RESULTSFigure 3 depicts time series of water elevation and burst-averaged velocity. Temporal breaks around low tide denotetimes when water levels fall below the height of the pressuretransducer. The time series covers four tidal periods, includingtwo low high (LH) and two high high (HH) tides. Along-channelcurrents are strongest during flood, and maximum speedsprecede maximum water elevation. Maximum speeds arestronger during the HH tides, because a larger volume ofwater must pass through the channel in the same amount oftime. Maximum flood currents are higher over the sandbank.Maximum along-channel ebb currents are weaker.The cross-channel current is weaker, but indicates a netlateral circulation on both sides of the channel that varies as afunction of bank elevation and tidal stage. During the first twotidal periods, an initial positive cross-channel current (towardthe eastern side of the bank) continues until around peak flood.The lateral current then switches direction to flow more alongthe channel axis or slightly toward the western side of the bank(oyster). During the last two tidal cycles, the same pattern isseen on the eastern (sand) side of the channel, but on thewestern (oyster) side, the current is directed up the bankthroughout flood. The net effect is a slight clockwise rotation ofthe horizontal current vector around mid-tidal stage when thespeeds are strongest. The rotation is related to overtopping ofthe channel banks, in which the current veers toward the westto flow over the submerged shoal that extends beneath theboardwalk. During ebb, the cross-channel current is highlyvariable and generally does not indicate a clear trend, except onFigure 3. Time series of the burst-averaged velocity. Water elevation isshown to illustrate corresponding tide stage. Interruption in the time seriesaround low tide denote periods when the water level dips below the height ofthe sensors. Water elevation is from the pressure sensor deployed on theoyster bank. From hour 0 to hour 24 the ADVs were deployed at the lowerelevation on either side of the bank. At hour 24 the sensors were repositionedto the higher locations on the banks.Journal of Coastal Research, Vol. 31, No. 4, 2015980Styles
the sandbank during the HH cycles, in which the current isdirected up the bank toward the east. This is consistent withthe direction of ebb in the system because the cross-channelcurrent flows from the shoal to the west. The lateral currenteventually vanishes as ebb continues and water levels fallbelow the top of the bank.The vertical velocity component at both locations during thefirst two tidal periods is negative around maximum flood andvery weak (,2.5 cm s±1). During the last two tidal periods,current speeds on the oyster bank remain negative but switchsign over the sandbank. This is consistent with a helical flowpattern, with downward flow on the western bank and upwardflow on the eastern bank. Like the horizontal components,vertical velocities are highly variable and generally weakerduring ebb.