Coastal Lab Project Sheet Week 7 Workshop 8 PDF

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Summary

For the coastal Lab project. It gives good practice to enhance QGIS skills....

Description

Faculty of Engineering and Information Technology ENGR10006 Engineering Modelling and Design

Workshop 7: Flood Modelling Project “Modelling the flooding of low-lying cities in extreme weather events” – From bare earth to built-up urban space –

Welcome to Workshop 7 for Engineering Modelling and Design. This workshop introduces Digital Surface Models (DSM). DSM are different from the DTM (Digital Terrain Model) we have introduced in Workshop 4 and used so far for flood modelling. In this workshop we will see how DSM can be used for flood modelling and compare the results to the previous results based on DTM.

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BACKGROUND: You have already learned that DEM is a 3-dimensional representation of the terrain's surface created from the terrain's elevation data as acquired by land surveying, photogrammetry, LiDAR or satellite-based methods. A DEM can be represented as a raster (a grid of squares) or as a vector-based Triangulated Irregular Network (TIN). It is important to distinguish between two forms of DEMs: 1. Digital Terrain Models (DTM) represent the bare ground surface without any objects like plants and buildings. Terrain data is simpler and therefore useful for many GIS analyses including modelling water flow for hydrology, flight simulation, precision farming or forestry. 2. Digital Surface Models (DSM) represent the terrain’s surface including all objects on it. DSM are significantly richer in detail and more complex to model, and hence used only in more demanding applications such as virtual city models, or noise or shadowing analysis. The DEM data we have used until now is a DTM in raster format. This week we will introduce a DSM dataset and see how the addition of the built objects to the DEM affects the flood modelling.

Aim: • • •

Understand what a DSM is Understand the differences between DTM and DSM Observe the effect of DSM on flood modelling

Objectives • • •

Obtain, explore, manipulate, and present height information from a DSM Compare DTM and DSM and visualise the difference in 3D Create the rainfall, burst pipe, and coastline flood maps using the DSM. Compare the results to the equivalent DTM based flood maps.

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RESOURCES RESOURCES::

QGIS Software Long-term Release version

https://qgis.org/en/site/forusers/download.html DSM Data

https://elevation.fsdf.org.au/ Manhole Data

https://aurin.org.au/ Coastline Data

https://canvas.lms.unimelb.edu.au/

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PROCEDURE: 1 – Preparation Come informed and inspired to the lab: Read this lab-sheet and answer the preparation quiz related to it.

2 – Download DSM for area of interest Head over to https://elevation.fsdf.org.au/ and search for Port Melbourne in Victoria. Use the first “Bounding Box” tool on the right to select the area of study highlighted in Figure 1 below. Tip: if you want to ensure your DSM will cover exactly the same area as the DTM from previous workshops, you can use the third tool on the website to define the coordinates of the bounding box. Here you should input the coordinates of your DEM layer extent which you can access in QGIS by right-clicking on the layer and then going to Properties  Information  Extent.

FIGURE 1 AREA OF STUDY, PORT MELBOURNE Under “Geoscience Australia” select “SRTM-derived 1 Second DSM Version 3” and download the data.

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In the next window, select the Coordinate System – Geocentric Datum of Australia 2020, Latitude-Longitude, Degrees (EPSG: 7844); Output Format – Geo TIFF Start extraction of dataset and you will receive a notification in your email momentarily. Click on the link in the email to download this data. Note that the downloaded data is a zipped file. You will have to unzip and extract the files within to begin using the DSM.

3 – Opening and reprojecting the DSM data Open up QGIS and make sure to set your coordinate system once the program loads up (EPSG: 7855). Load the data from previous workshops including the DEM from last week, the manhole layer, and the coastline layer. You can include any other layer(s) from previous workshops as a reference according to your preference. Now we need to add the DSM data by going to Layer  Add Layer  Add Raster Layer, use the dialog to navigate to where you have downloaded the DSM data and load the *.tif file. We need to reproject the DSM data into the projected coordinate system that we are using for our QGIS project above (GDA 2020 / MGA zone 55 – EPSG:7855). Follow these steps to reproject the DSM layer: •

Raster  Projections  Warp (Reproject…)

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Under “Input Layer” click on “…” and select the DEM TIF file

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For “Source CRS” select EPSG:7844 – GDA2020. Click on the globe on the right if you cannot find it from the dropdown menu

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For “Target CRS” select EPSG:7855 – GDA2020 / MGA zone 55

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Keep settings as default and scroll to the bottom of the window and look for “Reprojected”

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Under “Reprojected” select “…” and pick a name and location for this new DEM file

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You can enable the “Open output file after running algorithm” option to add the reprojected layer automatically. Otherwise, you will need to manually add the layer through Layer  Add Layer  Add Raster Layer when the reprojection is finished

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Click Run

Try presenting the DEM in 10 separate classes of elevation (see Figure 2 for example). Hint: Right-click layer  Properties  Symbology

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FIGURE 2 PORT MELBOURNE DSM VISUALISED IN INVERTED SPECTRAL COLOR RAMP Also, you will notice that the DSM data in Figure 2 has some values for the area covered by sea (i.e., height is 0 m). If you wish to replace these pixels with no-data values like we had in the previous DEM, you can use Processing Toolbox  SAGA  Raster calculator. Use the formula “ifelse(a=0, 0/0, a)” and do not forget to save your output layer.

