Relative Risk in Dune Hazard Areas = ( Sdune + D) + (LR x Tp)+(Lr x Tp) where Sdune = the total horizontal extent of shoreline erosion (wave undercutting) projected to occur during a design storm event or cluster of storm events (feet);
D = the dune topographic stability factor (feet);
LR = the average annual rate that the shoreline is projected to migrate landward due to sediment budget considerations (feet/year);
Lr = the average annual rate that the shoreline is projected to migrate landward due to relative sea level rise (feet/year); and
Tp = the planning period (years).
and where the distances determined through the application of the above formula are measured landward from the furthest landward of the following reference locations: the following reference locations: • The 'Ocean Shores Vegetation Line', which corresponds to the location of the 16 foot N.G.V.D. contour datum as surveyed in 1969; or • The 'vegetation line', which corresponds to the first line of stable natural vegetation.
Sdune. Komar et al. (1997) describe a simple geometric model that can be used to estimate the maximum extent of foredune retreat. The model translates the existing beach/dune form landward in response to elevated storm water levels according to the following relationship:
Emax = (WL - Hj) + _BL tanß where Emax, the maximum extent of foredune retreat, is equivalent to Sdune; WL is the total water level; Hj is the elevation of the beach/dune junction; _BL is vertical shift in the beach profile that can result from the presence of a rip current - in effect a safety factor; and tanß the beach slope. Values used here for the total water level are those associated with the 5 , 25, and 100 year storms; are approximately 16.5, 18.0, and 19.5 feet (Komar et al., 1997); and correspond to extreme, high, and moderate relative risk respectively. Values for Hj are: 15.2 feet for shoreline segment D1; 13.7 feet for for shoreline segment D2; and 11.2 feet for for shoreline segment D3. Tanß is equal to 0.0303 (1.75 degrees), a typical winter beach slope. _BL is set equal to zero.
D. This term accounts for the fact that storms cut a near-vertical slope in the foredune and that this slope tends to fail. Assuming that the extent of this failure is approximated by the stable angle of repose in loose sand, then D = Hdune _ 1.6 Hdune tan32o
where Hdune is the difference between the elevation of the dune toe (beach/dune junction) and the elevation of the primary dune crest.
LR. Because of the relative long term stability of many dune-backed shorelines along the Oregon coast (Komar et al., 1997), LR is set equal to zero and thus excluded from the analysis.
Lr. Estimates of the current rate of relative sea level rise along the central Oregon coast are 1-2mm/year (Komar et al., 1997). This value (~0.005 feet/per year) was added to the total water level (WL) when calculating Sdune using the geometric model.
Tp. Values used here for the planning period are 5 , 25, and 100 years for extreme, high, and moderate relative risk respectively