The Chagos Islands: Physical Environment - Key Facts‏

Prepared by Richard Dunne October 2013

The Chagos Islands

Physical Environment – Key Facts

Over the last 20 years the Chagos has been a relatively stable physical environment,

and these low-lying coral islands should continue to be able to support human

habitation, as they have done for much of the last 200 years (Dunne et al. 2012).

Island Morphology

• The Chagos islands are all of low elevation with only 18% of their land above 2 m and 7% above 3 m (Woodroffe 2008). They were formed between 6,000 and 4,000 years beforepresent (BP) (Perry et al. 2011) when sea level had risen to about its present level.
• •Reef islands are accumulations of sand and gravel (derived from the surrounding reef) that are transported and deposited by waves and wave-induced currents on reef surfaces. The location of islands therefore, results from the unique combination of hydrodynamic forces and the sediment available for transport and deposition. The maximum elevation of reef islands is controlled by wave runup against the shoreline, which controls the maximum level of sediment deposition. Wave overtopping (and associated inundation) therefore, is a natural mechanism that allows a fresh layer of sediment to be added to island surfaces, thus increasing land levels (Kench et al. 2006).
• Increasing land elevation through overwash processes, combined with differential erosion and accretion of island shorelines, controls the dynamic physical adjustment of islands on reef surfaces (Webb & Kench 2010) and provides mechanisms for islands to adjust to changing environmental boundary conditions.•
• A comparison of the shape of Diego Garcia atoll between 1824 and 1971 suggests that any changes to the size of the island over this timespan have been minor (Stoddart 1971a).
• A comparison of island morphology in Salomon and Egmont Atolls between 1905 and the present day suggests that some islands may have decreased in size to a small extent (Ile Boddam -4.7%, Ile de la Passe -2.8%) but that others have increased (Ile Takamata +6.3%; Egmont Atoll +36.5%). This is over a time period when GMSL rose by about 20 cm (Dunne unpublished).
• A survey of 27 Pacific Atoll islands over a 20 to 61 year period during which sea-level has risen at 2mm year-1 has demonstrated that 43% of the islands remained stable, 43% increased in size, and 14% decreased (Webb & Kench 2010). The study showed that these islands are robust and can increase in area despite sea-level rise, that erosion may be balanced by deposition on other parts of the shore, and that the islands are dynamic landforms that adjust in response to physical conditions. Furthermore the study results were applicable to islands in other reef settings and contradicted widespread perceptions that all reef islands are eroding in response to recent sea-level rise.

Seismic Activity

• •The Chagos lie in an area of the Indo–Australian tectonic plate where there is intense intraplate deformation, with high seismic activity.
• Large earthquakes occurred in the area in 1912 (M6.8) and 1983 (M7.6) with a “swarm” of moderate sized earthquakes in 1965–1968 (Wiens 1986).
• Since 1973, there have been 314 earthquakes greater than M4.5 within a 500 km (270 nm) radius of Diego Garcia (USGS/NEIC PDE, 2011. http: //earthquake.usgs.gov/ earthquakes/ eqarchives/epic/.Catalogue searched 13 Jun 2011).

Vertical land movement

• •Although in an area of seismic activity, there is no record of island subsidence, indeed onDiego Garcia minor crustal uplift of 0.63} 0.28 SE mm year−1 has occurred between 1996 and 2009 (Dunne et al. 2012).
• On Diego Garcia there is tentative evidence of ‘emergence’ of the atoll (Woodroffe 2005) but this is actually due to a slight lowering of the sea level since the mid-Holocene (3,000 years ago).
• There is no known evidence of subsidence in the archipelago in the past 100 years (Dunne et al. 2012).
• Tectonic plate movements and seismic activity can cause changes to island elevation and there is some inference of uplift having occurred on several islands (part of Coin du Mire in Peros Banhos atoll, and North Brother and Resurgent in the Three Brothers group).

Sea Level

• •Sea level rise (SLR) is an inescapable consequence of increasing greenhouse gas concentrations, whether these are due to anthropogenic causes or otherwise.
• Global mean sea level (GMSL) is controlled primarily by volume changes of the world’s oceans and is currently rising at a rate of 3.2 } 0.4 mm year-1 (Colorado University Sea Level Research Group - http://sealevel.colorado.edu/) although more recent analysis suggests that part of this rise might be due to the Pacific Decadal Oscillation and that the true rate of rise is 2.7 mm year-1 (Hamlington et al. 2013).
• Over the period, 1901-2010, GMSL has risen by between 17 and 21 cm as measured by tide gauges (IPCC 2013b; IPCC 2013a).
• There is no evidence that the rate of GMSL rise accelerated during the 20th Century (Gregory et al. 2013).
• Due to continued warming of the world’s oceans induced by increasing greenhouse gas (GHG) concentrations, global mean sea level is projected to rise by between 26–97 cm by the end of the 21st century, depending on which Intergovernmental Panel on Climate Change Representative Concentration Pathway (RCP) will be realized in the future. There are large uncertainties in the ranges projected by the models. Under RCP 4.5 or 6, both of which are considered realistic, GMSL will rise by a further 24 cm by 2050 and 64 cm by 2100 from today’s levels (2013) computed from figures in IPCC (2013b) and IPCC (2013a).
• Regionally, because of changes in wind- and buoyancy-driven ocean currents in response to global warming, local sea-level rise (SLR) could be higher or lower than the global mean, but on past history is likely to be lower in the Chagos region of the Indian Ocean (Han et al. 2010).
• In the Chagos Archipelago no significant SLR can be detected in the instrument record (tide gauge at Diego Garcia 1988-2011, and satellite altimetry 1993-2011) (Dunne et al. 2012).
• There is also no long term change in the incidences of the highest sea level (e.g., extreme spring tides) when island flooding or inundation is most likely to take place (Dunne et al. 2012).

