BEACH AND SHORELINE RESPONSE TO AN ARTIFICIAL ‘LANDSLIDE' AT ROCKY POINT, SOUTHERN OREGON COAST

In January 2006, a 200 m section of US Highway 101 located 3.5 km south of Port Orford on the southern Oregon coast slumped seaward causing its western lane to drop vertically some 6 m in response to a fill/block failure. In order to remediate the Highway, Oregon Department of Transportation engineers concluded that the entire fill section overlying Rocky Creek would need to be excavated, disposed of elsewhere, and replaced with much coarser fill material. After consulting with state, federal and local parties, the decision was reached to place 53,000m³ of the fill sediment directly on the beach below, where it would be subsequently redistributed by ocean waves and currents. Placement of fill sediment on the beach raised questions about the appropriate use of the public beach and intertidal region for sediment disposal, including questions regarding the rate of absorption and dispersal throughout the littoral system and effects on the marine biology.

In March 2006, staff from DOGAMI implemented a beach monitoring program using survey grade GPS to document the patterns of change at the fill placement site, and its eventual dispersal throughout the beach system. Grain-size analyses of the fill indicated that of the original 53,000 m³ of sediment added to the beach, approximately 19,700 m³ of the finer sediments (fine sand to silts) were removed to deep water, while the remaining 33,000 m³ of coarser sediment was added to the beach. Our monitoring surveys and analyses indicated that by mid-April 2006 (3 weeks after excavation began) about 7,050 m³ of fill sediment remained and by February 2008 there was less than 2,300 m³ remaining. Sediment eroded by ocean waves were transported into the inter-tidal zone offshore from Rocky Creek as well as carried by longshore currents along the shore to the north where it resulted in progradation of the beach. Not surprisingly, erosion of the landslide toe was greatest during the winter months when wave energy levels and swash runup elevations are highest and negligible in the summer. Our monitoring efforts at Rocky Creek indicated that placement of the fill did not have an adverse effect on the beach, having contributed cobbles, gravel and sand to the sediment budget similar to a naturally occurring landslide, and has not had a lasting effect in terms of the morphodynamic response of the beach.