Tidal Prism Model Research Articles

Simple Tidal Prism Models Revisited

Simple tidal prism models for well-mixed estuaries have been in use for some time and are discussed in most text books on estuaries. The appeal of this model is its simplicity. However, there are several flaws in the logic behind the model. These flaws are pointed out and a more theoretically correct simple tidal prism model is derived. In doing so, it is made clear which effects can, in theory, be neglected and which can not.

Tributyltin in Sydney Harbour and Georges River Waters

Water from 23 sites in Sydney Harbour and the Georges River estuary (N.S.W.) have been analysed for tributyltin (TBT) using gas chromatography with electron capture detection, after solvent extraction as the hydride. Analytical data for TBT in Sydney waters are consistent with other data reported for overseas estuaries, with the majority of samples containing less than 45 ng Sn L-1 as TBT. Samples from areas of high boating activity such as Rushcutters Bay and Garden Island gave the highest TBT concentrations at 90-190 ng Sn L-1. The mean ambient TBT concentration in each estuary was computed using a tidal prism model. Despite the broad assumptions made, calculated results agreed well with those measured at sites with no obvious local point source of TBT.

A Modified Tidal Prism Model for Water Quality in Small Coastal Embayments

A water quality model was developed for easy application to small coastal embayments. The simulation of physical transport processes is based on Ketchum's tidal prism theory, modified and expanded such that it becomes applicable to cases where the embayment is branched and/or freshwater discharge is negligibly small. A model coastal embayment was divided into segments of lengths equal to local tidal excursions. Instead of starting the segmentation from landward end with freshwater discharge and tidal prism as two non-zero parameters, the modified model subdivides the water body starting from the seaward end with the difference between tidal prism and freshwater discharge as a single parameter. The mass balance within each segment was formulated by considering the exchange of water with its neighboring segments due to the flushing of freshwater discharge, as well as the tidal prism on ebb cycle, and to the mixing of the tidal prism on flood tide. This results in an algebraic equation which may be solved for concentration in each segment by successive substitution. For a non-conservative substance, the biochemical reaction terms may be easily added to the algebraic equation without complicating the solution scheme. The model has been applied to a number of tidal creeks and coastal embayments in Virginia, USA. The application to the Lynnhaven Bay is presented.

Residence Time: Application to Small Boat Basins

Definitions are presented for transit time, age, flushing time and residence time. Using a continuous exchange model, analytical expressions for the transit time distribution function and the age distribution function are derived for a basin having a constant volume. The residence time for instantaneous as well as continuous dye injections equals the volume of the basin divided by the rate at which new water is added to the basin. Using the tidal prism model it is shown that the normalized residence time for an instantaneous dye injection is a function of three dimensionless parameters. These parameters are: (1) The average basin volume divided by the volume of new water returning on the flood; (2) the fraction of the flood volume (=tidalprism) that is new water; and (3) the phase of the tide at which the dye is injected. For a continuous dye injection, the normalized residence time is a function of the first two parameters only. Numerical computations show that, to a first approximation, the residence ti...

The Application of a Segmented Tidal Mixing Model to the Great Bay Estuary, N. H.

Measurements show that in general salt is vertically well-mixed everywhere in the Great Bay Estuary, New Hampshire except near the river entrances at the head of the estuary. Dyer and Taylor’s (1973) modified version of Ketchum’s segmented tidal prism model has been applied to the Great Bay Estuarine System in order to predict high and low water salinity distribution for a specified river flow. The theory has been modified here to account for the mixing which occurs at the junction of two branches of an estuary. The mixing parameter, which in this model is related to the tidal excursion of water in the estuary, has been determined for different segments in the estuary on the basis of a comparison between predictions and a comprehensive data set obtained for a low river flow period. Using a mixing parameter distribution based on the low river flow calibration procedure the salinity distribution has been predicted for high river flow. The resulting salinity distribution compares favorably with observations for most of the estuary. The corresponding flushing times for water parcels entering at the head of the estuary during periods of low and high river flow is 54.5 and 45.9 tidal cycles respectively.

A modification of existing simple segmented tidal prism models of mixing in estuaries

The paper describes a model which combines the simple, but effective ideas underlying previous models developed by Ketchum, and Dyer & Taylor. Ketchum's data on the Raritan Estuary is used to demonstrate the application of the model. An alternative interpretation of inter-segment transfers in terms of an average dispersion coefficient is shown to yield values consistent with those quoted in the literature.

A simple, segmented prism model of tidal mixing in well-mixed estuaries

Segmented tidal prism models have considerable merit, especially for pedagogical purposes, in that they can incorporate mixing processes in an easily visualized, if over simplified, way. The present paper attempts to expose some flaws in Ketchum's original model of this type and proposes a more general approach. This appears to give quite satisfactory results when applied to test cases, in particular to the prediction of salinity distribution for the Thames estuary.