Drone Surveys.
In an effort to better understand the dynamics of the open / closed status of blind estuaries in the Garden Route, Strandloper Project has been conducting drone mapping surveys of three estuaries, namely the Touw, Swartvlei and Goukamma estuaries.
Climate Change and Agriculture
In the past 30 years the open / closed cycle of the Swartvlei Estuary Mouth has changed from an open phase of between 12 and 16 mouth and a closed phase of 5 to 8 weeks to an open phase of 3 to 5 months and a closed phase of 3 to 10 months. In the same period the mean annual rainfall has decreased from 580mm to 480mm.
In the same period, agriculture in the Swartvlei catchment has transformed from predominantly dryland grazing and some vegetable production to center pivot irrigation of GMO crops and the establishment of Macadamia and Avocado orchards.
Sand Bar Management
Both the Touw and Swartvlei estuary mouths are managed by SANParks and are artificially opened when they reach a specified height above sea level. Preparation for the manual breaching of the sand bar involves excavating a channel and leaving a sand plug on both the seaward and estuary ends of the channel which can be easily opened when the water level rises to reach the trigger height above sea level.
Methodology
Survey missions of the estuary mouths are planned and conducted using Dronelink and flown using a DJI drone. Captured images are processed using WebODM and a geo referenced raster file is produced, which is then analyzed for channel metrics in QGIS.
Sand Bar Genesis
Our initial surveys of all three estuaries indicate that the development of the sand bar follows a process that is driven by a combination of water flow into the ocean and the deposition of sand from the inshore counter current onto the western banks of the estuary mouth.
The sand bar forms in a sequence of sand deposition, culminating a perpendicular wedge on the western bank that narrows the channel, and when the flow rate decreases, closes entirely.
Interim Results
When the study was initiated in August 2022, all three estuaries where closed. In the third week of December 2022, the region received heavy rains which resulted in the Touw River mouth to be manually breached by SANParks and for the Goukamma Estuary Mouth to breach naturally.
By contrast, the water level in the Swartvlei Estuary subsided by approximately 100mm and continued to subside till mid February. From mid February the water level rose and on the 20th February 2023, the sand bar was manually breached by SANParks.
Touws River : January 2024 to July 2024
The Touws River mouth closed in the first half of January 2024, then breached on the 4th April 2024, partially closed on the 3rd June, then breached on the 4th June and closed again in the first week of July 2024.
Swartvlei : August 2023 to August 2024
The sandbar breached on the 25th September 2023 after heavy rain, remaining open till the 15th August 2024. Following strong winds and large ocean swells, large volumes of sand were deposited in the channel upstream from the sandbar. |
