Transcript The Linear Instability Characteristics of the Coupled Wave

Marine radar wave observations at the mouth of the Columbia River
Merrick C. Haller, H. Tuba Özkan-Haller, Patricio Catalan, J.D. Lentine, Justin Brodersen, Hai-Ying Jao, and Lisa Andes,
Civil and Construction Engineering, College of Oceanic & Atmospheric Sciences, Oregon State University
New Observing Technologies for the NANOOS-Pilot Project
Wave Analysis
North jetty
Conventional marine radar technology is being adapted as a tool for wave observing in coastal areas. Recently, researchers within the Ocean
Engineering Program at Oregon State University in cooperation with Imaging Science Research, Inc. have developed a high-resolution wave
observation system using an X-band marine radar with a customized data acquisition system. These observation systems offer the potential
for providing real-time wave information over large nearshore areas (~20 km2). We expect that remote sensing technology such as this can
provide a considerable benefit at navigational entrances where wave conditions are often hazardous and highly variable and where
traditional in-situ sensors are often ineffective or cannot be safely deployed.
North Head
(USACE camera station)
north jetty
Radar
- Tp=10 sec
NDBC 46029 - Tp=10 sec
Radar
- Tp=16.7 sec
NDBC 46029 - Tp=13 sec
Radar
- Tp=16.0 sec
NDBC 46029 - Tp=14.3 sec
Radar
- Tp=13.1 sec
NDBC 46029 - Tp=14.3 sec
Top: Portions of single radar images expanded to show the area near Benson Beach, WA. Waves were consistently breaking as far out as 1
km from shore leading to the bright radar returns as far out as the end of the north jetty.
Bottom: Frequency spectra calculated from radar pixel time series located approximately 600 m offshore of Benson Beach. Estimated peak
periods are compared with those measured by NDBC Buoy 46029 located approximately 37 km to the southwest.
The flow out of the Columbia River varies depending on the stage of the
tide and during periods of strong currents the wave field can be
significantly modified. For example, during strong ebb flows waves over
the Columbia River Bar are highly steepened and often break, which leads
to dangerous navigational conditions.
south jetty
Radar images from the mouth of the Columbia River. Images
span 6 km in range distance from the radar (located at center of
each image). The left image covers 170 degrees in azimuth, the
right covers 260 degrees. North Head is visible directly north
from the radar, to the south are the Columbia River jetties.
Top left: Aerial photo of the north jetty; Top right: Temporary radar
tower and support vehicles; Bottom left: View of waves at Benson
Beach during the Jan. 16, 2006 storm. Bottom right: View of Benson
Beach and north jetty from atop the radar tower.
Left: Radar image taken Jan 18th during a mid-tide condition when
currents were not strong shows little breaking offshore.
Right: Radar image taken on Jan 18th at 705PM during a strong ebb tide
current shows a strong increase in wave breaking just offshore of the river
mouth.
Sampling capabilities
Image frequency
-
44 rotations per minute (1 image every 1.36 seconds)
Image resolution
-
3 m in range, 1o in azimuth
Image footprint
-
6 km radius (adjustable)
Duration
-
user specified, typical image sequences are 15 min
long (640 images) and repeated every hour
Wave Modeling
Wave heights (m) - SWAN
Wave directions - SWAN
N
Radar image time series provide a synoptic picture of the ocean surface over a large area. This is similar to having a wave buoy located
every 10 meters! For wave analysis, image pixels from areas of interest are selected and processed with Fourier analysis routines and wave
frequency and direction information can be determined at essentially the same resolution as the image pixels. This gives a detailed
description of the spatial variability of wave conditions throughout the area. In addition, with calibration, radar systems can provide direct
estimates of wave heights and signify the presence of breaking waves. Finally, large scale ocean density fronts can be identified in these
images due to the changes in radar scattering they induce.
Left: Wave heights predicted by the SWAN nearshore wave model, Jan. 17th 1pm.
Center: Predicted wave directions, colors represent degrees from North.
Right: Corresponding radar image from Jan. 17th 1pm.
Image rectification
Rectified image
In order to geo-locate the wave data, each radar
image needs to be rectified to the local real-world
coordinate system. The figure at left shows a raw
radar image and the image at left is after partial
rectification. Full rectification requires GPS
measurements of Ground Control Points visible in
the images (such as the jetties) and interpolation
onto a uniform grid.
Range distance
Civil, Construction
and Environmental
Engineering
Department
Raw image
azimuth
The Future
In the near term, we are assembling a mobile radar trailer and tower similar to the one depicted at right.
The tower is retractable and will extend 30 ft high, which will allow the radar to see over dunes and jetties.
We are also considering a renewable energy power system using a combination of solar and wind power
technologies. When completed, the mobile system will allow for long-term radar deployments and radar
wave observations can be made available in real-time to the NANOOS observatory.
Finally, model-data comparisons are ongoing in an effort to calibrate a high-resolution wave modeling
system for the Columbia River mouth.
Ocean Engineering