Dr Gary MockeDr Gary Mocke. No interference with coastal processes No need for any expenditure Social impacts (relocation, loss of beach access and.

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Presentation transcript:

Dr Gary MockeDr Gary Mocke

No interference with coastal processes No need for any expenditure Social impacts (relocation, loss of beach access and recreational value) Reinstatement of beach for recreation Minimal interference with coastal processes Dredging in the river mouth as a sand source improves navigation for marina users High ongoing maintenance Potential long term loss of sandy material to offshore Intensive operations and management requirements 9 4

Relatively easy construction using land-based equipment Potential continuation of loss of beach after severe erosion events No reinstatement of beach for recreational purposes Relatively easy construction using land-based equipment Reinstatement of beach for recreation Dredging in the river mouth as a sand source improves navigation for marina users Build-up of beach on the up drift side improving and returning amenity value Landward property and infrastructure as well as the spit have no immediate protection Rip currents can be induced adjacent to the structure, which can be a swimming hazard B + D 2 3

High level of protection to the spit Reinstatement of beach for recreation and therefore beach amenity value is retained and improved Formation of salient, thus adding some amenity value Dredging in the river mouth as a sand source improves navigation for marina users Rip currents can be induced and this is dangerous to swimmers Artificial reefs difficult to construct and marine based equipment is required Dangerous for small craft navigation Provide only partial protection Reinstatement of beach for recreation High level of protection to the spit Relatively easy construction using land- based equipment Protection of a longer section of coast possible than for single groyne option Reinstatement of beach for recreation and therefore beach amenity value is retained and improved Dredging in the river mouth as a sand source improves navigation for marina users Construction activity over greater length of beach Rip currents can be induced Interruption of beach interface by many structures 1 10

Reinstatement of beach for recreation High level of protection to the spit Relatively easy construction using land-based equipment Protection of landward property and infrastructure by revetment re- armouring at vulnerable locations Dredging in the river mouth as a sand source improves navigation for marina users Possibility of loss of beach after severe erosion events High ongoing maintenance Potential long term loss of sandy material to offshore Protection of landward property and infrastructure by the maintenance and reinstatement of vulnerable revetments High level of protection to the spit Relatively easy construction using land based equipment. Potential continuation of loss of beach after severe erosion events No reinstatement of beach for recreational purposes. 7 8

Beach amenity value is retained and improved Build-up of beach on the up drift side improving and returning amenity value Formation of salient, thus adding some amenity value Dredging in the river mouth as a sand source improves navigation for marina users Rip currents can be induced adjacent to the structures, which can be a swimming hazard Artificial reefs difficult to construct and marine based equipment is required Dangerous for small craft navigation B+D+F 4

CriterionDescription/Considerations Visual Overall acceptability and physical appearance (aesthetics) Cost Capital cost Cost effectiveness Environmental Accommodating environmental requirements Potential environmental impacts during construction Risk of vandalism Potential environmental benefits of the scheme Coastal ecology Littoral active zone Adaptability/Reversibility Removal of solution in future Adaptability or change of area of solution over time Enables addition or extension in the future Beach Amenity Sandy beach area for recreational purposes Facility of beach access Scope for which the intervention improves amenity value of the beach Health and safety (rip currents, rail crossing, etc.) Maintenance Frequency and extent of intervention Availability of suitable resources for repair (materials, plant, expertise), if relevant Expected maintenance costs and funding requirements Accessibility for construction plant Engineering Effectiveness Achievement of functional requirement (e.g. preventing erosion, retention of sand including beach width buffer, reducing wave overtopping) Changes in acceptable probability of failure over time Resistance to climate change Constructability Health and Safety issues Access of construction plant Construction materials Site area for storage of materials and operations Vulnerability during construction (e.g. storm frequency) Resilience Ability to recover after major storms Ability to withstand sequence of significant storm events

The above assessment shows that Beach nourishment in combination with dune protection and offshore breakwaters offers distinct benefits over other options. It maintains a beach to provide for public amenity, effective at preventing erosion of the spit, and facilitates the formation of a salient on the lee side of offshore breakwaters. Furthermore, as outlined by ASR (2006), presence of existing natural submerged reefs in the area creating wider beaches in their lee than the adjacent coast provides good evidence that offshore breakwaters are likely to be effective at stabilizing the beach at St Francis Bay. The assessment assumes the weighting as provided in the table above; however, alternative weighting can be applied depending on the requirements of the community, the municipality and other stakeholders.

Conclusions In the context of the collected data, especially the aerial photographs since 1942, the average shoreline movement indicate a steady seaward drift from 1942 through to about 1970’s, where after the shoreline starts eroding to the current position. This was mainly due to stabilization of the Santareme dunes and construction of the Impofu dam; The average wave height along St. Francis Bay coastline is relatively mild (less than 1 m) with a predominant wave direction from the south east. The south easterly direction is a result of waves diffracting around the St. Francis Bay headland as well as locally generated seas; Beach profile surveys of the past 20 years indicate that on average, there is a shoreline retreat or beach erosion rate of about 1.0 m to 2.5m per year along the St. Francis Bay beach; The required beach nourishment volumes in order to increase the beach width by an extra 40m are estimated at m3; Annual alongshore sediment transport has been estimated to be in the order of 50,000 m m3 at St. Francis Bay beach and appears to be predominantly in a W-E direction; Various coastal protection options were assessed and evaluated to protect a m stretch of St. Francis Bay beach and spit. The evaluation criteria incorporated cost, effectiveness, environmental impacts, and beach amenity and maintenance requirements. Beach nourishment in combination with dune/spit protection and offshore breakwaters was selected as the best solution for St. Francis Bay Beach and Spit erosion problem.

Recommendations Although beach nourishment in combination with dune/spit protection and offshore breakwaters is recommended as the most appropriate coastal management solution to protect the coastline at St. Francis Bay, further studies are required in order to optimize the layout configuration of the breakwaters. It is advisable that related field measurements and model studies commence as soon as possible to expedite the design of a sustainable solution for St Francis Bay; Given the significant erosion that has occurred at the spit area, including over the recent months, it is recommended that this area be immediately protected using a revetment. Subject to a cost comparison from tendered offers, geotextile sand bags are likely to be recommended over rock revetments. St. Francis Bay Local Municipality have to decide on the suitable material for dune protection in light of the advantages and disadvantages provided for both rock and geotextile sand bags. Regular surveys of the beach profile should be undertaken to monitor the foreshore changes and the effectiveness of the implemented solution. 1.Council to get proposal relating to Mocke Report and pass it so that the above conclusions and recommendations can be solved. 2.Beach issue belongs to all residents of St Francis Bay. 3.TOR for works is being drawn up by the KM. 4.Will take time 2 to 3 years. 5.Financing of works?