USCG Tide and Current Problems


#1

My post here is 2 fold…

  1. Provide some competitive banter amongst mariners about whether the USCG databank provide ANY realistic education for upcoming mariners - I would argue yes and no, but can make a strong case for no. *Of the 86 Tides and Current problems on lapware, I have successfully completed about 75.

  2. Educate mariners that want to learn more - This will consist of an excerpt from your boy Nathaniel Bowditch as well as small piece I put together on a pretty cool book I’m reading called Tides: The Science and Spirit of the Ocean by Jonathon White. I am about halfway through this one, but so far, I can recommend it for anyone looking to gain further knowledge on the matter. The author is passionate about Tides. He ran aground on a sailing vessel in his younger days due to tides to currents and vowed to never do it again by understanding this matter further (he discusses this in the intro). So he we go…

The tide tables are from 1983 - that’s a long time ago, lets update that. All of these “calculations” are inaccurate due to sea level rise. Also, your ECS or ECDIS does this automatically.
From Bowditch “One should exercise extreme caution in using general rules. The belief that slack occurs at local high and low tides and that the maximum flood and ebb occur when the tide is rising or falling most rapidly may be approximately true at the seaward entrance to, and the upper reaches of an inland tidal waterway. But generally this is not true in other parts of inland waterways. When an inland waterway is extensive or its entrance constricted, the slacks in some parts of the waterways often occur midway between the times of high and low tide. Usually in such waterways, the relationship changes from place to place as one progresses upstream, slack water getting progressively closer in time to the local tide maximum until at the head of the tidewater (the inland limit of water affected by a tide) the slacks occur at about the times of high and low tide.
The daily rise and fall of the tide is important to every mariner."
More from Bowditch “One should remember that heights given in the Tide Tables are predictions and that when conditions vary considerably from those used in making the predictions, the heights shown can be considerably in error. Heights lower than predicted are to be anticipated when the atmospheric pressure is higher than normal or when there is a persistent strong offshore wind. Along coasts where there is a large inequality between the two high or two low tides during a tidal day the height predictions are less reliable than elsewhere.
The current encountered in inland waters is due primarily to tidal action, but other causes are sometimes present. The tidal current tables give the best prediction of total current, regardless of the cause. The prediction of a river may be considerably in error following heavy rains or a drought.”

Some tidbits I found interesting the book mentioned above.
There are 370,000 miles of shoreline on the planet and all of them are affected by tides in some manner. Examples of coastline that have small tide ranges are the Gulf of Mexico, Mediterranean Sea and South Pacific Islands. Oppositely, the tide change is very large and noticeable in Northeastern Canada, northwestern Australia, Patagonia and the United Kingdom.
Sea Level Rising
The famous Piazza San Marco in Venice Italy occasionally can receive up to 18 inches of water in the square at high tide these days. While having the water occasionally jump the wall in the past hasn’t been unheard of, this has happened more frequently in recent years. In 2015, this famous square was recorded as receiving water over the seawall more than 100 times. The Italians call high water acqua alta.
Bay of Fundy, Nova Scotia
Bay of Fundy has the largest tidal range in the world, recorded officially at 54.5 feet at its greatest. At certain times of the year, the tide range will be 52 to 53 feet on average at its maximum. The mud shore at Bay of Fundy combined these unusual and drastic tide changes results in the unusual flight migration of the Sandpiper en route to its destination in Suriname near the equator.
Mont Saint Michel, Normandys Atlantic coastline in France
The 45 foot tidal range here brings tourism to its famous monastery that is a common ending spot to pilgrimage hikes across Europe. “Brochures today caution tourists not to walk on Mont Saint Michels flats without a guide. Besides the risk of quicksand and the tide, lightning is also a danger: anyone walking in the tidal zone is surely the tallest thing on the horizon. “Once the tide hits your feet, two minutes later it will be at your waist and take you away.”
Chinas Qiatang River Tidal Bore
A tidal bore is a tidal phenomenon in which the incoming tide forms a wave (or waves) of water that travels up a river against the direction of the river current. This strange phenomenon is present in many parts of the world, but here is it the most drastic and most violent, responsible for many deaths each year. This “tidal bore” can get up to 26 feet high and travel upriver at speeds of up to 20 mph in certain stretches of the river. The 2nd largest tidal bore known occurs in the Amazons Pororca and reaches heights of 12 feet and can travel upriver for almost 500 miles. “All tidal bores have two things in common, the first is a funnel-shaped river mouth with a shallow, gently sloping bottom. The second is a large tide at the rivers entrance.” The Qiatang River, is about 90 miles south-south west of Shanghai and the well known Yangtze River.


