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A simple empirical model is proposed for predicting extreme wave run-up on natural beaches during severe wave events (deep water wave heights H0 ≳8 m or return periods of about 50 years). The new model departs from traditional approaches that use the slope of the beach face βf and the Iribarren number ξ0 as parameters for predicting run-up and instead uses the distance offshore xh to water depth h to estimate a near-shore profile slope as S = h/xh, where the depth of closure is the proposed choice for h. Extreme run-up Rx is then expressed in terms of S as Rx/H0 = CS2/3. Observations from recent severe storm events in South Africa are used to estimate the dimensionless coefficient C≃7.5. The data are also compared with those of Holman [1986] and the results verify his regression equations and confirm they are valid for significant wave heights extending to 8.5 m for beach-face slopes around 0.1. The run-up predictions of Holman [1986], Nielsen and Hanslow [1991] and Stockdon et al. [2006] are compared to those of the proposed new model. The results suggest that the new model reduces the uncertainties in predicting wave run-up on natural beaches compared with previous models, and thus enables improved estimates of extreme wave run-up and the upper limit of beach change for coastal planning and management.

The tidal basin of the Kosi Bay lagoonal-lacustrine complex is dominated by fish traps that line the intertidal and subtidal environments. These have been considered significant sediment sinks, responsible for transformation of parts of the system from subtidal and intertidal sand-flat settings to mangrove habitats. Mangrove progradation has been attributed to sediment retention within the trap structures. Fish-trap counts since 1942 indicate no significant increase in trap number prior to 1980, but thereafter trap numbers have increased exponentially. Historical GIS analysis indicates a linear growth rate of mangrove areas since 1942, a trend not mirrored in the numbers of fish traps. Modelling of mangrove progradation indicates lateral extension rates per annum much lower than global figures for choked systems elsewhere. Modelling of choking thresholds for the Kosi Bay system yields a minimum date of ~2500 AD. By including sea-level rise within the model, newly formed mangrove communities may either be drowned or remain in step with predicted sea-level rises within the next 150–200 years. The role of fish traps in the conversion of subtidal environments to sedimentary environments dominated by mangroves is negligible, and this shallowing would not be of concern in light of the short-term nature of managerial timescales. Keywords: fish traps; GIS; Kosi Bay Estuary; sedimentation African Journal of Marine Science 2006, 28(3&4): 617–624

Although the importance of estuaries as ecological, geomorphological and sedimentological systems is well recognised and a number of estuarine classification schemes are referred to in literature, geomorphological behaviour of small estuaries such as those on the KwaZulu-Natal coast are not well understood. In particular the development and role of spits and bars in the functioning of small, perched estuaries needs clarification. The purpose of this study was to determine the mechanism of spit initiation and growth in the Mdloti estuary, a small perched estuary some 25km north of Durban, on the KwaZulu-Natal coast, with a view to developing an hypothesis which explains the procedure of split development and its role in the geomorphological life of the estuary. Seven generations of air photos from 1953 to 2000 were analysed, and sediment samples from four trenches and two cores located on the spit allowed recent sedimentological history to be interpreted and integrated with geomorphological development of the mouth over the last 50 years. Results showed that infrastructural developments in the upper estuary and artificial breaching of the of the mouth all had significance in determining estuarine behaviour, but that spit initiation and development after breaching was largely dependent on long-shore sediment transport. The results allowed the proposition of a sequential model for spit development for the Mdloti estuary subsequent to breaching during a flood. An offshore delta is formed by the flood and later joined by a spit bar built up by longshore drift. Ebb delta deposits are re-worked by longshore currents. As the new bar becomes emergent aeolian activity commences and the spit is gradually widened by overwash deposits, and growth continues until the estuary is completely closed off.

