SOLVING THE ‘MOUNTAIN-LAKE’ PUZZLE: APPLICATION OF SCUBA–ASSISTED VIDEOGRAPHY, SIDE-SCAN SONAR AND HIGH-RESOLUTION, ACOUSTIC DOPPLER, CURRENT METERING FOR DETERMINING BOTTOM FLOWS

Advances in electronic flow-meters have expanded the application-range of such devices from relatively fast moving open-ocean to much more quiescent lacustrine environments. In this study we evaluate use of a “Nortek Aquadopp Doppler Profiler-TM” (NADP) for measuring ultra-low velocities referenced to compass directions at “Mountain Lake” in Giles County, Virginia. One of only two natural lakes in Virginia, “Mountain Lake” has been documented as fluctuating from full-pond to a completely-dry lake bed and vice versa. Such significant fluctuations in water-level have had a direct negative impact on local economy through loss of revenue of over a million dollars a year for the past ten plus years. Even though such fluctuations at “Mountain Lake” have been studied by multiple universities (Kent State, Virginia Tech, and Radford University) and techniques – this study is innovative in being the first to measure water-column velocities using an acoustic doppler approach.

The hypothesis of this study was that water was exiting “Mountain Lake” through a series of piping holes in larger bowl-shaped depressions with very low velocities that could be documented using underwater videography and multiple deployments of a NADP. Methods involved: (i) Conducting detailed bathymetric mapping with side-scan “Search & Rescue SONAR”, (ii) Identification and under-water GPS tagging of piping holes, (iii) SCUBA diving in targeted depressions for obtaining video footage of bottom sediment transport in addition to hour-long deployments of NADP device, and (iv) Processing velocity and direction vector data sets in multiple layers of the water-column to determine dominant flow paths and velocities.

Results show a correlation of bottom flow (as documented via underwater videography) with flow speeds and direction data (obtained via NADP). Our data also highlights the usability of such a NADP for detecting low-velocity bottom flows in a lake environment. Lastly, a comparison of imagery from dry, lake-bottom times with dominant flow velocities and directions suggests a control of incised erosional channels on flow with increasing water-levels. However, further research is needed for establishing the magnitude of control of channel bathymetry versus rising lake level on bottom flows.