where Sbluff = the total horizontal extent of erosion projected to occur during a simple shallow sloughing event (feet);
LR = the average annual rate that the bluffline is projected to migrate landward due to mass wasting (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 'Ocean Shores Boundary Line', which corresponds to the location of the 16 foot N.G.V.D. contour datum as surveyed in 1969; or • The 'toe of the bluff', which is typically represented as a distinct change in foreshore slope.
Sbluff. Values for this term are based on direct measurements of the extent of bluff failure. Relationships between bluff height and the landward extent of failure were established by Gless et al. (1998). Specifically, for bluff-backed segments of shoreline south of Grant Creek (B1 -B11) Sbluff is calculated as 1.5 Hbluff, where Hbluff is the difference between the elevation of the bluff toe (beach/bluff junction) and the elevation of the bluff crest: For bluff-backed segments of shoreline north of Grant Creek (B12 -B19) Sbluff is calculated as 2.5 Hbluff. Such relationships take into account interactions between factors such as material composition, structure, and drainage that in combination control event character.
LR. Average annual recession rates (AARRs) are determined from changes in the position of the bluffline in consecutive sets of aerial photographs. Given that at different times different segments of bluff are in different phases of the 'bluff recession cycle', the AARRs that are of the greatest interest are the extreme values. It is these maxima, taken to represent individual bluff failure events, that most accurately represent the potential extent of bluff recession. Selecting the AARR maxima measured at each location and averaging these values along a segment of shoreline gives a projected AARR for that segment of shoreline*. Specifically, for bluff-backed segments of shoreline south of Grant Creek the following relationship is used to calculate LR: LR = 1.85 - (0.125 + Hj) where Hj is the average value for the elevation of the beach/bluff junction (bluff toe) along a given segment of shoreline. This relationship was derived from a linear regression of the observed relationship between the elevation of the beach/bluff junction and the maximum recession rates along a given segment of shoreline. R2 for this regression equals 0.87. Calculated values of LR for individual segments of bluff-backed shoreline are as follows**: B2=0.80 feet/year; B3=0.65 feet/year; B4=0.60 feet/year; B5=0.80 feet/year; B6=0.35 feet/year; B7=0.60 feet/year; B8=0.50 feet/year; B9=0.70 feet/year; B10=0.45 feet/year; and B11= 0.30 feet/year. For bluff-backed segments of shoreline north of Grant Creek LR was calculated as the observed average of the AARR maxima along a given segment of shoreline. These values are as follows: B12=0.50 feet/year; B13=0.50 feet/year; B14=1.0 feet/year; B15=0.50 feet/year; B16=0.50 feet/year; B17=0.80 feet/year; B18=0.50 feet/year; and B19=0.75 feet/year.
*Observed AARR maxima were typically used to calculate LR in the immediate vicinity of creeks. **Segment B1, which lies directly north of Seal Rock is an exception. Here LR = 0.30 feet/year.
Lr. Because an assessment of this factor suggests that at current rates relative sea level rise is a minor concern along bluff-backed shorelines of the central Oregon coast LR is set equal to zero and thus excluded from the analysis.
Tp. Values used here for the planning period are 0 , 25, and 100 years for extreme, high, and moderate relative risk respectively.