Reading Tide Tables

[wooster]

Hi,

I asked this elsewhere but it got confused with another question and I need to start a new thread.

I need a lesson on tide table reading. Maths isn’t my strong point so bear with me please.

According to my app, today the time Ludham Bridge is at low water is 11.37 and this is followed by  the reading 0.37m. High water is 18.10 followed by the reading 0.91m.  This is similar to the BA tide table format. I had thought the height readings related to water height above Chart Datum -with that being the lowest "normal" tide. It might not be the correct definition but at any event, CD will be a constant point from which tide height is measured. If BA use some other datum then it will be constant to make the height measurement relevant and comparable. It's only the difference between the height at low and high water I am talking about here.

What I don't understand is that if I’m right, then the water at Ludham today moves between 0.37m above CD and 0.91m above CD which is a rise and fall of 0.64m or roughly 25”. However everyone here tells me it only moves around 3 or 4" so what am I getting wrong?

I don't really care about the height of tides at Ludham and I don't care about how accurate the tables are but I'm really interested in the general method of making sense of tide tables and what the height numbers actually mean and why these seem to be so way off.

 

Thanks for your help

[Ray]

Interesting question! I agree with your reading and conclusions, also the fact that they bear no resemblance to actual conditions experienced.

I can only offer a suggestion for the reason, that is that the tide table assumes (in effect) that Ludham Bridge (in this case) is on the coast ie no allowance is made for the actual water of the river itself flowing downstream and meeting the tidal water. Thereby  creating a much smaller tidal range than would be the case otherwise.

Calculating actual rise and fall taking into account the flow of the river would be nigh on impossible I imagine because of the variables involved.

A truly useful Broads tide table would include an actual measured range at each location, with a warning about these being average and likely to fall somewhere within the range on any given day.

 

[ChrisB]

Remember, Tides are not an absolute science, they are a best approximation and local conditions re atmospheric pressure, wind have a considerable effect on predictions. Due to the constricting influence of the entrance to Great Yarmouth and the rivers thereafter tides are usually calculated on the basis of "hours after Gorleston" heights are above chart datum in similar unrestricted water. Really the only information the heights give you so far up river from their point of calculation is whether it is Springs, neaps or between and a summer deluge over a few days will wipe out any extrapolated calculations that you have made.

My advice would be:-  if you are concerned with bridge clearance on the Northern Rivers, Potter, Ludham, Wayford and Wroxham your best course of action is to monitor the various EA Monitoring Stations as the most accurate information available.

[ChrisB]

This type of real time information is far more use if your concerns are bridge clearance.

[marshman]

Shorebase are in my opinion are the most accurate but there are just too many variables relating to Broadland tides - no amount of studying tables will help!!!! The best way is to eyeball the BA height boards, which tend to be conservative, and approach slowly!!

I get what others say about "gunning" it as you go through, but get it wrong......!!

If you make a generalisation like you can get through Ludham , say 75% of the time, I will guarantee that the day you NEED to get through, the tide will be high enough to stop you!!

[TheQ]

if you want low water at the bridges, look to the weather forecast for Southerly /  Westerly winds and high air pressure.

The water levels will be higher than predicted with Northerly  / Easterly winds and low air pressure.. Especially if it's been that way for several days..

[Ray]

I think the op is really asking why there is such a disparity in published tidal range compared to that actually experienced... Ludham Bridge is an example but it applies to other places on the Broads. That's why I made a stab at a possible explanation above ( which could be entirely wrong of course 🙂)

[annv]

Hi Ray the BA tide table tells you this bridge heights at mean then subtract where your are normally 4 hours covers most short bridges then time of day then higher or lower depending on mean water level if higher mean and you are close to height approach with car site along top of boat to far side of bridge underside if seen you are clear if only front stop turn round or moor put kettle on and wait. John

[grendel]

are the numbers quoted the maximum recorded high and minimum recorded low by any chance?

