Graduate Thesis Or Dissertation
 

On the nearshore impact of wave energy converter arrays

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1c18dj08k

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  • Radiation stresses, defined as the excess flow of momentum due to the presence of waves (Longuet-Higgins, 1964), are the main drivers behind the cross-shore and longshore forcing that results in wave setup, set-down, and longshore currents (e.g. Svendsen, 2006). Longshore currents entrain and transport sediment and therefore play an important role in short- and long-term coastal evolution (Komar, 1998). Due to their importance in many nearshore processes, quantifying and analyzing nearshore radiation stresses is often an important component of coastal engineering and management projects. This thesis presents two manuscripts focusing on radiation stress in nearshore zones. The first manuscript describes a methodology for assessing the nearshore impacts of the presence of Wave Energy Converter (WEC) arrays based on the net change in the alongshore radiation stress gradients shoreward of the array. First, a threshold for nearshore hydrodynamic impact is established based on the observed relationship between nearshore radiation stress and the maximum alongshore current velocity in past field studies. A parametric study is then conducted on an idealized beach using the spectral model SWAN to analyze nearshore impact using a range of array configurations, locations, and incident wave conditions. WEC devices are represented in SWAN through the external modification of the wave spectra at the device location using an experimentally determined power transfer function. In the final section, the same methodology is applied to two wave energy test sites off the coast of Oregon to assess the applicability of the conclusions made in the parametric study to sites with more complicated bathymetries. Although the changes in wave height, wave direction, and cross-shore force in the lee of the array are similar to the changes seen in the parametric study, the changes in longshore radiation stress forcing are more heavily influenced by local bathymetry. The second manuscript examines how the total radiation stress is affected by directional asymmetry in the incident wave spectrum. The process of wave refraction in nearshore zones results in an asymmetric directional distribution of spectral energy when oblique, multi-directional wave fields propagate into shallow water areas with limited alongshore variability. In this study, net radiation stresses are calculated using a JONSWAP frequency spectrum with both symmetric and asymmetric directional spreading functions and then compared to those calculated from the monochromatic formulation. Because information on the full frequency and directional distribution of spectral energy is not always available in practice, the monochromatic formulation is often used as an approximation of the true radiation stress. Past studies have demonstrated that the use of the monochromatic approximation in radiation stress calculations results in a significant overestimation of the radiation stress components S[subscript xx] and S[subscript xy] in broad-banded seas. The present results show that the inclusion of directional asymmetry in radiation stress calculations reduces the difference between the full spectral S[subscript xx] and the monochromatic approximation but increases the difference between the full spectral S[subscript xy] and the monochromatic approximation for a range of dominant wave directions. The use of a monochromatic approximation of S[subscript xy] can therefore lead to an overestimation of the actual radiation stress in certain sea states and consequently an overestimation of associated parameters such as alongshore current velocities and alongshore sediment transport rates.
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