GIS is fun! At the very least, it becomes fun if you have time to develop your skills around the required techniques and then acquire the capacity to apply these techniques to fields/areas of study which are of interest to you.
Lest I forget, GIS stands for Geographical Information System, and it relates to the application of Information Technology (IT) to data which are spatial in nature, that is data which contain some form of coordinates which allows for the data to be positioned at specific locations on the planet. GIS can either be standalone software such as the public map applications found on your mobile phone or can be embedded in other types of software or web pages such as those who provide the location of shops, restaurants etc …
But obviously, the purpose of this article is not to talk about those public applications but rather as provided in the title to present a specialised application of GIS to the study of stream flows. This should be of interest to all those concerned with the subject of recurrent flooding in Mauritius. The information provided is visual in nature and has for sole objective to educate the general public on those issues affecting their houses and livelihoods.
Before we continue, it is important to understand 1.) the process used to reach the results which are presented herein and 2.) the assumptions used and inherent limitations of this process. These are presented in bullet form below:
1.All data used in the study are publicly available on the web (as the saying goes: “Google is your best friend” nowadays). Both the US and the EU have implemented open data platforms where satellite data can be downloaded and used for free.
2.A DEM (Digital Elevation Model) is a file containing data about the topography of a surface and is produced by different technological means (satellite or nowadays LIDAR technology). DEM can either be produced for bare terrains or include objects such as trees or buildings. The quality of a DEM is determined among others by its resolution. Publicly available DEMs for Mauritius have been produced through satellite technology and have a granular resolution of 30m per pixel size at best. This is largely enough for the study being presented. It is accepted though that a higher resolution DEM would provide more accurate results especially in coastal areas.
3.Traditional DEMs produced from Satellite Technology (to the contrary to those produced by LIDAR) struggle to map topography of land close to water bodies (such as reservoir, lakes, coastal areas). In the case of this current study, the limitations were more pronounced around the coastal areas of Mauritius especially Port-Louis and the Northern Plains.
4.A colorised DEM of Mauritius used for the study is provided here (source Copernicus, EU):
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5.A Watershed area / Catchment area / River basin / Drainage basin are different terms used to represent an area of land whereby rain water (surface runoff) flow on the land surface via streams, rivers and lakes to the sea. source: https://www.rcrcd.org/what-is-a-watershed
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6.Based on the above Mauritius DEM, the drainage basins of our island can be delimited as follows: The sheer amount of drainage basins for a small island bears testimony to the volcanic origin of the island and to the complexity of land management without impairing the surface flow of water. Note that the small drainage areas around the coastline are due to the DEM limitations mentioned above.
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7.A stream network is a group of streams within a drainage basin that are connected to one another. They allow rainwater flowing at land surface to reach the sea. It starts with small streams at the higher elevations of the watershed which become larger as they join while flowing downwards. The number of connections from small streams to the main river constitute the stream network. Illustration is provided of the Rivière des Galets stream network below:
8. Based on analysis of the DEM through GIS software and specialised modules contain therein, the stream network of Mauritius has been established. Note that 1.) most streams are ephemeral in nature and become alive only in case of heavy rains causing surface runoffs. 2.) it is beyond my competencies to explain the full characteristics of the stream network of Mauritius and would instead recommend the excellent book “Mauritius A Geomorphological Analysis” by Prof Prem Saddul (starting Pg 187). Extract from the book: “Most drainage basins in Mauritius are quite small compared to those in other parts of the world and they have little capacity to store rainwater. As a result, they have high peak flows with a sharp rise and quick recession after a rainfall event. The total volume of water per event is usually small, so the time required for water to flow through the drainage basin is very small. Therefore, during extreme rainfall events, … one of the chief hazards in Mauritius is flash floods.”
9.Looking at each catchment area from different perspectives already provides a jolly good idea of why some localities are heavily impacted by flash floods. Further analysis and calculations would provide a precise idea of the extent of flooding, but this is beyond the scope of this current article (it may be provided at a later stage though based on the availability of required data). Illustration below of Chemin Grenier (1st image is a colorised DEM indicated the slopes from Green to Blue (sea level) and 2nd Image shows the position of inhabited areas): (note the illustration includes smaller basins in the Chemin Grenier area).
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10.The analysis does not take into account man-made modifications of natural stream flows. In most cases and owing to the Mauritian topography, these modifications are bound to create havoc.
11.The accuracy of the model can be determined by the fidelity with which it has mapped known rivers.
It is even better to look at these streams in a 3D perspective. Since this is very time-consuming, 4 drainage basins are proposed through the following link :https://lemauricien.com/3d/streamflow/
I.The Insoluble Problem of Chemin Grenier
II.The Obvious man-made issue of Grand Baie (and Fond du Sac)
III.The Exclusive water path from Nouvelle France to Plaine Magnien
IV.The Lackadaisical urbanisation of Terre Rouge area