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The Sodwana reef system experiences short-term temperature fluctuations that may provide relief from bleaching and be crucial in the future survival of the system. These temperature fluctuations are best described as cold water temperature anomaly events that occur over a period of days and cause a drop in temperature of a few degrees on the reef. We explored the statistical link between the temperature anomalies and the regional hydrodynamics to elucidate the driving mechanisms of the temperature anomalies around Sodwana. Temperature measurements taken between 1994 and 2015 on Nine‑Mile Reef at Sodwana show that temperature anomalies occur on average three times per year at Sodwana and predominantly during the summer months. A conditional average of altimetry data at the peak of the temperature anomalies showed the emergence of a negative sea surface height (SSH) anomaly pattern and associated cyclonic eddy just offshore of the Sodwana region. The cyclonic eddies associated with the temperature anomalies originate on the southwestern edge of Madagascar and migrate westwards until they interact with the African coastline at Sodwana. Instantaneous altimetry SSH fields over the 21-year period were cross-correlated to the conditionally averaged SSH field within a 2° region around Sodwana. It was found that 33% of the temperature anomalies at Sodwana were not associated with the presence of cyclonic eddy systems. This finding suggests that an offshore cyclonic eddy interacting with the shelf is not the sole driving mechanism of the temperature anomalies. Significance: Cold water temperature anomalies that occur at Sodwana are believed to play an important role in the sustainability of the coral reefs; however, their driving mechanisms are not yet well understood. We identified the annual frequency and seasonal occurrence of the temperature anomalies at Sodwana. Conditionally averaged sea surface heights delineated regional hydrodynamic patterns associated with the temperature anomalies. These hydrodynamic patterns could be a possible driving mechanism to be explored in future research. Previous research associated these temperature anomalies with a single regional hydrodynamic pattern around Sodwana; however, additional hydrodynamic patterns associated with the temperature anomalies have been found based on the length of the data sets used in this study.

Many studies present hybrid algorithms to solve multiobjective uses of a reservoir. The reservoirs presented in this review are used for flood control, hydropower generation, ecological flow requirement and water distribution systems. While flood control and hydropower are main function of a reservoir, ecological flow requirement are shown to be an important part that should be incorporated into reservoir operation models. Evolutionary algorithms are shown to be capable of solving complex reservoir operation models with fast convergence rate. A review of different algorithms in solving different reservoir operation problems is presented. The results generated by these algorithms are effective, competitive, comparable and applicable. The algorithms present the solutions to the computationally expensive models in an efficient way. Systematic ways of solving different reservoir operation models are presented. Many models reviewed involve reservoirs operated in single objective, multiobjectives, single reservoir and multireservoir. Real time operation is shown to be superior to normal operation. The results generated by the evolutionary algorithms presented show that the algorithms are capable of solving complex and multidimensional problem of water resources. The non-dominated solutions generated are many and spread widely on the Pareto-optimal front. Keywords: Reservoir operation, Flood control, Ecological flow, Water distribution, Evolutionary algorithm, Hybrid

Episodic extreme waves due to sea storms can cause severe coastal erosion. The recovery times of such events are important for the analysis of risk and coastal vulnerability. The recovery period of a storm damaged coastline represents a time when the coastline is most vulnerable and nearby infrastructure is at the greatest risk. We propose that identification of the beach recovery period can be used as a coastal management tool when determining beach usage. As a case study, we analyse 37 yr of beach profile data on the east coast of South Africa. Considering beach length and cross-sectional area, we establish a global recovery period and rate and identify the physical characteristics of the coastlines that either accelerate or retard recovery. The beaches in the case study were found to take an average of two years to recover at a rate of approximately 90 m3 m−1 yr−1. Beach profiles with vegetated dunes recovered faster than urbanized beaches. Perpendicular beach structures have both positive and negative effects on beach recovery. Coastlines with rock outcrops in the surf zone tend to recover slowly and long-term sediment loss was identified in cases where storm damaged beaches have not recovered to pre-erosion levels.

Many catchments in developing countries are poorly gauged/totally ungauged which hinders water resource management and flood prediction in these countries. This study explored the application of the PyTOPKAPI model to South African (Mhlanga) and Ethiopian (Gilgel Ghibe) case study catchments to test its suitability for simulating stream flows from ungauged catchments. The aim is to extend the model application to poorly gauged/totally ungauged catchments in developing countries. The model uses digital elevation data and other spatial data sources to set up the model parameters and the forcing files. To generate reliable stream flows, models generally need to be calibrated, which typically relies on the availability of reliable stream flow data. We show how application of simple lumped models for average runoff ratios, such as that proposed by Schreiber in 1904, can be used as an alternative to detailed calibration with gauged flows. This approach seems to be new; and we show how the proposed method, together with the PyTOPKAPI model, can be used to predict runoff responses in ungauged catchments for water resource applications and flood forecasting in developing countries. Keywords : PyTOPKAPI, Schreiber formula, streamflow, runoff ratio, South Africa, Ethiopia

