The Phil-LiDAR 1.B.14 project (also called “CSU Phil-LiDAR 1”), is one of several projects under the “Phil-LiDAR-1. Hazard Mapping of the Philippines using LiDAR” program funded by the Department of Science and Technology (DOST). Projects under this program are being implemented nationwide by several state universities and colleges (SUCs) and higher education institutions (HEIs) for 3 years starting 2014.
An extension of the UP DREAM program, CSU Phil-LIDAR 1′s purpose is for SUCs/HEIs to undertake researches on the utilization of LiDAR technology to generate flood hazard maps of their assigned project areas.
CSU Phil-LiDAR 1 in particular will generate flood hazard maps of the river basins and watershed of Caraga Region (except the Agusan River Basin which has been covered by the UP DREAM program).
Taking advantage of LiDAR technology, the project aims to generate flood hazard maps that can be used to determine at the household level those houses and structures that are at risk of an impending flood disaster.
Generally, a hazard map for a specific flooding event or scenario would contain:
- the expected inundation level and flooding extent categorized into low, medium and high hazards;
- locations of structures (such as households, schools, commercial buildings, roads, bridges, land cover types, and evacuation centers, among others);
- political boundaries (provincial, city/municipal, and barangay); and
- information about the flooding event where the map is applicable (e.g., according to volume and intensity of rainfall).
Depending on how much rain has fallen in the past number of hours, flood hazard maps can tell us if our house, the houses of our relatives, a nearby road, our rice fields, etc. will be flooded in the next number of hours. Because of the advance information that flood hazard maps provide, there is an adequate time for us to prepare ourselves, secure our properties, and to evacuate, if necessary. It also helps local government agencies for localized emergency response (i.e. evacuation and access routes, road closures, selection of sites for key rescue facilities).
Areas Covered by the Project
For three years starting 2014, the project will cover twelve (12) river basins within Caraga Region , with a total drainage area of approximately 4,750 square kilometers. These river basins have been prioritized according to the severity of recent flood disasters (for example during Agaton and Seniang). For each river basin, the CSU Phil-LIDAR 1 project will generate: i.) LiDAR data products such as DTMs and DSMs; ii.) hazard exposure database derived from LiDAR data products; iii.) flood models; and iv.) flood hazard maps.
Table 1. List of river basins of Caraga Region covered by the CSU Phil-LiDAR 1 project.
|Year 1 (2014)||Year 2 (2015)||Year 3 (2015|
|Bislig||Malinao Inlet 1|
|GaasLulet (Malinao Inlet 2)|
FLOOD HAZARD MAPPING OVERVIEW
The series of steps to generate flood hazard maps are illustrated in Figure 2. First, information about the volume and intensity of rainfall is obtained. This information is then fed into a mathematical simulation model (or flood model) to compute how much volume of runoff or flood water is generated in the mountains when it rains, and to determine how this flood water flows downwards into the rivers and overflows into the floodplains. In order for the flood model to work, it would require the following information: 1) topography; 2) geometry of the river; 3) location of man-made structures; and 4) the types of land-cover. This information will be used by the model to compute the volume of flood water, determine the speed and direction of the flow, and finally determine the depth and extent of inundation. From the computed depth and inundation extent, a flood hazard map is then prepared. For it to be useful, flood hazard maps must be detailed in such way that it is possible to locate the areas at risk of flooding at the household level. It must also be accurate to ensure that the flood inundation and extent depicted in the map is as near as possible to what is occurring or what will occur in the ground.
LIDAR DATA AS INPUT IN FLOOD HAZARD MAPPING
The accuracy and detail of information depicted in a flood hazard map is dependent on the topographic data used during flood modelling. In the generation of flood hazard maps, the CSU Phil-Lidar 1 project utilizes topographic information extracted from LiDAR data acquired by the University of the Philippines Phil-Lidar 1 Data Acquisition Component. LiDAR or “Light Detection and Ranging” is a remote sensing method that uses light in the form of a pulsed laser to measure ranges (variable distances) to the Earth from the sensor. These light pulses—combined with other data recorded by the airborne system— generate precise, three-dimensional (3D) information about the shape of the Earth and its surface characteristics. Among many things, it is useful for such tasks as surface mapping, vegetation mapping, transportation corridor mapping, transmission route mapping, and 3-D building mapping.
LiDAR data provide the much-needed accuracy and topographic detail to model and delineate the potential extent of flooding due to the high accuracy of the data and the ability to resolve small features that influence flow paths. There are two LiDAR data products that are especially used in detailed flood hazard mapping: 1) Digital Surface Model (DSM) and 2) Digital Terrain Model (Figure 3). DTM and DSM are generally referred to as Digital Elevation Models (DEM). A DSM is a representation of the earth’s surface where all features including all man-made structures and vegetation are present. When these features are removed from the DSM, it will result to a DTM which is also called a “bare earth” DEM. Both the DTM and DSM provide topographical and surface feature information needed by the flood model in its computation of how much rainwater drains into the river and how much will overflows, and how this overflow will cause flooding in a certain locality. Because the underlying topography is finely represented by the LiDAR DTM, the flood model is fed with the right information to adequately account for the differences in elevation of the locality and this allow the model to determine which areas the flood water will flow and as to what extent it will flow through time.
