Comparison of recording current meters used for measuring velocities in shallow waters of San Francisco Bay, California

Abstract

Several recording current meters were field tested in South San Francisco Bay, California to determine their effectiveness for use in shallow (tidally effected) waters under the influence of wind-generated waves. Speed sensors employed by the meters included a horizontal-axis ducted impeller (Endeco-174), vertical-axis rotor (Aanderaa RCM-4), inclinometer (General Oceanics 6011 MKII), and electromagnetic sensor (InterOcean Systems S4). Meters were deployed from June to September 1984 at 1.2 m above bottom using bottom platforms and a taut-wire mooring. Water depth ranged between 2.0 and 5.1 m during the study. Comparison of velocity records showed that near slack water when wind speed was about 5 m/s or greater and water depth was about 3.5 m or less, Aanderaa and General Oceanics meters recorded higher current speeds than did Endeco and InterOcean meters. Endeco and InterOcean meters recorded speeds that approached zero near slack water regardless of wind and water depth. However, as current speeds increased after slack water, speeds recorded by the InterOcean meter were 20 to 50 percent higher than those recorded by the Endeco meter. During low wind periods or when water depth exceeded about 3.5 m, speed readings from Endeco and Aanderaa meters showed close agreement even at slack water. At higher current speeds, InterOcean and General Oceanics meters recorded consistently higher speeds than did the Endeco or Aanderaa meters. Directional data differences were insignificant. Whereas the Endeco and InterOcean meters (as tested) appear suitable for measuring low velocities in the wind wave zone, further testing is required to determine which meter type records mere accurately at higher current speeds.