SF Bayweb 2009 Part 2: Planting the seeds of an observing system in the San Francisco Bay

Using lessons learned during SF Bay Web part I, a second field experiment was conducted during July 24 - August 6, 2009 with the goals to: 1) Obtain data from Raccoon Strait, where the mooring failed during Bayweb I; and 2) To successfully move the data from the instrument to the laboratory in real-time. Since a few weeks of data to accurately describe tidal variability are extremely valuable, and good data had already been obtained from the Main Channel, the decision was made to place two real-time Stablemoor systems in Raccoon Strait (on either side of the sill) (Figure 1). This decision was further supported by the ship-board surveys conducted during SF Bayweb I, which suggested hydraulic control in the Strait and flow separation across the top of the sill, especially during the peak ebb tide. These two goals were partially realized: a 12-day record was obtained from ADCP 21 on the SW side of the sill, and four days were obtained from ADCP 22 on the NE side (Figure 2). The partial failure of ADCP 22 was curious: the ADCP was still pinging on recovery, but had stopped recording data on day four. We hypothesize this was caused by an extraneous character sent by the network, which contained a leading dollar sign ($) and confused the instrument. The real-time networking in Bayweb II, as in Bayweb I, was unsuccessful. The repeaters had difficulty communicating at less than 0.6 km, due to the very high ambient noise in the area. Following system repairs and relocation of the repeater nodes, there was a terminal failure on the gateway buoy. Much was learned however regarding acoustic networking in high-current, high ambient noise environments.

The preliminary scientific results are fascinating. The flow below 20 m was indeed close to zero on the SW side of the sill during surface ebb tides exceeding 1.5 m/s (Figure 4). This phenomenon was also evident, although much less so, on the NE side during flood. Strong fronts were also easily visible on the sea surface. Understanding these controls on the flow through the straight will provide important physical insight for the evolving community circulation model of the San Francisco Bay.

San Francisco Bay Web II Map

Map of San Francisco Bay with ADCP locations

Figure 1. Locator map of the San Francisco Bay showing Angel Island, the Tiburon Peninsula, and the positions of all the in-water assets deployed during SF Bayweb II. ADCP 21 ( Raccoon Strait West ) and ADCP 22 ( Raccoon Strait East ) were deployed on the SW and NE side of the sill, respectively. The gateway node deployed on a USCG channel buoy is indicated by the green triangle. The bottom repeater nodes are indicated by the yellow circles.

ADCP pressure plots

Figure 2. Pressure fluctuations observed by the two ADCPs, mounted in the Flotation Technologies Stablemoor buoys. The tides were mixed, semidiurnal dominant, with a clear fortnightly beat. These records result from a combination of mooring motion and the actual tide itself.

 

Temperatures from the ADCPs

Figure 3. Temperature as observed by the ADCPs at Raccoon Strait East (red) and West (Blue). Temperatures averaged 3 deg C warmer than during SF Bayweb I in May.

 

Raccoon Strait (West A21)

Contours of Velocity Along/Across Channel

Figure 4. The along- (top) and across-channel (bottom) currents at Raccoon Strait West. An optimal rotation of 31 degrees was determined via a principal component analysis.

Raccoon Strait (East A22)

Contours of Velocity Along/Across Channel  

Figure 5. The along- (top) and across-channel (bottom) currents at Raccoon Strait East. An optimal rotation of 48 degrees was determined via a principal component analysis.

Comparison plot of along channel currents

Figure 6. A comparison of the along-channel currents on the East (top) and West (bottom) side of the sill in Raccoon Strait. The currents were strongly influenced by topography, as discussed in the text.

 

 

Pitch and Roll data plots

Figure 7. Pitch (top) and roll (bottom) from the Raccoon Strait West (blue line) and East (red line) moorings in the San Francisco Bay. The zero-speed values showed a mean offset of about 9, 14 degrees for the pitch, roll respectively for both moorings. The tilt improved however as the currents increased and the buoys leveled out. The tilts did not degrade the ADCP performance.

Google Earth KML file of ADCP2 data

Note: You will need Google Earth with terrain on. Use the wrench tool to adjust the speed.

To watch both current profiles simultaneously in Google Earth

  1. Save both files to disk
  2. Open Google Earth
  3. Right click on the temporary places folder in the left-hand column of Google Earth (under Places)
  4. In the menu that pops up, mouse over Add and then mouse over Network Link
  5. Click Browse in the window that pops up and add the location of the kml files which you downloaded
  6. Run the animations using the time-slider in the upper left of the window with the Globe
Note: since ADCP 22 stopped working before ADCP 21 you will see one of the profiles disappear.

These instruments and moorings were provided by the Naval Postgraduate School. The communications network used to transmit the data to shore was a collaboration between the Navy and the U.S. Coast Guard.

DATA LINKS

The raw data files require the use of WaveMon3 from Teledyne RDI to read properly.

An explanation of the Matlab file variables used can be found here.

ADCP 21 was located at 37 51.86' 122W 26.88' in 35 m of water, times are UTC!

ADCP 22 was located at 37N 52.09' 122W 26.59' in 39 m of water, times are UTC.