The Secret Behind River Weirs: Mitigating Floods and Preserving Ecosystems
The effectiveness of building a wall in turbulent rivers to prevent downstream flooding lies in the principles of energy conservation.
When we simulate a turbulent river with a continuously flowing device, let’s say the water level is at 1.4 centimeters. Placing a weir in the middle of the river causes the water level upstream of the weir to rise from 1.4 centimeters to 2 centimeters, while downstream, the water level drops by as much as 70%, only by 4 millimeters.
According to the law of conservation of energy, energy cannot appear or disappear out of thin air but instead transfers from one object to another. In this case, the total energy of the fluid in the upstream and downstream areas remains constant. With the introduction of the weir, the increased potential energy of the fluid upstream due to the higher flow height and the larger overflow area slows down the upstream flow velocity. Meanwhile, downstream, the potential energy converts into kinetic energy, resulting in faster flow velocity as the overflow height decreases and the overflow area reduces.
This explains why the downstream water depth is lower than upstream. During heavy rainfall, the river’s water level rises, leading to flooding downstream. By strategically placing a weir, the downstream water level can be lowered, reducing the probability of downstream flooding. However, the continuously rising water level upstream severely affects the living environment of forest animals.
To mitigate this, engineers modify the weir into a zigzag shape, doubling the overflow area, allowing more water to pass through and lowering part of the still-water level. Additionally, it facilitates the deposition of suspended sediment on the riverbed, effectively preventing river channel sedimentation and serving the purpose of irrigating farmland.
It’s truly ingenious how the inventors of weirs have tackled the challenges of river management, simultaneously addressing flooding issues while considering environmental and agricultural needs.
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