Wind-driven sediment resuspension affects the physical and biological environment of the water column in shallow estuarine lakes. This study investigated the relationship between wind-driven waves and suspended sediment concentration (SSC) using the 33 km 2 South Lake basin of Lake St Lucia, South Africa as a case study. Five wave poles measuring significant wave height and turbidity were deployed over an aggregate period of twenty days at distributed locations where sediment substrate compositions varied from muddy to sandy and depths ranged from 0.7 m to 2.1 m. The resulting turbidity dynamics were used to test a simple depth-averaged model of suspended sediment concentrations. The model performed best in the muddy regions of the lake and was able to simulate the resuspension dynamics more accurately than the settling dynamics. Peak suspended sediment concentration levels were best captured for the deeper muddy locations. The model provides a means to make spatially explicit predictions of suspended sediment concentrations that can be used to understand the forcing mechanisms for primary producer growth and distribution or to improve sediment budget calculations.

The modelling of waves associated with extreme events is fundamental to coastal engineering design and coastal vulnerability assessments. The storm systems that drive extreme wave events are associated with specific atmospheric circulation patterns (CPs). In this paper the link between these circulation patterns and wave spectra is explored as a means to improve wave modelling in engineering applications. The methodology involves partitioning wave spectra into low frequency swell and locally generated wind waves. The origin(s) of the swell waves can then be estimated in order to link them to the atmospheric circulation pattern(s) that generated them. A method based on fuzzy logic and fuzzy sets is used to identify and classify the atmospheric circulation patterns. Finally the spectral characteristics associated with specific circulation patterns can be obtained. The methodology is tested using a case study on the east coast of South Africa. The atmospheric circulation patterns driving low frequency swell events resemble those previously identified as the dominant drivers of significant wave events in the region. The link between wave spectra and CPs can be used to study the impacts of specific CPs on the coastline. For example the spectra associated with swell produced by tropical cyclones can be used to model their potential impacts. This new methodology may improve the inputs to spectral wave models and aid studies of climate change impacts. It may also help in identifying statistically independent storm events and improve multivariate statistical models of such events.

Urbanisation is taking place at an unprecedented rate throughout the world, often outpacing plans and the capacity of city managers. As a result, natural open space areas in cities are being degraded and diminished, and problems such as flooding, air and water pollution are getting worse. The environmental problems associated with increased hardened surfaces and the loss of natural areas and ecosystem services are particularly acute in developing country cities, where a lack of regulation and resources has led to poor planning, the expansion of informal settlements in highrisk, marginal areas, and the inability to adequately manage the quantity and quality of surface water flows. While conventional storm water conveyance measures contribute to reducing flooding impacts, they have not been able to keep ahead of the problem and have also contributed to pollution and degradation of downstream aquatic systems. However, great strides have been made in the design of more sustainable engineering mechanisms to dealwith urban flooding and water quality problems, andthe management and planning of cities is increasinglyfocusing on a more holistic approach that includes theconservation of natural areas as part of a green urban development (GUD) strategy. A GUD strategy does notonly focus on surface water issues but also involves the maintenance of natural open space areas for recreationwhich is essential for human health and well being.One of the challenges of green urban development will be to find the right balance between natural, seminatural,innovative and conventional built infrastructure. Understanding the costs and benefits associated with the different types of measures is important and requires careful consideration of their potential benefits and cost effectiveness in managing urban environmental problems. Durban, located within the eThekwini Municipality onthe east coast of South Africa, is rich in biodiversity, butfaces a number of environmental and developmental challenges. Green urban development is an approach that aimsto minimize the impacts of urbanization on the environment, and tackles the core problems of pollution and waste, the consumption of natural resources, theloss of urban open space and the degradation and loss of biodiversity, as well as mitigation of the urban contribution to climate change. In addition to a range of policy interventions, this involves investing in naturalcapital as well as use of green structural engineering and conventional grey infrastructure. Green urban development includes (1) sanitation services and regulations to minise pollution, (2) applying “green engineering” approaches to urban problems such as stormwater management, (3) controlling consumption and carbon emissions, (4) protecting natural assets and(5) maintaining parks, street trees and gardens.The study involved modelling current flooding and water quality in the Umhlatuzana - Umbilo catchment,and determining the change in water quality and flood hydrographs under a series of hypothetical scenarios inwhich the past development of the area had involved different combinations and extents of green urban development measures including better sanitation,stormwater management and conservation measures.

