A.M. Smith

Vertical stacking of multiple highstand shoreline deposits from the Cretaceous to the present: facies development and preservation

ABSTRACT Cooper, J.A.G., Green, A.N. and Smith, A.M. 2013. BARDEX II: Vertical stacking of multiple highstand shoreline deposits from the Cretaceous to the present: facies development and preservation. A sequence of vertically stacked shoreline facies exposed by unprecedented water level lowering in Lake St Lucia, South Africa, records multiple occupation of the same shoreline (5–6m amsl) on at least eight occasions since the late Cretaceous. The sequence involves a basal wave-cut surface that is the outcrop of a regional unconformity cut into Late Cretaceous siltstone with occasional borings, representing a hardground (Facies 1). This is succeeded by a limestone unit indicative of sedimentation in a region of low terrigenous input quite different to today. This commences with a 10cm-thick unit comprising corals and giant clams that colonised the hardground as a shallow reef (Facies 2). The reef has an erosional upper surface that is overlain by a 30–50cm thick coquina (Facies 3) with characteristic sand-lined branching burrows, representing a coarse clastic beach unit. This is equated with the Uloa Formation of Miocene/Pliocene age. This unit has in turn been colonised by a patchy development of a coral reef of a single species, representing a renewed phase of reef development (Facies 4). The reef and the underlying Facies 3 have been waveplaned and eroded to form an erosional rocky shoreline with small potholes on a shore platform. The potholes are encrusted with barnacles and oysters to form a distinctive unit (Facies 5). The oysters and barnacles are encrusted with red algae suggesting a slight subsequent rise in sea level which is also associated with the formation of an erosional notch and a higher level shore platform with several small erosional gullies (Facies 6). These gullies are in turn encrusted by thick accumulations of serpulid worm tubes (Facies 7) into which two subsequent notches have been cut by wave action. The shorelines preserved represent a succession of sea level highstands within a few metres of the contemporary sea level since Late Cretaceous times. They survived intervening sea level lowerings and fluvial incision by virtue of their location on an interfluve between adjacent incised valleys. Early cementation would have also been key to their preservation. Each shoreline facies was in turn influenced by the antecedent conditions imparted by the preceding shoreline as well as the contemporaneous conditions of sediment supply, sea level change and the surrounding palaeogeography. The presence of limestone and the absence of clasts or storm beach deposits suggests a protected coastline. The intermittent occurrence of coral and the reduced coral assemblage suggests that the water may not always have been fully marine.

Forecasting lagoon outlet erosion: KwaZulu-Natal, southeast Africa

ABSTRACT Smith, A.M., Guastella, L.A., Goble, B.J., 2014. Forecasting lagoon outlet erosion: KwaZulu-Natal, southeast Africa. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 151–156, ISSN 0749-0208. The Amanzimtoti Lagoon is at the mouth of the Amanzimtoti River. The lagoon outlet changes systematically in response to the 18 year long Dyer-Tyson summer rainfall cycle (DTC). During the dry part of the DTC the lagoon is generally closed, opening only during heavy rainstorms. When closed the lagoon is separated from the sea by a baymouth bar, varying from 30 to 60 m wide. The open-days per year have increased dramatically as the DTC has entered the wet phase. Early in the DTC the lagoon rarely opened, and when this occurred it breached near the centre of the headland bound bay, but as the DTC-driven runoff cycle has progressed the outlet has remained open. However, during this open phase the outlet has been forced southward along the back beach until it deflected seaward by the southern headland outcrop. This appears to be brought about by longshore drift and an increased beach width. We suggest that this is due to the shoreward reworking of sediment by waves. This sediment has been brought down by the river in increasing amounts as the wet part of the DTC waxes, strengthening the seasonal rainfall and the river flow cycle. Wave reworking has forced the outlet along the back beach causing erosion of the coastal dune behind the back beach. Thus, the lagoon mouth enters a predictable erosion phase as the DTC progresses. This cyclicity can be used to forecast this type of lagoon outlet erosion. The Amanzimtoti Lagoon has strong similarities to the Hapua Lagoon-type recognized from New Zealand.

Cliff-top storm deposits (55-63m amsl) from Morgan Bay, South Africa

ABSTRACT Smith, A.M, Green, A.N, Cooper, J.A.G. et al., 2014. Cliff-top storm deposits (55-63m amsl) from Morgan Bay, South Africa. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 349–353, ISSN 0749-0208. Cliff-top storm deposits (CTSDs) occur south of Morgan Bay, South Africa at elevations varying from ±55- to 63m. These occur as a ±10m-wide horizontal fringe of shell breccia mixed into a very thin (>15cm) sandy soil on the cliff top platform. Visually it is evident that the shell breccia is of various ages. Comparisons with proven wave breccia from this area indicate the same source. We suggest that this breccia was deposited as fall-out from wave and wind-borne plumes produced by wave bores striking the cliff base. These bores would likely have been produced by waves at least ~40m in height. Alternative interpretations, such as a perched marine desposit or tsunamiite are rejected in favour of CTSDs produced by multiple large wave events.

Coastal management and mis-management: comparing successes and failures at two lagoon outlets in KwaZulu-Natal, South Africa

ABSTRACT Guastella, L.A., Smith, A, M., Breetzke T., 2014. Coastal management and mis-management: comparing successes and failures at two lagoon outlets in KwaZulu-Natal, South Africa. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 513–520, ISSN 0749-0208. KwaZulu-Natal (KZN) has a high energy, dynamic coastline. The coast is a significant asset, utilised for residential, industrial, transport, nature conservation and recreational purposes. It is also the discharge point for many steep gradient, short-headed rivers. These rivers have highly variable discharges and, together with variable coastal erosion and deposition cycles, place stress on coastal resources and infrastructure. Many of the rivers, particularly those with smaller catchments, discharge into back-beach lagoons before discharging into the ocean. Lagoon outlet dynamics are variable and, when combined with ocean swells and/or river flooding, can cause erosion. Coastal managers need to be aware of the risks to coastal assets and to this end, informed decision-making is vital to ensure sustainability of coastal resources and protection of coastal infrastructure, albeit often inappropriately located. While a strategy of non-intervention of lagoon outlets is always preferable, there comes a point when intervention may be necessary to protect coastal infrastructure, particularly if the intervention can replicate natural processes. Fixed coastal webcams, backed up by on-site photography, are used to illustrate two contrasting examples of coastal management on the KZN south coast at: (a) Margate where intervention resulted in the saving of coastal infrastructure, and (b) Amanzimtoti where a lack of intervention resulted in a train derailment, loss of infrastructure, injuries and significant repair costs. This paper critically reviews these contrasting examples and concludes that a greater understanding of lagoon outlet dynamics can significantly contribute to improved coastal management practices, and in this instance, could have prevented this derailment.

Failed Coastal Stabilization: Examples from the KwaZulu-Natal Coast, South Africa

Coastal stabilization, mostly involving coastal dune cordon destruction, has been practised on the KwaZulu-Natal coast for the last two decades. During this time no major swells occurred. The last Lunar Nodal Cycle (LNC) tidal peak was associated with a series of unusually high swells. The temporal coincidence of exceptionally high tide and large swell led to catastrophic coastal erosion and underlined the folly of coastal stabilization, especially at reversing erosion hotspots, many of which are urbanized and defended. These locations remain at risk and will experience future erosion.