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Scouring

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Massive thunderstorms that will dump serious amounts of rain are forecasted for the next few days. The rain and warm weather is expected to melt most of the snowfall that’s been on the ground upriver for months. This means that huge amounts of water will soon be flowing in the local river at speeds much faster than normal. It’s your job to add material to best support the bridge structure to keep the bridge from collapsing. Do you think that you can withstand the flood? Get ready, because the downpour is coming.

Scouring Brochure

Scouring Brochure

What is erosion?

Erosion is a process of breaking down soil and rocks, which are then carried away to another location. There are many types of erosion like water, wind, and waves, and they wear away the Earth's surface. Water erosion is one of the most common forms of erosion. Rain can create splash erosions because the raindrop moves tiny bits of soil as it splashes on the ground. Those droplets of water will collect into streams and create sheet erosion as soil is carried away by runoff. The water in a stream or river corrodes rocks by dissolving them, as well as eroding them as the water hits against them. The water then picks up the corroded material and transports it down the river. An example of water erosion is the majestic Grand Canyon. The Colorado River has run through the canyon for thousands of year, and over time the water has eroded the rock below. In deserts, wind erosion is very common. The powerful wind picks up the sand from the desert and can transport it to a different region. The wind can also use the sand in the wind to wear away rock formations just by the impact of the sand hitting the rock. This process is a form of abrasion. Abrasion is the scraping of a rock due to friction between the rock and the particle. Wave erosion produces coastal erosion due to the power of the wave as well as the substances dissolved in the water. Wave pounding is when a wave hits a cliff so hard that rock pieces break off. Abrasion can also occur if material from the sea crashes onto the cliff breaking bits of rocks off. Not only can the material of what is inside the ocean erode the sea cliff, but also what the water is composed of. In seawater, carbonic acid is dissolved in the water, which makes the water slightly acidic. Because of this acidity, the rock will corrode over time as it is in contact with the water. Limestone cliffs are significantly affected by this, as its main material calcium carbonate can be dissolved by acid. This process is called corrosion, which is a gradual destruction of a material due to a chemical reaction.

What factors increase the rate of erosion?

For soil erosion from rain, there are many factors that increase the rate of erosion. An important factor is the rain. The amount and intensity of rainfall can increase the rate of erosion because the more rain there is, the more soil erosion there will be. The type of rain also influences how much soil will be eroded because if the rain is acid rain, it will corrode the soil. The type of soil can also affect the rate of erosion. There are three types of soil, which are based on size: sand, silt, and clay. Having a soil material of silt will increase the rate of erosion since it is more prone to erosion. The slope length and steepness can also dramatically increase the rate of erosion. The longer the slope is, the more of a potential for erosion there is due to runoff building up. The steepness of the slope is also an important factor because the steeper the slope is, the faster the water will flow, which will likely cause erosion due to abrasion.

What factors decrease the rate of erosion?

The factors that increase the rate of erosion can also be used to decrease the rate of erosion. For example, the amount of rain will decrease the rate of erosion, because if there is less rain, there will be less run off. The material of the soil can also decrease the rate of erosion. Silt is the most erosive material, but sand and clay are less erosive soils. Clay soil, for example, erodes less easily because it is very cohesive. When clay soil gets wet the soil particles cling together to form a doughy consistency. This leads to less soil displacement. So if a soil has a high clay material, the rate of erosion will decrease. The slope length and steepness can decrease the rate of erosion. Slopes with short lengths that are relatively flat will greatly decrease the severity of erosion. Vegetation is also important in decreasing the rate of erosion. A lot of plants and trees will protect the soil from the raindrops due to foliage cover. The vegetation also binds the soil together as well as takes in some water through the roots.

What is the function of bridge pilings?

The function of bridge pilings is to support bridge towers in moving water such as a river. Piles are long poles made of wood, concrete, or steel that are hammered into the ground underwater. The piles are hammered into the soft soils until the compacted soil or rock level is hit. By placing the pile into such a large depth, the friction from the soil surrounding the pile will support the weight of the bridge.

What is scouring?

Scouring is when fast moving water removes the sand or rocks from around the bridge foundations on the river bottom. A hole is left behind when the sand is washed away from the river bottom. Scouring is most common during floods due to the fast flowing water, which can lift and carry the sediment away. About 60% of bridge failures are due to scouring, and it is one of the main causes of bridge failures. Because of scouring, bridges must be inspected periodically above and below the water to ensure that the bridge is safe.

How do engineers reduce scouring?

Engineers reduce scouring by using riprap, a method of placing large stones and other materials around the bridge foundation. The purpose of this is to absorb, deflect, and diffuse the water currents. The rocks absorb the impact of the current flow and the gaps between the rocks slow the water flow, which will lessen the amount of erosion.
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