Wave climates are fundamental drivers of coastal vulnerability; changing trends in wave heights, periods and directions can severely impact a coastline. In a diverse storm environment, the changes in these parameters are difficult to detect and quantify. Since wave climates are linked to atmospheric circulation patterns, an automated and objective classification scheme was developed to explore links between synoptic-scale circulation patterns and wave climate variables, specifically wave heights. The algorithm uses a set of objective functions based on wave heights to guide the classification and find atmospheric classes with strong links to wave behaviour. Spatially distributed fuzzy numbers define the classes and are used to detect locally high- and low-pressure anomalies. Classes are derived through a process of simulated annealing. The optimized classification focuses on extreme wave events. The east coast of South Africa was used as a case study. The results show that three dominant patterns drive extreme wave events. The circulation patterns exhibit some seasonality with one pattern present throughout the year. Some 50–80% of the extreme wave events are explained by these three patterns. It is evident that strong low-pressure anomalies east of the country drive a wind towards the KwaZulu-Natal coastline which results in extreme wave conditions. We conclude that the methodology can be used to link circulation patterns to wave heights within a diverse storm environment. The circulation patterns agree with qualitative observations of wave climate drivers. There are applications to the assessment of coastal vulnerability and the management of coastlines worldwide.

The spatiotemporal structure of wind-driven circulation patterns and associated water exchanges or residence times can drive important bio-hydrodynamic interactions in shallow lakes and estuaries. The St Lucia estuarine lake in South Africa is an example of such a system. It is a UNESCO World Heritage Site and RAMSAR wetland of international importance but no detailed research on its circulation patterns has previously been undertaken. In this study, a hydrodynamic model was used to investigate the structure of these circulations to provide insights into their role in transport and water exchange processes. A strong diurnal temporal pattern of wind speeds, together with directional switching between two dominant directions, drives intermittent water exchanges and mixing between the lake basins. “High speed flows in shallow nearshore areas with slower upwind counter-flows in deeper areas, linked by circulatory gyres, are key features of the circulation”. These patterns are strongly influenced by the complex geometry of St Lucia and constrictions in the system. Water exchange time scales are non-homogeneous with some basin extremities having relatively long residence times. The influence of the circulation patterns on biological processes is discussed.

Zikhali, V., Tirok, K., Stretch, D., 2014. Wind-driven waves in a shallow estuarine lake with muddy substrates: St Lucia, 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. 729-735, ISSN 0749-0208. Wind-waves in shallow lakes or estuaries with muddy substrates can drive sediment re-suspension and cause high turbidity levels that can negatively impact the productivity of photosynthetic organisms. This investigation evaluates the efficacy of a simple semi-empirical model (Young and Verhagen, 1996 : Coastal Engineering, 29, 47–78) for predicting the wave characteristics in these systems in order to include their effects in ecosystem models. The southern basin of the St Lucia estuarine lake in South Africa was used for a case study. Average depths are about 1 m with fetches up to approximately 10 km. Substrate materials vary from sandy to muddy with deeper locations pr...

Pringle, J. and Stretch, D.D., 2019. A new approach for the stochastic simulation of regional wave climates conditioned on synoptic scale meteorology. Journal of Coastal Research, 35(6), 1...

In order to provide realistic storm simulations and to quantify coastal risks the dependencies between storm parameters such as wave height, wave period and storm duration need to be considered. Copulas provide a means to achieve this by enabling the development of multivariate statistical models of sea storms. Although there are many families of copulas, Archimedean copulas are appealing to engineers because of their mathematical tractability. The dependencies between wave height, wave period, storm duration, water level and storm inter-arrival time (or calm period) were investigated in a case study on the east coast of South Africa using Kendall's tau correlation coefficient as a dependency metric. Three methods of creating multivariate copulas were applied and the results were compared using (1) Kendall's measure; (2) empirical multivariate distributions; and (3) simulations. Only the wave height, wave period and storm duration were found to be significantly associated. Hierarchical copulas provided the best trivariate model for the case study data. The trivariate analysis extends previous bivariate analyses and thereby enables a more detailed description of sea storms to be incorporated in the statistical model. A significant limitation of the current model is that it fails to link wave parameter statistics to physical forcing and physical constraints. Ways of overcoming these and other limitations are discussed.

Durban is located within a global biodiversity hotspot, and still contains a wealth of biodiversity. Some of this is protected in nature reserves, but much of it is in private hands or in communal lands on the city’s periphery. City managers are divided over the level of attention that should be given to preserving these remaining natural areas. While it is argued that they make a significant contribution to biodiversity conservation in the province, provide valuable ecosystem services and will contribute to the city’s resilience in the face of climate change, the counter argument is that much of this area should make way for development to alleviate the escalating problem of unemployment. The study of ecosystem services and their value to society has made significant advances since an estimate was made of Durban’s ecosystem services in the 1990s using early values from the international literature. The aim of this study was to provide estimates of the value of ecosystem services provided by natural open space areas within the eThekwini Municipal Area (EMA), and to map the geographic variation in these values as far as possible so as to be able to compare both areas and types of value, using available and locally-relevant data. This study was carried out as a desktop study based on available data. Modelling assumptions were based on data from within the study area, drawing on the regional, national or international literature only where necessary. The study focused on the direct values associated with the provision of natural resources, indirect use values associated with regulating services generated by ecosystem functioning, and the amenity values generated by ecosystem attributes.

