Abstract

Balboa Island was created from 1929 to 1935 by filling a sand shoal with dredge spoil from associated channels and berth areas in Lower Newport Bay. This east-trending island is approximately 1,160 m long and 460 m wide, and is protected by concrete bulkheads which surround it. Short-term, periodic erosion of an artificial beach along the south shore requires repeated replenishment to maintain an attractive and usable recreational beach. Although wind-induced waves formed within Lower Newport Bay and diffracted open ocean waves may contribute to erosion, their effects appear inadequate to develop sufficiently large and frequent forces to explain the dynamic changes in beach topography. The most frequently observed waves (69 percent) are southerly swell with wave heights of approximately 0.5 m and periods of about 16 seconds. The reflection of such incoming waves off vertical sea walls at Carnation Cove and the Coast Guard Station at the harbor are redirected toward the south shore of Balboa Island and produce sufficient current velocity (>14 cm/sec) to entrain and transport associated beach sand. Reflected waves and tidal currents both occur frequently and, in conjunction, can explain the short-term changes in shoreline configuration.

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