Sea cliff along the intersection of Seacliff Drive and San Benito Ave. Note the variable width of the beach here in Seacliff State Beach in front of the seawall and park road. The homes sit atop the Highway One Terrace (Santa Cruz Terrace), a former beach uplifted between 60,000 and 120,000 years ago.
The 1.2 km stretch of Seacliff State Beach included in this study is a portion of a continuous section of sea cliffs that extend 3 km from New Brighton State Beach in the north to Aptos Creek in the south. The cliffs at Seacliff State Beach are protected from waves by a seasonally dependent, variable-width sandy beach backed by a seawall. Waves only reach the base of the cliffs during extreme storms that occur on the order of once every several decades. Therefore, the sea cliff failures and resulting cliff retreat that occur along this stretch of coast are primarily a result of terrestrial processes (overland flow, groundwater flow, and seismic shaking).
The 28- to 36-m-high cliffs are composed of the Pliocene Purisima Formation, and are capped along most of their length by a 6- to 12 m-thick layer of unconsolidated Pleistocene terrace deposits. In this exposure, the Purisima Formation is moderately weathered, weakly to moderately indurated siltstone and sandstone that contain lenses of shell hash deposits. Joints are common along this exposure and are unevenly spaced. Along the northern half of the study area joints are separated by as much as 5 to 6 m, whereas in the southern half they become much more closely spaced (0.5 m). The joints are unfilled, with little to no separation. There are two distinct joint sets with average strikes of 35° and 275° and near vertical dips.
The wave climate is well documented for the northern Monterey Bay. Existing data show that deep-water waves have a mean height of 1.2 m and a mean period of 13 seconds. The waves most frequently arrive from the northwest, but during El Niño winters storm waves arrive more frequently from the west and southwest with heights of 3 m or greater. Wave refraction studies show that for the portion of coastline of northern Monterey Bay that includes Seacliff State Beach, waves approaching from the northwest diverge around Point Santa Cruz at the northwestern entrance to the Monterey Bay, changing to approach the shore from the west. Wave height (and consequently wave energy) is thus reduced before reaching the shoreline. However, waves approaching from the southwest undergo less refraction because there is no headland to dissipate wave energy. As a result, waves from the southwest have greater heights and more energy upon reaching the shoreline.
Tides in this region are diurnal and have a mean range of 1.6 m; the highest high water is 2.4 m and the lowest low is -0.8 m. The highest monthly tides occur in the winter and summer; it is not unusual for the highest tides to coincide with large, winter storm waves. Rainfall in this region occurs predominantly from December through March, and high rainfall frequently coincides with large waves. The average annual precipitation since 1895 is 53 cm, although large climatic perturbations such as El Niño can bring excessive precipitation to the area. Based on data compiled by Storlazzi and Griggs, 76 percent of historical storms that caused significant coastal erosion or damage occurred during El Niño years.
This study documents the impacts of earthquakes and large storms to the sea cliffs along Seacliff State Beach. The first event is the 1989 Loma Prieta earthquake, a M7.1 earthquake that caused widespread damage to the area stretching from Santa Cruz to the San Francisco Bay. The epicenter of the earthquake was located in the Santa Cruz Mountains, approximately 9 km inland from the coast. Extensive block and debris falls, induced by the seismic shaking, occurred along the sea cliffs in the study area.
The second major event considered in this study is the 1997-98 El Niño that brought increased winter storm activity to the coastline of the northern Monterey Bay. Associated with these storms, which began in force in late January of 1998, were increased wave energy from more westerly directions than in non-El Niño years, elevated sea level, and increased amount and duration of precipitation. While increased wave energy and elevated sea level potentially have significant impacts on those portions of the cliffs that are exposed to waves, increased rainfall leading to excessive surface wash and increased groundwater pore pressures promote erosion of the sea cliffs.
The amount of cliff retreat for Seacliff State Beach was determined by digitizing the top edge of the cliff on the rectified photographic stereo models from October 1989 and March 1998. Digitizing while viewing in stereo ensures that the true topographic break in the terrain is used as the cliff edge. The maximum retreat (7.1 m) is located at the northern end of this section. Other than this localized area of retreat, the northern half of the study area experienced few failures associated with El Niño storms. The amount that a particular section of cliff retreats in a given time period provides quantitative information that may be useful to land-use planning and land owners. While such retreat information is valuable, it provides little information on the processes of slope failure that lead to sea cliff retreat. ng rectified photographic stereo models. This method allows us to document the linear extent of cliff failures, the spatial and temporal relationship between failures, and the type or style of slope failure.
Five different types of slope failure were documented during the period from October 18, 1989 to March 6, 1998. These include debris falls, block falls, debris flows, slumps, and slaking. Rapid, seismically induced failures were either debris falls or slumps, resulting in failure of 244 m of the 1.3 km-long cliff section. Failures over the course of the decade, which include the failures associated with early (December and January) El Niño storms, lead to the greatest localized amount of retreat of the top edge of the cliff. However, spatially, the severe storm period of 1998 (January to March) and the Loma Prieta earthquake caused the most widespread slope failures.
This map has introduced new techniques of analyzing the short-term evolution of sea cliffs and the differential response of sea cliffs to seismic and climatic events. Using stereo models derived from softcopy photogrammetry, we are able to locate sea cliff failures and determine their spatial distribution and the geologic units involved for several different time periods in an area where sea cliff failure and retreat periodically threaten homes and community infrastructures. These data can be incorporated into a GIS database to examine the relationship of the failures to one another, to coastline morphology, and to field data (faults, joints, or lithologic variations). Spatial plots of the failures appear to show specific patterns; if additional data continue to support this observation, the technique of extracting failure signatures and analyzing the temporal and spatial distributions of the signatures may help to identify areas prone to future failures.
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Updated: April 23, 2007 (bwr, mfd)