cell dimensions, they can be modeled in WIFM as flow barriers placed at grid 

 cell faces. The hydrodynamics of flow over these barriers is computed by the 

 broad-crested weir formula (Chow 1959) . The parameters of barrier submergence 

 (head across the weir) and Manning's n in the formula dictate the flow rate 

 or "permeability" across a barrier in WIFM. The permeable jetties at St. Marys 

 Entrance were therefore simulated with submerged barriers in the tidal current 

 model. 



79. An ad hoc method determined barrier "permeability" parameters for 

 WIFM. Two initial assumptions were made to reduce the number of variables 

 involved in parameter estimations. First, the crest of the submerged barriers 

 used in WIFM was arbitrarily set to -4 ft msl. This depth ensured that the 

 barriers would not become exposed during low tide. Second, it was assumed 

 that the bottom friction in the study area, below -10 ft msl, could be 

 approximated with a set Manning's n of 0.025. These assumptions reduce the 

 variables affecting permeability to: (a) water velocity over the barrier, 



(b) water depth surrounding the barrier, and (c) the Manning's n of the bar- 

 rier. The relationships between these variables were determined by a simple 

 computational experiment. 



80. A horizontal flume with length scales of the same magnitude as 



St. Marys Inlet was modeled by WIFM. The flume is 16,000 ft long and 2,400 ft 

 wide, and it has a submerged barrier obstructing half the channel width at the 

 center of the flume. Plate 6 illustrates the plan view of the layout, and the 

 velocity pattern for a typical computation. WIFM was run for 128 different 

 combinations of flow velocity (1, 2, 3, and 4 fps) , water depth (10, 20, and 

 30 ft), and barrier Manning's n (varied from 0.025 to 0.050). Discharges 

 per unit width were measured at the inflow and over the barrier for each run, 

 and the permeability for the given conditions was computed as the percentage 

 ratio of the latter to the former. It was determined that permeability was 

 not a function of the flow velocity. 



81. Figure 11 shows the family of curves plotted from this experiment. 

 To set a desired permeability for a jetty barrier in the tidal current model, 

 the water depth at the jetty section is noted, and the appropriate Manning's 

 n is determined by interpolating between isobath curves in Figure 11. The 

 Manning's n values needed to simulate a 28 percent jetty permeability were 

 determined for each barrier segment in this fashion. 



47 



