Accounting for Sediment and Geomorphology in Flood Risk
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. 3 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 5 WP 5.1 Modelling flood impact of upland land use change 6 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 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 7 WP 5.1 Modelling flood impact of upland land use change contact: [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 8 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 gravelbed rivers, Progress in Physical Geography 34(1) 2345. 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 17 contact: [email protected] 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 18
contact: [email protected] 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 60.00 Stream Power Screening 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 ) HEC-RAS/SIAM 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 risk, to 60 erosion x higher and in restored could reduce flood 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]ngham.ac.uk 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] 25 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 apply theallmethod/ vary, but methods and model models and require 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 life; and loss of Extensive overbank deposition of sediments damaged farmland. coarse Channel and floodplain instability ecosystems and habitats. destroyed 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 Universit 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 http://frmrc.hw.ac.uk/
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