Transcript EWRG Colin Thorne May 2013 Sed Geomorph

Accounting for Sediment and Geomorphology
in Flood Risk Management
Colin Thorne
Chair of Physical Geography, Nottingham University
and
Faculty Affiliate, Portland State University
[email protected]
www.floodrisk.org.uk
EPSRC Grant: EP/FP202511/1
UPLAND CATCHMENTS
WP 5.1 Modelling flood impact of upland land use change
contact: [email protected]
Pontbren experimental catchment
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
1969
1997
Breeding ewes km-2
0 - 100
100 - 200
200 - 300
300 - 400
400 - 500
500+
Historical changes in sheep stocking density
Henshaw et al. (in prep.)
Pasture improved through drainage,
liming and reseeding
Increased sheep stocking levels in uplands
WP 5.1 Modelling flood impact of upland land use change
contact: [email protected]
Land use, Infiltration and Runoff
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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WP 5.1 Modelling flood impact of upland land use change
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contact: [email protected]
Land-use Runoff and Farm-scale Flooding
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
WP 5.1 Modelling flood impact of upland land use change
contact: [email protected]
Upland land use change impacts on peak flows
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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WP 5.1 Modelling flood impact of upland land use change
contact: p.e.o‘[email protected]
Land-use impacts on downstream flood peaks in
Large Catchments
Modelled impact on peak is small, only a few percent,
but uncertainty is high
95% prediction bounds
Pre-change
Post-change
Peat: blocked
Peat: drained
Peat: intact
Good
Fair
Poor
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Land-use and Flooding: Summary
Increasing scale
Increasing return period
1 – 5 Years:
local ‘nuisance’
floods
50 - 100 Years:
regional ‘catastrophic’
floods
Maximum effect
Minimum effect
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.
UPLAND CATCHMENTS
Catchment Sediment Yields: natural vs intensive pasture
Fine sediment yield
5x greater
Melin-y-grug
Coarse sediment yield
12x greater
Pen-y-cwm
Pontbren
Experimental
Catchments
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, midWales. Unpublished PhD thesis. University of Nottingham. Available online at http://riverscience.wikidot.com/alex-henshaw
UPLAND CATCHMENTS
Increased Sedimentation in
Engineered vs Natural Channels
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.
WP 5.2 Modelling sediment impacts of upland land use change
contact: [email protected]
Sediment Impacts on Conveyance, Channel Stability and Habitats
Reduced Water quality
Habitat degradation
Accelerated
Channel migration
Present
2050s climate
scenario
2002-2004
aggradation
Lane et al. (2007)
Reduced conveyance capacity
1-in-0.5 year flood
+12.2%
+5.7%
Combined: +38.2%
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.
Sediment Management:
Policy-related premises
1. There is a general
presumption against
removing sediment
from rivers.
2. The justification to move
or remove sediments
must be evidence-based.
3. 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.
Lowland Catchments
WP 5.3 Modelling flood impact of lowland land use change
contact: [email protected]
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Distributed hydrological model for the River Tone
Vertical Data Layers
Water Movement Procedures
(MIKE SHE/11)
Precipitation
Evapotranspiration
Canopy
Interception
Grid size – 100 metres
Overland Flow
Model
…Vegetation
…Topography
…Soil
River
(Channel flow model)
Root Zone Model
INFILTRATION
…Interflow
Reservoir
Interflow Storages
INTERFLOW (H)
PERCOLATION (V)
Baseflow Storages
Slower/Deeper Baseflow
…Baseflow
Reservoir
WP 5.3 Modelling flood impact of lowland land use change
contact: [email protected]
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Lowland land use change scenarios
The model shows limited impact of woodland planting, but greater
impacts from distributed flood retention storage
Woodland
planting
scenario
Flood
retention
storage
scenario
