A Geospatial Appraisal of Ecological and Geomorphic Change on Diego Garcia Atoll, Chagos Islands (British Indian OceanTerritory)

Abstract

This study compiled a wide range of modern and historic geospatial datasets to examine ecological and geomorphic change at Diego Garcia Atoll across a 38-year period . This remarkable collection of spatially referenced information offered an opportunity to advance our understanding of the nature and extent of environmental change that has taken place with the construction of the military airbase at Diego Garcia. Changes assessed included movements of the lagoon rim shorelines, changes in the terrestrial vegetation on the lagoon rim and amendments to the bathymetry of the lagoon basin through dredging activities. Data compiled included detailed shoreline and vegetation maps produced as part of the H.M.S. Vidal Indian Ocean Expedition (1967), three Ikonos satellite images acquired in 2005 that collectively covered the complete Atoll area, a ground truthing field dataset collected in the northern section of the lagoon for the purpose of seafloor mapping (2005), observational evidence of shoreline erosion including photographs and descriptions of seawater inundations and bathymetric soundings from five independent surveys of the lagoon floor (1967, 1985, 1987, 1988 and 1997). Results indicated that much of the change along the lagoon rim is associated with the expansion of the inner lagoon shoreline as a result of the construction of the military airbase, with an estimated increase in land area of 3.01 km 2 in this portion of the atoll rim. the lagoon, particularly in the vicinity of the Main Passage. As no previous record of benthic character exists, a complete benthic habitat map of the atoll was derived through classification of the three IKONOS satellite images. Management implications arising from this overall appraisal of geomorphic and ecological change at Diego Garcia included the need for ongoing monitoring of shoreline change at a representative set of sites around the atoll rim, monitoring of the water flow regime through the northern channels between the open ocean and the lagoon basin and an ongoing mapping campaign to record periodic changes in the character of the benthic surface ecology.

Response to Sheppard, C. Atoll Rim Expansion or Erosion in Diego Garcia Atoll, Indian Ocean? Comment on Hamylton, S.; East, H. A Geospatial Appraisal of Ecological and Geomorphic Change on Diego Garcia Atoll, Chagos Islands (British Indian Ocean Territory). Remote Sens. 2012, 4, 3444–3461

“…The remaining four photo records suggested no shoreline change, but our measurements suggested that rim widths had eroded over the longer 38-year period. Thus, our study [2] appears to indicate a greater amount of erosion than Sheppard's photographic evidence. Indeed, the wider study reported both erosion and accretion across 26 and 43 OPEN ACCESS transects respectively.…”

“…Sheppard provided eight independent photorecords of his inter-annual observations for inclusion in our study ([3]) with which moderate correspondence was found when compared to our findings [2]. Four photographs exhibiting erosion were also associated with erosion in analysis of rim width transects.…”

“…As 31 of the 43 transects for which accretion was reported fall outside the adjusted margin of error, we believe that our assessment [2] remains a useful indication of accretionary dynamics around several sections of the atoll rim. We provide here a new Figure 1 illustrating the results of a revised assessment of shoreline change incorporating only estimates falling outside the wider margin of error of 20.75m.…”

“…Unfortunately it is often the case that historical maps and aerial photographs represent the only available record of past reef island shorelines and, while these are of differing scales and quality [5] they offer the best opportunity available for determining reef island change over time [6]. Thus, as is frequently the case in analyses of reef island change, our study [2] sought to provide the best possible assessment of change given the available information. We calculate the additional margin of error associated with the small scale (1:125,000) of the historic map to be 20m, not the "more than 30 metres" claimed.…”

“…Whilst our assessment incorporated numerous aspects of change, including movements of the lagoon rim shorelines, changes in the terrestrial vegetation on the lagoon rim and amendments to the bathymetry of the lagoon basin through dredging activities [2], this comment solely addresses the estimates of shoreline change. Here we make some brief remarks relating to this shoreline assessment of Diego Garcia and elaborate on some of the complexities of the geomorphic processes that underpin shoreline dynamics.…”