Environmental Engineering Reference
In-Depth Information
Fig. 5.8
Flow diagram illustrating how
SIAM (Sediment Impact Analysis
Method) accounts for bed material and
wash load dynamics in the sediment
transfer system. (See the colour version
of this figure in Colour Plate section.)
commonly used for this purpose and, based on
experience gained in numerous SIAM applica-
tions, it is a reasonable 'default' value for the wash
load threshold. To apply HEC-RAS/SIAM it is
therefore necessary to sample the bed in each
sediment reach using a technique capable of ac-
curately representing the finer limb of the particle
size distribution. As theD
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changes downstream,
sediment that is wash material in one reach may
become bed material load in the next, and vice
versa (Fig. 5.8) and SIAM tracks and accounts for
these changes in calculating the reach-scale sed-
iment budget.
defined size fraction) for the wash load-bed mate-
rial load threshold);
.
sediment supply from local sources (average
annual yields and particle size distributions: to
support calculation of annualized sediment inputs
in SIAM).
Local sediment sources include diffuse catch-
ment erosion, landslides, eroding channel banks,
tributaries, and anthropogenic sources such as
arable fields, ditches, gullies and mines. Where
sediment is removed from a reach for flood risk
management or mineral extraction, the sediment
impacts can be explored by including this in HEC-
RAS/SIAM as a negative local source.
HEC-RAS/SIAM indicates the net sediment
balance in a sediment reach. It does not, how-
ever, indicate the types or patterns of morpho-
logical change likely to result from any
sediment imbalance. This is the case because
it is not a sediment routing or mobile boundary
model and there is no computational feedback
between the sediment movement, flow hydrau-
lics and channel form. Morphological interpre-
tation of the results of a HEC-RAS/SIAM study
requires additional information and the insights
that may be obtained from other types of sedi-
ment-related investigation based on some form
of stream reconnaissance, such as a Fluvial Au-
dit or a Geomorphic Dynamics Assessment
(Thorne et al. 2010).
Data requirements and outputs
The data required to run HEC-RAS/SIAM define
for each sediment reach the:
.
annualized hydrograph (discharge record and
flow duration curve: to support calculation of
quasi-steady, hydraulic parameters within HEC-
RAS);
.
channel form (geometry and roughness: to sup-
port calculation of quasi-steady, hydraulic para-
meters within HEC-RAS);
.
bed material properties (particle size distribu-
tion: to support calculation of annualized bed
material transport capacities by size fraction in
HEC-RAS and to identify D
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(or another user-
