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

 

Boat Races

If you have spent time in southern California, you may know what it means to “race for pink slips.” One fine day in May, the Falk-Guza (FG) and Britt-Steve (BS) groups will race boats for pink slips.  The FG boat, the “Pink Storm” already is pink (see photo below) and has a cool, but scary masthead.

The BS boat, the BAD boat (photo of a similar one is below), is not pink, but it is fasssssssssst.

 

Side bets are encouraged. We suggest an anti of an ADV, jet ski, or REMUS.

Remote sensing devices will be discounted appropriately.

 

Stayed tuned for exctiing photos of the PVLAB c-vans about to be shipped.

 

Steve

NEARCOM model analysis of Tides by Tom Hsu et al.

by Tom Hsu, Fengyan Shi, and Julie Chen (UDEL):  I just like to follow up with Falk’s comments on time series and phase lag between surface elevation and flow velocity magnitude. More information from the model run is attached. In this document, you will find time series at four different locations. Please also notice that in these figures, we show tidal velocity magnitude (not individual components), which is probably more representative. Basically, max ebb occurs 0.5~1 hour before low tide.

As Falk mentioned, the results seems to be sensitive to bottom friction used. We are evaluating the sensitivity and will give you an update soon. We also plan to run realistic tide, which can make a difference in phase. Once we have update, we will post on the blog (and we may provide a link where you can download more specific and detailed data from our own website).

Bathymetry:  from Summer 2011

Offshore Conditions:   Normally incident waves Hs=1 m.  Drag coef Cd=0.01, Tide Level  =0.623 m

Max EBB

Max Flood

Time series of tidal elevation and tidal velocity magnitude:

Point 1:

Point 2:

Point 3:

Point 4:

Schedule for Dye and Drifter Releases

The schedule for RIVET dye and drifter releases have been tentatively schedule for:

  • May 1, 2, 3, 4:  Drifter Release Experiments
  • May 5 : Turnaround Instrument Day
  • May 6,7,8 : Dye Release Experiments with Airborne Dye Observations
  • May 9th:  Turnaround
  • May 10,11, 12:   Dye Release Experiments with Airborne Dye Observations
  • May 13th : Turnaround
  • May 14, 15, 16, 17:  Drifter Release Experiments
  • May 18th:  Turnaround
  • May 19, 20, 21:  Dye Release Experiments:  no airborne dye observations

Some details on the drifter and dye releases:

  • SIO (Feddersen/Guza) have 30 surfzone capable GPS tracked (and transmitting to shore) drifters which will be released on drifter days.     The plan is to release on ebb tide and track the drifters for as long as possible – hopefully into the flood tide.  Please see earlier post on numerical simulations of drifters at New River Inlet.
  • On the dye release days, we will be releasing 20-30 gallons/day of 23% Rhodamine WT dye.    This will be tracked at fixed locations (WireWalkers, ADV, and ADCP locations) and via mobile platforms at the surface by jetskis (and plane), and subsurface by Boat and REMUS (Terrill).

WHOI (Raubenheimer/Elgar) & SIO (Feddersen/Guza) instrument plan (draft)

Below is a google earth image of the WHOI (Raubenheimer/Elgar) & SIO (Feddersen/Guza) sensor deployment plan.  We coordinated last week to get the best coverage from all of our sensors.   Steve Elgar made the nice map below.  It will change once we get the new survey after the dredging,  but this gives an idea of # of instruments and their distribution.  Note, the red symbols are WHOI (Raubenheimer/Elgar).

