Flood Risk Assessment in QGIS - Revision history

Smitch: /* Data */ Added a phrase to prevent reader from immediately going to Open Data Canada and trying to figure out how to download without realizing there are instructions provided further below.

2019-10-24T14:34:55Z

Data: Added a phrase to prevent reader from immediately going to Open Data Canada and trying to figure out how to download without realizing there are instructions provided further below.

OliverFarwell: /* Identifying Flooded Buildings */ correct sentence flow

2019-09-29T21:28:15Z

Identifying Flooded Buildings: correct sentence flow

OliverFarwell: /* Deleting Unwanted Polygons */ correct grammar

2019-09-29T21:25:24Z

Deleting Unwanted Polygons: correct grammar

OliverFarwell: /* Raster to Polygon */ correct grammar

2019-09-29T21:23:39Z

Raster to Polygon: correct grammar

OliverFarwell: /* Changing NoData Values */ correct grammar

2019-09-29T21:22:00Z

Changing NoData Values: correct grammar

OliverFarwell: /* Raster Calculator */ correct sentence flow

2019-09-29T21:20:19Z

Raster Calculator: correct sentence flow

OliverFarwell: /* Identifying Water level */ change sentence flow

2019-09-29T21:15:13Z

Identifying Water level: change sentence flow

OliverFarwell: /* Sink Fill */ correct sentence flow

2019-09-29T21:12:00Z

Sink Fill: correct sentence flow

OliverFarwell: /* Clipping */ correct sentence flow

2019-09-29T21:06:03Z

Clipping: correct sentence flow

OliverFarwell: /* Adding Vector Layers */ correct sentence flow

2019-09-29T21:03:19Z

Adding Vector Layers: correct sentence flow

← Older revision		Revision as of 14:34, 24 October 2019
Line 10:		Line 10:

	==Data==		==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).		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); specific download instructions are provided in the next section, below, but first we'll review different forms of elevation data models.

@@ Line 215: / Line 215: @@
 [[File:SelectByLocation_Step12.png|1400px]]
 *Click run.
-*The buildings that intersect with the flooded areas should now be selected.
+*The buildings that intersect with the flooded areas should now be selected and highlighted when you zoom in.
 *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.
@@ Line 221: / Line 221: @@
 *This step can also be repeated with the other vector layers to identify what pedestrian walkways or roads have been flooded.
 [[File:ID_FloodBuildings_Step12.png|1400px]]
+*The tutorial is now complete!
 == References ==

@@ Line 202: / Line 202: @@
 *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 [[File:SelectFeatures_By_Button.png|32px]], then click on the unwanted polygon.
+*In the middle toolbar at the top, click on the '''Select Features by Area or Single Click''' button [[File:SelectFeatures_By_Button.png|32px]], then click on the unwanted polygon.
 *The polygon should now be highlighted in yellow, click the '''Delete Selected''' button [[File:DeleteSelected_Button.png|20px]] to delete the polygon.
-*Right click the polygon layer again and click '''Save Edits''', then '''Toggle Editing''' so that it is turned off.
+*Right click the polygon layer again and click '''Save Edits''', then '''Toggle Editing''' so that editing is turned off.
+*The unwanted polygon should now be deleted from your flooded area polygon.
 === Identifying Flooded Buildings ===

@@ Line 189: / Line 189: @@
 '''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 '''Input Layers''' heading select the water only raster we 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.
 [[File:Raster_To_Polygon_Step10.png|1400px]]
 *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)
+*The output should be a polygon of the flooded areas as seen below (notice the gap in the raster image on the left has also been included in the new polygon layer, we will need to remove this next)
 [[File:Raster_To_Polygon_Output_Step10.png|1400px]]
 === Deleting Unwanted Polygons ===

@@ Line 176: / Line 176: @@
 '''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.
+*After creating the raster of the flooded areas, we can now turn 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.
@@ Line 184: / Line 184: @@
 *You should now have a raster of water only areas in black as below.
 [[File:WaterOnly_Step9.png|1400px]]
 === Raster to Polygon ===

@@ Line 163: / Line 163: @@
 '''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.
+*We will now use the '''Raster Calculator''' tool to establish what areas will be flooded given an increase in water level of 2 meters.
-*Navigate to: '''Raster - Raster Calculator'''.
+*Under the top banner 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).
+*Recreate the following expression under the '''Raster Calculator Expression''' section of the window: '''( ( "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).
 [[File:RasterCalculator_Step.png|600px]]
 *If entered correctly, Expression Valid will appear under the expression window, then select a save location and name for your output file.
@@ Line 171: / Line 172: @@
 *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).
 [[File:RasterCalculator_Flood.png|1400px]]
 === Changing NoData Values===

@@ Line 148: / Line 148: @@
 '''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.
+*To do this you will need to install a plugin though the '''Plugins''' tab in the top banner.
 *Click on '''Plugins''' -> '''Manage and Install Plugins''', scroll down until you see the '''Value Tool''', then Click Install Plugin.
 [[File:ValueTool_Plugin.png|1400px]]
-*After installing close the popup window and you should now see the '''Value Tool''' button on your main taskbar [[File:ValueTool_Button.png|32px]]
+*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)
+*By default the tool should be enabled, 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'''
+*Next click on the '''Identify Features''' tool [[File:Identify_Features_Button.png|32px]] on the main toolbar, then click on the '''Value Tool'''. [[File:ValueTool_Button.png|32px]]
 *Zoom in to “Little Lake” in the middle of the downtown Peterborough area.
 [[File:ValueTool_Step7.png|1400px]]
@@ Line 159: / Line 159: @@
 *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===

@@ Line 137: / Line 137: @@
 '''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.
+*In this step, the Sink Fill (Wang & Liu) tool 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 (''this is generally used to remove sinks from hydrologic analyses'').
-*Navigate to '''Processing''' at the top of the program, and from there select '''Toolbox''', in order to open the list of available tools.
+*Navigate to '''Processing''' in the top banner, select '''Toolbox''', this will open a 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)'''
+*At the bottom of the list select '''SAGA''', look for and select '''Terrain Analysis - Hydrology''' -> '''Fill Sinks (Wang & Liu)'''. The tool window should now open.
-*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''
+*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.
+*After the tool has finished, drag 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.
+*The final output will look very similar to the image in ''Step 5'', if everything has gone correctly (''if you look closely you may be able to see low points in the DTM that the tool has filled'').
 ===Identifying Water level===

@@ Line 127: / Line 127: @@
 *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'''.
+*Select the '''Hydrography''' layer as the ''Input Layer,'' and the '''AOI''' as the ''Clip Layer''. This clips the '''Hydrography''' layer to the boundary of the '''AOI''' layer.
-*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.
+*Under the ''Clipped'' section of the popup window select a save location for the output file and change the file type to shapefile (.shp), then 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.
+*You can now turn off the original '''Hydrography''' layer or remove it from the session (''right click the layer and select remove'').
 [[File:Clipped_Hydrology_Step5.png|1400px]]
 ===Sink Fill===

@@ Line 115: / Line 115: @@
 '''Step 4.'''
 *We can now add the vector Peterborough data to our QGIS session.
-*Navigate to the folder they were saved and import each of the shapefiles using the .
+*Navigate to the folder they were saved and import each of the shapefiles using the '''Data Source Manager'''.
+*Ensure the polygons are overlaying the merged raster layer, and edit the appearance of each feature to your liking (''right click on the layer and select properties to change symbology'').
-*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: