Mitigating Flood Losses: An Introduction to Implementing a Basin-wide Approach Using Remote Sensing and Geographic Information Systems (GIS)

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1 Vol. 1 No. 1 January 2011 ISSN: pp International Peer Reviewed Journal Mitigating Flood Losses: An Introduction to Implementing a Basin-wide Approach Using Remote Sensing and Geographic Information Systems (GIS) ALEJANDRO TONGCO al.tongco@okstate.edu Oklahoma State University Date Submitted: Sept. 1, 2010 Plagiarism Detection: Passed Final Revision Complied: Oct. 6, 2010 Flesch Reading Ease: Gunning Fog Index: Abtract - The paper is an introduction to implementing a basin-wide approach using remote sensing and geographic information systems with a purpose of mitigating flood losses in Cagayan de Oro City. The paper discusses remotely sensed imagery and explores the strategy in building the GIS database for the Cagayan de Oro flood loss reduction project. Keywords - base wide approach, geographic information system INTRODUCTION Losses due to flooding take many forms. These can include loss of life, property, jobs, productive time, and opportunities. Not only flood victims are the ones affected, but concerned relatives and friends as well. Quantifying the physical damage is difficult enough, but the emotional hardships are impossible to measure. The city government, government agencies, NGOs, and other charitable groups may need to spend substantial man-hours, 103

2 millions of pesos in resources, and emotional support to mitigate the difficulties experienced by victims of flood disasters. Those in the lowerincome bracket who unfortunately are the most vulnerable may need even more help. Imagine if these charitable acts could be translated into monetary terms. It could be put to use in many other ways. But losses and hardships can be mitigated, if thorough planning and preparation are done before flood events occur. Some sectors may already be involved in advocacy and awareness, project planning and implementation, as well as disaster response. Regardless, it is a must to study the flooding phenomenon on a holistic scope that is unique to Cagayan de Oro. Flooding mitigating efforts such as levee construction, drainage, and river dredging may help, but they do not address the root cause of the flooding problem. If the causes of flooding are known, and if potential floods can be assessed before they occur, mitigating efforts can be intelligently designed. Knowing about the problem can guide planners to synthesize mitigating strategies to apply before, during, and after the flooding. Nevertheless, one must address the challenges about flooding in CDO on a basin-wide scale. Topics such as general components of flooding, simulating flooding scenarios using new technologies and available satellite imagery, land-use planning and implementation, determining strategic points for early-warning systems, flood plains delineation, formulating guidelines for smooth coordination of various aid sectors during disasters, and assessing the socioeconomic implications of flooding, are just some of the areas that need to be studied in-depth. The Basin-wide Approach The basin-wide approach of analyzing the flooding phenomenon entails examining all the factors that can potentially contribute to the rise of water beyond the normal level within a basin or watershed, and examining their effects on the basin s population and sustainability. Thus, the whole flooding cycle needs to be comprehensively examined (Guidelines for Reducing Flood Losses 2006). Since flooding in the low-lying areas of the city occurs during prolonged and heavy rains within the basin or watersheds, one needs to look at the flooding cycle on a basin-wide scale. A basin or watershed is a low-lying region or area where river and its tributaries drain into (Figure 1). 104

3 Mitigating Flood Losses: An Introduction to Implementing a Basin-wide Approach Using Remote Sensing and Geographic Information Systems (GIS) A. Tongco Figure 1: A basin and its watersheds (also known as sub-basins) and stream network. Source: To tackle the problem basin-wide, a delineation of the basin and its watersheds must first be established. Remotely Sensed Imagery One way to determine the delineation of the basin and its watersheds is by the use of a satellite image called digital elevation model or DEM (Figure 2) and a capable GIS or image processing software. Every cell or pixel in a DEM contains an elevation value. The higher the resolution or the smaller the pixel size of the DEM, the greater is the accuracy of the measurements that can be derived. Besides delineating a basin, a DEM can be used to quantify low-lying areas, slopes, stream networks, and water volume flow (together with factors such as rainfall values and land cover) of a small or entire area. 105

4 Figure 2. Digital elevation model (DEM) of the CDO basin area clipped from 30-m spatial resolution elevation mosaic of ASTER GDEM (Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Digital Elevation Model). Source: philgis.org. DEMs (Figure 2) can be processed to produce an output as in Figure 3 using basin and watershed delineation procedures of BASINS (Better Assessment Science Integrating point & Non-point Sources) software program of the U.S. Environmental Protection Agency (USEPA) ( datait/models/basins/index.cfm) and ArcGIS 10. The boundary (yellow) of Cagayan de Oro City was copied from Global Administrative Areas (

