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Mossel Bay Sediment Supply Study
Technical Report · October 2015
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THINGSREPORT
Mossel Bay Sediment Supply Study
Client: Mossel Bay Municipality
Reference: IEMPB2808R001F214 Revision: 01/Final Date: 02 October 2015
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HASKONINGDHV UK LTD.
Rightwell House Rightwell East Bretton Peterborough
PE3 8DW United Kingdom Industry, Energy and Mining VAT registration number: 792428892
+44 1733 334455 T +44 1733 262243 F royalhaskoningdhv.com W
Document title: Mossel Bay Sediment Supply Study
Document short title: Mossel Bay Sediment Management Reference: IEMPB2808R001F214 Revision: 01/Final Date: 02 October 2015 Project name: Mossel Bay Sediment Supply Study Project number: PB2808 Author(s): David Brew and Rachel Jones
Drafted by: David Brew and Rachel Jones
Checked by: Greg Guthrie
Date / initials: 2nd October 2015
Approved by: Lance Dawkins
Date / initials: 2nd October 2015
Classification Open
Disclaimer No part of these specifications/printed matter may be reproduced and/or published by print, photocopy, microfilm or by any other means, without the prior written permission of HaskoningDHV UK Ltd.; nor may they be used, without such permission, for any purposes other than that for which they were produced. HaskoningDHV UK Ltd. accepts no responsibility or liability for these specifications/printed matter to any party other than the persons by whom it was commissioned and as concluded under that Appointment. The quality management system of HaskoningDHV UK Ltd. has been certified in accordance with ISO 9001, ISO 14001 and OHSAS 18001.
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Table of Contents
Introduction 1
1 The study area and scope of work 1 1.1 Scope of Work and Methodology 1 1.2 Coastal Setting 3 1.2.1 Geology and General Coastal Planform 3 1.2.2 Crenulate Bay Theory 10 1.3 Coastal Development and Infrastructure 11 1.3.1 Residential Development and Coastal Defence 11 1.3.2 Mossel Bay Desalination Plant 17 1.3.3 Coastal Municipal Infrastructure 18 1.3.4 Santos Beach 23 1.4 State of Dune 24 1.5 Legislation 26 1.5.1 Defining the Coastal Zone 28 1.5.2 Roles and Responsibilities 30 1.5.3 Coastal Management Programmes 31
2 Physical Processes 36 2.1 Wind 36 2.2 Wave Climate 38 2.3 Tidal Regime 44 2.4 Holocene Sea-level Rise 44 2.5 Historic Relative Sea-level Rise 45 2.6 Future Relative Sea-level Rise and Base Flood Elevations 45 2.7 Rainfall and River Discharge 47
3 Sediment Budget 49 3.1 Definition of the Vis Bay, Dana Bay and Mossel Bay Littoral Cells 49 3.2 Beach Morphology 50 3.3 Beach Sediment Particle Size 51 3.4 Offshore Bathymetry and Geomorphology 54 3.5 Sediment Supply from Rivers 57 3.5.1 Gouritz River 58 3.5.2 Rivers Entering Mossel Bay 58 3.5.3 Potential Barriers to River Sediment Supply 60 3.6 Net Longshore Sediment Transport 60 3.7 Coastal Erosion and Sediment Supply from Relict Dunes 62
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3.8 Future Erosion Rates in Response to Sea-level Rise 63 3.9 Foredune Formation and Destruction 65 3.10 Transport into the Nearshore Zone 66
4 Critical Areas of Erosion 68 4.1 Risk and Consequences of Erosion 68 4.2 Maintenance of Coastal Access 68 4.3 Infrastructure at Risk in the Dana Bay Littoral Cell 69 4.3.1 Vleesbaai 69 4.3.2 Boggomsbaai 70 4.3.3 Danabaai Western Parking Area 70 4.3.4 Danabaai between the Parking Areas 72 4.3.5 Danabaai Eastern Parking Area 72 4.4 Infrastructure at Risk in the Mossel Bay Littoral Cell 73 4.4.1 Diaz / Voorbaai 73 4.4.2 Bayview 75 4.4.3 Hartenbos 77 4.4.4 Klein Brak River 78 4.4.5 Reebok and Tergniet 78 4.4.6 Groot Brak River 79 4.4.7 Bothastrand and Outeniqua 79 4.4.8 Glentana 80 4.5 Summary 80
5 Regional Sediment Management 70 5.1 Allow Dune Erosion to Continue 71 5.2 Beach Nourishment 75 5.2.1 Potential Receiver Sites for Beach Nourishment in the Dana Bay Littoral Cell 75 5.2.2 Potential Receiver Sites for Beach Nourishment in the Mossel Bay Littoral Cell 76 5.2.3 Factors to Consider before Implementing Beach Nourishment 76 5.3 Beach Control Structures to Encourage Sand Deposition 77 5.3.1 Potential for Beach Control Structures in Dana Bay 78 5.3.2 Potential for Beach Control Structures in Mossel Bay 79 5.4 Adaptive Management 81 5.4.1 Decision Support Tool 86 5.4.2 Potential Monitoring 86
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6 Conclusions 88
References 91
Table of Tables
Table 1-1 Municipal infrastructure below the 5m MSL contour of the coast and rivers (Mossel Bay Municipality, tables provided by Warren Manuel) ...... 19 Table 1-2 Applicable Principles (adapted from DEA, 2014) ...... 27 Table 1-3 Objectives of the Integrated Coastal management Act (Celliers et al., 2009) ...... 28 Table 1-4 Defining the coastal zone in terms of the Integrated Coastal Management Act (adapted from DEA, 2014 and Celliers et al, 2009) ...... 28 Table 1-5 Local Government Roles and Responsibilities of the ICM Act (adapted from DEA, 2014)...... 31 Table 1-6 South African National Priorities for Coastal Management (adapted from DEA, 2014) ...... 31 Table 1-7 Western Cape Provincial Priorities for Coastal Management (adapted from Western Cape Government, 2004) ...... 32 Table 1-8 Eden District Management Objectives (adapted from Enviro Fish Africa, 2012) ...... 33
Table 2-1. Wave conditions simulated in the wave model for Mossel Bay (Hugo, 2013). Hm0 is significant wave height and Tp is peak wave period ...... 43 Table 2-2. Tidal datums for Mossel Bay (Admiralty Tide Tables, 2015) ...... 44 Table 2-3. Projected future sea-level rise for the design of new infrastructure (Mather and Stretch, 2012) ...... 46 Table 2-4. Areas vulnerable to coastal, estuarine and fluvial erosion and inundation between the Goutitz River and Groot Brak River (Umvoto Africa, 2010) ...... 46 Table 2-5. Rivers flowing into Mossel Bay and Dana Bay (adapted from CSIR, 2013 and River Health Programme, 2007) ...... 48 Table 3-1. Particle size of beach sediments in the 1980’s and 1990’s (Hugo, 2013). The 1988 and 1990 sieve analyses were carried out by CSIR and were obtained from Laurie Barwell. The 1996 sieve analyses were obtained from CSIR (2000) ...... 52 Table 3-2. Sedimentary facies offshore from Dana Bay and Mossel Bay (Cawthra, 2014) ...... 56 Table 3-3. Dams on rivers draining into Mossel Bay (River Health Programme, 2007) ...... 60 Table 3-4. Projected future erosion rates at five parking areas in Dana Bay and Mossel Bay (Mather, 2010) ...... 63 Table 3-5. Projected future sea-level rise and erosion rates ...... 65 Table 4-1. Risk categories used along the Mossel Bay Coastline (modified from PWA et al., 2008)...... 68 Table 4-2. Sources of risk to beach access points along the Mossel Bay Coastline (Mossel Bay Municipality, 2009) ...... 69
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Table 4-3. Risk and consequence of erosion in Dana Bay ...... 69 Table 4-4. Risk and consequence of erosion in Mossel Bay ...... 69 Table 5-1. Potential RSM options in Dana Bay and Mossel Bay...... 70 Table 5-2. Overview of the dune system and proposed dune rehabilitation measures (adapted from Ebersohn and Ebersohn, 2014) ...... 82
Table of Figures
Figure 1-1. Idealised process to implement Coastal RSM ...... 2 Figure 1-2. Methodology adopted in this study ...... 3 Figure 1-3. The south coast of South Africa showing crenulate bay morphology and location of Vis Bay, Dana Bay and Mossel Bay (adapted from Google Earth, 2015)...... 4 Figure 1-4. Crenulate shapes of Vis Bay, Dana Bay and Mossel Bay (adapted from Google Earth, 2015) ...... 5 Figure 1-5. Simplified geological map showing how the alternating relatively hard and soft rock exposures control the position of the crenulate bays (Cawthra, 2014)...... 6 Figure 1-6. Rock outcrops of Table Mountain Group sandstone at Hartenbos in Mossel Bay. Photograph taken by David Brew, 1st May 2015 ...... 7 Figure 1-7. Rock shore platform composed of Table Mountain Group sandstone at Reebok in Mossel Bay. Photograph taken by David Brew, 29th April 2015 ...... 7 Figure 1-8. Exposure of Bredasdorp Group sandstone at Groot Brak River. Photograph taken by David Brew, 29th April 2015 ...... 8 Figure 1-9. Exposure of Bredasdorp Group sandstone along the base of the cliffs at Danabaai. Photograph taken by David Brew, 1st May 2015 ...... 8 Figure 1-10. Shoreline geomorphology along the Mossel Bay Coastline (Jackson and Lipschitz, 1984)...... 9 Figure 1-11. Planforms of crenulate bays on the south coast and west coast of South Africa (Hugo, 2013). X and Y axes are normalised distances alongshore and cross shore, respectively ...... 10 Figure 1-12. Locations of defended portions of the coastline along Dana Bay (adapted from Google Earth, 2015) ...... 11 Figure 1-13. Coastal defences at Vleesbaai. Photograph taken by David Brew, 1st May 2015 .. 12 Figure 1-14. Gabion baskets and rock piling in front of Danabaai western parking area. Photograph taken by David Brew, 1st May 2015 ...... 12 Figure 1-15. Rock revetment and gabion baskets in front of Danabaai eastern parking area. Photograph taken by David Brew, 28th April 2015 ...... 13 Figure 1-16. Locations of defended portions of coastline along Mossel Bay (adapted from Google Earth, 2015)...... 14 Figure 1-17. Gabion baskets and wooden vertical posts in front of Hartenbos. Photographs taken by David Brew, 29th April 2015 ...... 15
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Figure 1-18. Low wall in front of Klein Brak River parking area. Photograph taken by David Brew, 28th April 2015 ...... 16 Figure 1-19. Fill and short length of rock revetment along the Reebok coast. Photograph taken by David Brew, 28th April 2015 ...... 16 Figure 1-20. Stacked gabion baskets at Bothastrand protecting individual properties. Photograph taken by David Brew, 29th April 2015 ...... 17 Figure 1-21. Coastal protection in front of properties at Glentana east. Photographs taken by David Brew, 29th April 2015 ...... 17 Figure 1-22. Layout of the seawater intake and brine discharge pipelines of Mossel Bay Desalination Plant (Killick et al., 2012) ...... 18 Figure 1-23 Examples of storm water outfalls in Mossel Bay subject to damage caused by wave action and erosion (MVD Consulting Engineers, 2011) ...... 19 Figure 1-24. Santos Beach in the lee of Mosselbaai breakwater (in the background). Photographs taken by David Brew, 29th April 2015 ...... 24 Figure 1-25. Dune vegetation types in Mossel Bay (Bgisviewer.sanbi.org, 2015) ...... 25 Figure 1-26 The Coastal zone of South Africa (Celliers et al, 2009) ...... 30 Figure 2-1. Wind rose compiled from 10 years of hindcast data (Hugo, 2013) ...... 37 Figure 2-2. Seasonal wind roses for the area bounded by 34o, 35oS and 21.75o, 22.75oE (CSIR, 2013)...... 38 Figure 2-3. Wave rose compiled from 10 years of hindcast data (Hugo, 2013). The data was obtained by Hugo (2013) from the United States National Centres for Environmental Prediction (NCEP). The wave rose was constructed from the NCEP global scale numerical climate model
WAVEWATCH III. Hs is significant wave height (the average height of the highest of one third of the waves in a given sea state). The rose shows the direction that the waves approach from ... 39 Figure 2-4. Seasonal wave roses compiled from 10 years of hindcast data (Hugo, 2013). The data was obtained by Hugo (2013) from the United States National Centres for Environmental Prediction (NCEP). The wave rose was constructed from the NCEP global scale numerical climate model WAVEWATCH III. Hs is significant wave height (the average height of the highest of one third of the waves in a given sea state). The rose shows the direction that the waves approach from ...... 40 Figure 2-5. Clustering of wave approach directions (Hugo, 2013). Each dot represents a measurement of wave direction with its corresponding wind direction ...... 41 Figure 2-6. Direction of wave approach in Mossel Bay (Hugo, 2013). Red arrows represent offshore southwesterly waves. Green arrows represent offshore easterly waves ...... 42 Figure 2-7. Simulation of significant wave heights in Mossel Bay for waves approaching from the southwest (Hugo, 2013). The left and right panels have winds input from the east and west, respectively. The scale is wave height in metres ...... 43 Figure 2-8. Simulation of significant wave heights in Mossel Bay for waves approaching from the east (Hugo, 2013). The left and right panels have winds input from the east and west, respectively. The scale is wave height in metres ...... 43
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Figure 2-9. Holocene sea-level rise along the south coast of South Africa (Ramsay, 1995; Compton, 2001, 2006). The x-axis is years before present multiplied by 1,000 and the y-axis is elevation in metres relative to present sea level ...... 45 Figure 2-10. 2.5 m above MSL (red), 4.5m above MSL (orange) and 6.5m above MSL (blue) swash and flood contour lines for the Hartenbos, Klein Brak and Groot Brak Estuaries (Umvoto Africa, 2010)...... 47 Figure 2-11. Average monthly rainfall recorded at Cape St. Blaize between 1972 and 2001 (CSIR, 2013) ...... 48 Figure 3-1. Components of a littoral cell ...... 49 Figure 3-2. Littoral cells of Vis Bay, Dana Bay and Mossel Bay (adapted from Google Earth, 2015)...... 50 Figure 3-3. Cross-shore beach profile in west Mossel Bay (Hugo, 2013). Location is shown on Figure 3.3 ...... 51 Figure 3-4. Locations of modelled wave climates (1 to 18) and cross-shore profiles and beach sediment samples (P1 to P8) (Hugo, 2013) ...... 53 Figure 3-5. Median particle size of beach sediment at eight locations around Mossel Bay (Hugo, 2013). The locations are shown on Figure 3.3 ...... 54 Figure 3-6. Bathymetry offshore from Dana Bay and Mossel Bay (Cawthra, 2014). The extent of the bathymetry describes the extent of the geophysical survey that was conducted to collect the data (about 255km2) ...... 55 Figure 3-7. Character of the seabed offshore from Dana Bay and Mossel Bay (Cawthra, 2014) 56 Figure 3-8. Vertical thickness of the unconsolidated sediment wedge offshore from Dana Bay and Mossel Bay (Cawthra, 2014) ...... 57 Figure 3-9. Sand bar across the mouth of the Hartenbos River. Photograph taken by David Brew, 29th April 2015 ...... 59 Figure 3-10. Sand body, rock platform and open mouth of the Klein Brak River. Photograph taken by David Brew, 28th April 2015 ...... 59 Figure 3-11. Sand bar at the mouth of the Groot Brak River. Photograph taken by David Brew, 29th April 2015 ...... 60 Figure 3-12. Modelled net longshore sediment transport rates in Mossel Bay (Hugo, 2013). Positive values represent transport to the south / southwest and negative values represent transport to the east / northeast. The distance along the coast is measured from a reference point at Diaz. The positions of the numbers in red squares are shown on Figure 3.3 ...... 61 Figure 3-13. Sediment transport within Mossel Bay (adapted from Google Earth, 2015) ...... 62 Figure 3-14. Shoreline movement in Mossel Bay between 1980 and 2010 (Hugo, 2013). Positive values of coastal change indicate accretion and negative values indicate erosion. Distances along the coastline are measured from a reference point at Diaz ...... 63 Figure 3-15. Low-lying discontinuous foredunes along the Mossel Bay coast. Photographs taken by David Brew, 28th April 2015 ...... 65 Figure 3-16. Growth of a large foredune field at Hersham between the relict dune edge (right) and beach (left). Photograph taken by David Brew, 29th April 2015 ...... 66
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Figure 3-17. Cross-section across Mossel Bay (right panel) showing the Cape St. Blaize submerged spit (Martin and Flemming, 1986). The left panel shows the location of the cross- section ...... 67 Figure 4-1. Low dunes fronting properties at Vleesbaai. (Photograph taken by David Brew, 1st May 2015). View looking south ...... 69 Figure 4-2. Wide area of vegetated dune fronting Boggomsbaai. Vleesbaai is in the background. (Photograph taken by David Brew, 28th April 2015). View looking south ...... 70 Figure 4-3. Low cliff in the top of the beach in front of Danabaai western parking area. (Photograph taken by David Brew, 30th April 2015). View looking west ...... 71 Figure 4-4. Cliff top properties set back from the vegetated dune edge at Danabaai. (Photograph taken by David Brew, 1st May 2015) ...... 72 Figure 4-5. Eroding dunes at the southern end of Diaz. (Photograph taken by David Brew, 28th April 2015) ...... 74 Figure 4-6. Hotels and condominiums along the central part of the Voorbaai coast. (Photographs taken by David Brew, 28th April 2015). Top left photograph view looking north. Bottom left and bottom right photographs view looking south...... 75 Figure 4-7. Properties set back along the Bayview coast. Note extended lawns in both photographs and eroding blow out in the right photograph. (Photographs taken by David Brew, 28th April 2015). Left photograph view looking south ...... 76 Figure 4-8. Eroding dunes in front of Bayview central parking area. (Photograph taken by David Brew, 28th April 2015) ...... 76 Figure 4-9. Facilities on the dune top at Hartenbos (central). (Photographs taken by David Brew, 29th April 2015). Left photograph view looking north. Right photograph view looking south ...... 77 Figure 5-1. Potential RSM options in Dana Bay (top panel) and Mossel Bay (bottom panel) (adapted from Google Earth, 2015) ...... 71 Figure 5-2. Relict dunes with no infrastructure between Vleesbaai and Boggomsbaai. (Photograph taken by David Brew, 1st May 2015). View looking north ...... 72 Figure 5-3. Relict dunes with no infrastructure north of Boggomsbaai. (Photograph taken by David Brew, 28th April 2015). View looking northeast ...... 73 Figure 5-4. Relict dunes with no infrastructure west of Danabaai. (Photograph taken by David Brew, 30th April 2015). View looking west ...... 73 Figure 5-5. Relict dunes with no infrastructure east of the Hartenbos River. (Photograph taken by David Brew, 29th April 2015). View looking northeast ...... 74 Figure 5-6. Photograph at Souwesia (between Tergniet and Southern Cross). (Photograph taken by Warren Manuel, 29th September 2015). View looking east ...... 74 Figure 5-7. An example of tombolos (left and centre) and a salient (right) formed by detached breakwaters at Sea Palling, UK ...... 79 Figure 5-8. An example of a terminal groyne at Langstone Rock headland, UK. Note the build-up of sand to the left of the groyne ...... 80
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Executive Summary
The coastline of Mossel Bay is located between the Gouritz River in the west and Glentana in the east. It contains three bays (Vis Bay, Dana Bay and Mossel Bay), comprising a suite of habitats, including relict vegetated dunes, active dunes, sand beaches, and nearshore subtidal areas, each with high recreational, environmental and economic values. Over the next 10 to 100 years, the relict dunes of the Mossel Bay Coastline are predicted to erode, placing oceanfront infrastructure and facilities at risk, and requiring mitigation measures to be implemented to prevent their loss.
This report describes the short-term (10-year), medium-term (50-year) and long-term (100 year) pressures to the coast within the context of the potential changes to the coastal geomorphology, processes and sediment budget, and recommends remedial measures to mitigate any potential adverse effects. It contributes to the body of literature that describes the wider physical and sedimentary processes along the Mossel Bay Coastline. This improved understanding will allow local and provincial government to make decisions that take into account regional processes alongside local factors.
Vis Bay, Dana Bay and Mossel Bay are crenulate bays in which sediment (sand) supply is constrained between erosion-resistant headlands, and their shapes are adjusting to orientations that are perpendicular to the main wave direction. This process means that longshore sediment transport rates are very low; estimated to be between 5,000m3/year and 15,000m3/year in Mossel Bay. The predominant supply of sand to the Mossel Bay Coastline is from erosion of the relict dunes. Average dune erosion rates are estimated to range from 0.1m/year to 0.4m/year. These rates are anticipated to increase in the future due to sea-level rise and increased storminess. Sediment contributions from the rivers entering the Mossel Bay Coastline are low, estimated to be between 14,000m3/year and 40,000m3/year.
The critical areas of erosion along the Mossel Bay Coastline were assessed using the following criteria:
What infrastructure / facility is at risk?
What is the probability that it will be impacted by coastal erosion over management planning horizons of 10, 50 and 100 years?
What are the consequences of loss of the infrastructure / facility (economic, safety and human health)?
The application of these criteria identified segments of the Mossel Bay Coastline that are subject to various levels of risk and consequence. In Dana Bay, high levels of erosion risk over all three planning horizons were identified at Vleesbaai and the Danabaai parking areas, with high and moderate consequences, respectively. In Mossel Bay, the consequence of loss of all facilities is considered to be high over all three planning horizons. However, the risk of erosion varied between facilities and over the different planning horizons. Facilities between Diaz and Hartenbos are considered at moderate risk of erosion in 10 years’ time, increasing to high risk in 50-100 years’ time. Facilities between Klein Brak River and Outeniqua are low risk in the short-term, increasing to moderate over the medium-term and moderate to high over the long-term. Facilities at Glentana are considered to be at moderate risk of erosion in 10 years’ time and at high risk in 50 and 100 years’ time.
Using a matrix of erosion risk and consequence and the conceptual assessment of the processes operating along the coast, four main potential sediment management options are identified that would encourage a more balanced system (Table E.1).
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Table E-1 Potential Management Approach
Bay Location Management Approach
Allow the natural process of dune erosion to Vis Bay Vis Bay continue without intervention Use beach restoration strategies particularly beach Vleesbaai nourishment to slow erosion rates Allow the natural process of dune erosion to Vleesbaai to Danabaai western parking area continue without intervention Install beach control structures to encourage sand Dana Bay Danabaai western parking area deposition Danabaai western parking area to Danabaai Allow the natural process of dune erosion to eastern parking area continue without intervention Install beach control structures to encourage sand Danabaai eastern parking area deposition Use beach restoration strategies particularly beach Diaz / Voorbaai, Bayview and Hartenbos nourishment to slow erosion rates Allow the natural process of dune erosion to Hartenbos to Klein Brak River continue without intervention Install beach control structures to encourage sand Klein Brak River, Reebok and Tergniet deposition Mossel Bay Allow the natural process of dune erosion to Tergniet to Southern Cross continue without intervention Install beach control structures to encourage sand Southern Cross to Glentana deposition Install beach control structures to encourage sand Glentana deposition
In areas where there is no infrastructure or facilities at risk of erosion, the recommended action is to allow the dunes to erode naturally without human intervention. Continued erosion of the dunes will supply large quantities of sand to the coastal system, helping to sustain the fronting beaches.
Where the beach and dunes are being lost through erosion subjecting the backing facilities to high risk of erosion, and the presence of coastal protection structures is likely to lead to a further loss of the beach, then beach nourishment should be considered as an option. This approach is a potential option at Vleesbaai in Dana Bay and along the Diaz / Voorbaai, Bayview and Hartenbos coasts in Mossel Bay. In order to effectively implement the beach nourishment option, further more detailed assessments will be required, including determination of the volume and texture of the sand, locations of the most efficient sources, and identification of environmental issues.
