FLORIDA GEOLOGICAL SURVEY
need that the resulting application will have As an example, suppose that we are spatial comparability (i.e., it can be analyzing and interpreting profile data to compared to the same application at any determine volumetric erosion of sandy other site), the result is a synerbistic beaches and coasts due to extreme event application. We shall discuss this latter class impact. Further, let us select as our reference of application first. water level datum Mean High Water, MHW.
That is, we shall assess erosion volumes
SYNERGISTIC TIDAL above MHW to an upland point that must be
DATUM PLANE APPULICA TIONS carefully deliberated depending upon whether the coast was non-flooded (interpretations are
It has been widely recognized, as normally straightforward) or flooded and/or demonstrated in the introduction to this paper, breached (interpretations can be problematic) that selection of the proper tidal datum as discussed by Balsillie (1985b, 1986). It depends upon the purpose to which it is to be must be recognized that MHW can be applied. The main purpose of this work is to assigned the status of a signature value for a determine the proper tidal datum for use in particular locality, representing its National coastal science and engineering for Tidal Datum Epoch. This assessment can be referencing littoral force and response levied because MHW can change significantly elements. Force elements include from locality-to-locality. For instance, in astronomical tides, storm tides, nearshore Florida MHW varies from +3.12 feet MSL (or currents, waves, etc. Respowme elements +3.36 feet NGVD; Balsillie and others, 1987a) include extreme event beach and coast along the northern portion of Nassau County erosion, foreshore slope changes, long-term on the Atlantic east coast, to +0.66 feet MSL shoreline changes, seasonal shoreline (or +0.90 feet NGVD; Balsillie and others, changes, etc. It became apparent during the 1987c) along the western portionof Franklin course of preparation of this paper that County on the northwestern panhandle Gulf of determination of the proper datum plane is Mexico coast of Florida. This embodies a probably best accomplished by discussing potential maximum difference of almost 2.5 application/use examples. feet in MHW elevation about the State of Florida. Suppose that for the above two
EXTREME EVENT IMPACT areas, profile conditions are comparable.
Furthermore, suppose that extreme events
From the preceding description of tidal embodying precisely the same magnitudes datum planes we must, from the scientific and characteristics producing identical force perspective, be quite careful in selecting a elements impacted the two areas, resulting in reference water level from which we define identical response elements, that is, the same such response elements as beach and coast erosion volumes (i.e., the area above the erosion due to extreme event impact, and dashed lines and below the solid lines of such force elements as the peak combined Figure 4). If, however, we reference the storm tide accompanying extreme events that, erosion volumes to MHW (shaded areas) as in part, induces such erosion. As noted illustrated in Figure 4, 8.12 cubic yards of previously, water level datum planes include sand per foot are eroded above MHW along certain insensitivities regardless of the the northern portion of Amelia Island, 33 per rigorous nature of statistical methods applied, cent less than the 12.05cubic yards of sand It is necessary that we do not further per foot eroded above MHW along western exacerbate these insensitivities, creating St. George Island. It becomes quite clear, additional variability and error through therefore, that erosion volumes around the selection of improper reference datums. state cannot be compared using MHW, since the MHW base elevation is not only
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