Flood Risk Assessment in QGIS

Introduction

Flood assessment is one of many practical applications of GIS. This analysis will focus on the city of Peterborough, a small city northeast of Toronto. The analysis is particularly relevant due to past flooding events in the Peterborough area. By a completing flood risk assessments, the city can identify at-risk infrastructure and populations, as well as develop stronger preparedness measures for future flooding events.

The purpose of this tutorial is to provide users with guidance in completing a flood risk assessment, as well as an introductory glance into QGIS.

Software Requirements

In order to begin this tutorial, you will need to download the QGIS software package from the following link (https://www.qgis.org/en/site/ ). This tutorial was created using QGIS version 3.8.1. The latest version of QGIS is freely available for Windows, MacOS X, Linux and Android.

All steps of the tutorial will be completed using this free and open source software, and will not rely on the use of any other programs.

Data

The primary source of data used for this analysis will be the High Resolution Digital Elevation Model (HRDEM) - CanElevation Series, produced by Natural Resources Canada for the city of Peterborough. The dataset is hosted on the Open Data Canada website here: (https://open.canada.ca/data/en/dataset/957782bf-847c-4644-a757-e383c0057995).

There are two main types of elevation datasets provided as part of the High Resolution Digital Elevation Model (HRDEM). Both datasets have been derived from airborne LIDAR data. The two datasets are in the form of a Digital Surface Model (DSM) and a Digital Terrain Model (DTM). It is important to know the difference between these two elevation datasets and how this can affect your analysis of flooded areas.

A Digital Surface Model or DSM is very similar to that of a Digital Elevation Model (DEM) or Digital Terrain Model (DTM). All three show elevation data of the earth’s surface relative to sea level. One major distinction between a DSM and a DTM is the presence of man-made and natural features. The DSM shows the tops of buildings, houses, highways, tree canopies, vegetated surfaces and all manor of other solid features above the surface of the ground. They are traditionally in the form of a raster image. DSM’s are generally used for purposes where it is necessary to see objects protruding from the surface of the earth. This includes tasks such as assessing the encroachment of vegetation or viewshed analysis for a new High-rise . They are not used for hydrologic modelling such as a flood analysis.

A Digital Elevation Model or DEM does not include features that protrude from the ground. A DEM represents the ground elevation from sea level. Therefore, the tops of trees, buildings and other features have been removed to reveal the ground underneath. This is very useful for the purpose of hydrologic mapping, such as flood analysis. DEM’s are generally in the form of a raster image .

A Digital Terrain Model or DTM can have varying definitions. In some cases, Digital Terrain Model and Digital Elevation Model are used interchangeably. This means that the DTM dataset would be a raster image of the Earth’s surface excluding buildings and vegetation. In other cases, a Digital Terrain Model can be essentially a DEM represented through point data. The dataset shows the elevation of the earth’s surface through a series of equally spaced points (although, in older datasets these points can be unevenly spaced). These points have an associated x and y value along with a third value for elevation. In this case the DTM would be in the form of a vector dataset, such as a shapefile .

The DTM dataset available through the High Resolution Digital Elevation Model link is in the form of a geotiff (.tif). This means it is essentially the same as a DEM and can be used for our flood mapping analysis. The DTM data that we will be using in this analysis will be at a resolution of 1m and will be projected in the UTM NAD83 (CSRS) coordinate system.

Additional vector data will also be used in this analysis from the Peterborough Open Data Hub. This includes building footprints, roads, pedestrian footpaths and hydrography polygons.

• See next section for instructions on how to download the files.

Downloads

Peterborough Digital Terrain Model Data

High Resolution Digital Elevation Model (HRDEM) - CanElevation Series

The High Resolution Digital Elevation Model (HRDEM) - CanElevation Series dataset hosted on the Open Canada Data web portal, where you can download different sets of DTM and DSM data across Canada

1. Click on the following link. This will take you to the directory where all of the DTM data is stored for the UTM 17 Zone.

2. Use the find function (Ctrl + F) to find each of the three images listed below in the file directory.

1m_utm17_e_20_90

1m_utm17_e_21_90

1m_utm17_e_21_91

3. Download each file into a folder on your computer. This will be a working folder for all of your flood analysis data.

Peterborough Vector Data

This data is hosted by Peterborough's GIS Open Data initiative on their Open Data web portal.

