FY 1983

Suspended particulate matter in Commencement Bay

Baker, E.T., and S.L. Walker

NOAA Tech. Memo. OMPA-26, 47 pp (1982)

The distribution and transport of suspended particulate matter (SPM) in Commencement Bay, an embayment of Puget Sound, was surveyed during September 9–11, 1980; November 12–14, 1980; and March 23–27, 1981. During each survey the SPM distribution consisted of: (1) a thin (<5 m) surface layer of variable SPM concentration; (2) a thick mid-depth low turbidity zone containing one or more high-turbidity layers extending seaward from the head of the Bay; and (3) a benthic turbidity layer of high variable concentration and thickness. The total mass of SPM in the Bay averaged 19.7 × 10⁸g and varied by <10 percent between surveys. Scatter plots of salinity versus the SPM concentration in the surface water had the highest correlation during high runoff periods (r = –0.86 in March and November, –0.59 in September). Correlations of these parameters in the bottom waters were uniformly poor. Horizontal transport of SPM was measured in the near-bottom waters at two stations with current meter/transmissometer combinations deployed from September 10 to November 13, 1980. Bottom-water flow was consistently clockwise with mean speeds ~5.4 cm/sec and mean attenuation values ~2.4/m. SPM transport averaged ~12.7 µg/cm²/sec at each station. Vertical particle flux, measured at the same stations from March 25 to May 4, 1981, averaged ~5.5 g/m²/day at 25 m and >80 g/m²/day in the bottom waters. Transport of SPM in the surface layer was controlled by the Puyallup River plume and a weak counterclockwise circulation. Mid-depth turbidity layers extending seaward from the head of the Bay were common and appeared to be generated by bottom erosion near the river mouth. Transport in the bottom water was governed by daily erosion/deposition cycles which added suspended particles to a strong and steady clockwise circulation.