Coastal adaptation to climate change: A case study in Durban, South Africa

COASTAL ADAPTATION TO CLIMATE CHANGE: A CASE STUDY IN DURBAN, SOUTH AFRICA

Ledden6

Abstract: Recent research done the IPCC (2007) working groups and other organizations has sparked global concern over the possible impacts of climate change and corresponding sea level rise (SLR) upon coastal communities. In reaction studies were done (for example by Nicholls et al., 2008) to assess the vulnerability of coastal regions andget an indication of the magnitude of the potential global impacts. However, most of these studies did not address the development of climate change adaptation designs to protect the coastline. In this paper it is demonstrated how a localised coastal vulnerability assessment could guide the development of conceptual coastal protection designs in an African context.

The overall aim of this paper is the appraisal of climate adaptation measures and coastal management strategies for Durban, South Africa. This is illustrated using a case study, for a coastal section along Durban´s central beaches. The case study is an example of how the vulnerability to coastal hazards could be assessed, for different SLR scenarios and should provide guidance for developing conceptual coastal protection designs.

A recent extreme storm event indicated that significant damage can be sustained from coastal hazards in Durban under the current conditions. A 1-in-100 year storm is shown to already affect the operations of critical infrastructure under current conditions at the case study site. The projected vulnerability increases significantly for future SLR scenarios.

Keywords: climate adaptation; coastal protection; sea level rise; developing world

INTRODUCTION

Much of the world’s population lives along the coast. Climate change, along with the corresponding rising sea levels, and population growth are putting pressure on existing coastal defences and could cause significant damage to unprotected coastlines (IPCC, 2007).

Some of the recent research has focussed on the impact of climate change on large port cities worldwide; Nicholls et al. (2008) investigated the physical and socio-economic vulnerability of these cities and Linham et al. (2010) the costs of adaptation to climate change. Of the 136 port cities analysed in the latter studies Cape Town and Durban, in South Africa, were included. These studies have brought the vulnerability of some large cities to coastal hazards to the forefront. It is estimated that 40 million people worldwide

1 MSc Graduate, Department of Hydraulic Engineering, Delft University of Technology, The Netherlands (previously) Marine Engineer at Aurecon, Cape Town, South Africa (currently) Email:

marli.geldenhuys@aurecongroup.com

2 Lecturer, Department of Hydraulic Engineering, Delft University of Technology, The Netherlands Email:

s.n.jonkman@tudelft.nl and also employed by Royal Haskoning

3 Project Executive: Coastal Policy, eThekweni Municipality, Durban, South Africa Email:

mathera@durban.gov.za

4 Associate Professor, Department of Hydraulic Engineering, IHE-UNESCO Institute of Water Education Email:

R.Ranasinghe@unesco-ihe.org /Lecturer Hydraulic department of Delft University of Technology 5 Coastal Chair and Professor, Department of Hydraulic Engineering, Delft University of Technology, The

Netherlands Email: M.J.F.Stive@tudelft.nl

6 Business Development Manager: Coastal and Rivers, Royal Haskoning, The Netherlands Email:

(which amounts to 0.6% of world population) could be exposed to a 1-in-100 year coastal flood event (Nicholls et al, 2008).

Very little published research relating to climate adaptation in Africa is available, particularly in terms of localised coastal protection solutions. Most current research is focussed at the country and city scale (such as the work by Kebede (2011a) relating to Mombasa, Kenya). The need for more regional or local research relating to climate adaptation in the coastal zone is increasingly.

Durban, a city on the east coast of South Africa, was chosen as the location for this study due to the fact that it sustained significant damage during a storm event in March 2007 which highlighted its vulnerability to coastal hazards. Durban is the third largest city in South Africa with a population of approximately 3.47 million (Statistics South Africa, 2007). The eThekweni Municipality administers the Durban metropolitan area and its 90 kilometres of coastline.

To date research in this field has focussed more on the vulnerability of the coastline to climate change and not at the development of actual designs to protect the coastline. This paper attempts to demonstrate how the development of conceptual designs could be guided by a localized coastal vulnerability assessment for a section of Durban´s central beaches. Empirical methods (rather than numerical models) are used in this study to account for the limitations in budget and time that is normally associated with initial vulnerability assessments in a developing world context.

