Siletz Bay is a drowned river valley filled with Holocene alluvial and estuarine sediments and is separated from the ocean by a sand spit 3.8 km in length. Since the area was settled by white man in the 1890's, the bay has apparently experienced rapid siltation, due to increased farming and logging. This along with the damming of the Siletz River sloughs has altered circulation patterns in the bay. Deflection of the Siletz River flow by the prograding Drift Creek delta has caused 105 m of erosion since 1912 on the east side of Siletz Spit. The ocean side of the spit suffers periodic erosional episodes separated by periods of accretion and dune building. The most recent and publicized erosion occurred during the winter of 1972-73 when it was feared that the spit might be breached; one partially constructed house was lost and three others were saved only by timely riprapping. A sand mining operation may have aggravated the recent erosion by disrupting the sand budget, the balance of sand additions and losses from the beach. All of the foredune on the spit has been stabilized by dune grass and much of it has been riprapped. The long term effects of stabilization and riprapping are uncertain. / Graduation date: 1975

Landsliding is a significant contributor to continuous erosion of the 150-mile northern Oregon coast. Direct loss of land to the sea by landslides occurs along 47 percent of the coast. The remaining 53 percent has minor shifting of sand along depositional areas such as spits and dunes. These minor movements alter coastal topography but do not erode material directly into the sea. The type of landsliding is principally controlled by the lithology of the coastline. Landslides are classified on the basis of two features: the lithology of the coastal material in the slide and the type of movement expressed by the overall shape of the slide. Slump occurs in deeply weathered sedimentary rocks and in marine terrace sands overlying seaward-dipping rocks. Rock and debris fall are mainly confined to headlands. Block glides develop along bedding planes of relatively unweathered sandstone. Debris shift occurs in thick terrace and dune sand deposits in which no slip surface is defined. Factors contributing to the cause of coastal landslides include high precipitation, easily weathered rock, and high coastal wave energy. Frequency of reported landslides is related to periods of high precipitation and high wave energy. Although rock weathering is continuous throughout the year, the final phase in disruption of slope equilibrium often occurs during winter storm conditions. Landslides on headlands and adjacent coastlines disrupt the most extensive land area. Wave refraction directly influences this relationship by focusing wave energy on promentories and on the coastline within one mile of the headlands. Distribution of coarse and fine beach material is affected by refraction in restricted coves. Severe coastal erosion has taken place in local areas. The average rates of retreat vary according to the lithology of the coastline, and have been determined as follows: unconsolidated sand and gravel - 23 feet per year; marine terrace sands overlying sandstone and clay - 20 feet per year; marine terrace sands overlying mudstone and sandy shales - 6.5 feet per year. Knowledge of the processes of marine erosion applied to local cliff protection measures can assist in increasing the effectiveness of erosion control attempts. / Graduation date: 1964

Beach shoreline retreat induced by coseismic subsidence in the Cascadia subduction zone is an important post-earthquake hazard. Sand on a beach acts as a buffer to wave attack, protecting dunes, bluffs and terraces. The loss of sand from a beach could promote critical erosion of the shoreline. This study was initiated in order to estimate the potential amount of post subsidence shoreline retreat on a regional scale in the Central Cascadia Margin. The study area is a 331 km stretch of coastline from Copalis, Washington to Florence, Oregon. Several erosion models were evaluated, and the Bruun model was selected as the most useful to model shoreline retreat on a regional scale in the Central Cascadia Margin. There are some factors that this model does not address, such as longshore transport of sediment and offshore bottom shape, but for this preliminary study it is useful for estimating regional retreat. The range of parameter input values for the Bruun model include: the depth of closure (h) range from 15 m to 20 m water depth; the cross-shore distance (L) range from 846 m to 5975 m; and the estimated subsidence amount (S) range from O m to 1.5 m. The minimum to maximum range of post-subsidence shoreline retreat is 142 to 531 m in the Columbia River cell, 56 to 128 m in the Cannon Beach cell, 38 to 149 m in the Tillamook cell, 25 to 91 m in the Pacific City cell, 11 to 126 m in the Lincoln City cell, 30 to 147 m in the Otter Rock cell, 0 to 165 m in the Newport cell, 0 to 76 m in the Waldport cell, and 0 m in the Winchester cell. Results of the study suggest that many of the beaches in the study area are at risk of beach and personal property loss. Beach communities could limit the amount of potential damage in these areas through coastal zone planning.