Tag: dams

Dam Failure
In a recent video, Practical Engineering tackles an important and often-overlooked challenge in civil engineering: dam failure. At its simplest, a levee or dam is a wall built to hold back water, and the higher that water is, the greater the pressure at its base. That pressure can drive water to seep between the grains of soil beneath the dam. As you can see in the demo below, seeping water can take a curving path through the soil beneath a dam in order to get to the other side. When too much water makes it into the soil, it pushes grains apart and makes them slip easily; this is known as liquefaction. As the name suggests, the sediment begins behaving like a fluid, quickly leading to a complete failure of the dam as its foundation flows away. With older infrastructure and increased flooding from extreme weather events, this is a serious problem facing many communities. (Video and image credit: Practical Engineering)
October 3, 2017
Spillways
Extensive rains in California have brought an unusual sight to Lake Berryessa – an overflowing spillway. The upper photo, taken in 2010, shows the concrete structure of the spillway’s entrance, known as a bellmouth – or, in the words of locals, a glory hole. When the water level rises above the concrete, water begins to cascade down the spillway to relieve flooding.
The flow is rather mesmerizing and beautifully laminar until it’s fallen many feet down the hole. This is intentional on the part of the designers – at least the laminar part. It means that the flow velocity at the entrance is slow, so that animals (or trespassing people) nearby are not going to get sucked down the spillway a la Charybdis. Nevertheless, the spillway does make quick work of excess water. The New York Times reported that on February 21st about two million gallons (7.5 million liters) of water a minute flowed down the spillway. (Image credits: J. Brooks; T. Van Hoosear; video credit: Lake Berryessa News; submitted by Zach B.)
March 1, 2017
How Dams Affect Rivers
This video shows how the installation of a dam can affect river flow and sediment transport. Before the dam is added, the flow is shallow and the sediment transport is uniform. The installation of the dam creates deep subcritical flow upstream and supercritical flow downstream. This means that wave information–like ripples–can propagate upstream on the subcritical side; on the supercritical side, the wave velocity is lower than the flow velocity and ripples cannot propagate upstream. This is analogous to sub- and supersonic flow in air. The critical flow over the dam is analogous to a shock wave. The lower velocity upstream of the dam is unable to carry sediment downstream and transport essentially ceases until the sediment builds up to a height where the depth of the water above the dam is roughly equal to that below the dam and sediment transport resumes, scouring the downstream supercritical section. Around 0:40, a gate is closed on the downstream side (off frame), creating a hydraulic jump. In the final section of the video, after sediment has built up on both sides of the dam, the downstream gate is re-opened and the jump reforms as sediment is blown out below the dam. (Video credit: Little River Research and Design, with funding from the Missouri Department of Natural Resources)
March 21, 2012