A flood is an overflow of water that submerges land that is usually dry. One of the pressing issues in the world today that has led to loss of lives and properties, is the prevalence of floods.
The prevalence of floods may be as a result of unusually high tides and storm surges, heavy rainfall, climate change, poor drainage system, melting snow or ice, among many others. As a result, social interventions and government agencies have laid out effective urban planning strategies that would help mitigate the occurrence of the natural disaster (flood) that addresses Sustainable Development Goals and Green infrastructure.
This tutorial makes use of spatial data science and remote sensing techniques to assess the susceptibility of floods using ArcGIS Pro. Many factors are considered during flood susceptibility analysis. However, the following factors will be considered in this tutorial; slope, hydrology, rainfall, and Land Cover.
Figure 1 below shows a summary of the geo-spatial methodological tools employed in this tutorial.
Deriving Hydrology from Digital Elevation Model (DEM) and Using “Euclidean Distance” Tool
The Digital Elevation Model can be accessed from the United States Geological Survey’s (USGS’s) website. After, the following steps can be repeated to delineate hydrology network from DEM.
- Open ArcGIS Pro, and create a new project from the “project” tab by selecting “blank.” From the insert tab on the menu bar, click on “New Map” to select a new base map.
- Click on “Add Data” from the “Map” tab to import the DEM onto the map canvas.
- From the search bar of the geoprocessing toolbox, type and search “fill.”
- From the window that appears, set “Input Surface Raster” to the DEM, and “Output Surface Raster” to preferred storage location and name. Click “Run.”
- From the search bar of the geoprocessing toolbox, type and search, “flow direction.” Set “Input Surface Raster” to the results of “fill” tool, and set “Output flow direction” to desired storage location and name. Click “Run.”
- From the search bar of the geoprocessing tool, search, “flow accumulation.” Set “Input Flow Direction Raster” to results of “flow direction,” and set “Output Raster” to be desired output location and name. Click “Run.”
- Type and search, “Raster Calculator” from the geoprocessing tool. From the calculation section of the calculator, type, “flow accumulation>500,” and “Run.”
- Type and search “stream order” from the search bar of the geoprocessing toolbox. Set “Input Stream Raster” to the results of “Raster Calculator,” and “Output Raster” to the desired output name and location. Also, set “Input flow direction raster” to the results of “flow direction.” Click “Run.”
- Type and search “stream to feature” from the search bar of the geoprocessing toolbox. Set “Input Stream Raster” to the results of “stream order,” and “Output raster” to the desired storage name and location. Also, set “input flow direction raster” to results of “flow direction,” and click “Run.”
The Euclidean Distance tool allows one to derive a set distance from a geometric feature (point, line, polygon), and also presents the output in a raster format. The tool will be used to draw a set distance from hydrology (river networks), which may be susceptible to floods.
The steps outlined below shows the use of Euclidean Distance in ArcGIS Pro;
- Type and search “Euclidean Distance” from the geoprocessing toolbox. Set “input feature source data” to results of “stream to feature,” and “Output Direction Raster” to desired name and location of processed raster. Also, set “maximum distance” to allowable distance around hydrology network. Click “Run.”
Deriving Slope from DEM
- To derive slope from DEM in ArcGIS Pro, type and search “slope” from the search bar of the geoprocessing toolbar.
- Set “input raster” to DEM, and “output raster” to the desired output name and location. Click “Run.”
Accessing and Importing Land cover data
Land Cover and land use data can be accessed freely from Esri or from WaterITech.com. After accessing the land cover data, the data will be imported onto the map canvas using the “Add Data” tool from the steps illustrated above.
Where the Landcover data needs to be streamlined to a specific Area of Interest, the “extract by mask” tool from the “Spatial Analyst tool” will be used. Steps are outlined below;
- Type and search “extract by mask” from the search bar of the geoprocessing toolbox.
- Set “input raster” to the “landcover raster data,” and “output raster” to desired name and location. Also, set “feature mask data” to the desired extent of study area. Click “Run.”
Accessing and Importing Rainfall Data
Rainfall data can be accessed from the CHRS data portal (CHRS Data Portal (uci.edu)). The data is available in a 1 hourly, 3 hourlies, 6 hourlies, daily, monthly, yearly or accumulative basis for any area of interest.
An area of interest (AOI) can be predefined by the user, or selected over a country, globe. Raster formats can be TIFF, ArcGRID or NETCDF. For the purposes of this tutorial, select the TIFF format. The accessed rainfall data can then be imported onto the map canvas, and clipped to the desirable AOI where necessary, using the “extract by mask” tool as illustrated above.
Using the “Reclassify” and “Weighted Overlay” Tools
The factors will be set into a defined number of classes (five classes), using the “reclassify” tool. After, weighted overlay analysis will be performed over the factors.
The following steps will be used in reclassifying the “Euclidean_Distance_Rivers” into five classes:
- Type and search “reclassify.” Set “input raster” to the “Euclidean_river.”
- Click “classify,” as highlighted in yellow marker in figure 8 below and set number of classes to 5.
- Set “output raster” to desired location and name of storage. Click “Run.”
Steps can be repeated for the other factors; rainfall, slope, and Landcover.
To perform weighted overlay, the following steps should be used;
- Type and search “Weighted overlay.”
- Values should be allocated to each of the factors in order of relevance; slope: 20, Euclidean_distance_hydrology: 30, rainfall: 30, Landcover: 20.
- Set “output raster” to desired output location and name, and click “Run.”