CHAPTER 6: METHODOLOGY, DATA COLLECTION AND MODEL SETUP
6.2 Steps of Methodology in Flow Chart
6.2.1 Preparation Phase
Various kinds of authentic data and information as per the requirement of the model collected to develop a good mathematical flood model. Both recent and previous year data were collected from BWDB and compiled as per model requirement. These data also form the basis for further analysis and interpretation of the model results leading to accurate assessment of hydrological condition of the Padma River and its floodplain. According to the modeling requirements, a significant amount of consistent data includes water level, discharge, cross-section, Digital Elevation Model (DEM) were collected and model setup has been made using these data. All collected data and information, sources and data location are summarized in the `table 6-1.
Table 6-1: Summary of Data Type, Data Sources, Data Locations and period of data collection Data Type Data Source Data Locations Periods (year) Discharge and Water Level data BWDB Baruria Transit (SW91.9L) 1996-2018
Water level BWDB Bhagyakul (SW 93.4L) 1996-2018
Cross sections BWDB PADMA River (RMP1-RMP 8) 2008
DEM Aster Dem Bangladesh 2014
Discharge and Water level Data
The water level and discharge time series data of the Padma River at Baruria Transit station were collected from BWDB for the year 1996-2018. The hydrograph comprises data at an interval of one day. The discharge data of Baruria Transit station was used as the upstream boundary condition in the model.
The water level data of Bhagyakul for the year 1996-2018 was used as the downstream boundary condition in the model. The data of year 2010 was used to calibrate and while 2012 data was used to validate the hydro-dynamic model. Locations of two stations for which the historical discharge and water level data of the Padma River was collected from BWDB are shown in Figure 6-3 (a).
River Cross-section
River cross-sections of Padma River of the model reach were collected for the years of 2008 from Morphology Department of BWDB. BWDB collects cross- section data at 10 different designated stations in the Padma River. These cross-sections depict the shape and morphology of a river.
Locations of the stations are shown in Figure 6-3 (b)
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Figure 6-3: (a) Locations of Discharge and Water Level Station of Padma River, (b) Locations of cross-sections of Padma River used in the mode
Figure 6-4: Location wise cross-section of the Padma River as per Image Map (Source: CEGIS) Digital Elevation Model (DEM)
A digital elevation model (DEM) is a digital model or 3D representation of a terrain's surface level, created from terrain elevation data. DEM data identifies the elevations of the earth surface and to locate natural and relevant features on it (Rouf, 2015). A DEM can be represented as a raster (a grid of squares, also known as a height map when representing elevation) or as a vector-based triangular irregular network (TIN). The DEM could be acquired through techniques such as photogrammetry, lidar, land surveying, etc. DEMs are commonly built using data collected using remote sensing techniques, but it may also be built from land surveying. This data is required to take care flood plain and to formulate mathematical models for the study area. Each cell, and cell face, of the computational mesh of 2D flow area is pre-processed in order to develop detailed hydraulic property tables based on the underlying terrain used in the modeling process (Brunner et al., 2015).
The present study explores the hydrological modeling with the help of the ASTER digital elevation model (DEM). The Digital Elevation Model (DEM) of Bangladesh in raster format was collected from the Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER), an imaging instrument onboard Terra, the flagship satellite of National Aeronautics and Space Administration (NASA)'s Earth Observing System (EOS). Figure 6-5 shows the DEM of Study Area.
Modification of Digital Elevation Model (DEM)
DEMs are increasingly used for visual and mathematical analysis of topography, landscapes and landforms, as well as modeling of surface processes. The accuracy of DEM is determined by data type and actual sampling technique of the surface during DEM creation. A DEM offers the most common way of showing topographic information and even enables the modeling of flow across topography, a controlling factor in distributed models of landform processes (Khan et al., 2017).
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The Digital Elevation Model (DEM) of Bangladesh was collected from the Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER), an imaging instrument onboard Terra, the flagship satellite of National Aeronautics and Space Administration (NASA)'s Earth Observing System (EOS). The DEM was in geographical coordinate system (GCS_WGS_1984). Geographic coordinate systems indicate location using longitude and latitude based on a sphere (or spheroid) while projected coordinate systems use X and Y based on a plane. Projections manage the distortion that is in evitable when a spherical earth is viewed as a flat map (Masood, 2011).
Figure 6-5: (a) Digital Elevation Model (DEM) of the Padma River reach and its surrounding model domain, (b) Digital Elevation Model (DEM) Modification.
All the data in the DEM were projected on to the geographical coordinate system (GCS_WGS_1984). The data comprises of a resolution of 30m x 30m grid. The elevation of the DEM were measured with respect to the mean sea level. All the elevations including topography of river cross sections, water surface elevation were measured from Public Work Datum (PWD).
PWD is a horizontal datum believed originally to have zero at a determined Mean Sea Level (MSL) at Calcutta. PWD is located approximately 1.5 ft below the MSL established in India under the British Rule and brought to Bangladesh during the Great Trigonometric Survey. To adjust this difference in elevation, a slight modification of the collected DEM was made. The DEM after modification has been shown in Figure 6-5 (a) and 6-5(b)
Clipping of Study Area and Triangulated Irregular Network (TIN) Generation
After taking the modified DEM of Bangladesh, the shape file of Harirampur upazilla adjacent to the Padma River was superimposed. The DEM of this upazilla was clipped from the modified digital elevation model using the Clipping Tool in Arc Toolbox. And after that, study area was clipped from the DEM of Harirampur using the shape file of study area.
The purpose of the Raster to TIN tool is to create a Triangulated Irregular Network (TIN) whose surface does not deviate from the input raster by more than a specified Z tolerance. It is used to convert raster from a DEM to a TIN surface model. It is done by using the Raster to TIN tool in the Arc Toolbox.
6.2.2 Execution Phase