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QUANTIFYING SHORELINE CHANGE AT THREE DIVERSE COASTAL GEOMORPHOLOGIES ON HAWAI‘I ISLAND

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Title: QUANTIFYING SHORELINE CHANGE AT THREE DIVERSE COASTAL GEOMORPHOLOGIES ON HAWAI‘I ISLAND
Authors: Hart, Rose
Advisor: Perroy, Ryan L.
Keywords: Geomorphology
Geographic information science and geodesy
Climate change
coastal vulnerability
coastal zone management
show 4 moreerosion
sea level rise
Small unmanned aerial systems
subsidence

show less
Issue Date: May 2018
Abstract: Hawai‘i Island’s coastal communities are in a weak position for adapting to the impacts of sea-level rise (SLR), coastal erosion, and subsidence. Though bounded by nearly 430 km of ecologically, culturally, and economically important coastline, Hawai‘i Island has never had a comprehensive assessment, or systematic monitoring, of long-term and short-term shoreline change rates to inform local coastal zone management policies. Consequently, occurrences of unsustainable coastal development have resulted in significant impacts to property and nearshore resources. To better understand and manage coastal vulnerabilities, we quantified shoreline change from the mid-twentieth century to the present for three diverse geomorphic coastal settings on Hawai‘i Island. These sites are a calcareous beach (Hāpuna State Beach Park), a sea cliff (Honoli‘i Beach Park), and a subsiding coastal lava field (Kapoho/Hawaiian Vacation Land). In order to quantify change, we produced shoreline position data using historic aerial photographs and three-dimensional datasets derived from monthly small unmanned aerial system (sUAS) surveys collected over a 12 month period. These data were merged with SLR and subsidence projections using GIS to estimate and visualize current and future shoreline locations at our three sites. From our monthly survey data at Hāpuna Beach, we found the shoreline to be highly dynamic, exhibiting a mean intra-annual shoreline positional variation of 7.33 ± 2.29 m. We also found that Hāpuna Beach experiences long-term erosion (1969-2018) at a rate of -0.18 ± 0.17 m yr-1. Along the Honoli‘i sea cliff, we quantified long-term erosion of -0.13 ± 0.26 m yr-1, with a maximum retreat of 9.5 m between 1964 and 2018. Our analyses for Kapoho found that present-day extreme flooding events (i.e. king tides) already cause tidal inundation 60 m inland from the current mean higher high water mark. If SLR and subsidence rates persist as expected, the entire Kapoho study site will experience flooding within 25 years. Through this study we were able to quantify, for the first time, shoreline changes exhibited across Hawai‘i Island’s diverse and dynamic coast. We also demonstrated the viability of sUAS as an effective tool for high resolution coastal monitoring. Our results provide insights to the chronic, seasonal, and episodic coastal processes that impact coastal communities and resources on Hawai‘i Island, and can help Hawai‘i County planners develop necessary adaptations to coastal management strategies.
Pages/Duration: 61 pages
URI/DOI: http://hdl.handle.net/10790/3501
Appears in Collections:Tropical Conservation Biology and Environmental Science



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