Multiple entry trafficsheds

Tabular simulation results for Boat Name Lanes can be seen in table 5.2 and Bird Key in table 5.3. In both cases, the single most popular destination from these trafficsheds was the ICW: the waterway was used to access the gulf in the winter, and the bay in the summer. The preference for the ICW is most pronounced on the weekends; on weekdays in the winter the traffic is more evenly split up between the ICW/gulf, ICW/bay, and the bay proper. In the summer, apart from a slight preference for boating in the bay, it is hard to detect a preference for any single destination, partially due to the small sample size (small number of boats that actually embark) in these runs. Note that for Bird Key, on a weekend in the summer, one run has six boats departing for Tampa Bay, making it by far the most popular destination on that day, and on another day there was only one. Bird Key shows greater variability in it's daily traffic summaries as a direct result of the greater diversity of boats moored in it. Greater diversity in boat populations means greater diversity in boating behaviors. The summaries for Boat Name Lanes is more consistent; half the boats moored in it originate from a single dry-docking facility, which mainly stores smaller power boats. Bird Key has a large number of boats moored in residents' back yards, ranging from small dinghies and personal watercraft up to 40'+ sailing yachts and power cruisers. It also has a single large facility (Bird Key Yacht Club), which contains many larger cruising boats. Larger cruising boats tend to have a greater range of trip types than smaller boats- some boats may be taken out several times a week, while others may sit idle apart from a cruise every year or so by an otherwise absentee owner.

  

Figure 5.5: Simulation summary for Boat Name Lanes

  

Figure 5.6: Simulation summary for Bird Key

Figure 5.5 shows the traffic layout for Boat Name Lanes. Approximately 60% of the traffic that leaves from this trafficshed embark for the bay, fishing flats, or north along the ICW, presumably through the northern channel as this is the most direct route and is adequately deep to allow unrestricted passage for all the boats in the trafficshed. The remaining portion of the traffic (roughly 40%) embarks for destinations that are either in the southernmost part of the bay, or destinations which require using New Pass or Big Sarasota Pass.

There is a high variability in the numbers and kinds of boats that embark in each simulated run. For this reason, it must be understood that exact or highly precise reporting of the results would only be meaningful in the context of a particular run; rounding of numbers is needed to draw any generalizations or inferences from the results of these runs. This variation is even more pronounced in Bird Key (figure 5.6). Bird Key is a complex system to begin with, and in the strictest definition of a trafficshed, it could be considered to be five or more separate source areas. However, the canal networks that comprise the trafficshed share essentially three entry/exit points at the southern, northeastern, and eastern edges of the island.

  

Figure 5.7: Oblique view of Bird Key and surrounding bathymetry

Bird Key is a dredge-and-fill island system: the area was occupied by shallow seagrass beds and mangrove islands prior to its development before 1970. Most of the land was created by dredging material from neighboring areas to convert the mangrove islands to a large area of compacted soil suitable for a housing development. This history accounts for the shape of the island, with its straight shoreline and canals, deep channels immediately adjacent to the shoreline, and a shallower ridge surrounding the island where side-cast spoil is deposited. If one were to view the island from the air with the water drained from the bay, this ridge would appear as a ring around the island (figure 5.7). This ridge ranges from 2 to 8 feet below the water, and can be an impediment to boat traffic where it is shallower than the draft of the boat. In the simulations, as boats were choosing the ``least cost'' route from their origin to their destination, the ridge was crossed wherever the most direct path could be achieved while avoiding areas shallower than its draft. This resulted in path diverging where boats first leave from their docks, and choose two or more paths around shallow seagrass beds. The paths converge once the shallower areas are circumnavigated; these convergence areas define the three man exit points that are mentioned above.

  

Figure 5.8: Boat paths of simulated traffic around Bird Key. Dark pixellated lines indicate paths taken by individual boats over the course of the simulation. Lines simply indicate whether a boat has travelled along a path, and is not indicative of how many boats have used a particular path.

In the simulated runs, most of the boat traffic (about 60%) embarked for destinations towards the north via the ICW. A smaller portion embarked towards the south along the ICW, which exited Bird Key via the east. An even smaller portion (typically 10%) left for the gulf or Siesta Key via the south. These boats traced paths around the shallow seagrass bed at the southwest end of the island, and converged as they headed out Big Pass (figure 5.8).