Flood Hazard Mapping

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Flood Hazard Mapping
Flood Risk Metric
Superposition of Flood Risk Layers
Historical Floods
Analysis of Historical Flood Marks
Hydrological Scenario Topographic
Methods Topologic Method Mass Balance
Method Hydrodynamic Floodplain Models
Methods for Mapping Flood Hazard
Probabilistic Framework Hydrological
Scenario Floodplain Analysis
Sources of uncertainty in flood hazard mapping
Structures People Mobiles Thresholds
Conditional Hazard
Flood Mapping vs Land Planning
Towards Harmonisation across Europe
Renzo Rosso Politecnico di Milano
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Flood Risk Metric
Flood risk is usually given as the product of 3
factors (see Varnes al. per la IAEG Commission
on Landslides, Landslide Hazard Zonation - a
review of principles and practice. UNESCO,
Paris, 1984) R  H E V with R denoting il flood
risk, as ranked from moderate to very high, e.g.
from R1 to R2, R3 and R4. H is a measure of
Natural Hazard, as given, e.g., by the
probability of occurrence of flood magnitude
(usually represented by the peak flow). For
example, probabilities of 0.02, 0.01, 0.005 and
0.002, are associated with return periods of 50,
100, 200 and 500 years, respectively. E is a
measure of Exposure, e.g. the potential damage
to people and structure in the floodplain, as
weighted from 0 to 1. V is a measure of
Vulnerability, i.e. the resistance of the
system to respond to the flood load, as
weighted from 0 to 1.
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Superposition of Flood Risk Layers
if V 1
From S. Fattorelli, M. Coccato, E. Frank A.
Ostan, Integrated water basin management and risk
assessment through advanced modeling techniques,
in Brebbia C.A. (ed.), River Basin Management
II, Proceedings of the, p. 411-421, WIT Press,
Southampton, 2003.
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Historical Floods
Inundated areas after the catastrophic flood of
the Arno river, Italy, in 1966
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Analysis of Historical Flood Marks
Flood-prone areas in the Arno river basin, Italy
low frequency
high frequency
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Methods for Mapping Flood Hazard
Flood Hazard Mapping
Hydrologial Scenario
Factors of Uncertainty
Floodplain Scenario
ApproximateTechniques
Floodplain Models
probabilistic framework
topographic methods

• hydrological scenario
• (a) flood peak (b) flood volume

1D
Pseudo-2D
topologic methods
2D
floodplain analysis
mass balance method
1D/2D
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Hydrological Scenario
Regionalised Flood Frequency Estimates
overflooding volume
overflooding threshold
Flood Hydrograph
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Topographic Methods
Assumption 1 infinite levees, confined steady
flow into the levees, countur level crossing the
transect
Assumption 2 effective levees, undetected
flow, countur level crossing the transect
Assumption 3 steady flow occurring in both the
river cross-section and the floodplain
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Topologic Method
flow paths are followed downsteam starting from
the overflooding river site, based on
geopotential (contour lines or raster)
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Mass Balance Method
overflooding volume
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Hydrodynamic Floodplain Models
combined 1D/2D staggered grid
combined finite mass volume for 1D/2D computations
From The Adige River Drainage Basin Flood
Mapping Plan, 2003 (by the courtesy of S.
Fattorelli)
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Sources of Uncertainty in Flood Hazard Mapping
probabilistic framework
hydrological scenario
floodplain analysis
flood peak
flood volume and hydrograph

• local effects
• (obstacles,.)
• spatial resolution
• (still unexplored)
• roughness
• (almost unexplored)
• 1D/2D
• (almost unexplored)

• return period ranking
• (high, moderate, low, )
• risk metric
• (complex probability convolution)
• link to damage evaluation
• (unexplored)

• flood duration curve
• (unrealistic)
• Joint Q-V distribution
• (unexplored)
• rational method
• (unflexible approach)
• Monte Carlo Simulation
• (pioneering stage)

index flood estimation (ungauged basins)
unexplored uncertainty
uncertainty trade-off
measurable uncertainty
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Uncertainty Probabilistic Framework

• In Flash-flood Prone Areas with
• high susceptibiliy to flood occurrence,
  including soil slip, debris flow and woody debris
  processes produced by high intensity storms with
  short duration, and
• high gradient of the growth curve describing the
  exceedence probability of peak flow (e.g. small k
  for the GEV distribution)
• one should consider relatively low return periods
  of the hydrological scenario (e.g. 20, 100 and
  300 years for high, moderate and low hazard,
  respectively).
• The 20-yrs, 100-yrs and 300-yrs are associated
  with exceedence probability of 5, 1 e 30/00,
  respectively.

• In Flood-Prone Areas with
• low susceptibily to flood occurrence, mainly
  associated with indundation only, and
• low gradient of the growth curve describing the
  exceedence probability of peak flow (e.g. k close
  to zero for the GEV distribution)
• one should consider high return periods of the
  hydrological scenario (e.g. 50, 200 and 500
  years for high, moderate and low hazard,
  respectively)
• The 50-yrs, 200-yrs and 500-yrs are associated
  with exceedence probability of 2, 50/00 e 30/00,
  respectively.

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Uncertainty Hydrological Scenario
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Uncertainty Floodplain Analysis
map resolution
river and floodplain survey
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Conditional Flood Hazard

• damage to buildings and other infrastructures
• people instability
• mobilisation of vehicles and other objects in the
  flood plain

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Buildings
studies based on scale models
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People
studies based on laboratory experiments
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Movable Objects
Scale tests (Cacioli Paciscopi, 1999)
Envelope of scale test results
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Damage Thresholds

• Comparison among
• building stability
• human stability
• vehicle mobilisation
• empirical
• hydrodynamic
• thresholds for conditional hazard evaluation

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Flood Mapping vs. Land Planning (1)
Current Criteria based on Flood Frequency
30 - 50 yrs
A-Zone Very High Hazard no new urbanisation
allowed nor renewal of ancient buildings
100 - 200 yrs
B-Zone High Hazard no new urbanisation allowed
A
B
this including
Sub_B-Zone new flood proofing buldings allowed
?
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Flood Mapping vs. Land Planning (2)
Integrated Criteria combining Flood Frequency
with Inundation Pattern
AA zone no new urbanisation allowed nor renewal
of ancient buildings
BB zone no new urbanisation allowed
B0 zone new flood proofing buldings allowed
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Flood Mapping vs. Land Planning (3)
30-yrs Flood
200-yrs Flood
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Flood Mapping vs. Land Planning (4)
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Towards Harmonisation across Europe
directive

• European Union
• Country
• Local

• Authority
• Region
• Municipal

guidelines
demonstration
validation