Quantcast Long-Term Sea Level Rise (cont.)

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Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 22
18,000 years) beach sands that have been pulled offshore primarily during storms and lost
to the littoral system, remaining in place as sea level continues to rise.
This is an important concept in that the available data suggests that, prior to the late
1970s, the last period of significant coastal retreat occurred in the late 1800s (1860s -
1890s), when the combined sediments from the upland watershed and erosion of the
coastal bluffs provided a stable, albeit dynamic, equilibrium profile, with the entire
nearshore shore platform, extending beyond the littoral zone, fully charged with sediment
due in part to significant offshore losses from the littoral zone from the many storms during
the later part of the last century (Kuhn and Shepard, 1984; May, 1987). With a healthy
beach berm, the more infrequent storms that occurred during the first part of the 20th
century would temporarily displace sand offshore to form the winter profile with the
protective offshore bars, with a net loss of the protective sand beach essentially following
that of the Marine Board sea level model.
The progressive and rather significant loss of upland sand sources within the Oceanside
Littoral Cell over the last 60 years, coupled with the offshore displacement of littoral
sediments deflected out of the littoral system by the Oceanside Harbor breakwater1, have
resulted in a significant and almost total loss of the protective transient sand beach and,
hence, future erosion rates should now more systematically follow the sea level erosion
model, only amplified somewhat due to the loss of the historical (pre-anthropic) protective
sand beach.
Referring back to the Marine Board study, the very real potential exists for the La Jolla data
to suggest the beginnings of a more rapid sea level rise. The Marine Board was also tasked
with developing a variety of sea level rise scenarios addressing global warming, with the
most conservative scenario predicting 1.5 meters (4.92 feet) of sea level rise by the year
2100 (Marine Board, 1987). This rather dire prediction would translate into an average
beach erosion rate of almost 3 feet per year, or approximately 295 feet of shoreline retreat
in the next century.
The USCOE (1987, 1991) Oceanside Littoral Cell Preliminary Sediment Budget Report
concluded that southerly littoral transport upon reaching the Oceanside Harbor breakwater is deflected
offshore via a strong rip current depositing the majority of the littoral sediment outside of the littoral zone.


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