African Wildlife and Environment Issue 66

DESTINATION

DESTINATION

movement south of around 220m, in the process washing away a pedestrian bridge which had joined the two sides of the estuary. At this time the barrier dune behind Lookout Beach was some four to six m high and had become heavily vegetated, primarily with invasive Port Jackson ( Acacia ), but also some indigenous plants. This growth can be seen in the December, 2004, picture in the collage, and prompted the belief that it appeared impossible that the beach could be eroded away. The first GPS survey was completed in December, 2005, and as a first effort was limited to low tide and a section of Lookout Beach. In August, 2006, a large flood removed almost 200m of the barrier dune with its supposedly resilient vegetation. It also removed a toilet/shower structure at the end of a path to the beach. These changes are depicted in Figure 2. After the flood the estuary mouth exited obliquely at the Lookout Rocks, and at high tide Lookout Beach was non-existent. Over the next year sand arriving from

It is estimated that more than 500,000 m 3 of sand was removed or displaced. Moreover, Lookout Beach was no more. These changes are depicted in Figure 3; the 2007 photo in the collage shows the extent of the wash-away. The southern end of the barrier dune was deflected seawards, and over the next few months this moved shorewards under the influence of prevailing waves. Unfortunately this exposed the whole section of shoreline from Milkwood Manor to the Lookout Rocks directly to ocean waves. This included the parking area, and in spite of the erection of coastal defensive measures such as gabions and sand bags by July, 2008, much of the parking area and the toilet/ shower facilities had been destroyed – see Figure 4. This also severed the connection between Salmack and Hill Streets. Over the next few years the ebb flow in the estuary continued to erode the southern section of the estuary, and the barrier dune consolidated its position, extending all the way from its northern origins. However, the flow also moved west before turning and exiting past the Lookout Rocks. In doing so, it started eroding the western bank in the Poortjies area, and there were fears that adjacent municipal infrastructure could be washed away. Then, over the four days 12 to 15 July, 2012, over 170mm of rain was recorded in the catchments and the estuary again changed dramatically. The existing channels could not cope with the resulting floodwaters which flowed straight over the barrier dune. An overwash more than one km north of Lookout Rocks proved to be a weak point and the dune was breached, releasing the pressure of rising floodwaters. Figure 5 is a photo taken at 5:10pm on 14 July, at about the time the breach occurred. By the next morning the waters had subsided and the new mouth was clearly evident. Figure 6 shows the sediments in the estuary some 3 or 4 days later: the tortuous exit channel past Poortjies to the oldmouth at Lookout Rocks is evident. The remaining flood waters and tidal exchanges were taking place predominantly through the wide new mouth. Over the next few months it became clear that the flood had been sufficiently big to scour out a new mouth through the barrier dune which could dominate the old mouth. Sand gradually started closing the channel at the Lookout Rocks, and a year later it was only knee-deep at low tide. By February, 2013, the channel was completely closed at low tide. This meant that Lookout Beach was again accessible to the public. A picture of the beach in July, 2014, is shown in the lower picture of the collage. As expected, the basic orientation and shape of the beach is the same as it

sand in the north. The barrier dune extends between these two limits, while the Keurbooms Estuary discharges its waters into the Indian Ocean through a mouth which splits the dune into a northern and southern section. Lookout Beach, extending off the Lookout Rocks, forms the southern section of this barrier dune. However, the actual position of the mouth can vary over the whole length of the barrier dune. It is determined by past history, vegetation, wind, waves and floods, as well as available sand. This sand, in excess of 100,000 m3 per year, is moved northwards in the surf zone by ocean waves. Some of it moves past the mouth, while some is also moved into the estuary by waves and tides. Historical accounts and drawings show the mouth in various positions along the length of barrier dune. It is thought that around 1900 it was about 1 km north of the Lookout Rocks, but by 1915 it had moved south right to the rocks. At that stage a large flood breached the northern end of the barrier dune, and the southern mouth subsequently closed. Records show that over the next 20 to 30 years the mouth hardly moved, but then between 1960 and 1980 it was situated about two km north of the Lookout Rocks, moving north and south over the years. In April, 1982, a large flood caused a sudden Figure 1. Geology of the Keurbooms Estuary based on the work of JSV Reddering (1981) The sedimentology of the Keurbooms Estuary. MSc Thesis, University of Port Elizabeth.

Figure 3. Estuarine morphology before and after the November, 2007, flood.

the south built up the low tide beach again, and even at high tide there were pockets of Lookout Beach available. In the estuary itself the ebb channel moved slowly southwards, limited by the mud and bioturbation structures built up in the backwaters of the lagoon since the flow was last in the area more than 100 years ago. At low tide the outflow turned right around as it was forced northward by the accumulating sand from the south. However, a massive flood hit the region over 21 to 23 November, 2007, with over 350mm of rain falling in the catchments of the Keurbooms and Bitou rivers. The ebb channel simply could not cope with the volume of water which took out the remaining dune bush and most of the barrier dune. Part of the parking area was washed away, and a rock revetment was quickly built to protect Milkwood Manor (MWM); fortunately it was protected by upstream rock sills exposed by the rampaging waters.

Figure 2. The morphology of the Keurbooms Estuary before and after the August, 2006, flood. Sediments at high and low tide are depicted, as well as a rough vegetation distribution – the colour representation is given in the list.

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