Field measurements of incident waves and the resulting run-up made at two different natural sand beaches along San Francisco Bay show low-frequency energy in the run-up not observed in the corresponding incident-wave spectra. The experiments were conducted in December 1977 at Alameda Beach and in May 1982 at Coyote Point Beach. Both experiments were designed to measure the run-up on dissipative beaches exposed only to high-frequency, narrow-band wind waves at the exclusion of lower frequency swell. Criteria for field-site selection included a single breaker zone located near the shoreline with incident wind waves that broke by plunging, thereby minimizing reflection and forming a low-energy dissipative beach system. Elimination of the long-wave components in the incident wave field would suppress reflection at the shoreline, minimizing external nonlinear modulation of the run-up by edge waves and standing waves. Amplitude time series measurements of offshore incident waves and the run-up on the beach face were made simultaneously at multiple locations on the beach using 16-mm photography. Coherence analyses indicate that the offshore incident waves and run-up on the beach are not linearly correlated. Correlation analyses conducted on the run-up at alongshore distances up to 3.5 m indicate that the run-up is coherent in the alongshore direction up to this distance.
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