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 Ottawa, ON. This analysis is particularly relevant due to the high number of flooding events and high-rainfall in Ottawa, especially in 2017. 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. The software that will be used is QGIS (ver. 2.18.10) as well as ArcMap (ver. 10.5.1) solely for preliminary data processing.

Data and Software Requirements

In order to begin this tutorial a download of the QGIS software is needed. The current version is QGIS 2.18.15 'Las Palmas' and was released on 08.12.2017. QGIS is available on Windows, MacOS X, Linux as well as Android. This can be found at QGIS Download

Moving forward, the elevation data that will be used for this tutorial can be found from this link. For the purpose of this analysis, 4 adjacent elevation tiles of the city of Ottawa were downloaded, which make a 4-tiled square. Be sure to download the files as shapefiles, not DEMs.

You may use whatever additional base layers you might like to add to this tutorial, but for this purpose, only roads, buildings, and water body data is used which can be found here

After all the data is downloaded, you may proceed to process the files in ArcMap.

ArcMap Processing

In order to create the proper DEM file needed to process the desired outcome of this tutorial, the four grid files previously mentioned must be loaded into ArcMap and then processed into DEMs using the Spatial Analyst tool: Inverse Distance Weighted (or IDW for short).