Erosion and Flooding

• There are recent documented reports of sea-water inundation and flooding in the Chagos islands, e.g., on Diego Garcia (Sheppard 2012a; Sheppard 2012b) which have been attributed to rising sea level. Sheppard (2012a) has also characterised this as “accelerating both on Diego Garcia and on northern atolls”
• Inundation is however a regular feature at different locations on these islands and there is no evidence to suggest that this is increasing over time on Diego Garcia (Dunne 2013).
• Erosion is a natural process on all low lying coral islands and is manifest in seasonal removal and deposition of sediments from different shorelines driven by the changing monsoonal winds and currents. These effects are evident in the Chagos over decades to hundreds of years e.g., (Woodroffe 2011).
• Inundation and erosion has regularly been reported by researchers for over 120 years, e.g., on Diego Garcia (Bourne 1886).
• On Diego Garcia, shoreline erosion has been particularly evident since major engineering work began in the 1970s and has been exacerbated by reef bedrock removal, shoreline engineering and vegetation clearance (EG&G Environmental Consultants 1980; Moffatt & Nichol 2008).

Cyclones, Storms, and Wind/Wave environment

• The Chagos lie outside the Indian Ocean cyclone belt and experience relatively low wind speeds (Dunne et al. 2012).
• The last known cyclone to pass over the islands was in 1891.
• There is no evidence of changes in the wind or wave environment in the past 20 years (Dunne et al. 2012).
• Projected changes in the wind/wave climate of the Indian Ocean for the years 2075-2100 compared to 1980-2009 are that significant wave height (Hs) will decrease by about 5% (IPCC 2013b; IPCC 2013a). Significant wave height is the mean of the upper ⅓ of the largest waves.
• There are no projected future changes in either the location of the tropical cyclone belt or the frequency of storms for the Indian Ocean (IPCC 2013b; IPCC 2013a).

Overtopping and flooding

• •The potential for wave overtopping of the island of Ile du Coin (Peros Banhos atoll) and Ile Boddam (Salomon atoll) which led to the conclusion that “both Ile Boddam and Ile du Coin are likely to experience severe overtopping” in the 2002 Phase 2B Feasibility Study (Posford Haskoning Ltd 2002) has since been demonstrated to be flawed (Kench 2012).
• Modelling of the response of low lying coral islands to increases in sea level demonstrates that, subject to sediment supply, beach ridges increase in height and migrate inland and retain their capacity to protect the island from wave overtopping (Cowell & Kench 2001).

Rainfall and Water resources

• The Chagos islands (Salomon & Peros Banhos atolls in particular) have the highest annual rainfall totals of all Indian Ocean atolls (Stoddart 1971b), approximately 3,750 and 4,000 mm year-1 respectively (Posford Haskoning Ltd 2002).
• Completely dry months do not occur on Salomon & Peros Banhos atolls (Stoddart 1971b).
• Freshwater on all the islands is contained in lenses trapped in the coral basement and is suitable for domestic consumption.
• The water supply ‘population capacity’ of the two principal islands in the northern atolls would be approximately 3,000 for Ile du Coin and 1,500 for Ile Boddam for an average water demand of 100 litres per person per day (Posford Haskoning Ltd 2002).
• Diego Garcia obtains all its freshwater supplies from the island lenses, sufficient to support a population of several thousand military and civilian contractors.

Phase 2B Resettlement Study 2002

• •The ‘General conclusions’ found in section 1.11 of the Executive Summary appear to be predicated largely on the Section 6 of the Feasibility Study and as such are considered to be flawed (Kench 2012).
• Reviews of the critical Section 6 of the Study were superficial and demonstrated that those tasked with assessing the robustness of the science by the FCO (Mr Little & Dr Sheppard) were not qualified to do so (Kench 2012) as a consequence they failed to detect scientific errors in the Study which led to the flawed ‘General conclusions’ (above).

References

Bourne, G. C. (1886) On the Island of Diego Garcia of the Chagos Group. Proceedings of the Royal

Geographical Society and Monthly Record of Geography 8(6): 385-393.

Cowell, P. J. & Kench, P. S. (2001) The Morphological Response of Atoll Islands to Sea-Level Rise. Part

2: Application of the Modified Shoreface Translation Model (STM). Journal of Coastal

Research (Special Issue 34): 645-656.