CASE STUDY SELECTION AND ANALYSIS METHODOLOGY

The analysis of the case study forms the body of this paper. A case study site for the proposed vulnerability assessment methodology was selected in terms of the following criteria:

x The location´s vulnerability to coastal storms

x The value of the location´s hinterland to the local government, property owner and/or general public

x The location´s representativeness as part of the larger coastline

x Sufficient information should be available for the chosen location, especially in the context of data limitations in developing countries

x Planned or existing critical infrastructure located in proximity to the site, such as nuclear reactors, hospitals or transport nodes

The chosen case study location (shown in Figure 1) is situated along the southern part of the Durban central beaches, which abuts the Central Business District, an area of valuable real estate and in close proximity to the crucially important Addington district hospital. The site also satisfies the rest of the abovementioned criteria.

Figure 1: Case Study Location

The case study site´s vulnerability to coastal hazards was assessed for the design storm. The empirical vulnerability assessment is done in terms of the dune erosion, possible coastal flooding and SLR recession of the coastline. Various SLR and storminess scenarios were tested to determine the sensitivity of these parameters and the increase in the vulnerability of the coastline with future climate change.

SELECTION OF CLIMATE CHANGE SCENARIOS

Various climate change scenarios were considered, with three SLR scenarios (+300 mm (LowSLR), +600 mm (MidSLR) and +1000 mm (HighSLR)) selected. The LowSLR scenario is used as a conservative SLR estimate to develop designs for a 50 year design life. The HighSLR scenario is used to develop designs for a 100 year design life to give an indication of a possible worst case scenario.

The conceptual designs are only developed for two alternatives, both with no storminess trend (due to a lack of research in this regard for the region), which should correspond to a worst case and best case scenario and a base case:

x BaseCase: no SLR or change in storminess

x LowSLR: the case with 0.3 metres SLR and no increase in storminess x HighSLR: the case with 1.0 metres SLR and no increase in storminess

The chosen scenarios for the development of concept designs are seen as reasonable for use with the 1 in 100 year design storm event. This should help to guide planning for the best and worst case scenarios, which could also be seen as shorter and longer term planning horizons and give an indication of the versatility of solutions in terms of their adaptability to higher water levels.

SETTING THE DESIGN CASE

The vulnerability assessment was done for a 1-in-100 year extreme storm event. The following boundary conditions were derived for the event:

Addington

Hospital

x Highest Astronomical tide of +1.4 m MSL

x Storm surge of +1.2 m MSL (including wind and wave set-up and hydrostatic pressure) x Significant wave height (Hs) of 9.4 metres in 30 metres of water (Corbella, 2011) x Peak wave period (Tp) of 16 seconds

x Storm duration of 24 hours

ASSESSING THE VULNERABILITY OF THE SITE

The vulnerability assessment is expected to give an indication of the coastline’s vulnerability to coastal hazards. The following indicators for vulnerability are proposed in this study:

• an estimation of erosion indicated by beach crest retreat (R(t)) in metres • flooding by runup (Ru) in metres

• overwash (q) in litres per second per metre width

Figure 2: Vulnerability assessment indicators

The impacts are mainly assessed independently with empirical methods, although the increase in overwash reach and potential damage for the eroded profile is considered in the evaluation of plausible options.

Cross shore erosion

To develop coastal protection designs for the case study it is necessary to estimate the potential storm and SLR related beach retreat that could be experienced for the 1 in 100 year design event for different scenarios. Dune retreat will be calculated for the following cases:

x the 1-in-100 year storm event with the convolution method (Kriebel and Dean, 1993) for the different SLR scenarios

x the long term recession of the LowSLR , MidSLR and HighSLR scenarios with the Bruun rule (Bruun, 1962)

The storm retreat (R(t)) ranges from 19 metres for the base case (BC) to 25 metres for the high SLR scenario (HighSLR). These values seem reasonable for such a storm event, when compared to the shoreline recession of 10 to 30 metres which took place during the extreme storm event in March 2007 (Mather, 2008 also referring to Ramsay, P pers. comm. 2008). Figure 3 gives a comparison of the retreat for the BaseCase, LowSLR and HighSLR cases; the conceptual designs will be developed for the latter two cases and compared to the base case.

