EnviroInfo 2009 (Berlin) Environmental Informatics and Industrial Environmental Protection: Concepts, Methods and Tools Introduction into Work Package Urban Flooding of the BMBF Megacity Research Project TP. Ho Chi Minh Nguyen Xuan Thinh, Anne Bräuer, Verena Teucher Leibniz Institut of Ecological and Regional Development Weberplatz 1, 01217 Dresden ng.thinh@ioer.de a.braeuer@ioer.de v.teucher@ioer.de Abstract Urban flooding in Ho Chi Minh City (HCMC) has been risen as one of the most concerned topics since recent decades. Thus, this topic is an important issue in the BMBF Megacity Research Project TP. Ho Chi Minh, coordinating by Brandenburg University of Technology Cottbus. The project is divided into two Action Fields Urban Environment and Urban Development with their own respective thematic Work Packages. The Action Field Urban Environment encompasses five Work Packages: Adaptation Planning Framework, Urban Flooding, Urban Climate, Urban Energy, and Urban Transport (see BTU 2007 and Storch & Schmidt 2008). The Leibniz Institute of Ecological and Regional Development is co-leader of the work package Urban Flooding. The objectives of the work package Urban Flooding are (1) to determine conflict areas and vulnerability for different flooding situations within HCMC, (2) to compile flood hazard maps for HCMC, and (3) to develop strategies and measures for efficient flood risk management in HCMC. Hereby one has to distinguish between hazard and risk maps. The hazard map describes the spatial distribution of flood damages for previous events, whereas the risk map displays the possible damage with a given probability of occurrence. In this paper, we report about the status quo of our work package. The main conducted work consists of (i) literature review, (ii) data collection and development of a geo-database, and (iii) initial GIS-analysis and statistical analysis of geo-database and first results. Keywords: Ho Chi Minh City, BMBF Megacity Research, Urban Flooding, Hazard Mapping, Climate Change 1. Motivation climate change and resulting flooding in Ho Chi Minh City Vietnam is one of the countries, which already today is suffering from the effects of climate change. An increase of extreme weather events such as flooding in the area of Ho Chi Minh City can be observed (Eckert and Waibel 2009, page 18). Because of its topographical location more than 50% of the city are situated lower than 1.5m elevation above mean sea level Ho Chi Minh City is extremely endangered by inundation. The sea level rise which is going along with climatic change therefore is a major thread for Ho Chi Minh City. Added to the danger of sea level rise there are several other hazards for the town: heavy rains during rainy season, especially in combination with high tides from the sea already today cause Ho Chi Minh City to sink in water at different times. Several times in the year especially the central districts of Ho Chi Minh City such as Quận 1, 5, 6, 8, and others experience water depths between 0.20m to 0.60m on the streets for a couple of hours. Streets and houses are flooded (Fig. 1), the traffic system collapses thus prolonging traffic jams, people are trapped inside houses, being forced to wait until waters have withdrawn and a passage on the streets is possible again. Not only that these events cause great inconvenience to the population, economical and environmental damage is, needless to say, another aspect. Five hectares of crops in district Thủ Đức were destroyed in December 2008 (Thanh Nien News 2009), earlier at an apartment block a sewage water overflow into an underground water reservoir contaminated the water (Thanh Nien News 2008). 223
Figure 1: Traffic collapse during flooding in Ho Chi Minh City (Source: Left picture VnExpress (2009b), right picture VnExpress (2009a)) Causes for floodings are not only heavy rains and high tides, but also the construction of the city which is paying little consideration to this topic: urban development projects are carried through in flood-prone areas or areas which used to take up the water in previous flood periods (Eckert and Waibel 2009, page 19). Urban areas are more and more covered with concrete, leaving no water absorbable area and thus worsening the situation (Doan Canh 2007). Also, urban sewage systems are unable to keep up with the rapid development of the population (Doan Canh 2007). Urban planning needs to put its efforts at protecting urban areas from flooding (Eckert and Waibel 2009, page 18). Solutions as building dikes and dredging drains are very costly, not only because of the large network of canals in the city. Some measurements as installing sluice gates have already showed effects and reduced floodings of streets (Thanh Nien News 2009). An important measure is certainly to increase the population s awareness on climatic change. In order to take serious steps to combat the effects of climate change, it is necessary to disseminate information on the topic. People, e. g. potential house owners, need to know about the problems faced in Ho Chi Minh City such as flooding. In particular, maps indicating conflict areas need to be published [Nguyen Dung, Urban Planner Starter Forum]. As maps convey a more direct and stronger impression of the spatial distribution of hazards such as inundation, they are a strong means to increase awareness of threats in the minds of general public as well as of appropriate authorities, thus making providence for risks possible. 