www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1
Accounting for Sediment and Geomorphology in Flood Risk Management
Colin Thorne Chair of Physical Geography, Nottingham University
andFaculty Affiliate, Portland State University
UPLAND CATCHMENTS
Pontbren experimental catchment
WP 5.1 Modelling flood impact of upland land use change
contact: [email protected]
Pontbren was a unique 6-year field experiment performed through collaboration between scientists, farmers and decision-makers.
3
Changes in land management
Increased sheep stocking levels in uplands
Henshaw et al. (in prep.)
1969 1997
Breeding ewes km-2
0 - 100100 - 200200 - 300300 - 400400 - 500500+
Historical changes in sheep stocking density
Pasture improved through drainage, liming and reseeding
Land use, Infiltration and Runoff
Arrows demonstrate relative magnitudes
At the field scale, effects of land-use on surface runoff are strong and responsive to management changes
WP 5.1 Modelling flood impact of upland land use change
contact: [email protected]
Land-use Runoff and Farm-scale FloodingAt farm scale, the effect of land-use on flows and flood
peaks is clear
WP 5.1 Modelling flood impact of upland land use change
contact: [email protected]
Flow gaugesLow ‘T’ indicates faster flow responses
Land use
6
Upland land use change impacts on peak flowsModels allow analysis of the effects of field-scale land
management on flood peaks
Scenario: Tree shelterbelts over 10% of the catchment
WP 5.1 Modelling flood impact of upland land use change
contact: [email protected]
Median change: -5%Uncertainty range: -2 to -11%
7
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
Peat: blockedPeat: drainedPeat: intactGoodFairPoorPre-change Post-change
Modelled impact on peak is small, only a few percent, but uncertainty is high
95% prediction bounds
8
Land-use and Flooding: Summary
Minimum effect
Maximum effect
Incr
easi
ng s
cale
Increasing return period
1 – 5 Years: local ‘nuisance’ floods
50 - 100 Years: regional ‘catastrophic’
floods
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.
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
Pontbren Experimental Catchments
Melin-y-grug
Pen-y-cwm
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
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.”
Increased Sedimentation in Engineered vs Natural Channels
UPLAND CATCHMENTS
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.
Sediment Impacts on Conveyance, Channel Stability and Habitats
Accelerated Channel migration
Reduced conveyance capacity
Reduced Water quality Habitat degradation
2002-2004 aggradation
2050s climate scenarioPresent
1-in-0.5 year flood +12.2% +5.7%Combined: +38.2%
Lane et al. (2007)
WP 5.2 Modelling sediment impacts of upland land use change
contact: [email protected]
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.
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.
Sediment Management: Policy-related premises
Lowland Catchments
Distributed hydrological model for the River Tone
WP 5.3 Modelling flood impact of lowland land use change contact: [email protected]
…Interflow Reservoir
…Vegetation
…Topography
…Soil
…Baseflow Reservoir
River(Channel flow model)
Slower/Deeper Baseflow
Precipitation
Evapotranspiration
Canopy Interception
Root Zone Model
Interflow Storages
Baseflow Storages
INFILTRATION
INTERFLOW (H)
PERCOLATION (V)
Water Movement Procedures Vertical Data Layers
(MIKE SHE/11)
Grid size – 100 metres
Overland Flow Model
17
Lowland land use change scenarios
Woodland planting scenario
Flood retention storage scenario
WP 5.3 Modelling flood impact of lowland land use change contact: [email protected]
The model shows limited impact of woodland planting, but greater impacts from distributed flood retention storage
18
Land use and Sediment Dynamics in the River Tone
Halse Water
114 T/km2/yr Halse Water GS10,000 T/yr Ham Weir6,000 - 16,000 River Tone River Tone
64 T/km2/yr
13,000 T/yr 70 T/km2/yr 57 T/km2/yr10,000 - 15,500 20,900 T/yr 18,000 T/yr 17,000 T/yr
19,000 - 25,500 12,000 - 27,000 12,000 - 27,000Bishops Hull GS
Upper River Tone
Downstream of Taunton
22,500 - 29,000 21,000 - 29,000
83 T/km2/yr 80 T/km2/yr 60 T/km2/yr25,000 T/yr 23,900 T/yr
Sediment Yield (Best Fit with limits)
Upstream of Taunton
