# 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.

![](https://resources.imis.gwsc.ait.ac.th/uploads/images/gallery/2025-03/embedded-image-sdc6ywh1.png)

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.