Erosion

RECENT EROSION STUDIES ON DIEGO GARCIA

There was a debate about how erosion might be affecting the island of Diego Garcia which was published in the Open Access journal "Remote Sensing".

In 2012 Sarah Hamylton and Holly East published a study entitled: "A Geospatial Appraisal of Ecological and Geomorphic Change on Diego Garcia Atoll, Chagos Islands (British Indian OceanTerritory)" .

They used geospatial datasets comparing a 38-year period from 1967 to 2005 to measure the extent of island and shoreline changes. Unsurprisingly they found 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 km² in this portion of the atoll rim.

They also made comparisons of 69 rim width transects which indicated that shorelines are both eroding (26 transects) and accreting (43 transects). Within a total vegetated area of 24 km², there was also a notable transition from Cocos Woodland to Broadleaf Woodland for a land area of 5.6 km².

In 2014 Charles Sheppard questioned their findings (Comment), stating that their "results contrast markedly with my own near-annual observations on the ground where erosion and increased seawater inundation, rather than land accretion, is evident in many places. Hamylton and East have never visited the atoll ....". In particular he criticised (1) their use of a 1967 map which he argued was quite inappropriate because of the scale where a pencil line thickness was equal or greater than much of their detected change; (2) their use of a 'vegetation edge' as a proxy for shoreline position because he argued that vegetation types had changed between the survey dates leading to errors. He then went on to highlight a recent discovery of a set of 1965 aerial mosaics from which he deduced that "a half dozen of Hamylton and East claimed substantial expansions of land are in fact general locations presently showing erosion", and argued that because his own observations over the years were that "brown earth" portions of the island were also eroding, then this was a "one-way process" and not simply temporal variation of mobile sediments.

Hamylton and East responded to this criticism (Response) saying that differences between Sheppard's in situ observations and their own analysis are overstated. They point out that they had looked at 8 of his photo-records. Four of these agreed with their results but 4 which showed no shoreline change corresponded to an eroding rim in their own work. Accordingly their study actually showed greater erosion than his photographs.

As to the use and accuracy of the 1967 map they calculated the error to be 20m compared to Sheppard's "more than 30 metres" (Sheppard also seems to have been using a different version of the map from them) and concluded that 31 of their 43 transects where they recorded accretion fell outside the margin of error, thus representing 'real' accretion. If one then inspects their revised map (Fig 1 opposite) it can be seen that for the eastern arm of the atoll where there has been no man-made land changes there are 16 sites where accretion is taking place compared to 6 where the rim is 'eroding'. They then continue to give a useful description of shoreline dynamics which are likely to be taking place, illustrating the complexities of these processes, and concluding with the words that: "Available evidence suggests that the sedimentary island rim of Diego Garcia atoll is a spatially heterogeneous, dynamic landform in continual adjustment with its changing surroundings, rather than simply washing away."

For the full debate and papers click on the links above. All may be downloaded free of charge.