USING BATHYMETRIC LIDAR TO EXPLORE THE RELATIONSHIP BETWEEN WAVE POWER, TIDAL DURATION AND NEARSHORE HYPSOMETRY IN PUGET SOUND

The recent collection of bathymetric lidar along two beaches in northern Puget Sound provides an opportunity to examine the geomorphology of over 30 kilometers of mixed sand and gravel shoreline in unprecedented detail. We have compared nearshore hypsometry (elevation-frequency diagrams) derived from these data to long-term tidal duration curves in order to establish a semi-quantitative relationship between wave power and beach morphology. For both locations, the hypsometry is bimodal, with a large peak just below Mean Lower Low Water (MLLW) and a lesser, more diffuse peak just above Mean Higher High Water (MHHW). These peaks correspond to fringing low-tide terraces and high-water berm crests respectively. In both the Camano Island and Whidbey Island study areas there is a break in the slope of the hypsometric integral at about 1–2 m MLLW. This feature is particularly strong in the Camano Island shoreline and appears to be related to a lower mid-tide break in beach slope that is pronounced on some shoreline reaches. This mid-tide feature occurs at the same elevation as a break in the slope of the tidal duration curves.

The apparent relationship between the beach foreshore morphology and long-term tidal duration curves is consistent with similar findings in other meso-tidal, coarse-clastic beach environments and supports the notion that northern Puget Sound beaches are responding to the modern wave climate and do not merely reflect antecedent morphology. However, it remains to be investigated whether these beaches are shaped by seasonal storm waves or less frequent events. The former possibility has intriguing implications for low wave energy sediment transport mechanisms while the later would imply that the beach morphology is not in equilibrium with normal conditions. In either case, the storm events probably occur frequently enough that over decades to centuries, the long-term tidal duration curve becomes a reasonable proxy for the distribution of wave energy across the beach.