2. Hazard and risk mapping Current techniques and developments Hazard and risk maps are considered to be the medium when dealing with natural disasters. Not only do they show where natural disasters can occur, but also to which extent, which intensity, their frequency and the scale of possible damages. They are the basis for interventions by the authorities in the field of spatial planning, for the organization of rescue operations during an emergency and to raise awareness for the seriousness of the situation among the public. Especially in areas prone to natural disasters the methodical development of hazard mapping has been promoted. Single events with severe impacts added to that. Usually there are different stages during the mapping process, where one map is derived from the other, resulting in maps with a higher level of information and detail at every stage. But although it is widely agreed on the importance of hazard maps no standardized nomenclature or coordinated procedure concerning the map production has emerged yet (Merz, Gocht, Thieken 2005, page 51). With special respect to flooding, Merz, Gocht and Thieken (2005) suggest following definitions of the central concepts of hazard mapping. 224
Risk The risk definition has been derived from the definition used in the insurance industry and the assessment of environmental and technological risks. It is the probability of suffering a certain damage or loss. In terms of natural disasters it stands for the probability of the occurrence of a natural phenomenon with a certain extent causing certain damage. Risk analysis is the field of knowledge, which detects and evaluates the probability of negative consequences of a natural, technological or industrial process. Mathematically it can be expressed as the actuarial expectation of damage within a predefined time span, although it is questioned, if this describes the term risk properly, because the tendency of society to avoid events with severe impacts is not taken into account. Risk itself is comprised of two elements - hazard and vulnerability. Hazard Flood hazard is the percentile of a potentially damaging flood situation in a certain area within a fixed time span. The main indicators are the probability of the occurrence and the intensity of the event, which can be specified with a distribution function of flow rate and time intervals as well as inundation depth for the intensity. That is why the results of the distribution function are generally transformed into flood scenarios. More indicators (flow velocity, duration of the flood, rise rate, transport of sediments and pollutants) can be used, but their calculation is usually considered to be too elaborate. Vulnerability There are different notions of vulnerability. Its key elements are the elements at risk, which are all elements of the society and the built and natural environment that are endangered by flooding in a certain area. The population belongs to it as well as buildings, infrastructure, economic activities and ecosystems, which can suffer from negative consequences as fatalities, injuries, psychological stress, damage, business interruption, disruption of traffic or pollution. Vulnerability is defined by the exposition and the sensitivity of elements at risk. The exposition includes all the objects or persons, which are affected by the flood, whereas they can be quantified by their number or value. The sensitivity indicates how or to which extend someone or something can be damaged. The assessment of vulnerability is often constricted to showing the direct monetary damage in relation to the land use and inundation depth. While there are more influencing factors and much more different kinds of damage, this is the accepted method for assessing vulnerability. The analysis of each item results in a series of maps, which can be used for different purposes (see Table ). Hazard maps are useful for an effective communication with the public and the determination of priority areas in regional and urban land use planning. Vulnerability maps are to be used for the designation of reserve areas within settlements, the planning of constructions for flood protection or the organization of emergency management. Risk maps help to communicate the advantage of flood protection to the authorities and to quantify insurance rates. Map type Definition Flood Danger Map Shows the spatial distribution of flooding without information about the probability of occurrence Flood Hazard Map Shows the spatial distribution of the hazard, i.e. the information about the flood intensity and the probability of occurrence for single or multiple flood scenarios Flood Vulnerability Map Shows the spatial distribution of the flood vulnerability, i.e. the information about the exposition and/or the sensitivity of the elements exposed to the flooding (population, built and natural environment) Flood Risk Map Shows the spatial distribution of the risk during the flood, i.e. the expected damage for one or multiple events with a certain probability of occurrence Table 1: Suggestion for systematic flood mapping (Source: Merk, Gocht, Thieken 2005, page 61) 225
