A. C. Redfield

I'm in the process of reading The Tides of the Waters of New England and New York. Woods Hole 

Oceanographic Institution. Taunton, Massachusetts: William S. 

Sullwold Publishing, Inc., by A. C. Redfield.

Yes, the double tops (plateaus) on the high tides are real, and described in this book with reference to Falmouth Harbor which is near by.

Redfield's book can be found on line here: 

https://darchive.mblwhoilibrary.org/handle/1912/1136

A great read.   

Here's an engineer's (not oceanographer) summary.  The tidal range at Falmouth in Vineyard sound is very small (around 1.5 feet).   As the tidal rise (wave) moves toward the coast, it gets into the sound through 2 major openings, one around Monomoy, the other near Cuttyhunk.   These waves destructively interfere (a node) around Falmouth.   Thus the small tidal range in Falmouth becomes progressively larger as you proceed east or west.

The tide consists of multiple harmonics.   Some are semidiurnal, and some are diurnal.   Semi are roughly 12 hr, diurnal are roughly 24 hr.   The harmonics are causing the double tops. 


The primary harmonics are:


M2: the moon which is semidiurnal - 12.42 hr period
S2: the sun which is semidiurnal - 12.00 hr period
K1: a lunar diurnal component - 23.94 hr period
O1: a lunar diurnal component - 25.83 hr period
P1: a solar diurnal component - 24.06 hr period
K1, O1, and P1 are related to declination.


M2 and S2 are the big components, but Vineyard Sound is tuned to reduce these components.   These tide components propagate from Cuttyhunk and Monomoy and nearly cancel near Falmouth.   The K1, O1, and P1 harmonics are smaller but are different period, so the Sound is not tuned to cancel them at Falmouth.   Their small amplitude appears relatively larger as M2 and S2 are reduced, resulting in the strangely shaped waves.

Any readers of this blog that are tide experts, please comment so I get this right!!

  

Remote LCD Monitor Packaging

Not beautiful, but better than exposed circuit boards, LCD display, and wires.

Filtering Artifacts and the Impact of Waves

Initially, when debugging the tide sensor, there were artifacts:

Each plotted tide level is an average of 10 samples taken at 1 second intervals in an attempt to average out wave action.  Something was causing the downward slices in the waveform.

The tide sensor is in a bay where strong winds can deliver up to a 1 foot chop.   When these strong winds occur, the data would exhibit the artifacts.   On an async terminal, I can look at the data stream and see the raw individual samples before averaging.   The individual samples where usually sensible; however, with high winds and wave action some samples would be 2X the others, or just very large numbers.

I believe the 2X numbers can be explained as follows:

When the ultrasound pulse hits a wave front, it's reflection can be directed by a wave under the dock.   The resultant return pulse hits the bottom of the dock, back down to the water, and back to the transducer.   The result is 2X the expected distance.   

Very large numbers can be explained by a total miss.   The wave front deflects the pulse off into space (away from the dock), and the sensor never sees the return.   

A filtering algorithm that has been running for the last week removes these artifacts out by tossing out samples that don't make sense.    The sensibility test involves looking at past tide heights, and assuming a very large change in a 15 minute interval is not possible (unless the moon explodes or something like that!).   

Here's the filtered data which I've been taking for the last week or so:

I am curious if the plateaus at high tide are real, or some other artifact of the sensor.   I'll be looking at that next.

The Storm

Here's another tide plot from the log at ioBridge.com.  

Yesterday, we had a very strong easterly wind associated with the large storm headed up the east coast.   Can we attribute the higher low, and higher high tides to the east wind?  See the last 2 hills and the valley between.   Note this AM's valley is much lower.    Barely a ripple on the water this AM.

A Few Days of Log Data

Here's a few days off the log.  We are having a good storm now with SE winds and lots of rain with flooding inland.  So far so good.

Data Log

IObridge.com provides an on line logging service.   Every 15 minutes, we are posting tide height in inches to the log.    Here is a graph of recent log output:

The tide level is measured in inches vs. a bottom reference at the measurement location.   Time is on the X-axis.

I'll be watching the tide in this coastal estuary on Cape Cod vs. reference tide information in Boston.

Some additional signal processing is required to average out wave action.   I'll review more of this in later posts.

Updated Block Diagram

The system now consists of 3 elements:

1.  A tide sensor on the dock.
2.  An internet interface.
3.  A remote tide level monitor with an LCD screen.

The interconnect is based on Freewave 900 Mhz spread spectrum serial radios.   They are running in multipoint mode.

TIDE SENSOR:

The tide sensor is powered by a 12 volt battery charged by a solar panel.   The sensor is based on a Parallax Ping (parallax.com) ultrasonic transducer, connected to a Arduino microcontroller (www.arduino.cc) with a custom program (sketch) developed by the author.  The program measures the distance from the Ping acoustic transducer to the water.   In oder to calculate depth, vs. a reference, the user  enters an offset (distance from the transducer to the bottom) in inches through the serial interface on the Arduino through a simple user interface.

The Arduino can be set to take continuous samples or samples every 15 minutes.   Continuous sampling mode is used during initial installation to make sure everything is working.  Each sample consists of 10 acoustic pings taken at 1 second intervals and averaged.   The averaging is an attempt to remove wave action from the tide calculation.    Samples are transmitted on the serial interface to the Freewave Radio (freewave.com).

















INTERNET  INTERFACE:

The internet interface is based on an IObridge IO-204 with a Serial Smart Board SB-5 (see IObridge.com).   Another Freewave radio receives the serial data stream from the TIDE SENSOR forwards it to the internet.   IObridge has excellent internet logging services, and the ability to tweet and/or email this information.   I'm tweeting tide levels now on twitter account Capecodda.


TIDE LEVEL MONITOR:

The tide level monitor is an Arduino micro-controller with an LCD interface connected to a 3rd Freewave Radio.   The serial data stream is parsed by the Arduino and a tide level is displayed.   A short custom program (sketch) that parses the serial data stream and displays tide height was developed by the author.
Next step is packaging.