Cape Hatteras Field Study
February 2010 Nearshore field study
The field study will focus on the processes responsible for alongshore delivery of sediment to the Cape point and the processes that transport the sediment offshore from the point. To investigate these processes, the study will conduct six different types of activities: Nearshore instruments, Offshore instruments, Lighthouse camera, WERA radar, beach surveys, and dye study. Several of the activities include deployment of oceanographic sensors in the surf zone and at offshore locations, on the dunes, and the lighthouse. The tentative deployment plan (Figure 1) identifies locations of the equipment for the Nearshore, Offshore, Lighthouse, and WERA sites. All six activities are described in the next sections.
Figure 1. Location map of Cape Hatteras, NC. Markers are placed to identify locations of instruments during the deployment: twelve sites for the Nearshore instruments (N1-N12), three sites for the Offshore instruments (O1-O3), one site for the Lighthouse camera (Lighthouse1), and one site for the WERA radar (WERA2). Additional activities (not shown) include beach surveys and a dye study.
The twelve (12) sites for the Nearshore instruments are identified as locations N1 to N12 on Figure 1. The sites are located on the east and south shorelines and on top of the large shoal that extends southward from Cape Point. Instruments will be deployed at these sites to measure ocean currents at various levels, water level, wave heights, and wave directions. Nine of these instruments will be Aquadopp 1 Mhz profilers and three will be RDI 1200 Khz ADCPs or equivalent. These instruments will be deployed on pipes that are 4 m long. The pipe will be jetted into the sea floor to a depth of ~ 3 m, leaving approximately 1 m or more projecting into the water. We are not intending for the top of the pipe to be visible above the water. The instruments are mounted on a cantilever arm attached to the pipe. We will use divers to deploy the equipment. The USACE Field Research Facility (FRF) maintains several specialized vehicles that are capable of deploying equipment and transferring gear from the beach into or out past the surf zone. One type of the vehicles is an amphibious water craft Lighter Amphibious Resupply Cargo (LARC) that is capable of driving from the beach into the water to deploy equipment in or beyond the surf zone. The divers will use the LARC to deploy, conduct periodic maintenance visits, and to recover the gear.
Figure 2. Example of equipment to be deployed at the nearshore sites consisting of instrumentation mounted on pipes to be jetted in using the LARC and divers.
Nearshore additional site:
One additional site may have an additional set of instruments mounted on a tripod system. The tripod would be mounted close to the shore and may be visible from the beach. This tripod system is being developed, and may be available for the deployment. If available, the tripod/frame will be jetted into the sand or supported with screw anchors to hold the frame in place. The instruments will measure surf zone turbulence, suspended sediment concentration, waves, currents, and water level.
Figure 3. Example of a nearshore tripod/frame that may be deployed at one of the sites. This piece of equipment is being developed and may be deployed for the study.
The three (3) sites for the Offshore instruments are identified as O1, O2, and O3 in Figure 1. These are the deepwater sites, and will require a vessel with a crane to deploy and recover them. The instruments will be deployed in depths of approx 10 m and will measure waves and vertical profiles of currents.
Figure 4. Example of offshore bottom mounted trawl-resistant adcp frames. Frames to be deployed by others in about 10 m water depth at 3 locations around Diamond Shoals.
The one (1) site for the camera is the Cape Hatteras Lighthouse. The lighthouse is approximately 59 m tall. This height will allow a vantage point to view the nearshore. We will mount a video camera system on the railing at the top of the lighthouse facing the ocean. The camera will image the surf zone. Processing of the images provides estimates of along shore ocean current speeds. Additionally we will install several (~6) targets on the beach and on the dunes that are in view of the camera. The targets will be 2m x 2m black plastic with a white X. We then locate these targets with GPS to exactly identify their locations. The targets help to geo-rectify the images during post processing.
Figure 5. Camera to be mounted at the top of the lighthouse on railing facing the water. Image of railing top right. Example of nearshore imaging from Cape Hatteras Lighthouse bottom left.
One (1) WERA radar site will be installed on the east side of the Cape Point, identified as WERA2 on Figure 1. The radar will be located near the old Lighthouse location along the dune line and consists of a series of transmit and receive arrays. The radar is used to measure the speed and direction of surface currents.
Figure 6. Example setup of WERA radar systems.
During the weeks of the deployment, we will be conducting several types of surveys to acquire subaqueous bathymetry and subaerial topography. Some of the work will be conducted during calm periods, and some needs to be performed during storms. We will use a zodiac type vessel in the nearshore and walking access to the beach and dunes areas. Also the USACE will conduct X-band radar and LIDAR surveys during the deployment using a 4wd vehicle (suburban type) truck equipped with a coastal LIDAR and Radar Imaging System (CLARIS). The CLARIS system provides measurements of wave heights, inferred bathymetry, and dune heights to provide a spatially continuous measurement of bathy and surface topography. The LARC may also be used to obtain measurements of bathymetry.
Figure 7. Other nearshore activities to occur during the deployment to measure bathymetry and surface waves.
A primary interest of this field investigations is the circulation dynamics at Cape Point. To help visualize these dynamics, a standard method is to place a non-toxic coloring agent into the water and observe its path. As the color spreads, we can infer lateral mixing rates, observe the lateral extent of mixing (is it limited to the surf zone), and also infer the transport speed as the color is carried alongshore with the currents.
We plan to inject Uranine dye into the surf zone on two separate tests. The dye is a non-toxic substance with a green fluorescent color. One test will be during winds from the NorthEast and one with winds from the SouthWest. If weather conditions allow, a third test will be conducted depending on the success of the earlier attempts. We are identifying the exact amounts of dye to use for each experiment, and this may vary with conditions.
To visualize the dye, we will take still pictures and video from a small plane. We will install several (~9) targets on the beach or dunes that are in view of the camera. The targets will be 2m x 2m black plastic with a white X. We then locate these targets with GPS to exactly identify their locations. The targets help to geo-rectify the images during post processing.
Figure 8. Example of a dye experiment.
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