Beach control structures may be considered in areas where the softer options to sediment management are not appropriate. This might include detached breakwaters at the Danabaai parking areas and a terminal groyne at Glentana. The breakwater structures operate by reducing wave energy and sediment transport in their lee and the groynes interrupt longshore sediment transport, locally building up sediment.
Where there is more uncertainty about how the coast will respond to sea-level rise in the future, then adaptive management is recommended. This option is considered to be appropriate in Mossel Bay; along the Klein Brak River, Reebok and Tergniet coasts and the Groot Brak River, Bothastrand and Outeniqua coasts. In support of an adaptive management strategy, a monitoring campaign should be implemented that considers changes in the morphology of the beaches and dunes.
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Introduction
The Mossel Bay Municipality retained Royal HaskoningDHV to complete a Regional Sediment Supply Study for the Mossel Bay Coastline, which consists of three crenulate bays: Vis Bay, Dana Bay and Mossel Bay. A number of important studies have previously been conducted along this coastline including sea-level rise and flood risk (Umvoto Africa, 2010), an assessment of Mossel Bay Municipality infrastructure (Mather, 2010), the Eden District Municipality Coastal Management Programme (Enviro Fish Africa, 2012) and the Mossel Bay Municipality Dune Management Plan (Ebersohn and Ebersohn, 2014). One of the aims of this report is to add to this existing body of knowledge. This report is structured as follows:
Section 1 sets out the scope of works and introduces the study area;
Section 2 describes the physical processes;
Section 3 describes the sediment budget;
Section 4 reviews the critical areas of erosion;
Section 5 recommends options for Regional Sediment Management; and
Section 6 concludes the report.
1 The study area and scope of work
Within the following sub-sections, the scope of work and key factors that have impacted upon the Mossel Bay Municipality coastline are considered.
1.1 Scope of Work and Methodology The municipal area which is under the jurisdiction of Mossel Bay Municipality includes a coastline which is approximately 122km long. Understanding the long-term shoreline evolution and the processes underpinning it are critical because the coastline is an important contributor to the economic, social and environmental well-being of Mossel Bay. Sections of the Mossel Bay Coastline have proven to be highly susceptible to erosion and at high risk of being negatively impacted by future sea-level rise (Umvoto Africa, 2010). Following an assessment of municipal infrastructure damaged by storms that was commissioned by Mossel Bay Municipality (Mather, 2010), it was decided that a Sediment Supply Study should be completed. The objectives of the study are to:
determine the current sources and sinks of sediment in the region;
define the movement of this sediment alongshore and onshore-offshore;
identify and prioritise regional sediment management needs and opportunities along the coast;
provide options for addressing the specific issues facing the coast such as coastal erosion, recreational opportunities, dredging, and sediment flow through coastal watersheds; and
provide this information in a user-friendly decision support format to resource managers and the general public to assist in addressing coastal sediment management issues, and develop strategies to streamline sediment management activities.
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This study aims to facilitate integrated decision making between the two levels of government (Local and Provincial), and contribute to the body of information available that relates to coastal processes in this region.
Management of coastal erosion needs to consider over a broader area, rather than purely at the local scale, acknowledging the importance of sediment supply and the movement of sediment within the coastal system. This approach is referred to as Regional Sediment Management (RSM). Coastal RSM focuses on restoring coastal habitat by eliminating or reducing disruptions to natural processes, which produce sediment imbalances and exacerbate coastal erosion. Hence, RSM solves sediment-related problems by designing solutions that recognize the regional nature of natural processes. Coastal RSM also recognizes that sediment is a resource integral to the economic and environmental vitality of coastal beaches, and that sustainability can be achieved through beneficial reuse of littoral, estuarine, and river sediments.
In an ideal situation where data can be collected to support analysis, Figure 1.1 describes the steps that would be taken to implement Coastal RSM. The first step would be to develop a conceptual regional sediment budget based on available information. This would be followed by an understanding of where the data gaps are and collection of bespoke data, site visits and numerical modelling to support refinement of the budget. Once a refined and updated budget has been completed then recommendations for Coastal RSM would be made.
Preliminary Sediment Budget
Numerical Models - Data Collection - Site Visit
Develop / Refine Regional Sediment Budget
Recommendations for Coastal RSM
Figure 1-1. Idealised process to implement Coastal RSM
Although this study was not able to produce a numerical model or conduct any data collection, the approach has been appropriately adapted. Under the revised approach the recommendations for Coastal RSM rely mainly on Step 1 of Figure 1.1, complemented by a site walk over (part of Step 2). A literature review of previous studies was completed to identify the local sedimentary and coastal processes, erosion
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rates, and sand budgets that are influencing the composition and behaviour of the Mossel Bay Coastline (Figure 1.2). This was followed by a site walk over by representatives from Royal HaskoningDHV and Mossel Bay Municipality. A series of photographs taken during this walk over are used throughout the report to illustrate some of the key findings. Using this data in combination with economic, environmental, and societal considerations; critical areas of erosion are identified and recommendations are proposed for Coastal RSM. These recommendations are meant to inform the local decision-making process to help maintain the beaches and dunes of the Mossel Bay Coastline in order to restore coastal sandy habitats, enhance public safety and access, and sustain recreation and tourism..
Establish baseline processes from a review of existing literature and walk over observations
Consideration of environmental, societal and economic factors
Identification of critical areas of erosion
Recommendations for Coastal RSM
Figure 1-2. Methodology adopted in this study
1.2 Coastal Setting This sub-section provides a general description of the study area and is an important first step in the Coastal RSM process. It was populated via a review of the available literature, in conjunction with the site visit. This allowed the study to ground truth previous analysis, focussing on the practical and real issues being faced.
1.2.1 Geology and General Coastal Planform The south coast of South Africa between Cape Town and Port Elizabeth is characterised by a series of crenulate bays created by differential erosion of rocks exposed along the coast (Figure 1.3). The resistant rocks that bound the crenulate bays form headlands with steep sea cliffs, whereas between the headlands, softer rocks have been eroded to create the crenulate bay forms, which are partially infilled with Quaternary sediments.
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Mossel Bay Dana Bay
Vis Bay
Figure 1-3. The south coast of South Africa showing crenulate bay morphology and location of Vis Bay, Dana Bay and Mossel Bay (adapted from Google Earth, 2015).
Vis Bay, Dana Bay and Mossel Bay form three of these crenulate bays along the central part of this coast (located in the Eden District Municipality and collectively named the Mossel Bay Coastline) (Figure 1.4). In this report, the three bays are referred to using their English names, whereas the towns and villages of the same name are referred to using their Afrikaans names (Danabaai and Mosselbaai).
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19
17 18
16
15 1. Gouritz River
2. Vis Bay
9 14 3. Vleesbaai
13 4. Boggomsbaai
12 5. Dana Bay 11
10 6. Danabaai
8 7 7. Cape St. Blaize 6 8. Mosselbaai
9. Mossel Bay
10. Santos Beach
11. Voorbaai 5 4 12. Seal Island
13. Bayview
3 14. Hartenbos
15. Klein Brak River
2 16. Reebok 1 17. Tergniet
18. Groot Brak River (Southern Cross, The Island and Hersham)
19. Glentana
Figure 1-4. Crenulate shapes of Vis Bay, Dana Bay and Mossel Bay (adapted from Google Earth, 2015)
Vis Bay is the westernmost, and the smallest, crenulate bay (about 4km wide) and is located between headlands immediately east of the Gouritz River mouth and at Vleesbaai (Figure 1.4). Dana Bay is about 18km wide and bounded by headlands at Vleesbaai and Cape St. Blaize which are composed of relatively hard sandstones (quartzites) of the Table Mountain Group (part of the Cape Supergroup, Roberts et al., 2012; Cawthra, 2014). Cape St. Blaize separates Dana Bay from Mossel Bay (which is about 23km wide) to the east, which is bounded further east by a hard protruding rocky outcrop immediately east of Glentana. The crenulate bays have formed where less resistant younger sandstones of the Bredasdorp Group are exposed at the coast and have been preferentially eroded (Figure 1.5).
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Figure 1-5. Simplified geological map showing how the alternating relatively hard and soft rock exposures control the position of the crenulate bays (Cawthra, 2014).
The sandstones (quartzites) of the Table Mountain Group are exposed as rock outcrops and intertidal shore platforms along the coast of Mossel Bay. Discontinuous rock outcrops occur in front of Hartenbos (Figure 1.6). More continuous shore platforms occur along the coasts between the Hartenbos River and Klein Brak River, between Klein Brak River and Groot Brak River (Reebok, Figure 1.7) and between Groot Brak River and Glentana. The sandstones of the Bredasdorp Group are exposed at the mouth of the Groot Brak River in Mossel Bay and along the coast of Dana Bay at Danabaai (Figures 1.8 and 1.9).
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Figure 1-6. Rock outcrops of Table Mountain Group sandstone at Hartenbos in Mossel Bay. Photograph taken by David Brew, 1st May 2015
Figure 1-7. Rock shore platform composed of Table Mountain Group sandstone at Reebok in Mossel Bay. Photograph taken by David Brew, 29th April 2015
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Figure 1-8. Exposure of Bredasdorp Group sandstone at Groot Brak River. Photograph taken by David Brew, 29th April 2015
Figure 1-9. Exposure of Bredasdorp Group sandstone along the base of the cliffs at Danabaai. Photograph taken by David Brew, 1st May 2015
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The crenulate bays that were initially formed by differential erosion of the alternating hard and soft rock exposures are partially filled with sediment deposited during the Quaternary. Relict (non-active or fixed) dune fields extend up to 10s of kilometre’s inland from the modern crenulate bay shoreline, deposited at former shorelines and across antecedent low-relief coastal plains associated with historic sea-levels (Pether et al., 2000; Roberts et al., 2006). Cawthra (2014) showed that the Holocene (last 11,700 years of Earth history) was a time of intense aeolian activity, when the latest dune fields developed. The modern shorelines within each bay are mostly composed of sandy beaches or sand beaches and rock platforms backed by these relict dunes (Figure 1.10) (Jackson and Lipschitz, 1984; Roberts et al., 2006).
Figure 1-10. Shoreline geomorphology along the Mossel Bay Coastline (Jackson and Lipschitz, 1984).
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1.2.2 Crenulate Bay Theory The formation of crenulate bays has been extensively studied and modelled world-wide since the 1940s. The current shapes of the crenulate bays and how they might change in the future are fundamental to defining a regional sediment management strategy. In order to predict how they might change in the future, it is important to understand how crenulate bays form and the processes that control their evolution.
A coast between two erosion-resistant points (headlands or coastal structures) will readjust its shape to an orientation that is perpendicular to the main wave direction. The natural equilibrium form of this type of shoreline is crenulate shaped and may be classified as either in static equilibrium or dynamic equilibrium (Silvester and Hsu, 1997; Hsu et al., 2008). A static equilibrium bay has near zero longshore sediment transport or near zero sediment input or output. Within this type of bay waves are refracted and diffracted around the headland or coastal structure and approach the beach perpendicularly. In a dynamic equilibrium bay, a balance between sediment input and sediment output maintains the bay planform at its equilibrium position. This type of bay generally occurs where a river enters into the closed cell between the headlands or when the boundaries of the bay-beach allow sediment bypassing.
Hugo (2013) compared the planform shapes of Dana Bay and Mossel Bay with the planforms of three other crenulate bays on the south coast of South Africa and one on the west coast (Figure 1.11). For the purposes of comparison, St. Helena Bay was rotated and mirrored, because it is located on the west coast and has a different orientation. Figure 1.11 shows that all the bays have similar shapes (the bays are scaled to match the size of Algoa Bay).
Figure 1-11. Planforms of crenulate bays on the south coast and west coast of South Africa (Hugo, 2013). X and Y axes are normalised distances alongshore and cross shore, respectively
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1.3 Coastal Development and Infrastructure This sub-section contains an overview of development along the coast of each of the three bays. The Mossel Bay Coastline consists mainly of residential development, municipal service infrastructure and the harbour. The assessment of coastal defence infrastructure is highlighted because it potentially affects the coastal dynamics in crenulate bays.
1.3.1 Residential Development and Coastal Defence There is no development in Vis Bay. However significant lengths of the Dana Bay and Mossel Bay coastlines have been developed. Coastal residential areas in Dana Bay include (from west to east), Vleesbaai, Boggomsbaai and Danabaai and in Mossel Bay include Mosselbaai, Hartenbos, Klein Brak River, Reebok, Groot Brak River, Bothastrand and Glentana (Figure 1.4).
Apart from short lengths of coastal defence at Vleesbaai and protection in front of two beach parking areas at Danabaai, the majority of the Dana Bay shoreline is undefended (Figure 1.12). At Vleesbaai, a wall protects the southernmost part of the town (Figure 1.13, left photograph) with a series of discontinuous ad hoc defences protecting low-lying properties further north (Figure 1.13, right photograph). At the Danabaai parking areas the shoreline is fixed forward by them, resulting in a loss of beach width. The protection consists of 130m of stacked gabion baskets and rock piling in front of the western park area (Figure 1.14) and about 100m of rock revetment and gabion baskets in front of the eastern parking area (Figure 1.15).
Parking areas at Danabaai
Defended portion of the frontage
Vleesbaai
Figure 1-12. Locations of defended portions of the coastline along Dana Bay (adapted from Google Earth, 2015)
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Figure 1-13. Coastal defences at Vleesbaai. Photograph taken by David Brew, 1st May 2015
Figure 1-14. Gabion baskets and rock piling in front of Danabaai western parking area. Photograph taken by David Brew, 1st May 2015
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Figure 1-15. Rock revetment and gabion baskets in front of Danabaai eastern parking area. Photograph taken by David Brew, 28th April 2015
In Mossel Bay, three types of dune / infrastructure relationship are recognised (Figure 1.16):
the dunes are undefended with no infrastructure;
the infrastructure is set back a distance from the dune front which is undefended; and
the infrastructure is at the dune front and protected by various types of coastal defence.
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Bothastrand Glentana
Reebok
Defended portion of the frontage
Hartenbos
Figure 1-16. Locations of defended portions of coastline along Mossel Bay (adapted from Google Earth, 2015).
In south Mossel Bay, residential development stretches continuously from Mosselbaai north through to the Hartenbos River. Coastal protection is most prevalent in front of Hartenbos, where infrastructure (shops, parking areas, holiday homes and apartments, caravan/camping sites) are built on the dune top slightly set back from the dune edge, which is protected by a variety of structures (Figure 1.17).
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Figure 1-17. Gabion baskets and wooden vertical posts in front of Hartenbos. Photographs taken by David Brew, 29th April 2015
Further north, at Klein Brak River, the parking area on the northeast bank of the river mouth is within the adjacent dunes that continue north along the bay. The front edge of the parking area is protected by a low wall (Figure 1.18). Further northeast, along the Reebok coast, the local road and residences are set back from the cliff top, but several parking areas and beach access points connect to, and are seaward of this road. Short lengths of coastal defences, including rock revetments and fill, have been constructed in front of the dunes to protect these small sections of infrastructure (Figure 1.19).
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Figure 1-18. Low wall in front of Klein Brak River parking area. Photograph taken by David Brew, 28th April 2015
Figure 1-19. Fill and short length of rock revetment along the Reebok coast. Photograph taken by David Brew, 28th April 2015
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The majority of Bothastrand is set back from the dune edge, apart from a few properties at its western end. Here, short lengths of stacked gabion baskets protect individual properties (Figure 1.20).
Figure 1-20. Stacked gabion baskets at Bothastrand protecting individual properties. Photograph taken by David Brew, 29th April 2015
At the eastern end of Mossel Bay, properties at the eastern end of Glentana are located on the dune edge and have suffered erosion in the past. These properties are now protected by gabion baskets behind which are areas of lawned fill constituting their front gardens (Figure 1.21).
Figure 1-21. Coastal protection in front of properties at Glentana east. Photographs taken by David Brew, 29th April 2015
1.3.2 Mossel Bay Desalination Plant In 2009 and 2010, the area was struck by the worst droughts experienced in over 100 years. In response to the massive shortfall in drinking water, the Mossel Bay Emergency Desalination Plant was constructed
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on the northern bank of the Tweekulien Estuary. The plant has the capacity to produce 15 million litres of clean drinking water every day. To produce such a large quantity of clean drinking water, 33 million litres of sea water has to be extracted from the bay and filtered. The salty brine which is a by-product of the filtration process is pumped back out to sea via an outfall fitted with a diffuser to encourage suitable mixing of the brine mixture with the sea water (Killick et al., 2012). The configuration of the nearshore infrastructure associated with the desalination plant is shown in Figure 1.22.
Figure 1-22. Layout of the seawater intake and brine discharge pipelines of Mossel Bay Desalination Plant (Killick et al., 2012)
1.3.3 Coastal Municipal Infrastructure Mossel Bay Municipality conducted an assessment of all municipal infrastructure that is below the 5m above mean sea level (MSL) contour of the coast and rivers (examples photographs are included in Figure 1.23). The results of the survey conducted by Aurecon showed that 167 pieces of municipal infrastructure are beneath the 5m above MSL contour (Table 1.1).