River Tone River Tone River Tone
French Weir
Firepool Weir
Knapp Bridge
New Bridge
Taunton
LOWLAND CATCHMENTS
Complex fines Complex fines sedimentation – especially sedimentation – especially
at structuresat structures
Elevated Elevated sediment yieldssediment yields
Localised coarse Localised coarse sedimentationsedimentation
Options for Modelling, Predicting and Managing Sediment-Related Flood Risk:
FRMRC Sediment Toolbox
Halse Water
90 T/km2/yr Halse Water GS8,000 T/yr Bathpool(estimated)
River Tone River Tone
75 T/km2/yr 41 T/km2/yr 28 T/km2/yr15,000 T/yr 12,000 T/yr 8,000 T/yr(SS No. 609) (SS No. 295) (SS No. 146)
Bishops Hull GS
Upper River Tone
Downstream of TauntonUpstream of Taunton Taunton
24,000 T/yr 12,000 T/yr 4,000 T/yr
River Tone
41 T/km2/yr 14 T/km2/yr
Knapp Bridge
83 T/km2/yr
(SS No. 113) (SS No. 182)
New Bridge
(SS No. 445)
French Weir
Firepool Weir
River Tone River Tone
Sediment Yield Analysis
Change in Stream Power d/s
0.00
10.00
20.00
30.00
40.00
50.00
60.00
0.0 1000.0 2000.0 3000.0 4000.0 5000.0 6000.0 7000.0 8000.0 9000.0 10000.0 11000.0 12000.0 13000.0
Chaniage (m)
Sp
ecif
ic S
trea
m P
ow
er (
Wm
-2)
Stream Power Screening
FRMRC Sediment Toolbox
HEC-RAS/SIAM
ISIS-Sediment
CAESAR – Cellular Automaton Evolutionary Slope and River model
www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1Carroll et al. (2004)
Infiltration rates close to zero in grazed pastures.Infiltration rates up to 60 x higher in restored
woodland areas within 2-6 years of planting!
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.
Could strategic tree planting reduce flood risk by disconnecting surface runoff pathways and increasing soil moisture storage?
Modelling future erosion, sediment and morphological responses to changes in climate and land use
Baseline
2050s tree strips
2050s current
2050s intensive
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
SEDIMENT FUTURES
Predicted future Pontbren sediment yieldsBaseline (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)
WP 5.1 Impact of upland land use on sediments contact: [email protected]
Climate change predicted to amplify sediment yield but problems could be offset through changes in land use management.
25
Habitat Connectivity
Hydrology
Farm productivity
Sediment Transport
Trade off Layer
POLYSCAPEMulti-functional Land-
use Management - areas are beneficial to
all services
SWP 5 Land use management negotiation tool
contact: [email protected]
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.
OPTIONS FOR MODELLING AND MANAGING SEDIMENT-RELATED FLOOD RISK
ProjectSuccess
ManagementResources
ManagementResources
StakeholderAttitudes
StakeholderAttitudes
ScienceScience
Credibility
Cognizance
Constraints
Support
ComplexitySimplicity
ProjectSuccess
ManagementResources
ManagementResources
StakeholderAttitudes
StakeholderAttitudes
ScienceScience
Credibility
Cognizance
Constraints
Support
ComplexitySimplicity
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.
Does Sediment and Geomorphology Really 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.
DOES SEDIMENT MATTER?
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
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.”
Environmental Regulation and Flood Risk Management
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.
FRMRC Sediment Researchers and AdvisorsAlex Henshaw – Queen Mary, LondonNick Wallerstein – Heriot-Watt UniversityEmma Raven – Durham UniversityIan Dennis – Royal HaskoningGemma Harvey – Queen Mary, LondonJorge Rameirez - - Hull University Phil Soar – Portsmouth UniversityJenny Mant – River Restoration CentreClifford Williams – Environment-AgencyChris Parker - University West of EnglandSteve Dangerfield – Nttm UniversityTim Meadows – Nottingham UniversityAndy Wallis - Black and Veatch
Paul Bates - Bristol UniversityPaul Brewer – Aberystwyth University Tom Coulthard - Hull UniversitySimon Gosling – Nottingham UniversityStuart Lane – Université de LausanneMark Macklin - Aberystwyth UniversitySuresh Surendran – Glamorgen
UniversityAdrian Collins - ADASMervyn Bramley – Independent Jon Rees - NERCMike Thorn – IndependentDavid Brown - Environment AgencyJim Walker - Environment AgencySean Longfield - Environment Agency
ACKNOWLEDGEMENTS
http://frmrc.hw.ac.uk/