HIGH RATES OF CROSS-SHORE SEDIMENT FLUX ON A HIGH-ENERGY ACTIVE MARGIN SHOREFACE

Along a high-energy active margin coast in the Pacific Northwest, USA, net cross-shore sediment flux estimates based on decadal-century scale volume change analysis and shoreface behaviour modeling of inter-centennial earthquake subsidence events range from 10¹ – 10³ m³ m^-1 yr^-1. These net transfers of sand between the inner shelf and the coast are much higher than those estimated by several authors from a variety of coastal settings around the world. Typical ranges of onshore sand flux over the late Holocene range from 10^-1 to 10¹ m³ m^-1 yr^-1. Possible regime-related explanations for the high rates sediment flux include include high wave climate, strong bottom currents, and fine sediments. However, high rates of onshore sediment flux are most likely due special cases of dis-equilibrium of the shoreface profile with the inherent energy regime.

Two different perturbation conditions are responsible for large signals of cross-shore displacements of sediment. First, the construction of jetties during the late 1800s and early 1900s at two estuary entrances with broad and shallow shore-connected ebb-tidal deltas created the situation of inner-shelf sand bodies out of equilibrium incident wave energy. Once constructed, the jetties blocked seaward directed tidal currents that had maintained a dynamic balance with the landward directed forcing from waves. These sand bodies represented a shoreface profile much shallower than equilibrium for the incident wave energy. This human-induced perturbation created a large scale experiment where the response of the shoreface could be observed in terms of its overall behaviour, rates of change, and relaxation properties. Second, earthquake-induced subsidence events occurring on the order of 500-yr intervals, represent a shoreface profile that is initially too deep following an instantaneous rise in sea level on the order of 2 m. Model simulations of these events suggest large initial displacements of sand from the barriers to the inner shelf, followed by large net onshore transfer of sand commensurate with rebound. Modeling results suggest that the rate of onshore sediment flux following earthquake event rebound is related to the accommodation space of the inner shelf.