Geotextile sand filled containers (GSC) have rapidly become the preferred coastal defence on the east coast of South Africa. Their growth can be attributed to political, social and environmental factors. This paper details South African experiences and the observed performance of GSCs. The main failure mechanism of geotextile sand bag seawalls is identified to be the movement of sand within the bags. This movement is observed to be a result of insufficient filling, bag elongation and sand leaking. Through a review of local and international manufacturing and construction techniques we identify methods of reducing the internal movements of sediment. The observed performance of local bags provide a practical full scale validation of the physical model findings of Hornsey et al. (2011) ; Recio and Oumeraci (2007) .

Storms and water levels are subject to seasonal variations but may also have decadal or longer trends that need to be included when estimating risks in the coastal zone. We propose a non-stationary multivariate generalised extreme value model for wave height, wave period, storm duration and water levels that is constructed using Archimedean copulas. The statistical model was applied to a South African case study to test the impacts of decadal trends on beach erosion. Erosion was estimated using three process-based models — SBEACH, XBEACH, and the Time Convolution model. The XBEACH model provided the best calibration results and was used to simulate potential future long-term trends in beach erosion. Based on the simulated erosion results of 5 beach profiles for storms with 25, 50 and 100 year return periods, it is estimated that the erosion rate could increase by 0.20%/year/storm and should therefore be a significant factor in long-term planning.

Sandy shorelines are dynamic with constant changes that can cause hazards in developed areas. The causes of change may be either natural or anthropogenic. This paper evaluates evidence for shoreline changes and their causative factors using a case study on the east coast of South Africa. Beach morphology trends were found to be location-specific, but overall the beaches show a receding trend. It was hypothesized that wave, tide, sea level and wind trends as well as anthropogenic influences are causative factors, and their contributions to shoreline changes were evaluated. Maximum significant wave heights, average wave direction, peak period and storm event frequencies all show weak increasing trends, but only the increases in peak period and wave direction are statistically significant. The chronic beach erosion cannot be attributed to wave climate changes since they are still too small to explain the observations. Instead, the impacts of sea level rise and reductions in the supply of beach sediments are suggested as the main causative factors. The analysis also identifies a trend in the frequency of severe erosion events due to storms that coincide with a 4.5-yr extreme tide cycle, which demonstrates the potential impact of future sea level rise.

The erosion of a beach depends on various storm characteristics. Ideally, the risk associated with a storm would be described by a single multivariate return period that is also representative of the erosion risk, i.e. a 100 yr multivariate storm return period would cause a 100 yr erosion return period. Unfortunately, a specific probability level may be associated with numerous combinations of storm characteristics. These combinations, despite having the same multivariate probability, may cause very different erosion outcomes. This paper explores this ambiguity problem in the context of copula based multivariate return periods and using a case study at Durban on the east coast of South Africa. Simulations were used to correlate multivariate return periods of historical events to return periods of estimated storm induced erosion volumes. In addition, the relationship of the most-likely design event (Salvadori et al., 2011) to coastal erosion was investigated. It was found that the multivariate return periods for wave height and duration had the highest correlation to erosion return periods. The most-likely design event was found to be an inadequate design method in its current form. We explore the inclusion of conditions based on the physical realizability of wave events and the use of multivariate linear regression to relate storm parameters to erosion computed from a process based model. Establishing a link between storm statistics and erosion consequences can resolve the ambiguity between multivariate storm return periods and associated erosion return periods.

The St Lucia estuarine lake in South Africa forms part of a World Heritage Site and is an important local source of biodiversity. Like many estuarine systems worldwide, St Lucia has experienced significant anthropogenic impacts over the past century. Abstractions have decreased fresh water inflows from the lake catchments by about 20%. Furthermore the Mfolozi river, which previously shared a common inlet with St Lucia and contributed additional fresh water during droughts, was diverted from the system in 1952 because of its high silt loads. The separated St Lucia mouth was subsequently kept artificially open until the onset of a dry period in 2002 when the mouth was left to close naturally. These changes and the current drought have placed the system under severe stress with unprecedented hypersaline conditions coupled with desiccation of large portions of the lake. Long-term simulations of the water and salt balance were used to estimate the occurrence and persistence of water levels and salinities for different management scenarios. The risks of desiccation and hyper-salinity were assessed for each case. The results show that the configuration of the Mfolozi/St Lucia inlets plays a key role in the physicochemical environment of the system. Without the Mfolozi link desiccation (of about 50% of the lake area) would occur for 32% of the time for an average duration of 15 months. Artificially maintaining an open mouth would decrease the chance of desiccation but salinities would exceed 65 about 17% of the time. Restoring the Mfolozi link would reduce the occurrence of both desiccation and hypersaline conditions and a mostly open mouth state would occur naturally. Integrating these modeled scenarios with observed biological responses due to changes in salinity and water depth suggests that large long-term changes in the biological structure can be expected in the different management scenarios.

