Page 246 - Soil and water contamination, 2nd edition
P. 246
Sediment transport and deposition 233
biofilms . Flocs may be formed when particles collide; the frequency of collisions between
particles increases with increasing turbulence, but the shear stress es induced by turbulence
around the flocs themselves can break up large flocs. Given a constant shear stress, the
particle size distribution in the water column will achieve a steady state , in which the creation
and destruction of flocs are in equilibrium. Increasing the shear stress increases the rate of
flocculation and shortens the time to steady state, but decreases the median floc size. As flocs
are very porous, they are considerably less dense than the individual particles. Nevertheless,
the effect of the larger particle size prevails and the settling velocity of the flocs is generally
faster than the settling velocity of the individual particles. Hence, flocculation results in an
enhanced sedimentation .
12.5 SEDIMENT EROSION
If the bottom shear stress at the soil–water interface exceeds the critical shear stress for
erosion , a net sediment flux from the soil or bed sediment to the water column occurs. Like
the deposition flux, the erosion flux can be formulated as a function of the shear stress:
J e M b 1 (12.21)
b ,e
-1
-2
where J = the erosion flux [M L T ], and M = the erosion flux when τ = 2τ . Just as in
e b b,e
the case of sedimentation , the erosion flux can be modelled as a linear function of the flow
velocity instead of the bottom shear stress . The critical bottom shear stress for erosion τ
b,e
increases with the degree of consolidation of the sediment (which is inversely related to the
water content of the sediment), the organic matter content , the clay content, and the activity
-2
of soil organisms. The value for τ is in the order of 0.05–0.5 N m (Van Rijn, 1989).
b,e
Equation (12.21) is only valid for consolidated sediments in which the critical shear
stress for erosion τ is constant over the sediment profile (Mehta et al., 1989). However,
b,e
in surface waters an unconsolidated sediment layer with a high water content (> 50 %) is
usually present between the consolidated sediment and the water column. Parchure and
Mehta (1985) described the erosion or resuspension rate for these soft cohesive sediment
layers using the following empirical expression:
J e b b e , (12.22)
e f
-0.5
-2
0.5
-1
where ε = resuspension flux constant [M L T ], and β = empirical constant [L T M ].
f
The critical shear stress for erosion usually increases with depth and if τ becomes larger, the
b,e
resuspension process can be described using Equation (12.21). Mehta et al. (1989) reported
-0.5
-0.5
values of 8.3 m N and 13.6 m N for β for lake sediments. The accompanying values for
-2 -1
-5
-5
-2 -1
ε range from 0.07⋅10 kg m s to 0.53⋅10 kg m s , respectively.
f
On slopes, erosion also occurs due to the impact of falling raindrops; this is also referred
to as splash detachment . The erosion flux as a result of splash detachment is a function
of the kinetic energy of the rainfall, the depth of the surface water layer, and the stability
of soil aggregates present at the soil surface. The kinetic energy can arise from both direct
rainfall and drainage from leaves (throughfall ). The LISEM model (LImburg Soil Erosion
Model) (De Roo et al., 1996; Jetten, 2013) uses the following equation to estimate splash
detachment (Jetten, 2013):
. 2 82 . 1 48 H
E K e . 96 P (12.23)
2
s e
A s
10/1/2013 6:45:08 PM
Soil and Water.indd 245 10/1/2013 6:45:08 PM
Soil and Water.indd 245