hazard modelling and risk assessment for urban flood scenario (presentation)

Maryam Izadifar, Alireza Babaee

M.Sc. Civil Engineering for Risk Mitigation

Dec. 2015

Hazard Modelling and Risk Assessment for

Urban Flood Scenario

Supervisor:

Professor Alessio Radice

Co-supervisor:

Professor Scira Menoni

Introduction

Flood Event in 1987:

• Sondrio was not flooded

• River channel was almost full (sediment aggradation)

• Peak discharge in river = 500 m3/ s

• Aggradation of up to 5 m at the Garibaldi Bridge

Garibaldi Bridge

• Return period = 60 years

• Flood duration = 60 hours

Methodology (Integrated Study)

Geological Assessment

Hydrology

River Modelling

(1D)

Sediment yield Urban Flood Modelling

(2D)

Outflow

hydrograph

River flood hydrograph

Risk Assessment

Flood extension,

depth, velocity

Modelling Validation “Idealised City”

Experimental Test Data vs 2D Modelling

Case Study: Town Sondrio

Flood Scenario:

• Peak discharge in river = 640 m3/s

• Return period = 100 years

Idealised City (Experimental Test)

• Université Catholique de Louvain (Belgium)

• Part of the European FLOODsite project

• Sudden transient flow of the dam-break wave

• A square city layout of 5 × 5 buildings aligned

with the approach of flow. Impervious blocks of

0.30 × 0.30 m; the streets were 0.10 m wide

• Validation of Modelling Procedure and

Parameters (Mesh size, Roughness,

Groundwater parameters)

• Validation of the Software (River2D)

• Uncertainties (Sensitivity Analyses in: Mesh

size, groundwater parameters and roughness)

What is Idealised City? Why We Used Idealised City Data?

Idealised City (Experimental Data)

4 sec

5 sec

6 sec

10 sec

Water Depth Water Velocity Recordings in 16 points along the

second longitudinal street

Data were recorded using water-level gauges and digital-imaging technique

Idealised City (Modelling Procedure)

River2D package:

1- Geometry Development in R2D_Bed

2- Mesh Generation in R2D_Mesh

3- Hydraulic Modelling in River2D

Model geometry

Adding Blocks

R2D_Bed:

R2D_Mesh:

4 Sizes Sensitivity Analysis

River2D:

16 Monitoring Points

River2D:

• Two-dimensional modelling

• Depth averaged SWE

Idealised City (Sensitivity Analysis for Mesh Size)

Two strategies:

1) Fine mesh everywhere in the model

2) Coarse mesh in the model with refinement in building block position

3.6 m

8 m

1 m

Mesh size 70 cm with refinement in blocks

Sensitivity Analysis for the Mesh Size:

4 sec

5 sec

6 sec

10 sec

6 sec

10 sec

Graphical results for mesh size 70 cm with refinement in blocks

- The Most Compatible Results

- Fastest Wave Front

The Largest Mesh Size:

Idealised City (Sensitivity Analysis for Mesh Size)

Groundwater Parameters in River2D:

Storativity (related to the volume needed to saturate the ground)

Tranmissivity (related to permeability and the ability to convey discharge)

Model 1: Storativity = 1, Transmissivity = 1;

Model 2: Storativity = 0.001, Transmissivity = 1;

Model 3: Storativity = 1, Transmissivity = 0.1;

Model 4: Storativity = 0.001, Transmissivity = 0.1.

Graphical results after 10 seconds of modelling

Model 1

Model 2

Model 3

Model 4 Fastest Wave Front

Idealised City (Sensitivity Analysis for Groundwater Parameters)

Faster Wave Front

Idealised City (Sensitivity Analysis for Roughness)

Idealised City (Conclusion)

Validation of River2D modelling procedure:

we can trust its results with approximation in the case study (Sondrio)

difference is the size of two models affecting the size of mesh

Mesh size:

the larger the mesh size, the more accurate the results there is a limit for mesh size

rational ratio between the mesh size and the size of blocks and streets

Groundwater parameters:

the smaller the values, the more compatible the results

no groundwater interaction in Idealised City (flume was sealed)