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.

Lake St Lucia in South Africa is part of a UNESCO World Heritage site and a Ramsar wetland of international importance. Like many coastal wetlands worldwide, anthropogenic activities including catchment land-use changes, water diversions/abstractions, and manipulation of the mouth state have significantly affected its functioning over the past century. Questions concerning its sustainability have motivated a re-evaluation of management decisions made in the past and of options for the future. A model for the water and salt budgets has therefore been used to investigate “what if” scenarios in terms of past anthropogenic interventions. In particular, simulations allow us to evaluate the effects of diverting the Mfolozi river from St Lucia on the functioning of the system and on the occurrence of various water level/salinity states that drive the biological functioning of the ecosystem. In the past, when the St Lucia estuary and the Mfolozi river had a combined inlet, the mouth was predominantly open. The lake had relatively stable water levels but variable salinities that increased during dry conditions due to evaporative losses and saltwater inflows from the sea. If the mouth closed, the Mfolozi flow was diverted into the lake which reduced salinities and maintained or increased water levels. Simulations indicate that without a link to the Mfolozi the lake system would naturally have a mainly closed inlet with lower average salinities but more variable water levels. During dry conditions water levels would reduce and result in desiccation of large areas of the lake as has recently occurred. We conclude that the artificial separation of the St Lucia and Mfolozi inlets underpins the most significant impacts on the water & salt budget of the lake and that its reversal is key to the sustainability of the system.

The interactions between shear-free turbulence in two regions (denoted as + and − on either side of a nearly flat horizontal interface are shown here to be controlled by several mechanisms, which depend on the magnitudes of the ratios of the densities, ρ+/ρ−, and kinematic viscosities of the fluids, μ+/μ−, and the root mean square (r.m.s.) velocities of the turbulence, u0+/u0−, above and below the interface. This study focuses on gas–liquid interfaces so that ρ+/ρ− ≪ 1 and also on where turbulence is generated either above or below the interface so that u0+/u0− is either very large or very small. It is assumed that vertical buoyancy forces across the interface are much larger than internal forces so that the interface is nearly flat, and coupling between turbulence on either side of the interface is determined by viscous stresses. A formal linearized rapid-distortion analysis with viscous effects is developed by extending the previous study by Hunt & Graham (J. Fluid Mech., vol. 84, 1978, pp. 209–235) of shear-free turbulence near rigid plane boundaries. The physical processes accounted for in our model include both the blocking effect of the interface on normal components of the turbulence and the viscous coupling of the horizontal field across thin interfacial viscous boundary layers. The horizontal divergence in the perturbation velocity field in the viscous layer drives weak inviscid irrotational velocity fluctuations outside the viscous boundary layers in a mechanism analogous to Ekman pumping. The analysis shows the following. (i) The blocking effects are similar to those near rigid boundaries on each side of the interface, but through the action of the thin viscous layers above and below the interface, the horizontal and vertical velocity components differ from those near a rigid surface and are correlated or anti-correlated respectively. (ii) Because of the growth of the viscous layers on either side of the interface, the ratio uI/u0, where uI is the r.m.s. of the interfacial velocity fluctuations and u0 the r.m.s. of the homogeneous turbulence far from the interface, does not vary with time. If the turbulence is driven in the lower layer with ρ+/ρ− ≪ 1 and u0+/u0− ≪ 1, then uI/u0− ~ 1 when Re (=u0−L−/ν−) ≫ 1 and R = (ρ−/ρ+)(v−/v+)1/2 ≫ 1. If the turbulence is driven in the upper layer with ρ+/ρ− ≪ 1 and u0+/u0− ≫ 1, then uI/u0+ ~ 1/(1 + R). (iii) Nonlinear effects become significant over periods greater than Lagrangian time scales. When turbulence is generated in the lower layer, and the Reynolds number is high enough, motions in the upper viscous layer are turbulent. The horizontal vorticity tends to decrease, and the vertical vorticity of the eddies dominates their asymptotic structure. When turbulence is generated in the upper layer, and the Reynolds number is less than about 106–107, the fluctuations in the viscous layer do not become turbulent. Nonlinear processes at the interface increase the ratio uI/u0+ for sheared or shear-free turbulence in the gas above its linear value of uI/u0+ ~ 1/(1 + R) to (ρ+/ρ−)1/2 ~ 1/30 for air–water interfaces. This estimate agrees with the direct numerical simulation results from Lombardi, De Angelis & Bannerjee (Phys. Fluids, vol. 8, no. 6, 1996, pp. 1643–1665). Because the linear viscous–inertial coupling mechanism is still significant, the eddy motions on either side of the interface have a similar horizontal structure, although their vertical structure differs.