1. Click on the following links and download the data as .shp files (a .zip file will be downloaded)

Hydrography Polygons: This is a polygon of all the waterways in Peterborough.

Buildings: This is a polygon layer of all the building footprints in Peterborough.

Roads: This is a line layer of all the road networks in Peterborough.

Pedestrian Network Polygon: This is a polygon layer of all the pedestrian footpaths and bike trails in Peterborough.

2. Go to the .zip files you previously downloaded and extract them to the same folder as you Peterborough DTM data.

QGIS Methods and Instructions

Importing and Merging

Step 1.

• Open Qgis.
• Upload the .tif DTM images in QGIS by clicking on the Data Source Manager button . The Data Source Manager window will then appear.
• Select the folder where your files are saved, (this folder can be added to your favourites by right clicking the folder and adding it to your favourites) drag in all four .tif files to the main window (ignore the .tif.ovr files and only use the .tif files). Alternatively, you can double click each file separately to add them to the project one by one.
• A popup window will appear when you load in the .tif files, select the first option and click Ok.

• The result should be the four raster images together in a square, as pictured below:
  • Note: If images are too hard to read, clicking on the image twice will result in a larger version of the image.

Merging Rasters

Step 2.

• Use the merge tool (Raster -> Miscellaneous -> Merge) to combine the .tif files into one working raster image.
• In the merge tool window, select the four .tif files through the Input Layers selection, directly under the Parameters tab.
• Under the Merged heading select the Save to File… option, give the output a name and save it in your working folder.
• Once the layers have been selected, click Run in Background in order to merge the files.
• The resulting merged file should look similar to the final image below:

Using the Merge tool.

Merged raster files.

Creating an AOI

Step 3.

• We will now create an Area of Interest or (AOI) polygon so we can clip our water layers to the boundary of our DTM data in the following step.
• Navigate to Layers -> Create Layer -> New Shapefile Layer
• At the top of the window, give a name to your new file (AOI is suitable), and select Polygon as the Geometry Type -> Click OK, At this point a new layer named AOI will be created.
• Right click on the newly created AOI layer, and select Toggle Editing. Once this is selected, navigate to the top panel and find the Add Polygon Feature button.
• Do your best to draw a border around the Merged layer (hint: use your numpad to move your screen when zoomed in or click and drag with the scroll wheel to pan), when complete, right click on the screen and when prompted to save changes to the layer, select Yes.
• You can now symbolize the polygon with an outline. Right click on the layer and select Layer Properties/ Symbology then select Outline Red. The final AOI and Merged layer should look similar to what is shown below:

Adding Vector Layers

Step 4.

• Once the four raster images have merged, and the AOI has been created around it, the Peterborough shapefiles that were downloaded earlier can be added into QGIS.
• Navigate to the folder they were saved in (same process as importing the .tif files) and drag in the features (vector data) that you want displayed.
• For the purposes of the tutorial, the polygons used were: Major Water, Buildings, Roads, Major Roads, Railroads, and finally Bridges.
• Ensure the polygons are overlaying the merged raster layer, and edit the appearance of each feature as you want (double click on the icon to the left of each layer name).
• The resulting file should appear similar to the image presented below:

Clipping

Step 5.

• Now that everything we want is added, the vector layers can be clipped to match the extent of the merged raster layer.
• Navigate to Vector -> Geoprocessing Tools -> Clip
• Select the Hydrography layer as the Input Layer, and the AOI as the Clip Layer. This clips all parts of the Hydrography layer that is outside the area of the AOI.
• Under the Clipped section of the popup window select a save location for the output file and change the file type to shapefile (.shp), the click Ok.
• You can now turn off the original Hydrography layer in the layers table of contents on the bottom left-hand side of your screen.

Sink Fill

Step 6.

• In this step, the Sink Fill (Wang & Liu) will be used from the QGIS toolbox. The tool will sequentially fill any significantly low pixels in the DTM where the flow of water could accumulate incorrectly.
• Navigate to Processing at the top of the program, and from there select Toolbox, in order to open the list of available tools.
• At the bottom of the list of tools is SAGA, select this and continue down until you reach Terrain Analysis - Hydrology -> Fill Sinks (Wang & Liu)
• Under the dropdown list for DEM select the layer Merged, and select save locations for the three output files (Filled DEM, Flow Directions and Watershed) then select Run
• Upon completion, move your mouse to the layers area, and move the three new layers (Filled DEM, Flow Directions and Watershed) underneath the vector layers.
• The final output will look the same as the image in Step 5, if everything has gone correctly.

