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11241 KiB

Open AccessArticle

Assessing Long-Term Changes in the Beach Width of Reef Islands Based on Temporally Fragmented Remote Sensing Data

by Thomas Mann and Hildegard Westphal

Remote Sens. 2014, 6(8), 6961-6987; https://doi.org/10.3390/rs6086961 - 25 Jul 2014

Cited by 40 | Viewed by 10129

Atoll islands are subject to a variety of processes that influence their geomorphological development. Analysis of historical shoreline changes using remotely sensed images has become an efficient approach to both quantify past changes and estimate future island response. However, the detection of long-term [...] Read more.

Atoll islands are subject to a variety of processes that influence their geomorphological development. Analysis of historical shoreline changes using remotely sensed images has become an efficient approach to both quantify past changes and estimate future island response. However, the detection of long-term changes in beach width is challenging mainly for two reasons: first, data availability is limited for many remote Pacific islands. Second, beach environments are highly dynamic and strongly influenced by seasonal or episodic shoreline oscillations. Consequently, remote-sensing studies on beach morphodynamics of atoll islands deal with dynamic features covered by a low sampling frequency. Here we present a study of beach dynamics for nine islands on Takú Atoll, Papua New Guinea, over a seven-decade period. A considerable chronological gap between aerial photographs and satellite images was addressed by applying a new method that reweighted positions of the beach limit by identifying “outlier” shoreline positions. On top of natural beach variability observed along the reweighted beach sections, we found that one third of the analyzed islands show a statistically significant decrease in reweighted beach width since 1943. The total loss of beach area for all islands corresponds to 44% of the initial beach area. Variable shoreline trajectories suggest that changes in beach width on Takú Atoll are dependent on local control (that is, human activity and longshore sediment transport). Our results show that remote imagery with a low sampling frequency may be sufficient to characterize prominent morphological changes in planform beach configuration of reef islands. Full article

(This article belongs to the Special Issue Remote Sensing in Geomorphology)

► Show Figures

<p>(<b>a</b>) Location of Takú Atoll in the tropical Western Pacific; (<b>b</b>) General map of the entire atoll; (<b>c</b>) Close-up of the eastern rim of the atoll.</p> Full article ">
<p>Pictures from seawalls on Nukutoa Island. (<b>a</b>–<b>d</b>) Traditional “rocks in net” walls (Gabion baskets). (<b>e</b>,<b>f</b>) Seawalls made of coconut logs. Pictures were taken in November and December 2008 and indicate that beach erosion prevails in front of the Gabion baskets. Photos courtesy of John R. Hunter.</p> Full article ">
<p>Climatic and hydrodynamic conditions for the area around Takú Atoll (156°E, 5°S). (<b>a</b>) Mean percent frequency of yearly wind directions for all months since 1991. Dashed lines represent April (black) and November (grey). (<b>b</b>) Mean annual wind speed. (<b>c</b>) Mean percent frequency of surface current direction since 1999 for May–November (black) and December-April (grey).</p> Full article ">
<p>(<b>a</b>,<b>b</b>) Morphological elements at the transition from the reef flat to the reef island on Takú Atoll. Photos courtesy of John R. Hunter.</p> Full article ">
<p>Derivation of the methodological approach. (<b>a</b>) Image section of Nukutoa Island in September 1943 exemplarily showing the baseline (yellow), edge of vegetation (green), toe of beach (red) and shoreline transects. The Shoreline Change Envelope is defined as the distance between the edge of vegetation and the toe of beach. (<b>b</b>) Table exemplarily showing the Shoreline Change Envelope at Transect 950 of Nukutoa Island for each year. Note the two temporal groups with considerably different Shoreline Change Envelope values at this transect. (<b>c</b>) Frequency distribution of the Shoreline Change Envelope suite for Nukutoa Island in 2003. The Shoreline Change Envelope suite is defined as the entirety of all Shoreline Change Envelope values for each time.</p> Full article ">
<p>(<b>a</b>) Location of the depicted islands on the reef flat. <b>(b</b>–<b>g)</b> Morphological changes in planform beach area for Kapeiatu Island over the study period. The dashed lines represent the toe of beach. Aerial photograph (b) was provided by the Bernice Pauahi Bishop Museum ( <a href="http://www.bishopmuseum.org" target="_blank">www.bishopmuseum.org</a>), satellite images (c–g) are © DigitalGlobe, supplied by European Space Imaging.</p> Full article ">
<p>Takú Island in 1943 (<b>a</b>,<b>b</b>) and 2012 (<b>c</b>,<b>d</b>). A comparison, especially of the close-ups (b and d) reveals a reduction of beach width along the lagoonward shore. Aerial photograph (a) was provided by the Bernice Pauahi Bishop Museum ( <a href="http://www.bishopmuseum.org" target="_blank">www.bishopmuseum.org</a>), satellite image (<b>c</b>) is © DigitalGlobe, supplied by European Space Imaging.</p> Full article ">
<p>Nukutoa Island in 1943 (<b>a</b>,<b>b</b>) and 2012 (<b>c</b>,<b>d</b>). The island has developed considerably since 1943 and is densely populated now. The lagoonward beaches virtually disappeared over the study period (cp. b and d). Aerial photograph (a) was provided by the Bernice Pauahi Bishop Museum ( <a href="http://www.bishopmuseum.org" target="_blank">www.bishopmuseum.org</a>), satellite image (c) is © DigitalGlobe, supplied by European Space Imaging.</p> Full article ">
<p>(<b>a</b>) Illustration of superposed toe of beach positions for Farafatu, Kapeiatu and Karuteke. Selected transects are marked in light grey to indicate their position on the map <b>(b)</b> Changes in total beach width as a function of Shoreline Change Envelope. Dark grey backgrounds in a and b display the range of transects that comprised extreme high SCE values. <b>(c)</b> Box plots showing the 10th, 25th, 50th, 75th and 90th percentiles of reweighted beach width. <span class="html-italic">n</span> = number of transects.</p> Full article ">

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