4 m. In the numerical computation, 5 vertical levels are generally used. 

 Although somewhat idealized, the model basin does include the major barrier 

 islands (Dauphin Island, Petit Bois Island, Horn Island, Ship Island, and 

 Chandeleur Island) and part of the Mobile Bay. An open boundary extends along 

 the South (x=L) and the East (y=L). 



Initially, the entire basin is assumed to be quiescent with c=0 

 everywhere. Flow is forced by the following boundary conditions along the 

 open boundaries: 



2Tr[t - (t.(x)] . ^f^^ _ L-x 



^g 



CoSin -- i '-/'-'^ ; *M-^=. H-io) 



where Hgyq is the average depth between x and x=L along the open boundary. In 

 the following computation, Cq is assumed to be 30 cm and T is taken as 24 hr. 



The three-dimensional model with the boundary conditions given by 

 Eqs. (2.31) and (2.32) was run for 4 days until the results reached a 



2 



quasi-Steady state. Constant vertical eddy viscosity of 200 cm /sec and C^j of 

 0.04 were used in the computation. We will present detailed velocity 

 distribution at the flood tide. 



The tidal currents within the Mississippi Sound and adjacent waters at 

 the end of 4 days are shown in Figures 4.10. Due to the shallow depths, the 

 mass fluxes in the Mississippi Sound and the Chandeleur Sound are much weaker 

 than those in the deeper offshore waters. However, as shown in Figures 

 4.10(b) and 4.10(c), the horizontal velocities in the shallow Sound and the 

 deeper offshore waters are actually quite comparable in magnitude. Currents 

 on the order of 15 cm/sec exist within the major passes. Since we are 

 concerned with the transport of sediments in the area, it is interesting to 

 examine the distribution of bottom shear stress. Figure 4.10(d) indicates 

 that relatively high shear stresses exist within the major passes and around 

 the Chandeleur Sound. It is expected that during spring tides, shear stresses 

 in these areas may be sufficient to cause entrainment of the sediments, thus 

 leaving behind the coarser sediments. This is consistent with the sediment 

 distribution map (Figure 1.3) which shows coarser sediments around these 

 areas. 



66 



