chapter 10

conclusions and discussion

The objectives of this study are:

·        To investigate the effectiveness of ADAM model in simulating the tidal flow in Great Bay with wetting and drying on the tidal flats.

·        To calibrate ADAM model by adjusting the bottom friction coefficient for, M₂, M₂S₂, and M₂S₂N₂ tidal forcing, respectively.

·        To explore the frictional effects of eelgrass distribution on the flow regime in Great Bay.

These goals are achieved. Simulation of Great Bay Estuary with ADAM model is good in general. ADAM model resolves the wetting/drying process on the tidal flats well. However, when the whole Great Bay Estuary system is modeled, some problems are observed in the upper estuary. Either the surface elevation amplitudes or the phase values did not compare well with the predictions from Swift and Brown (1983) data. This problem is solved by applying the model only to the Great Bay/Little Bay  section in the upper estuary, which is the area of interest for exploring the eelgrass effects. Simulation of the Great Bay section works well. The Great Bay section is characterized by a network of channels with tidal flats on the sides. A transition from ebb dominance in the channels to flood dominance in the shallow tidal flats, which is the real dynamics in that section, is obtained with ADAM model simulations. Thus, the assumptions made in ADAM model are verified.

The bottom friction coefficient distribution is adjusted for the M₂, M₂S₂ and M₂S₂N₂ tidal forcing. The results are compared with the predictions from Swift and Brown (1983) data where possible. 

After a satisfactory bottom friction coefficient distribution is found for each tidal forcing, 1990 eelgrass distribution is added to the system. The eelgrass beds treated as extra dampers and the friction coefficients are increased at those locations. Addition of eelgrass causes the following changes in the model results:

·        the velocity over the eelgrass beds are reduced,

·        the velocity in the channels are increased,

·        eelgrass blocks the water, lets less water enter the system during flood, and  lets less water exit the system during ebb.

·        eelgrass holds water and increases the water surface area, with a maximum increase at low water,

·        eelgrass decreases the average depth at low water due to the increase in water surface area.

 

The change in water surface area and average depth caused by the eelgrass distribution is given in Table 10-1.

 

Table 10-1. Water surface area and averaged depth values for various simulations.

 

 

 

The lack of detailed bathymetry information and velocity measurements in the Great Bay section makes the modeling efforts difficult. However, modeling the eelgrass effects on the tidal flow by increasing the bottom friction is a good approximation and gives physically realistic results.