REVIEW OF INTERNATIONAL GEOGRAPHICAL EDUCATION ISSN: 2146-0353 ● © RIGEO ● 11(3), SUMMER, 2021 www.rigeo.org Research Article The Development of the Flood Inundation Area Model in the Way Sekampung Sub- Watershed in Lampung Aprizal1 Sofia W. Alisjahbana2 Doctoral Program in Civil Engineering, Civil Engineering, Bakrie University, Jakarta Tarumanagara University [email protected] [email protected] Any Nurhasanah3 Civil Engineering, Bandar Lampung University [email protected] Abstract Flood disasters often occur in most areas in Indonesia. Floods are the most common disasters that cause losses and casualties. Flood disaster mitigation needs to be done to overcome and reduce the impact of losses. Mitigation of flood disasters will be helpful if you can predict in advance the potential for inundation caused by flooding, namely by modeling flood inundation. So far, research on modeling the area of flood inundation in Indonesia can be categorized as relatively rare. It is due to the many obstacles that surround it. The problem faced is the unavailability of complete data caused by limited funds and the complexity of most of the watersheds in Indonesia. The study aims to develop a flood inundation area model based on land use conditions and rain variables. It is done by maximizing minimal data and integrating empirical and 2D hydrodynamic modeling formulated with statistical models. The method used is to collect secondary data in the form of rain and land use data and primary data by conducting field surveys of flood inundations that have occurred. Then, it is followed by hydrological modeling to obtain flood hydrographs by integrating HEC-RAS, HEC-GEO RAS, and Arc GIS to generate variables to be modeled statistically using the Multiple Linear Regression approach. The modeling uses samples in some Way Sekampung sub-watersheds in Lampung Province, each with two rain time series data. The independent and dependent variables are the variables to be modeled where the independent variables are Watershed Area (X1), % Forest Area (X2), % Residental Area (X3), % Agricultural / Rice Field Area (X4), % Mixed Plantation Area (X5), % Other Area (X6), River Slope (X7), River Length (X8), Rainfall (X9), Flood Peak Time (X10), Flood Discharge (X11) and one dependent variable, namely Flood Inundation Area (Y)Four independent variables do not affect statistical modeling, namely X4, X6, X7, and X9. The model obtained is Y = 150,442 -0,242 X1 -0,412X2-0,515X3- 0,241X5+ 2,050 X8 -0,704 X10+ 0,020 X11 4. MAPE test results (Mean Absolute Percentage Error) The model equation shows a value of 4.672% of the data. Thus, the model is in the Class 1 category that is Very Accurate. Keywords Flood inundation, Land Use, Modeling, HEC RAS, ARC GIS To cite this article: Aprizal; Alisj ahbana, S,W, ; Nurhasanah , A. (2021) The Development of the Flood Inundation Area Model in the Way Sekampung Sub-Watershed in Lampung. Review of International Geographical Education (RIGEO), 11(3), 1246-1256. Doi: 10.48047/rigeo.11.3.115 Submitted: 06-03-2021 ● Revised: 13-04-2021 ● Accepted: 20-03-2021 Aprizal, ; Ali sjahbana, S,W, ; Nurhasanah, A. (2021) The Development of the Flood Inundation Area … Background As development growth increases, land use also increases, leading to changes in land cover in a watershed (Bedient, Huber, & Vieux, 2008). As a result, erosion and landslides are increasingly uncontrollable in the upstream watershed area, and there is a narrowing of the river channel in the middle watershed area. Meanwhile, the existing rivers and canals (drainage systems) can no longer accommodate rainwater runoff in the downstream watershed area, resulting in flooding. Floods that are very detrimental almost every year also occur in the Sekampung watershed. The term modeling is known to understand natural phenomena, including floods. Modeling is the miniaturization of a complex biological phenomenon into a simple one or vice versa. Ravindran, Phillips, and Solberg (1976) in operations research. Phillips states that what is meant by a model is a simple representation of something real. Modeling is an option in understanding the phenomenon related to flooding. According to J. Teng et al., research on flood phenomena through flood modeling can be grouped into two groups, namely: 1) empirical, conceptual approach and 2) hydrodynamic approach consisting of 1D, 2D, and 3D modeling. (Teng et al., 2017) 3 Dimention Spatial Flood Inundation Modeling Method 2 Dimention Measurement 1 Dimention Empirical Hydrodynamic Method method Figure 1. Classification Diagram of Flood Inundation Area Modeling Each approach has advantages and disadvantages. The conceptual, empirical approach is supported by measurements, surveys, and remote sensing—statistical models developed from these data-based methods (Schumann et al., 2009; Smith, 1997). Hydrodynamic model approaches such as one-dimensional (1D) (Brunner, 2016; DHI, 2003), two-dimensional (2D) (e.g., DHI, 2012; Moulinec et al., 2011), and three-dimensional (3D) (e.g., Prakash et al., 2014; Vacondio et al., 2011) simulates the movement of water by solving equations supported by computer technology.