THE POSITION OF THE LAST HIGH TIDE SWASH COMPARED TO MEAN HIGH WATER ON A MICROTIDAL, WAVE-DOMINATED MIXED-ENERGY COASTLINE

The Rhode Island south shore is a microtidal wave-dominated, mixed energy shoreline. The addition of wave induced swash combined with tidal phase complicates the relationship between the last high tide swash (LHTS) and the intersection of mean high water (MHW) with the shoreline. The state defines MHW as the official boundary between state and private property. Seaward of MHW, the public has constitutionally granted access laterally along the shore. The law is based on a ruling regarding an extremely low wave energy shoreline, where LHTS is landward and seaward of MHW approximately 50% of the time. The law fails to consider the effect of storm surge and wave swash on the profile, which directly affects the point where LHTS intersects the shoreline. Furthermore, public use of the shore is limited, since lateral access is seaward of MHW more than half the time.

The last high tide swash is a suitable proxy for determining the boundary between state and private property in a wave-dominated environment. A demonstration of how the LHTS line is landward of MHW due to wave swash was undertaken. Data from a near-weekly 35 year beach profile from Charlestown, Rhode Island collected using the modified Emery method, was used to determine the spatial location of the LHTS relative to the intersection of MHW and the shoreline. The vertical intersection of LHTS and MHW on the shore were taken from the each profile from 1977 to the present. The relationship between the LHTS and MHW along the Rhode Island south shore is quantified and demonstrates that LHTS is higher on the profile than(landward of) MHW 100% of the time, with an average horizontal offset of 19 meters as a result of wave swash and storm surge.