#2

All the calculations are accurate, you need to use the practice edition of the book, not the current year’s.


#3

I agree. I worked them all of out 86 except for about 10 of them. The 10 or so that I haven’t I simply have no care to because they have no practical utility. My argument is lets update this 1983 book to lets say 2018 where these calculations will be more “accurate”.


#4

I’m all for updating the exams for modern times, but I’m not sure I’m following. Wouldn’t it be the same calculations whether you use a 2018 almanac or an 80s almanac?


#5

[quote=“LI_Domer, post:4, topic:48822, full:true”]
I’m all for updating the exams for modern times, but I’m not sure I’m following. Wouldn’t it be the same calculations whether you use a 2018 almanac or an 80s almanac?
[/quote
Good point, I imagine these numbers should change between now then though, no?

Then again, maybe I am overthinking this which THEN raises the same question, are these questions even helping out? especially if even I’m missing the point and I’ve spent a lot of time figuring them out.


#6

The numbers might change, but the process does not. It doesn’t matter what the numbers are. Sure, maybe the ECDIS does it for you, but when the ECDIS shits the bed when you need it, you’ll be able to do a tide or current calculation with no problem (hopefully).


#7

You’re being tested on the mathematics and processes for applying the information from the tide tables. They could be from 1949 and the methods would still be the same. Celestial Nav Exam is also based on the 1983 nautical almanac. I think you’re overthinking it as you said.


#8

The calculations are perfectly accurate, as sea level rise will have absolutely no effect on the calculations for 1983’s tides and currents, which is what the test questions and answers are calculated from. You’ll also be referring to tables in the 1981 edition of Bowditch. Those tables are equally accurate for the Coast Guard exam questions.

Would I be correct in guessing that you’re trying to check your work on some sort of tide computer or ECDIS that is having trouble calculating that far in the past and that’s why you want them to redo them with newer tables and updated questions? As others have said, the math and procedures are the same even if the tables were from 1883 (or calculated ahead to 2083 even, since the tide tables are based off of MLLW and don’t take into account for sea level rise regardless).


#9

As @New3M said, the process remains the same. The continued use of these volumes for testing provides a standard for training, and many things in the USCG exams may be outdated or obsolete, there is certainly value in knowing how to calculate tides and currents correctly. It’s a baseline of professionalism that should be expected of a deck officer. There’s a lot of things you can do with modern technology, but that doesn’t mean you shouldn’t know how to do them yourself. There are two real goals in the exams:

  1. Demonstrate a baseline of proficiency in the subjects.
  2. Demonstrate an ability to use the resources available to produce the correct answer.

I view it as a sense of pride that I know how to calculate tides, manually calculate stability without cargomax, determine the weather without an app, or find my position without GPS. Sure, I could, but then couldn’t anyone?


#10

It’s more of the case that I didnt understand them as well as I thought I did, you guys helped to clear that up, so thanks. I still believe that these calculations “can be condiserably in error” like Bowditch has said, as in the complete opposite of what you calculate them. Much of what we now know about tides and currents we have figured out in the past 20-30 years. I would argue will still don’t fully understand deep ocean currents that well. I think these calculations are “guesstimates” at best. Also, they factor in perfect weather conditions, which we sailors know rarely is the case. Heavy rains would be a good example. Much like geographic range of lights, last night I’m looking out of the window and know for damn sure with the overcast skies and fog, geographic range won’t do me much good, at least there is an alternate calculation for luminous range. I’m more playing devil’s advocate here, I do think understanding how to do these is fundamental in understanding tides and currents, just don’t think the calculations hold much weight.


