RUNUP KINEMATICS ON A NATURAL BEACH

Runup kinematics on a gently sloping natural beach are examined with d etailed measurements from video images, resistance wires deployed at f ive elevations (between 5 and 25 cm) above and parallel to the beach f ace, and pressure sensors located in the inner surf zone. As suggested in a previous study comparing a single-level resistance wire and manu ally digitized films, runup measurements are sensitive to the sensor e levation above the bed, owing to the elongated shape of the runup tong ue. The measured mean runup elevation (setup) and vertical excursion i ncrease as the sensor elevation decreases, with the video-based runup estimates having the maximum means and variances. For the six data run s the average ratios of the video-based setup and significant runup ex cursion to estimates based on wires elevated 15 cm above the bed are 2 .7 and 1.5, respectively. These trends, combined with the high coheren ce and small phase difference between the video and the lowest wire, d emonstrate that the video-based estimates correspond to a very near-be d (less than a few centimeters elevation) wire measurement. The measur ed increase in runup excursion with decreasing sensor elevation and th e cross-shore variation in the amplitudes of pressure fluctuations at infragravity frequencies, are consistent with the theory for linear, i nviscid, normally incident standing waves. For example, valleys in the pressure spectra occur at approximately the predicted standing wave n odal frequencies. Also in accord with small-amplitude wave theory, obs erved swash excursions are nearly identical to pressure fluctuations a t the location of the measured runup mean (for pressure sensors locate d seaward of the most offshore bed-level rundown). However, at very lo w frequencies, where reflection is typically assumed complete and diss ipation negligible, the observed, near-bed swash magnitudes are overam plified relative to a best fit of the linear standing wave model based on the amplitude and phase of the seaward observations.