Accounting for Sediment and Geomorphology in Flood Risk Management

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Accounting for Sediment and Geomorphology in
Flood Risk Management

• Colin Thorne
• Chair of Physical Geography, Nottingham
  University
• and
• Faculty Affiliate, Portland State University
• colin.thorne_at_nottingham.ac.uk

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UPLAND CATCHMENTS
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Pontbren experimental catchment
WP 5.1 Modelling flood impact of upland land use
change contact n.mcintrye_at_imperial.ac.uk
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Pontbren was a unique 6-year field experiment
performed through collaboration between
scientists, farmers and decision-makers.
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Changes in land management
Pasture improved through drainage, liming and
reseeding
Increased sheep stocking levels in uplands
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Land use, Infiltration and Runoff
WP 5.1 Modelling flood impact of upland land use
change contact n.mcintrye_at_imperial.ac.uk
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At the field scale, effects of land-use on
surface runoff are strong and responsive to
management changes
Arrows demonstrate relative magnitudes
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Land-use Runoff and Farm-scale Flooding
WP 5.1 Modelling flood impact of upland land use
change contact n.mcintrye_at_imperial.ac.uk
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At farm scale, the effect of land-use on flows
and flood peaks is clear
Land use
Low T indicates faster flow responses
Flow gauges
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Upland land use change impacts on peak flows
WP 5.1 Modelling flood impact of upland land use
change contact n.mcintrye_at_imperial.ac.uk
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Models allow analysis of the effects of
field-scale land management on flood peaks
Median change -5 Uncertainty range -2 to -11
Scenario Tree shelterbelts over 10 of the
catchment
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Land-use impacts on downstream flood peaks in
Large Catchments
WP 5.1 Modelling flood impact of upland land use
change contact p.e.oconnell_at_newcastle.ac.uk
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Modelled impact on peak is small, only a few
percent, but uncertainty is high
95 prediction bounds
Peat blocked Peat drained Peat
intact Good Fair Poor
Pre-change
Post-change
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Land-use and Flooding Summary
Increasing return period
1 5 Years local nuisance floods
Increasing scale
50 - 100 Years regional catastrophic floods
Maximum effect
Minimum effect
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How Drainage Network Morphology Controls Flood
Impacts at Large Catchment Scale

• Hydrodynamic Dispersion channel and floodplain
  size, shape and roughness attenuates Flood Peaks
  and their impacts.
• Geomorphological Dispersion sediment dynamics
  and geomorphology of drainage network controls
  flood arrival times and impacts at Flood Receptor
  locations.

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UPLAND CATCHMENTS
Catchment Sediment Yields natural vs intensive
pasture
Fine sediment yield 5x greater Coarse
sediment yield 12x greater Most excess sediment
generated from within channel network
Henshaw, A.J. (2009) Impacts of land use changes
and land management practices on upland catchment
sediment dynamics Pontbren, mid-Wales.
Unpublished PhD thesis. University of Nottingham.
Available online at http//riverscience.wikidot.
com/alex-henshaw
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Increased Sedimentation in Engineered vs Natural
Channels
UPLAND CATCHMENTS
Foresight on Future Flooding found that  a
year and a half of aggradation produced an
increase in the flooded area equivalent to nearly
half a century of climate change.
E.K Raven et al. 2010. Understanding sediment
transfer and morphological change for managing
upland gravel-bed rivers, Progress in Physical
Geography 34(1) 23-45.
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WP 5.2 Modelling sediment impacts of upland land
use change contact c.thorne_at_nottingham.ac.uk
Sediment Impacts on Conveyance, Channel Stability
and Habitats
2002-2004 aggradation
2050s climate scenario
Present
Lane et al. (2007)
1-in-0.5 year flood
12.2
5.7
Combined 38.2
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Reconciling goals for flood risk and ecological
status

• National trends in ecological indices in
  managed reaches
• Reduced instream habitat heterogeneity
• Reduced riparian habitat complexity

Harvey, G. L. and Wallerstein, N. P. (2009)
Exploring the interactions between flood defence
maintenance works and river habitats the use of
River Habitat Survey data.  Aquatic Conservation
Marine and Freshwater Ecosystems 19 689-702.
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Sediment Management Policy-related premises

• There is a general presumption
  against removing sediment
  from rivers.
• The justification to move or
  remove sediments must be
  evidence-based.
• When sediment actions are justified best practice
  must be employed with the aim of maximizing
  benefits to habitats and ecosystems while
  avoiding or at least minimising damage to the
  environment.