Wastewater discharges affect the functioning of small temporarily open/closed estuaries (TOCEs) through two main mechanisms: (1) they can significantly change the water balance by altering the quantity of water inflows, and (2) they can significantly change the nutrient balance and hence the water quality. This study investigated the bio-physical responses of a typical, small TOCE on the east coast of South Africa, the Mhlanga Estuary. This estuary receives significant inflows of treated effluent from upstream wastewater treatment works. Water and nutrient budgets were used together with biological sampling to investigate changes in the functioning of the system. The increase in inflows due to the effluent discharges has significantly increased the mouth breaching frequency. Furthermore, when the mouth closes, the accumulation of nutrients leads to eutrophication and algal blooms. A grey water index, namely the proportion of effluent in the estuary and an indicator of the additional nutrient inputs into the estuary, reached high values (≳50%) during low flow regimes and when the mouth was closed. In these hyper-eutrophic conditions (DIN and DIP concentrations up to 457 μM and 100 μM respectively), field measurements showed that algal blooms occurred within about 14 days following closure of the mouth (chlorophyll-a concentrations up to 375 mg chl-a m−3). Water and nutrient balance simulations for alternative scenarios suggest that further increases in wastewater discharges would result in more frequent breaching events and longer open mouth conditions, but the occurrence of hyper-eutrophic conditions would initially intensify despite more frequent openings. The study indicates how water and nutrient balance simulations can be used in the planning and impact assessment of wastewater treatment facilities.

In this paper, we derive a general relationship for the turbulent Prandtl number Prt for homogeneous stably stratified turbulence from the turbulent kinetic energy and scalar variance equations. A formulation for the turbulent Prandtl number, Prt, is developed in terms of a mixing length scale LM and an overturning length scale LE, the ratio of the mechanical (turbulent kinetic energy) decay time scale TL to scalar decay time scale Tρ and the gradient Richardson number Ri. We show that our formulation for Prt is appropriate even for non-stationary (developing) stratified flows, since it does not include the reversible contributions in both the turbulent kinetic energy production and buoyancy fluxes that drive the time variations in the flow. Our analysis of direct numerical simulation (DNS) data of homogeneous sheared turbulence shows that the ratio LM/LE ≈ 1 for weakly stratified flows. We show that in the limit of zero stratification, the turbulent Prandtl number is equal to the inverse of the ratio of the mechanical time scale to the scalar time scale, TL/Tρ. We use the stably stratified DNS data of Shih et al. (J. Fluid Mech., vol. 412, 2000, pp. 1–20; J. Fluid Mech., vol. 525, 2005, pp. 193–214) to propose a new parameterization for Prt in terms of the gradient Richardson number Ri. The formulation presented here provides a general framework for calculating Prt that will be useful for turbulence closure schemes in numerical models.

Mixing efficiency in stratified flows is a measure of the proportion of turbulent kinetic energy that goes into increasing the potential energy of the fluid by irreversible mixing. In this research direct numerical simulations (DNS) and rapid distortion theory (RDT) cal- culations of transient turbulent mixing events are carried out in order to study this aspect of mixing. In particular, DNS and RDT of decaying, homogeneous, stably-stratified turbulence are used to determine the mixing efficiency as a function of the initial turbu- lence Richardson number Rit0 = (NL0/u0) 2 , where N is the buoyancy frequency and L0 and u0 are initial length and velocity scales of the turbulence. The results show that the mixing efficiency increases with increasing Rit0 for small Rit0, but for larger Rit0 the mixing efficiency becomes approximately constant. These results are com- pared with data from towed grid experiments. There is qualitative agreement between the DNS results and the available experimental data, but significant quantitative discrepancies. The grid turbulence experiments suggest a maximum mixing efficiency (at large Rit0 )o f about 6%, while the DNS and RDT results give about 30%. We con- sider two possible reasons for this discrepancy: Prandtl number effects and non-matching initial conditions. We conclude that the main source of the disagreement probably is due to inaccuracy in determining the initial turbulence energy input in the case of the grid turbulence experiments.

Mouth breaching is a recurrent event in temporarily open/closed estuaries (TOCEs). Such disturbances result in flushing and sediment scouring, reducing the microalgal biomass stock. The depletion of these microalgae may have negative repercussions in the form of depleted stocks of commercial fish, game fish, crustaceans and mollusks. The aim of this investigation was therefore: (1) to monitor the recovery of microalgal biomass and production following a breaching event; and (2) to determine the key environmental parameters influencing primary production during the open and recovery phases. Phytoplankton and benthic microalgal production was measured ( 14 C-uptake method) successively during the closed, open and recovery phases of the Mdloti TOCE (South Africa). Upon breaching, 94–99% of microalgal biomass was washed out to sea through flushing and sediment scouring. A temporary recovery of phytoplankton and benthic microalgal biomass was observed during the open phase, but this was not sustained because of continual flushing and scouring of the sediment. During the re-closure (recovery phase), microalgal biomass immediately increased, reaching pre-breaching levels 35–40 days following the breaching event. In contrast to biomass, autochthonous pelagic primary production reached a maximum level (341 mg C m −2 h −1 ) during the open phase. Pelagic primary production normalized to biomass (P B ) significantly increased during the open phase. This is attributed to a favorable combination of optimum light conditions, high influx of macronutrients and high water temperatures (33 °C). Similarly, benthic primary production normalized to biomass (P B ) peaked during the open phase (35 mg C mg chl- a −1 h −1 ). Multivariate analysis showed that major variations in primary production were mainly controlled by temperature, dissolved inorganic nitrogen (DIN) to phosphorus (DIP) molar ratios (water-column and pore-water) and light extinction ( K d ), all of which were regulated by the state of the mouth.

