Flood Risk Mapping and Damage Analysis Using GIS

ABSTRACT

Natural and manmade hazards and their destructive nature can seriously restrict economic development due to the loss of life and the destruction of property. It is the premise of this study that Geographic Information System (GIS) can be a useful tool for hazard identification and analysis for implementing mitigation strategies. Flooding is a natural phenomenon that occurs in Bangladesh almost every year with varied intensity. In this study GIS was used as a tool for flood hazard analysis, taking Nabinagar Upazila of Brhamanbaria District as a case study because of its geo-physical characteristics.

The leading objectives of this study were to determine the Highest Flood Level (HFL) for a determined return period (25 years) and to analyze both the physical and economic damage caused by the determined flood level.  Using a deterministic approach of flood level prediction, highest flood level was determined by analyzing the previous 40 years’ highest water level data of Nabinagar Water Station (BWDB-298). This analysis revealed that when considering 25 years as a return period the highest level (HFL) for 2029 would be 6.36 m+PWD.   A Digital Elevation Model (DEM) and contour map of the study area were also generated through   interpolation of the 3987 no. of spot heights for GIS analysis.  GIS analysis showed that, almost 85 percent of the total area becomes flooded by different flood depth. In addition, 52.22 percent of total population, 52.11 percent of total households, 86.69 percent of total agricultural area and 78.76 percent of total road would be also affected by floods of different depths; shallow, medium and high.

In order to fulfill the other research objectives, an economic damage analysis was also done which covered the direct damages of different sectors such as household, agriculture, road and other infrastructures. A total of 58.56 billion taka was found as the total damage cost, including residential sector damage of 10.46 billion taka, Infrastructure sector damage of 10.71 billion taka and agriculture sector damage of 37.38 billion taka. There is no doubt that this can be considered as a huge economic loss for a country like Bangladesh.

Using GIS potentiality, productive measures can be taken which can respond effectively pre, during and post disaster period. Taking flood as a common phenomenon for Bangladesh this type of non-structural measurement should be emphasized for better response.  

CHAPTER ONE

INTRODUCTION

1.1          Background

Flood is more or less a recurring phenomenon in Bangladesh, and usually occurs within tolerable limits, although occasionally it can be devastating. In 1997, 1988, 1998, 2000 and 2004 Bangladesh faced unprecedented floods, causing massive loss of life and property. Bangladesh is a land of many rivers, and heavy monsoon rains. Therefore, the country is subject to inundation by overflow from the riverbanks due to drainage congestion, rainfall run-off, and storm-tidal surges. In the years 1988 and 1998, two devastating floods inundated more than 65 per cent of the geographical area of the country. In the year 2000, Bangladesh faced an unusual flood over its normally flood-free southwestern plain, which also caused loss of life and massive damage to property. In the year 2004, 39 districts of Bangladesh were affected by flood, which resulted in an economic loss of $ 200 (CPD, 2004).

Recently Geographic Information System (GIS) has been widely used over the world to a great extent for natural disaster planning, mapping and management using its optimum ability of data capturing, retrieving and processing power. GIS has proved it can be a very efficient tool for disaster planning, management and mitigation for decision makers or planners using its spatial decision support. The most basic and interesting geographical data type is the digital elevation model (DEM). A Digital Elevation Model is a binary file that contains only spatial elevation data in a regular girded pattern in raster format. Digital elevation model (DEM) is a digital file consisting of terrain elevations for ground positions at regularly or irregularly spaced horizontal intervals, and is a way by which the variation of surface elevation over an area can be modeled. In GIS DEMs are modeled by regular/irregular grids, Triangulated Irregular Network (TIN) and contour lines. DEM can be used for serving different purposes such as storage of elevation data for digital topographic map, creation of digital and analogue orthophoto maps, three dimensional (3D) display of landforms, for planning routes, locations of dams, for statistical analysis and comparison of different kinds of terrains, as a background for displaying thematic information and spatial decision support for disaster and hazard management such as flood, earthquake, etc (Jones, 1997). 