Dunne, R. P. (2013) Sharks on the lawn in Diego Garcia - but is rising sea level to blame? Ocean

Challenge (Summer 2013).

Dunne, R. P., Barbosa, S. M. & Woodworth, P. L. (2012) Contemporary sea level in the Chagos

Archipelago, central Indian Ocean. Global and Planetary Change 82-83: 25-37.

EG&G Environmental Consultants. (1980) Environmental survey of construction and dredging related

activities on Diego Garcia, Indian Ocean. pp. 1-107. Hawaii.

Gregory, J. M., White, N. J., Church, J. A., Bierkens, M. F. P., Box, J. E., van den Broeke, M. R., Cogley,

J. G., Fettweis, X., Hanna, E., Huybrechts, P., Konikow, L. F., Leclercq, P. W., Marzeion, B.,

Oerlemans, J., Tamisiea, M. E., Wada, Y., Wake, L. M. & van de Wal, R. S. W. (2013)

Twentieth-Century Global-Mean Sea Level Rise: Is the Whole Greater than the Sum of the

Parts? Journal of Climate 26(13): 4476-4499.

Hamlington, B. D., Leben, R. R., Strassburg, M. W., Nerem, R. S. & Kim, K. Y. (2013) Contribution of

the pacific decadal oscillation to global mean sea level trends. Geophysical Research Letters:

2013GL057297.

Han, W., Meehl, G. A., Rajagopalan, B., Fasullo, J. T., Hu, A., Lin, J., Large, W. G., Wang, J. w., Quan, X.

W., Trenary, L. L., Wallcraft, A., Shinoda, T. & Yeager, S. (2010) Patterns of Indian Ocean sealevel

change in a warming climate. Nature Geosci 3(546): 550.

IPCC. (2013a) Working Group I contribution to the IPCC 5th Assessment Report "Climate Change

2013: The Physical Science Basis - 30 September 2013. pp. 2216. Stockholm.

IPCC. (2013b) Working Group I Contribution to the IPCC Fifth Assessment Report, Climate Change

2013: The Physical Science Basis - Summary for Policymakers - 27 September 2013. pp. 36.

Kench, P. S. (2012) Review of the Coastal Processes and Ocean Study: Feasibility Study for

Resettlement of the Chagos Archipelago (Royal Haskoning, 2002). pp. 15. Auckland:

University of Auckland.

Kench, P. S., McLean, R. F., Brander, R. W., Nichol, S. L., Smithers, S. G., Ford, M. R., Parnell, K. E. &

Aslam, M. (2006) Geological effects of tsunami on mid-ocean atoll islands: The Maldives

before and after the Sumatran tsunami. Geology 34(3): 177-180.

Moffatt & Nichol. (2008) Diego Garcia Shoreline Erosion Study. p. 120. Long Beach, CA: Moffatt &

Nichol, 2780 Kilroy Airport Way, Suite 600, Long Beach, CA 90806.

Perry, C. T., Kench, P. S., Smithers, S. G., Riegl, B., Yamano, H. & O'Leary, M. J. (2011) Implications of

reef ecosystem change for the stability and maintenance of coral reef islands. Global Change

Biology 17(12): 3679-3696.

Posford Haskoning Ltd. (2002) Feasibility Study for the Resettlement of the Chagos Archipelago:

Phase 2B. In: London.

Prepared by Richard Dunne October 2013

Sheppard, C. (2012a) Seawater inundations in the Chagos archipelago at high tides, and shoreline

erosion. CCT unpublished report.

Sheppard, C. R. (2012b) Sharks on the lawn - The reality of rising sea-level on Indian Ocean islands.

Ocean Challenge (Autumn 2012): 17.

Stoddart, D. R. (1971a) Geomorphology at Diego Garcia Atoll. In: Atoll Research Bulletin, Geography

and Ecology of Diego Garcia Atoll, Chagos Archipelago, eds. D. R. Stoddart & J. D. Taylor, pp.

7-26. Atoll Research Bulletin: The Smithsonian Institution.

Stoddart, D. R. (1971b) Rainfall on Indian Ocean Islands. Atoll Research Bulletin 147: 1-22.

Webb, A. P. & Kench, P. S. (2010) The dynamic response of reef islands to sea-level rise: Evidence

from multi-decadal analysis of island change in the Central Pacific. Global and Planetary

Change 72(3): 234-246.

Wiens, D. A. (1986) Historical seismicity near Chagos: a complex deformation zone in the equatorial

Indian Ocean. Earth and Planetary Science Letters 76(3-4): 350-360.

Woodroffe, C. D. (2005) Late Quaternary sea-level highstands in the central and eastern Indian

Ocean: A review. Global and Planetary Change 49(1-2): 121-138.

Woodroffe, C. D. (2008) Reef-island topography and the vulnerability of atolls to sea-level rise.

Global and Planetary Change 62(1-2): 77-96.

Woodroffe, S. A. (2011) Sea-level changes and shoreline dynamics. Unpublished report