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Structure 12 at Dwarsweg strandoord Structure21 at Tergniet Structure24 Reebok
Figure 1-23 Examples of storm water outfalls in Mossel Bay subject to damage caused by wave action and erosion (MVD Consulting Engineers, 2011)
Table 1-1 Municipal infrastructure below the 5m MSL contour of the coast and rivers (Mossel Bay Municipality, tables provided by Warren Manuel) Type Infrastructure type Infrastructure name Latitude Longitude Electricity Ground Mounted Transformer Ground Mounted Transformer_Gmt1 - 500Kva -34.0300 22.2229 Electricity Ground Mounted Transformer Ground Mounted Transformer_Gmt23 -2000Kva -34.0854 22.1383 Electricity Ground Mounted Transformer Ground Mounted Transformer_Gmt17 -34.0533 22.1352 Electricity Ground Mounted Transformer Ground Mounted Transformer_Gmt16 -34.0664 22.1460 Electricity Ground Mounted Transformer Ground Mounted Transformer_Gmt22 -2000Kva -34.0854 22.1383 Electricity Ground Mounted Transformer Ground Mounted Transformer_Gmt18 -34.0399 22.2202 Electricity Ground Mounted Transformer Ground Mounted Transformer_Gmt8 -200Kva -34.0512 22.2166 Electricity Ground Mounted Transformer Ground Mounted Transformer_Gmt7 - 200Kva -34.0542 22.2170 Electricity Ground Mounted Transformer Ground Mounted Transformer_Gmt2 A -34.0419 22.2219 Electricity Ground Mounted Transformer Ground Mounted Transformer_Gmt19 -34.0808 22.1366 Electricity Mini Sub-Station Atkv 1 Mini Sub-Station -34.1230 22.1168 Electricity Mini Sub-Station Sand Hoogte Mini Sub-Station -250Kva -34.0526 22.2135 Electricity Mini Sub-Station Lang Str Mini Sub-Station -34.0490 22.2145 Electricity Mini Sub-Station Klein Brak 1 Mini Sub-Station - 315 Kva -34.0887 22.1457 Electricity Mini Sub-Station Mac Mini Sub-Station -34.0854 22.1383 Electricity Mini Sub-Station Klein Brak 1 Mini Sub-Station - 200Kva -34.0832 22.1366 Electricity Mini Sub-Station Lang Str 2 Mini Sub-Station -34.0419 22.2193 Electricity Mini Sub-Station Mokodoor 6 Mini Sub-Station -34.1513 22.1003 Electricity Mini Sub-Station Die Dekke Mini Sub-Station -34.0539 22.2261 Electricity Mini Sub-Station Total Mini Sub-Station -34.0548 22.2249 Electricity Mini Sub-Station I&J Fish Mini Sub-Station -34.1793 22.1497 Electricity Mini Sub-Station Viking Mini Sub-Station -34.1779 22.1489 Electricity Mini Sub-Station Island Sub Mini Sub-Station -34.0528 22.2392 Electricity Mini Sub-Station Island Cove Mini Sub-Station -34.0501 22.2381 Electricity Mini Sub-Station Stasie Weg Mini Sub-Station -34.0442 22.2249 Electricity Mini Sub-Station Mossie Nes Mini Sub-Station -34.0416 22.2230 Electricity Mini Sub-Station Ssio Mini Sub-Station -34.0385 22.2222 Electricity Mini Sub-Station Oudshoorn Riool Mini Sub-Station -34.1181 22.0974
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Type Infrastructure type Infrastructure name Latitude Longitude Electricity Mini Sub-Station Hartenbos Landgoed 2 Mini Sub-Station -34.1141 22.1176 Electricity Mini Sub-Station Sonskynvalley 1 Mini Sub-Station -34.1158 22.0876 Electricity Mini Sub-Station Orion Str Mini Sub-Station -34.0565 22.2351 Electricity Mini Sub-Station Atkv 2 Mini Sub-Station -34.1198 22.1167 Electricity Mini Sub-Station Medium Voltage Mini Sub Station -34.0476 22.2163 Electricity Mini Sub-Station Kusweg Mini Sub-Station -34.0813 22.1645 Electricity Mini Sub-Station Port Natal Mini Sub-Station B -34.1190 22.1185 Electricity Mini Sub-Station Port Natal Mini Sub-Station A -34.1234 22.1161 Electricity Mini Sub-Station Mini Sub-Station -34.0498 22.3147 Electricity Mini Sub-Station Mini Sub-Station -34.0470 22.2159 Electricity Mini Sub-Station Mini Sub-Station -34.1786 22.1491 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm45 -34.0722 22.1136 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm11 B -34.1154 22.1164 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm30 -100Kva -34.0502 22.2371 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm12 -100Kva -34.1147 22.1114 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm27 315Kva -34.1119 22.0844 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm21 B -34.1146 22.1045 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm19-315Kva -34.1112 22.0995 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm31 16Kva -34.1171 22.0952 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm30 -25Kva -34.1166 22.0933 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm46 -34.0736 22.1431 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm10 - 50 Kva -34.1131 22.1122 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm18 -100Kva -34.0394 22.2152 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm28 -25Kva -34.1103 22.0841 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm34 -34.0464 22.2162 Electricity Pole Mounted Transformer Mv Pole Mounted Transformer -34.0679 22.1280 Electricity Pole Mounted Transformer Mv Pole Mounted Transformer -34.1144 22.1151 Electricity Pole Mounted Transformer Pole Mounted Transformer_Pm42 -34.0541 22.1310 Electricity Ring Main Unit Morrison_Ring Main Unit -34.0514 22.2333 Electricity Ring Main Unit Erica Park_Ring Main Unit -34.0538 22.2291 Electricity Ring Main Unit Port Natal_Ring Main Unit -34.1192 22.1168 Electricity Ring Main Unit Te Bakke Charlyes_Ring Main Unit -34.1731 22.1288 Electricity Mv Substation Mac Sub S/S Circuit Breaker -34.0854 22.1383 Electricity Mv Substation Kleinbrak1 S/S Building_32M2 -34.0874 22.1449 Electricity Mv Substation I&J S/Scircuit Breaker -34.1801 22.1502 Parks Picnic Area Picnic Area -34.0561 22.2368 Parks Sports Ground Kleinbrak Tennis Courts -34.0866 22.1435 Parks Sports Ground Groot Brak Sports Ground -34.0509 22.2156 Parking Area Parking Area Parking Area -34.2073 22.0303 Parking Area Parking Area Parking Area -34.0734 22.1842 Parking Area Parking Area Parking Area -34.0756 22.1794 Parking Area Parking Area Parking Area -34.0559 22.2364 Parking Area Parking Area Parking Area -34.2052 22.0485
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Type Infrastructure type Infrastructure name Latitude Longitude Parking Area Parking Area Parking Area -34.0898 22.1501 Road Transport Retaining Wall Bakke Swem Area 2 -34.1712 22.1261 Road Transport Retaining Wall Bakke Swem Area 3 -34.1718 22.1268 Road Transport Retaining Wall Bakke Swem Area 4 -34.1719 22.1269 Bakke Swem Area Stone Wall From Bakke Road Transport Retaining Wall -34.1727 22.1292 Beach To Santos Beach Santos Pavillion Restaurant Brick Wall At Back Road Transport Retaining Wall -34.1764 22.1359 Of Restaurant Santos Pavillion Restaurant Stone Wall At Back Road Transport Retaining Wall -34.1764 22.1359 Of Shellay 49 Santos Pavillion Restaurant Stone Wall From Road Transport Retaining Wall Restaurant To Public Beach Toilets Near Santos -34.1784 22.1382 Express Restaurant Road Transport Retaining Wall Point Village Retaining Blocks Left Of Stairs -34.1807 22.1553 Road Transport Retaining Wall Point Village Stone Retaining Wall Left Of Stairs -34.1807 22.1553 Point Village Concrete Retaining Wall Right Of Road Transport Retaining Wall -34.1811 22.1561 Stairs Point Stone Retaining Wall All Along Footpath Road Transport Retaining Wall -34.1850 22.1595 And Parking Road Transport Retaining Wall Dana Bay 2 Beach Parking Bottom Gabion -34.2074 22.0296 Road Transport Retaining Wall Dana Bay 2 Beach Parking Upper Gabion -34.2074 22.0296 New Glentana Storm water Upgrade Road Transport Retaining Wall -34.0501 22.3153 (Restaurant) A New Glentana Storm water Upgrade Road Transport Retaining Wall -34.0504 22.3153 (Restaurant) B New Glentana Storm water Upgrade Road Transport Retaining Wall -34.0495 22.3147 (Restaurant) C Road Transport Retaining Wall Bakke Swem Area 1 -34.1715 22.1264 Road Transport Retaining Wall Santos Pavillion Restaurant Stone Wall At Stairs -34.1762 22.1359 Road Transport Retaining Wall Gabion Structure -34.0402 22.2200 Sanitation Ablution Facility Ablution Facility -34.0506 22.3143 Sanitation Pump Station Delfino'S Pump Station -34.1832 22.1570 Sanitation Pump Station Restaurant Pump Station -34.1858 22.1590 Sanitation Pump Station Beach Street Pump Station -34.1799 22.1529 Sanitation Pump Station Harbour Pump Station -34.1788 22.1434 Sanitation Pump Station Draai Circle Pump Station -34.1789 22.1383 Sanitation Pump Station Pavillion Pump Station -34.1769 22.1364 Sanitation Pump Station Santos Caravan Park Pump Station -34.1729 22.1306 Sanitation Pump Station Bakke Strand Pump Station -34.1715 22.1262 Sanitation Pump Station Bakke Shalay'S Pump Station -34.1698 22.1225 Sanitation Pump Station Bakke Shalay'S 1 Pump Station -34.1673 22.1197 Sanitation Pump Station Twee Kuiling Pump Station -34.1523 22.1033 Sanitation Pump Station Pst-102 -34.0499 22.2387 Sanitation Pump Station Pst-104 -34.0419 22.2218 Sanitation Pump Station Pst-105 -34.0393 22.2219 Sanitation Pump Station Pst-106 -34.0512 22.2165 Sanitation Pump Station Pst-115 -34.1141 22.1042 Sanitation Pump Station Sonskyn Valley Sewer Pump Station -34.1154 22.0879 Sanitation Pump Station Oudtshoorn Road Sewer Pump Station -34.1181 22.0974 Sanitation Pump Station Hartenbos Landgoed Pump Station 3 -34.1146 22.1192
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Type Infrastructure type Infrastructure name Latitude Longitude Sanitation Pump Station Hartenbos Landgoed Pump Station 2 -34.1124 22.1161 Sanitation Pump Station Hartenbos Landgoed Pump Station 1 -34.1110 22.1153 Sanitation Pump Station Riviera Sewer Pump Station Pump 2 -34.1190 22.1173 Sanitation Pump Station Rivirea Overflow Sump Pump Station -34.1193 22.1173 Sanitation Pump Station Loerie Park Sewer Pump Station -34.1180 22.1161 Sanitation Pump Station Cloete Se Gat Sewer Pump Station -34.1246 22.1197 Sanitation Pump Station Eric Warner Sewer Pump Station -34.1350 22.1141 Sanitation Pump Station Sanitation Pump Station -34.0857 22.1406 Sanitation Pump Station Sanitation Pump Station -34.0857 22.1406 Sanitation Pump Station Hartenbos Riviermond Sewer Pump Station -34.1185 22.1221 Sanitation Pump Station Terrein B Sewer Pump Station Pump 2 -34.1282 22.1173 Sanitation Pump Station Bay Dunes Sewer Pump Station -34.1425 22.1113 Sanitation Pump Station Mascador Pump Station -34.1517 22.1009 Sanitation Pump Station Vbps Voor Baai Pump Station -34.1518 22.1016 Sanitation Pump Station Diaz Beach (Trio Towers) Pump Station -34.1544 22.1100 Sanitation Pump Station Sanitation Pump Station -34.0554 22.2349 Mossel Bay Regional Waste Water Treatment Sanitation Sewage Treatment Works -34.1077 22.1005 Plant Storm water Major Culvert Voorbaai / Gouriqua Park Culvert -34.1515 22.1003 Storm water Major Culvert Harbour Storm water (Main Outlet) -34.1799 22.1475 Storm water Major Culvert De Bakke Chalets Major Culvert -34.1706 22.1243 Storm water Major Culvert Gleniqua Beach Irin Culvert -34.0504 22.2965 Storm water Major Culvert Santos Beach Culvert -34.1796 22.1402 Storm water Major Culvert Santos Museum Culvert -34.1798 22.1407 Storm water Major Culvert Pienaar Strand Culvert -34.0525 22.2643 Storm water Major Culvert Stander Street 2 Culvert -34.0529 22.2139 Storm water Major Culvert Botha Street Major Culvert -34.0391 22.2142 Storm water Major Culvert Stander Street 3 Culvert -34.0537 22.2128 Storm water Major Culvert Glentana/Oudeweg Culvert B -34.0502 22.3152 Storm water Major Culvert Stander Street 1 Major Culvert -34.0520 22.2150 Storm water Major Culvert Glentana/Oudeweg Culvert A -34.0502 22.3147 Storm water Major Culvert Glentana (Oudeweg) Major Culvert -34.0495 22.3149 Storm water Major Culvert Beach Major Culvert -34.0500 22.2843 Storm water Minor Culvert Minor Culvert -34.0856 22.1422 Storm water Minor Culvert Glentana Sea Bottom Culvert -34.0509 22.3197 Storm water Minor Culvert Muller Street Minor Culvert 1 -34.0396 22.2159 Storm water Minor Culvert Muller Street Minor Culvert 3 -34.0394 22.2174 Storm water Minor Culvert Mossienes Street -34.0415 22.2228 Storm water Minor Culvert Muller Street Minor Culvert 2 -34.0397 22.2159 Storm water Minor Culvert Amy Searle Street Culvert - 900Mm -34.0403 22.2167 Storm water Minor Culvert Amy Searly Street Culvert 1 -34.0399 22.2144 Storm water Minor Culvert Suiderkruis Street Culvert -34.0540 22.2303 Storm water Minor Culvert Stasieweg 1 Lower Culvert -34.0434 22.2241 Storm water Minor Culvert Morrison Weg Culvert -34.0494 22.2387
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Type Infrastructure type Infrastructure name Latitude Longitude Storm water Minor Culvert Lang Street/Sandhoogte Street Minor Culvert -34.0515 22.2149 Storm water Minor Culvert De Bakke Parking Minor Culvert -34.1718 22.1274 Storm water Minor Culvert Santos Bay Culvert -34.1782 22.1373 Storm water Minor Culvert Santos Strand Minor Culvert -34.1779 22.1375 Storm water Minor Culvert Stasieweg 1 Higher Culvert -34.0432 22.2242 Storm water Minor Culvert Stasieweg / High Level Way Culvert -34.0441 22.2249 Storm water Minor Culvert Voorbrug Weg Culvert -34.0412 22.2240 Storm water Minor Culvert Botha Street Minor Culvert 2 -34.0390 22.2142 Storm water Minor Culvert Glentana/Oudeweg Minor Culvert -34.0503 22.3146 Storm water Minor Culvert Glentana Sea Top Culvert -34.0506 22.3165 Storm water Minor Culvert Botha Street Minor Culvert 1 -34.0389 22.2143 Storm water Minor Culvert Diaz Strand Culvert -34.1599 22.1102 Water Supply Water Treatment Works Klein Brak Water Treatment Plant -34.0837 22.1418
1.3.4 Santos Beach Santos Beach is located immediately west of Mosselbaai Harbour. Before the harbour breakwaters were built (1980s), the beach was predominantly composed of cobbles. After the breakwaters were completed, sand began to build up on Santos Beach to the point where it is now a relatively wide stable sand beach (Figure 1.24). The breakwaters act as a barrier to high energy waves that use to approach Santos Beach from around Cape St. Blaize headland, allowing sand to remain in the intertidal zone. There are future plans to extend the eastern breakwater of the harbour. The extension is likely to provide additional shelter to the beach from waves and provide further stability.
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Figure 1-24. Santos Beach in the lee of Mosselbaai breakwater (in the background). Photographs taken by David Brew, 29th April 2015
1.4 State of Dune This sub-section provides an overview of the dune vegetation and the current condition of the dunes (Ebersohn and Ebersohn, 2014). Dune vegetation provides an important habitat and is a stabiliser of the dune system. Maree and Vromans (2010) reported that Mossel Bay falls within the Cape Floristic Region of South Africa, and is a biodiversity hotspot. Maps of the predominant dune vegetation along the Mossel Bay Coastline are provided in Figure 1.25 (reproduced from the 2006 SANBI National vegetation map).
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FS 9 Groot Brak Dune Strandveld
FFs15 North Langeberg Sandstone Fynbos
FFd9 Albertinia Sand Fynbos
FFl 3 Canca Limestone Fynbos
AZd 3 Cape Seashore Vegetation
FRs 14 Mossel Bay Shale Renosterveld
Vis Bay and Dana Bay FS 9 Groot Brak Dune Strandveld
FFs15 North Langeberg Sandstone Fynbos
AZd 3 Cape Seashore Vegetation
FFl 3 Canca Limestone Fynbos
FFb 4 Central Coastal Shale Band Vegetation
Mossel Bay
Figure 1-25. Dune vegetation types in Mossel Bay (Bgisviewer.sanbi.org, 2015)
The dunes are commonly home to the Groot Brak Dune Strandveld comprising dense and tall (up to 3m), scrub with gaps supporting shrub. This vegetation is only found along the coast, occupying stabilised dunes and the steep coastal slopes, from the Gouritz River to Victoria Bay (approximately 25km northeast of Glentana) (Killick et al., 2012; Ebersohn and Ebersohn, 2014). Ebersohn and Ebersohn (2014)
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indicated that the largest areas covered by this vegetation type occur in Mossel Bay extending up to 17km inland.
The Groot Brak Dune Strandveld is fronted by a narrow strip of Cape Seashore Vegetation (Ebersohn and Ebersohn, 2014). This type consists of beaches, coastal dunes, dune slacks and coastal cliffs of open grassy, and to some extent also dwarf-shrubby vegetation, often dominated by a single pioneer species. Various plant communities reflect the age of the substrate (dune mobility), distance from the upper tidal mark and the exposure of dune slopes.
In 2014, a Dune Management Plan was drafted for the coast between Vis Bay and Glentana (Ebersohn and Ebersohn, 2014). The Plan identified a number of factors that were negatively impacting upon the dune system:
inappropriately located developments within the dune fields;
alien vegetation;
failing beach access points; and
failing beach infrastructure.
The shoreline between Mosselbaai and Glentana was compartmentalised into eight Coastal Management Units. For each of these Units, a detailed breakdown of the current state of the dunes, as well as proposed dune rehabilitation measures were produced (Ebersohn and Ebersohn, 2014). The Plan made the following recommendations to protect and rehabilitate the dune systems:
The approach to erosion management should aim for a naturally balanced system, with minimal human interference, whereby the sand is still able to exchange between the beach and the sea. Impacts on ecology, geomorphology and the visual amenity should not be threatened;
Soft defences are the favoured approaches. Continuous defences along the dune face are considered to be too destructive to the natural processes at play and also to the visual amenity of the landscape;
Construct defence structures above the high water line;
Maintain public access points and potentially create new ones in order to discourage visitors from walking over the dune system;
Slow down the discharge of water from the numerous storm water outlets along the coast line, and extend the outlets away from the primary dune system;
The removal of alien vegetation should also consider methods to stabilise exposed sand; and
In line with the Integrated Coastal Management Act, Act 24 (2008), inappropriate developments within the primary dunes should be prohibited.
1.5 Legislation The coastal zone is a multi-use, diverse, dynamic and sensitive area that needs to be managed in an integrative and inclusive manner. This is proposed to be undertaken via Integrated Coastal Management (ICM), a multi-disciplinary management vehicle which proposes to ensure that both development and the use of natural resources within the coastal zone are socially and economically justifiable, as well as being ecologically sustainable. ICM is described as a both an integrative and collaborative process in respect to the development, management and use of the coast within a framework that facilitates the integration of interests and shared responsibilities.
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The use of ICM as the legislated instrument in the context of the South African Coastal Zone originally began with the formulation of South Africa’s pre-eminent environmental legislation, the National Environmental Management Act (NEMA) in 1998. This Act’s primary objective includes the promotion of co-operative governance, as well as ensuring the priority development of co-ordinating mechanisms and institutions (DEA (Department of Environmental Affairs), 2014). Shortly thereafter, South Africa’s first ICM policies emerged in the form of the Coastal Policy Green Paper in 1998 and the White Paper for Sustainable Coastal Development in 2000. After some delay, this policy was enacted via a specific environmental management act (SEMA) under the umbrella of the NEMA, namely National Environmental Management: Integrated Coastal Management Act (ICM Act, Act No 24 of 2008).
The principles contained in the ICM Act mirror the principles contained in the Green and White Papers, and provide a coastal specific interpretation of the NEMA principles. These coastal specific principles (Table 1.2) must be applied in a balanced manner in order to complement the application of the NEMA environmental management principles. Implementation should attempt to promote the conservation, protection or sustainable development of the coastal environment. The complexity of decision making in the coastal environment does need to be acknowledged and, therefore, only those principles or objectives relevant to the particular decision or action contemplated must be applied. Objectives of the ICM Act are detailed in Table 1.3.
Table 1-2 Applicable Principles (adapted from DEA, 2014)
The coast must be retained as a national asset, with public rights to access and benefit from the National Asset opportunities provided by coastal resources.
Coastal economic development opportunities must be optimised to meet society’s needs and Economic Development to promote the wellbeing of coastal communities.
Coastal management efforts must ensure that all people, including future generations, enjoy the Social Equity rights of human dignity, equality and freedom.
The diversity, health and productivity of coastal ecosystems must be maintained and, where Ecological Integrity appropriate, rehabilitated.
The coast must be treated as a distinctive and indivisible system, recognising the Holism interrelationships between coastal users and ecosystems, and between the land, sea and air.
Risk Aversion and Coastal management efforts must adopt a risk averse and precautionary approach under Precaution conditions of uncertainty.
Accountability and Coastal management is a shared responsibility. All people must be held responsible for the Responsibility consequence of their actions, including financial responsibility for negative impacts.
All people and organisations must act with due care to avoid negative impacts on the coastal Duty of Care environment and coastal resources.
Integration and A dedicated, co-ordinated and integrated coastal management approach must be developed and Participation conducted in a participatory, inclusive and transparent manner. Partnerships between government, the private sector and civil society must be built in order to Co-operative ensure co-responsibility for coastal management and to empower stakeholders to participate Governance effectively. Recognising that the implementation of integrated coastal management is contextual. While a Differentiated Approach generic standardised) management framework is important, mechanisms of implementation cannot be rigid (“fit-for-all”). Adaptive Management Incrementally adjusting practices based on learning through common sense, experience, Approach experimenting, and monitoring (“learning-by-doing”).
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Table 1-3 Objectives of the Integrated Coastal management Act (Celliers et al., 2009)
Objective Description
Previously a number of different and often conflicting boundaries were being used to control different activities along the coast. The ICM Act clearly spells Determine the coastal zone of South Africa. out the boundaries of the different zones and describes procedures for adjusting these boundaries.
Previous management efforts in the coastal zone have lacked co-ordination, Provide within the framework of the NEMA, common purpose and accountability due to, among others, poorly defined for the coordinated and integrated responsibilities, sectoral approaches, fragmented legislation and inadequate management of the coastal zone by all enforcement of legislation. The ICM Act thus provides for ICM within the spheres of government in accordance with framework of the NEMA and this is provided for throughout all three spheres the principles of co-operative governance. of government. Preserve, protect, extend and enhance the Coastal public property is held in trust by the State for the benefit of all South status of coastal public property as being Africans, including present and future generations (inter-generational and held in trust by the State on behalf of all intra-generational equity). South Africans, including future generations.
The ICM Act ensures that the public has the right of physical access to coastal public property, as well as access to the benefits and opportunities Secure equitable access to the opportunities provided by the coastal zone. While not advocating unrestricted access and benefits of coastal public property. under any circumstances, the ICM Act describes the manner in which such access is to be managed.
Give effect to South Africa's obligations in The ICM Act provides for compliance with international laws relating to terms of international law regarding coastal coastal management and the marine environment. management and the marine environment.
An amendment to the ICM Act was recently published and came into effect in April 2015 (Act No. 36 of 2014). Some of these amendments are more significant than others, and include amendments relating to coastal public property, the coastal protection zone, the National Coastal Committee, coastal access and coastal use permits – which replaces coastal leases and concessions.
1.5.1 Defining the Coastal Zone The ICM Act also defines the coastal zone by providing uniform national definitions to support effective and practical implementation (Figure 1.26). The coastal zone is made up of coastal waters, coastal public property, the coastal protection zone; coastal access land; coastal protected areas, special management areas and estuaries. The defined areas of the coastal zone, their constituents, characteristics, responsible authority and authority responsible for adjustment, if applicable, is detailed in Table 1.4.
Table 1-4 Defining the coastal zone in terms of the Integrated Coastal Management Act (adapted from DEA, 2014 and Celliers et al, 2009) Coastal Waters Marine waters that form part of the internal waters or territorial waters of the Republic referred to in Constituents sections 3 and 4 of the Maritime Zones Act (Act No. 15 of 1994) (Maritime Zones Act), respectively, and, subject to section 26, any estuary. Characteristics Intention is for the State to control activities in coastal waters in the interests of all South African citizens. Responsible Authority National or provincial conservation agencies Authority responsible for N/A adjustment Coastal Public Property . Coastal waters; . Land submerged by coastal waters; . Any island in coastal waters; Constituents . The seashore; . Any Admiralty Reserve owned by the State; . Any other State land declared as coastal public property; and . Any natural resources. . Marks the shift away from resource centred management to people centred approach; and Characteristics . Aims to improve access to coastal resources, protect sensitive coastal ecosystems, promote functioning of natural coastal processes.
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Responsible Authority The State which includes all three spheres of Government. Minister of Environmental Affairs in accordance with section 27 of the ICM Act by notice in the Authority responsible for Gazette (the power of the Minister to determine or adjust the inland coastal boundary of coastal adjustment public property in terms of section 27, includes the power to make any consequential change to an adjoining coastal boundary of the coastal protection zone or coastal access land). Coastal Protection Zone . Sensitive coastal areas, as defined by the Environment Conservation Act (Act No. 73 of 1989, section 21 [1]); . Any part of the littoral active zone that is not coastal public property; . Any coastal protected area, or part of such an area, which is not coastal public property; Constituents . Any rural land unit that is situated within one kilometre) of the HWM which is zoned as agricultural or undetermined; . Any urban land unit that is situated completely or partly within 100 metres of the HWM; and . Any coastal wetland, lake, lagoon or dam which is situated completely or partially within a land unit situated within 1000 metres . To protect the ecological integrity, natural character, and the economic, social and aesthetic value of the neighbouring coastal public property; Characteristics . To avoid increasing the effect or severity of natural hazards; and . To protect people, property and economic activities from the risks and threats which may arise from dynamic Responsible Authority The State which includes all three spheres of Government MEC (Member of Executive Council) of a coastal province who is responsible for the Authority responsible for designated provincial lead agency) in accordance with section 28 of the ICM Act by notice in the adjustment Gazette Coastal Access Land Constituents Land designated as such in terms of section 26 of the ICM Act. Intention of coastal access land is to ensure that the public can gain access to coastal public property via Characteristics public access servitudes. Responsible Authority Municipalities Authority responsible for Municipality in accordance with section 29 of the ICM Act by notice in the Gazette adjustment Coastal Protected Areas A protected area that is situated wholly or partially within the coastal zone and that is managed by, or on Constituents behalf of an organ of state, but excludes any part of such a protected area that has been excised from the coastal zone in terms of section 22 of the ICM Act. . Coastal protected areas are managed via the Protected Areas Act; and Characteristics . Intended to augment the coastal protection zone. Responsible Authority Responsible conservation authority Authority responsible for N/A adjustment Special Management Areas Constituents An area declared as such in terms of section 23 of the ICM Act. . May prohibit certain activities from taking place within such a management area in order to: o Achieve the objectives of a coastal management programme; Characteristics o Facilitate the management of coastal resources by local communities; o Promote sustainable livelihoods; or Conserve, protect or enhance coastal ecosystems and biodiversity Responsible Authority National Government (may appoint a Manager) Authority responsible for Minister of Environmental Affairs in accordance with section 23 of the ICM Act by notice in the adjustment Gazette Estuaries . Estuarine Functional Zone (EFZ) as defined in the National Estuaries Layer, available from the South African National Biodiversity Institute's BGIS website (http://bgis.sanbi.org) (Government Gazette No. 33306, Notice No. R 546, 10 June 2010); and Constituents . This layer maps the estuarine functional zone for South Africa’s estuaries. The estuarine functional zone is defined by the 5m topographical contour (as indicative of 5m above mean sea level). The estuarine functional zone includes: Open water area; Estuarine habitat (sand and mudflats, rock and plant communities); and Floodplain area. . Estuaries are rich in resources, biodiversity and habitat provision; . The provide the link to the hinterland and the catchment; Characteristics . Their state is often referred to as the report card for the catchment as a whole; . They require integrated and dedicated management tools; and . Local input in an advisory capacity. Responsible Authority National, Provincial, Local Government or conservation agencies Authority responsible for N/A adjustment
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Figure 1-26 The Coastal zone of South Africa (Celliers et al, 2009)
1.5.2 Roles and Responsibilities The ICM Act assigns roles and responsibilities in respect to the management of the coastal zone to all three spheres of government. These are unpacked in the National CMP with assigned municipal roles and responsibilities detailed in Table 1.5. It should be noted that while the ICM Act defines a municipality as a metropolitan, district or local municipality established in terms of the Local Government: Municipal Structures Act (Act No. 117 of 1998) in respect to implementation if an area which falls within both a local municipality and a district municipality, responsibilities are assigned to the district municipality; or the local municipality, if the district municipality, by agreement with the local municipality, has assigned the implementation of that provision in that area to the local municipality.
As such, coastal management responsibilities within Mossel Bay are legally assigned to the Eden District municipality, with the Mossel Bay Municipality undertaking certain roles, with the approval and support of the District, should they have the capacity to do so.
