High energy coastal sedimentary deposits; an evaluation of depositional processes in Southwest England

Sediments left by coastal flooding have been observed worldwide and have been variously ascribed to the action of storm surges and tsunami waves. To date, no study has attempted to unequivocally establish on stratigraphical, sedimentological, and palaeoecological grounds the means by which these two different processes might be distinguished in coastal sedimentary sequences. This paper examines the evidence for historical storm surges and tsunamis and shows that both high magnitude events have been documented over the past 250 years in southwest England. Sand layers of varying thickness are present within Holocene lagoonal and peat sequences of several shallow lakes of the Scilly Isles. Detailed analysis of Big Pool, St Agnes, indicates that the basal peats date from around 1000 BP. Within the basal peats are numerous thin sand layers. Above the basal peat is an extensive sand layer 15 to 40 cm thick. The base of this latter layer probably dates from the early to mid 18th century. Particle size, grain surface morphology, diatom, and mineral magnetic studies are used to try and determine the most likely mode of deposition. The results of all analyses are inconclusive, but the weight of evidence suggests that the increasing frequency of thin sand layers in the upper part of the basal peat may be related to the increasing frequency and intensity of Atlantic storms during the Little Ice Age superimposed upon a rising sea level. The thick sand layer may have been deposited by the tsunami wave generated by the Lisbon earthquake of November 1,1755. Due to the difficulties in distinguishing depositional processes in coastal environments known to have been affected by storm surges and tsunami waves, it is suggested that generally accepted sedimentological techniques are inadequate for discriminating depositional processes in coastal environments.     

Evaluation of various satellite sensors for waterline extraction in a coral reef environment: Majuro Atoll, Marshall Islands

The ability of five satellite sensor bands (IKONOS band 4, Terra ASTER bands 3 and 4, and Landsat ETM+ bands 4 and 5) was examined to extract the waterline at coral reef coasts (Majuro Atoll, Marshall Islands) using different wavelength regions (near infrared [NIR] and shortwave infrared [SWIR]) and different spatial resolutions (4, 15, and 30 m). After performing georeferencing and normalization of the images, density slicing was used to extract the waterline. Comparisons of extracted waterline positions with ground-level data for eight transects and global positioning system (GPS) tracks of the island shorelines showed that NIR bands were superior to SWIR bands because of the characteristics of the coral reef coasts, including a lack of foam and suspended sediments (which can affect the NIR wavelength region, if present) and the presence of remnant water on reef flats during low tide (which can affect the SWIR wavelength region). A linear relationship was found between the estimation errors of waterline positions and the spatial resolutions of the NIR sensors. Analysis on estimation errors and image costs showed that Terra ASTER band 3 was the most cost-effective sensor for extracting waterlines with reasonable accuracy. The results serve as general guidelines for using satellite-derived data to estimate intertidal topography and detect and monitor shorelines in coral reef environments.