(RHIE) Water Lesson

Water

Water erosion is basically the wearing away of soil, by water.

Rain splash erosion is caused by the impact of water striking the surface. Rain splash erosion generally takes place in two steps. As precipitation is absorbed by the surface it fills the pore spaces, loosening soil particles and driving them apart. The impact of other rain drops hitting the surface splash the particle away from the point of impact.

Sometimes the soil in a location can't hold anymore water. Surface runoff forms when the rainfall from a storm is more than the soil can contain. This is when sheet erosion happens. It is caused by the unconfined flow of water running across the surface of the land. The effects of sheet erosion often go unnoticed because such thin layers of soil are being removed. It isn't until several years later that people begin to to notice that a great deal of soil is missing.

Rill erosion is caused by water concentrating into innumerable, closely-spaced small channels. Left unchecked, rills can cut vertically and horizontally and when joined form gullies.  Gullies are steep-sided trenches formed when many rills combine. Once started they are difficult to stop.

Soil erosion by water is the result of rain detaching and transporting vulnerable soil, either directly by means of rainsplash or indirectly by rill and gully erosion. Rain may move soil directly- this is known as 'rainsplash erosion' (or just 'splash erosion'). Spash is only effective if the rain falls with sufficient intensity. If it does, then as the raindrops hit bare soil, their kinetic energy is able to detach and move soil particles a short distance. Rainfall may also move soil indirectly, by means of runoff in rills (small channels) or gullies (larger channels, too big to be removed by tillage). In many parts of the world, rill and gully erosion is the dominant form of water erosion.

Running water is the main force of erosion. Rivers, streams, and runoff change the surface of the earth. Runoff is rainwater that flows over the earth's surface. During heavy rains there is a lot of runoff. As runoff flows over the earth's surface, it carries away soil pieces. Runoff empties into rivers and streams. The action of running water has made more changes on the surface than any other force.

When it comes to water - speed makes a difference. Streams that move very quickly will carry more pieces and larger pieces of soil and rock eroded from the bottom and sides of the stream. Streams flow very fast when they go down steep slopes. Streams flow more slowly when they go down gently slopes. Heavy rainfall makes a difference also. During heavy rains, streams may swell with water speeding up the flow. As the flow speeds up it will also increase that water's ability to carry pieces of weathered rock and materials with it.

Low Impact Development - Stormwater Management

Stormwater is rainwater and melted snow that runs off streets, lawns, and other sites. When stormwater is absorbed into the ground, it is filtered and ultimately replenishes aquifers or flows into streams and rivers. In developed areas, however, impervious surfaces such as pavement and roofs prevent precipitation from naturally soaking into the ground. Instead, the water runs rapidly into storm drains, sewer systems, and drainage ditches and can cause-

• Downstream flooding
• Stream bank erosion
• Increased turbidity (muddiness created by stirred up sediment) from erosion
• Habitat destruction
• Changes in the stream flow hydrograph (a graph that displays the flow rate of a stream over a period of time)
• Combined sewer overflows
• Infrastructure damage
• Contaminated streams, rivers, and coastal water

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