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Table 1-5 Local Government Roles and Responsibilities of the ICM Act (adapted from DEA, 2014) Aspect Description Ensuring that the public has equitable access to coastal public property by designating coastal access land, designate in by-laws strips of coastal access land to Access to coastal public 1 promote access to CPP along the coast, withdraw inappropriate coastal access land property and Follow an environmentally sensitive and socially responsible process in designating coastal access land. Delineate coastal management lines in municipal zoning schemes maps (should participate in the provincial coastal management line determinations. Province will Coastal management line 2 have to Gazette). demarcation on zoning maps Proposed coastal overlay zones associated with coastal management lines will also need to be adopted and implemented. Determining and adjusting Ensure specified considerations are taken into account when determining or adjusting 3 coastal boundaries of a coastal boundary of coastal access land. coastal access land Marking coastal boundaries Delineate coastal boundaries determined or adjusted in terms of S26 on zoning 4 on zoning maps scheme maps. Prepare and adopt a municipal CMP for managing the coastal zone or specific parts 5 Municipal CMPs of the coastal zone in the municipality. Ensure that any plan, policy or programme adopted by an organ of state that may affect coastal management is consistent and aligned with municipal coastal Consistency and alignment management programmes, which in turn is aligned with provincial coastal 6 between Municipal CMPs and management programmes and the national coastal management programme and other statutory plans ensure that IDPs (including its spatial development framework) is consistent with other statutory plans [See S52 (1) (a-f)] adopted by either a national or a provincial organ of state. Consultation and public Adequate consultation and public participation precede the exercising of a power by a 7 participation municipality, which this Act requires to be exercised in accordance with this section. In implementing any legislation that regulates the planning or development of land, in Implementation of land use a manner that conforms to the principles of co-operative governance contained in 8 legislation in coastal Chapter 3 of the Constitution, apply that legislation in relation to land in the coastal protection zone protection zone in a way that gives effect to the purposes for which the protection zone is established as set out in section 17.
1.5.3 Coastal Management Programmes South Africa’s National Coastal Management Programme (NCMP) was released in 2014, and provides the direction and guidance towards a structured and standardised approach to coastal management in South Africa, including an appropriate cooperative governance framework (DEA, 2014). It is strongly founded on the initial precepts of the White Paper such that the Vision and principles of integrated coastal management are the same, and the national mandate in terms of the ICM Act. Key priorities included in the NCMP are detailed in Table 1.6.
Table 1-6 South African National Priorities for Coastal Management (adapted from DEA, 2014) Key Priorities Description Ensuring that all planning and decision-making tools applied by all organs of state within the coastal zone address coastal vulnerability by taking into account the dynamic nature Priority 1 – of our coast, sensitive coastal environments, health and safety of people, protection of Effective planning for coastal property rights, illegal structures within coastal public property, and appropriate vulnerability to global change placement of infrastructure not to compromise fiscal investment by the state, as well as the rehabilitation of coastal ecosystems. Ensuring that the public has safe and equitable access to coastal public property by Priority 2 – virtue of establishing sufficient coastal access land that is cognisant of the sensitivity of Ensuring equitable public coastal ecosystems, the needs and livelihoods of coastal communities or other socio- access in the coastal zone economic considerations, as well as the removal of inappropriate and unsafe coastal access points. Ensure that all estuaries along the South African coast are management in an integrated, Priority 3 – holistic manner in accordance with the National Estuary Management Protocol and the Integrating management in extent to which activities within estuaries are consistent with the other key priorities for estuaries coastal management Priority 4 – Ensure effective management of waste and wastewater into the coastal zone and Managing pollution in the minimizing adverse effects on the health of coastal communities, and on coastal coastal zone ecosystems and their ability to support the sustainable uses of coastal resources in a manner that is socially, economically and ecologically justifiable.
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Key Priorities Description Ensure the development and implementation of a dedicated, cooperative, co-ordinated Priority 5 – and integrated coastal monitoring and reporting system that includes compliance Establishing coastal monitoring and reporting in accordance with laws and policies, performance monitoring monitoring and reporting and reporting to measure progressing coastal management, and descriptive monitoring systems to inform decision- and reporting to measure variability and trends in biophysical, social and economic making characteristics and processes in the coastal zone. Establish a committed compliance and enforcement system for coastal management in Priority 6 – alignment with related laws and policies, and inclusive of cooperation and coordination Mechanisms for effective between organs of state with enforcement responsibilities and NGO appropriate compliance and enforcement capacity
To have an effective national information system and research framework to support Priority 7 – integrated coastal management, that is able to promote a dedicated, cooperative, Provision of coastal coordinated and integrated planning management approach accessible to all information and research stakeholders, in particular, decision-makers and the general public to ensure meaningful
participation. Priority 8 – Ensuring that the general public and decision-makers are appropriately aware, educated Strengthening awareness, and trained, so as to be able to take collective responsibility for managing and protecting education and training to the coastal environment in a manner that is socially, economically and ecologically build capacity justifiable. To ensure that institutional partnerships and mechanisms for ICM are established Priority 9 – amongst all sectors and spheres of government, the private sector and civil society in a Strengthening partnerships collaborative, problem-solving and consensus-building manner that promotes dialogue, for ICM cooperation, coordination and integration.
The inaugural provincial Coastal Management Programme (PCMP) for the Western Cape Province was published in 2004. This PCMP was updated in 2015 but has not, as yet, been approved. It builds upon the strengths and successes of the inaugural 2004 CMP (Western Cape Government, 2004), the starting point for the cycle of ICM in the Western Cape, and includes the requirements of updated policy direction identified specifically in respect to creating a clearly mandated transversal system closely linked to the green as well as blue economy, to unlock the economic potential of the coastal environment underpinned by sustainable development, explicitly enables Local Government and protects the coastal environment. Direct links are proposed with the implementation of Operation Phakisa, which focusses on unlocking the economic potential of South Africa’s oceans. Key priorities identified in the PCMP are detailed and described in Table 1.7.
Table 1-7 Western Cape Provincial Priorities for Coastal Management (adapted from Western Cape Government, 2004) Key Priorities Description A cornerstone of effective Integrated Coastal Management is the promotion of a balance between sustainable, viable and appropriate development and the protection of coastal Priority 1 - Social, Economic resources/assets, including the natural, social and cultural environments. A focus on Development and Planning social up-liftment and economic development and effective planning is critical in the continued fight to alleviate poverty and to generate sustainable livelihoods and is considered as the first priority in the Western Cape Province. Co-operative government and governance, mandated by the South African Constitution, is prescribed in the coastal environment by the ICM Act. The Act promotes stakeholder engagement and co-operation via the implementation of the provincial and municipal Priority 2 - Cooperative cross-sectoral, multi-actor CMPs and the formal cooperative governance structures Governance and Local established via the ICM Act, namely coastal management committees. Co-operative Government Support governance, in contrast to cooperative government, includes collaboration and partnerships between all forms of government and business, the private sector, research institutions and civil society (including traditional leadership). While the facilitation of coastal access is a municipal function in terms of the ICM Act, the Western Cape Government is a key role-player in building commitment and providing Priority 3 - Facilitation of guidance and support to municipalities to allow them to effectively implement, maintain Coastal Access and monitor coastal access. This priority area includes ensuring that the public has an equitable right of access to the coast and its resources as well as the management of such access.
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Key Priorities Description The vulnerable and sensitive Western Cape coastal zone is increasingly being threatened as a result of increasing demands for development, the exploitation of resources and the effects of global climate change (e.g. flooding, sea level rise and Priority 4 - Climate Change, increase in storminess). Resilience of both the environment and the communities living Dynamic Coastal Processes there is the focus of this priority area which proposes that development is both properly and Building Resilient planned and managed to avoid exposure to the significant risks associated with dynamic coastal processes. A uniform response is required to assessing and responding to coastal vulnerability as well as in respect to the rehabilitation of coastal areas. Coastal areas are particularly vulnerable to the negative impacts of pollution, being the end or collection point in various solid and liquid waste streams. This pollution emanates from both the marine environment, as a result of shipping and commercial fishing Priority 5 - Land and Marine- activities, as well as from land-based sources, as a result of effluent discharges, urban Based Sources of Pollution stormwater and the ‘throw-away’ mentality that pervades our society. This priority area and Waste aims to minimise the impacts associated with pollution in the coastal environment by proposing and implementing appropriate pollution control and waste management measures. The ecosystem goods and services of the Western Cape coastal zone contribute an enormous amount of tangible and intangible benefits to residents and visitors. These benefits cannot be measured in purely economic terms, and the protection, conservation Priority 6 - Natural capital and and continued ecological functioning of this natural capital is an asset to the province Natural Resource beyond measure. It is thus critically important that the natural functioning of the Western Management Cape coastal system, and its resources, be allowed to continue with minimum anthropogenic interference. Of particular conservation concern are environmental assets that promote sustainable livelihoods, which must be sustainably utilised, adequately protected and appropriately rehabilitated. Estuaries, which are under increasing pressure from human interference, modification and degradation, are considered amongst the most threatened ecosystems in the world. These sensitive, yet highly productive and diverse ecosystems, are of critical importance in the provision of ecological social and economic benefits in the Western Cape. This Priority 7 - Estuaries priority area focusses on the requirements of the National Estuarine Management Protocol and proposes the development and implementation of a Provincial Estuary Management Programme that will prioritise the development of estuary management plans and provide provincial direction for the establishment and operation of estuary advisory forums. The recognition of the value of the coast, shared ownership of the coastal zone and accompanying shared responsibility and need to facilitate co-operation can only be Priority 8 - Advocacy, effectively implemented if awareness is created and coastal managers and stakeholders Education, Monitoring and are effectively trained. Applied training and capacity-building of coastal managers and Capacity Building other stakeholders as well as accessible and co-ordinated research are additional requirements to ensure effective co-operative governance and government under this Priority Area. A final key priority of this CMP, in keeping with the NCMP, is compliance and Priority 9 - Compliance and enforcement of the ICM Act and the exploration of new and innovative ways to Enforcement strengthen capacity and collaboration in respect to monitoring within the coastal zone of the Western Cape.
The Eden District Municipal Coastal Management Programme (CMP), developed in 2012, is in accordance with the requirements of the ICM Act, but was developed prior to the development of the National Guideline as well as the NCMP. As such, the CMP does not include specific priorities but rather 13 specific coastal management objectives, as detailed below (Table 1.8) as well as a significant number of strategies, presented as management actions plans which include: issues, action required, legislative context, mandate, time frame, estimated cost and performance indicators.
Table 1-8 Eden District Management Objectives (adapted from Enviro Fish Africa, 2012) Coastal Management Description Strategies / Issues Objective
Coastal public access is recognised as a . Location and condition of existing legal coastal access human right, and access at Nautilus land . Additional coastal access land Public access Bay, Pinnacle Point, Mossel Bay Golf Club and Dana Bay were assigned as . Protection of the environment “hot spots” during stakeholder . Coastal access through future developments consultation . Illegal coastal access land
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Coastal Management Description Strategies / Issues Objective . Limit infrastructure development for coastal access land to designated coastal access land . Limit development in the coastal zone through land use planning and decision making processes Infrastructure, . Protect property against natural disasters and climate spatial planning Infrastructure that falls within the coastal change affects and development zone should be maintained or upgraded . Protect sensitive coastal habitats . Maintain the coastal 'Sense of Place' . Critical Biodiversity Area Maps and planning . Desalination plants . Estuary management . Alien vegetation removal . Restoration and rehabilitation of biodiversity . Biodiversity Monitoring . Rehabilitation of illegal activities or structures Biodiversity Biodiversity must be protected and . Pollution control and coastal clean-up strategies or protection, where possible enhanced, and alien pollution and control of water resources conservation and species should be removed and . Critical Biodiversity Area Maps and biodiversity enhancement indigenous fauna re-introduced conservation . Fire management . Financial measures to protect, conserve or enhance biodiversity . Protection of fish species Heritage Heritage sites should be protected and Heritage resources in the Eden coastal zone need to be resources available to the community and visitors managed, protected and shared by all . Effective and coordinated disaster management is required for ensuring human safety . Estuary breaching protocols must ensure human safety, protection of property and infrastructure and Disaster Each municipality must produce a the maintenance of ecosystem functioning management Disaster Management Plan . Procedures for whale entanglements and beached whales & dolphins . Awareness amongst recreational users of dangers associated with the sea . Pollution of water sources (estuary and marine) . Contamination of groundwater for human use or consumption at Jongensfontein . Estuaries being deprived of freshwater due to abstraction of groundwater from fountains/springs Water quality and Water Resource quality should be (Stilbay) or boreholes, abstraction of water directly quantity managed to ensure a healthy environmen from rivers and . construction of dams that reduce base flows . Developments in the coastal zone exceed the carrying capacity of water resources for human use or consumption . Implementation of the CMP . Interaction between organs of state (cooperative governance) . Capacity of Municipal entities to fulfill mandates Institutional Institutions should adopt a co-operative . Availability of GIS data for spatial planning, EIAs and arrangements approach to operate, manage and prioritise coastal management conservation initiatives . Funding for implementation of CMP strategies . Dissemination of the CMP . Drafting of additional by-laws . Enforce existing legislation and planning schemes . Incidents of non-compliance with National and Provincial legislation Compliance and Enforcement of the legislation should be . Illegal structures and activities in the coastal public enforcement effectively implemented property and coastal protection zone . Municipal courts . Mandates and contact details
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Coastal Management Description Strategies / Issues Objective . Create awareness of coastal management issues and solutions. Education and Education within the local communities . Education of public prior to issuing of recreational awareness to encourage understanding and fishing licenses. ownership of the coastline . Education of the judiciary with regards the severity of environmental transgressions. . Promote private sector investment. Economic . Mariculture opportunities and development of Still Bay development Job creation harbour . Micro-economic activities and opportunities. . Additional and maintained coastal access to stimulate tourism . Provide recreational activities within the coastal zone . Promote organized events . Blue flag beach and marina programme Tourism and The tourism industry should be protected . Safety and security recreation and “Eden should be recognised as the . Dogs and beaches jewel of the Garden Route” . Use of the beach by horses . Access to harbours . Tourism websites . Maintain the coastal 'Sense of Place' to benefit tourism . Allocation of launching rites for fishing jet-skis Sustain (and where possible enhance) Sustainable Existing activities need to be controlled and additional the livelihoods of those that rely on the livelihoods opportunities explored coastline Ensure that management approaches Research Management decisions based on sound scientific research are area specific
The Mossel Bay Municipalities’ Coastal Management Strategy, which was adopted in February 2012, seeks to implement these key Coastal Management Objectives for the Mossel Bay area and comply with the ICM Act responsibilities assigned to local municipalities. The strategy proposes not supporting development applications within 100 metres of the high water mark as well as funding several coastal management projects. Projects include:
The preparation and adoption of a municipal coastal management programme to manage the coastal zone in the Mossel Bay municipal area;
A sediment supply study (this assessment);
An assessment of all key municipal infrastructure below the 6.5 metre average mean sea level and 4.5 meter river contour to see how resilient they are to the rising sea level or flooding;
A sea level rise/flooding action plan to be incorporated into the Municipality’s Disaster Management Plan; and
The determination of setback lines.
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2 Physical Processes
Physical processes are the mechanisms that adapt the environment by either positive or negative feedback. These include wind, waves, tides, currents, surges and extreme events, each of which influences the coastal environment in a different way. In the coastal zone, these processes are responsible for the distribution of sediment that constructs landforms. Hence, the examination of possible future patterns of sediment movement and deposition cannot be achieved without a thorough understanding of the underlying physical forcing factors.
2.1 Wind Understanding wind is important for two reasons. First, wind acts as a medium for direct aeolian transport of finer particles, which can accumulate to form coastal dunes. Second, when the wind blows, energy is transferred to the water surface. As waves form, the surface becomes rougher, increasing the surface area over which the wind acts and increasing the potential energy input. As the wind energy is increased, waves become larger.
Hugo (2013) summarised a 10-year hindcast wind dataset from 85km south of Cape St. Blaize obtained from the United States National Centres for Environmental Prediction (NCEP). Figure 2.1 presents the wind rose which shows that the prevailing winds are dominantly bimodal from the west and the east (Carr and Botha, 2012). CSIR (2013) analysed Voluntary Observing Ships (VOS) data for a rectangle bounded by 34.0oS, 35.0oS, 21.75oE and 22.75oE (up to about 100km south of Mosselbaai) for the period 1960 to 2013. The wind distribution from this data is shown in Figure 2.2 and also describes an east-west distribution. Winds from the west predominate in winter, both in terms of strength and direction, and to a lesser degree, in autumn and spring. In summer, winds from the east are more common.
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Figure 2-1. Wind rose compiled from 10 years of hindcast data (Hugo, 2013)
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Figure 2-2. Seasonal wind roses for the area bounded by 34o, 35oS and 21.75o, 22.75oE (CSIR, 2013)
2.2 Wave Climate With respect to RSM, the wave climates of Vis Bay, Dana Bay and Mossel Bay are important for two reasons. First, differential wave energy alongshore causes variations in the magnitude of erosion due to wave impacts at the dune toe. Second, the direction and magnitude of wave approach relative to the shoreline orientation, controls the direction and strength of alongshore sediment transport.
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The Mossel Bay Coastline is a wave-dominated coast with the prevailing direction of wave approach from the southwest (Davies, 1980; Heydorn and Tinley, 1980). The wave climate has a distinct seasonality. The long southwest fetch means that the coast is dominated by winter swell waves with large wave heights and long periods generated in the southern oceans. During summer, the storm tracks shift southward. The wave climate is still dominated by waves from the southwest, but wave heights are generally smaller with shorter wave periods. A wave rose for a location about 85km south of Cape St. Blaize, constructed from a 10-year (January 1997 to June 2008) wave dataset, is shown in Figure 2.3 (Hugo, 2013). Seasonal wave roses are shown in Figure 2.4.
Figure 2-3. Wave rose compiled from 10 years of hindcast data (Hugo, 2013). The data was obtained by Hugo (2013) from the United States National Centres for Environmental Prediction (NCEP). The wave rose was constructed from the NCEP global scale numerical climate model WAVEWATCH III. Hs is significant wave height (the average height of the highest of one third of the waves in a given sea state). The rose shows the direction that the waves approach from
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Figure 2-4. Seasonal wave roses compiled from 10 years of hindcast data (Hugo, 2013). The data was obtained by Hugo (2013) from the United States National Centres for Environmental Prediction (NCEP). The wave rose was constructed from the NCEP global scale numerical climate model WAVEWATCH III. Hs is significant wave height (the average height of the highest of one third of the waves in a given sea state). The rose shows the direction that the waves approach from
A comparison of the wind direction with the corresponding wave direction shows two main offshore wave directions; a relatively small set clustered around 90° and a relatively large set around 215° (Figure 2.5). Although the offshore southwesterly waves approach from approximately 215o, they actually approach the shore of Mossel Bay from 90o in the western corner and 180o along the eastern end of the bay after refraction, directional spreading and loss of energy around Cape St. Blaize. This causes a decreased wave angle at the coast (Hugo, 2013). Any storm waves that are generated with an east to west trajectory
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do not encounter the headlands and maintain much of their original energy. Figure 2.6 describes the wave rays caused by this refraction.
Figure 2-5. Clustering of wave approach directions (Hugo, 2013). Each dot represents a measurement of wave direction with its corresponding wind direction
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Figure 2-6. Direction of wave approach in Mossel Bay (Hugo, 2013). Red arrows represent offshore southwesterly waves. Green arrows represent offshore easterly waves
Hugo (2013) used the SWAN (Simulating Waves Nearshore) numerical wave model to simulate the nearshore wave climate in Mossel Bay. The offshore wave climate was transformed into the nearshore using four wave and wind conditions (Table 2.1):
Condition 1: waves from the southwest, wind from the east; Condition 2: waves from the southwest, wind from the west; Condition 3: waves from the east, wind from the east; and Condition 4: waves from the east, wind from the west.
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Table 2-1. Wave conditions simulated in the wave model for Mossel Bay (Hugo, 2013). Hm0 is significant wave height and Tp is peak wave period o -1 o Condition Hm0 (m) Tp (s) Wave Direction ( N) Wind Speed (ms ) Wind Direction ( N)
1 2.45 11.0 215 7.0 90
2 2.45 11.0 215 4.2 254
3 2.45 7.0 95 6.9 91
4 2.45 7.0 95 3.8 250
The significant wave heights simulated for the four combined wind and wave conditions at mean sea level are shown in Figures 2.7 (waves from southwest) and 2.8 (waves from east). Overall, the predicted significant wave height increases from in the lee of the Cape St. Blaize headland towards the more exposed coast at the eastern end of the bay. Given their similar orientation and planform to Mossel Bay, it is likely that similar wave conditions exist in Vis Bay and Dana Bay.
Figure 2-7. Simulation of significant wave heights in Mossel Bay for waves approaching from the southwest (Hugo, 2013). The left and right panels have winds input from the east and west, respectively. The scale is wave height in metres
Figure 2-8. Simulation of significant wave heights in Mossel Bay for waves approaching from the east (Hugo, 2013). The left and right panels have winds input from the east and west, respectively. The scale is wave height in metres
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The two southwesterly wave conditions in Figure 2.7 describe the important effect of wind direction. The inclusion of an easterly wind (left panel) results in a higher wave energy in the lee of Cape St. Blaize than a westerly wind (right panel). This is primarily due to the easterly generated wind waves being less refracted and dispersed. The two different wind conditions added to the waves approaching from the east have a much less pronounced effect on the penetration of waves in the lee of the headland (Figure 2.8).
2.3 Tidal Regime The Mossel Bay Coastline experiences semidiurnal micro-tides, with a large spring to neap fluctuation. The mean spring tidal range is about 1.8m and the mean neap tidal range is about 0.6m (Table 2.2). The tidal range determines the extent of beach exposure and inundation throughout the tidal cycle and hence redistribution of sediment. Of particularly importance are the timing and height of high tides coincident with maximum wave heights and surge developed during storms.
Table 2-2. Tidal datums for Mossel Bay (Admiralty Tide Tables, 2015) Tidal Datum LAT (m)
Highest astronomical tide (HAT) 2.4
Mean high water spring (MHWS) 2.1
Mean high water neap (MHWN) 1.5
Mean sea level (MSL) 1.2
Mean low water neap (MLWN) 0.9
Mean low water spring (MLWS) 0.3
Lowest astronomical tide (LAT) 0.0
Killick et al. (2012) showed that average tidal current velocities in Mossel Bay are 0.04-0.06m/s, with a general current direction from north to south, although Seal Island has a localised influence within the bay. Also, it is possible that the thicker sand body in the southwest part of Mossel Bay (see Section 3.4) may have formed in response (partly) to formation of a tidal gyre (a rotating tidal current) induced by the headland of Cape St. Blaize. As a consequence, the area in the lee of Cape St. Blaize acts as a sediment trap, where the configuration of the tidal currents encourage deposition of sand.
2.4 Holocene Sea-level Rise The modern sediments of the coast are the culmination of present interglacial (Holocene) sedimentation that began around 11,700 years ago. These sediments were deposited in response to sea-level change, and natural coastal processes, and contain a detailed record of the sedimentology, geomorphology, climate and sea-level evolution. This Holocene sedimentary record can be used to improve our understanding of the modern coastline by placing it in the context of its long-term evolution, allowing better predictions of likely future change.
Two different models of sea-level rise during the Holocene (last 11,700 years of Earth history) have been described for the south coast of South Africa (Ramsay, 1995; Compton, 2001, 2006) (Figure 2.9). According to Ramsay (1995), sea level gradually rose after 7,000 years ago to reach a high stand of about 3m above present level around 4,500 years ago. Sea level then fluctuated, with a significant low stand (around 3,000 years ago) and a further high stand (around 1,500 years ago), until it reached its current elevation around 1,000 years ago. Compton (2001, 2006) suggested that the Holocene high stand (about 3m above present level) was earlier, around 6,500 years ago followed by a fall to its present level around
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5,500 years ago. After 5,500 years ago, sea level oscillated near present levels with occasional minor low stands.