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Lowland Catchments
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Distributed hydrological model for the River Tone
WP 5.3 Modelling flood impact of lowland land use
change contact i.d.cluckie_at_swansea.ac.uk
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Water Movement Procedures
Vertical Data Layers
(MIKE SHE/11)
Precipitation
Grid size 100 metres
Evapotranspiration
Overland Flow Model
Vegetation
Canopy Interception
Topography
Soil
River (Channel flow model)
Root Zone Model
INFILTRATION
Interflow Reservoir
Interflow Storages
INTERFLOW (H) PERCOLATION (V)
Baseflow Reservoir
Baseflow Storages
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Lowland land use change scenarios
WP 5.3 Modelling flood impact of lowland land use
change contact i.d.cluckie_at_swansea.ac.uk
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The model shows limited impact of woodland
planting, but greater impacts from distributed
flood retention storage
Woodland planting scenario
Flood retention storage scenario
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Land use and Sediment Dynamics in the River Tone
LOWLAND CATCHMENTS
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Options for Modelling, Predicting and Managing
Sediment-Related Flood RiskFRMRC Sediment
Toolbox
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FRMRC Sediment Toolbox
CAESAR Cellular Automaton Evolutionary Slope
and River model
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Could strategic tree planting reduce flood risk
by disconnecting surface runoff pathways and
increasing soil moisture storage?
Strategic woodland restoration in agriculturally
intensified catchments could reduce flood risk,
erosion and sediment transfer by disconnecting
surface runoff pathways and increasing soil
moisture storage.
Carroll et al. (2004)
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SEDIMENT FUTURES
Modelling future erosion, sediment and
morphological responses to changes in climate and
land use
Selective woodland planting can reduce flood
peaks in small catchments Strategic land use
management can substantially reduce erosion and
sediment yields Land use changes buffer rivers
from the worst impacts of climate change
2050s intensive
Baseline
2050s current
2050s tree strips
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Predicted future Pontbren sediment yields
WP 5.1 Impact of upland land use on sediments
contact Colin.Thorne_at_nottingham.ac.uk
Baseline (1961-90) 2050s (low emissions) 2050s (medium emissions) 2050s (high emissions)
Present-day (with Pontbren tree strips) - 9.3 28.3 35.3
1990s (pre-Pontbren tree strip s) 4.1 15.3 30.0 53.8
Tree strips in all grazed pastures -58.2 -37.6 -22.4 -11.4
Climate scenario
Land use scenario
Change in 30-year sediment yield from baseline
climate/present-day land use scenario
(percentages represent difference in median
sediment yield calculated from 50 UKCP09 weather
generator rainfall sequences)
Climate change predicted to amplify sediment
yield but problems could be offset through
changes in land use management.
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SWP 5 Land use management negotiation
tool contact afse0c_at_bangor.ac.uk
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POLYSCAPE
Multi-functional Land-use Management - areas are
beneficial to all services
Trade off Layer
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OPTIONS FOR MODELLING AND MANAGING
SEDIMENT-RELATED FLOOD RISK
FRMRC Sediment Tool Box A range of sediment
methods and models is available. The relative
contributions of interpretative and analytical
approaches vary, but all methods and models
require both.
Successful uptake depends not only on the
strength of the science base but also
availability of management resources to apply the
method/model and stakeholder attitudes.
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Does Sediment and Geomorphology Really Matter?
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Cumbrian floods - 2009
DOES SEDIMENT MATTER?

• Sediment and vegetation reduced conveyance
  capacity of engineered channels
• Bank scour damaged properties
• Bed scour led to the collapse of bridges
  and loss of life
• Extensive overbank deposition of
  coarse sediments damaged farmland.
• Channel and floodplain instability
  destroyed ecosystems and habitats.

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SEDIMENT FLOOD VICTIMS

• Drop collect questionnaires interviews
• Carlisle (2005)
• Cockermouth (2009)
• Boscastle (2004), Lostwithiel, St Blazey (2010)
• Cockermouth initial results
• 55 respondents stated damage costs
• mean damage/household 83,000
• 52 of damage attributed to water
• 30 of damages attributed to sediment
• 18 of damage attrributed to debris
• 85 respondents rated life satisfaction
• (0 extremely dissatisfied 1 extremely
  satisfied)
• Interviews thematic analyses
• High anxiety concerning future flooding
• Stakeholders believe that sediment management for
  Conservation pre-empts sediment management for
  Flood Control

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Environmental Regulation and Flood Risk Management
Drivers of Future Flood Risk
The Foresight project found that a clash
between FRM and environmental objectives could
lead to a 3-fold increase in flood risk in the
2050s, rising to a 4-fold increase in the
2080s (Evans et al. 2008). They concluded
that under Global Sustainability, lower
climate change and economic growth combined with
greater environmental consciousness result in
River Vegetation and Conveyance, Environmental
Regulation, and River Morphology and Sediment
Supply topping the table in the 2050s.
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TAKE HOME MESSAGES

1. Land use is significant to downstream flood risk
   and flood victims understand this even if not all
   hydrologists do.
2. Land use management can substantially increase or
   decrease flood and sediment-related flood risks.
3. Unless we act, future flood and sediment impacts
   are likely to increase due to climate and land
   use changes.
4. Land use management for flood risk reduction must
   be properly aligned with agricultural,
   environmental and planning policies, legislation
   and regulation.

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FRMRC Sediment Researchers and Advisors
ACKNOWLEDGEMENTS

• Paul Bates - Bristol University
• Paul Brewer Aberystwyth University
• Tom Coulthard - Hull University
• Simon Gosling Nottingham University
• Stuart Lane Université de Lausanne
• Mark Macklin - Aberystwyth University
• Suresh Surendran Glamorgen University
• Adrian Collins - ADAS
• Mervyn Bramley Independent
• Jon Rees - NERC
• Mike Thorn Independent
• David Brown - Environment Agency
• Jim Walker - Environment Agency
• Sean Longfield - Environment Agency

• Alex Henshaw Queen Mary, London
• Nick Wallerstein Heriot-Watt University
• Emma Raven Durham University
• Ian Dennis Royal Haskoning
• Gemma Harvey Queen Mary, London
• Jorge Rameirez - - Hull University
• Phil Soar Portsmouth University
• Jenny Mant River Restoration Centre
• Clifford Williams Environment-Agency
• Chris Parker - University West of England
• Steve Dangerfield Nttm University
• Tim Meadows Nottingham University
• Andy Wallis - Black and Veatch

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http//frmrc.hw.ac.uk/