Accounting for Sediment and Geomorphology in Flood Risk Management

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

[email protected]

mailto:[email protected]

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.

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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



Land-use Runoff and Farm-scale FloodingAt farm scale, the effect of land-use on flows and flood

peaks is clear



Flow gaugesLow ‘T’ indicates faster flow responses

Land use

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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



Median change: -5%Uncertainty range: -2 to -11%

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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

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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

http://riverscience.wikidot.com/alex-henshaw

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


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

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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

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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

http://www.halcrow.com/isis/default.asp

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.

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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


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


ScienceScience

Credibility

Cognizance

Constraints

Support

ComplexitySimplicity

ProjectSuccess

ManagementResources

ManagementResources



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/

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

Documents

Accounting for Sediment and Geomorphology in Flood Risk Management