Sarah Petty
ERTH 425- Geomorphology
Fall 2015
Landscape Report
Coastal Equilibrium
According to the Merriam-Webster dictionary, equilibrium is the state of adjustment
between opposing or divergent influences or elements. Waves may be the most
constant factor contributing to both the equilibrium and disequilibrium of sediment
transport in regard to beach landscapes, especially at Ocean Beach in San Francisco.
I focused on a study done over 4 years implementing the Equilibrium Shoreline
Model. The hypothesis is that a beach exposed to steady wave conditions evolves
toward a unique, wave condition dependent equilibrium beach profile, with no
further change after equilibrium is reached. The rate of change is dependent upon
the disequilibrium of wave conditions to the present beach profile. I found in the
study that Coastal beaches seem to be constantly striving for equilibrium, and only
achieving it over a fairly large time scale.
There are several factors that must be taken into account from many aspects of a
coastal landscape. Ocean Beach has medium sand size grains and high-energy
waves. Both the underwater and subaerial landscapes play a large role in the beach
configuration. Tidal currents are strong, bay flushes occur through the adjacent
Golden Gate tidal inlet. There is a large sand bar just off the coast and incoming
waves are measured using buoys. Ocean Beach has strong seasonal cycles with more
energetic waves in the winter, and calmer (yet still high energy relatively speaking)
waves in the summer. Strong winter waves pull sediment from the beach and bring
it offshore and summer waves allow for sediment accumulation back onshore. On
shore, the backbeach boundary caries between sea walls, bluffs and vegetated
dunes. The overall study area covers 7 km of shoreline. Seasonally averaged beach
slopes are determined by measuring the distance from the backbeach boundary to
the mean sea level water line. The North end of the beach has a 130 m width and the
south end is about 20 m wide and near an erosional hot spot. The beach is widest in
the summer and narrowest in the winter.
The equilibrium shoreline change model relates the rate of horizontal shoreline
displacement to the hourly wave energy and wave energy disequilibrium. This
model can predict seasonal shoreline changes because of the high energy winters
and lower energy summers. The model has a built in assumption that the shoreline
contour displacement depends on the energy of the waves and not the direction
from which the waves hit the beach, implying that cross-shore sediment flux is what
affects the shoreline change. The equilibrium shoreline change model was
developed over several studies at Torrey Pines State Beach, which has
proportionally smaller grain size and wave energy than Ocean Beach. Ocean Beach
has an average wave energy 8 times greater than Torrey Pines, however sand level
and shoreline displacement changes are only by a factor of 2. Response coefficients
depend on grain size and can be used at different sites. What is not transferable
between sites are the shoreline changes to the relative mean shoreline position
because they were defined independently as fluctuations of the temporal mean
shoreline position.
Constant wave height change occurring at Torrey Pines took about 2 months, took 4
months at Ocean Beach. The model can only be used for similar geomorphic
shorelines, since beaches with similar wave, sand, and alongshore transport
characteristics tend to form similar looking beaches. The model is limited due to the
specifics of particular characteristics.
Wave conditions vary faster than the beach changes. Over a short timescale, the
beach is always in disequilibrium, but it can be perceived to reach equilibrium if
evaluated over a multi season timeline. Coarse grain size at ocean beach act to
stabilize the shoreline requiring higher energy waves to change the profile and
slower transport times. A beach with smaller or finer sediment may have a faster
change rate than a beach with coarser sand. Calibrating the Shoreline Change Model
from lower energy beaches to higher energy beaches validates previous studies of
equilibrium of particular shorelines. Though the model does not fit to every beach
setting, Ocean Beach and Torrey Pines State Beach both work as study sites that can
in fact reach a state of equilibrium over the long term.
References
Yates, M.L., Guza, R.T., O'reilly, W.C., Hansen, J.E., and Barnard, P.L., 2011,
Equilibrium shoreline response of a high wave energy beach: J. Geophys. Res. Journal of
Geophysical Research, v. 116.
Horn, D. P., 1992, A review and experimental assessment of equilibrium grain size
and the ideal wave-graded profile