Characterization of geologic and oceanographic conditions at Pleasure Point, Santa Cruz County
- Curt Storlazzi
United States Geological Survey (USGS)
End Date: September 30, 2007
The Santa Cruz County Department of Public Works, the Santa Cruz County Redevelopment Agency and the California Division of Boating and Waterways requested a proposal from the U.S. Geological Survey (USGS) Western Coastal and Marine Geology Team (WCMGT) to provide baseline geologic and oceanographic information on the coast and inner shelf off Pleasure Point, Santa Cruz County, California. To meet these information needs, the USGS has proposed a study to collect baseline scientific information on the morphology and waves at Pleasure Point.
This study will provide high-resolution topography of the coastal bluffs and bathymetry of the inner shelf off East Cliff Drive between 32nd Avenue and 41st Avenue and out to water depths of 20 m (60 ft) (Figure 1). Further, we will document the spatial and temporal variation in waves at the study site. Although this project will not actively investigate the impacts of the proposed bluff stabilization project, these data will provide the baseline data needed for future studies directed toward predicting the impacts of stabilization on the seacliffs, beach and nearshore sediment profiles, natural rock reef structures, and offshore habitats and resources. It will also provide a basis for calculating potential changes to wave transformations into the shore at Pleasure Point.
Summary to DateThe following is a summary of the project goals and methods. Results will be updated as they become available:
Task 1 – Mapping
High-resolution maps of the Pleasure Point area will be compiled for both the terrestrial and subaqeous parts of the study area (Figure 1) from a combination of historical and newly collected data. The morphology of the seacliffs will be documented using historic airborne LiDAR (Light Detection and Ranging) data and terrestrial LiDAR data. The bathymetry of the inner shelf will be collected using single-beam and multibeam fathometers.
Subtask 1.1 - Historical Data
Historic airborne scanning LiDAR surveys were collected to measure shoreline and seacliff volume changes throughout the study area as a consequence of the 1997-1998 El Niño-influenced winter storms. These data sets provide excellent topographical coverage for determining regional shoreline position and seacliff morphology for comparison with our present survey work.
Subtask 1.2 – Terrestrial LiDAR
Terrestrial LiDAR is the newest and most accurate technology being used to monitor coastal bluff stability and has several advantages over aerial scanning (Figure 2). Because ground based LiDAR scanning is performed horizontally, not only is the point density much higher, but geologic features such as sea caves and wave cut notches are also captured. Current terrestrial LiDAR scanners also have integrated cameras that allow instantaneous image draping over the digital elevation model. This method is also more cost effective for localized coastal bluff monitoring where more effort can be placed on areas of interest. Current scanners have a range of 360 degrees horizontal and 80 degrees vertical with an accuracy level of 25 millimeters. Volumetric and spatial morphology can then be used to investigate the actual bluff failure mechanism(s). The terrestrial LiDAR will be collected along the Pleasure Point study area to create a high-resolution digital elevation model of the seacliffs of interest.
Subtask 1.3 – Shallow Nearshore Bathymetry
The historic airborne LiDAR and the proposed terrestrial LiDAR will be coupled with bathymetric profiles conducted using survey personal watercraft (PWC) with kinematic GPS and echo sounder equipment to create a three-dimensional model of the seacliffs, beach and shallow nearshore at Pleasure Point (Figure 3). A kinematic GPS base station is set up onshore that allows us to measure water depths in real-time with centimeter accuracy. The cross-shore survey lines will be run from approximately 0.5 kilometer offshore through the surf zone to depths approaching mean sea level. Along-shore profiles will be run to image cross-shore oriented features, such as subaqeous bedrock ridges often observed in aerial imagery. A higher-resolution bathymetric survey grid along the study area will be nested in a coarser grid that will extend a few kilometers to both the east and west of the study area to put the high-resolution data collected off Pleasure Point into the context of the region’s bathymetric variability.
Subtask 1.4 – Deeper Nearshore Bathymetry
A multibeam/side-scan survey will be run offshore Pleasure Point, the first ever high-resolution bathymetric survey in this region, to link the shallower PWC surveys with USGS side-scan sonar and single-beam fathometer surveys collected farther offshore in 1996. The new bathymetry collected by the PWC (see Subtask 1.3) and the multibeam cruise will be the baseline data for any future modeling efforts and a valuable resource as management decisions for Pleasure Point are being made.
Task 2 – Wave Characterization
The spatial and temporal variation in both the incoming waves and the resulting breaking wave patterns at Pleasure Point will be documented from a combination of in situ instrumentation and remote sensing techniques. The information on the incident wave and current field at the study site will be collected by way of oceanographic instrumentation deployed just offshore of the coast. Wave breaking patterns will be documented using a web-based camera system.
Subtask 2.1 – Temporal Variation in Currents and the Incident Wave Field
An Acoustic Doppler Current Profiler (ADCP) will be deployed for up to 12 months offshore of Pleasure Point to document the range of wave energy conditions observed over a year. ADCP data will provide a vertical profile of current velocity and acoustic backscatter (a proxy for suspended sediment) through the water column along with tide, temperature and wave (height, period and direction) information. This sensor will allow us to determine the link between the offshore wave conditions measured by the deep-water NDBC Monterey Bay directional wave buoy and the resulting wave breaking patterns at Pleasure Point imaged by the web-based camera system (see Subtask 2.2). This data, in conjunction with the proposed nearshore bathymetry (see Subtasks 1.3 and 1.4), is crucial if accurate modeling of waves in the study area under a range of scenarios (engineering, climate, etc) is desired by resource managers in the future.
Subtask 2.2 – Spatial and Temporal Variation in Breaking Wave Patterns
A web-based camera system will be installed to document the patterns of breaking waves across the study area in real-time. The system is comprised of an analog video camera and a digital still camera, housed in a single pan tilt unit (Figure 4), linked to a computer such that the camera can be controlled remotely from the USGS office in Santa Cruz. This video monitoring makes it possible to track wave breaking patterns, rip channel development and potentially infer sand bar location(s) under a range of wave conditions. These data can then be compared to offshore deep-water offshore wave conditions measured by the deep-water NDBC Monterey Bay directional wave buoy and the proposed USGS ADCP (see Subtask 2.1).
- Substrate characterization
- Geological characterization
- Optical properties
Figures and Images
Figure 1: Map of the study area, between 32nd and 41st Avenues along East Cliff Drive.
Figure 2: Terrestrial LiDAR scanner used to make a high-resolution topographic map of the seacliffs.
Figure 3: Personal watercraft (PWC) with kinematic GPS and echo sounder equipment used to make a high-resolution bathymetric map of the shallow rock reefs and sand channels.
Figure 4: Web-based camera system to document wave breaking patterns.