Flood inundation mapping with QGIS
A flood model is just a hydrological model that estimates the rise in water level during a flood event. Modeling urban flood inundation is becoming increasingly important due to the amount of possible losses and disruptions to human life.
The intent was to model flood situations in a range of locations using publicly available information and field-verified data sets. The efforts led to the creation of a flood model that can be replicated using simple open-source tools and data sets.
The model was verified in a village in Kerala, India where a flooding event had already happened in recent years. Here I will demonstrate the methodology I used for creating the model.
QGIS with additional plugins(Version 2.18 and upwards), SRTM DEM (30m) and OpenStreetMap datasets.
The steps for creating a simple inundation model within QGIS with open data are as follows:
1. Open a blank project in QGIS.
2. From the plugin menu, install QuickMapServices to render background maps and locate the region of interest.
Any background maps can be chosen from the services, I used a combination of Bing Aerial Imagery and OpenStreetMap Services
3. Now, to download SRTM, the SRTM downloader plugin can be used. Zoom to the required canvas extent and download the DEM files and save a local copy in the system
4. For better visualization, style the DEM layer in the properties of the layer.
5. Now for deriving flood maps from DEM we can use the raster calculator in the raster option
6. Before generating the floodplain, identify the actual MSL value of the point/plain from the DEM. This will help to model the flood inundation based on water level rise during an event.
For instance, If the elevation value of the plain is 23m, we add a rise of 2m to the SRTM to generate a flood plain of 2m.
7. The flooded area can be identified by filtering out the other values in the raster styling option. By adjusting the transparency level we can clearly see the flood level rise in the region. Multiple flood levels can be generated using this method and overlaid for better visualization
Creating such viable models provides us with a better understanding of the places that must be evacuated during a disaster, lowering the number of victims during a flood. Furthermore, we can easily identify which roads or localities are at high risk, and a thorough catastrophe plan may be developed based on such models.
8. For 3D viewing of the inundation model, we can use the qgis2threejs plugin for a better perception of the area.
