SPECIAL PUBLICATION NO. 43
12 ,MHW is not the proper reference
12 water level datum to apply for 10 case A- Nortywn An+1 ht erosion volumes. It also becomes
- Nssau County, M.W = +3.12 ft MSL
S\ Ge =-A12 yd /ft apparent that it is not proper to
6 use the datum for reference for .such a force element as the peak 4 .W4 w combined storm tide. Similar logic results in the conclusion S- MSL that use of the MHHW, MLW,
10 and MLLW datum planes would
SWestern St. George also be improper. It should, in
. Fra kin County, MHW = +0.6 ft MSL fact, be clear that MSL (or MTL) S= -12.05 yd3/ft is the only tidal datum that is to .. .. be used for reference.
S- ML-- LONGER-TERM BEACH
-2 RESPONSES
I I II l l I I I I I I I
120O -1o0 4 0 -40 -20 0 20 40 6 It is clear why the MSL datum
Dist tm "M MW MWWpt (ftt) is the desired convention to apply Fgure 4. Erosion volumes, Q, above MHW for identical for extreme event impacts to which proxies impacted by identlical storm events, but with force and response elements are different local MHW planes. to be referenced. MSL datum should also be applied to longergeographically variable, but significantly so. term force and beach responses. One will note further that, for other North Notwithstanding the need for a standardized American MHW datums (see Table 2), the convention already required for extreme event problem can become even further impact, there is sound reasoning that it applies exaggerated. In fact. it has been to longer-term scenarios, although, such demonstrated that MSL is the best datum application is more subtle than for the extreme from which to reference erosion volumes; event impact case. The preceding extreme "... at the seaward extremity of the post- event impact scenario has dealt with physical storm profile, some material of the seaward beach and coast conditions of a sort which sink (also including some degree of post- transcend certain physiographic limitations. storm beach recovery) may reside above That is, the energetics associated with storms MSL (determined to be about 6% of the and hurricanes so exceed physical stability seaward sink volume from 245 analyzed constraints that individual gradients comprising profile pairs), the analytical method is fairly the beach and coast (e.g., shoreface, unbiased since it is applied equally to all foreshore slope, berm(s), dune or bluff stoss profiles investigated' (Balsillie, 1986). slope; see Figure 5) do not, in themselves, Seaward datums or depth of profile closure impose limiting conditions. Under normal are not suitable references, if only because littoral force conditions, however, survey response is slow compared to the physiographic slope characteristics become response of subaqueous sand-sized more nearly a limiting condition. Perhaps the sediments in the energetic force element surf most important of these gradients is the environment (e.g., Pugh, 1987; Lyles and foreshore slope, a subject that needs some others, 1988). discussion prior to addressing two additional synergistic application/use examples,
It becomes apparent, therefore, that namely, seasonal beach changes and long37