Response of the Length and Stratification of the North River Estuary to Changes in Forcing
Monday, 15 December 2014
The response of the length and stratification of the North River estuary (MA) to variations in river discharge Qf and tidal amplitude UT is analyzed using time series from four along-channel locations in conjunction with cross- and along-channel boat board surveys collected from April 4 to June 22, 2006. The North River is a generally well-mixed estuary with a channel depth of 5 m and width of 70 m. Its length L5 is defined as distance between the mouth and the location where bottom salinity reaches a value of 5 psu, determined by approximating a linear along-channel salinity gradient. During the period of low discharge (Qf < 5 m3/s) spanning April, the average length of the estuary is relatively high and varies based on tidal amplitude. Lengths are typically 7-9 km for neap, 8-11 km for intermediate, and 10-14 km for spring tides. During two storm events marked by high discharge (Qf >15, reaching 85 m3/s) mean length remains shorter, at 6-9 km. Length of the estuary has a weak dependence on discharge and a positive relationship to tidal amplitude, L5 ~ Qf -0.135 UT 0.611. Stratification exhibits an increase with increasing discharge and a decrease with increasing tidal amplitude at mid-estuarine locations as ΔS ~ Qf 0.443 UT -1.22. Within tidal cycles, peak stratification is typically observed at the start of ebb. Using a one-dimensional salt balance, the along-channel diffusivity K is defined as K = Qf L/A, where A is cross-sectional area. K has a strong dependence on river discharge, and less so on tidal amplitude. It ranges from 100-200 m2/s during low discharge periods and peaks at 1200 m2/s during storm events. Although a simplified linear model of the salinity gradient was assumed, insight was gained as to how the estuary responds to river discharge and tidal variability.