By Craig Bennett
Teledyne RDI's unique five-beam configuration Sentinel V ADCP proved to be the perfect tool for Saltwater Intrusion Analysis. The Sentinel V's vertical fifth beam and the data it produced gathered a more complete picture of the conditions that influence saltwater intrusion.Read more
The Department of Civil and Coastal Engineering at the University of Florida Analyzes Saltwater Intrusion Conditions and the Impact on Coastal Communities
Freshwater Freshwater for many coastal communities across the globe is derived from groundwater aquifers, and that freshwater is being threatened by saltwater intrusion. Experts in the field argue that this threat will substantially increase as the Earth’s sea levels rise. To get a more complete picture of the scope of intrusion at a particular site, researchers must analyze the outflow rate and discharge from freshwater springs. Previous studies show that the outflow rate from freshwater springs is moderated by three processes: tides, waves and remote currents. Spring groundwater discharge to the coastal ocean is suppressed during periods of active waves, high tidal elevations, or coastal setup from weakened remote currents.
To study turbulence in coastal environments, Acoustic Doppler Current Profilers (ADCPs) with four Janus beams have been used traditionally to calculate the Reynolds stress terms with the assumption of homogenous turbulence.
Ongoing studies show that ADCPs with a vertical fifth beam, such as the five-beam Sentinel V, allow calculation of the vertical component of the turbulent kinetic energy. The fifth beam also provides information to test the assumption of horizontal homogenous turbulence.
Location: Puerto Morelos, Quintana Roo, Mexico
A recent study led by Dr. Arnoldo Valle-Levinson of the University of Florida analyzed the conditions that influence saltwater intrusion at the Puerto Morelos reef lagoon in Quintana Roo, Mexico’s easternmost state, on the Yucatán Peninsula. The Sentinel V’s five-beam configuration—and the raw data it produced—was required to properly analyze turbulence parameters at the site. The Sentinel V’s unique vertical fifth beam allowed researchers to measure the vertical velocity (flow direction) at the site. This is important because the fluctuation of the vertical velocity is used to calculate the changes in the intensity of the turbulence and infer the mixing of the plume discharge.
The Sentinel V was deployed in 5.5 meters depth with its heading reference beam 3 aligned with magnetic north for six days from September 11th through September 17th, 2014. It was configured to collect velocity profiles in 10-minute bursts (1200 pings) every 30 minutes with a 0.3 meter vertical resolution. The raw data stored in the beam coordinate frame allows the calculation of turbulence parameters in post-processing.
Figure 1: Averaged Sentinel V data from Teledyne RDI's velocity software
Figure 2: Mean vertical velocity data using Teledyne RDI's software
Figure 3: Raw single-ping data as shown using Teledyne RDI's software
The raw ADCP data were processed using Teledyne RDI’s velocity software to produce depth-averaged current speed and direction. The background currents at the site were consistently less than 15 cm/sec directed to the south (180 degrees) with episodic pulses of up to ~30 cm/sec that do not appear to be correlated with the waves or tides.
Additionally, evaluation of the mean vertical velocity from the Sentinel V’s vertical fifth beam showed researchers large pulses of water flowing toward the surface. These large pulses correlated with the pulses in the mean horizontal velocity.
Lastly, single-ping data on September 13th, from 7:30 to 7:40 in the morning, revealed to researchers extremely high temporal variability in the echo strength as the plume moved in and out of the Sentinel V’s fifth beam. Of interest are the apparent downward velocities immediately after the plume passes.
The Sentinel V’s vertical fifth beam—and the data produced from it—allowed researchers from the University of Florida’s Department of Civil and Coastal Engineering to gather a more complete picture of the conditions that influence saltwater intrusion. Further post-processing of the vertical beam data from the Sentinel V by the team will allow a better understanding of the relationship between the intensity variations in the turbulent mixing of the submarine groundwater discharge and the risk of intensified saltwater intrusion with rising sea levels.