#11

In making a prediction typically we will start with an estimate and then making corrections in order of magnitude of effect. For example to estimate ship’s speed start with rpm then correct that result using wind, sea, current, trim an so forth. Doesn’t make sense to start with trim or whatever, trim results in a correction, not an estimate.

Predicting the water depth at a location the starting point is going to be predicted height at the reference station. Then make any know corrections until the answer is sufficiently accurate. There will always be some error.

The Internet says sea height change due to climate change is about 0.32 cm/year. The tide tables assume a atmospheric pressure of 1013 mb however 1 mb in barometric pressure will change sea height by 1 cm. so for example 1030 mb - 1013 mb = 17 mb or a decrease in depth of 0.17 meters, that’s by far a more important correction if things are being cut close.

EDIT: Here is: NOAA provides real-time water level information that is updated every 6 minutes.

Here is Portland Maine - Both predicted and measured on the same graph. - today the predicted range is about 8.5 feet (2.6 meters), the largest error looks to be about 0.3 feet (0.1 meters).


#12

ESPN article on a Brazilian surfer riding the tidal bore on the Amazon.


#13

:facepalm: Yes, that is correct. Even if you were to use the tide tables book for today, it would only be factoring in the tidal effect of the moon on the spheroid and based on past observations for the location. For the exam there’s no correction for storm surge, or wind effects, or even rising sea levels. Your calculations are going to be solely for the tidal effect of the moon in a given location.

See the link below regarding Harmonic Constituents

NOAA Tides and Currents Products Information


#14

The harmonic constant is determined by observation:from Bowditch:

The nature of the tide at any place can best be determined by observation. The predictions in tide tables and the tidal data on nautical charts are based upon detailed observations at specific locations, instead of theoretical predictions.

Tidal elevations are usually observed with a continuously recording gage. A year of observations is the minimum length desirable for determining the harmonic constants used in prediction. For establishing mean sea level and long-term changes in the relative elevations of land and sea, as well as for other special uses, observations have been made over periods of 20, 30, and even 120 years at important locations. Observations for a month or less will establish the type of tide and suffice for comparison with a longer series of observations to determine tidal differences and constants.

Mathematically, the variations in the lunar and solar tide-producing forces, such as those due to changing phase, distance, and declination, are considered as separate constituent forces, and the harmonic analysis of observations reveals the response of each constituent of the tide to its corresponding force. At any one place this response remains constant and is shown for each constituent by harmonic constants which are in the form of a phase angle for the time relation and an amplitude for the height. Harmonic constants are used in making technical studies of the tide and in tidal predictions on computers. The tidal predictions in most published tide tables are produced by computer.


#15

Bowditch again;

  1. Meteorological Effects

The foregoing discussion of tidal behavior assumes normal weather conditions. However, sea level is also affected by wind and atmospheric pressure. In general, onshore winds raise the level and offshore winds lower it, but the amount of change varies at different places. During periods of low atmospheric pressure, the water level tends to be higher than normal. For a stationary low, the increase in elevation can be found by the formula
image

in which R0 is the increase in elevation in meters and P is the atmospheric pressure in hectopascals. This is equal approximately to 1 centimeter per hectopascal depression, or about 13.6 inches per inch depression. For a moving low, the increase in elevation is given by the formula

image

in which R is the increase in elevation in feet, R0 is the increase in meters for a stationary low, C is the rate of motion of the low in feet per second, g is the acceleration due to gravity (32.2 feet per second per second), and h is the depth of water in feet.

Where the range of tide is very small, the meteorological effect may sometimes be greater than the normal tide. Where a body of water is large in area but shallow, high winds can push the water from the windward to the lee shore, creating much greater local differences in water levels than occurs normally, and partially or completely masking the tides. The effect is dependent on the configuration and depth of the body of water relative to the wind direction, strength and duration.

hectopascal = hPa = mb = millibar