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

Abstract

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.