Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 53

caused largely by raindrop impact. The other part of the process is the transportation of soil particles,
largely by flowing water. The primary erosion locations are overland slopes, gullies, agricultural areas,
mining operations, and construction sites. Various forms of water erosion are related to river management
and can be controlled by planting vegetation:
Splash erosion— When vegetative cover is stripped away, the soil surface is directly exposed to
raindrop impact. On some soils, a very heavy rain may splash as much as 22,400 t/km 2 of soil (Buckman
and Brady, 1969). If the soil is on a slope, gravity will cause the splashed particles to move downhill.
When raindrops strike bare soil, the soil aggregates are broken up. Fine particles and organic matter are
separated from heavier soil particles. This pounding action destroys the soil structure. A hard crust often
forms when the soil dries. This crust inhibits water infiltration and plant establishment, and runoff and
future erosion are thereby increased. Splash erosion is closely related to raindrop size. Large raindrops
have a much greater impact than small raindrops.
Sheet erosion— Sheet erosion is caused by shallow "sheets" of water flowing over the soil surface
(Fig. 2.4). These very shallow moving sheets of water are seldom the detaching agent, but the flow transports
soil particles that have been detached by raindrop impact. The shallow surface flow rarely moves as a
uniform sheet for more than a few feet before concentrating in the surface irregularities (VSWCC, 1980).
Ril1 erosion— Rill erosion begins when shallow surface flow starts to concentrate in low spots in the
soil surface (Fig. 2.4). As the flow changes from sheet flow to deeper flow in these low areas, the velocity
and turbulence of flow increase. The energy of this concentrated flow is able to both detach and transport
soil particles. This action begins to cut tiny channels called rills. Rills are small but well-defined
channels that are at most only several to several tens cm deep (VSWCC, 1980).
Gully erosion— Gully formation is a complex process that is not fully understood. Some gullies are
formed when runoff cuts rills deeper and wider or when the flows from several rills come together and
form a large channel. Gullies can enlarge in both uphill and downhill directions. Water flowing over the
headwall of a gully causes undercutting. In addition, large chunks of soil can fall from a gully headwall
in a process called mass-wasting. This soil is later carried away by storm water runoff. A heavy rain can
transform a small rill into a major gully almost overnight (Reader, 1975; Goldman et al., 1986). Figure 2.5
shows the rills being developing into gullies due to rainstorm erosion. Once a gully is created, it is very
difficult to stop it from growing, and it is costly to repair.
Fig. 2.4 Types of water erosion