  •  Black center = ADCP + pressure
  • Red = ADV + P

The other symbols are SIO (Feddersen/Guza)

  • White Squares = ADV (vector, denoted SIO-V*)
  • Magenta circ = Aquadopps  (denoted SIO-A*)
  • White Diamonds = WireWalker (CTD+F) + ADCP:  the WireWalker are set up in a cross-shore array from the entrance of the dredged channel.  One in ~8 m depth and another in ~ 12 m depth.  Each WW will measure dye and turbidity with an Ecotripplett (ET)
  • Additional: 13 WetLabs EcoTriplett (ET) (not shown in image) to deploy across these locations to meet two goals 1) measure the dilution downstream from the release point 2) measure something like the total flux in and out of the inlet.  To this end one plan is to deploy ET near: a) for Downstream Dilution:  04, 0a, 06, 08, 0c;  and (b) for Flux:  85, 86, 87, 78, SIO-V7, SIO-V6, SIO-A3, 17

The thick white line is the location of the new (to be dredged) channel.  The very thin white lines mark the location of the “current” channel.    The medium white contour marks an educated guess of the 4 m depth contour.

There are many other instruments being deployed by various groups (NPS/UMiami, UNH, SIO-Terrill, WHOI-Traykovski/Geyer) among others.

 

drifters on NEARCOM model currents

In order to get an idea of the offshore extent of the tidal jet, exchange, and overall dispersion, Matt Spydell has taken the NEARCOM (tide+waves) model output provided by Tom Hsu, Fengyan Shi, and Jia-Lin Chen @ UDEL and released numerical drifters inside the inlet at the start of the ebb tide.  These drifters are advected by the currents and also randomly dispaced in a random walk (for details see Spydell et al. JGR 2012 in press) with a diffusivity of 0.1 m^2/s.   Arguably this is small for the inlet but ….

The top panel shows the tidal elevation.  time=0 is 1.5 hr after high tide.  The panels below show the numerical drifters as red dots.  At t=1min after release, they are tightly clustered near the release location.   The drifters then get ejected out the inlet after 1 hour.   They spread and eventually some start to get pulled back to the inlet on a flood tide (t =480 min, 8 hrs after release).

It is interesting to compare this to releasing drifters right at high tide (1.5 hours before the release above).   Now, the drifter rush all the way up the inlet before turning around on ebb tide and being ejected roughly 6 hours after release.

Note, in this simulations after 8 hrs, some drifters are more than 3 km from shore!

NEARCOM tidal simulation results

In order to plan drifter and dye deployments as part of RIVET at new river inlet, we’ve been looking at model simulations from UDEL (Hsu et al. – NEARCOM),  U Miami (Reniers et al. – Delft3D) as part of the DRI, and also UNC (Leuttich et al. – ADCIRC) who have graciously given us some model simulation data.    We’ll be posting results from each of the simulations here.

First up is NEARCOM (U Del).    They ran 3 scenarios.   They are:

  1. M2 Tides + normally incident waves (offshore Hsig = 1 m)
  2. M2 Tide only (no waves)
  3. M2 Tide + waves but with drag coefficient 10X larger

The (U,V,eta) over the inlet domain was decomposed into principal components to get major axis and minor axis tidal ellipses.     Then the phase lag between the tide level (eta) and the major axis tidal velocity was computed (zero is a progressive wave, 90 deg is a standing wave).   Below is shown (left panels) the phase difference and (right panel) std of major axis velocity (multiply by sqrt(2) to get peak amplitude)  for the 3 cases above.

Things to note:

  1. The waves do not seem to have a strong effect on the tidal currents
  2. Friction makes a big difference
  3. It is hard to tell how many hrs the velocity lags the tide level in this plots.   Soon we will have ADCIRC and Delft3D runs analyzed to compare to.   This will make it easier to see.

For the tide+waves NEARCOM (#1 above) run a time series is a little clearer.  Below is a eta (blue), U,V (red,green) timeseries over 12 hours of at one location inside the inlet.

Note how the maximum in tide occurs at t=4 hrs, but the velocity does not start to ebb (switch sign, see red) until t=5.75 hrs or so.

RIVET Blog Site and RIVET coordinate system

The idea of this blog site is to have a place to put all the pre and post experiment ideas and analysis for the entire RIVET teams to share.   This should serve as a supplement to the dri-nr mailing list.

WE’ll be posting pre experiment model simulation results soon.

Also: don’t forget the RIVET coordinate system information can be found at:

http://iod.ucsd.edu/~falk/RIVET/