Transcript Flooding: An Avoidable Hazard or An Increasing Risk? Professor Adrian McDonald University of Leeds with contributions by Professor Stuart Lane.

Flooding: An Avoidable Hazard
or An Increasing Risk?
Professor Adrian McDonald
University of Leeds
with contributions by
Professor Stuart Lane
between 1991 and 2000
over 665,598 people
died in 2,557 natural
disasters
torrential rain – two years
worth over two days, led to
15 million metres3 of mud,
rocks and trees detaching
from mountain sides and
descended into the urban
areas of Venezuela killing
30,000 people
In 2,000, 153 flood events
included disastrous floods in
Mozambique and along the
length of the Mekong,
The 1998 monsoon
season brought with it
the worst flood in living
memory to Bangladesh,
placing some 65 percent
of the country
underwater
Third World and First World
•
•
Floods in the first world cost money.
Floods in the third world cost lives.
But
•
3rd world flooding is also an opportunity
–
–
Crop diversity
Crop security
These conclusions come from field studies by
Matt Chadwick of the 1998 floods in India.
The first question
Is there evidence that flooding is
getting worse?
1 80
Floods
1 60
1 40
Aval an ches /l and s li de s
D ro ugh ts /fa m in es
Number of Events
1 20
Ea rthq uake s
1 00
Extrem e te m peratures
Fl ood s
80
Fo re s t/s crub fires
Vo lcan ic erup ti ons
60
W i nd s torm s
O the r na tura l dis as ters
40
20
0
199 1
1 992
19 93
199 4
1 995
199 6
1 997
199 8
1 999
20 00
Year
Reported Disasters IFRC 2001
1991-2000
1981-1990
1971-1980
1961-1970
1951-1960
1941-1950
1931-1940
1921-1930
Decadally-averaged annual precipitation (mm)
900
Data from the
River Ouse
850
800
750
700
650
600
550
500
Decadally averaged annual maximum
floods (m above Newlyn)
7
1991-2000
1981-1990
1971-1980
1961-1970
1951-1960
1941-1950
1931-1940
1921-1930
1911-1920
1901-1910
1891-1900
1881-1890
10
9.5
9
8.5
8
7.5
Number of peaks over 8.058 m threshold
1991-2000
1981-1990
1971-1980
1961-1970
1951-1960
1941-1950
1931-1940
1921-1930
1911-1920
1901-1910
1891-1900
1881-1890
50
45
40
35
30
25
20
15
10
5
0
Annual rainfall (mm)
2000
1500
1000
500
0
1965
1975
1985
1995
Hydrological year
To summarise
• Hidden in a time series of more variable
annual rainfall totals is a picture of
• declining rainfalls yet
• bigger floods
• more floods
There are many types of floods
• Just heavy rain
• Accidents
• Floods from rivers
• Floods from the sea
• Sewage floods
November 2000
Key Facts
•
•
•
•
•
8% of England is at risk of river flooding.
1.7 million dwellings.
130,000 commercial premises.
4.5 million people.
12% agricultural land (river and sea floods).
‘A national appraisal of assets at risk from flooding’ MAFF 2000.
Yorkshire
•
•
•
•
189,000 ha land at risk (94% agricultural).
217,000 houses.
280 hectares of industry/commerce.
Substantial length of road and rail.
Regional flood defence committee.
Easter Floods 1998
• £400 million.
• 5 deaths.
• 1,500 evacuated.
• National damage costs are £600 million / yr.
• National costs to maintain defences £300 million / yr.
• Actual spend £200 million.
What makes a flood a hazard?
• Not all floods are hazards
• We can consider river flows as some sort of timeseries
• Occasionally there will be floods (when the river goes
out of bank)
• A flood becomes a hazard when the flood passes some
sort of human threshold
• This resource band is best defined by ‘an ability to
cope’
What makes a flood a hazard?
• Floods are out of regime behaviour
Rivers can (and do) go anywhere!
What makes a flood a hazard?
Engineering
Flood proofing
infrastructure
T IME
ST AT IC
DYNAMIC
(natural)
DYNAMIC
(human)
Resource band
The future
Flooding will get worse:
– Climate change
– Past development
– Urbanisation
Flood potential is increasing. Flood potential is the
term used to define the increased damage caused
(£ usually) by the same flood discharge .
Ouse Floods: First Renewables
Floods hamper
communication.