Figure 2-9. Holocene sea-level rise along the south coast of South Africa (Ramsay, 1995; Compton, 2001, 2006). The x-axis is years before present multiplied by 1,000 and the y-axis is elevation in metres relative to present sea level
The rise in sea level during the Holocene deposited the extensive wedge of sediment across the offshore zone (see Section 3.4), which continues onshore in the form of the now relict dunes in Vis Bay, Dana Bay and Mossel Bay (Martin and Flemming, 1986).
2.5 Historic Relative Sea-level Rise Mather (2009) showed that relative sea levels have risen around the south coast of South Africa over the last 50 years. However, Mather (2009, 2010) argued that data recorded at the tide gauge in Mossel Bay is unreliable and measured relative sea-level rise should be based on the Simon’s Town (Cape Town) and Knysna tide gauges. The regional rate of relative sea-level rise over the period 1960-2008 has been about 1.5mm/year (Mather, 2009, 2010; Umvoto Africa, 2010).
2.6 Future Relative Sea-level Rise and Base Flood Elevations Recommendations regarding how sea-level rise projections should be incorporated into the design of new infrastructure were developed by Mather and Stretch (2012) (Table 2.3). Using the design criteria of Mather and Stretch (2012), future sea-level rises of 0.1m, 0.6m and 1.0m at 10 years, 50 years and 100 years, respectively are used for RSM in each of Vis Bay, Dana Bay and Mossel Bay.
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Table 2-3. Projected future sea-level rise for the design of new infrastructure (Mather and Stretch, 2012) Value of Life of Impacts of Failure of Planned Sea-level Rise (m) Infrastructure (ZAR) Infrastructure Infrastructure
Low (up to 2 million) Less than 20 years Low: minor inconvenience 0.3
Medium: local impacts, loss of Medium (2-10 million) 20 to 50 years 0.6 infrastructure and property High: regional impacts, loss of High (20-200 million) 50 to 100 years 1.0 significant infrastructure and property Very high (greater than Greater than 100 Very high: failure of key national 2.0 200 million) years infrastructure
Umvoto Africa (2010) modelled results for sea-level rise identified several areas that are most susceptible to coastal, estuarine and riverine erosion and inundation along the Dana Bay and Mossel Bay coastline. These include Mosselbaai, Hartenbos, Klein Brak River and Groot Brak River (Table 2.4 and Figure 2.10).
Table 2-4. Areas vulnerable to coastal, estuarine and fluvial erosion and inundation between the Goutitz River and Groot Brak River (Umvoto Africa, 2010) Area Coastline, Estuarine and Fluvial Erosion and Inundation A 2.5-4.5m above MSL swash run-up can be expected at Gouritsmond due to the relatively rocky nature of the coastline, with a small portion of coastal property along Voelklip Gouritz possibly being affected by higher run-ups. Extensive Goukou River floodplain inundation along the lower reaches of the river may occur as a result of flooding and back flooding. Cape Vacca and Vleespunt may focus large storm swell energy, resulting in possible swash run-ups in the range of 4.5-6.5m above MSL in the vicinity of Visbaai and Vleesbaai town. Visbaai is unpopulated, and swash run-ups will cause possible beach erosion, dune Cape Vacca to Vleesbaai undercutting and collapse, and damage to the road along Cape Vacca. The majority of Vleesbaai town is currently protected by a rocky coastline, with only the northernmost sandy coastline section of Vleesbaai (first line of coastal developments) being possibly damaged during a 6.5m above MSL swash run-up. A 2.5-4.5m above MSL swash run-up is likely in this area during an extreme event, causing beach erosion, dune undercutting and collapse, and erosion of the Blinde Estuary Vleesbaai to Danabaai sand bars. Dune undercutting and collapse at Danabaai may cause damage to coastal property and infrastructure, with only approximately 30m of the shoreward section of the vegetated foredune remaining undeveloped. The area is currently protected by rocky coastline and a swash run-up of even 6.5m above MSL (although 2.5m above MSL would be expected in the case of an extreme storm) Pinnacle Point to Cape St. would have no affect other than possible cliff undercutting. Pinnacle Point Cave 13B, which Blaize is a very important Middle Stone Age (about 40,000-165,000 years before present) archaeological site, may be damaged over time however. Cape St. Blaize may focus large storm swell energy, resulting in possible swash run-ups in the range of 4.5-6.5m above MSL along Mossel Bay. These run-ups would cause damage Mosselbaai to coastal developments towards Cape St. Blaize, as well damaging Mossel Bay Harbour. Large swell may inundate the N2 road and railway line, as well as damage them through cliff undercutting. A small portion of undeveloped foredune (about 10-15m high and 10-70m wide) currently Diasstrand, Bayview and protects coastal and residential areas, and would prevent a swash run-up of 6.5m above Voorbaai MSL causing damage. The Hartenbos River estuary and floodplain are highly vulnerable to flooding, with a large portion likely to be inundated if estuary flood waters reach 2.5m above MSL (Figure 2.10). Larger swash run-ups would also damage coastal property along northern Hartenbos Hartenbosstrand due to development removing the protective foredune barrier. Beach erosion and dune collapse might occur along the undeveloped foredune section towards Klein Brak River.
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Area Coastline, Estuarine and Fluvial Erosion and Inundation The Klein Brak River Estuary and floodplain are highly vulnerable to flooding, and as with Hartenbos Estuary a large portion is likely to be inundated if estuary flood waters reach 2.5m above MSL (Figure 2.10). Klipheuwel would be flooded by a 2.5m above MSL flood, Klein Brak River while the developed eastern bar of the Klein Brak Estuary could be damaged by a 4.5- 6.5m above MSL swash run-up. Klein Brak Riverstrand, Reebok and Tergniet are currently protected by a thin undeveloped shoreward section of foredune (about 10-20m high and 20-70m wide). The Groot Brak River estuary and floodplain are also highly vulnerable to flooding, with central Groot-Brakrivier, Bergsig, The Island and N2 road all below the 2.5m above MSL flood line (Figure 2.10). The western Groot Brak Estuary sand bar and suburb of Suiderkruis are also vulnerable to erosion and damage by a 4.5-6.5m above MSL swash Groot Bak River run-up. Groot-Brak Riverstrand, Outeniquastrand and Glentana Beach are currently protected by a thin undeveloped shoreward portion of vegetated foredune (about 10-20m high and 20-50m wide), except in parts where it has been removed through development and is vulnerable to erosion by 4.5-6.5 m above MSL swash run-ups.
Figure 2-10. 2.5 m above MSL (red), 4.5m above MSL (orange) and 6.5m above MSL (blue) swash and flood contour lines for the Hartenbos, Klein Brak and Groot Brak Estuaries (Umvoto Africa, 2010)
2.7 Rainfall and River Discharge The region experiences a fairly temperate climate with mild to hot summers and cold, generally wet winters. The average monthly rainfall at Cape St. Blaize is between 25mm and 48mm with a tendency for peaks in April, August and October-November (Figure 2.11) (CSIR, 2013).
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Figure 2-11. Average monthly rainfall recorded at Cape St. Blaize between 1972 and 2001 (CSIR, 2013)
The mean annual runoffs of the rivers flowing into Mossel Bay and Dana Bay are very small compared to those of the Gouritz River (Table 2.5) (1,680 Mm3/year, DWAF, 2004b).
Table 2-5. Rivers flowing into Mossel Bay and Dana Bay (adapted from CSIR, 2013 and River Health Programme, 2007) River Bay Catchment Area (km2) River Length (km) Mean Runoff (Mm3/year)
Blinde Dana 36 10.4 0.56
Hartenbos Mossel 205 34.2 5.00
Small Brak Mossel 550 49.9 59.53
Groot Brak Mossel 190 31.5 38.79
Gouritz Gouritzmond 45,702 267 695
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3 Sediment Budget
Sediment (sand) budgets are important tools in understanding regional sediment processes and quantifying the sediment sources (inputs) and sinks (outputs) from a littoral cell; a defined length of shoreline along which the cycle of sediment erosion, transportation, and deposition is essentially self- contained. Sediment enters a cell from one or more rivers (the Hartenbos, Klein Brak and Groot Brak Rivers in Mossel Bay) draining the coastal watersheds, and/or from erosion of beaches and coastal bluffs (coastal dunes in Dana Bay and Mossel Bay), and possibly transport from offshore. A littoral cell includes the beach above the highest tides, wind-blown sand, and any sediment within the surf/swash zone and out to the depth on the shoreface at which wave energy stops transporting sediment (so-called closure depth,
Hc). The boundaries between littoral cells are delineated by a change in the rate of longshore sediment transport. These may include rocky headlands or submarine canyons that intercept transport paths. Some of the sediment inputs and outputs are not well defined, such as those in cross-shore directions, and these components are often treated as an unknown and estimated by the residual in the budget. The principal components involved in development of a littoral cell sediment budget are shown in Figure 3.1.
Figure 3-1. Components of a littoral cell
3.1 Definition of the Vis Bay, Dana Bay and Mossel Bay Littoral Cells Dana Bay and Mossel Bay constitute the two primary littoral cells bounded and separated by fixed headlands (Figure 3.2). Vis Bay is a smaller littoral cell to the west of Dana Bay. The Dana Bay littoral cell is confined between headlands at Vleesbaai and Cape St. Blaize. It is considered that no (or very little) sand enters the bay from around the Vleesbaai headland (i.e. an absolute or limited partial sediment transport boundary). Although no evidence exists, this inference is based on the Hugo (2013) assertion that no sediment (or very little sediment) enters Mossel Bay around Cape St. Blaize, which has similar
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morphological characteristics to the situation at the Vleesbaai headland. Mossel Bay is between Cape St. Blaize and the rock shore at Glentana. This littoral cell is considered separate from Dana Bay because no sediment (or very little sediment) enters from around Cape St. Blaize (Hugo, 2013) (i.e. also an absolute or limited partial sediment transport boundary). It is possible, given the indistinct nature of the eastern boundary of the Mossel Bay littoral cell at Glentana, that some sediment may be bypassing east of this boundary into the adjacent cell (i.e. the boundary is fixed and partial). The rate at which sediment is bypassing the Glentana headland is not known.
Mossel Bay
Dana Bay Gouritz River
Vis Bay
Figure 3-2. Littoral cells of Vis Bay, Dana Bay and Mossel Bay (adapted from Google Earth, 2015)
Hugo (2013) estimated the closure depth in Mossel Bay (the depth on the shore face at which wave energy stops transporting sediment) to be in approximately 6m of water in the lee of Cape St. Blaize to 12m at the exposed coast at Glentana.
3.2 Beach Morphology The beaches of Vis Bay, Dana Bay and Mossel Bay are accumulations of loose sand that change shape in response to changes in wave energy. The movement of beach sand, in turn, dissipates some of the energy of a wave breaking on the shore. The beaches are therefore able to maintain themselves in a state of dynamic equilibrium with their environment, due to the mobility of sediments. The morphology of the beaches changes on a seasonal basis because of differences between shallower swell-dominated waves in summer, and steeper storm waves in winter. Sediment is moved up the foreshore during the summer to build berms and moved seaward in winter to build a longshore bar. The bar acts as a submerged breakwater dissipating wave energy and further erosion of the foreshore is reduced. This offshore sediment movement has the effect of narrowing the beaches. In the summer, the sediments are
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transported onshore and a berm-type profile is gradually formed and the beach will eventually be restored and widened. According to Mather (2010), this seasonal variability in beach width is typically 30m, but can be as much as 50m, depending on the extent of available sediment and the relative strengths between the summer and winter waves.
Hugo (2013) collected beach profiles at eight positions along the Mossel Bay coastline from the top of the dunes to 1.4m below mean sea level, supplemented by nearshore bathymetric data. Beach profile 2 (the only one published by Hugo, 2013) in the western part of Mossel Bay is shown in Figure 3.3. The slope of beach in profile 2 between the elevations of mean high water spring (which is 0.9m above mean sea level, Table 2.2) and mean low water spring (which is 0.9m below mean sea level, Table 2.2) of about 1:50.
Figure 3-3. Cross-shore beach profile in west Mossel Bay (Hugo, 2013). Location is shown on Figure 3.3
3.3 Beach Sediment Particle Size One of the most important parameters controlling the erosion, transport and deposition of sediments is their particle size. In addition, the beaches and shoreface of the Mossel Bay Coastline might be potential receiver sites for beach nourishment (one of the Coastal RSM options that may be considered) and it is therefore important to characterize their particle size distribution.
th Hugo (2013) compiled sediment particle size data (median d50 and 90 percentile d90) from beach samples collected in the late 1980’s and early to mid 1990’s at Hartenbos, Klein Brak River and Groot Brak River (Table 3.1). The results show that the sand is medium-grained at all three locations, but the average medium grade increases in size from the western part of Mossel Bay (0.28mm at Hartenbos) to the central part of the bay (0.35-0.38mm at Klein Brak River and Groot Brak River).
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Table 3-1. Particle size of beach sediments in the 1980’s and 1990’s (Hugo, 2013). The 1988 and 1990 sieve analyses were carried out by CSIR and were obtained from Laurie Barwell. The 1996 sieve analyses were obtained from CSIR (2000) Location Hartenbos Klein Brak River Groot Brak River
Particle size (mm) d50 d90 d50 d90 d50 d90
0.24 0.37 0.35 0.50
1988 0.30 0.45
0.28 0.43
1988 Average 0.24 0.37 0.31 0.46
0.26 0.37 0.36 0.56 0.31 0.40
0.23 0.31 0.39 0.66 0.28 0.37 1990 0.27 0.36 0.37 0.51 0.30 0.37
0.28 0.40 0.33 0.46 0.34 0.42
1990 Average 0.26 0.36 0.36 0.54 0.31 0.39
1996 Average 0.34 0.44 0.46 0.61 0.40 0.54
Overall Average 0.28 0.39 0.38 0.54 0.35 0.47
Hugo (2013) collected sediment samples at the eight profile locations shown in Figure 3.4. A low resolution sieve analyses was completed (i.e. only a low number of sieves less than 1mm in size were th used) to calculate the median (d50) and 90 percentile (d90) particle diameters. In addition, a visual analysis was undertaken with a grading tool to estimate the size of the average particle. The results of Hugo (2013) are presented in Figure 3.5 and show median particle sizes of 0.25mm (medium-grained sand) south of the Hartenbos River, increasing to 0.3-0.4mm (medium-grained sand) around the rest of the bay. The coarsest beach is located in front of Glentana at the eastern end of the bay. The increasing particle size of the beaches from west to east is positively correlated with wave height (Figures 2.7 and 2.8).
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Figure 3-4. Locations of modelled wave climates (1 to 18) and cross-shore profiles and beach sediment samples (P1 to P8) (Hugo, 2013)
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Figure 3-5. Median particle size of beach sediment at eight locations around Mossel Bay (Hugo, 2013). The locations are shown on Figure 3.3
Mather (2010) collected beach samples from five parking area sites in Dana Bay and Mossel Bay and found fine- to medium-grained sand with an overall median diameter of 0.25mm. At the mouth of the Gouritz River, the beach sand was sampled in 1987 (Heydorn, 1989). The results indicate uniform medium-grained sand, the diameter of which ranged from 0.21mm to 0.47mm, with an average diameter of 0.34mm.
3.4 Offshore Bathymetry and Geomorphology Coastal geomorphology is concerned with a range of space scales. The width of the coastal zone can vary substantially, and in many cases it may be necessary to incorporate the bathymetry of the offshore area and in other cases it may only be necessary to consider the immediate shoreline area. Hence, the morphology of the coast could be dictated to an extent by the bathymetry and morphology of features that exist offshore.
The nearshore seabed is relatively flat and smooth offshore of both Dana Bay and Mossel Bay (Cawthra, 2014) (Figure 3.6) with no major valleys or canyons. In Mossel Bay, the inner part of the nearshore zone seaward to about 45m below mean sea level is relatively wide (up to 8km) and shallow, whereas in Dana Bay it is narrower (about 1.5km) and relatively steep (averaging 0.86o). The offshore area of both bays, extending seaward of 45m below mean sea level, has a significantly lower gradient (averaging 0.05o in the mapped area).
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Figure 3-6. Bathymetry offshore from Dana Bay and Mossel Bay (Cawthra, 2014). The extent of the bathymetry describes the extent of the geophysical survey that was conducted to collect the data (about 255km2)
Cawthra (2014) mapped the nearshore seabed sediments into five different facies (Figure 3.7 and Table 3.2). The mapping is based on three marine geophysical surveys supported by scuba diving to ‘ground truth’ the geological deposits on the seabed. The majority of the seabed that was mapped is covered by medium-grained sands (77.8% or 208km2) with smaller areas of exposed sandstone (8.9% or 28.6km2) in the eastern part of Mossel Bay, attached to Cape St. Blaize and in a linear strip offshore from Dana Bay. Patches of coarser grained seabed sediment (5.5% or 6.6km2) also occur associated with the sandstone outcrops.
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Figure 3-7. Character of the seabed offshore from Dana Bay and Mossel Bay (Cawthra, 2014)
Table 3-2. Sedimentary facies offshore from Dana Bay and Mossel Bay (Cawthra, 2014) Facies Description
a Silicified sandstone and calcarenite seafloor outcrops
b Subdued morphology/eroded outcrops of Acoustic Facies a
c Coarse grained bioclastic sediment
d Planar shelf sands
e Wave rippled shelf sands
f Fine grained sediment: silty mud
g Low reflectance, fine-grained sediment: mud
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The seabed sands form the active surface of an unconsolidated thickness of sediment (wedge-shaped) deposited during the Holocene (Martin and Flemming, 1986). The wedge is thickest in Dana Bay and in the lee of Cape St. Blaize headland (Mossel Bay side) (Cawthra, 2014) (Figure 3.8). It thins to the east across the nearshore zone of Mossel Bay until it disappears and is replaced by sandstone outcrops. The sand body adjacent to Cape St. Blaize has accreted here in response to the dominant longshore transport from west to east in combination with the possibility of a tidal gyre formed in the lee of the headland (see Section 3.2).
Figure 3-8. Vertical thickness of the unconsolidated sediment wedge offshore from Dana Bay and Mossel Bay (Cawthra, 2014)
3.5 Sediment Supply from Rivers The quantity of beach sand that is contributed to the littoral cells by the rivers is an uncertainty in the sediment budget of Dana Bay and Mossel Bay. The volume of sediment discharged depends on the size and geology of the catchments and the amount of runoff (see Table 2.4). Low flow down the rivers results in smaller volumes of transported sediment. In addition, dams and sand mining in the catchments reduce the river sediment supply. Delivery of sand to the bays is also likely to be episodic. During many years, the mouths of the rivers are blocked by sand bars, which change morphology with seasonal changes in wave climate and rainfall. During periods of low river discharge the bars grow through longshore sediment transport and interrupt sediment supply from the rivers. Artificial breaching of the bars may take place to prevent flood damage to the surrounding up-river areas, particularly in the Groot Brak and Hartenbos Rivers (Maree and Vromans, 2010).
Rooseboom (1975) estimated maximum sediment discharges of about 14,000m3/year for the Hartenbos River and Groot Brak River, and 40,000m3/year for the Klein Brak River. Given that only a small proportion of this total sediment is beach-sized, combined with the presence of dams on both the Klein Brak and
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Groot Brak Rivers, the amount of bedload sediment entering Mossel Bay from these rivers is very small. Indeed, the bathymetry does not describe any ebb-tide deltas along the bay (Figure 3.6) and it is likely that marine processes rework the limited delivered sediment into the general sediment wedge. The supply of beach sand from even smaller water bodies entering the coastal system, including the Blinde River (Dana Bay) and Tweekuilen River (Mossel Bay) is negligible compared to other supplies. This means that Vis Bay and Dana Bay receive no sediment from rivers entering the system within their crenulate bay shapes. Any suspended mud that is discharged by the rivers is likely distributed further offshore and transported away by tidal currents.
3.5.1 Gouritz River The largest river in the area (by far) is the Gouritz River, which enters the coast immediately west of Vis Bay (outside the defined littoral cells). Its mouth is permanently open but can adopt different geomorphological configurations depending on discharge (Heydorn, 1989). The river is one of the main sources of riverine sediment to the coastal zone along the south coast of South Africa (Birch, 1980). Martin and Flemming (1986) reported that the Gouritz River is responsible for contributing over 13 million m3 of sediment per year to the coast. This sediment is then transported east with wave-driven longshore transport and is eventually trapped off eastward facing headlands where there is accommodation space and protection from the further longshore transport. The sediment pile off the Gouritz River is up to 20m thick (Cawthra, 2014).
3.5.2 Rivers Entering Mossel Bay The Hartenbos River is located immediately north of Hartenbos. A large sand bar at the mouth of the river prevents exchange with the sea for the majority of the time (Figure 3.9). The Klein Brak River enters Mossel Bay between Hartenbos and Groot Brak River. The mouth of the river has historically been open 90% of the time, and has been permanently open since 1991 (SSI Environmental, 2010). However, reduced flows have resulted in an increase in sedimentation at the mouth (Figure 3.10). Flood events in the past have removed this build-up of sediment (River Health Programme, 2007). The Groot Brak River is located in east Mossel Bay. Its mouth is only periodically open (Figure 3.11), and the permanent reclaimed bank within the river mouth forms a residential area known as The Island (River Health Programme, 2007). If the mouth is allowed to remain closed for extended periods, the water quality in the river suffers (Anchor Environmental, 2013). Water abstraction from the Groot Brak River is extensive, and can reduce annual run off to as little as 16 million m3/year (Anchor Environmental, 2012) (compared to the average shown in Table 2.5). Management of the estuary identifies planned breaching to promote healthy flow regime and emergency breaching to prevent flooding.
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Figure 3-9. Sand bar across the mouth of the Hartenbos River. Photograph taken by David Brew, 29th April 2015
Figure 3-10. Sand body, rock platform and open mouth of the Klein Brak River. Photograph taken by David Brew, 28th April 2015
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Figure 3-11. Sand bar at the mouth of the Groot Brak River. Photograph taken by David Brew, 29th April 2015
3.5.3 Potential Barriers to River Sediment Supply The River Health Programme (2007) identified that upstream abstraction of water by damming has substantially reduced the flow into the downstream portions of the rivers. The impact of water abstraction (along with invasive species constricting the river corridor), particularly through the summer months, is in places, reducing the flow to almost nothing. River Health Programme (2007) described four main dams on rivers draining into Mossel Bay (Table 3.3).
Table 3-3. Dams on rivers draining into Mossel Bay (River Health Programme, 2007) Name Location Capacity
Wolwedans Dam 2km upstream of the Groot Brak River mouth 24 million m3
Ernest Robertson Dam 25km upstream of the Groot Brak River 400,000 m3
Hartebeeskuil Dam 10km upstream of the Hartenbos River mouth 7 million m3 Water extracted from the Moordkuil River; one of the major tributaries of the Klipheuwel Dam 4 million m3 Klein Brak River (SSI Environmental, 2010)
3.6 Net Longshore Sediment Transport The net longshore sediment transport rates within the Vis Bay, Dana Bay and Mossel Bay littoral cells are low. This is because waves approach the shoreline at near-normal angles due to refraction across offshore bathymetric contours including around the headlands (Figure 3.6), and the evolution of the shoreline in response to the wave climate. This is in line with Mather (2010), who suggested that the regional net longshore transport rates along the south coast of South Africa are very low.
Longshore sediment transport in Dana Bay and Mossel Bay is driven predominantly by waves (see Section 2.2). Hugo (2013) used the UNIBEST (UNIform BEach Sediment Transport) sediment transport
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and coastline stability model to simulate the longshore transport and coastline position in Mossel Bay. Two modules were used; UNIBEST-LT which calculates the longshore sediment transport based on a specified wave climate and UNIBEST-CL which calculates the coastline position based on the longshore gradients in sediment transport. The nearshore wave climate was determined from the SWAN wave transformation model (see Section 2.2) from which wave climates were extracted at 18 positions along the 10m depth contour (Figure 3.4). Hugo (2013) specified a zero sediment transport condition at the western boundary and due to the presence of rock cliffs at Glentana, the eastern boundary was assumed to be stable in the long term and fixed.
