GG3021: Rivers

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GG3021 Rivers Landscape

• Flood Prediction The Flood Ranger Game

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Reading

• 1. Jones, J.A (1997) Global Hydrology. Chapter 6.
  Longman
• 2. Smith, K Ward,R (1998) Floods. Chapter 8.
  Wiley.
• 3. Discovery Software Ltd (2003) FloodRanger
  Manual (Provided)

3
Flood Forecasts

• Discharge/Stage Forecast reliability is required
• Flood stage forecast Too Low gt avoidable
  damage
• Flood Stage Forecast Too High gt Loss of
  credibility with
• Vulnerable populations
• Timing of flood Peak gt Estimates for evacuation
  time etc

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Flood Forecasts

• Input to catchment rain/snowmelt
• Input to channel runoff
• Conversion runoff volume to flow hydrograph
• Routing of hydrograph to estimate Qpeak, Shape,
  inundation time as flood wave moves downstream.

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Session 3 Flow Estimation

• First attempts to estimate floods in 1840s and
  led to ..
• The Rational method Flow Size linked to.
• SIZE OF CATCHMENT (A)
• RAINFALL INTENSITY (I)
• A COEFFICIENT OF RUNOFF (c) (Integrates catchment
  variables such as infiltration rate, catchment
  resistance etc)
• Q c x I x A
• The Rational method is still a useful flood
  estimator but is VERY DEPENDENT on choice of
  RUNOFF COEFFICIENT.
• Alternative methods today make use of EMPIRICAL
  relations between existing Flood Records and
  catchment variables sych as area, soil factors
  etc (Flood Estimation Handbook 1999)

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Unit Hydrograph Method for catchment hydrograph
synthesis(Source J.A.A.Jones, 1997 Chapter 6))
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-Estimating the flood hydrograph Unit Hydrographs
8
Simulation Models

• Attempt to simulate real-world processes
  accurately.
• LUMPED MODEL Treat the whole drainage basin as
  a homogenous whole.
• SEMI-DISTRIBUTED MODELS Attempt to calculate
  flow conditions from sub-basins, which are
  treated as homogeneous within themselves.
• DISTRIBUTED MODELS Whole basin is split into
  small areas (eg Grid net) and flows passed from
  one grid cell to another. Spatial and temporal
  variations built in.

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Catchment Models- Flow routing(SourceShaw )
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Estimating the flood hydrograph
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Flood Hydrograph Formation Drainage net
variations due to sub-catchment lithology
(Source Smith Ward)
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Types of Catchment runoff models(Source Jones,
1997)
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Elements of Catchment simulation models Lumped
Distributed systems. (Source Jones, 1997)
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FLOW ROUTING Hydrodynamic Routing Models

• Continuity Equation
• DQ/ DX DA/Dt q 0, Q output discharge, A
  cross sectional area of the flow, q lateral
  inflow.
• Combined with a momentum equation which relates
  rate of change in momentum to applied forces.
  This may be simplified to the manning roughness
  equation
• N (roughness factor) R0.67S0.5 / V, where R
  hydraulic radius (Cross section area of
  flow/wetted perimeter) S channel gradient V
  mean velocity.
• This simplification is called the KINEMATIC WAVE
  MODEL.

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Hydrologic Routing Models

• Based on the continuity equation and an
  empirical method used to cover momentum elements.
• DS/Dt Qin Qout , Change in section storage
  input discharge output discharge.
• Storage consists of that defined by input/output
  discharges (prism storage) and that defined by
  waves within the reach (wedge storage).

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Flood Propagation Elements of flood attenuation
due to floodplain storage
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Flood Propagation Relationship of flood wave
velocity (Celerity) to stage.
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FloodRanger

• FloodRanger is a flood simulator.
• Outcome of the Foresight Flood coastal defence
  project
• 200 billion in assets and 1.7m households are in
  flood threatened locations.
• The FloodRanger simualation is designed by
  Discovery Software Ltd.
• AIM To inform understanding and debate about the
  underlying factors affecting risk from flooding.
• Aimed at non-specialists as well as professionals.

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FloodRanger Aims

• To manage the flood risk in a virtual landscape
  over the period 2000-2100. Aim is to minimise
  risks of flooding
• The Ranger can use Sea defences and River flood
  defence systems such as groynes, sea walls,
  managed retreat, beach replenishment, river flood
  walls, reservoirs, weirs, locks and demountables
• The Ranger must also meet demands for new
  housing, industrial development and water demand
  to ensure political survival in a democracy.

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Flood Ranger Economic Background

• Two world futures scenarios are built in
• 1. World Markets Future based on short term
  consumerism, globalisation of governance systems.
  Social values are materialistic with high
  consumption and mobility. Strong pressure to
  reduce taxes. Public opinion strongly influenced
  by economic success.
• 2. Local Stewardship Furure based on community
  and conservation. Diverse political and economic
  systems at regional levels. Cooperative social
  values with high degree of environmental
  awareness. Large degree of public provision but
  hard to levy taxes for flood defence etc. Public
  opinion affected by successful regional economy
  and healthy environment.

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FloodRanger Background

• Climate Change Scenarios Based on Carbon
  emissions from UK Climate Impacts Programme 2002
  and the Hadley Climate Models.
• 1. Low Medium Low Medium High High
• 2. Climate Parameters in these scenarios include
  air temperature, winter upland precipitation
  winter lowland precipitation rainstorm events
  sea level rise and storm surges.
• 3. Events are sampled randomly over a decade for
  1/5,1/10, 1/20, 1/30, 1/50, 1/75, 1/100, 1/150
  and 1/200 year events.

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FloodRanger Background

• Sea Level rise depends on climate scenario and
  adds to tidal height. Max value 1m for High
  emissions scenario.
• Runoff controlled by permeability of the
  underlying geology from 0.1 (Peat) to 0.8
  (Carboniferous Limestone.
• Drainage Net Created by accumulating flow from
  each grid square in the landscape elevation
  model. Flow follows steepest path to the sea.
• Precipitation modified by altitude and
  north-south gradient. Input to each grid square.

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FloodRanger background

• Indicative Floodplain derived by using volume of
  accumulated water and distributed perpendicular
  to the flow of the river.
• Indicators
• Health of environment Area lost to building
  from SSSIs etc Length of new roads etc
  (Dependent on economic model chosen)
• Public Opinion Based on unemployment, Housing
  need, insurance premiums, water demand unmet
  health of environment
• Flood Risk
• Insurance Premiums
• Water Demand f(water demand of people, industry,
  agriculture. Figures depend on Economic scenario
• Water Available depends on summer rainfall,
  reservoir capacity etc.