Exchange Flows in Tributary Creeks Enhance Dispersion by Tidal Trapping

Adrian Mikhail P Garcia,Noa Randall,W Rockwell Geyer

doi:10.1007/s12237-021-00969-4

Abstract

The North River estuary (Massachusetts, USA) is a tidal marsh creek network where tidal dispersion processes dominate the salt balance. A field study using moorings, shipboard measurements, and drone surveys was conducted to characterize and quantify tidal trapping due to tributary creeks. During flood tide, saltwater propagates up the main channel and gets “trapped” in the creeks. The creeks inherit an axial salinity gradient from the time-varying salinity at their boundary with the main channel, but it is stronger than the salinity gradient of the main channel because of relatively weaker currents. The stronger salinity gradient drives a baroclinic circulation that stratifies the creeks, while the main channel remains well-mixed. Because of the creeks’ shorter geometries, tidal currents in the creeks lead those in the main channel; therefore, the creeks never fill with the saltiest water which passes the main channel junction. This velocity phase difference is enhanced by the exchange flow in the creeks, which fast-tracks the fresher surface layer in the creeks back to the main channel. Through ebb tide, the relatively fresh creek outflows introduce a negative salinity anomaly into the main channel, where it is advected downstream by the tide. Using high-resolution measurements, we empirically determine the salinity anomaly in the main channel resulting from its exchange with the creeks to calculate a dispersion rate due to trapping. Our dispersion rate is larger than theoretical estimates that neglect the exchange flow in the creeks. Trapping contributes more than half the landward salt flux in this region.

Highlights

The salinity distribution of an estuary impacts circulation patterns by introducing horizontal density gradients, which drive baroclinic exchange flows (Pritchard 1952a), and vertical density gradients, which can inhibit turbulent mixing (Bowden 1964)
To obtain a local estimate of s0∕ x at NR2, we estimated its value from the maximum salinity range per tidal cycle divided by the tidal excursion (Banas et al 2004; MacVean and Stacey 2011)
Using an average value of 3 psu m 3 s−1 for the tidal cycles when the net salt flux was landward in Cove Brook and salinity ranges of 20 psu and 15 psu, we estimate a net landward salt flux of 2.2 psu m3 s−1 in Cove Brook for the period of shipboard surveys on July 27

Summary

Introduction

The salinity distribution of an estuary impacts circulation patterns by introducing horizontal density gradients, which drive baroclinic exchange flows (Pritchard 1952a), and vertical density gradients (stratification), which can inhibit turbulent mixing (Bowden 1964). While the steady baroclinic circulation (Hansen and Rattray 1965; Chatwin 1976) is the primary driver of the landward salt transport in long estuaries where the salinity intrusion length extends much further than a tidal excursion, tidal dispersion processes (Hughes and Rattray 1980) are the dominant driver of the landward salt transport in short estuaries, where the salinity intrusion is similar in length to the tidal excursion (Chen et al 2012). A notable distinction is that in long estuaries, the salinity intrusion decreases during spring tides due to greater vertical mixing which inhibits the baroclinic circulation (Lerczak et al 2006), while in short estuaries, the salinity intrusion increases during spring tides due to stronger tidal currents (Ralston et al 2010).

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