LOWLAND CATCHMENTS
Land use and Sediment Dynamics in
the River Tone
Sediment Yield (Best Fit with limits)
Upstream of Taunton
Taunton
Downstream of Taunton
Halse Water
114 T/km2/yr
10,000 T/yr
6,000 - 16,000
64 T/km2/yr
13,000 T/yr
10,000 - 15,500
Halse Water GS
River Tone
83 T/km2/yr
25,000 T/yr
22,500 - 29,000
Bishops Hull GS
French
Weir
River Tone
80 T/km2/yr
23,900 T/yr
21,000 - 29,000
Firepool
Weir
Knapp
Bridge
Ham Weir
New
Bridge
River Tone
River Tone
River Tone
70 T/km2/yr
20,900 T/yr
19,000 - 25,500
60 T/km2/yr
18,000 T/yr
12,000 - 27,000
57 T/km2/yr
17,000 T/yr
12,000 - 27,000
Upper River Tone
Elevated
sediment yields
Localised coarse
sedimentation
Complex fines
sedimentation – especially
at structures
Options for Modelling, Predicting and
Managing Sediment-Related Flood Risk:
FRMRC Sediment Toolbox
FRMRC Sediment Toolbox
Upstream of Taunton
CAESAR – Cellular Automaton Evolutionary Slope
and River model
French
Weir
Halse Water GS
River Tone
75 T/km2/yr
15,000 T/yr
(SS No. 609)
Downstream of Taunton
Sediment Yield Analysis
Halse Water
90 T/km2/yr
8,000 T/yr
(estimated)
Taunton
83 T/km2/yr
24,000 T/yr
(SS No. 113)
Bishops Hull GS
River Tone
41 T/km2/yr
12,000 T/yr
(SS No. 182)
Firepool
Weir
Knapp
Bridge
Bathpool
New
Bridge
River Tone
River Tone
River Tone
41 T/km2/yr
12,000 T/yr
(SS No. 295)
14 T/km2/yr
4,000 T/yr
(SS No. 445)
28 T/km2/yr
8,000 T/yr
(SS No. 146)
Change in Stream Power d/s
Stream Power Screening
60.00
ISIS-Sediment
Specific Stream Power (Wm-2)
50.00
Upper River Tone
40.00
30.00
20.00
10.00
0.00
0.0
1000.0
2000.0
3000.0
4000.0
5000.0
6000.0
7000.0
Chaniage (m )
ST:REAM
Sediment Transport:
Reach Equilibrium
Assessment HEC-RAS/SIAM
Method
8000.0
9000.0
10000.0
11000.0
12000.0
13000.0
Could strategic tree planting reduce flood risk by
disconnecting surface runoff pathways and
increasing soil moisture storage?
Infiltration rates close
Strategic woodland restoration in agriculturally intensified
to zero incatchments
grazed
Infiltration
rates
up to
60 erosion
x higher and
in restored
could
reduce
flood
risk,
sediment transfer
by disconnecting
pastures.
woodland
areas
within
2-6 years
planting! soil moisture storage.
surface
runoff
pathways
andofincreasing
www.floodrisk.org.uk
EPSRC Grant: EP/FP202511/1
Carroll et al. (2004)
SEDIMENT FUTURES
Modelling future erosion, sediment and morphological responses to
changes in climate and land use
Selective woodland
2050s intensive
Baseline
2050s current
2050s tree strips
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
WP 5.1 Impact of upland land use on sediments
contact: [email protected]
Predicted future Pontbren sediment yields
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.
SWP 5 Land use management negotiation tool
contact: [email protected]
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POLYSCAPE
Habitat Connectivity
Hydrology
Multi-functional Landuse Management areas are beneficial to
all services
Farm productivity
Sediment
Transport
Trade off Layer
OPTIONS FOR MODELLING AND MANAGING
SEDIMENT-RELATED FLOOD RISK
FRMRC
Sediment Tool
BoxSuccessful
uptake depends
A range of sediment
not only on the
methods and models
isstrength
available. of the
science base but
The relative
also
availability of
contributions
of
management
interpretative and
resources to
analytical approaches
the
vary, apply
but all methods
method/model
and
models require
and stakeholder
both.
attitudes.
Credibility
Stakeholder
Stakeholder
Attitudes
Attitudes
Constraints
Project
Success
Management
Management
Resources
Resources
Cognizance
Support
Simplicity
Science
Science
Complexity
Does Sediment and Geomorphology
Really Matter?
DOES SEDIMENT MATTER?
Cumbrian floods - 2009
• 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.
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
Environmental
Regulation and Flood
Risk Management
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.”
Drivers of Future Flood Risk
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.
ACKNOWLEDGEMENTS
FRMRC Sediment Researchers and Advisors
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
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
http://frmrc.hw.ac.uk/