5 Mitigating Flood Losses: An Introduction to Implementing a Basin-wide Approach Using Remote Sensing and Geographic Information Systems (GIS) A. Tongco Bohol Sea Figure 3. The Cagayan de Oro Basin System and various basins that comprise it. Figure 3 shows that several independent basins are contained in the Cagayan de Oro boundary area. The CDO river and its tributaries are in one distinct basin which is composed of several watersheds. The CDO river basin (dark red) is independent of the basins adjacent to it. The latter basins have their own stream network not connected with the CDO river basin network. The purpose of including the adjacent basins in the CDO basin system as shown in the illustration is to show that all these individual basins can potentially contribute to the flooding of metropolitan CDO. The enormity of CDO river basin and its narrow elongated outlet suggest a high likelihood that flooding can happen downstream. The stream network gives a clue where to install early warning sensors. Higher-resolution DEMs and photography, however, are needed to determine the most ideal location after calculating the estimated amount of rainfall that may fall upstream of the sensor. On the other hand, the role of factories and agricultural and animal 107

6 farms in various locations of the watersheds during heavy rainfall needs to be studied to devise efforts to mitigate poisoning the landscape and the population downstream especially during flood events. The flooding magnitude in the basin can further be examined by factors such as land cover and utilization which influence flood volume flow. Land cover and utilization of the entire basin can be determined from multispectral imagery, as in Figure 4. Like DEMs, high-resolution imagery gives more accurate results. Recency of the imagery is likewise important. Figure 4. A multispectral image of the CDO basin system. (Source: Landsat 7 ETM+ [Enhanced Thematic Mapper Plus] 15-m spatial resolution panchromatic band, with 3 spectral bands, taken in nasa.gov/mrsid/mrsid.pl; In Figure 4, the urban areas and bare earth are shown in magenta. Areas in various shades of green are vegetative cover. Clouds are shown as cottonlike white areas. The blue area at the top of the image is Bohol Sea. The blue squiggly lines are streams and rivers. Thus, to assess the possible extent of city flooding before it occurs, one needs to know some basic information such as: where the flood waters may come from within the basins; how much rain may fall at a span of time, what coverage, and where; how fast water rushes to the city at what volume; the size and slope of the watershed; the land cover and land utilization of a specific area of the basin; and flow measurements from gauging sensors at 108

7 Mitigating Flood Losses: An Introduction to Implementing a Basin-wide Approach Using Remote Sensing and Geographic Information Systems (GIS) A. Tongco various converging points of the tributaries. Soil type also plays a part in the calculation of surface runoff and resulting flood flow. All these require investments in hardware, software, data, and manpower and are necessary to ultimately lessen the devastating and expensive outcome of floods. This is where the application of a geographic information system (GIS) can play a highly significant role throughout the project cycle and beyond. GIS A GIS is basically an information database consisting of geographically referenced data layers, wherein data in each layer is linked to a graphic (Figure 5). Because of its visual feature, GIS is effective in presenting information that is easy to understand by planners, designers, coordinators, managers, and other stakeholders in flood disaster mitigation projects. A basin-wide GIS can well illustrate the comprehensive nature of the flooding problem and is therefore a valuable tool in explaining pre-disaster scenarios. A GIS allows faster and easier exchange of data to all who may wish to do flood disaster projects. Figure 5. Layers of data simulating a real world. (Source: 109

8 In a GIS, each dataset or layer is tied to a uniform geographic coordinate system. The idea is for all layers to stack on top of each other seamlessly. A combination of same-referenced layers of the basin may be used as desired. A popular example of a layer is a political boundary, such as national, regional, provincial, municipal, barangay, or purok, and densely populated areas. These are called areas or polygons. Another example is river, stream, or roads. These are normally represented as lines at some elevation. A layer can also be a point. An example is the location of early-warning sensors or water depth gages. Raster images such as DEMs, Landsat satellite images, and aerial photos can also be used as layers. By overlaying several layers, one can simulate real-world situations. This is one major usefulness of GIS to simulate and analyze events before they actually happen, avoiding investments which otherwise could be very costly. Building the GIS Database for the CDO Flood Loss Reduction Project A basin-wide GIS database is necessary to support and supply the geospatial data needs of individual, comprehensive, or integrated projects that deal with flood loss mitigation in Cagayan de Oro City. Planners and investigators of flood-related projects need geospatial data to design, implement, manage their projects, as well as to assess their projects performance. These data include the following: administrative boundaries, population density and distribution, basin and watershed delineation (as shown above) and characterization, river network, flood plain delineation, flood-control structures, desired locations of early-warning systems, location and time of rainfall forecasts, and city drainage system. Furthermore, data should include planned evacuation and temporary shelter areas, evacuation routes, capacity of transport vehicles, and locations of hospitals and clinics. As in any information system, GIS requires data as input to produce information. GIS biggest challenge, therefore, is in database construction. Data needs to be collected, verified, processed, standardized, and managed to conform to the needs of project investigators. For a basin-based project, characterizing the watershed or basin could be time-consuming. This entails delineating and assessing the watershed and its resources, including vegetative cover, extent of developed areas, soil type, slope, runoff, waterholding capacity, and geology of the area. Evaluating the basin s land cover and land-use may have to be done regularly every year or two to reflect their timeliness and recalculate flood flow. 110

