Wave-induced sediment transport on and off of beaches is difficult to understand and predict without thorough knowledge of the nearshore wave conditions. Wave data is commonly provided by a buoy located offshore in deep water that measures waves prior to shoaling and refraction. Irregular bathymetry causes dissimilar refraction and shoaling and can lead to variable wave conditions in the nearshore environment. To account for wave propagation over varying bathymetry, numerical wave models are good tools for estimating the nearshore wave climate from offshore wave data. Ocean Beach in San Francisco, CA is an energetic, intermediately sloping beach that was the subject of frequent sub-aerial topographic surveys in 2005 and 2006, with some surveys being as close as two days apart. Sediment volume change derived from these surveys was correlated to nearshore wave heights estimated from offshore buoy measurements and the application of the numerical wave model SWAN (Simulating WAves Nearshore). The SWAN model was used to create a "look-up" table of nearshore wave heights from over 4500 combinations of offshore wave heights, periods, and directions. The model was run using a nested grid scheme using three separate spatial resolutions, with the finest being closest to shore. Correlations between the sub-aerial beach volume data at five morphologically different reaches of Ocean Beach and the SWAN derived wave heights from just outside of the surf zone (in 5, 7.5, or 10 m of water depending on wave height) are generally low, with R2 values less than 0.5, with the highest being 0.61. Although the coefficients of determination are low in most instances the significance exceeds 90%. The reason for the low coefficients of determination is not known but is currently being investigated; some possible reasons are improper characterization of the lengthy time series of wave data between surveys (up to 28 days), or the ignored effect of strong along-shore directed tidal currents (O(~1m/s)) on breaking waves in the surf zone.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 4534 Hydrodynamic modeling;
- 4546 Nearshore processes;
- 4558 Sediment transport (1862);
- 4560 Surface waves and tides (1222)