Identifying Water level

Step 7.

• The next step is to determine the current water level of the filled DTM.
• To do this you will need to install a plugin though the Plugins tab in the top taskbar.
• Click on Plugins -> Manage and Install Plugins, scroll down until you see the Value Tool, then Click Install Plugin.

• After installing close the popup window and you should now see the Value Tool button on your main taskbar
• By default, the tool should be enables, however if it is not navigate to the View -> Panels -> Value Tool Panel (and make sure the box is checked)
• Next click on the Identify Features tool on the main toolbar, then click on the Value Tool
• Zoom in to “Little Lake” in the middle of the downtown Peterborough area.

• You should now see the pixel value of the water surface on your right under the Identify Results window.
• You can click around the lake to compare values and see if they change, the Value Tool will continue to select values from the topmost raster image, even if there are polygons over-top.
• We can now see that the elevation of the water level is approximately 188m.

Raster Calculator

Step 8.

• Raster Calculator is used in this step to establish what areas will be flooded given an increase in water level of 2 meters -- visualized in the example is an increased water level of 1m.
• Navigate to: Raster - Raster Calculator.
• Use the Raster Bands and Operators to recreate the following expression: ( ( "Filled_Merge@1" <=190 ) = 1 ) AND ( ( "Filled_Merge@!" >= 190 ) = 0 ) (Note: the expression will not run properly if you copy and paste it in you will need write it out manually in the popup window).

• If entered correctly, Expression Valid will appear under the expression window, then select a save location and name for your output file.
• Click Ok and run the tool.
• You should end up with an output raster with two values 1 (flooded area, white) and 0 (non-flooded area, black). The output should look like the following image (you may need to rearrange the order of your layers so that the roads, buildings, etc. polygons are on top).

Changing NoData Values

Step 9.

• After creating the raster of the flooded areas, it is now possible to this layer into a polygon with a few intermediate steps.
• First the 0 value pixels (land) must be turned into no data values. Right click the water raster created in step 8 and click Export -> Save as… a popup window should appear.
• Give the output a file name and save location, then scroll down and check the No Data Values box.
• Click on the green plus symbol to add a new value manually, then in the box above type in 0, the left box will autofill to 0 as well, then click OK.

• You should now have a raster of water only areas in black as below.

Raster to Polygon

Step 10.

• Go to the top toolbar and click Raster -> Conversion -> Polygonize (Raster to Vector)…
• Under the Input Layers heading select the water only areas raster created in the previous step.
• Under the Vectorized heading select a save location for your output polygon and change the file type to save as a .shp file.

• Click OK
• The output should be a polygon of the flooded areas as seen below (notice the gap in the raster image has also been included in the new polygon layer)

Deleting Unwanted Polygons

Step 11.

• We can now get rid of the no data area that was mistakenly included in our water only polygon.
• Right click the polygon layer and click Toggle Editing.
• In the middle toolbar at the top click on the Select Features by Area or Single Click button , then click on the unwanted polygon.
• The polygon should now be highlighted in yellow, click the Delete Selected button to delete the polygon.
• Right click the polygon layer again and click Save Edits, then Toggle Editing so that it is turned off.

Identifying Flooded Buildings

Step 12.

• We are now able to identify vector objects that are impacted by the flooded areas.
• To identify all the buildings that have been flooded go to the search bar in the bottom left-hand corner of the screen and type in Select by Location, select the tool form the menu and a popup window should appear.
• Under the Select Features From heading select the Building polygon layer, under the By Comparing Features From heading select your flood water polygon.

• Click run.
• The buildings that intersect with the flooded areas should now be selected.
• Right click the building shapefile and click Export -> Save Selected Features As…
• You can now save all the overlapping features as their own polygon layer.
• This new layer can then be symbolized differently to highlight what buildings have been flooded.
• This step can also be repeated with the other vector layers to identify what pedestrian walkways or roads have been flooded.