www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1

Land-use, Sediment and Flood Risk

delivered by:Colin Thorne, Nottingham University

on behalf of:

FRMRC Sediment Research Team

Catchment Sediment Yields: natural vs intensive pasture

Coarse sediment yield 12x greater

Fine sediment yield 5x 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 Unpublished PhD thesis.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

River Wharfe: sedimentation in engineered channels can lead to significant increases in flood risk. Review of Foresight on Future Flooding commissioned by Sir Michael Pitt (Evans et al., 2008) stated that:

 “approximately a year and a half of aggradation produced an increase in the flooded area equivalent to nearly half a century of the impact of climate change on catchment runoff.”

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.

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.

TRAGIC EVIDENCE

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 deposition – Complex fines deposition – especially at structures especially at structures

Elevated Elevated sediment yieldssediment yields

Localised coarse Localised coarse sedimentationsedimentation

Modelling future erosion, sediment and morphological responses to changes in climate and land use

Baseline

2050s tree strips

2050s current

2050s intensive

Strategic land use management can substantially reduce erosion and sediment yields

Land use changes buffer rivers from the worst impacts of climate change

Uncertainties are high and further research is needed

SEDIMENT FUTURES

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

TAKE HOME MESSAGES

1. Sediment-related flood risks and damages are significant and flood victims recognise this.

2. Land use management can substantially increase or decrease sediment-related flood risks.

3. Unless we act, future sediment impacts are likely to increase due to climate change.

4. Sediment management for flood risk reduction must be aligned with environmental legislation.

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