Basis of Design Book 2 - City of St. Petersburg

6.0 EVALUATION OF FLUSHING IN MARINA
6.1 FLUSHING
To evaluate the flushing within the proposed marina basin, a mass of dye [i.e., concentration of
100 milligrams per liter (mg/L) in all cells within the marina basin] was placed in the area of the
proposed boat slips, and the model run for a period of 4 days and the mass of dye in the marina
basin remaining over time was recorded.
The proposed design of the marina calls for a wave fence on the east side of the basin, as
detailed in figures 2-3a and 2-3b. This design will leave an approximately 2.4m tall opening
between the seafloor and the bottom of the wave fence, all along the fence perimeter, through
which water can flow. In the model, the marina wave fence is represented as a thin barrier that
completely blocks flow between adjacent grid cells. Thin barriers are commonly used in EFDC
models to establish certain details that would otherwise require an excessive and often time
unmanageable level of detail in the model grid. This modeled representation is analogous to a
wave fence that extends from above the water surface, down to the seafloor. This approach has
been taken due to the inability of the model to partially block flow from one cell to another,
relative to the z-axis.
The location of the wave fence in relation to the model grid cells is shown in Figure 4-2. The
assumption that the wave fence completely obstructs the flow is a conservative one, in that the
analysis will substantially underestimate the flushing rate of the marina area.
Figures 6-1a through 6-1f provide graphics showing the initial dye concentration in the area of
the marina and the subsequent total concentrations over time after the period of release based
upon the differing times of dye release outlined in Section 4.1. Table 6-1 summarizes flushing
performance.
GNV/2012/122381A/8/8/2012
6-1