3.2 Assessments of Waterlogging/Flood Prone Settlements

3.2.1    Data Requirements
1.    Elevation data
2.    Waterbodies/ Rivers
3.    Past Flood data with location
4.    High Flood Line for rivers

3.2.2    Stepwise Process Flow Details
-    Step 1: Import Data
o    Open QGIS and load your Digital Elevation Model (DEM) data representing the topography of the study area.
o    Import building data and any other relevant spatial data layers, such as rivers, flood-prone areas, and historical flood level data, if available.

-    Step 2: Clip the Digital Elevation Model
o    Use the "Clip raster by extent" tool to clip the DEM to your study region. This tool can be found in the Processing Toolbox (Processing > Toolbox) under the GDAL > Raster extraction menu.

-    Step 3: Reclassify the DEM
o    Utilize the "Reclassify by table" tool in the Processing Toolbox (Processing > Toolbox) under the SAGA > Raster tools menu.
o    In the tool dialog, select the clipped DEM as the input raster, and define your flood threshold values in a reclassification table (e.g., set values less than the High Flood Line (HFL) to 1 and the rest to 0).
 
-    Step 4: Identify Flood-Prone Areas
o    After reclassifying the DEM, the resulting raster image will represent
potential flood-prone areas, where cells with a value of 1 are susceptible to flooding.

-    Step 5: Use Historical Flood Levels (if available)
o    If you have access to historical flood level data, repeat Steps 2 and 3 with this data to create an additional raster representing historical flood-prone areas.

•    Step 6: Overlay Buildings
  o    Add the building data to QGIS and ensure it aligns correctly with the other layers.
  o    Use the "Intersect" tool in the Processing Toolbox (Vector overlay >
  Intersection) to overlay the buildings layer with the flood-prone areas layer. This will help identify buildings falling within the flood-prone regions.
  

Figure 8 Illustrations of identifying of water logging prone settlements, Sherpur (Bangladesh)
Source – CWIS spatial analysis, Innpact Solutions and GWSC

3.2.3    Output Application
Areas with waterlogging risks present operational challenges for the smooth functioning of toilet units. These challenges include toilet back-flow and non-functional soak pits. Understanding such risk areas may aid in understanding such risk hot spots on at city scale and integrating them with building bye-laws could also assist in the development of an appropriate monitoring framework.