Although blowouts in sand dunes have been extensively studied elsewhere, little is known about those within the Holocene dune fields of the Maputaland coastal margin in KwaZulu-Natal, on the east coast of South Africa. Research in this paper involved measuring, describing and analysing wind flow patterns at a single blowout near Mabibi and related the results to the wind dynamics. From this, a preliminary hypothesis of blowout formation was made. Topographic steering through both the logarithmic spiral form of the bay adjacent to the blowout and the morphology of the dune had a significant influence on wind patterns in both pre-and post-frontal wind conditions. As the blowout widens, further channelling accelerated a jet around the headland, which would ultimately result in complete breakthrough of the dune, leaving a small isolated residual dune fragment, cut off by a mobile arc of sand. The paper concluded that such blowouts are characteristically formed under highly negative beach budgets, and that blowout formation along this coast is more likely at sites in the proximity of rocky headlands.

Although dunes fronted by sandy beaches constitute approximately 80 per cent of South Africa's coastline, few studies have addressed the formation and life cycle of coastal foredunes, the small, ephemeral shore-parallel dune ridges typically less than 5 m high and 20 m wide, which form seaward of the storm line. This study used regular, detailed topographic surveys of embryo and foredunes at Tugela mouth, an aggrading stretch of shoreline on the subtropical east coast of South Africa, over a 32-month period, to gain insight into the formation and motion of these highly mobile landforms over the short term. Average wind drift potential at Tugela mouth during the study period, at 2·35 m s−1, was an order of magnitude lower than that typical of most parts of the eastern South African coast. The dominant sand-moving wind for the region was from the southwest to west-southwest at 10·7 to 13·8 m s−1, with a secondary vector from north to north-northeast at 10·8 to 13·8 m s−1. Significant shoreline retreat, a result of the low sediment yield of the Tugela River during the study period, was one of the main results. This provided the context for redistribution of sand from the inland to the seaward side of the study area, a consequence of the dominant wind direction, and for frequent creation and destruction of short-lived embryo dunes. Those foredunes which survived the whole study period tended to increase in height, but there was no consistent directional trend in foredune crest movement throughout the 32 months. The study results generally supported Psuty's model of foredune development, but could not confirm his contention of landward retreat of dune forms under conditions of shoreline erosion. This may be due to the relatively short duration of the study, or possibly to low wind drift potential at the site. Copyright © 2003 John Wiley & Sons, Ltd.

The paper traces the major themes in southern African geomorphology from its early roots in both geography and geology at the start of the twentieth century through to the present. The development ...

Completed in 1988 at a site some 32 km from the Mgeni river mouth, the Inanda Dam may have influenced bed load transport and deposition. The aims of this study were to ascertain whether the dam had affected fluvial sediment delivery to the lower channel, whether the sediment characteristics and bedform of the estuary had been modified and, within this context, to predict the likely effects of continued sand mining on the Mgeni estuary. Re-examination of estuarine cross sections originally surveyed prior to dam completion, comparison of present estuarine sediment characteristics with those of pre-dam sediments and a bottom survey of the Inanda Dam itself were key elements in the analysis. Mean annual discharge of the Mgeni river has reduced by 4% since dam construction and changes in discharge regime caused by the water release policy resulted in an increase in competent discharge events. Retention of sediment behind the dam wall had led to reduction in available downstream sediment. Sediment volume in the estuary had not changed significantly since 1989, although sediment calibre is now much finer. These changes will probably signal the end of sand mining in the lower river and will ultimately cause the estuary to deepen as residual pre-dam sediment is flushed through and not replaced by new material from upstream. Since Mgeni sediment is a source of material for natural beach nourishment, it is likely that beaches will become narrower.

Hazelmere Dam, located on the Mdloti river 12 km from the KwaZulu Natal coast, was completed in 1975. Since then it has lost more than 25% of its original design capacity through siltation, a fact which could threaten its economic life. This has occurred partly because of high annual average sediment accumulation rates under normal climatic conditions, and partly as a result of the 1987 floods, an extreme event with a return period of between 50 and 200 years. The objective of this work was to indicate why sediment production rates per km- of catchment area as indicated by dam siltation were higher than in most other KwaZuku Natal dam catchments. Basic rainfall, soil and slope conditions cause the Mdloti catchment to have a high erosion potential, which has been exacerbated in recent years by large scale population increases, vegetal degradation and road construction activities. These together created conditions for massive sediment transport during the 1987 floods. In terms of the theories of fl...