The results of the model (Figure 3.12) show four zones of longshore sediment transport. Predicted transport rates along the Voorbaai to Klein Brak River coast in the lee of Cape St. Blaize are very low (less than 5,000m3/year) and directed to the south / southwest (Figure 3.13). Cape St. Blaize forms an important boundary in the net longshore sediment transport regime of Mossel Bay. Wave directionality drives regional sediment transport to the east, but because of its sheltered location, the transport component is to the west between Klein Brak River and Voorbaai.
Along the Reebok and Tergniet coasts, the predicted transport is also very low but directed in a northeast direction (about 5,000m3/year), in line with the regional transport direction. This means that there is a minor sediment transport divide in the vicinity of Klein Brak River. Along the Groot Brak River, Bothastrand and Outeniqua coast, predicted sediment transport rates are higher, reaching a maximum of 10,000m3/year, directed to the west-southwest. The reasons for this reversal to a westerly direction are not immediately obvious. The result of the reversal is a minor sediment transport convergence in the vicinity Groot Brak River / Bothastrand.
At Glentana, the predicted sediment transport rate is the highest (approximately 15,000m3/year) and directed to the east, indicating a second minor transport divide in the Outeniqua / Glentana region. The transport is higher and reverts to the regional direction at Glentana, because this stretch of coast is fully outside the shelter to waves approaching from the southwest provided by Cape St. Blaize. It is likely that some of the sediment is lost around the headland at the eastern end of Mossel Bay.
Figure 3-12. Modelled net longshore sediment transport rates in Mossel Bay (Hugo, 2013). Positive values represent transport to the south / southwest and negative values represent transport to the east / northeast. The distance along the coast is measured from a reference point at Diaz. The positions of the numbers in red squares are shown on Figure 3.3
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3 10,000m3/ 15,000m / year year 5,000m3/ year
<5,000m3/ year
Predominant wave driven regional sediment transport to the east
Figure 3-13. Sediment transport within Mossel Bay (adapted from Google Earth, 2015)
3.7 Coastal Erosion and Sediment Supply from Relict Dunes The largest input to the sand budget of the Vis Bay, Dana Bay and Mossel Bay littoral cells is from erosion of the coastal dunes, which are composed of relict dune sand with low cohesion. Erosion is likely to occur mainly during winter when storm events allow waves to overtop the beach and undercut the base of the dune causing the overlying sand to slump. The ability of the dunes to recover from erosion is limited. An important feature of the relict dune erosion is that it is irreversible. This is because, once eroded, the relict dune sand cannot be replaced in its consolidated form. The eroded sands are transported and then deposited within the littoral cell, where they form the surface of beaches and nearshore areas. While onshore winds can re-build active foredunes (such as those at Hersham) (see Section 3.9), the heights and volumes of the relict dunes cannot be re-established at current sea levels. These relict dunes therefore form eroding sandy bluffs behind the beaches.
Based on an analysis of aerial photographs of Mossel Bay flown in 1980 and 1991 and Google Earth images from 2004/2005 and 2010, Hugo (2013) described changes in the position of the dune vegetation line of up to +/-10-15m between 1980 and 2010 (Figure 3.14). Although Hugo (2013) argued that there were inaccuracies in the measurements, Figure 3.14 appears to show that there is a general trend of erosion between Mosselbaai and Groot Brak River, accretion from Groot Brak River east along the Hersham and Bothastrand coast and then erosion again at Glentana. The lengths of erosion are located where the relict dune front has been lost and there has been no recovery by new dune growth at the top of the beach. This is particularly the case at Glentana, where the greatest retreat of the dune front was recorded.
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Figure 3-14. Shoreline movement in Mossel Bay between 1980 and 2010 (Hugo, 2013). Positive values of coastal change indicate accretion and negative values indicate erosion. Distances along the coastline are measured from a reference point at Diaz
Although it is appreciated that there is uncertainty in the absolute values of historic erosion and accretion established by Hugo (2013), the results are used here to provide a high-level indication of how the Mossel Bay shoreline has changed over the past 30 years. Between Mosselbaai and Groot Brak River the average erosion rate is estimated to be approximately 0.1-0.2m/year. Between Groot Brak River and Bothastrand, an accretion rate of 0.1-0.2m/year is estimated, whereas towards Glentana, the coast reverts to erosion of about 0.3-0.4m/year. These estimated average rates are lower than those published by Mather (2010) at the location of individual pieces of infrastructure (1.3m/year to 3.2m/year at five parking areas, two in Dana Bay and three in Mossel Bay, Table 3.4). These estimates are higher than the average rates due possibly to increases induced by the infrastructure itself being close to the coastline.
Table 3-4. Projected future erosion rates at five parking areas in Dana Bay and Mossel Bay (Mather, 2010) Parking Area Shoreline Erosion (m/year 2004-2009)
Dana Bay Western 2.6
Dana Bay Eastern 1.3
Bayview Central (Hartenbos) 3.2
Reebok Western No observable change but erosion is taking place
Reebok Eastern 1.3
3.8 Future Erosion Rates in Response to Sea-level Rise One of the most important long-term concerns for RSM in Vis Bay, Dana Bay and Mossel Bay is the physical response of the shoreline to future sea-level rise. Predicting shoreline erosion rates is critical to planning a sediment management strategy and forecasting future problem areas. One solution is to assume that historic rates can be projected into the future. However, it is likely that the future erosion rate of the dunes will be affected by higher rates of sea-level rise than historically. Higher baseline water levels would result in a greater occurrence of waves impacting the toes of the dunes, increasing their
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susceptibility to erosion. However, without proliferation of coastal armouring in the three bays, beach widths will be maintained as the dunes erode.
An approximate quantitative approach for predicting shoreline response to sea-level rise is to use the Bruun Rule. The Bruun Rule is a two-dimensional cross-shore concept relating shoreline recession to water- (sea-) level rise. It attempts to define the erosional transgression of the shoreline, primarily due to erosion under conditions of wave attack and negligible sediment supply, based on offshore-directed transport. The rule assumes that with a rise in sea level, the equilibrium profile of the beach and surf zone moves upward and landward. According to the Bruun Rule the shoreline recession (R) can be expressed as:
R = S.LC(1+P)/(B+HC)
Where S = relative sea-level rise, LC = cross-shore distance to the position of closure depth HC, B = height of the eroded section of profile above initial mean sea level, and P = percentage of fines (<0.06mm) in the sediment eroded from the upper beach.
In this study a derivative of the Bruun Rule is used whereby the recession rate is estimated by multiplying the amount of sea-level rise by the ratio of the beach width to beach height.
R = (Y/X).S
Where R = recession rate, Y = horizontal dimension of beach, and X = vertical dimension of beach, and S = sea-level rise.
The derivative assumes that the amount of fines is zero (a logical assumption based on the particle size of the beach sediments, Section 3.3). It also assumes that the closure depth and height of the eroded section are uncertain and not well defined, and therefore relies on the measured slope of 1:50 determined from survey data (Section 3.2).
This simplified approach is considered valid, given the regional scale of the analysis, and the uncertainties inherent in predicting both the future rates of sea-level rise and the potential response of the shoreline to the rise. Hugo (2013) noted that although the Bruun Rule has received much criticism, Mather and Stretch (2012) provided some confidence in its applicability along the South African coast. In a study by Mather (cited in Mather and Stretch, 2012), the Bruun Rule was used to predict coastal erosion caused by historic sea-level rise. The predictions were then compared to observed erosion by measuring the difference between 14 successive historical shoreline positions. The study indicated that the Bruun Rule could predict the setback caused by sea-level rise within 10% accuracy.
Based on a single profile in the southern part of Mossel Bay (Hugo, 2013), an assumption is made that all the beaches of the three bays slope at approximately 1:50. Using the recent historic relative sea-level rise rate of about 1.5mm/year (Mather, 2009, 2010; Umvoto Africa, 2010), the method yields an estimated shoreline erosion rate of approximately 0.1m/year (similar to the measured rates along most of the coast of Hugo, 2013).
Sea-level rise is expected to accelerate in response to climate change (see Section 3.7). Assuming a future relative sea-level rise of up to 0.05m in 10 years, between 0.05m and 0.6m in the 10-50 year period and between 0.6m and 1.0m in the 50-100 year period (Table 2.3), the corresponding estimates of shoreline erosion would be up to 2.5m in 10 years, 2.5m to 30m in 50 years and 30m to 50m in 100 years (Table 3.5). Given that relative sea-level rise and erosion are expected to accelerate but rates and
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responses are uncertain, it may be more appropriate to identify indicative ranges to approximate shoreline erosion due to sea-level rise for each period; approximately 1.0m in 10 years’ time, 15m in 50 years’ time and 40m in 100 years’ time.
Table 3-5. Projected future sea-level rise and erosion rates Time Sea-level Rise Shoreline Erosion Indicative Erosion Extrapolation of Historic Erosion Period (m) (m) (m) (m) 10 years 0.05 0.05-0.6 1 1
50 years 0.05 to 0.6 2.5-30 15 5
100 years 0.6 to 1.0 30-50 40 10
This analysis indicates that projected sea-level rise based on climate change forecasts will result in increased shoreline erosion in Vis Bay, Dana Bay and Mossel Bay. In 10-years’ time, sea-level rise will have a minimal impact on erosion rates. Erosion over the next 50 years could amount to 15m which is an increase of 200% over historic erosion rates. In 100 years’ time, the application of the historic erosion rate would amount to 10m of erosion. This compares to the indicative erosion with sea-level rise of 40m, an increase of about 300%.
The future long-term sea-level rise estimates compare favourably with those of Umvoto Africa (2010). That study suggested that if a 2m rise in sea level was realised by 2100, then the sandy sections of Vis Bay and Dana Bay could experience 5-20m of shoreline retreat, whereas the sandy sections of Mossel Bay could retreat by as much as 30-40m.
3.9 Foredune Formation and Destruction At numerous locations along both Dana Bay and Mossel Bay coasts, the zone between the top of the beach and the face of the relict dunes is occupied by modern active foredunes (Figures 3.15 and 3.16). Sand has been blown landward from the beach and is settling at the base of the relict dunes where it becomes vegetated. The largest foredunes are at Hersham (Figure 3.16) which correspond with the zone of accretion east of Groot Brak River (Figure 3.14), which in turn corresponds approximately with the minor longshore sediment transport convergence zone (Figure 3.12). Most of the foredunes are low (apart from Hersham) and the capacity for them to grow and develop into more mature dune forms is limited by the height of the relict dunes behind them and the space available in the supratidal zone in front of them.
Figure 3-15. Low-lying discontinuous foredunes along the Mossel Bay coast. Photographs taken by David Brew, 28th April 2015
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Figure 3-16. Growth of a large foredune field at Hersham between the relict dune edge (right) and beach (left). Photograph taken by David Brew, 29th April 2015
The foredunes are highly unstable; their fate and form is governed by both wave and wind energies, and they will only have a chance of survival if they develop in an area above the mean high water spring mark. Hence, it is likely that the modern dunes of Dana Bay and Mossel Bay are transient morphological features. They will tend to form during quieter periods when constructive conditions encourage transfer of sediment from the beach to the dunes, and will be destroyed during higher energy storm periods when waves erode them. Hence, although the presence of the foredunes indicates accretion, the system overall is eroding. This is because during storms, the relict dunes and the modern dunes are both eroded and even though the erosion of the modern dunes can be reversed through subsequent accretion, the erosion of the relict dunes is irreversible. This leads to a continually eroding coast; accretion of new foredunes will take place further landward at the base of the new position of the relict dune edge. The cycle of relatively high erosion and relatively low accretion then repeats itself.
3.10 Transport into the Nearshore Zone Some of the eroded sediment may also be lost offshore due to high waves with long periods. Mather (2010) suggested the late 2008 storms moved large amounts of sediment into deeper water. In addition, some of the sediment transported east along the headlands is deposited in submerged spits that grow out into subtidal zone at the tips of the headlands. The formation of a submerged spit has been reported at Cape St. Blaize (Figure 3.17). The submerged spit is composed of fine-grained and medium to fine- grained sand (Martin and Flemming, 1986).
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Figure 3-17. Cross-section across Mossel Bay (right panel) showing the Cape St. Blaize submerged spit (Martin and Flemming, 1986). The left panel shows the location of the cross-section
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4 Critical Areas of Erosion
This section provides an analysis of critical areas of erosion within the Vis Bay, Dana Bay and Mossel Bay littoral cells. In order to delineate these areas, two criteria are adopted that are used to prioritize erosion responses over three different planning horizons (10, 50 and 100 years into the future); risk of erosion and consequences of erosion. Most of the information on critical areas of erosion was put together from the results of the site visit and meetings attended with the client and steering group. The 10-year, 50-year and 100-year time horizons are classed as short-, medium-, and long-term. These time horizons were agreed following discussions with local and provincial government representatives, who collectively are tasked with making decisions about how best to manage the Mossel Bay Coastline.
4.1 Risk and Consequences of Erosion The risk of erosion is a modified version of the risk analysis developed by PWA and Griggs (2004) and PWA et al. (2008) in southern Monterey Bay, California, and is applicable to sandy coasts backed by relict dunes. Their method establishes the first level of risk assessment over the three planning horizons (note that the PWA et al. (2008) method only used a 50-year time horizon, and that the 10-year and 100-year risk categories were added for the Mossel Bay Coastline to take into account the need to extend planning horizons):
what facility is at risk?
what is the probability that it will be impacted by erosion?
These risk categories are determined by assuming that historic erosion rates would continue over the next 10, 50 and 100 years. For assessment of critical areas of erosion, the historic erosion rate results of Mather (2010) and Hugo (2013) are used (Figure 3.12) with no increment after 10 years. Increments of 200% and 300% are added over 50 and 100 years, respectively, to account for potential increases due to future sea-level rise. The risk categories are shown in Table 4.1.
Table 4-1. Risk categories used along the Mossel Bay Coastline (modified from PWA et al., 2008) Level of Risk Explanation
Low risk Infrastructure with a low probability of being impacted by erosion over the next 10, 50 or 100 years Infrastructure not likely to be affected by chronic erosion over the next 10, 50 or 100 years, but potentially Moderate risk susceptible to short-term storm event erosion within those planning horizons Infrastructure that is located seaward of the shoreline position anticipated in 10, 50 or 100 years or presently vulnerable to short-term event-based erosion. A high risk designation also applies to High risk infrastructure with shore protection, where erosion is impacting public safety and access, and reducing the shoreline recreational value
All infrastructure is then qualitatively assessed as to its future value. This assessment is based on an evaluation of the economic (potential loss of infrastructure) and safety and human health (potential loss of life) consequences of loss of the infrastructure. The infrastructure is designated as high consequence, moderate consequence, or low consequence.
4.2 Maintenance of Coastal Access Maintaining access to coastal public property is not just a priority for the Municipality, it is also an obligation under the Integrated Coastal Management Act (2008). In 2009, Mossel Bay Municipality conducted a condition assessment of all the access points along the Dana Bay and Mossel Bay coasts that were assigned to their responsibility (Mossel Bay Municipality, 2009). This review identified that at the time of the survey, the access points described in Table 4.2 caused the most concern. They primarily consist of constructed wooden, concrete or plastic staircases.
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Table 4-2. Sources of risk to beach access points along the Mossel Bay Coastline (Mossel Bay Municipality, 2009) Location Number of Access Points Biggest Source of Risk
Vleesbaai 1 Damage during storm events
Danabaai 4 Erosion to the supporting dune system Diaz / Voorbaai, Bayview, Reebok, Tergniet 40 Erosion to the supporting dune system and Glentana Hartenbos 2 Flood events
4.3 Infrastructure at Risk in the Dana Bay Littoral Cell
4.3.1 Vleesbaai Vleesbaai is located in the southern corner of Dana Bay in the lee of Vleesbaai headland. Approximately 800m of the town is located on the relict sand dunes stretching north from the rock cliffs of the headland. The coastal properties here are served by a short coastal road which is located behind a low seawall. The coast in front of the dunes is composed of sand beach with exposed rock patches at mid- to low-tide level. Anecdotal evidence suggests that there has been historic erosion of the beach in front of the sea wall and the dunes in front of the coastal properties further north. For this reason a series of ad hoc defences have been completed in front of some of the properties (Figure 1.13), whereas other properties still remain fronted by a narrow strip of undefended dune (Figure 4.1). It is possible that erosion of the beach has been caused by construction of the defences, and it may be exacerbated into the future with sea-level rise.
Figure 4-1. Low dunes fronting properties at Vleesbaai. (Photograph taken by David Brew, 1st May 2015). View looking south
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Risk and Consequence The historic erosion rate at this location has not been measured. However, based on its location in a sheltered part of the bay, a rate of about 0.1-0.2m/year is estimated (similar to the erosion rates of the equivalent location in Mossel Bay). With relative sea-level rise, this rate is expected to increase, and after 10, 50 and 100 years, approximately 1-2m, 15-30m and 40-80m, respectively, of potential erosion is anticipated. Future dune erosion would mean that some of the coastal properties in Vleesbaai could be compromised in less than 10 years’ time, with many of the remainder compromised in less than 50 years’ time. This part of Vleesbaai is therefore designated as having infrastructure at high risk of erosion in the short- to medium-term. The loss of this infrastructure would have high economic consequences to the town as it is a tourist destination.
4.3.2 Boggomsbaai The Boggomsbaai frontage is about 1.2km long with the most seaward properties set back from the vegetated dune edge by greater than 30m (Figure 4.2). Assuming similar future rates of coastal erosion to those at Vleesbaai, they are therefore set back far enough to not be at risk over a 50-year time frame. However, there is the potential for these properties to be compromised in 100-years’ time. Hence, Boggomsbaai is designated at low risk of erosion in the short- to medium-term and at moderate risk of erosion in the long-term. The consequence of erosion would be high to the local residents as the loss of their properties would be economically damaging.
Figure 4-2. Wide area of vegetated dune fronting Boggomsbaai. Vleesbaai is in the background. (Photograph taken by David Brew, 28th April 2015). View looking south
4.3.3 Danabaai Western Parking Area Danabaai western parking area is located at the western end of Danabaai, directly in front of the relict dune face, and elevated above beach level. In 2009, the seaward edge of the parking area was at risk of
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collapse due to encroachment of waves (Mather, 2010) and since that time the seaward face of the parking area has been further defended with various types of structure (Figure 1.14). Mather (2010) implied that the erosion of the beach and the undermining of the parking area structures was due to the loss of sediment from the beaches during the 2008 major storm event (and the potential for its total loss beyond the more typical closure depth). The western and central sections have been moved seaward and are fronted by stacked gabion baskets. The section between these two appears to have not been improved. The eastern section of the parking area has been defended by piles of rocks. The presence of the parking area has narrowed the beach, and anecdotal evidence suggests that the beach is fluctuating in elevation in front of the new defences. Currently, there is a small erosional cliff at the top of the beach in front of the parking area (Figure 4.3) as the beach is being squeezed in front of the new defences that protect the parking area.
Figure 4-3. Low cliff in the top of the beach in front of Danabaai western parking area. (Photograph taken by David Brew, 30th April 2015). View looking west
Risk and Consequence Mather (2010) indicated that the historic erosion rate at Danabaai western parking area was 2.6m/year between 2004 and 2009. Applying this rate over the next 10 years equates to an estimated 26m of potential erosion at this location, which would place the parking area at high risk of erosion in the short- term. The beach is likely to narrow further concomitant with a reduction in elevation (it appears to be already underway evidenced by the cliff, Figure 4.3) leading to potential undermining of the coastal protection structures fronting the parking area. The beach is also vulnerable to storms, where sand will be removed offshore reducing the beach width and allow waves to potentially impinge on the defences.
The loss of the parking area would have relatively low economic consequences. However, continued loss of beach would reduce public access, public safety and the recreational value of this shoreline. Lateral access along the beach in front of the parking area would potentially be lost and the beach would be
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hazardous at high tide. Hence, taking into account all factors, the loss of the parking area after 10 years is considered to be of moderate consequence.
4.3.4 Danabaai between the Parking Areas The Danabaai coast between the western and eastern parking areas (about 1.5km) and for 1km east of the eastern parking area is composed of relict dunes on top of rock which is exposed at the top of the beach. The beaches are wide with small pockets where the dune front is eroding, supplying sediment to the beach. Otherwise, the coast is controlled by the presence of the rock outcrops at beach level. This means that erosion rates are likely to be very low and the coast is relatively stable (now and into the future). The seaward properties of Danabaai are set back by greater than 20m from the vegetated dune edge (Figure 4.4) and the erosion risk is designated as low in the short-, medium- and long-term. The consequence of erosion would be high to the local residents as the loss of their properties would be economically damaging.
Figure 4-4. Cliff top properties set back from the vegetated dune edge at Danabaai. (Photograph taken by David Brew, 1st May 2015)
4.3.5 Danabaai Eastern Parking Area Danabaai eastern parking area is located in a small bay (pocket beach) between two minor headlands at the eastern side of Danabaai. It appears to be backed by relict dunes although it is not immediately obvious whether they extended to beach level prior to construction. In 2009, the seaward edge of the parking area was at risk of collapse due to encroachment of waves. In a similar way to Danabaai western parking area, the front face of the parking area has been strengthened by a rock revetment (more than half the western side) and a shorter length of gabion baskets along the eastern side (Figure 1.15). Anecdotal evidence suggests that the beach morphology is fluctuating, with lower elevations in winter and higher elevations in summer. It is likely that the equilibrium form of the small bay in which the parking area
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is situated is crenulate, and the structure is not allowing this form to materialise, because it is un-naturally holding the coast forward. This has caused the beach to narrow in front of the parking area and it is likely to be inducing accelerated erosion.
Risk and Consequence The historic erosion rate at Danabaai eastern parking area is estimated to be around 1.3m/year (2004- 2009) (Mather, 2010). In 10 years’ time this will cause approximately 13m of potential erosion to occur at this location, placing the parking area at high risk over the short-term. Further narrowing and lowering of the beach is likely, which would lead to potential undermining of the coastal protection structures fronting the parking area. The beach is also vulnerable to storms, where sand will be removed offshore reducing the beach width and allow waves to potentially impinge on the defences.
The loss of the parking area would have relatively low economic consequences. However, continued loss of beach would reduce public access, public safety and the recreational value of the shoreline. Lateral access along the beach in front of the parking area would potentially be lost and the beach would be hazardous at high tide. Hence, taking into account all factors, the loss of the parking area after 10 years is considered to be of moderate consequence.
4.4 Infrastructure at Risk in the Mossel Bay Littoral Cell
4.4.1 Diaz / Voorbaai The beach and dunes of Diaz / Voorbaai are about 2.3km long and stretch from the southern corner of Mossel Bay to a small creek immediately north of the depot. The coast consists of a wide sandy beach backed by relict dunes with intermittent modern foredunes at the top of the beach. The dunes are cut by three creeks, two of which are located either side of the depot at the north end of the beach and one towards the south end of the beach. A variety of infrastructure is located on the top of the dunes set back at various distances from the vegetated dune edge.
Along the first 300m (Diaz), the dunes are relatively high but are eroding on their front face (Figure 4.5). There are no foredunes here and anecdotal evidence suggests that the beach is narrowing. A railway line and then the main N2 road are located at the back of these dunes, and in places the railway line is within 50m of the vegetated dune edge.
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Figure 4-5. Eroding dunes at the southern end of Diaz. (Photograph taken by David Brew, 28th April 2015)
Further north (for 1.2km to immediately south of the depot), the tops of the dunes are lined with hotels and condominiums, and a parking area (Figure 4.6). These facilities are at various set-back distances from the edge of the vegetated dunes. However, occasional buildings are on stilts which extend over the dune front. The beach is wide and modern foredunes of varying heights and widths indicate a degree of stability. In front of the depot and the Mossel Bay Desalination Plant, the relict dunes which are about 800m long, are well vegetated and greater than 50m wide.