4 – Difference between DSM and DTM We will now assess the difference between the DSM layer and the DTM layer we have used in previous workshops. One way of comparing two elevation models in raster format is to subtract the values of one raster from the other and see the difference in height data between two models. Since DSM contains built objects in addition to the terrain elevations, we will subtract the values of the DTM from the values of the DSM. We can do this in QGIS with the Raster  Raster Calculator tool. Once in this tool, double click on the DSM layer under Raster Bands and then double click on the DTM layer. Now edit the Raster Calculator Expression so that your expression is equivalent to: “DSM_layer@1” – “DEM_layer@1”. Now specify where you want to save the Output layer (click the … button) and press OK to create the raster. Try to visualise the difference between DSM and DEM in 3D. Refer to the Workshop 4 instructions for a reminder on how to do this. See the Figure 3 below for an example of the visualisation. 6

FIGURE 3 THE DIFFERENCE BETWEEN DSM AND DTM VISUALISED IN 3D D ISCUSSION 1 •

What are the differences you can notice between the DSM and DTM?

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What do you think is the meaning of the red hotspots in the visualisation in Figure 3?

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Try to overlap the layer showing the DSM – DTM difference over a basemap such as OpenStreetMap or Google Maps and make it transparent. Observe the correlations between objects on the map and the values of DSM - DTM layer.

5 – Floods based on DSM Now that you understand the difference between DSM and DTM, let us see how DSM will affect the results of flood modelling. We will create the rainfall, burst pipe, and coastal flood maps using the DSM layer as our DEM. Follow the instructions below for creating each of these flood maps. (1) Rainfall flood map a. Use the Raster Calculator tool to find the inundated area after a specified level of rainfall b. For detailed steps of this process, refer to Workshop 5. (2) Burst pipe flood map a. Load in the manhole layer and specify a manhole where the flood is going to start b. Use the rasterization function to convert manhole from vector to raster format. Previous rasterized manhole cannot be used because it matches the DTM raster which had different resolution from our DSM raster. c. Use the r.lake (GRASS) function in the QGIS Processing Toolbox to create simulate a flood from a manhole. d. For detailed steps of this process, refer to Workshop 5. (3) Coastal flood map

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a. Load in the coastline layer and use the rasterization function to convert it from vector to raster format. Previous rasterized coastline cannot be used because it matches the DTM raster which had different resolution from our DSM raster. b. Use the r.lake (GRASS) function in the QGIS Processing Toolbox to create simulate a flood from the coastline. c. For detailed steps of this process, refer to Workshop 6.

FIGURE 4 BURST PIPE FLOOD MAP USING DSM

DISCUSSION 2 •

What is the main difference between the DTM and DSM flood maps you have created?

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How do results compare for similar water levels in rainfall, burst pipe, and coastal flood maps?

DISCUSSION 3 •

What are the advantages and disadvantages of each model when you want to perform a realistic flood simulation?

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C HALLENGE •

Imagine there was a tsunami that has increased the sea level by 8 meters in Port Melbourne. Create two flood maps of this event using DTM and DSM. Calculate the inundated area in both maps and compare the results. Which model gave more realistic results when you consider the expected outcomes of a tsunami with 8m run-up presented in this NZ government fact sheet (https://www.civildefence.govt.nz/assets/Uploads/exercises/Tangaroa/what-isa-tsunami-fact-sheet.pdf)?

ASSESSM ASSESSMENT ENT CHECKLI HECKLIST ST: What do we need to submit? •

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At the end of the class, make sure your team submits the following to your team’s file repository on Microsoft Teams: o

Final map(s) from this week’s workshop (pdfs or images)

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QGIS project save file (*QGZ) and any important layers used and produced. You can find the location of where each layer is located by right-clicking the layer > Properties > Information > Path

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Reflections on workshop Discussion and/or Challenge questions (pdf, no formatting required)

Each team needs to set up and maintain the following folder structure under the Files tab in Microsoft Teams: Layer 2 Folders

Layer 1 Folders Description

Workshop 2 Workshops

Folders under Workshops will contain data / information gathered during workshop classes

Workshop 3 Workshop 4 …. Meeting Minutes

Project

Folders under Project will contain all content related to the respective project component

Team Management Plan Video Presentation Final Report

This will be your active working directory for the whole semester. 9

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There is no assessment this week.

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You are encouraged to explore beyond the scope of this lab and critically reflect on what you have learnt.

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