Mapping Coastal Changes Along Northern Monterey Bay, California, to Aid Planning for Future Storms
When U.S. Geological Survey (USGS) scientist Curt Storlazzi was standing near Moran Lake Beach east of Santa Cruz, California, in January 1998, he witnessed waves from an epic storm wash onto the roadway and straight into a bus, hitting it hard enough to push it into the oncoming lane. Luckily no one was hurt, but the village of Capitola a few miles away suffered severe damage from waves and wave-driven logs bursting through the ocean-facing windows of restaurants and businesses. Normally, a wide beach protects Capitola’s Esplanade from the ocean waves. But in that El Niño year, much of the beach had already been eroded by winter storms when the February tempest hit.
Big storms are inevitable on the central California coast, and climate change and rising sea level are expected to intensify them, particularly in El Niño years, when atmospheric conditions bring heavy rains. For safety’s sake, and to protect resources such as businesses, beaches, and harbors that bring a coastal community like Santa Cruz much of its revenue, there’s a great need to understand how big storms can shape and affect the coast.
To address this need, a team of USGS scientists led by research geologists Patrick Barnard and Jonathan Warrick are running repeated surveys of beaches and the nearby ocean bottom to compile three-dimensional maps of how beaches in northern Monterey Bay change over time. The first survey was conducted October 20–24, 2014, from Mitchell’s Cove in Santa Cruz to Moss Landing, where the head of Monterey Canyon comes close to shore. Staff from the USGS Pacific Coastal and Marine Science Center in Santa Cruz used high-precision Global Positioning System (GPS) receivers carried on foot and mounted on all-terrain vehicles (ATVs) to measure beach and swash-zone elevations (topography), and GPS receivers and 200-kilohertz echosounders mounted on personal watercraft to measure underwater elevations (bathymetry).
Scientists carrying GPS receivers in backpacks walked transects perpendicular to the shore, from the inland edge of the beach—for example, the base of the sea cliff—as far into the water as they could safely go. The personal watercraft drivers also surveyed transects perpendicular to the shore; most of these were extensions of the onshore transects, running from as close to the beach as possible to approximately 2 kilometers offshore. The ATV drivers surveyed beach transects parallel to the shore. Over the 5 days of the October survey, 15 mappers and support personnel collected a total of 513 kilometers of trackline data along the coast: 219 kilometers of personal-watercraft data, 210 kilometers of ATV data, and 84 kilometers of backpack data (see map, below).
The October survey proved its worth just 2 months later, when a large storm, dubbed the “Super Soaker,” brought heavy rain and big waves to the Santa Cruz area on December 11, 2014. To capture the effects of this storm, the scientists conducted small-scale surveys near the mouth of the San Lorenzo River in Santa Cruz and the mouth of Soquel Creek in Capitola—two areas known to undergo large changes in response to storms. For these surveys, a terrestrial lidar scanner was added to the mix of instruments. Lidar (light detection and ranging) uses laser light to measure distances and produce highly accurate three-dimensional maps and images of terrain. (To learn more about lidar, and to view a lidar “flight” over the beach near the San Lorenzo River mouth, visit “Climate Check in our Santa Cruz Backyard.”)
Because lidar had not been part of the October survey, USGS personnel collected pre-storm lidar data on December 10, the day before the storm was forecast to hit. On December 18, a week after the storm, they surveyed the two river-mouth areas with the lidar scanner plus all the instruments they had used in the October survey.
The December survey results are currently being compared with the October survey data. Among the preliminary findings is evidence that the storm eroded sediment from the beach near the San Lorenzo River mouth and deposited sediment offshore (see change map, below). Overall, the volume of sediment near the river mouth increased; the researchers hypothesize that the additional sediment was delivered to the area by the river, possibly augmented by along-shore movement of sediment from the west.
Another full-scale survey, from Santa Cruz to Moss Landing, will take place in March 2015, and regular surveys will be run in the fall and spring of subsequent years to capture seasonal fluctuations and extreme events, such as flooding from the San Lorenzo River. USGS scientists will also create beach maps from video captured during aircraft flyovers, and they will attach time-lapse cameras and tide and wave gauges to local piers for a multidimensional understanding of coastal processes.
Collecting these data over many years will ultimately provide a detailed picture of how sand moves along the northern Monterey Bay coast, and how the position of the coastline changes in response to changes in sand input and waves. Studying vulnerable and dynamic zones such as the San Lorenzo River mouth and the Soquel Creek mouth before winter storms hit will enable scientists to measure how the beaches change, and will aid the understanding of how big storm events, such as those occurring during El Niño years, shape and erode the coast. The survey results can be incorporated into future scenarios of sea-level rise and climate change, contributing directly to Monterey Bay communities working on how and what to protect along their coastlines.
For more information about this project, please visit the USGS webpage “Climate Check in our Santa Cruz Backyard.”
in this issue:
Mapping Coastal Changes in Monterey Bay to Aid Planning for Future Storms