Drone Mapping – Goukamma Estuary Mouth

Introduction

In the garden route of South Africa, most of the rivers are blind rivers, being closed by a sand bar during periods of of low river flow. Between Wilderness and Knysna, there are three blind estuaries, namely the Touw Estuary, Swartvlei Estuary and the Goukamma Estuary. The latter breaches through natural events while the Touw and Swartvlei estuaries are managed and manually breached.

To better understand the dynamics of the channel formation pre and post breaching, monitoring using drone mapping has been initiated.

Methods

Using the Dronelink application, a transect was used to plan an automated transect mission using a DJI drone. After a mission was completed the images were stitched using WebODM software and exported as a rasterfile which was imported into QGIS for analysis of channel with and sequential location. A JPG images was then exported with the date and a scale bar.

Images

Figure 1 : The first image was not calibrated for the SE portion of the survey area. The survey was subsequently modified as can be seen in the latter images. The black lines are reference lines from the dune east of the mouth on the 19th February 2023.

Figure 2. The extent to which the water rose can be seen clearly. When zoomed in, it is possible to see where an attempt was made to dig a channel using a spade.

Figure 3 .The extent to which the dune east of the mouth eroded can be seen clearly. There is also evidence of beachgoers setting up on the eastern bank of the mouth which created a disturbance to the gull colony.

Figure 4 : The reference lines indicate the erosion of the dune between 23rd January 2023 and 19th February 2023.

Figure 5 : 12th March 2023 – the channel has shifted west and the mouth deflected east.

Figure 6 : 28th March 2023 – the deposition of sand on the western side of the mouth has increased in size and the width of the exit channel has begun to narrow.

Figure 7 : 12th April 2023 – the size of the sandbar has grown 47m in the past 2 weeks and the mouth channel has deflected slightly south west.

Figure 8 : 28th April 2023 – the sand bar has growth an additional 34m and sand has been deposited on the northern side of the channel. The mouth has constricted and indicates that the mouth will close shortly.

Figure 9 : 1st may 2023 – the mouth has closed and the estuary water level has commenced to rise as a result of the inflow of water from the catchment area.

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