Delft3D dye simulations of New River Inlet (w. Ad Reniers & Patrick Rynne)

The youtube movie below shows Log10 dye concentration (no units) at New River Inlet released just up the inlet over t=2-4 hr.    This simulation is forced by the M2 tide and has no stratification, no wind, and no waves.  Top: time series of tidal elevation at (x,y)=(0,0) m.   The red dashed line represents the current time location.    Bottom:  plan view of Log10 surface dye concentration.   Only a subset of the model domain is shown.   Land values are white and dashed lines represent bathymetry contours (2, 4, 6, 8, 10, 12 m depth).    The colorbar gives log10 concentration: Check the video below:

I also looked at the bottom dye concentration.   Although there are subtle differences the movies look pretty much the same.   For these purposes this unstratified simulation is pretty well-mixed.

At first release, the dye jets about 2 km offshore of the inlet mouth.    There is a significant amount of dye flux back into the inlet.    However, this may be a result of no wind, no waves, no  alongshore tidal propagation, and no other shelf-scale forcing mechanisms that drive shelf-scale alongshore currents.

If folks are interested in the .mp4 video file (better resolution) it can be downloaded via NRI_dye_surface_delft3d

Please feel free to leave comments and suggestions.   -falk

NEARCOM tidal analysis – delta t from high tide to water ebb (w. Matt Spydell)

In the previous post on Delft3d model output,  we showed in one of the panels, the time it takes from high tide until the water starts to ebb.    Notationally this could be written as t_{high tide} – t_{Uebb},  where t_{Uebb} is the time when the alongchannel tidal velocity switches to ebb (out of the inlet).     This time lag is related to the M2 eta/V phase lag, but is slightly different due to skewness in the tidal velocities related to the water level going up and down.    To better compare the two models, we made the same calculation with the 3 NEARCOM model runs – see previous posts.  They are  (a-top) waves+tide, (b-mid) tide only, and (c-bottom) 10X drag coefficient, waves+tide.   The results are shown below.  The left panels are the time lag from high tide to velocity, the R panel is the std of alongchannel velocity (the same as in previous posts).

The results of the tide-only run are broadly consistent with the analysis of the Delft3D model output.   Although the coordinate system that NEARCOM and Delft3D are using are different, the pattern of time-lag is similar for the tide only case.   Note also that for 10X higher drag coefficient, the tidal velocities and phase radically change.    Both Delft3D and NEARCOM model data are output at 1/2 hr intervals (99% sure).   This limits the resolution of estimating this time lag.    However, based on the various simulations, it appears that an average of 2 hr lag inside the inlet is appropriate.   -Falk

Delft3D model tidal analysis (w. Ad Reniers & Patrick Rynne)

In order to understand how to time drifter and dye releases, I’ve become preoccupied with the New River Inlet tidal currents and the phase lag relative to the tide level (call this eta).   As is well known a standing wave has a 90 deg (pi/2) phase lag between eta and velocity and a progressive wave has a 0 deg lag.    In a tidal inlet, the tide is somewhere in between the two.    Another way of asking the question is how soon after high tide will the water velocity ebb (go out the inlet) – that is.  time of max (eta) – time when U switches to flood.   Here, numerical models can help us, although one must be careful as the tide-velocity phase relationship depends on the friction inside the inlet.

In a previous posts, NEARCOM model results were shown.    Ad Reniers and Patrick Rynne at Univ. Miami have kindly provided me with some Delft3D M2 tidal model simulations of the New River Inlet.  I believe this is the same bathymetry as for the NEARCOM model runs.     Patrick sent me the data over 4 M2 tidal cycles output every 1/2 hour.    Results are shown below.     The panels are (top,L ) bathymetry, (top,R) M2 tide magnitude [i.e., eta = eta_0 cos( omega t) ] ,  (mid, L)  major axis velocity magnitude, (mid,R) minor axis velocity magnitude, (bottom, L) phase lag between eta and major axis velocity and (bottom, R) time in hrs from high tide to when water starts ebbing.

Note that there are similarities and differences between the Delft3D and NEARCOM simulations.    From these plots it is hard to tell what the tide magnitude, currents, and phase is within the inlet.    Below is the same quantities at the deepest portion of the inlet at any cross-shore position.  The bathymetry panel (top, L) marks the deep channel locations with white asterisks (squint cause they are hard to see).

As expected, the tide magnitude decreases as one goes up the channel (top, R).  This is fairly linear.   The major axis velocities (~ 1 m/s) also sorta decrease but oscillates, because the channel depth is not regular.    The minor axis velocities are small.    The phase lag is interesting.   Near the mouth (x ~ 0 m), the lag is ~21 deg – far closer to a progressive wave than a standing wave.   This corresponds to a 2.5 hour lag between high tide and water velocity beginning to ebb (the phase difference and time lag are not excactly related because of nonlinearities and skewness in tidal velocities).   The phase slowly increases up the inlet (more negative x).  And the time to go from high tide to ebb velocity also decreases.

We will compare these results with the other models (NEARCOM and ADCIRC) coming soon.  -Falk