Assessing the Impacts of Flooding Caused by Extreme Rainfall Events Through a Combined Geospatial and Numerical Modeling Approach
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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLI-B8, 2016 XXIII ISPRS Congress, 12–19 July 2016, Prague, Czech Republic ASSESSING THE IMPACTS OF FLOODING CAUSED BY EXTREME RAINFALL EVENTS THROUGH A COMBINED GEOSPATIAL AND NUMERICAL MODELING APPROACH J. R. Santillana∗, A. M. Amoraa, M. Makinano-Santillana,b, J. T. Marquesoa, L. C. Cutamoraa, J. L. Servianoa, R. M. Makinanoa a CSU Phil-LiDAR 1 Project, Caraga Center for Geo-Informatics, College of Engineering and Information Technology, Caraga State University, Ampayon, Butuan City, Agusan del Norte, Philippines - [email protected] b Division of Geodetic Engineering, College of Engineering and Information Technology,Caraga State University, Ampayon, Butuan City, Agusan del Norte, Philippines - [email protected] KEY WORDS: Extreme rainfall, Flooding, Impact assessment, 2D Flood modeling, LiDAR, Landsat ABSTRACT: In this paper, we present a combined geospatial and two dimensional (2D) flood modeling approach to assess the impacts of flooding due to extreme rainfall events. We developed and implemented this approach to the Tago River Basin in the province of Surigao del Sur in Mindanao, Philippines, an area which suffered great damage due to flooding caused by Tropical Storms Lingling and Jangmi in the year 2014. The geospatial component of the approach involves extraction of several layers of information such as detailed topography/terrain, man-made features (buildings, roads, bridges) from 1-m spatial resolution LiDAR Digital Surface and Terrain Models (DTM/DSMs), and recent land-cover from Landsat 7 ETM+ and Landsat 8 OLI images. We then used these layers as inputs in developing a Hydrologic Engineering Center Hydrologic Modeling System (HEC HMS)-based hydrologic model, and a hydraulic model based on the 2D module of the latest version of HEC River Analysis System (RAS) to dynamically simulate and map the depth and extent of flooding due to extreme rainfall events. The extreme rainfall events used in the simulation represent 6 hypothetical rainfall events with return periods of 2, 5, 10, 25, 50, and 100 years. For each event, maximum flood depth maps were generated from the simulations, and these maps were further transformed into hazard maps by categorizing the flood depth into low, medium and high hazard levels. Using both the flood hazard maps and the layers of information extracted from remotely-sensed datasets in spatial overlay analysis, we were then able to estimate and assess the impacts of these flooding events to buildings, roads, bridges and landcover. Results of the assessments revealed increase in number of buildings, roads and bridges; and increase in areas of land-cover exposed to various flood hazards as rainfall events become more extreme. The wealth of information generated from the flood impact assessment using the approach can be very useful to the local government units and the concerned communities within Tago River Basin as an aid in determining in an advance manner all those infrastructures (buildings, roads and bridges) and land-cover that can be affected by different extreme rainfall event flood scenarios. 1. INTRODUCTION as well as in evaluating adaptation strategies if such kind of flood- ing will actually occur in the near future (Few, 2003). 1.1 Background 1.2 Flood Risk Assessment Flooding is one of the most destructive natural disasters in the Assessing the risk of present and future flooding, especially those Philippines. Excessive quantity of rainfall brought by tropical brought by extreme rainfall events, is a very challenging task, storms is the most common cause of flooding, just like what hap- especially that the components of flood risk, such as the expo- pened in various provinces in Mindanao Island when Tropical sures and hazards are subject to fast changes in time due to eco- Storms Lingling (Local name: Agaton) and Jangmi (Local name: nomic development and the possible effect of the changing cli- Seniang) caused rivers and lakes to overflow in 2014 (NDRRMC, mate (Alfieri et al., 2015). In many studies (e.g. Alfieri et al., 2014; 2015). In the advent of climate change which has caused 2015; Gilbuena et al., 2013; Ortiz et al., 2016), deriving potential tropical storms and the rains that it brings along becoming fiercer damages caused by flooding includes combining and intersecting and extreme, the need to become more prepared for flood dis- flood hazard maps with the exposed population and land cover asters has also become more urgent (Few, 2003; Vidal and Car- features. These methods are done with the use of Remote Sens- rington, 2013). It is in this need that simulating and assessing ing (RS) and Geographic Information System (GIS) techniques, the impacts of various flood scenarios is important not only for which has significantly improved the efficiency of flood disas- the purpose of establishing a baseline information where