GG3021 Session 2 Flood Hazard I

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GG3021 Session 2 Flood Hazard I

• Processes of flooding and the flood hazard
  context of the UK
• Key Issues in this lecture
• 1. Recent Flood crises in the UK 2000, 2007
• 2. The UK Foresight Report - some outputs
• 3. Flood typology
• 4. Flood hydrograph flood propagation Basics

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Autumn 2000 Floods in the UK

• The map shows rainfall totals for
  September-December 2000.
• Percentage or normal rainfall also given.

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Autumn 2000 Floods Events mapped by number of
properties affected.
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Rainfall in Autumn 2000

• Wettest n-month periods for England Wales.

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Year 2000 Floods

• Peak Flows and associated Return Periods for
  floods in 2000/2001.
• Figures are subject to revision.

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Year 2000 Floods Soil Moisture in Catchments

• Top Soil Moisture deficit (SMD) for Kent in 2000
• Red Line Average 1961-90
• Blue line Year 2000
• Lower Hydrologically effective rainfall in Kent
• Blue Year 2000.

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Historic Floods 2000. The Three orange
hydrographs give combined flow for rivers Thames,
Trent, Severn, Dee Wharfe.
Feb-Apr 1947
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Tewkesbury Summer 2007
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Flood Hazard

• Flood Risk
• flood risk is a combination of two components
  the chance (or probability) of a particular flood
  event and the impact (or consequences) that the
  event would cause if it occurred
• (Environment Agency 2003)
• Flood risk management can reduce the probability
  of occurrence through the management of land,
  river systems and flood defences, and reduce the
  impact through influencing development in flood
  risk areas, flood warning and emergency response
  (EA, 2003)

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UK Foresight Report into Flood Hazard

• AIMS
• 1. Identify assess relative importance of
  threats which will influence long term planning
  of flood defence
• 2. Construct a set of risk-based scenarios over a
  30-100 year timescale.
• 3. Provide an overview of possible responses
• 4. Inform long term needs for flood defence
  research
• 5. Provide an overview of responses available to
  combat flood hazard

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Foresight Project The flood system
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Foresight Project Definitions of the flood
hazard system
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ForesightHydraulic perspective of the physical
flooding system nesting of urban system in the
catchmentDrivers1. Socio-economic2.
Environment pressures3. Impacts leading to
policy responseSOURCESPATHWAYS RECEPTORS
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Foresight Geographic distribution of changed
flood risk in England Wales 2002
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Foresight RASP Results for 10x10km grids.
Changes in flood inundation probability . Note
increase for every region
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Floods Typology

• Fluvial flooding floodplains or washlands. Flow
  exceeds channel capacity.
• Groundwater flooding water table rises. Mainly
  in low-lying terrain on permeable rocks eg Chalk
• Pluvial flooding surface water or rainwater
  flood, when surface conditions reduce
  infiltration of runoff. Critical factors
  rainfall location intensityhard to predict.
  Urban areas.
• Stormwater drain flooding Urban areas, where
  drains become surcharged overflow. Often in
  intense rainfalls (convection). Sheffield (2007)
  London (2007)

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Flood generation

• Non-coastal floods occur most commonly when
  discharge exceeds bankfull and flow spills onto
  the floodplain.
• This stormflow is derived from catchment runoff
  and is controlled by incoming precipitation and
  the hillslope hydrological cycle.
• The flow diagrams show the key runoff models of
  Horton Hewlett.

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Flood Hydrograph Measures of Lag
(Time of concentration)
T1
Tp
T2
Bankfull Discharge
Qpeak
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Flood Hydrograph Flood Hazard

• Measures of LAG
• Time from start of rainfall to peak flow (T1)
  discharge
• Time from onset of flood producing (overbank)
  flow to peak discharge (T2)
• T2 is shorter than T1
• Time to Peak (Tp) time from start of hydrograph
  rise to Peak discharge
• Controls
• main stream length,
• main stream gradient,
• urbanised area of catchment
• net rainfall excess.
• (Flood Studies Report, NERC)

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Flood intensifying factors

• 1. Catchment factors
• 2. Drainage network factors
• 3. Channel factors
• Factors control
• speed of water movement and accumulation
• Antecedent moisture accumulation and development
  of overland flow/rapid sub-surface flow sources.

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River flood intensifying factors

• The shape of the flood hydrograph is influenced
  by
• A) catchment characteristics
• B) Propagation factors
• Left Basin size, shape and runoff contributing
  areas and their control on runoff hydrographs

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Flood producing rainfalls- Scale (Source Smith
Ward)

• Scale Example Size (km2) Duration
• Synoptic Front/Trop Storm 25000-1m days
• Large Meso Convective Line gt50000 6-24 hours
• Mesoscale Multi-cell storm lt2500 1-12 hours
• Mesoscale Thunderstorm lt750 0.5-3 hours
• Microscale Convective cell lt10 lt0.5 hours

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Flood hydrographs weather systems

• Location and movement of rainfall systems in
  relation to the catchment will influence the
  shape and peakiness of the event hydrograph
  through their influence on timing of runoff
  concentration at a specific point in the
  catchment.
• Flood hydrology depends on whether the rainfall
  area covers ALL or only PART of the catchment.

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Determinants of flood damage

• 1. flow depth
• 2. flow velocity
• 3. area inundated the flood outline for given
  discharge
• 4. Water quality
• 5. Socio-economic factors
• 6. flood frequency
• Return period/recurrence interval (1 in 10 yrs)
  etc
• Flood probability () eg 10 chance in any year
• Design criteria for flood defences/drainage
  systems eg design flood protection.

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Flood Wave Propagation

• Flood wave movement is represented by the
  hydrograph at successive locations downstream
  along the main channel.
• Flood wave translation measured by velocity of
  movement of the flow peak downstream (celerity of
  flood wave)
• Extension of hydrograph time base caused by
• Area of drainage basin contributing to channel
  flow
• Attenuation of the flood hydrograph by floodplain
  storage and channel/floodplain FRICTION
• Duration and severity of flood producing rainfall.

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Severn at Bewdley, 2007
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Flood dynamics Flow routing and storage effects
on flow hydrographs
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River Severn, July 2007
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Flood Routing Wave speed (Celerity) and the
effects of flow stage.
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Flood records are derived from river gauging
stations
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Flow levels in UK Catchments 2000

• The colour of the circles indicates levels in
  relation to normal flow.
• Figures indicate of normal flow.

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Autumn 2000 Floods Soil Moisture deficits

• Shaded area shows long term average SMD (Soil
  Moisture deficit) for grid-squares for period
  1961-1990.
• Dark line Shows Yr 2000.
• NB Seasonal differences.

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Flood Hazard

• Flood Warning
• Flood warning comprises several processes
  including monitoring weather, river and coastal
  conditions, forecasting river and sea levels,
  disseminating flood warnings, and influencing
  those at risk to take effective action to prepare
  for, and respond to, flood warnings
  (Environment Agency 2003).

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River levels on the Medway downstream of Yalding
rainfall at Dunks Green Oct Nov 2000
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Regional distribution of flooded property in 2000
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Foresight Scenarios Socio-economic
UKCIP02 scenarios economy World Markets High
CO2 Emissions National Enterprise Medium-High
Emissions Local Stewardship Medium-low
emissions Global Sustainability Low emissions