NOAA CO-OPS conducted an ADCP (acoustic Doppler current profiler) survey of Portsmouth Harbor and Great Bay Estuary during the summer 2007 field season. Eleven stations were occupied during the deployment. Two of these, PIR01 and PIR02, are located within my study area. My objective is to harvest the time-series of current magnitude and direction from these two stations, in order to model bed-load transport in the estuary under the observed current conditions.
But first, a little about the data. Each station was instrumented with a 600 kHz RD Instruments (RDI) Workhorse Sentinel ADCP with trawl-resistant bottom mount and Benthos acoustic transponding release. Each ADCP was equipped with pressure, pitch, roll, heading and temperature sensors. The data (current velocity) were acquired in earth coordinates and were corrected for platform attitude and magnetic variation at the time of acquisition. For more about ADCPs in general, check out the RDI-published Principles of Operation, which is a very good resource (I think).
Here are some stats about the two stations I’m using:
Station PIR01
Location: (large bedform field south of Fort Point)
Deployment period: 9 may 2007 – 12 july 2007
ADCP serial #: 7452
Water depth: 15.5 mDistance from seabed to center of first bin: 2.96 m
Accuracy given by PLAN: 0.46 cm/s
Station PIR02
Location: (100m ESE of small bedform field north of Fort Point)
Deployment period: 9 may 2007 – 19 june 2007
ADCP serial #: 3030
Water depth: 14.4 m
Distance from seabed to center of first bin: (**check this**)
Accuracy given by PLAN: 0.46 cm/s
Soo….. how to get the data off the ADCP and into a useful form? I used the following steps:
- Convert ADCP binary file to ASCII format. Jim Irish did this using BBList. This outputs an ASCII file with date/time, measured depth, velocity north/east/west, backscattered amplitude for beams 1-4, current speed, current direction, and range from transducers.
- Sort ASCII data into a more friendly format (a MATLAB structure) for processing. I modified Jim Irish’s code (sortadcp.m) to do this.
- Process ADCP data. This includes calculating current magnitude and direction, and trimming off all of the “bad” bins (e.g. above the water surface). Jim Irish’s script calculates magnitude as the RMS value of northgoing, eastoing, and vertical velocity. Jim’s script also calculates current direction, but it looks kind of wacky, so I used the ADCP-calculated values instead, which seem about right. As for trimming, I ditched everything above Bin 12, which is centered on 14.96m above the seafloor (MLLW is 15.5m).
- Plot data. Woo-hoo!
