Coastline dynamics at Thukela Mouth and the Amatigulu and Umlalazi river estuary areas KwaZulu-Natal

Abstract

The objectives of the study were to observe, record and determine coastal dune changes in northern KwaZulu‐Natal. The areas considered in the study were the Thukela River mouth (29° 13’ 29.05” S; 31° 30’ 17.58” E), the Amatigulu‐Nyoni estuary (29° 06’ 44.25” S; 31° 36’ 52.23” E) and the Umlalazi estuary (28° 56’ 41.13” S; 31° 48’ 56.63” E). The study focused on survey data and the movement of the relevant dunes adjacent to the coastline to determine whether these dunes were moving in a certain direction and whether there was an increase or decrease in the dune area and/or volume. South Africa’s entire coastline is subject to the force of high‐breaking waves on the beaches (littoral), cliffs and shores, tides, wind (aeolian) and runoff from rivers. The estuaries on the KwaZulu‐Natal North Coast consist of sandy barriers in the form of wave‐built sandy ridges that run parallel to the coastline. The barriers are relatively shallow and water can flow easily between the sea and the estuaries. Some of the dunes studied in this research, such as those in the Amatigulu estuary, have approximately 10% vegetation growth on them and are thus stable, but the remainder of the dunes along this coast are not vegetated at all. The movement of the dunes was monitored using sophisticated land surveying instruments, such as Global Positioning Systems (GPS) and total stations. Assessments were done with the help of survey instruments and land surveying calculation methods, in combination with the relevant surveying programs and Geographic Information Systems (GIS) to obtain area and volume results over several years. For example, a baseline was measured at the Umlalazi beach using a total station, and all the surveys that followed were done using GPS instruments, and these surveys were based on and connected to the initial survey. The Umlalazi assessment used three methods. The first was comparing old aerial photographs to how the area appeared at the time of the study. The second was GIS methods to geo‐reference aerial and satellite images showing the movement of the dunes at the Umlalazi beach site. The third was establishing a fishnet grid to break down the already surveyed data to yield a more accurate database, containing three times more data than the original database. With the help of aerial photographs, geomorphological changes could be determined for a period of 70 years for the Umlalazi beach site. This comparison was undertaken for practical reasons to check the movement and stability of the dune and to assess the degree of geomorphological change. These methods measured only relative changes or movements over time. To survey and understand dune movement better, a solid and very reliable survey network was established around the dunes. These networks were established using different surveying methods: GPS, remote sensing, triangulation, traversing, scanning, as well as light detection and ranging (Lidar). A review of the scientific reasons for dune movement provided a better understanding of the amount of movement that takes place, which helps establish prediction methods. Findings from the assessment of the Thukela River showed, firstly, that the southern and northern beaches at the Thukela River mouth varied only slightly in size and in volume and, secondly, that the southern beach seemed more stable than the northern beach. Both changed in shape, but not much in size or volume. At the Amatigulu estuary, a full survey was done in 2016, and aerial photos from 1953 and Google Earth data up to 2017 were also used. From the survey data, a typical sine graph could be plotted of the changes in size and volume for the calculated areas. At theUmlalazi beach, physical surveys were undertaken during 2016 and 2017. The percentage change in size and volume was positive in 2016, and negative in 2017. It can therefore be assumed that over time, all barriers vary in size, shape and volume, and that whatever is eroded, will build up again to maintain the cycle between beaches, oceans and rivers.