Figure 3: Comparison of calculated dune retreat for BaseCase, LowSLR and HighSLR

The coastline recession is calculated to vary from 14 metres for the LowSLR case to 45 metres for the HighSLR case.

Runup

Runup was calculated for the design storm with the preferred methods by Stockdon et al. (2006), Mather et al. (2011) and Mase (1989). The runup calculated with Mase’s (1989) method for the runup which the significant wave is expected to reach (R1/3) is considered to be the best to use for a design value in this instance as it is somewhat more conservative than the values calculated by the Mather et al (2011) method and somewhat lower than that of Stockdon et al. (2006). The measured values for the March 2007 storm compared well to this method’s predictions.

The potential runup for both the best and worst case scenarios, LowSLR and HighSLR, ranges from 4.2 to 4.5 metres above the still water level in comparison to four metres for the BaseCase. Therefore the actual increase in runup level should also include the still water level increment corresponding to the SLR increase. Significant flooding of the walkway and road is expected for all scenarios with potential maximum runup reaching the Addington hospital.

Overtopping

According to the EurOtop Manual (Pullen et al., 2008) wave overtopping is the process that occurs when the maximum runup height exceeds the crest height of a structure, dune or beach. In this instance runup exceeds the crest of the beach for a third or even more of the waves; therefore it is important to get an idea of the expected overtopping. Overtopping (in litres per second) was calculated by the following methods:

x Tanaka et al. (2002) which estimates overtopping in relation to the runup height, tidal variation and dune crest height

x The EurOtop manual formula by Van der Meer (2010) in relation to the dune freeboard and wave height

x The maximum overtopping per wave is estimated using a rule of thumb (Van der Meer, 2010) which gives Vmax as equal to 1000 times the qmax for small q values

The overtopping values calculated with the method of Tanaka et al. (2002) and the EurOtop manual gives relatively similar results. The results for q and q0 are all between 3 and 7 litres per second per meter (l/s/m) for the different scenarios, which means that erosion could take place depending on the surface material.

Vulnerability Assessment conclusions

Figure 4 shows assets adjacent to the project site and the areas that could be exposed to inundation and erosion from the design storm. As can be seen this includes a beach road, the newly built pedestrian walkway, the Addington district hospital (also a training facility for medicine students and interns) and various tourist facilities such as the uShaka Marine Park.

Figure 4: Assets potentially exposed to coastal erosion and flooding in a 1 in 100 year event Initial coastal hazard setbacks were calculated in terms of the estimated storm related dune retreat, the runup and overwash buffer and SLR recession. As is shown on Figure 4 the setback calculated for HighSLR reaches far into the hospital area. However, this does not mean that it is immediately at high risk, but rather that future investment and development in this zone is not recommended as risk would increase with rising sea levels. The area that could potentially be flooded is also estimated to be quite large and the beach road is expected to be out of operation.

The main causes of coastal damage for this design case are expected to be beach erosion (and a shift of the beach crest landward), coastal flooding (particularly on account of overwash of the beach crest) and pollution corresponding to storm debris and damage to infrastructure such as the sanitation network.

This output could be used as guidance to develop designs to decrease the vulnerability of the coastline. In theory, coastal protection designs proposed should be able to withstand a 1-in-100 year event. In practise, this might not necessarily be feasible to implement due to financial or structural limitations (e.g. lack of space or lack of sediment).

According to the vulnerability assessment, dune retreat (R(t)) during a 1 in 100 year event, could increase by approximately 2 or 6 metres respectively for a SLR increase of 0.3 metres and 1.0 metres from the base case of 19 metres of retreat. This shows the large impact of water level rise on the dune retreat. However, the dune retreat does not change much with a change in the significant deep water wave height (Hs). DEVELOPING CONCEPTUAL DESIGN SOLUTIONS

This section describes the design process followed during the development of conceptual coastal protection designs.

Potential protection solutions

The different coastal protection solutions described in the sections above have been summarized in Table 1. Whether an option will be eliminated or be developed further as a plausible option is also indicated.

