Dataset: Tidal water velocities in Groves Creek salt marsh, Skidaway Island Georgia, USA, 2013-2014

ADP field sampling:
Nortek Aquadopp 2MHz ADPs (AQD 6204 & AQD 9331) were used to  measure water velocities and tidal height in the tidal creeks at stations S0, S2, and S8.  ADP transducer heads were coated with a mixture of cayenne pepper and petroleum jelly to discourage hard fouling.    

Units were affixed to a concrete plate that was lowered from a small boat by hand and rope to the bottom of the creek channel.  Deployments were blind: effort was made to assure that the units were oriented channel-parallel and on flat bottom, but the exact location and orientation of each deployment depended greatly on the swing of the boat’s anchor in the current.  The a priori objective of each deployment was to locate a unit in the center of a straight section of creek seaward of the water quality sampling stations.  Each ADP mount was fixed to a weighted line that ran to the nearest shoreline where it was anchored and marked with a buoy.  Individual deployments lasted 8-10 weeks.  Units were recovered by retrieving the line at the shore and hand-hauling it to the surface. 

ADPs were returned to the lab for cleaning, downloading data, charging batteries and setting up the unit for the next deployment.  Units were re-deployed at the next convenient opportunity.  Gaps in the record were on the order of a week to a month.

Over the study, the ADP measured temperature (°C), velocity (u, v, w in m s-1), and pressure (decibars converted to water level (m) relative to Mean Sea Level) at a 10-minute interval. Velocity was measured across 18 to 20 vertical bins with each bin covering 30 to 40 cm. This bin setup allowed maximum water level (2 m above Mean Sea Level) to be captured by the ADP. To correct for minor differences in ADP positioning on the creek bed between deployments, velocity and water level records underwent alignment.

ADP data processing:
To process in situ data, ADP deployment records were linearly interpolated to the same 10-minute time grid across the study, and the start and end of individual ADP deployments were determined (first or last measurement above 0.35 m relative to channel bottom). Furthermore, out of water ADP measurements (pressure <2 decibar) were eliminated as were vertical ADP bins at depths above the water column and untrustworthy surface bins impacted by side-lobe interference. Average velocity (u, v) was calculated across the remaining bins. Using average binned velocity provided a continuous velocity record. A vertical average was an appropriate approximation for velocity in a well-mixed system such as Groves Creek (Blanton et al., 2010; Sullivan et al., 2015). Resulting velocity measurements (u, v) were converted to along and cross channel velocity by rotating velocity vectors per deployment by their respective principal axis angles.