Environmental Engineering Reference
In-Depth Information
Limitations and uncertainties
Sediment Toolbox, rather than as a stand alone
alternative to any of them.
There are several sources of uncertainty in REAS,
related to issues of data quality, processing and
interpretation. Given the limited data availability
in the UK and the perhaps unavoidable sparsity of
hydrological, morphological and sediment data
sampling sites when working at the catchment
scale, uncertainty is inevitably associated with
measurement and interpolation of channel dimen-
sions, slopes and sediment sizes, the assignment
of roughness values and the synthesis of flow
duration curves, which often come from donor
sites. However, with data availability dictated by
(often quite limited) project resources, a degree of
uncertainty is an inevitable outcome of this type
of scheme and should not necessarily be treated as
a limitation to its application.
Additional sources of uncertainty in the proces-
sing stage stem from assumptions made in the
application of the various equations and the de-
lineation of reaches of energy similarity (see Soar
et al., in press), although the latter is optional as
each cross-section in the scheme can be treated
as its own reach, if this is preferred to reach
averaging.
Finally, the scheme is based on interpretation
and characterization of imbalances in annual geo-
morphic energy as being indicative of discontinu-
ities in sediment transfer. However, this premise
neglects the expenditure of energy in lateral ero-
sion and the transport of material derived from
bank erosion and sourced from outside the chan-
nel, which in some situations could constitute a
significant proportion of the sediment load. In
such cases REAS results might be discordant with
reaches identified as sediment sources, transfers
and sinks in a Fluvial Audit.
In light of these points, Soar et al. (in press)
stress that results should be interpreted as being
indicative, with a view to guiding sediment in-
vestigations towards reaches where limited re-
sources are best targeted to develop a fuller
understanding of sediment dynamics and the sen-
sitivity of rivermorphology to disturbance. Hence,
application of REAS should be seen as being com-
plementary to the use of other tools in the FRMRC
Sediment Impact Accounting Method
embedded in Hydraulic Engineering Centre
River Analysis System (HEC-RAS/SIAM)
Background and utility
The Sediment Impact Accounting Method (SIAM)
is available in the 'Hydraulic Design' module of
version 4.0 of the US Army Corps of Engineers'
Hydrologic Engineering Center, River Analysis
System (HEC-RAS) (Biedenharn et al. 2006; Gib-
son and Little 2006). It uses hydrological and
hydraulic information computed using the one-
dimensional, quasi-unsteady flow model in HEC-
RAS to calculate the reach-averaged rate of bed
material load transport by grain size, for the re-
corded range of discharges. Computed transport
rates are integrated over the recorded range and
durations of flow to compute an annualized bed
material load transport capacity for each user-de-
fined geomorphic reach. The capacity of the reach
to transport bed material load is then compared to
the annualized input of bed material load from
upstream and local sediment sources (bank ero-
sion, landslides, gullies, field erosion, etc.) to es-
timate the balance between bed material load
supply and transport capacity in the reach for each
size class. In SIAM, it is assumed that the move-
ment of wash load is supply, rather than transport,
limited and so no transport capacity is computed
for wash load.
The Sediment Impact Accounting Method
tracks the wash and bed material loads separately
as the calculations progress downstream through
the fluvial system. Each size fraction is classified
as either wash or bed material load on a reach-by-
reach basis, following convention usually attrib-
uted to Einstein (1950), that wash load is that part
of the sediment load that is not found in signifi-
cant quantities in the bed. In HEC-RAS/SIAM the
threshold between wash and bed material loads is
defined by the user for each sediment reach by
selecting the appropriate bed material grain class.
In practice, the grain size for which ten percent of
the sample is finer (D 10 ) of the bedmaterial ismost
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