Table 1: Elimination of potential coastal protection solutions

Option Eliminated Motivation

Do Nothing No Base case is a reference and should be included

Managed realignment No Will form part of 'do nothing' option

Setback lines or zoning No Will form part of all preferred options

Wetlands Yes The case study area does not contain wetlands

Kelp or seagrass Yes Big sea grass beds with dissipative potential are not indigenous

habitat due to the exposed location

Beach drainage Yes Has not been proven for many instances in practice

Disaster management Yes Not part of scope of project

Nourishment No Potentially good solution from coastal protection,

environmental, recreational and touristic point of view Dune building or

stabilization No

Potentially good solution from coastal protection, environmental, recreational and touristic point of view

Sea walls No Potentially good solution in terms of coastal protection and risk

reduction

Revetments No Potentially good solution in terms of coastal protection point of

view

Groynes Yes Does not solve cross shore problem, downstream problem

Detached breakwaters Yes Too large environmental and recreational impact

Dikes/Levees No Potentially good solution in terms of coastal protection and risk

reduction

Detailing plausible options

Initial plausible options are developed, compared and preferred options selected. Each option is developed for both the LowSLR and HighSLR cases (with no increase in storminess).

The development of plausible design options was guided by the following general assumptions to facilitate comparison:

• Develop preliminary cross sectional designs for comparative purposes

• The longshore transport gradient will be accommodated by the sand bypassing system. Durban has an operational sand pumping scheme; see Mather et al (2003).

• The design storm is a 1-in-100 year event with boundary conditions as stated earlier • The LowSLR case is assumed to coincide with the 50 year design life and a SLR of 0.3 metres • The HighSLR case should give an indication of a 100 year design life and a SLR of 1.0 metres The following design options were developed further by schematizing a conceptual cross section and estimating the main construction material volumes; ´Do Nothing´, ´Nourishment only´, ´Nourishment and dune´, ´Large dune´, ´Buried geotextile sand container (GSC) revetment´, ´Sea wall´ and ´Dike in dune´. The evaluation of the different plausible coastal protection solutions proposed is an essential part of the analysis of the case study. The assessment was done in terms of the following proposed evaluation criteria using a colour comparison spreadsheet:

x Impact of implemented protection measure on vulnerability of the coastline: such as the local impact of the protection solution (decreasing vulnerability for instance) as well as the regional effect (erosion might be increased in adjoining coastline

x Cost: the approximated capital expenditure required to implement the solution

x Environmental: the impact on the biodiversity of the area and the sustainability of the design x Utilisation: the social, touristic and recreational aspects of the site which would be affected by the

visual or health and safety impacts of the solution

Table 2 shows a basic colour comparison of the different options in terms of the selected evaluation criteria. The storm impact upon the case study location is first compared in terms of infrastructure and property, operations, health and safety and the regional or downdrift impacts. This is determined by considering the possible reduction in vulnerability that the implementation of a particular solution could produce. A sea wall could for instance limit the damage to property behind it and the storm impact could be minimal. Thereafter the variation in cost, environmental implications and impact upon the beach and facilities utilization of the different plausible options is compared. For example, building a larger dune is expected to have a positive impact upon the biodiversity of the area, but a negative impact upon the recreational utilization of the walkway.

Table 2: Colour comparison of plausible options: Potential impacts relating to the implementation of coastal protection measures

Evaluation Criteria Impact of storm Cost Environmental Utilization

Option Infr as tr u ct ur e a nd pr ope rt y O p e rati o n s H e al th an d saf e ty Re gi ona l/ Dow ndr ift C apit al Ma int e n an ce Biodiv e rs it y Su st aina bilit y Vi su al Re cr e at iona l Tour is ti c

Option 1: Base Case

Option 2: Nourishment only

Option 3: Nourishment with

small dune

Option 4: Large dune

Option 5: Buried GSC

Option 6: Sea wall

Option 7: Dike in Dune

Legend: Impacts

Positive

Minor

Moderate negative

Significant negative

From the colour comparison the “Sea wall”, “Dike in Dune” and BaseCase” seem to be the most negative options, whereas the sand based and “GSC revetment” options seem to be the most positive for this particular location.

