Evaluation of Sandsavers for Wailua Beach, Kauaʻi using the Coastal Modeling System

Abstract

Hawaiʻi spends millions, each year, on beach nourishment projects due to coastal erosion. With little beach width in many areas, the encroaching sea is often directly upon aligning shoreline structures. Wailua Beach, Kauaʻi, Hawaiʻi experiences the impacts of coastal erosion and has been chosen to be the first beach in Hawaiʻi to have the Sandsavers structures installed to help build back the beach and prevent further erosion. The Sandsaver is a trapezoidal structure with conical holes, with the larger holes facing the ocean and the smaller holes facing the shore. The goal of the Sandsaver is to have sediment flow through the holes and settle on both sides to build back the beach. Two separate arcs, where the Sandsavers will be “tied” together, are planned for installation at Wailua Beach in the near future. The performance of the Sandsavers is analyzed during two periods, a non-storm and storm event. Hurricane Iniki was the largest storm event to hit the island on September 11, 1992, therefore, the modeling period for the storm event is set for September 8, 1992 at 0:00 to September 14, 1992 at 23:00 (7 days). The non-storm event is also the same month/day/time but a year prior, 1991, to understand normal conditions during that time of year. The Surface-water Modeling (SMS) is used with the Coastal Modeling System (CMS) to perform the simulation. The model is set up for 1991 and 1992 conditions using a 2013 - present Digital Elevation Model (DEM). Each study period is modeled with and without the structures to determine if there are changes in the results when the structures are used. CMS is used to simulate water elevation, waves, current velocity, morphology change, and total sediment transport. Observation points are placed on the seaward and landward sides of each arc to determine if the Sandsavers dampen wave energy. Sediment budget areas are analyzed landward and seaward of the structures to determine if the Sandsavers are accreting or eroding the beach. Based on the analysis, the Sandsavers show an increase in sediment on their landward sides and a decrease in wave height when the structures are installed. The Sandsavers are a new solution to coastal erosion.