how can communities be better prepared for floods?
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“Floods - Past and Present Issues” Address given to The Royal Scottish Society of Arts 11th February 2002 Edinburgh by Professor George Fleming FREng FRSE FICE FASCE. - PowerPoint PPT PresentationTRANSCRIPT
“Floods - Past and Present Issues”
Address given to The Royal Scottish Society of Arts
11th February 2002Edinburgh
by
Professor George Fleming FREng FRSE FICE FASCE
Professor of Civil Engineering, University of StrathclydeManaging Director of EnviroCentre
Chairman, ICE Presidential Commission Undertaking a Technical Review of Flooding in
England and Wales
• How can communities be better prepared for floods?
• Why is it not yet a requirement of the Building regulations that the lowest floor of a building on a flood plain is above the 1 in 100 year flood level?
• Should all buildings on flood plains be constructed of flood resistant materials?
• Why is there no national strategy for such mitigation systems?
• Could a national flood insurance programme be linked with these requirements?
-V-
INTRODUCTION
• England and Wales - 2000
• River Tay - 1993
• River Ness - 1989
• The Clyde - 1977
• Bristol - 1968
• Lynmouth - 1952
• Medway - 1814
Risk – Expenditure Cycle
No flood
Complacency
Reduced expenditure
Increasing risk
Major flood
Damage, distress, disruption, deaths
Review
Expenditure
Reduced risk
Risk – Expenditure Cycle
No flood
Complacency
Reduced expenditure
Increasing risk
Major flood
Damage, distress, disruption, deaths
Review
Expenditure
Reduced riskNo flood
Complacency
Reduced expenditure
Increasing risk
Major flood
Damage, distress, disruption, deaths
Review
Expenditure
Reduced risk
Depute Prime Minister
John Prescott MP described the severe weather and flooding
that dramatically disrupted parts of the
country in Autumn 2000 as a “wake-up call” to the impacts of climate
change.
THE HISTORICAL CONTEXT
A flood is a “great flow of water, causing overflow and inundation” (Chambers, 2000(2)). The factors
causing a flood to occur are extremes in meteorology and hydrology, coupled with changes to river
hydraulics caused by land use and alterations to river geomorphology
The Forth & Clyde and Union Canal
floodplain
early towns & settlements
normally on local elevated areas or where the channel abuts higher ground on edge of floodplain
floodplain
later extensions to town
on natural floodplain in part of the natural flood channel of the river
Development in Flood Plains
HISTORICAL FLOOD ESTIMATION
River levels
Rainfall
River flow
None Limited Widespread
1600 1800 2000
Historical Data Collection
“Risk of a one in 100 year flood event”
• Only possible once in 100 years
• One in 100 chance of flooding
X
Regional Flood Frequency Curve: Scotland ( Biswas & Fleming, 1966)
Regional Flood Frequency Curve: Great Britain (NERC, 1975)
Regional Flood Frequency Curves
Regional Flood Frequency Curve: Scotland (Biswas & Fleming, 1966)
Regional Flood Frequency Curve: GB(NERC, 1975)
1.0
2.0
3.0
4.0
5.0
1 2 3 4 5 6 7 80-1
T
y1
2 5 10 25 500100 1000
x
x
x x
x
x
xQ /Q
x
Analysis of Flood
Frequency Curves
1800 1850 1900 1950 2000 2050
Data Availability Empirical methods
Statistical flood frequency analysis Modelling
Flood Estimation Techniques
THE CURRENT PROBLEM
economic••commercial disruption•services disruption•
flood type• depth of flooding• flood velocity• rate of flood rise• wave action
flood prediction• source of flooding• reliability of forecasting• warning time
social• flood awareness• resilience of population• nature of housing• social disruption
flood protection• type of defence• d• c
background conditions•• number of properties at risk
environmental• climate change • environmentally sensitive areas• long and short term impacts
flood risk
economicinfrastructure at risk
••potential agricultural losses
flood type••••
flood prediction•••
social••••
flood protection•• design standard• condition of defence
background conditions•
environmental•••
flood riskflood risk
Factors Affecting Flood Risk
flood history
inflow
outflow
inflow
outflow
inflow
outflow
natural flood plain storage
flood bank
outlet
attenuation
lag
Flow
Time
attenuation
lag
Flow
Time
typical catchment
inflow
outflow
inflow
outflow
inflow
outflow
inflow
outflow
inflow
outflow
inflow
outflow
inflow
outflow
natural flood plain storage
flood bank
outletoutlet
attenuation
lag
Flow
Time
attenuation
lag
Flow
Time
attenuation
lag
Flow
Time
attenuation
lag
Flow
Time
typical catchment
Effect of Constraining Flood Plains
Flow
Time
inflo
w
Inflow hydrograph to Community AHydrograph at Community B under natural conditionsHydrograph at Community B with Community A defended
Flood peak exaggeration
Earlier response time inflow
natural conditioncommunity A
community B
defended conditioncommunity A
community B
Flow
Time
inflo
w
Inflow hydrograph to Community AHydrograph at Community B under natural conditionsHydrograph at Community B with Community A defended
Flood peak exaggeration
Earlier response time Flow
Time
inflo
w
Inflow hydrograph to Community AHydrograph at Community B under natural conditionsHydrograph at Community B with Community A defended
Inflow hydrograph to Community AHydrograph at Community B under natural conditionsHydrograph at Community B with Community A defended
Flood peak exaggeration
Earlier response time inflow
natural conditioncommunity A
community B
inflow
natural conditioncommunity A
community B
defended conditioncommunity A
community Bdefended condition
community A
community B
Effect of Constraining the Flood Plain
Leigh Barrier in Operation
Flood Defences at Gainsborough
FUTURE FLOODING ISSUES
Pacific Quay
Daldowie Station
THE CLYDE CATCHMENT
Projected Effects of Climate Change on Run-Off for Clyde Catchment
Flood chance in any year(return period)
5 10 20 30 50 70 100 200 500
Year 2000 runoff(m3/s)
678 798 931 1005 1114 1191 1254 1411 1680
Predicted 2050s runoff(m3/S)Design event rainfall +10%
750 884 1033 1116 1239 1324 1396 1572 1875
Increase in runoff (%) 9.6 9.7 9.9 9.9 10.0 10.1 10.1 10.2 10.4
Presidential Commission to Review Flooding in England and WalesInstitution of Civil Engineers
One Great George StWestminster
London, SW1P 3AA
Tel: 0207 665 2232www.icenet.org.uk/presidential.html
CONCLUSIONS
• How can communities be better prepared for floods? A: For flood risk to be more clearly explained to both professional and lay person.
• Why is it not yet a requirement of the Building regulations that the lowest floor of a building on a flood plain is above the 1 in 100 year flood level? A: Building on the flood plain needs to be carefully assessed. There is more to do than a simple building regulation.
• Should all buildings on flood plains be constructed of flood resistant materials? A: Existing buildings need flood proofing. New buildings on flood plains need to have flood resistance designed in as well as flood impact designed out.
• Why is there no national strategy for such mitigation systems? A: There is an emerging strategy in England and Wales to manage flood risk. A similar strategy has to be developed in Scotland which takes us beyond just flood warning.
• Could a national flood insurance programme be linked with these requirements? A: Flood insurance programme must be consumer-driven and be flexible. A national flood insurance programme would be as complicated as it was ineffective.
Warning systems will be important, but a flood
management system must be introduced in order that
community and profession can work together and learn to live
with our rivers.