systems.
What Are Levees?
Earthen levees – a.k.a. flood banks, dikes, and artificial embankments – are created by piling up dirt along the river banks and packing it down hard so that the dirt won’t easily wash away and the walls won't give way when the river swells. They are constructed with a broad base that tapers as it rises moderately thick wall with a flat top. The flat top makes allowances for sandbag stacking to add height to the levee walls during periods of heavy rains and snow melts.
Problems Leading to Levee Failures
One of the main problems is silt. Rivers carry silt downstream, to the sea. Over time, some of this silt gets deposited along the river bed, slowly making the river channel shallower. Since the river channel now holds less volume, the likelihood of the river flooding its banks at the first heavy rain or snow melt grows exponentially.
Another reason levees fail following heavy rains is that, the water is moving at such a high rate of speed and with such intensity, that the water starts carrying the dirt away, eroding the earthen walls. With the erosion, the earthen walls weaken, and the levee breaks. Poorly constructed levees (improperly packed) also lead to such catastrophic failures.
Once the levee systems are in place, maintaining the properly is the primary concern for all river communities. Levee system maintenance and flood prevention involves river widening efforts, dredging to increase river depth, and sandbagging to increase the height of the walls during emergencies. Taking care of the people living along rivers means road closures near the river, when flooding is imminent, as well as updating rescue plans and accelerating clean up efforts to prevent disease afterwards.
For anyone choosing to live near a river, flooding is always a concern. But each person can be part of the solution by paying attention the weather, listening to the authorities when road closures and evacuations are issued, assisting with sandbagging efforts, and attending city hall meetings whenever river levees and related issues are discussed to keep abreast of the issues.
Science Daily: Levee
US Army Corps of Engineers. Levee System Program.
Tulane University: River Flooding
Reuters: Mississippi River Lever Breaks, June 14, 2008
Feather River Setback Levee: TRLIA Levee Improvement Projects