Watershed Scale Physically Based Water Flow, Sediment and Nutrient Dynamic Modeling System - Landscape Ecology for Sustainable Environment and Culture - page 145

Fine particles often behave in a cohesive manner. If the behavior is cohesive,
flocculation may occur. Floc size and settling velocity depend on the conditions
under which the floc was formed (Krishnappan 2000 ; Haralampides et al. 2003 ).
As a result of turbulence and other factors, not all sediment particles settling
through a column of flowing water will necessarily reach the sediment-water
interface or be incorporated into the sediment bed. Beuselinck et al. ( 1999 ) sug-
gested this process also occurs for the overland plane. When flocculation occurs,
settling velocities of cohesive particles can be approximated by relationship of the
form (Burban et al. 1990 ):

v s ¼ a d f ð 8 : 8a Þ

v se ¼ P dep v s ð 8 : 8b Þ

where v s = floc settling velocity [L/T], a = experimentally determined constant,
d f = median floc diameter [L], m = experimentally determined constant,
v se = effective settling (deposition) velocity [L/T], and P dep = probability of
deposition.

8.2.2.3 Upper Sedimentation Processes

The upper soil and sediment bed play important roles in the transport of con-
taminants. Once a particle erodes, it becomes part of the flow and is transported
downstream within the watershed. The fluxes of the channel erosion and sedi-
mentation control the dynamics of the upper most contaminated layer. Particles
and associated contaminants in the surficial sediments may enter deeper sediment
layers by burial or be returned to the water column by scour. Whenever burial/
scour occurs, particles and associated contaminants are transported through each
subsurface sediment segment within a vertical stack. In response to the difference
between bed form transport, erosion, and deposition fluxes, the net addition
(burial) or net loss (scour) of particles from the bed causes the bed surface ele-
vation to increase or decrease. The rise or fall of the bed surface is governed by the
sediment continuity (conservation of mass) equation, various forms of which are
attributed to Exner equation (Simons and Sentürk 1992 ). Julien ( 1998 ) presents a
derivation of the bed elevation continuity equation for an elemental control vol-
ume that includes vertical and lateral (x- and y-direction) transport terms.
Neglecting bed consolidation and compaction processes, and assuming that only
vertical mass transport processes (erosion and deposition) occur, the sediment
continuity equation for the change in elevation of the soil or sediment bed surface
may be expressed as:

@ z
dt þ v se C ss v r C sb ¼ 0

q b ð 8 : 9 Þ