(Teng et al., 2017). Currently, floods often occur in developing countries such as Indonesia, which has many rivers. Flood discharge data and flood history are complicated to obtain so that the empirical and hydrodynamic modeling of the inundation area is experiencing considerable obstacles. This study tries a new development to model the flood inundation area, which can overcome the problem by generating flood data from rain data by preparing DEM and SHP maps using HEC-RAS 5.0. 2 dimentional Empirical Hydrodinamic method modeling Slice 2. Modeling Method using Statistics Figure 2. Model Development Novelty Diagram 1247 © RIGEO ● Review of International Geographical Education 11(3), SUMMER, 2021 Study Area Lampung Province is one of the provinces in Indonesia, which is located on the island of Sumatra. Lampung has three river areas, namely Mesuji-Tulang Bawang, Seputih-Sekampung, and Semangka. The study area is focused on the Way Sekampung sub-watershed, as shown in Figure 3 below. Bandar Lampung – Kalianda Watershed Figure 3. Way Sekampung Lampung Sub-watershed Study Area Source: (Syafri, 2013) Topography Map DEM map shows the elevation of an area, which helps know the boundaries of a watershed, river elevation, and the earth's shape to help model a watershed. Figure 4. DEM Study Area Sub Watershed Way Sekampung Lampung Shapefile (SHP) Shapefile is a data format for storing vector-based non-topological spatial data. Shapefiles are used to store digital map data in geographic information systems. This data format is developed by ESRI (Aronoff, 1989) This data format is capable of storing spatial data, such as fields (islands, provinces), lines (roads, rivers), points (cities, buildings), and information about the three spatial data (the name of a town, type of a street, etc.). 1248 Aprizal, ; Ali sjahbana, S,W, ; Nurhasanah, A. (2021) The Development of the Flood Inundation Area … Figure 5. SHP for Way Sub-watershed Study Area, Sekampung Lampung Problem Identification Based on the above background, the identification of the problem in this study is: 1. What are the variables of a watershed that affect changes in flood inundation areas? 2. What is the correct model for the flood inundation area in the Way Sekampung sub-watershed? Methodology The method used in conducting this research is as follows: 1. Collecting data: Digital Elevation Model (DEM) is obtained from download on Digital Elevation Model National (DEMNAS) Indonesia. Shapefile (SHP) is obtained from download on Shapefile (SHP) Indonesia Info Geospatial. Two time-series data, 1996-2007 and 2004-2015, in the four Way Sekampung sub- watersheds for calculating the Nakayashu hydrograph debit. Inundation Event Map. 2. Processing Digital Elevation Model (DEM) data for the four Way Sekampung sub- watersheds using geometry modeling in the ArcGIS 10.3 application and Global Mapper version 20.0 for use in the HEC-RAS modeling version 5.0.7. 3. Analyzing and modeling the 2D flood distribution in the four Way Sekampung sub- watersheds (without any water structures), based on the river geometry data generated by the Ras Mapper with the Nakayshu hydrograph debit parameters in 2007 and 2015. After the data formation process carried out by ArcGIS version 10.3 and Global Mapper version 20.0 is complete, the next step is to enter the data into the HEC-RAS version 5.0.7 application. It is done to correct the imperfect geometry due to the weak level of DEM accuracy by making a two-dimensional model. After the geometry data is fixed and the new DEM is formed into two dimensions, the researchers enter data for unsteady flow, which means that the water depth can change with time. In the unsteady flow stage, the input data is the debit data calculated by the Nakayashu hydrograph. After all the geometry data and flow data are entered, the authors analyze and model the flood distribution by clicking the Run Unsteady flow menu. The author then clicks compute to complete the calculation of the water level profile. The last step in modeling the distribution of floods is to display the results of the flood inundation simulation. 4. Map and calculate the area of flood inundation that occurs. The author uses the help of the RAS Mapper feature in the HEC-RAS 5.0.7 program to create a flood inundation map, with the following steps: 1249 © RIGEO ● Review of International Geographical Education 11(3), SUMMER, 2021 After the results of the hydraulics computation with the input discharge time are complete, the results can be seen in the RAS Mapper feature by activating the map layer from google satellite or others. The author then computes the inundation boundary, which is the area value of the flood inundation simulation. After that, it can be concluded. The final preparation of flood inundation maps in the four Way Sekampung sub-watersheds can be carried out.