Flood is such a disaster where GIS can easily contribute to effective management, mitigation, forecasting and planning. Geographic information systems provide a broad range of tools for determining areas affected by floods or for forecasting areas likely to be flooded due to high river or sea levels. For an effective management of flood water in low lying flood-prone areas, GIS technology is proving to be a useful and efficient instrument (Wagner, 1989, Wu, Bingfang et al., 1990 and Rahman, 1992). Spatial data stored in the digital database of the GIS, such as a digital elevation model (DEM), can be used to predict the future flood events. The GIS database may also contain agriculture, socio-economic, communication, population and infrastructural data. This can be used, in conjunction with the flooding data to adopt an evacuation strategy, rehabilitation planning and damage assessment in case of a critical flood situation.

This research is an empirical study where probable flood level was determined for 25 years as a return period or for the year 2025 using year 2000 as a base year. Direct physical and economic damage was also calculated along with preparation of a flood risk map of different land uses using determined flood level considering GIS as an effective tool.

1.2          Objectives

1. To determine the highest flood level (HFL) for a determined return period by analyzing previous records.

2. To create a Digital Elevation Model (DEM) and inundation map showing flood depth of the study area.

3. To calculate the area that might be flooded by determined gauge level.

4. To prepare the flood risk map for different land uses.

5. To analyze direct damages (both physical and economical) due to inundation.

1.3          Data and Data Sources

Data Types	Sources
Study Area Coverage (Base Map)	Survey of Bangladesh (SoB) Topographic Map and LGED Base Map
Spot height of the study area	Institute of Water Modelling (IWM)
Demographic Features	Bangladesh Bureau of Statistics (1991)
Land use and other infrastructural data	LGED Base Map (1993)
Hydrological Data	Institute of Water Modelling (IWM)
Previous Flood Record	Institute of Water Modelling (IWM)
Relevant Information	Journals, Publications, Internet

1.4          Rationale of the Study

Flood is more or less a recurring phenomenon in Bangladesh, and often within tolerable limits occasionally, it becomes devastating. Moreover 25 to 40 percent of the Bangladesh floodplains are inundated annually at various depths and durations during the monsoon months of June through September. This is due to the flat topography and overbank flow from three major rivers that converge in the delta plains; the Ganges, Brahmaputra and Meghna. In 1988 and 1998 about 61 percent and 68 percent of the total area of the country was inundated respectively, by severe floods (EGIS, 1999). The recurrent devastating floods cause substantial losses in agricultural productivity, infrastructure, life and property. Economic growth of the country significantly depends on efficient structural and non-structural measurement against flood. As a poor country Bangladesh cannot afford huge amounts of money for structural measurement.  That is why it is more rational to place emphasis on non-structural measurement such as development GIS based flood risk mapping, damage mapping and estimation procedure etc. DEM as a digital representation surface can be a very efficient tool for digital flood risk mapping and damage estimation, which provides greater accuracy than manual risk mapping or damage calculation. Ultimately this GIS based procedure may help policy makers to make effective response against possible flood in future.           

1.5          Review of Literature

±    Ahmed, Taufiq. (1998), in his research paper Application of GIS for Flood Impact Assessment showed that  GIS would make the authority more efficient to cope with the floods. From his research, it is identified that assessment of flood risk with the help of GIS tool is possible and  can help in  reducing vulnerability to flood, can make the assessment process easier  saves time and gives an optimum result.

±    Burrough, P. A. and McDonnell, R. A. (1998), in their book Principles of Geographic Information System tried to provide an introduction to the theoretical and technical principles that needed to be understood to work effectively and critically with GIS. This book also provides a detailed concept of working with continuous surface or DEM and its application.

