The islands consist of reef debris (coral shingle and fragments) in lower
strata, and primarily sands and gravels in upper strata deposited on the hard
intertidal reef flat by waves and currents. Figure 2 shows geologic logs taken
from three bore holes drilled on Bikini Island in 1947 (after Emery, Tracey and
Ladd, 1954). It is expected that the shallow subsurface geology of the other
islands in the atoll, while varying somewhat in detail, generally is consistent
with the lower elevation strata of Bikini Island as shown in Figure 2.
Beach rock and occasionally reef conglomerates form most of the intertidal
and supra-tidal shorelines of the islands, but sandy beaches are common along
many depositional shorelines, including the ocean sides of Bikini and Eneu
Islands the lagoon sides of most of the other islands. A soil layer with
organics seems to be well developed only on the larger higher islands (Bikini
and Eneu}, and observations suggest soil is poorly developed or absent on the
smaller islands (also see Stone and Robison, Appendix B).
Bikini Atoll is situated in a very dynamic oceanic environment, and hence
reef materials are continuously being eroded, especially on the windward side.
However, the erosion is more than balanced by rapid biological growth, and sand
and other reef debris are constantly transported to the lagoon side of the
reefs and washed into the lagoon.
In their comprehensive study of the geology
of the Bikini Atoll Emery, Tracey and Ladd (1954) observed the islands to be
fairly stable under conditions which existed at the time, although there has
been some recently cbserved minor losses and gains of land area.
During a site visit to Bikini in May 1984 two members of this Committee
(Peterson and Maragos) made the following observations concerning general
island stability and susceptibility to wave overwash:
(1)
Bikini and Eneu, because of their relatively large size and
elevation and wide expanse of ocean reef flat, appear relatively stable and
show little evidence of recent shoreline erosion or wave overwash.
Minor
shoreline erosion is evident only on the southern end of Eneu Island.
(2) .If anything, the northwest tip of Bikini and its northern and
eastern ocean shoreline for the most part appear to be areas of net sand
deposition.
A sandspit over 1 km long off the northwestern tip of Bikini
appears quite stable and a gently sloping beach averaging between 8 and 12m
wide along the ocean shoreline also appears stable. Undoubtedly these
depositional features owe their stability to the very wide expanse of reef
flat on the ocean side of Bikini Island. The reef flat, which averages | to
1.5 km wide here, is an excellent dissipator of wave energy and protects the
island's shoreline.
(3) Conversely, the 12-km long stretch of reef flat separating Eneu
and Bikini Islands is an area of high erosive energy. The several small islets
on this reef flat are all narrow and low, and show extensive evidence of
erosion and wave overwash. ‘This reef flat is alsa an important area of ocean-
lagoon water exchange and strong wave driven and tidal currents (estimated at
1-3 knots depending upon tide) usually flow across it from the ocean (eastern)
side.
Any structure built on this stretch of reef flat (such as a causeway)
would be constantly exposed to very high energy erosive forces particularly
during tropical storms and associated high waves during high tide. A causeway
there would be exposed to lagoon wave action from the south and west and ocean
wave action from the northeast to southeast.
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