Intermittent breaching of sand barriers at temporary open estuaries plays a key role in the functioning of these systems. In addition to their ecological impacts, breaching events can cause significant morphological changes because high breach outflows result in the scouring of significant amounts of accumulated sediments from an estuary. Estimation and modeling of these processes requires insight into the parameters that determine features of the breach such as its size and the timescales for the breach formation. The latter is particularly important for characterizing the outflow hydrograph and for estimating sediment transport effects. Simple laboratory experiments are reported that investigated the temporal evolution of the breach and the scaling of the breach formation time TF and peak outflow QP. The experiments were specifically designed to investigate the influence of the outflow volume S, the hydraulic head H, and the barrier breadth B. A scaling is suggested that gives a good description of the experimental data. The scaling is shown to be consistent with observed breach characteristics for actual estuaries (or coastal lagoons) and earth-dam failures where outflow volumes are several orders of magnitude larger than the models.

The mouth dynamics of small temporary open estuaries play a key role in their overall functioning. Intermittent breaching of the sand barriers of these systems leads to large changes in the physico...

List of contributors Foreword Alan Whitfield Preface 1. South Africa's first World Heritage Site Roger N. Porter 2. Management history Ricky H. Taylor 3. Geological history Greg A. Botha, Sylvi Haldorsen and Naomi Porat 4. The marine environment Allan D. Connell and Sean N. Porter 5. Catchment hydrology Derek D. Stretch and Andrew Z. Maro 6. The Wetlands William N. Ellery, Suzanne E. Grenfell, Michael C. Grenfell, Marc S. Humphries and Kirsten B. Barnes 7. Estuary and lake hydrodynamics Derek D. Stretch, Clint P. Chrystal, Robynne A. Chrystal, Christopher Maine and Justin J. Pringle 8. Groundwater hydrology Bruce E. Kelbe, Ricky H. Taylor and Sylvi Haldorsen 9. Physico-chemical environment Renzo Perissinotto, Nicola K. Carrasco and Ricky H. Taylor 10. Microalgae Renzo Perissinotto, Guy C. Bate and David G. Muir 11. Macrophytes Janine B. Adams, Sibulele Nondoda and Ricky H. Taylor 12. Benthic invertebrates Deena Pillay, Sarah J. Bownes and Holly A. Nel 13. Zooplankton Nicola K. Carrasco, Renzo Perissinotto and Hendrik L. Jerling 14. Penaeid prawns Anthony T. Forbes and Nicolette T. Forbes 15. Fish and fisheries Digby Cyrus 16. Birds Jane Turpie, Ricky H. Taylor, Meyrick Bowker and Caroline Fox 17. Crocodiles Xander Combrink, Jonathan Warner and Colleen T. Downs 18. Hippopotamuses Ricky H. Taylor 19. Alien and invasive species Nelson A. F. Miranda and Janine B. Adams 20. Food webs and ecosystem functioning Ursula M. Scharler and Fiona C. MacKay 21. Climate change impacts Andrew A. Mather, Derek D. Stretch and Andrew Z. Maro References Appendix: web page database, 1900-2010 Nuette Gordon Index.

The estimation of a critical dissipation rate, ec, below which vertical transport ρw is completely suppressed, is examined in the context of laboratory experiments on shear‐free, decaying, and stably stratified grid‐generated turbulence. It is shown how the use of a criterion based on ρw=0 in these transient flows is not appropriate for obtaining a universal value for the critical dissipation rate expressed nondimensionally as ec/νN2. It remains unclear how any universal value of ec may be inferred from these laboratory experiments.