Bridges will carry gas,
electricity, telephone
cables, etc
Ouse Wrack Lines
Wrack lines give us
evidence of the peak
height of the flood.
Ouse Autumn 2000
Flood Management by
Development Control
This section refers only to
development control
Based on a River Ouse Project Board paper
by Phil Lawton requested with
amendments by the Commons
Environment Committee
22 November 2000
The Government Has No
Statutory Duty to Protect Land
or Property Against Flooding.
All flood defence works
under permissive powers.
The national approach:
“..reduce the risk to people and the natural and developed
environment from flooding….by technically, economically
and environmentally….sound defence measures.”
Three objectives:
• Adequate and cost effective flood warning.
• Adequate flood defence.
• Discourage inappropriate development.
Approaches to Development
Control
• Planning System
• Developer Action and Contribution
• Legal and Fiscal
Implementing Development Control
• Planning Policy Guidance Note 25 (May 2000).
Recommends Precautionary Principle. Development
should not increase flooding elsewhere.
• Regional Planning Guidance. Recognises more frequent
flooding of defended areas. Promotes zonation.
Requires best available risk assessment.
• Structure Plans. Drainage control. Compensatory
volume. Managed retreat.
• Local Plans. As yet tend to differ from authority to
authority. Multiple authorities in a river basin. May,
in future, show flood risk on statutory maps.
Flood proofing in the U.K.: how
effective?
Emergency
• The problem of vulnerability
evacuation.
– "...Remember, it's your responsibility to look after yourself,
your family and your possessions. Because floods don't just
happen to other people...".
– What do you do if your elderly, have young children or
suffer from poor health?
• The problem of knowledge
– do you know the EA’s floodline number
– which catchment you are in?
– Try it!
Types of Response
• Ecological
• Engineering
• Economic
Ecological Responses
• Centered in the catchment
• Keep water in the basin
– increase interception
– increase evapotranspiration
– increase infiltration
– slow down overland flow
Problems with Ecological
Response
• Can have a detrimental effect on flood
conditions
– e.g. forests
• Lack of public recognition
– => political interference
Engineering Responses
Centered on:
• flood site
• upstream storage
• Both attempt to increase the capacity of the
channel to contain floods
Engineering Approaches
• Simple
– increase slope
– increase cross-sectional area
– decrease resistance to flow
• Complex
– flood levees
– bypass
– reservoir
Engineering infrastructure
• Construction of levees and flood retaining
walls
Levee
• Flood compensation reservoirs
• River realignment
Flood retaining
wall
Areas prone to
most frequent
flooding
Problems of Ecological and
Engineering Responses
• State intervention =>capital /maintenance /
planning / legislation
• Approaches fail or are too slow
• Engineering changes publics perception
Economic Response
• Loss bearing and change in use
• Emergency planning
• Land elevation and flood proofing
• Insurance
• Floodplain zoning
Loss Bearing
• Most widespread response
• Minimisation of flood damage
• Ignores social constraints
– skills
– poor data
Contingency Planning
• Reschedule to minimise stock at risk during
perceived hazardous times
• Dependent on adequate warning
• Easier for larger basins
• Needs public input
Land Elevation
• Individual response
• Raise land so above flood levels
• Only possible at start of building
Flood Proofing
• Available at any time
• Seal property against floodwaters
• Need adequate flood warning
• Limited to maximum depth
• Property needs to be suitable
• Will raise general flood level
Insurance
• Flooding is localised and intermittent
• Many people at risk, but unlikely to all be
flooded
• But consequences are severe
• For a premium those at risk are covered
against a specified risk
• Success depends on take up
Floodplain Zoning
•
•
•
•
Delineate floodplain into a number of areas
Each has different flood risk
Oversimplifies
Three types of zone
– Exclusion zones
– Regulatory zones
– Advisory zones
Exclusion Zone
• Development is against public interest
• Zone needs to be kept clear for flood waters
• Any development would increase depth of
flood
Regulatory Zone
• Development is permitted, but is regulated
• Flood insurance, flood proofing and
contingency planning maybe mandatory
• Flood risk is lower but likely
Advisory Zone
• Advice is provided on degree of risk
• Flood insurance, but at a lower premium is
encouraged
These many different approaches must
be integrated into site specific plans
because the Water Framework Directive
requires this but also because we are
dealing with problems that really do
require individual solutions
Choosing the mix of actions that gives
a sustainable solution