Environmental Engineering Reference
In-Depth Information
Discharge
computing transport capacity for the next sedi-
ment routing iteration.
Computation
Time Step
Flow
Increment
Physical constraints to erosion and deposition
HEC-RAS applies temporal erosion and deposition
modifiers as well as sorting and armouring rou-
tines to augment the simple continuity computa-
tions. Physical process constraints are necessary
because simply solving the Exner equation trans-
lates 100% of computed sediment surplus or def-
icit immediately into deposition or erosion. This
does not reflect the fact that both deposition and
erosion take time. Therefore, HEC-RAS applies
time-dependent modifiers to the surplus or deficit
calculated for each cross-section.
Deposition efficiency is calculated by grain size,
based on the fall velocity and the expected centre of
mass of the material in the water column - based
roughly on Toffaletti's depth-concentration rela-
tionships (Vanoni 1975). A similar relationship is
implemented to limit erosion temporally. HEC-
RASuses a 'characteristic length' approachadapted
from HEC-6, which includes the assumption that
erosion takes a distance of approximately 30 times
the depth to develop fully.
t
Time
Fig. 5.10
Schematic to illustrate quasi-steady flow
division used in characterizing hydraulics for sediment
transport calculations in HEC-RAS 4.0 (Hydrologic
Engineering Center River Analysis System 4.0).
Wilcock and Crowe (2003) and Yang (1972). HEC-
RAS simulates graded sediment transport by di-
viding the sediment gradation curve into up to 20
discrete, editable size classes. HEC-RAS calcu-
lates an overall transport capacity by computing
independent transport potentials for each size
class. The sediment transport function is applied
to eachgrainclass as if itwere the onlymaterial in
the channel (with the exception of the Wilcock
equation, which includes hiding functions and
other inter-grain class dependencies). The trans-
port capacity is computed for each grain class by
multiplying the computed potential for that grain
class by the relative fraction of that grain class in
the active layer of the bed.
Bed elevation rises and falls in response to a
sediment supply deficit or surplus in the control
volume, i.e. the positive or negative difference
between capacity and supply. HEC-RAS solves the
Exner equation separately for each grain size -
adding material to, or removing it from, the active
layer. At the end of each computational time step,
aggradation or degradation is translated into a
uniform bed change over the entire wetted perim-
eter of the cross-section. HEC-RAS updates eleva-
tion information for each cross-section and
performs new hydraulic computations before
Sorting and armouring
The other major process considered in the com-
putation of continuity is potential supply limita-
tion as a result of bed sorting. Currently, HEC-
RAS has two options to compute the effects of bed
sorting processes: Exner-5, a 'three layer' algo-
rithm taken from HEC-6 (Fig. 5.11), and a simple
'two-layer' active layer method. Exner-5 divides
the active layer into two sublayers, simulating
bed coarsening by removing fines initially from a
thin surface layer. During each time step, the
composition of this surface layer is evaluated and
if, according to an empirical relationship, the bed
is partially or fully armoured, the amount of
material available to satisfy excess capacity can
be limited.
The simplified, 'two layer', active bed approach
- with the Toro-Escobar et al. (1996) exchange
increment method - is designed for simulating