Recent coastal storms in southern Africa have highlighted the need for more proactive management of the coastline. Within the southern and eastern African region the availability of coastal information is poor. The greatest gap in information is the likely effects of a combination of severe sea storms and future sea level rise (SLR) on the shoreline. This lack of information creates a barrier to informed decision making. This research outlines a practical localized approach to this problem, which can be applied as a first order assessment within the region. In so doing it provides a cost effective and simple decision support tool for the built environment and disaster professionals in development and disaster assessments. In a South African context the newly promulgated Integrated Coastal Management Act requires that all proposed coastal developments take into consideration future SLR, however such information currently does not exist, despite it being vital for informed planning in the coastal zone. This practical approach has been applied to the coastline of Durban, South Africa as a case study. The outputs are presented in a Geographic Information System (GIS) based freeware viewer tool enabling ease of access to both professionals and laypersons. This demonstrates that a simple approach can provide valuable information about the current and future risk of flooding and coastal erosion under climate change to buildings, infrastructure as well as natural features along the coast.

[1] Quantifying diapycnal mixing in stably stratified turbulence is fundamental to the understanding and modeling of geophysical flows. Data of diapycnal mixing from direct numerical simulations of homogeneous stratified turbulence and from grid turbulence experiments, are analyzed to investigate the scaling of the diapycnal diffusivity. In these homogeneous flows the instantaneous diapycnal diffusivity is given exactly by K d = ∈ ρ /(∂ρ/∂z) 2 where ∈ ρ is the dissipation rate of density fluctuations, and ∂ρ/∂z is the mean density gradient. The diffusivity K d may be expressed in terms of the large scale properties of the turbulence as K d = γL 2 E /T L , where L E is the Ellison overturning length-scale, T L is the turbulence decay time-scale, and γ is half the mechanical to scalar time-scale ratio. Our results show that L E and T L can explain most of the variations in K d over a wide range of shear and stratification strengths while γ remains approximately constant.

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

Existing models for urban runoff water quality and dispersion in the coastal zone are cumbersome for application to everyday management of beach use. A simplified model is therefore proposed and tested using a case study. The model captures the key physical processes involved in mixing and dispersion of pathogenic pollution at beaches, and should therefore have some generality. Simulations using the model are shown to adequately reproduce measurements at the case study site. The utility of the model is demonstrated by analysing a specific case of poor water quality at one of the beaches and by using it to estimate the reduction in pollution loadings needed to meet water quality guidelines. Key words: beach-water quality model, pathogenic pollution, storm-water runoff, E. coli Water SA Vol.31(1) 2005: 47-52

The pathogenic pollution of Durban\'s beaches is reviewed on the basis of local and international guidelines by analysing concentrations of indicator micro-organisms (E. coli and enterococci). The average water quality is generally acceptable according to South African guidelines, but assessments based on international guidelines indicate poor water quality at many beaches during some seasons (especially summer). The reason for this inconsistency is the absence of any enterococcus criteria in the SA guidelines, which was found to be particularly significant when the pollution loadings are relatively low. This result confirms epidemiological studies that have shown enterococcus to be a more sensitive indicator of pathogenic pollution in marine environments. South African guidelines should therefore be updated to incorporate enterococcus as the preferred indicator for marine waters. WaterSA Vol.30 (3) 2004: 317-323

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.

A set of objective criteria based on the local strain rate, vorticity and pressure have been found to describe regions in which the streamlines circulate, converge or diverge, and form streams of high velocity flow. The homogeneous and sheared turbulent flow fields are made up of characteristic flow zones — eddy, shear, convergence and streaming zones. These are studied in turbulent velocity fields produced by different methods of simulation, including the novel method of Kinematic Simulation of homogeneous isotropic turbulence are summarised. We derive and explain the zonal algorithm to classify structures and then use this classification to compare the results of two different numerical simulations (DNS, KS) both qualitatively (turbulence structure, physical processes) and quantitatively (turbulence statistics) for homogeneous isotropic turbulence and then we apply the zonal algorithm to a turbulent shear flow. New conclusions are reached about the significant regions in turbulent flows for dynamical and kinematical processes.

Direct numerical simulations are used to study mixing and dispersion in decaying stably stratified turbulence from a Lagrangian perspective. The change in density of fluid particles owing to small-scale mixing is extracted from the simulations to provide insight into the mixing process. These changes are driven by temporally and spatially intermittent events that are strongly suppressed as the stratification increases and overturning motions disappear. This occurs for times Nt > 2n, i.e. after one buoyancy period, where N is the buoyancy frequency. The role of small-scale mixing processes in the density (or buoyancy) flux is analysed. After an initial transient, we find that diapycnal displacements due to mixing dominate the dispersion of fluid particles, even in weak stratification. The relationship between the diapycnal diffusivity and vertical dispersion coefficients is found to be strongly dependent on stratification. Models for the mixing following fluid particles are investigated. The time scale for the density changes due to small-scale mixing is shown to be approximately independent of N and instead remains linked to the energy decay time scale which is relatively insensitive to stratification. There are large changes in the structure of these flows as they evolve under the influence of buoyancy forces. We investigate these changes and their relationship to mixing. We find that strong mixing events are closely linked to the presence of overturning regions in the flow, and that they occur close to (but not within) these regions. The results reported here have implications for the development of improved models of diffusion in stably stratified turbulence.