Page 132 - Biosystems Engineering

Soil and W ater Conservation 113

holding sediment particles intact and by making the detachment of
soil particles either by rainsplash or by sheet flow harder. Some veg-
etation covers and practices, such as filter strips and riparian buffers,
filter sediment from sheet flow, and therefore help mitigate the ero-
sion problem. Soils with no land cover (i.e., barren land), have no
erosion protection. By intercepting rainfall, forest canopy and ground
cover decrease kinetic energy applied to the soil by raindrops. By
transpiring soil water, forest canopy and ground cover increase soil
storage capacity and reduce surface runoff. They also provide organic
matter to the soil, which makes soils more resistive against erosion.
Ground cover slows down sheet flow by increasing resistance against
the flow. Land grazing, silvicultural activities, and forest operations
all have negative effects on soil erosion. Similarly, construction sites
are often sediment sources unless precautionary measures are taken.
Topography effects both soil erosion and sediment transport. Soils
on steeper slopes are eroded easier. Furthermore, water flows in steeper
slopes faster, resulting in higher shear stress. Watershed areas and drain-
age density (total length of channels and streams per unit area) known
to affect sediment yield. As the watershed area and drainage density
becomes larger, sediment delivery ratio gets smaller. This is due to the
increased distance between the watershed outlet and eroded areas
increasing the likelihood of sediment deposition. Also, smaller headwa-
ter watersheds are, in general, steeper compared to larger watersheds.

3.6.3 Types of Soil Erosion
The erosion process in a watershed takes place at four different spa-
tial scales: rill, interrill, gullies, and channels. Interrill and rill erosion
takes place in upland (hill slope) areas. Agricultural practices mainly
affect these two types of erosion processes. Channel erosion is highly
dependent on rill and interrill erosion, which are a source of sediment
supply to channels. The amount of sediment supply to the channel
dictates whether there will be further erosion or sediment deposition
in the channel.
Rill Erosion Rills are areas in overland flow planes where water flows
in a concentrated fashion. They act like microchannels. Rills are not
permanent features. They may form as a result of one rain event, but
another rain event may destroy them. Soil detachment, transport,
and deposition in rills are due to concentrated water flow. The amount
of erosion in rills depends on the distance between the rills. The
greater the distance between rills, the greater the amount of rill ero-
sion and vice versa. Net soil detachment in rills is given by the fol-
lowing relationship (Flanagan and Nearing 1995):
⎛ Q ⎞
s
) 1
E = K (τ f − τ c ⎜ ⎝ − T ⎠ ⎟ (3.25)
r
r
c