COASTAL PROCESSES, COASTAL MORPHOLOGY, AND COASTAL ENGINEERING PRACTICE ON TWO HAWAII SHORELINES: THE NORTH SHORE OF OAHU AND THE WEST SHORE OF MAUI

Hawaii’s vigorous wave climate and varied geography produces uniquely different coastal morphologies. The North Shore of Oahu faces directly into energetic waves from the northwest during the winter season, and is also affected by smaller, shorter period, oblique trade wind waves. The beaches have an abundant supply of sand, and a recent profile study (2015-2016) shows that the morphology is characterized by a mobile low beach that responds to both trade wind waves and distant swell, and a relatively stationary high storm berm. The berm is particularly well developed on the North Shore due to the winter wave climate and the lack of wind-generated dune features that can mask the berm presence. The storm berm at Sunset Beach is a remarkable feature that stretches over two miles with minor interruptions. It provides robust natural shore protection for communities of single family homes built along the shoreline. The storm berm is examined as an energy dissipating, cross-shore morphology built by overwash during extreme wave events. While stable in the presence of giant winter waves, erosional hot spots can develop in the berm due to longshore processes. At present, the preferred erosion mitigation is sand pushing, which is inexpensive, effective, and preserves the storm berm morphology.

The west side of Maui is shadowed from direct wave approach from the northwest by the island of Molokai and from trade wind waves by the island itself. The wave climate is also seasonal, but characterized by highly oblique waves from opposing directions. Coastal processes are dominated by longshore transport that changes direction with the winter and summer wave climates. The seasonal shift results in dramatic changes in sand distribution within the local beach cells. Terrestrial erosion predominantly occurs at the ends of the cells during the season in which that part of the beach is deflated, with a tendency to migrate toward the cell center with increasing severity of sand loss and wave impact. In contrast to the North Shore morphology, the shoreline in these areas is highly reflective, and characterized by steep wave-cut scarps carved into the terrestrial soils. Appropriate erosion mitigation methods in West Maui are presently subject to widespread discussion, as high value condominium and hotel properties are threatened.