Environmental Engineering Reference
In-Depth Information
Management ResearchConsortium (FRMRC) was
directed at developing new tools to account for
sediment in rivers, concentrating particularly on
semi-quantitative and indicative characterization
of the dynamics of the sediment transfer systemat
the catchment scale, and simulating system re-
sponse to the impacts (intentional or unintention-
al) of existing and proposed interventions in the
fluvial system that are related to flood risk
management.
Computation of sediment movement is con-
ventionally approached through application of the
equations of fluid flow, sediment transport capac-
ity and sediment continuity in a hydraulic or
hydrodynamic model with a sediment module.
However, the resources and field data required to
apply these models restrict their use to the reach
rather than the catchment scale, while extended
run times mean that they cannot readily be used
for the types of long-term simulation required to
investigate sediment movement over protracted
periods or through long reaches. Also, reliable
sediment modelling demands both specialist
training and prior experience on the part of the
modeller, not only in hydraulic/hydrodynamic
modelling, but also in the selection and appropri-
ate use of different sediment transport equations.
It is in the context of the limitations of conven-
tional qualitative and quantitative methods of
accounting for sediment that the FRMRC assem-
bled a toolbox of sediment methods and models
that can be used by practitioners faced with the
need to account for sediment or to solve sediment-
related problems in flood risk management.
Strategically, use of the toolbox should aid un-
derstanding of the interactions between flood de-
fence infrastructure and sediment dynamics,
knowledge of which is vital in assessing the sed-
iment impacts of existing flood alleviation
schemes; appraising options during project plan-
ning and supporting detailed design of new
schemes. Further, the toolbox also provides a
means for end users to understand, and therefore
account for, the impacts on flood defence infra-
structure of changes in catchment sediment sup-
ply and adjustments of the fluvial system that
seem ever more likely in the coming decades in
sediment impacts of a flood alleviation scheme
may trigger channel instability elsewhere in the
fluvial system (Sear et al. 2010). In this respect,
recent research suggests that the effects of, for
example, removing sediment by dredging for flood
defence purposes may be much more damaging
than previously realized (Wishart et al. 2008).
Currently, the standard method for investiga-
tion of catchment-scale sediment dynamics in UK
rivers is the 'Fluvial Audit' (Thorne et al.2010).In
this approach, field and documentary investiga-
tions are used to divide the fluvial system into
geomorphic reaches designated as sediment source
(scouring), sediment transfer (dynamic equilibri-
um) or sediment sink (depositional) reaches. The
approach rests on detailed field reconnaissance of
the entire drainage network by experienced fluvial
geomorphologists (Thorne 1998). The Fluvial
Audit has proven very useful in river conservation
and restoration projects, but it does not yield the
quantification of the sediment dynamics required
to interface effectively with the engineering com-
ponents of strategic flood risk assessments, Catch-
ment Flood Management Plans (CFMPs) or River
Basin Management Plans (RBMPs).
Also, the insights into catchment sediment
dynamics provided by a Fluvial Audit are reliant
on accurate interpretation of field and archive
evidence, which is always equivocal, by an expe-
rienced geomorphologist. Consequently, the out-
comes are necessarily related to the quality of the
expert judgement exercised by the person respon-
sible for the investigation. However, the main
limitation of the Fluvial Audit is that it has no
inherent predictive capacity and so cannot simu-
late system response to different, proposed flood
risk management actions - reducing its utility in
options appraisal. The Fluvial Audit is now evolv-
ing to incorporate a modelling dimension (Thorne
et al. 2010), but finding a quantitative approach to
representing sediment dynamics that can be rou-
tinely applied across a range of catchment scales
remains a difficult challenge that is yet to be
solved in the context of practical applications.
Recognizing the practical limitations of exist-
ing, qualitative approaches, a component of the
research pursued during Phase 1 of the Flood Risk
