Executive Summary  

Section I.  Quantification of Total Freshwater Input and Flushing Time

A method was developed to quantify total freshwater input and flushing time in estuaries using measured flow and salinity data at the estuary-ocean boundary.  Fischer et al.’s (1979) formulation was used to calculate the amount of new ocean water that enters the estuary on the flood tide.  A new formula was derived to calculate the amount of mixed estuarine water that leaves the estuary on the ebb tide.  These two quantities were then applied in the water balance equation to yield the freshwater input.  The calculated amount of mixed estuarine water was also used to quantify the flushing time.  The developed method was applied to Barnegat Bay, New Jersey.  For the studied period of January 1995, it was found that (1) the total freshwater input was 2.0 million cubic meters per day, (2) the flushing time was 24 days, and (3) the amount of direct groundwater seepage to the bay was insignificant.  An assessment was made on the sensitivity of calculated values to the quality of measured data at the estuary-ocean boundary.  The results estimated from the developed method were highly reliable when there was a significant salinity difference between flood and ebb tides.

Section II.   Analysis of Wind –Induced and Other Subtidal Currents

Time series data collected during the Year I project were analyzed for wind-induced and other subtidal currents for the primary purpose of better calibration and verification of a numerical circulation model for Barnegat Bay.  The analytical method used and its findings are summarized below:

1.      Separation of Signals in Time Domain.  The measured current velocity was separated into a tide-induced current, through harmonic analysis; a wind-induced current by assuming a linear relationship between wind speed and wind-induced current; and a background current.  The magnitude of the wind-induced current was comparable to that of tide-induced current inside the bay.  The background current flowed out of the bay at Barnegat Inlet, flowed into the bay from the northern end of the bay at Mantoloking, and flowed into the bay from the southern end of the bay at Surf City.  The background current is significantly stronger than the freshwater flow-induced current.

2.      Correlation Analysis in Time Domain.  The wind-current correlation analysis yields the following conditions:  An eastward wind caused a southward current at Barnegat Inlet (leaving the bay); a northward wind caused a northeastward current at Mantoloking; a northward wind caused a southward current at Silver Bay; a northward wind caused a northward current and an eastward wind caused a westward current at Loveladies; a northward wind caused an eastward current and an eastward wind caused a northwestward current at Surf City.  A northward wind caused water to pile up toward the north both inside and outside the bay.  An eastward wind caused water surface elevations inside and outside the bay to go down.  The subtidal water elevation within the Barnegat Bay went up or down in phase with the outside subtidal water surface elevation.  In June/July 1995, the rising or lowing of ocean water surface elevation caused filling and draining of the bay. 

3.      Spectral Analysis in Frequency Domain.  More than two-third of the spectral energy in terms of water surface elevation inside the bay was contained in signals with low frequencies of less than 0.6 cycle per day.  Up to one-third of the spectral energy in terms of current velocity inside the bay was contained in signals with low frequency of less than 0.6 cycle per day.

4.      Coherence Analysis in Frequency Domain.  An advantage of the use of coherence analysis in the frequency domain is that one or more distinct frequencies at which the coherence occurred can be identified.  All the findings from coherence analysis are consistent with those derived from correlation analysis.  Additional findings are: only long period (10 to 20 days) subtidal water surface elevation outside the bay affected water surface elevation inside the bay; the subtidal water surface slope at Silver Bay affected subtidal current velocity at the same location.            

Section III.  Collection and Analysis of Additional Field Data

            An extensive field data collection was conducted on May 1, 1997 to examine velocity distribution, salinity distribution, and the representativeness of S-4 data for the entire channel cross section at Barnegat Inlet.  The assumptions regarding the normal velocity distribution, well-mixed salinity distribution, and the representativeness of S-4 data for the entire cross section were confirmed.

            A dispersion coefficient must be determined before a water quality model is used to predict fate and transport of contaminants in the bay.  As described in Section I, using the newly developed method, the amount of groundwater seepage to the bay was estimated to be small in comparison to the total amount of freshwater from surface streams.  Thus, the amount of freshwater input can be quantified with a reasonable accuracy.  As a result, the measured salinity distribution in the bay from the Year I project, along with the estimated freshwater input from this Year II project, can be used to calibrate the dispersion coefficient inside the bay.

A comparison among subtidal water surface slope, wind shear stress, and bottom shear stress indicates that the bottom return current did not exist at Silver Bay.  The opposite direction of wind and wind-induced current at Silver Bay is a result of the wind-induced horizontal circulation pattern.  Water flows in the same direction as the wind at the shallow side of the bay, but it returns in an opposite direction at the deep side.  Therefore, a vertically-integrated two-dimensional model can be selected to simulate circulation patterns in Barnegat Bay.