Page 133 - Biosystems Engineering

114 Cha pte r T h ree

2 −1
where E is the rill detachment rate (kg m s ), K is the rill erodibility
r r
−4
−1
of soil (kg s m ) with typical values varying from 10 to 10 , τ is the
−4
f
hydraulic shear stress (Pa) given by γR with γ being the specific
s
weight of water (~9.81 kN m ), R being the hydraulic radius of the
−3
rill (m), and s being the slope of the rill flow. τ is the critical shear
c
stress below which no rill erosion occurs (Pa), Q is the sediment flux
s
−1 −1
in the rill (kg m s ), and T is the sediment transport capacity of the
c
1.5
−1 −1
rill (kg m s ) and is given by k τ . The parameter k here is a trans-
t t
port coefficient and generally varies from 0.01 to 0.1.
Interrill Erosion Interrill areas are the regions in overland flow planes
between rills. Water flows in these zones as very shallow sheet flow.
Soil detachments in rills are mainly due to rainsplash and, to a smaller
extent, to sheet flow. Transport and deposition of eroded sediments
happen via sheet flow. Soils eroded from interrill areas are carried to
rills and from rills to channels. Flanagan and Nearing (1995) estimate
interrill erosion as
E = K iqS C (3.26)
i i f v
2
−1
where E is the interrill erosion rate (kg m s ), K is the interrill erod-
i i
−4
ibility of the soil (kg s m ) with typical values between 10 and 10 , i
7
5
is the rainfall intensity (m/s), q is the runoff rate (m/s), C is the cover
v
adjustment factor ranging from 0 to 1, and S is the interrill slope fac-
f
tor given as S = 1.05 − 0.85e −4sinθ with θ being the interrill slope angle.
f
Gully Erosion Gullies are natural deep channels with very steep side
slopes, and they carry ephemeral flows; that is, they convey water
only during storm events and are dry other times. Formation of gul-
lies is a complex process and is not yet totally understood. Gullies are
often a result of accelerated erosion that is triggered by significant
changes in the hydrologic regimes of upland drainage areas, such as
urbanization and forest clear cutting.
Channel Erosion Channel erosion involves erosion of the streambed,
the stream bank, and the flood plain. Streambed erosion produces
coarse materials such as sand that moves along the streambed. Fine
materials are carried out in suspended form. When the transport
capacity of the stream is not exceeded, then the sediment deficit can be
replenished by further erosion of the stream banks, which usually
consists of fine sediments deposited in previous events and if the flow
level exceeds bank-full conditions by erosion of the flood plain. When
stream flows contain sediments above their transport capacity, this
surplus sediment is deposited to the streambed or the flood plain.
3.6.4 Universal Soil Loss Equation
The universal soil loss equation (USLE) and its later revision, the
revised universal soil loss equation (RUSLE), are widely used methods