The shelf sources of estuarine inflow

Abstract

The inflow to an estuary originates on the shelf. It flushes the estuary and can bring in nutrients, heat, salt, and hypoxic water, having consequences for estuarine ecosystems and fjordic glacial melt. However, the source of estuarine inflow has only been explored in simple models that do not resolve interactions between inflow and outflow outside of the estuarine channel. This study addressed the estuary inflow problem using variations on a three-dimensional primitive equation model of an idealized estuarine channel next to a sloping, unstratified shelf with mixing provided by a single frequency, 12-hour tide. Inflow was identified using particle tracking, momentum budgets, and Total Exchange Flow. Inflow sources were found in shelf water downstream of the estuary, river plume water, and shelf water upstream of the estuary. Downstream is defined here with respect to the direction of coastal trapped wave propagation, which is to the right for an observer looking seaward from the estuary mouth in the northern hemisphere. Downstream of the estuary and offshore of the plume, the dynamics were quasi-geostrophic, consistent with previous simple models. The effect of this inflowing current on the geometry of the river plume front was found to be small. Novel sources of inflow were identified which originated from within the plume and upstream of the estuary.

Effect of sloping bottom on river plume dynamics on a laboratory-scale rotating table

“…Even during a period of downwelling favorable winds, the freshwater transport in the coastal current is less than 1/2 of the estuarine freshwater outflow. Brasseale and MacCready (2021) found that the gentle slope promoted the downstream transport, while the upstream transport on the steep slope disappeared, which is consistent with our results. Therefore, we can conclude that bottom attachment promotes alongshore transport.…”

“…On the one hand, the sloping bottom in laboratory experiments cannot be made at a similar slope as the actual continental shelf. It is impossible to make the slope as gentle as 0.01 (or even 0.001) in the numerical models (Yankovsky and Chapman, 1997;Brasseale and MacCready, 2021), so the bottom-attached plume we observed still has a noticeable bulge. Although we cannot simulate the phenomenon in reality, our conclusion is still applicable when the slope is very gentle.…”

“…Garvine (1999) found that the downshelf penetration decreases for increased slope due to the freshwater transport offshore through the numerical model. Garvine (1999) and Brasseale and MacCready (2021) both found that gentle slope promotes alongshore transport. In Brasseale and MacCready (2021), the shelf slope were set as 2 × 10 -3 for steep slope case and 5 × 10 -4 for gentle slope case.…”

“…Garvine (1999) and Brasseale and MacCready (2021) both found that gentle slope promotes alongshore transport. In Brasseale and MacCready (2021), the shelf slope were set as 2 × 10 -3 for steep slope case and 5 × 10 -4 for gentle slope case. Due to the limitations of laboratory experiments, we cannot make such a gentle slope to simulate the real continental shelf.…”

Wave Enhanced Overtides in an Idealized Tidal Inlet System

“…𝐴𝐴 𝐴𝐴𝑜𝑜 is a reference density; 𝐴𝐴 𝐴𝐴 𝑆𝑆𝑆𝑆 , 𝑣𝑣 𝑆𝑆𝑆𝑆 and 𝐴𝐴 𝐴𝐴 𝑆𝑆𝑆𝑆 are the x, y and z Stokes drift velocities; 𝐴𝐴 𝐴𝐴 𝐵𝐵 𝑥𝑥 and 𝐴𝐴 𝐴𝐴 𝐵𝐵 𝑦𝑦 are the non-wave body forces; 𝐴𝐴 𝐴𝐴 WEC 𝑥𝑥 and 𝐴𝐴 𝐴𝐴 WEC 𝑦𝑦 are the non-conservative wave terms; and 𝐴𝐴 𝐴𝐴 is the eddy viscosity(Brasseale & MacCready, 2021;Kumar et al, 2012).…”