Components of Water Level along the Coastal Margin and Associated Effects on the Nearshore Storm Wave Environment

This paper describes how the nearshore storm-wave climate along coastal margins of northern Oregon and southern Washington, USA was established to evaluate life-cycle wave loading for coastal infrastructure. The approach could also be applied for other locations and project types, as discussed at the end of this paper with regard to New South Wales, Australia. The stochastic procedure described in this paper utilized a conditional subset of offshore storm wave observations and nearshore water level components, and transformed the subset of offshore wave events to various nearshore locations using an established wave model. Appropriate probability density functions (PDFs) were used to generate stochastic estimates for offshore wave height, period, and direction for each storm event within a given simulation year. Appropriate water level component PDFs were developed and used to generate a total nearshore water level associated with each realized storm event. The above stochastic approach combines aspects of wave and water level environment at a given nearshore location for a given storm event and accounts for variation of: 1) Annual storm climate (numbers of storms per year), 2) Individual storm wave parameters (height, direction, period), and 3) Superposition of water level components (tide, storm surge, sea-level change, and infragravity transients). Because of the modulating effect that water level can have on nearshore wave height, there may be little correlation between the rank (return period) for a given offshore wave event and the corresponding rank for the same wave event within nearshore waters where wave height becomes depth-limited. The return period for wave action within a given nearshore location can be more a function of the water level than the offshore wave height. Inaccurate representation of the extreme water levels can lead to an underestimate of the design and loading climate for nearshore coastal areas. The occurrence of nearshore infra-gravity transients can play a significant role in increasing short-term water level, and associated depth-limited wave height.