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Figure 4-6. Hotels and condominiums along the central part of the Voorbaai coast. (Photographs taken by David Brew, 28th April 2015). Top left photograph view looking north. Bottom left and bottom right photographs view looking south
Risk and Consequence The historic average erosion rate of the Diaz / Voorbaai coast is estimated to have been about 0.1- 0.2m/year over the past 30 years. With relative sea-level rise, this rate is expected to increase, and after 10, 50 and 100 years, approximately 1-2m, 15-30m and 40-80m, respectively, of potential erosion is anticipated. Future chronic dune erosion would mean that most of the coastal properties are at low risk of erosion over the next 10 years. However, a significant storm could impact those properties that are relatively close to the dune edge, and these are elevated to moderate risk of erosion in the short-term. Given the set-back distances, some of the properties could also be compromised in less than 50 years’ time and are designated as high risk of erosion over the medium-term. There is potential for all of the infrastructure (including the railway line, N2 road, depot and desalination plant) to be compromised over a 100-year time frame and so these facilities are designated as high risk of erosion over the long-term.
The loss of holiday facilities along the Diaz / Voorbaai coast would have high economic consequences to the region, as this part of Mossel Bay is a popular tourist destination. Also, the desalination plant produces clean drinking water, and hence the plant needs to remain in full operation indefinitely. Therefore erosion would have significant economic, environmental, and human health impacts. A breach of the depot would cause adverse environmental impacts to the dunes and beaches, and water quality impacts along the bay. Overall, the Diaz / Voorbaai infrastructure is designated as high consequence of loss over all time periods.
4.4.2 Bayview The Bayview coastline is 1.3km long between the north end of the depot in the south and Bayview central parking area (Mather, 2010) in the north. This coast comprises relatively high dunes, which in many
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places have been ‘reclaimed’ and turned in to lawns in front of large residential properties and hotels (Figure 4.7). Some sections of dune have suffered ‘blow-out’ where the dune vegetation has been lost, forming local hotspots of erosion (Figure 4.7, right photograph). Low-lying modern foredunes are forming in some places. Bayview central parking area, located on the dune-top, marks the northern end of this beach. Here, the dune front is in poor condition and subject to erosion (Figure 4.8). Most of the seaward- facing walls of the properties are set back between 20m and 30m from the vegetated dune edge. However, some of the lawns extend down the dune front to beach level (Figure 4.7).
Figure 4-7. Properties set back along the Bayview coast. Note extended lawns in both photographs and eroding blow out in the right photograph. (Photographs taken by David Brew, 28th April 2015). Left photograph view looking south
Figure 4-8. Eroding dunes in front of Bayview central parking area. (Photograph taken by David Brew, 28th April 2015)
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Risk and Consequence The historic average erosion rate along the Bayview coast is estimated to be about 0.1-0.2m/year over the past 30 years. However, Mather (2010) suggested that local to the parking area the erosion rate has recently been as high as 3.2m/year (2004-2009). With relative sea-level rise, the average erosion rate is expected to increase, and after 10, 50 and 100 years, approximately 1-2m, 15-30m and 40-80m, respectively, of potential erosion is predicted. If the rate of 3.2m/year is realised, then around 32m of dune front could be lost locally over the next 10 years, and properties compromised. Hence, the Bayview properties are designated as moderate risk of erosion over the short-term and high risk of erosion in the medium- to long-term.
Many of the properties in Bayview are privately owned and the consequences of their loss would be economically damaging to individual owners. Also, loss of the parking area would remove the main public access to the beach which is popular with local residences and tourists alike. The Bayview properties and parking area are therefore considered to be high consequence of loss facilities.
4.4.3 Hartenbos Along the Hartenbos coast (north of Bayview central parking area for about 1.8km) the dunes gradually lower in elevation, approaching the Hartenbos River. The dunes are very low elevation for the 500m of coast immediately southwest of the Hartenbos River rising significantly along the remainder of the coast. There is a mix of facilities on the dune top including residences, holiday lets, camping and caravan sites, public access areas and promenades. These facilities are set back at various distances from the vegetated dune edge (Figure 4.9). The camping sites are located immediately landward of the low dunes at the Hartenbos River end of the frontage (Figures 1.17). Further southwest, in the centre of Hartenbos, the 1.1km-long dunes are higher and protected at their toe by low gabion baskets or lines of wooden posts (Figure 1.17). At the southwest end of Hartenbos (for about 200m), the dunes are undefended and the footpath (connecting to Bayview central parking area) and residential properties are located approximately 15m and 20-25m behind the vegetated dune edge, respectively.
Figure 4-9. Facilities on the dune top at Hartenbos (central). (Photographs taken by David Brew, 29th April 2015). Left photograph view looking north. Right photograph view looking south
Risk and Consequence The facilities along the northeast part (500m) of this coast sit less than 10m inland from the low-lying dune edge. Historic erosion rates (over the past 30 years) are estimated to have been about 0.1-0.2m/year. With relative sea-level rise, these rates are expected to increase, and after 10, 50 and 100 years, approximately 1-2m, 15-30m and 40-80m, respectively, of potential erosion are predicted. Therefore the dunes towards the northeast end are at high risk of erosion over the next 50 years. In addition, the
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facilities could be vulnerable to wave damage and flooding due to the low elevation of the fronting dunes over a shorter period of time. Hence, they are considered to be at moderate risk of erosion over the next 10 years.
The protected central part of the Hartenbos coast contains essential infrastructure for the tourist industry (Figure 4.9). The dunes are protected at the toe and are likely to erode at less than the predicted average, as long as the protection remains intact. However, as a consequence of the protection, the beach has likely been narrowed and lowered, and is likely to continue to do so as it is squeezed between the low water mark and the fixed structures. Narrowing and lowering of the beach would lead to potential undermining of the coastal protection structures at the base of the dunes.
This is a very popular tourist destination supporting facilities and promenades along the entire length of the dune top. Continued loss of the beach would reduce public access, public safety and the recreational value of the shoreline resulting in high economic and high recreational consequences. Lateral access along the beach in front of Hartenbos may also become restrictive as a lower and narrower beach would be more hazardous at high tide.
4.4.4 Klein Brak River Infrastructure at Klein Brak River is confined to the northeast side of the river mouth and extends for about 1.2km in that direction. Most of the seaward properties are set back from the vegetated dune edge by greater than 60m. Wide and relatively high foredunes are located in front of the Klein Brak River properties indicating a degree of recent stability. There is a parking area at the river mouth, which has been built into the dunes, so its seaward face is in line with the vegetated dune edge to the northeast. The parking area has eroded in the past, so a low wall was built with gabion baskets about three years ago to protect from further erosion. The parking area now has foredune formation in front of it (Figure 1.18), but it is difficult to determine whether the recovery would have taken place regardless of whether the wall had been built.
Similar future rates of coastal erosion apply to Klein Brak River as those along the southern half of Mossel Bay. Hence, the properties are set back far enough to not be at risk of erosion over a 50-year time frame. However, there is the potential for these properties to be compromised in 100-years’ time. The Klein Brak River properties are designated as low risk of erosion in the short- to medium-term and at moderate risk of erosion in the long-term. The parking area is at a low elevation (Figure 1.18), and although the fronting beach appears to be accreting, it could revert to erosion with sea-level rise and be vulnerable to flooding in the future. The consequence of erosion would be high to the local residents as the loss of their properties would be economically damaging.
4.4.5 Reebok and Tergniet Reebok and Tergniet represent a 4.2km-long continuous strip of development east-northeast of Klein Brak River. Rows of residential properties are located immediately landward of a coastal road which has a seaward kerb set back from the vegetated dune edge by about 15-35m, and extends along the entire length of the frontage. Several parking areas, including Reebok western and Reebok eastern extend seaward from the road and cross the dune front to approach to within 5m of the dune edge (Mather, 2010). There are numerous other access points from the road to the beach including steps and walkways. Ad hoc discontinuous defences including boulders and short lengths of artificial fill have been placed at the dune toe to counteract erosion (Figure 1.19). However, most of the dunes along this stretch of coast are undefended.
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Risk and Consequence The Reebok and Tergniet coast is estimated to have eroded at average rates of 0.1-0.2m/year over the past 30 years. With relative sea-level rise, these rates are expected to increase, and after 10, 50 and 100 years, approximately 1-2m, 15-30m and 40-80m, respectively, of potential erosion are predicted. This means that the coastal road and properties are at low risk of erosion in 10 years’ time. The road will be at high risk and the properties at moderate risk of erosion in 50 years’ time, and the properties will be subject to high risk of erosion in 100 years’ time.
Mather (2010) indicated that the parking areas, which are much closer to the dune edge, have eroded at rates up to 1.3m/year between 2004 and 2009. They are at high risk of erosion over the next 10 years because it is possible that 13m of erosion could take place in that time. They may be affected by storm erosion within the short-term planning horizon.
According to Mather (2010), the Reebok parking areas are used relatively infrequently as this stretch of coast is less popular with tourists. Hence, the consequence of loss of the parking areas is considered to be low. However, the road provides access to the residential properties and its loss would have high consequences with respect to local economics and public safety. Also, the variety of steps provides access to the beach for residents and their loss would provide inconvenience for the local population. They are designated as moderate consequence of loss.
4.4.6 Groot Brak River Groot Brak River extends about 1km to the west and about 1km to the east of the river mouth. Western Groot Brak River comprises a coastal road with a seaward kerb set back about 30-50m from the vegetated dune edge. Landward of the road are residential properties known as Southern Cross. Eastern Groot Brak River is also residential (Hersham), but these properties are located on very high dunes and set back further from the vegetated dune edge (20-80m). The dunes at Hersham are the highest along the Mossel Bay coast (Figure 3.14). They are in a healthy condition with wide stretches of foredune in front of the relict dunes.
The future erosion rates in front of Southern Cross are predicted to be similar to Reebok and Tergniet. Given these rates, the coastal road and properties are at low risk of erosion in 10 years’ time. The road will be at moderate risk of erosion in 50 years’ time, and the road and properties will be at high risk of erosion in 100 years’ time. The properties in Southern Cross are privately owned and the consequences of their loss and the loss of the access road would be economically damaging to individual owners, and hence they are designated as high consequence facilities.
The coast of east Groot Brak River (Hersham) has historically accreted (about 0.1m/year) (Figure 3.14). If this accretion continues into the future and is able to keep pace with relative sea-level rise, then the properties on the cliff-top will be at low risk of erosion over the foreseeable future. However, with sea-level rise, it is likely that this coast will at some point revert to erosion as the capacity for the modern dunes to accrete (and recover from potential future erosion) will become less over time. So, it is assumed that the properties will be at moderate risk of erosion in 50 and 100 years’ time. The consequence of their loss would be high, in a similar way to the properties in Southern Cross.
4.4.7 Bothastrand and Outeniqua Bothastrand and Outeniqua represent a 3km-long continuous strip of developed area east of Groot Brak River. Most of the residential properties making up this strip of coast are set back from the vegetated dune edge by between 35m and 60m, with occasional properties closer than this. The most seaward properties
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are at the western end of Bothastrand, where they are located at the dune edge and are protected by coastal structures including gabion baskets (Figure 1.20).
Risk and Consequence Historical data suggests that the Bothastrand and Outeniqua coast has been accreting at low rates (approximately 0.1m/year) over the past 30 years. This means that the properties would be at low risk of erosion into the foreseeable future, if accretion continued. However, reversion to erosion is a possibility with relative sea-level rise and so the properties are designated as moderate risk of erosion in 50 and 100 years’ time (in a similar way to the Hersham coast to the west). The consequence of their loss would be high for similar reasons to Southern Cross and Hersham.
4.4.8 Glentana The 2.1km-long Glentana frontage occupies the eastern end of the Mossel Bay crenulate bay with the beach in the lee of the eastern headland. The coastal properties form a continuation of those to the west in Outeniqua. Significant erosion took place here during the 2007 storms. Seafront properties and the parking area were affected and the residents built their own protective coastal defences (including gabion baskets backed by fill which has been grassed to create lawns) (Figure 1.21). The gabion baskets were constructed 2m deep and currently only the top 1m is showing above the beach. The beach has therefore partially recovered after the storms with growth of low foredunes in front of the defences. The front walls of the properties are set back about 10m from the defences. Elsewhere, properties are set back between 30m and 60m from the vegetated dune edge.
Risk and Consequence The Glentana coast is estimated to have eroded at an average rate of 0.3m/year over the past 30 years. With relative sea-level rise, this rate is expected to increase, and after 10, 50 and 100 years, approximately 3m, 45m and 120m, respectively, of potential erosion is predicted. This means that pressure will be applied to the defences over the next 10 years. However, the properties will be at low risk of chronic erosion in 10 years’ time, but they could be vulnerable to wave damage and flooding due to the low elevation of the fronting dunes. Hence, the coastal defences and properties (including the parking area) are considered to be at moderate risk of erosion in the short-term. Over 50-year and 100-year timescales most of the properties along the Glentana coast will be at high risk of erosion.
The majority of the properties in Glentana are privately owned and the consequences of their loss would be economically damaging to individual owners. Also, loss of the parking area would remove the main public access to the beach. The Glentana properties and parking area are therefore considered to be high consequence of loss facilities.
4.5 Summary Table 4.3 summarises the risk and consequence of erosion at all the critical areas of erosion along the Dana Bay coast at 10, 50 and 100 years. Table 4.4 provides the same details for Mossel Bay.
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Table 4-3. Risk and consequence of erosion in Dana Bay 10-year Time Horizon 50-year Time Horizon 100-year Time Horizon Location Potential Risk of Consequence Potential Risk of Consequence Potential Risk of Consequence Erosion Erosion of Erosion Erosion Erosion of Erosion Erosion Erosion of Erosion Vleesbaai 1-2m High High 15-30m High High 40-80m High High
Boggomsbaai 1-2m Low High 15-30m Low High 40-80m Moderate High
Danabaai Western Parking Area 26m High Moderate >26m High Moderate >26m High Moderate
Danabaai between Parking Areas 1-2m Low High 15-30m Low High 40-80m Low High
Danabaai Eastern Parking Area 13m High Moderate >13m High Moderate >13m High Moderate
Table 4-4. Risk and consequence of erosion in Mossel Bay
10-year Time Horizon 50-year Time Horizon 100-year Time Horizon Location Risk of Consequence Risk of Consequence Risk of Consequence Potential Erosion Potential Erosion Potential Erosion Erosion of Erosion Erosion of Erosion Erosion of Erosion
Diaz / Voorbaai 1-2m Moderate 15-30m High 40-80m High
15-30m (>32m at 40-80m (>32m at 1-2m (32m at the Bayview Moderate the central parking High the central parking High central parking area) area) area)
Hartenbos 1-2m Moderate 15-30m High 40-80m High
Klein Brak River 1-2m Low 15-30m Moderate 40-80m Moderate High High High Reebok and 1-2m (up to 13m at 15-30m (>13m at 40-80m (>13m at Low Moderate High Tergniet the parking areas) the parking areas) the parking areas) 1-2m (west), 15-30m (west), 40-80m (west), High (west), Groot Brak River accretion of 1-2m to Low erosion to the east Moderate erosion to the east Moderate the east (rate unknown) (rate unknown) (east) Bothastrand and erosion (rate erosion (rate accretion of 1-2m Low Moderate Moderate Outeniqua unknown) unknown)
Glentana 3m Moderate 45m High 120m High
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5 Regional Sediment Management
This section focuses on potential RSM approaches as solutions to the coastal erosion problems in Vis Bay, Dana Bay and Mossel Bay. Based on the information outlined in Sections 1 to 4, four main approaches to RSM are considered appropriate. These are:
allow the natural process of dune erosion to continue without intervention;
use beach restoration strategies particularly beach nourishment to slow erosion rates;
install beach control structures to encourage sand deposition; and
adaptively manage the coast with a view to implementation of future strategies.
The RSM options that are considered feasible in Dana Bay and Mossel Bay are summarised in Figure 5.1 and Table 5.1.
Table 5-1. Potential RSM options in Dana Bay and Mossel Bay Bay Location Management Approach
Vis Bay Vis Bay Allow dune erosion to continue
Vleesbaai Beach nourishment
Vleesbaai to Danabaai western parking area Allow dune erosion to continue
Dana Bay Danabaai western parking area Beach control structure
Danabaai western parking area to Danabaai eastern parking area Allow dune erosion to continue
Danabaai eastern parking area Beach control structure
Diaz / Voorbaai, Bayview and Hartenbos Beach nourishment
Hartenbos to Klein Brak River Allow dune erosion to continue
Klein Brak River, Reebok and Tergniet Adaptive management Mossel Bay Tergniet to Southern Cross Allow dune erosion to continue
Southern Cross to Glentana Adaptive management
Glentana Beach control structure
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Allow dune erosion to continue Beach control structure
Allow dune erosion to continue
Beach control structure Allow dune erosion to continue
Beach nourishment
Allow dune erosion to continue
Beach control structure Adaptive management
Allow dune erosion to continue
Adaptive management
Allow dune erosion to continue
Beach nourishment
Rock headland
Figure 5-1. Potential RSM options in Dana Bay (top panel) and Mossel Bay (bottom panel) (adapted from Google Earth, 2015)
5.1 Allow Dune Erosion to Continue This ‘no action’ approach allows the natural processes of dune erosion to continue without human intervention over the short-, medium- and long-term. Section 4 shows that within Vis Bay, Dana Bay and
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Mossel Bay there are significant stretches of coastline (just over 20km) where there is no infrastructure or facilities at risk of erosion (Figure 5.1). These are:
all of Vis Bay (about 4km);
between Vleesbaai and Boggomsbaai (about 1.5km) (Figure 5.2);
between Boggomsbaai and Danabaai western parking area (about 10km). This stretch of coast can be divided into two morphologies related to the amount of modern wind-blown sand. The western morphology is northeast of Boggomsbaai where the relict dunes are adjacent to the beach with a narrow strip of fronting foredunes (Figure 5.3). Further east and along the coast to Danabaai western parking area, the front of the relict dunes is covered by piles of mobile un-vegetated sand (Figure 5.4);
between Hartenbos and Klein Brak River (about 3km) (Figure 5.5); and
between Tergniet and Southern Cross (west of Groot Brak River) about 2km (Figure 5.6).
Figure 5-2. Relict dunes with no infrastructure between Vleesbaai and Boggomsbaai. (Photograph taken by David Brew, 1st May 2015). View looking north
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Figure 5-3. Relict dunes with no infrastructure north of Boggomsbaai. (Photograph taken by David Brew, 28th April 2015). View looking northeast
Figure 5-4. Relict dunes with no infrastructure west of Danabaai. (Photograph taken by David Brew, 30th April 2015). View looking west
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Figure 5-5. Relict dunes with no infrastructure east of the Hartenbos River. (Photograph taken by David Brew, 29th April 2015). View looking northeast
Figure 5-6. Photograph at Souwesia (between Tergniet and Southern Cross). (Photograph taken by Warren Manuel, 29th September 2015). View looking east
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At these locations, the dunes are also sufficiently wide and high so there is no immediate threat of flooding. Erosion of the dunes along these stretches of shoreline will continue to provide large quantities of sand to the coastal system, helping to maintain the dynamic equilibrium of the bays and keep the beaches in a healthy condition, providing benefits for coastal protection and recreation.
Two locations, at Boggomsbaai and Danabaai between the parking areas, are considered to be at low risk of erosion over the next 50 years (Tables 4.3 and 4.4). Hence, at these locations the dunes should be allowed to erode naturally over this time period. After 50 years it may then be appropriate to implement adaptive management (see Section 5.4).
5.2 Beach Nourishment Beach nourishment can be defined as the introduction of sand on to a beach to supplement a diminished supply of natural sediment, for the purpose of beach restoration, enhancement, or maintenance. In so doing, the beach will provide additional buffer to waves approaching the base of the relict dunes and slow their erosion rate. The implementation of beach nourishment strategies has occurred frequently throughout the United States and Europe and is an established sediment management technique. In Dana Bay and Mossel Bay, there are three main situations where beach nourishment could be used:
where the beach is experiencing continued loss of sediment and there is a shortage of new sediment supplied through longshore or cross-shore transport;
where the dunes are eroding behind the beaches, and the beaches are acting as a self-contained system, with a balanced budget; and
to enhance the recreational value of the beach by widening it.
There are numerous factors that need to be taken into account before beach nourishment is implemented in either Dana Bay or Mossel Bay. These include locating a suitable receiver site(s), sourcing nourishment sand, selecting sand texture/particle size, determining nourishment volume, transporting and placing sand at the site and environmental considerations. Beach nourishment is often used in combination with beach control structures, such as groynes and detached breakwaters. These structures would be designed to reduce net long-term loss of beach sediment due to alongshore or offshore transport.
Beach nourishment is considered an appropriate potential RSM measure along the Vleesbaai coast of Dana Bay and the Diaz / Voorbaai to Hartenbos coast of Mossel Bay.
5.2.1 Potential Receiver Sites for Beach Nourishment in the Dana Bay Littoral Cell The only location in Dana Bay where beach nourishment is potentially feasible (and necessary) as an RSM solution is at Vleesbaai, where future loss of the beach and coastal properties could occur over the short-term with a high consequence of loss. Here, the continued presence of coastal protection structures is likely to lead to a gradual loss of the beach. This would lead to greater landward encroachment of waves, which could exacerbate erosion of the dunes that remain undefended along this coast or may undermine the existing defences. A healthy beach at Vleesbaai is also important for recreation and tourism.
The physical and sedimentary processes (see Sections 2 and 3) suggest that the shoreline at Vleesbaai has a relatively low wave energy (being in the lee of a headland) and very low net sediment transport rates. These relatively ‘quiet’ physical conditions provide potential benefits for implementation of a beach nourishment strategy along this stretch of coast:
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sand would remain at the receiver site for a longer period of time, leading to a reduced frequency of maintenance of the site through further nourishment;
the impacts of the nourishment would only be felt locally with little or no far-reaching impacts to the rest of Dana Bay; and
the nourishment would benefit multiple facilities along the Vleesbaai frontage. Given that the need to mitigate erosion at Vleesbaai is immediate (short-term, Table 4.3), a detailed feasibility study should be completed over the next few years and a strategy developed shortly after that.
5.2.2 Potential Receiver Sites for Beach Nourishment in the Mossel Bay Littoral Cell The continuously developed coast of Diaz / Voorbaai, Bayview and Hartenbos is considered to be at moderate risk of erosion over the short-term with a high consequence of loss of the facilities on top of the dunes. The beaches along this stretch of coast appear healthy (i.e. they are acting as a self-contained system, with little input or export) but continued erosion of the relict dunes is placing the dune-top facilities at risk. The implementation of beach nourishment would increase the width and height of the beach providing additional protection to the frontage. The receiver site(s) would need to be carefully targeted to provide the most benefit to the properties most at risk (the properties closest to the vegetated dune edge). Beach nourishment at this location in the lee of Cape St. Blaize would be in an area of low wave energy and very low sediment transport rates (see Sections 2 and 3). This would allow the sand to be retained for a longer period of time (and provide the same benefits for implementation as those for Vleesbaai, although at a larger scale).
Beach nourishment along this part of Mossel Bay would also improve the preservation and potentially enhance the modern active foredunes that occur at the top of the beach (Figures 3.15 and 3.16). By virtue of the new sand, the beach profile would be widened and raised, and more sand would remain dry for longer periods of time and be available to supply the dunes (as long as onshore winds blow to entrain the sand). The availability of wind-blown sand would depend on the initial volume placed and how the beach profile changes as the sand is re-distributed by waves. The extra supply of sand to build the foredunes into higher and wider forms could be supplemented by dune management techniques such as grass transplanting to encourage and support accumulation of dune sand.
Given that the general risk of erosion in the short-term is moderate along the Diaz / Voorbaai, Bayview and Hartenbos coast (Table 4.4), a detailed feasibility study should be completed over the next few years and a strategy developed shortly after that.
