Section 7.0 Shore Erosion and Debris Production 1960050-03
7.0 SHORE EROSION AND DEBRIS PRODUCTION
Relationship Between Landslide and Shore Erosion
Since the reactivation of the Portuguese Bend Landslide in 1956, a sediment plume has been
visible in the nearshore waters of Portuguese Bend. This plume has been created by the
continual erosion of the landslide shoreline by wave action. Suspended sediments from the active
landslide toe can be seen up to 1.5 nautical miles offshore, and over an area of 771 hectares
(Stevens, 1990). Section 3 of this report discusses the thickness of these sediments as identified
by Dill and Norall (1995).
Charles Abbott Associates, Inc. (1997) conducted a study to determine the total volume of
landslide materials lost due to wave erosion. The study determined that 5.834,000 cubic yards of
landslide material has been eroded from the landslide toe since the reactivation of the Portuguese
Bend Landslide. Cumulative material loss over the past 41 years is shown in Figure 24.
The average yearly loss of material was calculated for several intervals of time since the start of
failure (see Figure 25). The first interval spans 21 years, from 1955 to 1976, with a total loss of
.approximately 1.8 million cubic yards. This yields an annual average loss of 85,633 cubic yards.
• The next period from 1976 to 1984, an interval of 8 years, had a total loss of approximately 1.5
million cubic yards. The average yearly loss for this period is 187,259 cubic yards. The time
interval from 1984 to 1987 had a total loss of 875,440 cubic yards, with a computed yearly
average loss of 291,813 cubic yards. Finally, the period from 1987 to 1995 had a total loss of
approximately 1.6 million cubic yards. The average yearly loss during this eight year period is
207,795 cubic yards.
In summary, beginning in 1976 the rate of loss increased, reaching a peak in the period between
1984 and 1987. This period of the highest erosion rate corresponds to the highest rate of
landslide movement discussed in Section 6 of this report. The end of 1987 marked the
completion of grading for interim stabilization of the active landslide and a corresponding
reduction of the rate of landslide movement. The period after 1987 showed a modest decrease in
the loss of landslide mass. Review of these data show a variable rate of loss over the
measurement period, that has generally had an increasing trend. The overall increase probably
results from: the rate of movement of the landslide mass, the long term change in the volume of
material at the beach, and disaggregation of landslide mass due to landslide movement. In
addition, gabions placed at the shoreline and variation in winter storm waves also had a
temporary effect on the rate of erosion.
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Volumetric Calculations 111
The methodology of calculating these numbers was based on cross-sections drawn through the
active Portuguese Bend Landslide. These cross-sections were drawn through topographic maps
of the area from 1955, 1976. 1984, 1987, 1988, and 1995 (CAA, 1997). By comparing
corresponding cross-sections from the different maps, the difference in landslide mass was
determined (Figure 26). To find the amount of material lost over time, CAA subtracted the fill
volume from the removal volume. The remainder from this calculation is the landslide debris
that was eroded into the ocean. These calculations showed that 3,584.000 cubic yards of
landslide debris has been lost since 1955.
However, loss of density (bulking) within the landslide debris accounts for additional yardage
eroding into the ocean (CAA, 1997). The bulking is proportional to the rate of movement of the
landslide debris (CAA, 1997). For the calculations, CAA assumed a constant density decrease of
10%. This was based upon the 10% to 15% bulking of loosened material that is commonly
observed in earthwork construction (CAA, 1997). The bulking results from internal fracturing
and distension of the landslide mass as it moves downslope, increasing the volume while the
mass remains constant. This increase in volume for a constant mass decreases the density, since
the density is inversely proportional to the volume when the mass is constant. This is analogous
to the bulking observed between a 10 pound block of ice and 10 pounds of ice cubes. To
calculate the density loss, CAA estimated the original mass of the landslide before reactivation to
be 22,480,000 cubic yards. The additional material loss into the ocean, resulting from10%
the
density loss, would then be 2,250,000 cubic yards. Therefore, the total volume of material lost
into the ocean is 5,834,000 cubic yards (CAA, 1997).
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