Seasonal, Spring‐Neap, and Tidal Variation in Cohesive Sediment Transport Parameters in Estuarine Shallows

Abstract

Numerical models for predicting sediment concentrations and transport rely on parameters such as settling velocity and bed erodibility that describe sediment characteristics, yet these parameters are rarely probed directly. We investigated temporal and spatial variation in sediment parameters in the shallows of San Pablo Bay, CA. Flow, turbulence, and suspended sediment data were measured at sites located at 1 and 2 m below mean lower low water (MLLW) from November 2013 through April 2015, supplemented by monthlong periods in 2011, 2012, and 2016. Maximum current velocities were 0.40–0.47 m s-1 at these depths; the strongest currents decreased to 0.27–0.34 m s-1 during neap periods. Winters 2013–2014 and 2014–2015 experienced strong drought conditions, limiting the potential for seasonal impact on sediment conditions during this experiment. Despite this, the more storm‐influenced site showed clear changes during the winter: the roughness parameter decreased from 10−4 to 10−5 m, from hydrodynamically rough to smooth conditions, and bed erodibility increased by an order of magnitude. Median settling velocity was 2.05·10−4 m s-1; it varied twofold within a tidal cycle, decreasing as current velocity grew during flood and ebb. This tidal control on floc size affected settling velocity on the spring‐neap timescale, possibly driving a spring‐neap oscillation in erodibility. Our findings highlight variation in sediment dynamics that is commonly ignored in numerical models and the need for field observations to ground truth ongoing modeling efforts.

Bottom Drag Varies Seasonally With Biological Roughness

“…This hypothesis is consistent with the GM1979 analysis (Table 1), the vertical structure of the Vectrino mean velocity (Figure 3a), the increased wave momentum flux over the roughness elements during summer (Figure 3c), the reduced mean velocities measured by the ADV (Figure 2) and ADP (Figure 1), and the abundance of feeding tubes observed in sediment bed images collected in summer compared to winter and spring. These results are also consistent with recent studies in the region, where it was found that bottom roughness was enhanced in summer relative to winter (Allen et al., 2019; Lacy & MacVean, 2016). And although our benthic surveys were far from comprehensive, previous studies over the shoals of San Francisco Bay found that Ampelisca abdita populations peak in summer and fall and decline in winter and spring as salinity decreases during the rainy season (Nichols & Thompson, 1985).…”

Seasonal Variation in Sediment Delivery Across the Bay‐Marsh Interface of an Estuarine Salt Marsh

“…Figure shows grain size distributions for disaggregated particles; however, in estuarine waters these small cohesive particles typically aggregate into larger flocs, which settle more quickly than the individual particles. Measurements of particle size in suspension with a LISST during S16 in the San Pablo Bay shallows (station elevation −2 m MLLW) showed a wide and tidally varying range of floc sizes (Allen et al, ). The median floc size was 88 μm, with an estimated settling velocity w s =2×10 −4 m/s.…”

“…A range of particle sizes is imported to the marsh from the shallows (Allen et al, ); while it would take the median 88 μm flocs 15.6 min to settle 0.2 m, a typical small particle (23 μm) requires 89 min, long enough to be exported from the marsh by the ebb tide. The quiescent marsh environment is conducive to flocculation and settling, both of which alter the incoming distribution of particle sizes in suspension.…”

“…Within the marsh, we assumed that measured SSC was a reasonable estimate of depth‐averaged SSC, due to the shallow water depths. For the shallows and mudflat stations, we determined depth‐averaged SSC ( ) from vertical profiles of SSC calculated with the Rouse equation: where SSC r is SSC at a reference elevation z r , z is elevation above the bed, κ =0.41 is the von Kárman constant, settling velocity w s was set at 2×10 −4 m/s, based on our observations of median w s in San Pablo Bay shallows (Allen et al, ), is shear velocity due to currents, and depth‐averaged velocity was estimated from measured velocities assuming a logarithmic velocity profile.…”

“…where SSC r is SSC at a reference elevation z r , z is elevation above the bed, = 0.41 is the von Kárman constant, settling velocity w s was set at 2 × 10 −4 m/s, based on our observations of median w s in San Pablo Bay shallows (Allen et al, 2019), u * c = C d 0.5ū is shear velocity due to currents, and depth-averaged velocitȳ u was estimated from measured velocities assuming a logarithmic velocity profile.…”