Thematic Area

Flood Hazard
& Risk Assessment

Overview

Flood is one of the most destructive natural hazards. With climate change and urbanization, the frequency and magnitude of flood and inundation events are increasing in many parts of the world. In particular, the ASEAN region is especially prone to floods. One of the main causes is heavy monsoon showers accelerate by high tide and drainage channels that lack the water conveyance capacity. As well as antecedent conditions of river networks, flood control facility, soil condition, from other phenomena such as storm surge, tsunami, dam failure, watercourse-blocked by landslide, etc.

The case study focuses on quick-rising floods caused by heavy rainfall over a short period, with integration of climate change impact on rainfall and sea level rise. Flood risk is calculated by a function of the flood hazard, the exposed values and their vulnerability

Flood risk is defined as following equation in this study:

Here, the flood hazard is a component of natural event damaging humans and human property, the exposed values is a component of people or asset values under the flood hazard, and the vulnerability is a component of how susceptible to the flood hazard.

Flood Risk = Hazard x Exposure x Vulnerability

Characteristics of the risk assessment in the Project is:

Scenario-based analysis (climate change impact scenarios)
Hazard mapping with different occurrence probabilities and risk mapping based on the average annual damage
Preliminary flood hazard maps and risk maps were created with limited available data to enhance flood hazard and risk mapping even under poor observation data

Overview of the procedure of flood hazard and risk analysis for this case study.

Framework of modeling for flood hazard and risk analysis

Characteristics of Approach

Build framework

identify starting point and select target river basin

Set framework of flood hazard and risk analysis

based on project objectives, data availability and characteristics of target river basin

SCS Curve Number Method

In rainfall-runoff modeling, Semi-distributed model applying the SCS curve number method was created by HEC-HMS.

The coupled 1D and 2D unsteady flow model

In flood inundation modeling, the coupled 1D and 2D unsteady flow model was created by HEC-RAS

Selecting suitable GCMs for the river basins:

Select highest and lowest severe case from the suitable models (CMIP5 models) from NASA Earth Exchange (NEX) Global Daily Downscale Projections (GDDP) for the study area

Develop future rainfall increase ratio

Develop future rainfall increase ratio for three time slice (2030, 2050 & 2080)

Vulnerability Assessment

Vulnerability assessment; element-at-risk identification and qualitative and quantitative indicators.

Prepare Flood Hazard Maps

Prepare Preliminary Flood Hazard and Risk Maps Prepare flood hazard and risk maps using QGIS, to identify areas at risk of flooding, and consequently to improve flood risk management and disaster preparedness. In this case study, preliminary flood hazard maps of river basin scale were created in accordance with 7 different probabilities and the 6 climate change scenarios. Preliminary flood risk maps were created based on average annual damage for 6 climate change scenarios.

In this project, the following free software of Hydrological Engineering Center (HEC) under the US Army Corps of Engineers is used for flood risk analysis:

HEC-HMS

Hydrological Modeling System

HEC-RAS

River Analysis System

HEC-FIA

Flood Impact Analysis

ASEAN DRR-CCA

ASEAN Project on Disaster Risk Reduction by Integrating Climate Change Projection into Flood and Landslide Risk Assessment (ASEAN DRR-CCA) is supported by the Government of Japan through the Japan-ASEAN Integration Fund (JAIF).

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