A 3.55 km2 catchment in Orangekloof Nature Reserve, Hout Bay, here conceptualised as an assemblage of landforms, is valued with respect to its ability to attenuate flood discharges derived from pre...

Topography, geology and a humid, sub-tropical climate make the Durban region on the east coast of South Africa particularly prone to sliding mass movements. Records of individual landslides and landslide events between 1970 and 1990 were compared with 22 rainfall variables calculated for the same period. Correlation coefficients demonstrated statistically significant relationships between annual and wet season landslide frequencies and maximum 3-day and maximum monthly wet season rainfall, whereas precipitation for shorter and longer periods was unrelated. Envelopes of critical values for cycle and total coefficients based on the percent of mean annual rainfall were identified for both the timing and magnitude of landslide events. They suggest a two-stage equilibrium condition for the rainfall-mass movement relationship, controlled by average wet season rainfall. Moreover, once the pattern of rainfall for any particular wet season is established, the envelopes allow the prediction of rainfall conditions necessary for landslides to occur. The likelihood of an inaccurate prediction is approximately 10%.

Stereoscopic air photos from 1944/5 and 1976–81 were used to conduct a temporal comparison of area affected by erosion in the Tugela catchment. The results show that the amount of eroded terrain had decreased with time. There was no significant difference in eroded area between KwaZulu and Natal in either time period, and both administrative regions showed a decline in eroded area from 1944/5 to 1976–81. This and other relevant studies indicate a decline in erosional activity in the area. This may be due to land use changes or improved conservation techniques, but could also result from medium or long term changes in variables in trinsic to the erosion system.

The tidal records of existing South African and Namibian tide gauges are examined and corrected. Regional sea level trends vary, with the West Coast rising by +1.87 mm y–1 (1959–2006), the southern coast by +1.48 mm y–1 (1957–2006) and the East Coast by +2.74 mm y–1 (1967–2006). The effects of barometric pressure and vertical crustal movement changes on these trends are examined. The derived relationship between sea levels and barometric pressure changes varied between 5.71 and 7.67 mm hPa–1, significantly less than the theoretical inverse barometric correction. Barometric pressure has been dropping along the west coast at 1.63 hPa per decade (1987–2006), has remained fairly static along the southern coast and is rising at 0.30 hPa per decade (1970–2007) along the east coast of southern Africa. The West Coast barometrically corrected sea level trends show that most of the change can be attributed to falling barometric pressure, whereas along the East Coast, the barometric pressure increase is suppressing sea level by 0.2 mm y–1. Vertical crust movements vary, with the largest recorded movements of +1.11 ± 0.25 mm y–1 found along the East Coast. Movement rate reduces southwards. Eustatic sea level trends vary from +3.55 mm y–1 along the East Coast and +1.57 mm y–1 along the southern coast to +0.42 mm y–1 along the West Coast. Keywords: barometric pressure; sea level rise; tide levelsAfrican Journal of Marine Science 2009, 31(2): 145–156

One of the most pressing problems facing wilderness managers in the ecologically fragile Drakensberg Mountains, South Africa, is that of path erosion, since it detracts from the wilderness experience and is very costly to remediate. As increasing demand for wilderness and outdoor recreation places greater pressure on paths and trails, it will be necessary to increase path network size and capacity, and it is important that new paths are routed such that maintenance requirements are minimal.

