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

Abstract

Sediment transport across bay-marsh interfaces depends on wave energy, vegetation, and marsh-edge morphology and varies over a range of timescales. We investigated these dynamics in a tidal salt marsh with a gently sloped, vegetated edge adjacent to northern San Francisco Bay. Spartina foliosa (cordgrass) inhabits the lower marsh and Salicornia pacifica (pickleweed) predominates on the marsh plain. We measured suspended-sediment concentration (SSC) and hydrodynamics in bay shallows and along a 100-m cross-shore transect in the marsh, during winter and summer. Four-year averaged accretion measured with marker-horizon plots was twice as great along the marsh transect as adjacent to a tidal creek, 50 m from the bay. We estimated deposition and trapping efficiency from the time series data to assess its variation with season and wave energy. At high tide the transition zone (between cordgrass and pickleweed) was usually erosional, the pickleweed zone was depositional, and both erosion and deposition increased with wave energy, as did the landward position of maximum deposition. Erosion from the transition zone accounted for approximately one third of the sediment flux into the pickleweed. In the pickleweed zone, SSC, the difference between flood-and ebb-tide SSC, and trapping efficiency were greater in summer than winter for comparable wave conditions, which we attribute to increased sediment trapping by dense summer cordgrass. Moderate waves in summer (46%) accounted for more annual accretion in the pickleweed zone than larger waves in winter (28%), although the contribution of winter storms was diminished by the dry winter during the study.

Modeling Marsh Dynamics Using a 3-D Coupled Wave-Flow-Sediment Model

“…The large area of accretion due to biomass production shows that this process is the principal mechanism through which marsh complexes vertically accrete similar to the observations (Neubauer, 2008;Morris et al, 2016). The inorganic sediment (both estuarine and marsh) gets redeposited close to the marsh edge with a reduced deposition in the interior of marsh replicating the aspects of previous field observations (Reed et al, 1999;Temmerman et al, 2003;Roner et al, 2016;Lacy et al, 2020;Smith et al, 2021). Nonetheless, these results suggest that deposition on eroded edges help maintain elevation as the marsh contracts at the seaward edge (Hopkinson et al, 2018).…”

Cohesive Sediment Modeling in a Shallow Estuary: Model and Environmental Implications of Sediment Parameter Variation

“…To generalize: near‐shore deposition is not strongly dependent on wind direction, whereas the amount of sediment present in the water column is. Observations show that the amount of sediment present in the water column at the boundary of the marsh is directly tied to deposition within the marsh (Lacy et al., 2020); onshore winds can increase the amount of sediment available to be transported on to a marsh. Despite the significance of near‐shore SSC for accumulation on marshes, increases in suspended concentration do not imply trends in erosion or deposition at the estuary margins.…”

Divergence of Sediment Fluxes Triggered by Sea‐Level Rise Will Reshape Coastal Bays

“…Without accounting for the sediment import from the ocean, we cannot close the sediment budget for the entire system (Hopkinson et al, 2018). In many systems (e.g., Virginia, USA, and island of Sylt, Germany), it was found that intense storms can resuspend sediments along the shelf and transport them to shallow bays and salt marshes (Castagno et al, 2018;Lacy et al, 2020;Schuerch et al, 2012). The Labrador Current, Warm Core Rings, and the Gulf Stream can also affect the along shelf transport and therefore the marine fluxes of sediment in Plum Island Sound (Townsend et al, 2015;Zhang et al, 2016).…”

“…Predicting the morphological response of costal systems to SLR is crucial, because of the alarming global SLR projections with a maximum scenario reaching 2 m by 2100 (Parris et al, 2012; Ranger et al, 2013; Sweet et al, 2017). Determining whether costal bays are stable and in equilibrium under SLR requires the quantification of sediment fluxes, and in particular the exchange of material between different geomorphic units like tidal flats and salt marshes (Duvall et al, 2019; French, 2006; Lacy et al, 2020). Sediment inputs are required to accrete tidal flats and salt marshes in addition to organic matter contribution and maintain constant water depths in a period of accelerated SLR (Horton et al, 2018; Schuerch et al, 2019).…”