View source for Flood Risk Assessment in QGIS

==Introduction==
Flood assessment is one of many important purposes for which GIS capabilities can be applied in a practical manner. The scope of the analysis in the tutorial includes a portion of the downtown core of the Ottawa-Gatineau area. This analysis is particularly relevant due to the high number of flooding events and high-rainfall in Ottawa. By completing flood risk assessments, the city can identify at-risk buildings/populations, as well as develop stronger preparedness measures for when a flood does occur. 

The purpose of this tutorial is to provide users with an introduction/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 and is 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) dataset and a Digital Terrain Model (DTM) dataset. 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 about the earth’s surface. One major distinction between a DSM and a DTM is the presence of man-made and natural features. This means 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 Highrise. They are not used for hydrologic modelling such as 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. It therefore does not include the tops of trees or buildings. 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 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. 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.<ref>''[https://gisgeography.com/dem-dsm-dtm-differences/ GISGeography]'', 2018-02-20, Retrieved 2019-09-21 </ref>

 

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 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 [http://ftp.maps.canada.ca/pub/elevation/dem_mne/highresolution_hauteresolution/dsm_mns/1m/ON/2016-17-Peterborough/utm17/ link]

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


;'''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)

::[http://data-ptbo.opendata.arcgis.com/datasets/hydrography-polygon?geometry=-78.665%2C44.246%2C-78.012%2C44.332 '''Hydrography Polygons:'''] ''This is a polygon of all the waterways in Peterborough. ''
::[http://data-ptbo.opendata.arcgis.com/datasets/building '''Buildings:'''] ''This is a polygon layer of all the building footprints in Peterborough.''
::[http://data-ptbo.opendata.arcgis.com/datasets/roads '''Roads:''' ] ''This is a line layer of all the road networks in Peterborough.'' 
::[http://data-ptbo.opendata.arcgis.com/datasets/pedestrian-network-polygon '''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.''' 
*Upload the .tif files into QGIS by navigating through the '''Data Source Manager ''' window in the top left-hand corner [[File:Datasource_Manager_Button.png|32px]] 
*Select the folder where your files are saved, (can be added to favourites by right clicking the folder) and drag in all four .tif files to the main window (ignore the.tif.ovr files). Or alternatively, you can double click each file separately to add them to the project. 
*A popup window will appear when you load in the .tif files, select the first option and click Ok. 
[[File:Import Popup.png|700px]]

*The result should be the three 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.''


[[File:Import_Step1.png|700px]]


===Merging Rasters ===

'''Step 2.''' 
*Use the merge tool ('''Raster''' -> '''Miscellaneous''' -> '''Merge''') to combine the .tif files into one working layer feature.
*In the merge tool, 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 folder with your other DTM data. 
*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 image below:


[[File:Merge_Popup_Step2.png|700px]]
''Using the '''Merge''' tool.


[[File:Merge_Step2_Example.png|700px]]
''Merged raster files.

===Creating an AOI===

'''Step 3.'''
*At this stage an ''Area of Interest'' will be created, so we can clip our Ottawa layers in the next 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''' icon. [[File:Add_Polygon_Feature_Button.png|32px]]
*Do your best to draw a border around the '''Merged''' layer (hint: ''use your numpad to move your screen when zoomed in''), when complete, right click on the screen and when prompted to save changes to the layer, select '''Yes'''.
*The final step of creating an AOI is to right click on the icon/ Layer Properties/ Symbology  for the layer, and select '''Outline Red'''. The final '''AOI''' and '''Merged''' layer should look similar to what is shown below:


[[File:CreatingAOI_Step3.png|700px]]

=== 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:


[[File:Vector_Layers_Step4.png|700px]]

=== 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.


[[File:Clipped_Hydrology_Step5.png|700px]]

===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.
[[File:ValueTool_Plugin.png|700px]] 
*After installing close the popup window and you should now see the '''Value Tool''' button on your main taskbar [[File:ValueTool_Button.png|32px]]
*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 [[File:Identify_Features_Button.png|32px]] on the main toolbar, then click on the '''Value Tool''' 
*Zoom in to “Little Lake” in the middle of the downtown Peterborough area.
[[File:ValueTool_Step7.png|700px]]  
*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.




== References ==
{{reflist}}