9 Mitigating Flood Losses: An Introduction to Implementing a Basin-wide Approach Using Remote Sensing and Geographic Information Systems (GIS) A. Tongco Using remotely-sensed multi-spectral (multi-band) satellite image data and through image analysis and classification, the concentration, distribution, and types of vegetative cover can be assessed. For output that requires greater detail and recency, one needs to acquire higher-resolution commercially available imagery. Elevation points and contour lines can also be derived from DEMs. A new technology called LiDAR (Light Detection and Ranging) can handle both vegetation canopy classification and elevation variation measurements. Aerial photography is also valuable to assess a landscape. It does not, however, show variation in elevation. But it is quite useful as backdrop for creating vector files and for verification and display. Aerial photographs of landscapes are also used for draping over DEMs to show a realistic 3-D view. Google Earth and ArcGIS Explorer, both free viewers, have produced detailed aerial photography of selected areas of the country, including parts of CDO (Figure 6) that are also viewable in 3-D. Figure 6. A satellite photo of a portion of CDO taken in (Source: Google Earth). Processing these various imageries requires specialized GIS and/or image processing software and skilled manpower. Free software are available but offer limited capabilities. The commercial ones have extensive capabilities but normally at steep price. 111

10 A GIS for the CDO basin system equipped with a comprehensive library of basin-wide geospatial data is definitely a worthwhile investment for CDO. GIS demands centralized data standardization and management. It is a valuable component in any mitigation project or in almost any other basin-based project. Having a central GIS database, delivery of data is easier and faster, data revisions are less expensive, and data duplication efforts are avoided. All these advantages hasten intelligent centralized and uniform planning, implementation, and assessment of disaster mitigation projects that could potentially reduce loss of life and property and lessen the disruption of people s lives. Flooding is a natural occurrence; it will come again. It could come next year or the year after next or sooner. It is not a matter of if but when the next will occur. Note: Pursuant to the international character of this publication, the journal is indexed by the following agencies: (1)Public Knowledge Project, a consortium of Simon Fraser University Library, the School of Education of Stanford University, and the British Columbia University, Canada:(2) E - International Scientific Research Journal Consortium; (3) Journal Seek - Genamics, Hamilton, New Zealand; (4) Google Scholar; (5) Philippine Electronic Journals (PEJ);and,(6) PhilJol by INASP. Acronyms ASTER GDEM Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Digital Elevation Model BASINS Better Assessment Science Integrating point and Non-point Sources DEM Digital Elevation Model CDO Cagayan de Oro ESRI Environmental Systems Research Institute GADM Global Administrative Areas GIS Geographic Information Systems Landsat 7 ETM+ Enhanced Thematic Mapper Plus LiDAR Light Detection and Ranging NGO Non-government Organization USEPA U.S. Environmental Protection Agency 112

11 Mitigating Flood Losses: An Introduction to Implementing a Basin-wide Approach Using Remote Sensing and Geographic Information Systems (GIS) A. Tongco LITERATURE CITED Guidelines for Reducing Flood Losses. United Nations International Strategy for Disaster Reduction isdr-publication/flood-guidelines/isdr-publication-floods.htm. About the author: Alejandro Tongco has more than six years of experience in GIS. He holds a doctoral degree in engineering and currently works as a research specialist focusing on GIS at Oklahoma State University, U.S.A. He is the founder and project director of the Philippine GIS Data Clearinghouse ( philgis.org), a non-profit portal for free distribution and sharing of Philippine geospatial datasets. Dr Tongco is helping Liceo de Cagayan University build its GIS capability. He can be contacted at al.tongco@okstate.edu. 113