5.2.3 Factors to Consider before Implementing Beach Nourishment Sourcing Nourishment Sand The specific source of sand that is chosen for nourishment depends on several factors. The volume of sand required will influence the source, with the smaller the volume needed, the greater the opportunities for choice of source. The location of the nourishment is also important; the closer it is to existing sources, the easier the supply will be. Also, when a potential source has been identified, it is important to obtain a realistic assessment of its availability to the nourishment project. Selecting the appropriate sand texture is the most important aspect of design, as it will influence beach stability, sediment dynamics and durability. Sorting of the natural beach sediment will have taken place for many years prior to nourishment, and the particle size of this sediment provides a good indication of what sediment type will be stable. It is therefore usual to nourish using a sediment particle size similar or coarser to the natural material, to ensure stability.
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Large volumes of sand exist offshore of Dana Bay and Mossel Bay (see Sections 3 and 5.2) (Figures 3.6 and 3.7). This offshore sand is a potential source that could be dredged (offshore of the closure depth) and placed either on the beach or in the nearshore. Particle size analyses of the beach sediments and offshore sediments in both bays reveal a degree of compatibility; all are in the medium-grained sand range (see Section 3). Therefore, the offshore seabed is a potential source of sand for beach nourishment in Dana Bay and Mossel Bay.
In addition, the potential future dredging of the Port of Mossel Bay and its navigation channel might provide an opportunistic source of sand. The infilling sediment is likely to be locally derived and would not be plentiful. Hence, it would provide only a very small portion of the necessary volume to significantly nourish the beaches of the Mossel Bay Coastline, and would therefore need to be supplemented with sand from other sources. The port sediments may be subject to minor contamination including wastewater and oil discharge from vessel operations, and these would have to be tested before consideration for beach nourishment.
Determining Nourishment Volume The determination of nourishment volume needs to take into account coastal defence and amenity considerations. The new beach profile is usually designed to ensure that it provides protection during storm events and to minimise losses of sediment. Allowance needs to be made for losses due to sediment transport processes, and a balance has to be struck between the amount of sand initially placed and future maintenance commitments.
Transporting and Placing Materials at the Site The methods of transporting and placing sand at the site from marine sources are known as hydraulic methods. If the site is close to its marine source then dredged sand may be pumped through a pipe directly from the borrow area to the site. More commonly, the dredged sand is transported by barge or hopper from the borrow area to a remote discharge point where it is pumped ashore through a pipe. The sediment is delivered with a large amount of water, which then drains from the beach, and from which the sediment settles out.
Environmental Considerations Beach nourishment should be preceded by an assessment of the potential impacts (both adverse and beneficial) of the proposals on the physical, natural and human environment. The main potential impact on the physical environment is the alteration of coastal processes. Increased or reduced movement of sediment in an alongshore direction may affect the natural or human assets elsewhere along the coast. Impacts on the natural environment typically include potential smothering of flora, invertebrates, birds and fish. The human environmental considerations are varied and may include impacts on recreation, access, safety, landscape, commercial activities, archaeology, navigation and infrastructure. Also, removal of sand from subtidal habitats to support nourishment disturbs and removes benthic habitat and results in elevated turbidity with potential impacts to invertebrates and fish in nearshore and offshore environments.
5.3 Beach Control Structures to Encourage Sand Deposition The main human factor that exacerbates beach erosion in Dana Bay and Mossel Bay is the presence of structures at the top of the beach (base of the relict dunes) or structures set forward on to the beach. These types of structures are prevalent at Vleesbaai, Danabaai western parking area, Danabaai eastern parking area, Hartenbos and Glentana (see Section 1.4). Beach nourishment is considered a feasible RSM option at Vleesbaai and Hartenbos (see Section 5.2). However, at Danabaai western parking area, Danabaai eastern parking area and Glentana, beach nourishment is not considered feasible. This is because these locations are towards the eastern end of their respective bays where they are exposed to
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greater wave energy and relatively high rates of longshore sediment transport (see Sections 2 and 3), which would make retention of placed sediment difficult. These locations also have some of the highest rates of beach erosion in each of the bays suggesting that further nourishments would have to take place on a regular basis. At these locations, beach control structures that encourage deposition of beach sand by reducing wave energy (detached breakwaters) or that present a barrier to longshore sediment transport (terminal groyne) could be considered.
5.3.1 Potential for Beach Control Structures in Dana Bay The two parking areas at either end of Danabaai are considered to be essential features of the coast by the local population, allowing easy access to the beach. The continued protection of the parking areas without further action is going to lead to loss of the usable beach width caused by the footprint of the structures, which will actually limit beach access. Both parking areas are causing a peninsula effect whereby they are jutting out into the beach and reflected wave energy from the structure interacting with the incoming waves causes enhanced erosion of the beach. In order to mitigate this erosion, whilst retaining the parking areas in their current position, the feasibility of using detached breakwaters immediately offshore to encourage sand deposition should be investigated.
Detached breakwaters are shore-parallel structures that provide coastal protection by reflecting, dissipating, diffracting and refracting incident waves, thereby reducing wave energy and sediment transport in their lee. They are usually constructed of rock armour. At Danabaai, sediment would be deposited in the lower energy zone between the breakwater and the front of each parking area forming a bulge or salient. A salient can be defined as an accumulation of sediment extending seawards towards an island, breakwater or other obstruction, but not connecting to it. This salient may eventually become attached to the breakwater forming a tombolo, effectively forming a shore-connected breakwater. A tombolo can be defined as an accumulation of sediment or spit extending from the shore and connected to an offshore island or breakwater. An example of tombolos and salient formed by detached breakwaters is shown in Figure 5.7. These structures would be very efficient at sediment trapping and could potentially halt the passage of sand past them and exacerbate erosion further down the longshore sediment transport pathway.
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Figure 5-7. An example of tombolos (left and centre) and a salient (right) formed by detached breakwaters at Sea Palling, UK
The key parameters that control the build-up of sand in the lee of an offshore breakwater that should be assessed in a feasibility study include:
shore-parallel length of the breakwater;
distance offshore of the pre-project shoreline;
wave transmission characteristics of the breakwater, i.e. amount of wave energy that can pass over and/or through the breakwater; and
local wave and tide climate.
5.3.2 Potential for Beach Control Structures in Mossel Bay The coast at Glentana, immediately west of the headland at the eastern end of Mossel Bay is likely to suffer high rates of erosion in the near future. Its open coast position means that it is susceptible to direct wave attack during storms, hence the damage that took place here during the 2008 storm event. The properties have since been protected, but they are close to the beach and at high risk of erosion in the short-term with a high consequence of loss. The longshore sediment transport rates are the highest of any in the bay and sediment is moving east (see Sections 2 and 3). It is probable that the headland is ‘leaky’ and sediment is being lost from the system that is not being replaced by sand from the west where the transport rates are lower. This differential in sediment transport rates is likely to be exacerbating the erosion problem. Hence, in order to build a wider and higher beach to provide additional protection at Glentana, the use of a terminal groyne attached to the headland should be considered.
Groynes are long, narrow structures built approximately normal to the shoreline. They can be built in groups, which are designed to allow continued longshore transport, or as in this case of Glentana as a larger single terminal groyne to stop all longshore transport. The purpose of the terminal groyne would be
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to completely interrupt longshore sediment transport, locally building up sediment on its west side in front of Glentana. In addition to widening the beach, the groyne will also alter the orientation of the beach so it is more closely aligned to the dominant incoming wave direction, reducing longshore transport. The groyne would need to be long enough to prevent sand bypassing (at least in the medium term). However, it is possible that over a period of time, depending on longshore transport rates and groyne length, sand will eventually bypass the tip of the groyne and partially re-establish transport around the headland. The construction of the groyne and the total interruption of sand transport would not have detrimental effects elsewhere because the blocked sediment wouldn’t be feeding any critical beaches otherwise; the coast east of the headland is rocky and not easily accessed. An example of a terminal groyne and its effect is shown in Figure 5.8.
Figure 5-8. An example of a terminal groyne at Langstone Rock headland, UK. Note the build-up of sand to the left of the groyne
Given that the need for action at Glentana is immediate (Table 4.4), a detailed feasibility study to determine the use of a beach control structure should be completed over the next few years, and a strategy developed shortly after that. The key parameters that should be assessed in a feasibility study include:
cross-shore length and position of the groyne;
local wave climate;
longshore sediment transport rate; and
potential for accretion and bypassing of sediment.
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5.4 Adaptive Management The potential implementation of the management options outlined in Sections 5.1 to 5.3 is based on a degree of certainty about how the coastline will function in the future at those particular locations. However, at other locations, flexibility in how they are managed is required due to uncertainty in future natural changes and how those changes would impact on existing and future developments. As a result, it is not possible to determine a set RSM approach that is best over the next 100 years. A better approach is to implement adaptive management that sets out a monitoring programme to determine how the coast is changing, the possible timescale and nature of necessary management decisions, and the factors that will trigger those decisions. Hence, adaptive management is a structured iterative process of robust decision making in the face of uncertainty, with the aim of reducing uncertainty over time using system monitoring. In this way, decision making either passively or actively accrues information needed to improve future management. The stretches of coast where adaptive management is considered appropriate are both in Mossel Bay:
Klein Brak River, Reebok and Tergniet; and
Groot Brak River, Bothastrand and Outeniqua.
The recommendations from the Dune Management Plan (Ebersohn and Ebersohn, 2014) for these particular areas could be used as part of an overall adaptive management strategy. The measures recommended seek to take advantage of the seasonal variation in accretion, with build-up of active dunes during summer months so that erosion of the relict dunes is reduced. Hence, adaptive management could consist of installation of dune fences / brush matting during the summer months, followed by monitoring of any accretion, then removing dune fences in winter and monitoring rates of erosion. Although the adaptive management should seek to minimise interference to natural processes, it may be appropriate to implement short-term works to manage or repair storm-related erosion within the longer term policy of controlled adaptation.
It is unlikely that any attempt to maintain or strengthen a dune system will be successful if the beach in front of the dune is eroding. Therefore, good dune management has to be linked to good beach management. Therefore, a healthy natural dune system linked to a healthy beach is the best possible form of management, because it is adaptable to the prevailing forcing conditions. Hence, when the regional sediment management options for Dana Bay and Mossel Bay are determined, they should aim to support and enhance the management options developed in the Dune Management Plan (Table 5.2).
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Table 5-2. Overview of the dune system and proposed dune rehabilitation measures (adapted from Ebersohn and Ebersohn, 2014) Management Current State Rehabilitation Exemplar Photographs Area (2014) Recommendations
Erosion to the Protection of the toe of the seawall. sea wall.
Numerous Single, maintained access informal point would reduce damage Die Bakke access points to the dune. (Coastal over the dune. Management
Unit M1 in the Dune Management Dune and Do not remove small dune Plan) seawall in that has developed good adjacent to the public condition. facilities.
Localised slope failure Stabilise and re-vegetate caused by with native species. gully erosion.
Sea wall is Santos Bay generally in a Fill voids in slope and re- (Coastal good vegetate. Management condition, Unit M2 in the some erosion Instate artificial wetlands in Dune at the toe of areas above water outlets Management the vegetated where possible. Plan) slope. Santos Bay Erosion Stabilise the slopes via toe (Coastal encroaching protection, backfill with Management on the fence in similar material to reduce Unit M3 in the areas. steepness and re-vegetate Dune Steep slopes, for the worst case, Management unstable in alternatively brush matting Plan) places if the fence line isn’t at risk.
Voorbaai (Coastal Frontal dune Construct raised board Management in fair walks and discourage Unit M4 in the condition. footpaths. Dune Management
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Management Current State Rehabilitation Exemplar Photographs Area (2014) Recommendations Plan)
Informal Install dune forming fences access is at top, middle and toe of eroding the the primary dune and brush primary dune. matting.
The dune is stable either Alien vegetation to be side of the removed in stages to avoid Tweekuilen erosion of exposed dunes. Estuary and Cover exposed areas in the Petro SA brush matting as indigenous site. Patches vegetation re-establishes. of alien
vegetation. Properties located on top of the dune, Not Applicable. with numerous wooden access steps.
Dune forming fences at the Dune erosion. top, middle and toe of the Bay View dune, and brush matting. (Coastal Management Unit M4 in the Dune Management Storm water Plan) accumulation Blockage within the pipe within the addressed. parking area.
Seawall, paved boardwalk and municipal Not Applicable. parking areas in good condition.
Hartenbos Municipal (Coastal parking area Management and paved Unit M5 in the Not Applicable. boardwalk in a Dune good Management condition. Plan)
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Management Current State Rehabilitation Exemplar Photographs Area (2014) Recommendations
Municipal parking area is in a good Not Applicable. condition.
Beach on the primary dune is in a good condition, vegetation is Not Applicable. in a fair condition and no signs of erosion. Parking area Klein Brak susceptible to (Coastal wind-blown Management sand as the Re-vegetate exposed Unit M6 in the dune only has areas, and brush matt. Dune approximately Management 60% Plan) vegetation coverage.
Dunes well Construct a raised vegetated, but boardwalk and prevent Reebok informal unauthorised vehicular (Coastal access points. access on to the beach. Management
Unit M6 in the Dune Management Erosion of the Construct a raised Plan) dune due to boardwalk and prevent informal unauthorised vehicular access points. access on to the beach.
Dune erosion is present along the stretch of Dune-forming fences at the coast at top, middle and bottom of Reebok and Reebok and the primary dune, and Tergniet Tergniet. brush matting. (Coastal Exacerbated Management in areas by Unit M6 in the storm water Dune outlets. Management Plan) Municipal gabion wall is Not Applicable. in a good condition.
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Management Current State Rehabilitation Exemplar Photographs Area (2014) Recommendations
East of Tergniet, the primary dune Not Applicable. is in good condition.
East of De Vette Mossel Brush Matting to re- Restaurant, stabilise the dune. Southern Cross dune erosion (Coastal is visible. Management Unit M6 in the Dune Management Plan) Wind-blown Not Applicable. sand.
Groot Brak (Coastal Management Primary dune Unit M7 in the in good Not Applicable. Dune condition. Management Plan)
The Island to Bothastrand Areas of dune (Coastal Dune-forming fences at the erosion Management top, middle and toe of the between The Unit M7 in the primary dune, and brush Island and Dune matting. Bothastrand. Management Plan)
Bothastrand (Coastal Areas of dune Dune-forming fences at the Management erosion along top, middle and toe of the Unit M7 in the the shoreline primary dune, brush Dune at matting and board walks. Management Bothastrand. Plan)
Outeniqua Strand (Coastal Dune-forming fences at the Management Areas of dune top, middle and toe of the Unit M7 in the erosion along primary dune, brush Dune the shoreline. matting. Management Plan)
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Management Current State Rehabilitation Exemplar Photographs Area (2014) Recommendations
Dune-forming fences at the Areas of dune top, middle and toe of the erosion along primary dune, brush the shoreline. matting.
Primary dune at parking Glentana Not Applicable (Coastal area in good Management condition. Unit M8 in the Dune Management Private Plan) seawall east of car park in a Not Applicable good condition.
Dune erosion Brush matting and sand east of the bags to temporarily private stabilise the dune. seawall.
Adaptive management is iterative, evaluating actions through carefully designed monitoring and subsequently proposing adjustments. The adjustments are, in turn, tested with appropriate, and perhaps redesigned monitoring. The adaptive management process would apply to each stretch of coastline as a whole, but management actions can be identified and implemented on individual elements of the system, as needed. The process is flexible as it allows for a wide range of management actions but it also imposes a structured approach as the need for intervention must derive from the monitoring results.
The coastal properties that occupy the Klein Brak River to Tergniet coast and the Groot Brak River to Outeniqua coast are considered to be at low risk of erosion over the next 10 years (Table 4.4). However, after 50 years, they are considered to be at moderate risk of erosion (Table 4.4). Hence, an adaptive management strategy should be developed within the next five years, with implementation of monitoring soon after that, to gather the baseline data (mainly beach morphology and nearshore bathymetry, see Section 5.4.2) to support future management actions beyond 10 years.
5.4.1 Decision Support Tool A Decision Support Tool for adaptive management is provided as a separate deliverable to this report.
5.4.2 Potential Monitoring In development of an adaptive management strategy for Mossel Bay, three main methods of monitoring should be considered to determine changes in the morphology of the beaches and dunes; beach profiling, aerial photographs and Airborne Laser Induced Direction and Range (LiDAR). A brief overview is provided
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here, but details of the specific methods and their location and frequency of collection should be worked out in the strategy.
Beach Profiling Beach and dune morphology could be monitored using cross-shore profile data to assess changes in width, slope and volume, and to describe beach behaviour and its variability. These data can be used to identify trends and areas of high net change and high variability. The temporal and spatial frequency of profiles depends on the specific aim of the monitoring and in areas where more information is required the temporal and spatial frequency can be increased. Several techniques of varying sophistication are available for collecting beach survey data. The least sophisticated method (although not necessarily the least accurate) is survey using a quick set level, staff and chain. More advanced methods include using a total station with electronic distance measurement to a survey reflector prism and computer logging of data points. Current best practice involves the use of GPS (Global Positioning System). The use of Real Time Kinematic (RTK) GPS can be very fast and efficient, entailing the establishment of a base station at a control point and surveying the profile using a separate GPS rover unit. The surveyor is able to pre- programme the profile details in to the GPS computer, which enables the surveyor to keep accurately to the correct line by monitoring the LCD display on the rover unit. A survey of this kind enables vertical accuracies of 30mm on hard surfaces and 50mm on soft surfaces, and horizontal accuracies of 20mm.
Aerial Photography Vertical aerial surveys of the coast would provide quantitative data on large-scale changes of the coast, such as movement of the beach crest or movement of the vegetated dune edge. The process of reviewing and assessing geomorphology from aerial photography would generally require time-series analysis. This is easiest achieved through the registration of the data into digital systems such as geographic information systems (GIS) that allow the data to be correctly spatially located and allow accurate location and measurement to be achieved. This process is called geo-rectification and allows the image to be fixed in the horizontal plane in relation to a standard spatial geo-referencing framework. A vertical aerial photograph image can be displayed in the system, relative to other data sets and the features of interest can be digitised to permit time series analysis of change/variation.
LiDAR LiDAR is a remote sensing technique used for the collection of topographic data. It uses laser technology to ‘scan’ the ground surface, taking up to 10,000 observations per square kilometre. These observations are then converted to the local co-ordinate and elevation datum by the use of differential GPS. The system routinely achieves vertical accuracy of 11-25cm and plan accuracy of 45cm, with a very rapid speed of data capture (up to 50km2 per hour). This rapid data capture, coupled with the relatively automatic processing system can result in quick delivery of results. The system is cost effective, and for larger areas (>10km2) the cost is an order of magnitude smaller than traditional survey techniques. The data can be used to develop a time series of digital terrain models for Mossel Bay to provide data on broad-scale changes to this coast.
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6 Conclusions
This study has considered the physical and sedimentary processes operating in three crenulate bays within the jurisdiction of Mossel Bay Municipality. The aim of the investigation was to understand the pressures of coastal erosion in the bays, and through the application of Coastal Regional Sediment Management, identify restoration measures that aim to re-balance the coastal dynamics.
A conceptual geomorphological ‘model’ of the Mossel Bay Coastline was completed using existing literature sources and a site walk, in order to understand the baseline conditions that control coastline behaviour, and indicate how these may change in the future. The results show that the crenulate bays are self-contained sedimentary systems with no (or very little) sediment entering Dana Bay and Mossel Bay around their bounding headlands. Some sediment may be transported out of Mossel Bay to the east of Glentana, although this has not been quantified. Longshore sediment transport rates are very low, ranging from predicted values of 5,000m3/year to 15,000m3/year.
The largest input of sediment to the coastal zone is from erosion of relict coastal dunes, which are composed of sand with low cohesion. In Mossel Bay, between Diaz and Groot Brak River the average historic erosion rate is approximately 0.1-0.2m/year. Towards Glentana, the average erosion rate increases to about 0.3-0.4m/year. Between Groot Brak River and Bothastrand, the coast has accreted at an average rate of 0.1-0.2m/year. Here, a series of modern dunes is actively prograding behind the beach and protecting the relict dunes from erosion.
Future erosion rates along the Mossel Bay Coastline are predicted to increase due to a potential rise in sea level and increased storminess. Applying a derivative of the Bruun Rule, future dune erosion rates for sections of the coastline in Dana Bay and Mossel Bay were estimated. A simple risk and consequence analysis of how these sections of coastline will respond to future erosion was then applied over 10, 50 and 100 year planning horizons (Tables C.1 and C.2).
Table C-1.Risk and consequence of erosion in Dana Bay Risk of Consequence of Location Potential Erosion Erosion Erosion Risk and consequence of erosion in 10 years’ time Vleesbaai 1-2m High High Boggomsbaai 1-2m Low High Danabaai Western Parking Area 26m High Moderate Danabaai between Parking Areas 1-2m Low High Danabaai Eastern Parking Area 13m High Moderate Risk and consequence of erosion in 50 years’ time Vleesbaai 15-30m High High Boggomsbaai 15-30m Low High Danabaai Western Parking Area >26m High Moderate Danabaai between Parking Areas 15-30m Low High Danabaai Eastern Parking Area >13m High Moderate Risk and consequence of erosion in 100 years’ time Vleesbaai 40-80m High High Boggomsbaai 40-80m Moderate High Danabaai Western Parking Area >26m High Moderate Danabaai between Parking Areas 40-80m Low High Danabaai Eastern Parking Area >13m High Moderate
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Table C-2. Risk and consequence of erosion in Mossel Bay Risk of Consequence of Location Potential Erosion Erosion Erosion
Risk and consequence of erosion in 10 years’ time Diaz / Voorbaai 1-2m Moderate
Bayview 1-2m (32m at the central parking area) Moderate
Hartenbos 1-2m Moderate
Klein Brak River 1-2m Low High Reebok and Tergniet 1-2m (up to 13m at the parking areas) Low
Groot Brak River 1-2m (west), accretion of 1-2m to the east Low
Bothastrand and Outeniqua accretion of 1-2m Low
Glentana 3m Moderate
Risk and consequence of erosion in 50 years’ time Diaz / Voorbaai 15-30m High
Bayview 15-30m (>32m at the central parking area) High
Hartenbos 15-30m High
Klein Brak River 15-30m Moderate High Reebok and Tergniet 15-30m (>13m at the parking areas) Moderate 15-30m (west), erosion to the east (rate Groot Brak River Moderate unknown) Bothastrand and Outeniqua erosion (rate unknown) Moderate
Glentana 45m High
Risk and consequence of erosion in 100 years’ time Diaz / Voorbaai 40-80m High
Bayview 40-80m (>32m at the central parking area) High
Hartenbos 40-80m High
Klein Brak River 40-80m Moderate High Reebok and Tergniet 40-80m (>13m at the parking areas) High 40-80m (west), erosion to the east (rate High (west), Groot Brak River unknown) Moderate (east) Bothastrand and Outeniqua erosion (rate unknown) Moderate
Glentana 120m High
The outcome of this erosion risk analysis is a set of four potential Coastal Regional Sediment Management options in order to mitigate the effects of the ongoing erosion (summarised in Figure C.1).
allow the natural process of dune erosion to continue without intervention;
use beach restoration strategies particularly beach nourishment to slow erosion rates;
install beach control structures to encourage sand deposition; and
adaptively manage the coast with a view to implementation of future strategies.
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Allow dune erosion to continue Beach control structure
Allow dune erosion to continue
Beach control structure Allow dune erosion to continue
Beach nourishment
Allow dune erosion to continue
Beach control structure Adaptive management
Allow dune erosion to continue
Adaptive management
Allow dune erosion to continue
Beach nourishment
Rock headland
Figure C-1. Potential Coastal Regional Sediment Management options in Dana Bay (top panel) and Mossel Bay (bottom panel) (adapted from Google Earth, 2015)
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