Sea level rise and coastal erosion in the Hawaiian Islands

Abstract

Time series and the power spectral distributions of relative sea levels are analyzed at selected tide-gauge stations in the western and central North Pacific between equator and about 30°N, in association with different time scales of motions. Coastal response to these sea-level dynamics is discussed in detail, based on the aerial photographs of shoreline changes. Wave climate around the Hawaiian Islands as well as surf conditions on Oahu are examined for simulating cross-shore beach erosion processes with an energetics-based sediment transport model. Long-term trend of relative sea-level rise during the past several decades (+1 to +5 cm/decade at most of the tide-gauge stations) is primarily affected by the local tectonism such as volcanic loading, plate movement and reef evolution, and subduction at the plate boundaries. Continual volcanic loading at Kilauea, Hawaii results in consequential subsidence of the Hawaiian Islands. Secondary reason for sea-level rise is the thermal expansion of sea surface waters due to global warming by increasing greenhouse gases, which may be potentially more significant in the near future. Interannual sea-level fluctuations, associated with ENSO (El Nino Southern Oscillation) phenomena, seem to be the primary factor to cause serious beach erosion (up to 10 times the long-term trend). Mean annual cycle of sea level (H ≈ 10 cm) and alternate annual wave conditions are the main causes of the cross-shore oscillation of sediment transport, although there is still some loss of sediments to deep-water region. Short-term change of beach profiles is basically caused by incoming wave conditions as well as sea-level height, sediment characteristics, and underlying geology. Simulations by a cross-shore sediment transport model show that higher waves result in faster offshore transport and deeper depth of active profile change, and that beach recovery process is usually much slower than the erosion process, especially after a storm surge. Deep erosion during a storm surge can not be recovered for much longer duration by mild post-storm waves, but may be partly recovered by non-breaking long waves such as longer-period swells.