Roughness:

the lower the roughness, the higher the water velocity and the lower the water depth

expectable results

Hazard Modelling for the Scenario

Input Data (100-year Flood Scenario)

Inflow Hydrograph – 8 hours

Mallero basin and its position in Italy and Lombardia region

Inflow Hydrograph – 34 hours

adapted from previous studies (2014)

Mallero river in Sondrio

Flood Scenario:

Return period =

100 years

Peak discharge =

117 m3/ s


Input Data (Bed Generation)

Aerial view of Sondrio including buildings

Ground

level

(a.s.l)

Garibaldi

Bridge

36 Monitoring Points

Simplified geometry for

urban blocks and streets

Sondrio River2D model including building blocks

Model dimensions and bed elevation variation


Input Data (Monitoring Routes)

Three monitoring routes in order to better understanding of water propagation

Via Alessi

Via Parolo

Via Parolo

Via Caimi

Via Caimi

Via Caimi

Corso Vittorio Veneto

Corso Vittorio Veneto

Piazzale Giovanni Bertacchi


Sensitivity Analyses (Mesh Size)

Sensitivity Analysis for Mesh Size: 20-40-60-80-100-120 (Q=117 m3/s , Ks=0.3 m)

slow water propagation

not realistic water propagation

acceptable result

acceptable result

acceptable result acceptable result


Sensitivity Analyses (Roughness)

(Mesh Size=80 m , Peak Discharge =117 m3/s)

Differences in water depth for roughness height (Ks) 0.3 m and 2 m in 480 min after flood

Differences in water depth for roughness height (Ks) 0.3 m and 2 m at Point No. 1


Hazard Map

Qualitative Results: Quantitative Results:

Max Recorded Water Depth and Velocity in 36 Monitoring Points Water Depth (m)

Water Velocity (m/s)

Compromise

between

qualitative and

quantitative

results

Hazard Map


Hazard Map

ArcGIS Flood extension

Water depth intervals (m):

(0 ˂ h ≤ 0.5), (0.5 ˂ h ≤ 1.5), (1.5 ˂ h ≤ 2)

Flood Risk Assessment

Sondrio Damage Assessment Steps (Buildings)

Steps:

• The depth of flooding is determined using the

flood hazard map

• Simplified USACE damage curve

• Categorize buildings based on the number of

building storey and presence of basement. (Site visit, Google Street View, previous damage study

for 50 buildings in town Sondrio)

Estimate the potential damage

(Micro-Scale, Direct, Tangible)

According to a significant shift from hazard

centered perspective to understanding of risk

there is a need to quantify flood risk in terms

of damage.

Flood Risk Assessment

Sondrio Damage Assessment Map (Buildings)

Damage Levels:

• Very High (50 % ≤ Damage rate)

• High (40 % ≤ Damage rate < 50 %)

• Moderate (25 % ≤ Damage rate < 40 %)

• Low (Damage rate < 25 %)

• No expected damage (out of flood extension zone)

Grand Hotel Della Posta in Piazza Garibaldi

Multi-storey with basement

Residential building

Multi-storey without basement

Sport Facility

One- storey without basement

Railway Station

Multi-storey with basement

Damage rate in 4 levels assigned according to building type and the level of hazard

Scenario (Limitations, Suggestions)

Improve geometry of the model

Scenario of flood in the west part of the city

can be considered

Explore other kind of software packages

Detailed study of uncertainties (roughness,

groundwater interaction, upstream

hydrograph, inflow location, etc.)

Hazard Modelling: Damage Assessment:

Lack of reliable data

Finding appropriate damage model for case study

Limited transferability of damage curves designed

for one geographic area to another area

Uncertainty in damage curve estimation is high

Flood damage depends, in addition to building type

and water depth, on many other factors (building

age, material, foundation type…, flow velocity,

duration of inundation, contamination…)

Different urban patterns and building typologies

that are typical of Italy make it difficult to

generalize damage functions or to obtain large

enough data sets

Thank You For Yours Attention

Hazard Modelling and Risk Assessment

for Urban Flood Scenario