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Ferry-Based Monitoring of Puget Sound Currents
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This project is in collaboration with the Washington Department of Ecology, which has a mandate to understand water quality in Puget Sound. Part of understanding water quality is understanding where the water’s coming from.
By having our instruments on the ferries, now we can start to get a total estimate as to how much water is intruding at the bottom from the ocean and how much water is leaving at the top from the rivers and the freshwater outflow.
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Research Results
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Tidal current observations through Admiralty Inlet from ferry-mounted current profilers Guerra, M., J. Thomson, T. Prusa, C. Falkenhayn Maloy, C. Krembs, and B. Sackmann, "Tidal current observations through Admiralty Inlet from ferry-mounted current profilers," J. Ocean Eng. Mar. Energy, 5, 159-172, doi:10.1007/s40722-019-00135-w, 2019. |
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More Info
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1 May 2019
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Admiralty Inlet is a narrow sill located at the northern end of Puget Sound (WA, USA). Circulation through Admiralty Inlet is complex, with tidal currents exceeding 3 m s-1, large variations in fresh water input to the system, and seasonal ocean water intrusions. Long-term observations of the currents across the entire inlet are crucial for understanding circulation through Puget Sound. In this context, the Washington State Department of Transportation (WSDOT) Ferries, which run year round through Admiralty Inlet, provide a cost-effective platform to mount instruments and obtain long time series of currents distributed across the inlet. Through the Ferry-Base Monitoring of Puget Sound Currents project, two down-looking acoustic Doppler current profilers (ADCPs) are installed on board two WSDOT ferries, providing depth profiles of velocities across the inlet since May 2014. All data are quality controlled and organized in an horizontal and vertical grid across the inlet. Data within each grid cell are analyzed to capture tidal current harmonic components. Results agree well with data from fixed bottom-mounted ADCPs, and show large spatial variability in the amplitude of harmonic components, probably related to the bathymetric features of the inlet. Further analysis provides estimates of tidal asymmetry and residual currents through the inlet, which are relevant to water quality within the Puget Sound.
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Research Objectives
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To improve and calibrate water quality models that are being developed by the Washington State Dept. of Ecology to better understand and predict water quality throughout the Puget Sound, we will expand the ferry monitoring network to include public ferries run by the Washington State Department of Transportation (WSDOT) on the Port Townsend to Coupeville route. Installation of automated instruments to measure current velocities on select WSDOT vessels will allow us to record measurements continuously as ferries makes their multiple daily runs.
WSDOT ferries occupy strategic cross-sections in Puget Sound – often at the very constriction points between basins that would let us most easily measure water exchange and circulation between those basins. We need to understand the water quality condition, circulation, and exchange of water masses within and between natural sub-basins of Puget Sound, as well as exchange with the ocean, in order to manage water quality in Puget Sound (e.g., nutrient enrichment, low dissolved oxygen conditions, the transport of toxic chemicals, harmful algal blooms, and ocean acidification).
We are installing an instrument known as an Acoustic Doppler Current Profiler (ADCP) on two ferries. These sensors, which primarily measure current velocities in a continuous transect along the ferry route, will provide unprecedented surface-to-bottom measurements of water-mass movement and transport between basins.
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Download the bulletin (PDF, 483 KB)
A PDF file of the Did you Know? bulletin |
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Research Bulletins aboard the Kennewick and Salish
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Goals
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These measurements from WSDOT ferry platforms are key to understanding overall water quality and for improving the performance of numerical models in Puget Sound. To understand and manage water quality it is imperative to measure:
- water quality condition, circulation, and exchange of water masses within and between natural sub-basins of Puget Sound
- movement and exchange of freshwater inputs from rivers and streams
- input and exchange of saltwater from the ocean
Environmental data obtained as an outcome of this research includes full-cross-section current velocities at a key transect across Puget Sound. These data will help measure flushing, net exchange, and circulation of water between Puget Sound and the ocean. The data will be used to improve and calibrate water quality models that are being developed to better understand and predict water quality throughout the Sound.
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New Assets
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Existing Ferry-Based Assets
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In cooperation with the WSDOT and the Puget Sound Partnership, we will add instruments to two ferries the Kennewick and Salish that occupy the route between Port Townsend and Coupeville. This is an especially valuable cross-section characterizing the overall influx and outflow of water to/from the ocean and greater Puget Sound. The instrument, known as an Acoustic Doppler Current Profiler or ADCP, primarily measures current velocities throughout the water column in a continuous transect along the ferry route. These sensors will provide unprecedented surface-to-bottom measurements of water mass movement and transport between basins.
These two ferries incorporate design features that will make the initial installation of the instruments less complicated than other ferry classes. Installation will occur when individual ferries are scheduled for dry-dock, which is mandated to occur only twice every five years per ferry.
Inside of thru-hull mount, with data cable and vent tubes for freshwater reservoir
Exterior view of instrument in 'sea chest' thru-hull mount, which is filled with fresh water and capped with a transparent polycarbonate window
Inside of junction box which connects instrument to ship's ethernet network and provides power to the instrument
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Weather observing equipment is placed on five Washington State ferries. Currently, each boat is equipped with weather sensors reporting wind speed, wind direction, air pressure, temperature, and humidity. These quantities are measured every minute, averaged over a period of 4 seconds. Ship heading and speed information is also recorded and used to correct the measured winds for ship motion.
The weather data are transmitted via radio to land, and then sent over the Internet to the University of Washington where they are decoded, archived, and used to create the Ferry Web page. The Ferry Web page also includes weather reports from other sources that are on or near the waters of western Washington.
