Environmental Engineering Reference
In-Depth Information
When investigating applications of these ap-
proaches, it soon became apparent that, due to the
scarcity of sediment data for UK rivers and the
complexity of sediment dynamics even in rela-
tively simple watercourses, uncertainties in the
results of quantitative, analytical sediment trans-
port calculations would remain stubbornly high,
so that the outcomesmust customarily be labelled
as 'indicative'. In light of this, elements of both
analysis and interpretation remain essential to all
sediment studies, regardless of the modelling tool
employed, and the methods and models included
in the toolbox all start by assuming that the user
has a sound, qualitative understanding of the flu-
vial and sediment systems, gained from a Fluvial
Audit or some equivalent methodology (Thorne
et al. 2010).
While all the methods and models rely on
elements of interpretation and analysis of the sed-
iment transfer system, the relative contributions of
these two components of scientific study vary be-
tweenmethods. In this regard, Figure5.1 illustrates
where each tool lies in the continuum between
purely interpretive and fully analytical approaches.
response to climate change, changes in land use
management and continued socioeconomic devel-
opment in floodplains (Lane and Thorne 2008).
Finally, the toolbox can support evaluation of the
implications of infrastructure-sediment interac-
tions for in-channel habitats and the ecosystems
they support. The capability to link infrastructure,
morphology, sediments and habitats when man-
aging flood risk is essential in coordinating flood
risk management and its operational delivery to
the requirements of the Water Framework Direc-
tive and so relieving tensions thatmight otherwise
develop between flood management and environ-
mental legislation, to the detriment of river man-
agement practices that are holistic and sustainable
(Lane and Thorne 2007, 2008).
Experience gained in using the toolbox will also
provide a window on the future research needed to
support improved understanding of sediment dy-
namics. The aimhere is to help guide development
of the next generation of broad-scale or whole-
system flooding models by indicating how they
might be made capable of recognizing and ac-
counting for the impacts of sediment on future
flood risks.
Selecting tools for the toolbox
Dealing with uncertainty
From the outset, it was recognized that the need
was to build on, rather than replace, qualitative
assessments including the Fluvial Audit. In es-
sence this called for selection of methods and
models capable of providing an analytical basis
for characterizing the sediment transfer system
that minimized subjectivity and reduced reliance
on expert judgement on the part of the user. On
this basis, themethods andmodels selected for the
toolbox were:
.
Stream Power Screening Tool
.
River Energy Audit Scheme (REAS)
.
Sediment Impact Analysis Method, embedded
in HEC-RAS (HEC-RAS/SIAM)
.
Hydrologic Engineering Center River Analysis
System (HEC-RAS version 4.0)
.
iSIS Sediment
.
Cellular Automaton Evolutionary Slope and
River Model (CAESAR).
It is now accepted that uncertainty must be rec-
ognized and dealt with appropriately in all aspects
of flood risk management, and accounting for
sediment is no exception. No matter how sophis-
ticated the analytical approach adopted, the fact is
that sediment and fluvial processes cannot be
perfectly observed or understood, let alone pre-
dicted. Uncertainty can be reduced through im-
proved knowledge of the governing physical
processes, by collecting sufficient data to repre-
sent the fluvial system and by ensuring that the
model selected is properly designed, calibrated and
verified. However, uncertainty due to natural var-
iability is a property of the river, not the model,
and it is to a degree irreducible. Once it has been
accepted that some uncertainty is unavoidable, it
is necessary to decide whether the existing level of
uncertainty justifies the additional effort needed
to reduce it further.