±    EGIS, (2000), in their publication Geo-spatial Tools for Analysis of Floodplain Management describes two spatial analytical techniques for computation of flood depths and mapping. Among those techniques DTM and DEM is incorporated. This book stated that those techniques can be applied to flood depth measurement and mapping for local level.

±    Government of Bangladesh (GoB), (1994), in their publication Flood Management Model under Flood Action Plan 25 (FAP 25) describes a methodology aimed at generating flood depth and flood impact maps. The result of FAP 25 was a modelling system for flood management, which integrated flood models, GIS and when combined with agriculture, fisheries, society, infrastructure and other data, opened up new avenue for multi-sectoral flood management practices.   

±    Hasan, Mehedi. (1996), in his research paper Assessment of crop damage due to flood using GIS technology tried to assess the crops damage vulnerable for flood using GIS techniques. Here GIS is also used for estimation of damage of crops from economic point of view as well.

±    J. A. M. de Brouder. (1994), in his paper  Flood study at Megna-Dhanghada Polder, Bangladesh tried to describe cooperative GIS and RS approach to manage flood prone areas. In this study some flood maps of 1988 flood were prepared with the help of RS image and DEM. This paper also suggested a methodology for preparing flood maps which can escape several complexities.

±    Jones, Christopher B. (1997), in his book Geographical Information Systems and Computer Cartography, tried to define the term Spatial Decision Making as Making Decisions on the phenomena directly related with space or locations integrating all geo-demographic information from a variety of sources in a spatial context.

±    Kiyingi, K. G. (1998), in his MSc thesis titled Integration of Digital Elevation Models and GIS: A Potential for Planning and Design of Sewer Systems in Dar es Salam, has described the characteristics, structure, methods of DEM creation with advantages and disadvantages of the methods and accuracy of DEM. In this thesis a successful application of DEM to delineate sewerage systems for Dar es Salam, Tanzania have shown.

±    Lek, Prapasajchavet. (1992), in his article Flood Vulnerability Assessment and Landuse Planning tried to describe flood risk analysis procedure which included flood risk mapping and damage analysis through basic survey and hydrological and hydraulic analyses with the help of information system.

±    Rana, Kaisar. (2000), in his research paper Development of Digital Elevation Model and its Application in Physical Planning Decision, tried to obtain an insight of relevancy of Digital Elevation Models and a demonstration of the capability of GIS in spatial planning with specific reference to drainage and sewerage design, layout of road alignment and so on to take the advantage of natures of a selected area of Khulna City. GIS application used by him was to look at terrain characteristics, types of infrastructure and other aspects of spatial planning in a more integrated manner.

±    Saini, N.S. (1987) in his edited book Urban and Regional Planning Information System tried to invent the scope of utilizing information system in Urban and Regional Planning including disaster management.

±    Steven, P. French (1991), in his paper Utilization of GIS for Landuse Planning described that risk as an interaction between some causative mechanism and something of value. Thus risk analysis is procedure which can combine these two procedures. In his paper three component such as, Information of hazard, inventory of property at risk, damage from hazard are highlighted boldly. This paper also identified three different scales of risk analysis using GIS.

Above literature reveals that, GIS can be an efficient and effective tool for flood forecasting, modelling and management. But most of the literature is based on theoretical knowledge rather than realistic application. The aim of this research is to use GIS as a capable tool for flood risk mapping, damage analysis and calculation using a case study in Bangladesh where other literature does not prove the GIS efficiency by taking such a case study. 

1.6          Limitation of the Study

Research is a never-ending process that demands adequate labor, time, data and money etc. The major limitation of this research was time; such a study requires greater inputs which the time limit on the study made difficult to achieve.  A GIS based study also requires a lot more data, but in context of Bangladesh it is difficult to get necessary data for analysis in the proper format. So it is very much complicated process to convert the data into GIS format and time consuming as well. Again for accurate damage assessment necessary socio-economic data and indicators were not available and as a result the assessment was made only for direct damages keeping indirect and intangible damages outside the study extent.