The Effects of Varying Fresh Water Discharge on Dispersion in an Estuarine Hydraulic Model of the Lafayette River, Norfolk, Virginia

Three experimental tests were conducted in the Lafayette River branch of the Chesapeake Bay Hydraulic Model, each successive test with an increase in the amount of fresh water discharged into the head of the river. This was done to study the response from varying river discharge on mixing parameters including the longitudinal dispersion coefficient (E). The model generated a tide of constant range and period. Batch releases of Rhodamine WT dye were made in the model and sampled throughout the river for ten tidal cycles. Samples were taken simultaneously at selected high and low water slack. The calculation of the Estuary Number, Estuarine Richardson Number, and the Hansen-Rattray Model showed that the degree of stratification increased with fresh water discharge. The results showed good agreement between experimental and theoretical results. There is evidence from the experimental data showing that trapping of dye in side embayments could be a major mechanism for estuarine mixing. Three methods were used to calculate the slack water approximation of the longitudinal dispersion coefficient. The salinity intrusion method showed that E varied directly with increased discharge. The dynamic relationship method had only a slight increase for the low water slack approximation. The change in moment method displayed irregular results. The values were not in good agreement for the different methods. Half-life values were calculated from the total dye mass and the maximum dye concentration. No difference was found in the half-life values for the low and medium discharge runs, but a substantial decrease in time for the high discharge run