Accounting for Sediment and Geomorphology in Flood Risk Management

1. 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@nottingham.ac.uk

2. UPLAND CATCHMENTS

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

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

5. WP 5.1 Modelling flood impact of upland land use change contact: n.mcintrye@imperial.ac.uk 5 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

6. WP 5.1 Modelling flood impact of upland land use change contact: n.mcintrye@imperial.ac.uk 6 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

7. WP 5.1 Modelling flood impact of upland land use change contact: n.mcintrye@imperial.ac.uk 7 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

8. WP 5.1 Modelling flood impact of upland land use change contact: p.e.o‘connell@newcastle.ac.uk 8 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 Peat: blocked Peat: drained Peat: intact Good Fair Poor Pre-change Post-change

9. 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

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. UPLAND CATCHMENTS Catchment Sediment Yields: natural vs intensive pasture Pontbren Experimental Catchments Melin-y-grug Fine sediment yield 5x greater Coarse sediment yield 12x greater Most excess sediment generated from withinchannel network 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

12. 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.

13. WP 5.2 Modelling sediment impacts of upland land use change contact: c.thorne@nottingham.ac.uk Sediment Impacts on Conveyance, Channel Stability and Habitats Reduced conveyance capacity Accelerated Channel migration 2050s climate scenario 2002-2004 aggradation Present Habitat degradation Reduced Water quality Lane et al. (2007) 1-in-0.5 year flood +12.2% +5.7% Combined: +38.2%

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. 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.

16. Lowland Catchments

17. WP 5.3 Modelling flood impact of lowland land use change contact: i.d.cluckie@swansea.ac.uk Slower/Deeper Baseflow 17 Distributed hydrological model for the River Tone Vertical Data Layers Water Movement Procedures (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

18. WP 5.3 Modelling flood impact of lowland land use change contact: i.d.cluckie@swansea.ac.uk 18 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

19. LOWLAND CATCHMENTS Land use and Sediment Dynamics in the River Tone 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. FRMRC Sediment Toolbox CAESAR – Cellular Automaton Evolutionary Slope and River model Sediment Yield Analysis Stream Power Screening HEC-RAS/SIAM ISIS-Sediment ST:REAM Sediment Transport: Reach Equilibrium Assessment Method

22. Could strategic tree planting reduce flood risk by disconnecting surface runoff pathways and increasing soil moisture storage? 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. Carroll et al. (2004)

23. 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 2050sintensive Baseline 2050s current 2050s tree strips

24. WP 5.1 Impact of upland land use on sediments contact: Colin.Thorne@nottingham.ac.uk Predicted future Pontbren sediment yields 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.

25. SWP 5 Land use management negotiation tool contact: afse0c@bangor.ac.uk Habitat Connectivity 25 Hydrology POLYSCAPE Multi-functional Land-use Management - areas are beneficial to all services Farm productivity Sediment Transport Trade off Layer

26. 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.

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. 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.”

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

32. ACKNOWLEDGEMENTS FRMRC Sediment Researchers and Advisors 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

33. http://frmrc.hw.ac.uk/