Estuarine Exchange Flow Variability in a Seasonal, Segmented Estuary

Abstract

Small estuaries in Mediterranean climates display pronounced salinity variability at seasonal and event time scales. Here, we use a hydrodynamic model of the Coos Estuary, Oregon, to examine the seasonal variability of the salinity dynamics and estuarine exchange flow. The exchange flow is primarily driven by tidal processes, varying with the spring–neap cycle rather than discharge or the salinity gradient. The salinity distribution is rarely in equilibrium with discharge conditions because during the wet season the response time scale is longer than discharge events, while during low flow it is longer than the entire dry season. Consequently, the salt field is rarely fully adjusted to the forcing and common power-law relations between the salinity intrusion and discharge do not apply. Further complicating the salinity dynamics is the estuarine geometry that consists of multiple branching channel segments with distinct freshwater sources. These channel segments act as subestuaries that import both higher- and lower-salinity water and export intermediate salinities. Throughout the estuary, tidal dispersion scales with tidal velocity squared, and likely includes jet–sink flow at the mouth, lateral shear dispersion, and tidal trapping in branching channel segments inside the estuary. While the estuarine inflow is strongly correlated with tidal amplitude, the outflow, stratification, and total mixing in the estuary are dependent on the seasonal variation in river discharge, which is similar to estuaries that are dominated by subtidal exchange flow.

Mechanisms of Exchange Flow in an Estuary With a Narrow, Deep Channel and Wide, Shallow Shoals

“…With ω = 1.4 × 10 −4 s −1 and the average tidal velocity amplitude u T = 0.61 m/s for spring tides and 0.44 m/s during neaps (averaged over the channel cross section), we obtain values of α for the range of discharge cases of 0.04 to 0.07 for spring tides and 0.1 to 0.4 during neap tides. The spring tide values for the lower‐discharge cases are very close to the optimal value of 0.05 observed in previous studies, including in another channel‐shoal estuary that is shorter and shallower than the Delaware and has an average ν of 0.6 (Conroy et al, 2020). We suggest that the lateral exchange of salt that occurs at tidal time scales between the shoals and the channel provides near‐optimal conditions for oscillatory shear dispersion, explaining the values of α close to 0.05.…”

Estuarine Circulation, Mixing, and Residence Times in the Salish Sea

“…The TEF variables give the total inward and outward salt flux through a section as Q in S in + Q out S out regardless of whether the salt flux was due to steady exchange flow or tidal pumping. TEF has been found to give a clear representation of the exchange flow in many systems (Burchard et al., 2018; Chen et al., 2012; Conroy et al., 2020; Geyer & MacCready, 2014; Lorenz et al., 2020; Rayson et al., 2017; D. A. Sutherland et al., 2011). Using the TEF method is especially appropriate in the Salish Sea because the system is known to have large basins with exchange organized by depth (deep saltier inflow, shallow fresher outflow) punctuated by energetic sills like Tacoma Narrows where the exchange is organized by time (saltier flood, fresher ebb).…”

“…The TEF variables give the total inward and outward salt flux through a section as Q in S in + Q out S out regardless of whether the salt flux was due to steady exchange flow or tidal pumping. TEF has been found to give a clear representation of the exchange flow in many systems (Burchard et al, 2018;Chen et al, 2012;Conroy et al, 2020;Geyer & MacCready, 2014;Lorenz et al, 2020;Rayson et al, 2017;D. A. Sutherland et al, 2011).…”

Exchange Flows in Tributary Creeks Enhance Dispersion by Tidal Trapping

“…The enhancement of the tidal trapping dispersion by the exchange flow in the creeks represents a mechanism which may play an important role in other multi-channel estuary systems, such as salt marsh tidal creek networks like the Coos Estuary (Conroy et al 2019) and the Plum Island Sound Estuary (Vallino and Hopkinson 1998), as well as in urbanized estuaries with various human-made ports and side channels like Newark Bay (Corlett and Geyer 2020). However, the impact of the exchange flow in the side channels on dispersion in the main channel will depend strongly on the geometry of the side channel-if it is too small, then the volumetric exchange with the main channel will be minimal and thus the total contribution to the dispersive salt flux will be negligible, but if it is too large, then the tidal velocities can become similar to the main channel and thus the axial salinity gradient-which drives the exchange flow in the side channel-will not be significantly enhanced.…”