Suspended sediments measured in the surf zone

Abstract

Abstract : Suspended sediments were measured optically within the surf zone at Torrey Pines Beach, California. Sediment laden water was sampled through three intake nozzles which were mounted on a tower along with the optical sensor (nephelometer) which was in line with the sediment laden water which was pumped to the shore. The nephelometer gave a time series of the suspended sediments. The water pumped to the beach was filtered to obtain total sediment concentration. Horizontal velocities were measured simultaneously with an electromagnetic current meter mounted on the same tower. During the experiments the breaker height ranged between 1 and 2 meters and the mean period between 8 to 16 seconds. The peaks of the nephelometer spectra occurred at approximately twice the peak frequency in the velocity spectra indicating two or more maximas per wave period. Cross spectra were computed between suspended sediments and horizontal velocity. A maximum coherence ranging above .7 occurred at the first harmonic of the peak wave frequency. The suspended sand was well sorted quartz with a mean grain size of 0.15 mm. Suspended sand concentration appeared to decrease exponentially with height above the bottom with the rate of decrease and total concentration related to the mean bed shear stress. Mean sand concentration ranged between 0.05 to 0.32 grams of sand per liter of sea water. (Author)