Ferry Weather
Current weather reported by Washington State DOT ferries, served from the University of Washington |
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The Washington State Dept. of Ecology employs a multi-pronged strategy to monitor marine waters of Puget Sound, relying on monthly samples collected by seaplane across a network of long-established sampling stations throughout the Sound (but sampled only once/month), instruments that measure water properties continuously at fixed sites (but a limited number of sites), satellite imagery (limited mostly to surface conditions), and as opportunity allows measurements taken by boat or ship along strategic transects (which provide a continuous record of observations across gradients and boundaries between basins).
For the last four years, Ecology has partnered with Clipper Navigations, Inc., to collect a suite of water quality measurements along the main axis of Puget Sound as the Victoria Clipper IV runs between Seattle and Victoria, B.C. This is accomplished through the installation of automated instruments on the ferry, which record measurements continuously as the ferry makes its once-daily (during the off season) or twice-daily (during the peak season) trips. The data collected by the instruments are logged onto an on-board laptop computer and then transmitted daily to Ecology for data processing and evaluation.
The Clipper instruments collect surface water quality data algal biomass, turbidity, temperature and salinity which allows measurements of water conditions along the main axis of Puget Sound. Installing instruments on the WSDOT ferries will allow data on water currents to be collected across strategic cross-sections of Puget Sound, often at the very constriction point between basins that will let us measure water exchange and movement between those basins, which is a key measurement to understand how water masses move and circulate between basins. These instruments are all harmless to marine life.
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Ferry and Victoria Clipper Data Available via NVS Data Explorer
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WSDOT Port Townsend/Coupeville Ferry Data
http://nvs.nanoos.org/Explorer?action=oiw:mobile_platform:WSDOT_Ferry1:details |
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Victoria Clipper Data
http://nvs.nanoos.org/Explorer?action=oiw:mobile_platform:WADOE_Ferry1:observations:H1_WaterTemp |
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Ferry-based ADCP Data for Puget Sound Now Available via OPeNDAP
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The complete near-realtime STA (15-second, Short Term Averaged) ADCP dataset from the WSDOT M/V Salish is now available via OPeNDAP. The data have been re-packaged into CF-compliant NetCDF files, which are being distributed through a THREDDS server.
THREDDS server for Salish data
http://138.68.225.121:8080/thredds/catalog/Salish_L1_STA/catalog.html |
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THREDDS server for Kennewick data
http://138.68.225.121:8080/thredds/catalog/Kennewick_L1_STA/catalog.html |
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Full-resolution STA data can be accessed on a per-file basis or as a single aggregated dataset (Salish_L1_STA.ncml) depending on user needs.
Using OPeNDAP-compliant client software (e.g., MATLAB, Python, R, ArcGIS, etc.) you can now easily request subsets of the complete dataset for subsequent product development on an as-needed basis. For those unfamiliar with OPeNDAP there a variety of online tutorials that have been developed for different software packages. Included here are a couple of simple MATLAB routines for those who are interested. The example m-file will issue a new data request and overlay bathymetric data on a GoogleMaps base layer. For those who have used MATLAB's linking/brushing capabilities, these features can be enabled once the plot finishes rendering.
Salish data files can also be downloaded via HTTP
http://138.68.225.121/Salish66/ |
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Kennewick data files can also be downloaded via HTTP
http://138.68.225.121/Kennewick52/ |
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nb: Level0 files remain in the original RDI format as received from the ship; Level1 files (STA only) have been re-packaged into a CF-compliant NetCDF format.
STA data are provided at full-resolution, but data collected while the ferry is not underway have been removed (i.e., the dataset only includes data collected while the vessel is underway).
In addition, ancillary datasets have been incorporated into the NetCDF output to ensure that the ADCP data can be projected into a standard vertical reference frame (particularly useful if ADCP data are to be used for hydrodynamic model calibration/validation). The following variables have been included in the Level1 NetCDF output:
pstide_prediction_mllw: PSTide Prediction (MLLW)
psdem_bathymetry_navd88: Puget Sound Digital Elevation Model Bathymetry (NAVD88)
vdatum_offset_mllw_to_navd88: VDatum Offset (MLLW to NAVD88)
Water velocities (*_absolute_water_velocity) are initially provided assuming a stationary instrument. To account for the movement of the instrument through the water it is necessary to estimate the ship's speed and direction.
There are two sources of data that can provide this information:
- Bottom-tracking velocities derived from the ADCP itself (*_bottom_tracking_velocity)
- Navigation-based velocity estimates derived from the ship navigation system (*_navigation_velocity)
To correct the water velocities for the motion of the ship simply sum the *_absolute_water_velocity and the reference velocity of your choice (i.e., either *_bottom_tracking_velocity OR *_navigation_velocity). This adjustment was not done a priori so that the end-user could choose which correction to apply.
OPeNDAP_demo.m (MATLAB, 1 KB)
OPeNDAP_demo.m |
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OPeNDAP_demo.pdf (PDF, 829 KB)
OPeNDAP_demo.pdf |
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plot_google_map.m (MATLAB, 14 KB)
plot_google_map.m |
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Disclaimer and Citation
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These data are provided 'as is' for use in oceanographic research. They are not intended to aid navigation or for operational use. When using the data, please credit the Environmental Protection Agency for funding and the project team of: Washington Dept. of Ecology, Univ. Washington, Washington Dept. of Transportation, and Integral Consulting.
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