The influence of seasonal patterns on a beach nourishment project in a complex reef environment

Abstract

Royal Hawaiian Beach, located in Waikiki, Hawaii received sand nourishment of 17,551m3 during the spring of 2012. Carbonate sand, dredged from a reef-top sand field 0.6km offshore, was placed along 520m of shoreline. Post-nourishment monitoring of the beach and offshore quantifies performance and provides transferable information for future nourishment projects in the study area and in regions with similar fringing reef environments and wave climates. Elevation data were collected along cross-shore profiles prior to sand placement and quarterly thereafter for a period of 2.7years. The time-sequence of profile data was used to construct digital elevation models (DEMs); a method designed for this study to achieve heightened spatial accuracy relative to two-dimensional profile comparisons that often ignore measurement inconsistency. Various analyses were performed using survey DEMs, including empirical orthogonal function (EOF) analysis and surface comparison. Monitoring data were analyzed in concert with seasonal incident wave conditions to further understand processes that drive beach movement. Overall, the beach lost volume at a rate of 760±450m3/yr over the entire monitoring period, consistent with the design rate of 1070m3/yr. Seasonal cycles caused beach volume to fluctuate between 2000m3 to 4000m3, i.e., 15% to 30% of total nourishment additions.In agreement with preceding studies, we confirm predominant westward transport that we describe as counterclockwise rotation. Cross-shore sand transport through an offshore channel is also evident, as we observe the channel acting as both a sediment source and sink depending on seasonal wave conditions: a source during seasonal and storm-related swell events, and a sink otherwise.