Processes of sea-cliff erosion on the Oregon coast : from neotectonics to wave run-up

Graduation date: 1993Sea-cliff erosion is a significant problem along the Oregon coast in that many\ud communities have been built on terraces affected by bluff retreat. There is considerable\ud coast-wide variability in the rates of cliff erosion. This variability is attributed in part to\ud tectonic activity that is causing differential interseismic uplift along the coast. Analyses\ud of geodetic survey data and tide gauge measurements have established rates of local\ud sea-level rise along the entire coast, including areas lacking direct tide measurements.\ud A littoral cell around the Lincoln City area on the central Oregon Coast is experiencing\ud the smallest degree of tectonic uplift and this results in the highest rate of local sea-level\ud rise and significant sea-cliff erosion. High cliffs cut into a Pleistocene marine\ud terrace, consisting of semi-consolidated sands, back the beaches over the length of the\ud littoral cell and supply coarse-grained sands to the beaches in the south of the cell.\ud Dissections of multimodal grain-size distributions of the beach and cliff sands have\ud shown that coarse-fraction modes are resistant to longshore wave dispersion, and this\ud produces a marked longshore variation in the coarseness of beach sand, in the beach\ud morphology, and in the nearshore processes affecting the cliff-toe erosion.\ud Two years of monthly beach-profile surveys at eleven beaches along the Lincoln\ud City littoral cell have shown that there is a significant difference in volumetric changes between beaches of different sand sizes. The coarse-grained reflective beaches are\ud much more dynamic in profile changes, and the total quantity of sand moved under a\ud given storm is much greater than on the fine-grained dissipative beaches. Rip-current\ud embayments are also more important to cliff erosion o the reflective beach, producing\ud bluff retreat that has a high degree of spatial variability and is extremely episodic. Risk\ud assessments based on the probability curve of the extreme run-up have demonstrated\ud that the height of the cliff-beach junction and the beach slope are important factors in\ud controlling the risk of cliff-toe erosion. Run-up measurements using video techniques\ud on three beaches having contrasting morphologies suggest that the maximum run-up\ud calculation based on the empirical relationship derived by Holman and Sallenger (1985)\ud appears to be valid, although the permeability effects might have contributed to a\ud significance deviation in the prediction of maximum run-up on a sediment-starved\ud beach