Why does river straightening reduce flood risk?

Answer 1

Straightening of river reduces floods in the immediate area because it allows the water to go straight instead of having to turn through curves. A natural meandering river has a curvy or sinusoidal shape to its path. A straight river would travel only a kilometer per kilometer of valley floor. Water associated with a curvy river will have to follow the natural sinusoidal curves and will travel considerably more distance per kilometer of valley floor. Also, water in a natural river will have a slower velocity because the course of the river is altered with every curve. A straight river sheds water faster from the immediate area because it has less distance to travel and is not slowed by the curves, however the straightening a river is associated with negative consequences including increased risk of flooding downstream and increased erosion.

Increasing the velocity causes more erosion. If a river is straight then erosion digs the river into a deeper and deeper channel. A curvy river is typically has a slower velocity and erodes less. Also the erosion in a curvy river is associated with the 'cut bank' or outside of each curve not with a deepening channel as seen in straightened rivers. If you imagine yourself as a water molecule traveling down the river when you arrive at the outside of a curve your inertia will cause you to run into the 'cut bank' until the outer bank slows your speed and alters your course. You may have caused a slight bit of erosion but because you keep hitting curves you can not build up the speed needed to induce much erosion. After eroding from the "cut bank" sediment is often deposited in the 'point bar' or inside of the next curve. This causes the curves to slowly meander sometimes leaving 'ox bows' or multiple channels. Building roads or structures in these meandering curves is unwise as inevitably erosion will destroy the structure. The fast moving waters of a straight river can carry far more sediments down stream because of the increased velocity. Straightened streams often had to be lined with concrete or 'rip rap' where the original curvy river did not because the high velocity induced severe deepening of streams and both banks would then collapse in.

Straitened rivers increase the risk of flooding downstream. A straight river or stream holds less water per distance unit of valley floor than a natural and curvy river. In a rain event a natural curvy river will hold more water. Also, since it is not slowed by the curves of a natural river, a straight river will shed what little water it holds faster. This is exacerbated by the draining of wetlands upstream and the tiling of agricultural lands to shed standing water. Wetlands and intermittent pools act as large sponges, holding and delivering rain and snow-melt slowly to downstream rivers. Furthermore, the loss of flood planes by building levies causes rivers to rise higher during flood events and again delivers more water downstream in a short amount of time.

Floods downstream have become more 'flashy' and acute because of straightening of rivers, building of levies and draining of wetlands. A rain event in the Midwest of North America prior to European settlement would have induced far less of a flood height down stream than the same rain event would cause today. This is because the water would be delivered to downstream locations slowly over a greater duration of time.