disas- ter monitoring and management (Haq et al., 2012; Van Westen, ter managers can make a reference to when doing pre- and post- 2013). Utilizing these technologies, a quicker and precise moni- disaster management and recovery efforts, but also for getting a toring and mapping of flooding can be done by using satellite im- detailed picture of how and why such kind of flooding can oc- ages and state-of-the-art modelling or simulation software (Hal- cur or have occurred. These assessments are vital in figuring out dar and Khosa, 2015; Haq et al., 2012). strategies that can minimize, or even avoid, the impacts should similar events occur in the future. Flood impact assessment can In the last few decades, numerical modeling has been signifi- also help communities and local government units to be empow- cantly enhanced and utilized in flood mapping due to the exis- ered in finding ways to mitigate the negative impacts of flooding, tence of reliable numerical methods and innovative topographic survey techniques such as those provided by Light Detection ∗Corresponding author and Ranging or LiDAR technology (Costabile and Macchione, This contribution has been peer-reviewed. doi:10.5194/isprsarchives-XLI-B8-1271-2016 1271 The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLI-B8, 2016 XXIII ISPRS Congress, 12–19 July 2016, Prague, Czech Republic 2015). Typically, flood mapping through numerical simulations 2. METHODS AND MATERIALS comprises two components known as the hydrological simulation which quantifies the size, duration and probability of the flood 2.1 The Combined Geospatial and 2D-Numerical Modeling event; and the hydraulic simulation which employs the mapping Approach of inundated areas (Dimitriadis, et al., 2016). Flood mapping can be done in either 1-dimensional (1D) or 2-dimensional (2D) ap- Figure 2 summarizes the combined geospatial and numerical proach, but despite the efficiency if 1D flood modeling approach, modeling approach as implemented in this study for flood impact it has been progressively encouraged recently the use of 2D flood assessment. simulation, since it can give detailed description of the hydraulic behaviour of the river’s flow dynamics (Costabile and Macchione, The geospatial component of the approach involves extraction 2015). of exposure datasets from remotely-sensed high spatial resolu- tion elevation models and images. These exposure datasets are 1.3 The case of Tago River Basin in Mindanao, Philippines the man-made features (buildings, roads, bridges) and land-cover which are extracted through manual digitization and image clas- Tago River Basin (Figure 1) is located in the province of Surigao sification, respectively. The numerical modeling consisted of de- del Sur in the island of Mindanao, Philippines. It is geograph- veloping the hydrologic model of the river basin which is then ically facing the Pacific region wherein typhoons usually come used to determine the volume and discharge of water entering from. The basin’s large catchment area of approximately 1,444 the floodplains; and the 2D hydraulic model which simulates the km2 and a very wide floodplain traversed by a dense network of flow of water entering on the rivers and on the floodplains as rivers and streams make it vulnerable to flooding. Flooding in the rain pours to the area. The development of the hydrologic and river basin due to overflowing of Tago River and its tributaries oc- hydraulic models utilizes high spatial resolution elevation mod- curs almost every year, especially during the rainy seasons from els and were parameterized using the information extracted from the month of December to February. In 2014, the basin was one the land-cover map. The models simulate flooding events by in- of the many areas greatly affected by flooding due to heavy to tor- putting rainfall data which can either be actual or hypothetical rential rains brought about by the passing of tropical storms Aga- rainfall events. ton and Seniang (NDRRMC, 2014; 2015). These flood events affected not only the communities living in the flood plain but 2D numerical modeling is very advantageous compared to 1D also the sources of income which include large cropland areas. modeling especially when dealing with a complicated river sys- tem that has multiple streams with complex flows. 2D modeling A preliminary assessment of the impacts of Agaton and Seni- also removes subjective techniques usually employed in develop- ang in the Tago River Basin has recently been conducted by ing a 1D model, such as cross-section orientation. Makinano-Santillan (2015). Using an integrated approach involv- ing the use of LiDAR datasets, land-cover from Landsat images, The 2D hydraulic model uses flow information computed by the and one-dimensional (1D) flood models based on HEC HMS and hydrologic model to generate flood maps corresponding to hypo- HEC RAS, the study was able to estimate 52.57% and 22.29% thetical rainfall events with return periods of 2, 5, 10, 25, 50 and of the buildings situated in the floodplains to have been flooded 100 years.