Erosion

EROSION is the removal and simultaneous transportation of earth materials from one location to another by water, wind, waves, or moving ice.

DEPOSITION is the placing of the eroded material in a new location. All material that is eroded is later deposited in another location.

Characteristics

Erosion and deposition are occurring continually at varying rates over the earth’s surface. Swiftly moving floodwaters cause rapid local erosion as the water carries away earth materials. Deposition occurs where flood waters slow down, pool or lose energy in other ways and the materials settle out. Similarly, wind erosion can occur from exposed areas such as fields, tailings and desert areas when the wind is strong and the materials are deposited when the wind diminishes. Another factor that controls the amount of erosion is the ease with which material can be dislodged. Hard granites erode very slowly while soft silts and sands erode very quickly. Vegetation that holds soils in place can decrease significantly the rates of erosion from water and wind.

Mitigation

The processes of erosion and deposition cannot be stopped totally. They can be reduced and controlled by surface drainage management, revegetation of disturbed lands, controlling stream-carried eroded materials in sediment catchment basins, and riprapping of erosion-prone stream banks, especially adjacent to structures. Understanding these processes and taking preventative action can lead to development and land-use methods that minimize losses.

Land Use

Ordinarily, erosion and deposition do not curtail land use, especially if efforts are made to minimize them.

Erosion can result in minor inconveniences or total destruction. Severe erosion removes the earth from beneath bridges, roads and foundations of structures adjacent to streams. By undercutting it can lead to increased rockfall and landslide hazard. The deposition of material can block culverts, aggravate flooding, destroy crops and lawns by burying them, and reduce the capacity of water reservoirs as the deposited materials displace water.

Erosion may adversely affect the respiratory functions of humans and livestock by reducing air quality from airborne dust, as in the famous Dust Bowl. Furthermore, there is an increased risk of pollution to surface and ground waters due to use of nutrients and pesticides from agricultural and residential treatment of vegetation.

Aggravating Circumstances

Human activities greatly influence the rate and extent of erosion and deposition. Stripping the land surface of vegetation, altering natural drainages, and rearranging the earth through construction of highways, subdivision development, farmland preparation, and modification of drainage channels for water control projects are significant factors in increased erosion and deposition. All the geologic processes that make available more material for erosion and deposition tend to increase the rates of each process. This is particularly true for landslides, mud flows, debris flows, earthflows, rock falls, and physical and chemical weathering. These processes also involve erosion and deposition while frequently make more material vulnerable to erosion.

Near Larkspur in Douglas County an access road and shallow borrow ditch were cut to serve an airport runway uphill from the access road. During construction of the road and borrow pit a large area was stripped of vegetation. Heavy water runoff from above the runway and the runway itself was channeled down the borrow ditch. There were no control features to slow the velocity of the water or retard erosion. Within five years the borrow ditch was eight feet deep. Properly designed and installed water control structures, revegetation of the graded area, detention ponds, drop structures, and other measures would have paid for themselves in later maintenance and repair costs.