The downside of hazard mapping is the difficulty to validate those maps. There are uncertainties in the quality of the data, the algorithms and methodical approaches or the extrapolation of infrequent events. The theoretical and empirical base for the models is weak. Detailled risk maps are only of a fictitious accuracy, because statements refering to single objects cannot be made (Merk, Gocht, Thieken 2005, page 65-66). In 2003 a flood vulnerability analysis has been carried out for the city of Hue in Central Vietnam by members of the Geomatics Business Park in Marknesse, the Netherlands. The analysis was based on a concept called Extreme Events Engineering and Monitoring (EEEM), which is a combination of analytical tools and activities for flood prevention, flood protection and rescue. In between those two elements stands the so-called Integrated Vulnerability Analysis (IVA) as a central defence strategy (Hien et al. 2005, p. 67). The database for the vulnerability mapping has been provided by Earth Observation using stereo satellite imaginery, GIS data and local knowledge. To determine the flood extent radar data from of an already occurred flood was used. Together with a coarse land use map on the basis of Landsat 5 and 7 data, information on topography by the Shuttle Radar Topography Mission (SRTM) and a Digital Elevation Model (DEM) made out of ASTER data the flood vulnerabilitiy map has been produced (Hien et al. 2005, p. 81). 3. Methodology The work package Urban Flooding aims at identifying conflict areas and damage potential for different levels of flooding as well as at providing hazard and risk maps for the research area Ho Chi Minh City. Whereas hazard maps show the spatial distribution of actual flood events, risk maps visualise possible damages of potential flooding with a given occurrence probability. The basis of this work package and the generation of risk maps is the development of urban structure types for Ho Chi Minh City. Urban structure types will be identified in high resolution radar satellite data, such as ERS, TERRA-SAR-X, and ALOS. The derived urban structure type maps will serve as the basis of risk analyses. These maps will be used to support risk management in flood endangered areas. The use of high resolution radar data will allow a detailed analysis of flood events. The gathered information can be used to improve flood protection as well as for achieving a more efficient flood management. The work package Urban Flooding is split in two parts, Remote Sensing and Risk Mapping, which will be executed by the Free University Berlin, Germany and the Leibniz Institute of Ecological and Regional Development (IOER), Germany, respectively. All work steps will be realized in close cooperation with Vietnamese partners. The following steps will be undertaken in order to accomplish the set requirements (see also Figure 2): Searching and acquisition of Data The first necessity is the research of historical flood events in Ho Chi Minh City and the acquisition of available data and maps. If necessary, the data is to be pre-processed by scanning, mosaicing, georectification etc. Furthermore historic shorelines and extents of historic floods have to be digitized. Part of this step is also the geo-rectification of radar satellite data and the classification of flooded areas by means of remote sensing. If possible, the duration of flood events is to be extracted from radar data as well. Development of urban structure types This works step comprises the classification of the existing buildings of Ho Chi Minh City into urban structure types. Urban structure types may have to be modified according to the quality of the available aerial or remotely sensed pictures (radar, IKONOS-2, QUICKBIRD-2). For each urban structure type an analysis of representative areas will be implemented. By doing so specific values for each urban structure type will be obtained, such as the degree of ground-sealing, proportion 226
of green space, evaporation and runoff. These parameters will be used for a spatially differentiated estimation of possible damages of flood events (vulnerability). They will also be implemented in modelling energy systems (work package Urban Energy ), in modelling urban climate, and in the evaluation of the climatic impact of planning options (work package Urban Climate ). Geoprocessing and analysis By overlaying urban structure types and flooded areas the buildings and constructions of Ho Chi Minh City which are affected by floods can be identified. An overlay of flooded areas with a Digital Elevation Model (DEM) allows the extraction of flood height. Figure 2: Workflow and tasks of the work package A milestone in work package Urban Flooding is the development of an automatic process of subscribing satellite pictures. At the beginning of the project, aerial and satellite images are to be acquired and interpreted manually. In a later stage aerial pictures are to be ordered automatically depending on weather and water level of the receiving stream. By doing so a routinely survey and analysis of flood areas will be possible. Also, this method will be highly cost efficient. The extraction of hazard and risk maps is a major aim of this work package. For the generation of risk maps it is necessary to identify potential flood areas including the occurrence probability of flooding. Potential flood areas and occurrence probability are determined by hydrological and hydraulic modelling, which is chiefly in the responsibility of Vietnamese partners. Flood hazard is described by flood probability and flood reoccurrence period. Flood height and flow velocity are important indicators for possible damages. In work package Urban Flooding we aim at calculating these indicators for different flood events (HQ10, HQ20, HQ50, and HQ100) with approved hydraulic models. If adequate input data is available, potential flood areas will be identified with the software programme FloadArea from the company geomer. If it is not possible to obtain the data, the estimation of vulnerability at different flood situations will be describe only in theory. 227