GENERATING FLOOD HAZARD MAPS OF CARAGA REGION RIVER BASINS
Within the CSU Phil-LiDAR 1 project, the steps involved in flood hazard map generation were streamlined into 3 major activities namely, field surveying, LiDAR data processing, and flood modelling.
All field data collection activities of the project are being handled by the Field Survey Team (FST). Gathering information from the field ensures that the actual characteristics of the rivers and its floodplains are captured. Knowing how much rain falls into the watershed, how wide and deep the river is, how fast the water flows, and how much water flows along the river would allow us to determine the capacity of the river and what levels it would take for the river to overflow. To gather all this information, the FST conducts river cross-section surveys through the use of Geodetic Engineering techniques and high precision instruments, and flow measurements through installation of rain gauges, water level sensors, and velocity meters
The Data Processing Team
LiDAR datasets of the rivers systems covered by the project are being processed by the Data Processing Team (DPT) to generate DTMs and DSMs. All LiDAR data used by the project are being collected by the Phil-LiDAR 1 Data Acquisition Component of the University of the Philippines Diliman. These datasets were pre-processed by the Phil-LiDAR 1 Data Pre-Processing Component (DPPC) before Validation and river bathymetry data gathered and provided to the project by Phil-LIDAR 1 Data Validation and Bathymetry Component of the UP Diliman are also processed by the DPT to assess accuracy of the LiDAR products and to incorporate river bed data into the DTMs. The DPT also extracts from the DSMs features such as houses, buildings, roads and bridges that will be utilized for mapping structures at risk to flooding.
The Flood Modeling Team
The Flood Modeling Team (FMT) uses computer models to predict flooding due to rainfall of varying volume and intensity. The FMT uses the DTM and information extracted from the DSM during the flood modeling process. The main outputs of the FMT are flood models and flood hazard maps.
The flood simulation software/programs used are the Hydrologic Engineering Center Hydrologic Modelling System (HEC HMS) version 3.5 and HEC River Analysis System (HEC RAS) version 4.1. Various geospatial datasets were utilized in the development of flood simulation models. In HEC HMS model development, a 10-m Synthetic Aperture Radar (SAR) Digital Elevation Model (DEM) was used for sub-basin delineations and for derivation of topography-related parameters of the model such as slope and elevation. Images acquired by the Landsat 8 satellite were also utilized to derive a landcover map using Maximum Likelihood classification. The landcover map is necessary for the derivation of land-cover-related model parameters such as surface roughness coefficient, and runoff/infiltration capacities. River width and cross-section data obtained from field surveys as well as those extracted from 1-m resolution LiDAR-derived Digital Terrain Model (DTM) were also used to estimate the channel routing parameters of the model. For HEC RAS model development, river bed topography (obtained from bathymetric surveys), sea bed topography (obtained from a NAMRIA topographic map), LiDAR DTM, building footprints (with top elevation) extracted from LiDAR DSM, and the same landcover map derived from Landsat 8 OLI satellite image were used as major inputs.
The HEC HMS-based hydrologic model computes for the volume of water coming from the upstream watersheds caused by rain falling in these areas. Rainfall depths recorded by rain gauges within and in the vicinity of the river basin are being used as input into the HEC HMS to compute discharge hydrographs for specific locations in the river basin, specifically at those locations where the upstream watersheds ends and the floodplain portions begin. The discharge hydrographs depict the volume of water per unit time (in m3/s) that drains into the main river at these locations. These hydrographs are then used as inputs into the HEC RAS hydraulic model to generate the flood depth and hazard maps. HEC RAS is a one-dimensional flood model that utilizes river and flood plain geometric data (from topographic and hydrographic surveys and LiDAR Digital Terrain Model – DTM), land-cover and surface roughness (from remotely-sensed images), and discharge hydrographs in order to compute water levels all throughout the river. Once these water levels are computed, the flooded or inundated areas along the river and in the floodplains are estimated by intersecting the water surface profiles into a high resolution LiDAR DTM.
For each river basin, the project will generate flood hazard maps representing actual flood events and hypothetical flooding events. The actual flood events represent maximum flood level and extent caused by heavy to torrential rains brought by Tropical Storms Agaton and Seniang. The rainfall data used in the flood simulations those recorded by rain gauges installed by the Advanced Science and Technology Institute – DOST. On the other hand, the hypothetical flooding events represent maximum flood levels and extent caused by rainfall events with varying intensity and duration (i.e., varying return periods). The rain return periods considered are 2-, 5-, 10-, 25-, 50-, 100-year. Rainfall Intensity Duration Frequency (RIDF) curves generated by Philippine Atmospheric, Geophysical and Astronomical Services (PAGASA) for various locations in Caraga Region were utilized as inputs in the flood simulation.
The FMT incorporates all the information from the field surveys to assess whether the flood hazard maps generated are realistic and accurate or not. The flow measurement datasets are being used to calibrate and validate the upstream hydrological model to ensure that its computations of flow during rainfall events are accurate within acceptable levels of accuracy. The FMT together with the FST also conducts validation surveys to check the accuracy of the flood hazard maps. The teams visit random locations within a river basin and interview the nearest household about the occurrence of flooding in their locality. Information gathered during the surveys are the geographic locations of the households, the maximum depth of flood, the estimated dates when the flooding occurred, and the causes of flooding (i.e. if it was due to overflowing of the river or an accumulation of rainwater).