The options were also compared in terms of basic material cost and their advantages and disadvantages. From the cost comparison the Base Case option would clearly be most expensive by far if it includes the relocation of the Addington Hospital. The relocation is estimated to be approximately 10 times more expensive than nourishment and up to 10 maintenance periods of 10 years each could be paid with this money, it is therefore not considered to be a viable solution at present; however, a cost benefit analysis is recommended for this option in a future design phase. The dike and sea wall options are considered second and third most expensive from an implementation point of view, although the maintenance and damages after a large event would be less.

Preferred options

The following two preferred options are selected and are refined in this section: the “Dune and Nourishment” Option and the “GSC revetment” Option. The preferred option designs are finally compared in terms of their potential impact upon the determined setback lines, their implementation cost and the feasibility of their implementation from an environmental, recreational, touristic and visual point of view.

The options are compared in terms of the basic cost for the main items. This includes the bulk materials such as sand, geotextile and the dune vegetation, but not other landscaping items. The material costs estimated for these options are quite similar.

Both preferred options proposed in this instance could be similar in terms of outer appearance, with a similar dune height, but somewhat wider dune crests for the two “Dune and Nourishment” options. Therefore it is not possible to choose between them at this stage of the design process. This exercise is expected to form part of the feasibility or detailed design phase. However it should be noted that the GSC design methodology used in this instance was a simple method and does not account for deformation of the bags, this option would necessitate more research into using the process based methods for this case. The schematic cross section for the LowSLR GSC option is presented in Figure 5.

Figure 5: Preferred Option 5L: The cross section and plan view of the proposed preliminary GSC revetment protection solution

Main findings of developing designs

The indicative costs of both of the mainly sand based preferred solutions are quite similar for a 500 metre stretch. Currently, the ´Dune and Nourishment´ Option, is favoured, but more research such as the following is required; an environmental impact assessment, cost benefit analysis, modelling the cross shore storm erosion using a two dimensional vertical model, modelling the longshore transport effects and development of detailed protection designs.

CONCLUSIONS AND RECOMMENDATIONS

In conclusion the case study site (a section of Durban´s central beaches) is vulnerable to a 1-in-100 year storm and this vulnerability is shown to increase significantly with future SLR. It is also shown how a vulnerability assessment could provide input to guide the design process. Softer sediment based solutions are preferred over hard options for the case study site. A representative study could prove valuable in terms of guiding coastal planning locally and regionally.

The main uncertainties of this study are related to data limitations; particularly a lack of storm surge data, limited period of wave data measurements, uncertainty regarding SLR predictions and limited measurement of storm related dune retreat, runup and overwash. Main limitations of the empirical methods available was a lack of methods relating to the reach of overwash on beaches and the fact that the process based method for GSC design (Recio and Oumeraci, 2008) was not calibrated for the conditions experienced at the site.

The following recommendations are given to local governments of coastal cities considering Climate Change adaptation interventions:

• Gather local data about the marine climate and coastal processes for your region • Determine local boundary conditions

• Assess the vulnerability of the coastline, focussing on critical coastal sections • Develop an SLR strategy to plan for future vulnerability increases

• Develop protection solutions for critical parts of the coastline

Some recommendations, of future research which is needed to help streamline future empirical vulnerability assessments, are summarised:

• Verifying empirical methods for greater variation in locations and conditions • Expansion of process based formulas for GSC´s for larger wave heights and periods • Develop a generic vulnerability assessment methodology for initial coastline reviews

• Integrate vulnerability studies of different focus levels so that accuracy of higher level studies increase

ACKNOWLEDGEMENTS

I would like to thank my supervisors at Delft University of Technology that guided the research of my M.Sc. thesis work: Prof. Marcel Stive, Dr. Bas Jonkman, Dr. Mathijs van Ledden, Prof. Rosh Ranasinghe and Dr. Jill Slinger. I would also like to thank the eThekweni Municipality in Durban and particularly Andrew Mather for advising me and assisting me in terms of sourcing essential information. Thanks also to PIANC-COPEDEC for selecting my paper for its 2012 conference and for providing me with a fellowship so that I can attend.

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