Using the Acoustic Doppler Velocimeter (ADV) in the MUDBED real-time observing system

Abstract

As part of the National Science Foundation MUDBED (Multi-Disciplinary Benthic Exchange Dynamics) project, we have deployed 5 MHz Sontek ADVs at two muddy sites along the York River estuary for the last 3 years. One of the two MUDBED Observing System sites is more biologically dominated, whereas the other is more physically dominated. At both sites, internally recorded ADV data have proven invaluable in allowing reliable long-term estimates of water velocity, bottom stress, suspended sediment concentration, sediment settling velocity, and bed erodibility under spatially and seasonally variable conditions. Nonetheless, it has been challenging to reliably collect these ADV data in a real-time mode. Working with Franktronics, Inc., an automated terminal emulator has been developed to allow ADV data to be logged internally and burst data to be automatically transferred off the internal logger every 15 minutes in near real-time. To facilitate wireless data transmission, we have placed a serial-to-Ethernet converter in an underwater housing on our benthic tripod. This allows us to transmit near-bed ADV data via an Ethernet cable up to a relatively small surface buoy, wirelessly transmit the signal via an Ethernet radio and omni-directional antenna on the buoy to a nearby stationary platform, and relay the ADV data via a second Ethernet radio and a uni-directional antenna back to VIMS. At VIMS, the data stream is received into a local intranet, which isolates the wireless Ethernet links from general internet traffic. To date, the results of ADV deployments at the MUDBED observing system sites indicate that settling velocity tends to be higher at the biological site, whereas suspended sediment concentration and seabed erodibility tend to be higher at the physical site. In addition, sediment settling velocity and bed erodibility are inversely correlated in both time and space. Finally, settling velocity and erodibility remain more consistent in time at the biological site, whereas erodibility increases and settling velocity decreases at the physical site following winter/spring increases in river water discharge.