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Presentation on theme: " EPSRC Grant: EP/FP202511/1 Accounting for Sediment and Geomorphology in Flood Risk Management Colin Thorne Chair of Physical Geography,"— Presentation transcript:

1 EPSRC Grant: EP/FP202511/1 Accounting for Sediment and Geomorphology in Flood Risk Management Colin Thorne Chair of Physical Geography, Nottingham University and Faculty Affiliate, Portland State University


3 Pontbren experimental catchment WP 5.1 Modelling flood impact of upland land use change contact: Pontbren was a unique 6-year field experiment performed through collaboration between scientists, farmers and decision-makers. 3

4 Changes in land management Increased sheep stocking levels in uplands Henshaw et al. (in prep.) 1969 1997 Historical changes in sheep stocking density Pasture improved through drainage, liming and reseeding

5 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: 5

6 Land-use Runoff and Farm-scale Flooding At 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: Flow gauges Low ‘T’ indicates faster flow responses Land use 6

7 Upland land use change impacts on peak flows Models 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: Median change: -5% Uncertainty range: -2 to -11% 7

8 WP 5.1 Modelling flood impact of upland land use change contact: p.e.o‘ Land-use impacts on downstream flood peaks in Large Catchments Peat: blocked Peat: drained Peat: intact Good Fair Poor Pre-change Post-change Modelled impact on peak is small, only a few percent, but uncertainty is high 95% prediction bounds 8

9 Land-use and Flooding: Summary Minimum effect Maximum effect Increasing scale Increasing return period 1 – 5 Years: local ‘nuisance’ floods 50 - 100 Years: regional ‘catastrophic’ floods

10 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.

11 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 UPLAND CATCHMENTS

12 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.

13 Sediment Impacts on Conveyance, Channel Stability and Habitats Accelerated Channel migration Reduced conveyance capacity Reduced Water quality Habitat degradation 2002-2004 aggradation 2050s climate scenario Present 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:

14 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.

15 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

16 Lowland Catchments

17 Distributed hydrological model for the River Tone WP 5.3 Modelling flood impact of lowland land use change contact: …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

18 Lowland land use change scenarios Woodland planting scenario Flood retention storage scenario WP 5.3 Modelling flood impact of lowland land use change contact: The model shows limited impact of woodland planting, but greater impacts from distributed flood retention storage 18

19 Land use and Sediment Dynamics in the River Tone LOWLAND CATCHMENTS Complex fines sedimentation – especially at structures Elevated sediment yields Localised coarse sedimentation

20 Options for Modelling, Predicting and Managing Sediment-Related Flood Risk: FRMRC Sediment Toolbox

21 Sediment Yield Analysis Stream Power Screening FRMRC Sediment Toolbox ST:REAM Sediment Transport: Reach Equilibrium Assessment Method HEC-RAS/SIAM ISIS-Sediment CAESAR – Cellular Automaton Evolutionary Slope and River model

22 EPSRC Grant: EP/FP202511/1 Carroll 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?

23 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

24 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) WP 5.1 Impact of upland land use on sediments contact: Climate change predicted to amplify sediment yield but problems could be offset through changes in land use management.

25 25 Habitat Connectivity Hydrology Farm productivity Sediment Transport Trade off Layer POLYSCAPE Multi-functional Land- use Management - areas are beneficial to all services SWP 5 Land use management negotiation tool contact:

26 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 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.

27 Does Sediment and Geomorphology Really Matter?

28 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?

29 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

30 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

31 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.

32 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 ACKNOWLEDGEMENTS


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