4. Status quo of our work package Urban Flooding Regarding to literature review, it is important to conduct research and acquisition of documents detailing the large scale flood events that have occurred in HCMC. The next one is to make research documentation of completed investigations into preventive flooding protection in HCMC. We do this work in cooperation with our Vietnamese partners (Sub-Institute of Hydrometeorology & Environment of South Vietnam SIHYMETE, Mr. Bao Thanh, Dr. Truong Van Hieu, and others; Vietnam National University, University of Technology HCMC, Department of Water Resources Engineering, Mr. Ho Long Phi et al.). Examples for these are Doan Canh (2007), Truong Van Hieu (2007), and Ho Long Phi (2007). Data collection and development of a geo-database, specialized on inundation in HCMC, is of major importance. At first, the spatial distribution and proportion of different land use types provide the base context to identify micro climate and urban climate change. We have got digital land use data in MicroStation-format (DGN-file, scale 1:25,000, status 2005) from Department of Natural Resources and Environment in HCMC. The conversion of these data into ESRI-format was demanding and time-consuming for our project team. This work has been accomplished by our Vietnamese partner Le Thanh Hoa from University of Social Sciences and Humanities, Department of Geography. Secondly, we need for flood modeling a Digital Elevation Model (DEM) with appropriate accuracy for HCMC. University of Technology HCMC, Department of Water Resources Engineering has provided rough data for a DEM of HCMC. We have mosaiced and made geoprocessing for these data (see Figure 2). As a first step we have accomplished the research and acquisition of data. The following data of importance to work package Urban Flooding have been acquired from different sources: Height data of Ho Chi Minh City (TIN) Land use of Ho Chi Minh City Flood events in Ho Chi Minh City Flood sites in Ho Chi Minh City Flood heights of flood occurrences in Ho Chi Minh City A general overview over the data had to be obtained. Quality and completeness were checked, preprocessing and rectification were undertaken if necessary. The height data had to be mosaiced, some geometries had to be completed. First analyses have been performed on these data. In a first step an overview over the different height layers of Ho Chi Minh City was gained in order to find out the areas affected by different levels of flooding heights. These analyses were undertaken for assumed flood levels of 0.5m, 1.0m, 1.5m, 2.0m elevation above mean sea level as well as for sea level. According to Ho Long Phi, a Vietnamese flood expert, 1.5m is the crucial level which should be looked at carefully when analysing flooding. Different flood levels were intersected with the land use to determine what kind of areas are mainly affected by flooding. By doing so highly endangered areas can be determined in a later working stage and the costs of the flood damage can be estimated. Table 2 shows in detail the type of land use affected by different flood levels. Figure 3 visualizes affected land use for an assumed flood level of 1.5m. 228
Table 2: Overview of land use proportions affected by different flood levels 229
Figure 3: Land use in Ho Chi Minh City in areas below 1.5m elevation above mean sea level 230
Furthermore, analyses of flood sites, the amount of flood occurrences (see example in Figure 4), as well as the mean heights of flood occurrences have been performed. Next steps will direct towards exploring the spreading of flood at different height levels. Figure 4: Amount of flood occurrences caused by tide in Ho Chi Minh City (Extent city centre) Acknowledgements This research project is funded by the German Federal Ministry of Education and Research (BMBF). The authors thank for the support. We wish also to acknowledge the useful advice and input of Dr. Harry Storch (BTU Cottbus), Nguyen Thi Cam Van (DoNRE HCMC), Ho Long Phi (HCMC UT), Dr. Truong Van Hieu (SIHYMETE), and Le Thanh Hoa (USSH HCMC). References Brandenburg University of Technology Cottbus (BTU) (2007): Adaptation Planning Framework to Climate Change for Urban Environment and Development in Ho Chi Minh City. BMBF Research Proposal. Doan Canh (2007): Application of Ecological Engineering Approach for Establishing a Sustainable Urban Drainage System in Order to prevent Urban Flooding and Infrastructure Subsiding and to Minimize 231
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