W orleyParsons

resources & energy

KOUGA MUNICIPALITY IN V E S T IG A T IO N OF THE ST. FRANCIS BAY SPIT AND BEACH E R O SIO N P R O B L E M S

Investigation of the St. Francis Bay Spit and Beach Erosion Problem s

Review of Previous Studies and Investigations and Assessment of Potential Remedial Options

278960-00-C S -R E P -0001

15 October 2013

Dr Gary Mocke

Study

Estimated Annual Sediment Transport Rate (m3/yr)

Estimated Beach Erosion Rate (m/yr)

Estimated Beach Nourishment Volumes Required

CSIR (1992) 50,000 – 100,000 - - WPR (1993) 100,000 - 1 000,000 m3 for a beach width

of 40m Entech (2002) 50,000 – 100,000 2.5 – 3.0 1 000,000 m3 for a beach width

of 30m Illenberger & Associates (2001)

80,000 Up to 3.0 -

ASR (2006) 72,000 – 76,000 1.5 – 3.0 500,000 m3 could be placed in combination with offshore reefs

WorleyParsons (The current study)

50,000 - 100,000 (refer to section 5)

1.0 – 2.5 900,000 m3 for a beach width of 40m

No interference with coastal processesNo 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 maintenancePotential long term loss of sandy material to offshoreIntensive operations and management requirements

9

4

Relatively easy construction using land-based equipment

Potential continuation of loss of beach after severe erosion eventsNo reinstatement of beach for recreational purposes

Relatively easy construction using land-based equipmentReinstatement of beach for recreationDredging in the river mouth as a sand source improves navigation for marina usersBuild-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 protectionRip 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 improvedFormation of salient, thus adding some amenity valueDredging 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 navigationProvide 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 beachRip currents can be inducedInterruption of beach interface by many structures

1

10

Reinstatement of beach for recreationHigh level of protection to the spitRelatively easy construction using land-based equipmentProtection of landward property and infrastructure by revetment re-armouring at vulnerable locationsDredging in the river mouth as a sand source improves navigation for marina users

Possibility of loss of beach after severe erosion eventsHigh ongoing maintenancePotential long term loss of sandy material to offshore

Protection of landward property and infrastructure by the maintenance and reinstatement of vulnerable revetmentsHigh level of protection to the spitRelatively easy construction using land based equipment.

Potential continuation of loss of beach after severe erosion eventsNo reinstatement of beach for recreational purposes.

7

8

Beach amenity value is retained and improvedBuild-up of beach on the up drift side improving and returning amenity valueFormation of salient, thus adding some amenity valueDredging 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 hazardArtificial reefs difficult to construct and marine based equipment is requiredDangerous for small craft navigation

B+D+F

4

Coastal Protection Option

Total Cost

(with geotextile sa n d bag as m a te ria l

op tio n )

(with rock as m a te ria l op tio n )

B

Beach Nourishment O n ly

R 54 ,00 0 ,00 0

R 54 ,00 0 ,0 0 0

C

Spit protection o n ly

R 22 ,00 0 ,00 0

R 28 ,60 0 ,0 0 0

B+C

Beach nourishment + sp it p ro te c tio n

R 75 ,70 0 ,00 0

R 82 ,30 0 ,0 0 0

B+D

Beach nourishment + sin g le g ro yn e

R 66 ,50 0 ,00 0

R 80 ,20 0 ,00 0

B+C+F

Beach nourishment + sp it protection + artificial ree fs

R 94 ,20 0 ,00 0

R 135 ,80 0 ,0 0 0

B+C+E

Beach nourishment + sp it protection + groyne fie ld

R 1 0 3 ,5 0 0 ,0 0 0

R 140 ,40 0 ,0 0 0

B+C+C 1

Beach nourishment + sp it protection + revetment re - armouring at vu lne ra b le lo ca tio n s

R 88 ,80 0 ,00 0

R 95 ,50 0 ,0 0 0

C+C 1

Spit protection + revetment re - armouring at vu lne ra b le lo ca tio n s

R 35 ,20 0 ,00 0

R 41 ,80 0 ,0 0 0

B+D+F

Beach nourishment + sin g le groyne + artificial ree fs

R 85 ,00 0 ,00 0

R 133 ,70 0 ,0 0 0

Criterion Description/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.

Activity

Year 1

Year 2

Year 3

Months

1

2

3

4

5

6

7

8

9

10

11

1 2

13

14

15

1 6

17

18

19

2 0

21

22

23

2 4

25

26

Project

management

Additional

Studies

Detailed

Design

Construction

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 900 000 m3;

• Annual alongshore sediment transport has been estimated to be in the order of 50,000 m3 - 100 000 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 2 800 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?