Design of successful and cost-effective beach nourishment schemes requires a sound understanding of the hydraulic character of both borrow material and native beach sand. Techniques for assessing borrow material compatibility exist, but have proven inadequate in some situations. Most methods use a particle size parameter to represent sand transportability, although hydraulic theory and the nature of post-erosional lag deposits on Durban beaches suggest that mass density may also be a factor of importance. Results of a flume test on natural and artificialy constituted samples of Durban beach sand showed that transport of sand particles was unrelated to median particle size, but inversely correlated with sample mass density. The study concludes that where beaches consist of mineralogically heterogeneous material, a density factor should be included in any attempt to assess sediment transport rates and borrow material compatibility.

Wave run up is important for quantifying risks to infrastructure in the coastal zone. The performance of global wave run up models are assessed by applying them to two significant storms along the South African coastline in 2007 and 2008. The models produced mixed results and therefore the development of a new wave run up model was undertaken. This model uses the distance offshore to a point on the bathymetric profile, located approximately at the cut off depth, as a proxy for the underwater beach profile. This new wave run up model has been calibrated for open coastlines as well as large and small embayments. The new model outperforms most of the current wave run up models and gives a good first order approximation of wave run up on natural beaches.

Land erosion in wilderness, conservation, and recreational, areas may be accelerated by certain non-agricultural land management practices. Examples from the Natal Drakensberg are used to demonstrate how certain types of land modification—such as burning, the construction of unpaved access roads, and the creation of paths and tracks—may result in rapid acceleration of erosion rates.

Despite frequent occurrences of slope failure in Berea red sand, it is commonly used as fill or embankment material in the Durban area. An investigation of a small rotational slip in this material showed that it could be explained in terms of rainfall, soil moisture content and soil properties.

Although Land Systems mapping is one of the most important aspects of applied geomorphology, its value to civil engineering projects is questionable at scales greater than 1 : 10000. This paper describes two examples of reconnaissance engineering geomorphological maps suitable for larger scales. The need to adapt cartographic representation to the -requirements of the project and the physical nature of the terrain is stressed, and it is concluded that photo-interpretation is a particularly useful tool in this kind of work, since it reduces time spent in the field.

In a three-year study at Kamberg Nature Reserve, measured annual rates of soil loss from an experimental path showed that under some circumstances path erosion can exceed soil loss tolerance and mean annual sediment yield from the Tugela river cachment, in which the experimental site was located. Further, burning of vegetation adjacent to the path caused a disproportionate increase in rates of soil erosion, and quantities of soil eroded were significantly related to some rainfall parameters. The study concludes that design of path networks must take burning regimes and spatial variation of rainfall into account if path maintenance costs are to be minimized.

Recent reviews and textbooks show that South Africa has contributed very little to the field of soil erosion research. Strongest areas of research are the study and estimation of sediment yield. and the evaluation of rainfall erosivity. whilst major weaknesses include the lack of knowledge of the nature and extent of erosion. and a poor understanding of the mechanics of erosional processes. General co-ordination of research policy is required if research is to make a significant contribution to the solution of problems caused by soil erosion.

Footpath erosion in mountain wilderness areas is generally undesirable, and maintenance and rehabilitation of eroded paths are costly. Although path erosion results from a complex interactioin between recreational and geomorphological forces and each path's resistance, the processes involved are not well understood.

Changes in land-use can have an important effect on rates of erosion and denudation. In order to avoid accelerating erosion rates, decision makers in land-use planning require adequate information on the contemporary geomorphological processes and hydrology of areas where modifications in the land-use pattern are envisaged. The first phase in the acquisition of the information is the reconnaissance survey, which shows where and how the erosional status of an area is likely to be affected, and also acts as a foundation on which to base more detailed work in the future.The Solitude area is an active erosional system, dominated by mass movement and fluvial processes. The rate of removal of material is likely to be increased by modifications in land-use which, from other points of view, would be considered entirely suitable for the area. Therefore, if accelerated denudation is to be avoided, projects involving land-use changes should be implemented only by those having an adequate understanding of the mechanism of geomorphological processes operating in the area.