Proposal for

Measures to Control Erosion

in front of Terrain Basset, Grand Gaube

 October 2009

Table of Content

Executive Summary......................................................................................................................... 3

Table of Content................................................................................................................................ 4

List of Figures, Map, Pictures and Tables......................................................................................... 6

Chapter 1 Overview......................................................................................................................... 7

Chapter 2 The Physical Environment at the site........................................................................... 12

2.1. Climatological Factors................................................................................................................ 12

2.1.1. Winds....................................................................................................................................... 12

2.1.2. Rainfall.................................................................................................................................... 12

2.1.3. Sea-surface temperature........................................................................................................... 12

2.1.4. Atmospheric pressure............................................................................................................... 13

2.1.5. Tides........................................................................................................................................ 13

2.1.6. Relative changes in mean sea-level.......................................................................................... 13

2.1.7. Sea-Level Changes during Cyclone Events............................................................................. 13

2.2. Oceanographic Parameters.......................................................................................................... 14

2.2.1. Tides at the site........................................................................................................................ 14

2.2.2. Waves and storm surges.......................................................................................................... 14

2.2.3. Nearshore wave transformation............................................................................................... 16

2.2.4. Longshore current.................................................................................................................... 16

Chapter 3 Survey Methods and Observations.............................................................................. 18

3.1. Survey Methods.......................................................................................................................... 18

3.2. Observations............................................................................................................................... 19

3.2.1. Sediment.................................................................................................................................. 19

3.2.2. Beach Topography & Bathymetry........................................................................................... 19

3.2.3. Appearance of Water............................................................................................................... 20

3.2.4. Corals....................................................................................................................................... 20

3.2.5. Seagrass................................................................................................................................... 21

3.2.6. Algae........................................................................................................................................ 23

3.2.7. Fish.......................................................................................................................................... 25

3.2.8. Invertebrates............................................................................................................................ 25

3.2.9. Currents................................................................................................................................... 26

Chapter 4 Proposals for control of erosion................................................................................... 29

4.1. Permeable breakwater................................................................................................................. 29

4.1.1. Proposal for permeable breakwater.......................................................................................... 29

4.1.2. Maintenance............................................................................................................................. 33

4.2 Elevating height of rocky outcrop............................................................................................... 33

4.2.1. Proposal for elevating the rocky outcrop................................................................................. 33

4.2.2. Maintenance............................................................................................................................. 34

4.3. Beach Reprofiling....................................................................................................................... 34

4.3.1. Proposal for beach reprofiling.................................................................................................. 34

4.3.2. Maintenance............................................................................................................................. 36

4.4 Fencing........................................................................................................................................ 36

4.4.1. Proposal for fencing................................................................................................................. 36

4.4.2. Maintenance............................................................................................................................. 36

Chapter 5 Impacts and mitigation measures................................................................................. 37

5.1. Impacts....................................................................................................................................... 37

5.2. Mitigation................................................................................................................................... 37

5.3. Environment monitoring plan..................................................................................................... 38

Chapter 6 Alternatives.................................................................................................................... 39

6.1.1. The ‘Do Nothing’ Option........................................................................................................ 39

6.1.2. Prevention................................................................................................................................ 40

6.1.3. Non structural interventions.................................................................................................... 40

6.1.4. Structural intervention............................................................................................................. 40

Conclusion........................................................................................................................................ 45

References......................................................................................................................................... 46

Annex 1 Species List

Annex 2 Sediment sieve analysis

Annex 3 Bathymetry and Topography