 

[ranworthbreeze]

1 hour ago, grendel said:

are the numbers quoted the maximum recorded high and minimum recorded low by any chance?

 

On the BA tide tables they are and it is why owners with higher air drafts use them.

 

[wooster]

Is that definitely the case, Alan? If so I don't understand why they give different numbers for each tide?

[wooster]

16 hours ago, ChrisB said:

This type of real time information is far more use if your concerns are bridge clearance.

These were suggested by someone when I asked the question as part of another thread which got confused and I believe that's the case. I will be using these in future. It seems it's not me reading the BA tables wrongly, just that they are of little use for accurate water height prediction.

[grendel]

to be fair I only ever use the tide information to let me know if its high tide, or low tide, or somewhere between, no app or predictions will ever be able to give you exact figures for any particular spot at a given day or time (even the water gauge at Repps doesnt give live data, I have found the aweigh app to be full of flaws, from predicting the tides a whole hour wrong to just not being able to connect and get tide data at all, ok its better than nothing, but the good old fashioned mark 1 eyeball when you arrive gives far more accurate information.

[RS2021]

I think the various information is all useful at different stages. When you are booking a boat or (if you are lucky enough to own one) when you are planning your trips, the tide tables will let you know times of high/low times accurately enough to know if you can cross Breydon at a reasonable time, does low tide at Potter occur when the pilots are on duty, or in the case of Ludham if its off season and you might not get through at high tide, is low tide a reasonable time etc. A day or two before the EA gauges plus weather forecast is great. You can know whether its worth heading for Ludham, or going somewhere else. On the day - as Grendel says - good old fashioned mark 1 eyeball.

[Smoggy]

Even if you take the rivers out of the equation the best tide predictions can vary by an hour if the weather doesn't suit it, I well remember a well timed arrival at the orford haven fairway buoy left me circling for over an hour watching the depth sounder waiting for an extra metre of water to appear before attempting to cross the bar when everything said we should have been able to go straight in, it was obviously not just our calculations as we watched a bilge keeled yacht sit bolt upright and then start slowly hopping it's way in with each wave after going aground on the way in.

Mk1 eyeball on the day every time, if it helps I have never failed to get under ludham with 8'8" of airdraft (in summer) without taking much notice of the tables although we did stop and do the tape measure trick under the bridge a couple of times.

[rightsaidfred]

For the Northern Broads the easiest guide to approx water levels alongside high and low times is the BB webcam at Wroxham, tides are not an exact science but you can get a pretty good idea from that reading as to the general levels in the North bearing in mind that the amount of rise and fall can vary day to day depending on conditions.

Fred

 

 

 

 

[vanessan]

We use a collection of tide tables, the BA booklet, the Norfolk-Broads.org tables and the Aweigh app. They’re never more than half an hour apart. The BA booklet is particularly useful for tide rise and fall information, if you know whether it’s spring tides or neaps you have a pretty good idea of bridge heights across the system (and mooring possibilities on the southern rivers too!). 

[Speleologist]

It strikes me that 

On 28/09/2021 at 00:30, wooster said:

 

I don't really care about the height of tides at Ludham and I don't care about how accurate the tables are but I'm really interested in the general method of making sense of tide tables and what the height numbers actually mean and why these seem to be so way off.

 

Thanks for your help

It strikes me that the first issue here is you cannot make sense of a tide table unless you know the source of the data and it's reliability. These days I am predominantly a coastal and offshore sailor, although I have sailed extensively on the Broads over many (50+) years.

There are many tidal apps available for the coastal sailor, but they are not necessarily good. I would only use one based on the Admiralty data. This has been developed from around 200 years of observation and is the best there currently is. (Developments in live tide forecasting may change this before too long). For the Broads we have coastal date modified by observation, but the observations are nothing like as comprehensive or as well co-ordinated/documented. An Admiralty Standard Port tide table (e.g. Lowestift) will give the best possible prediction of tide times and heights. These can be modified for environmental factors. The change can be accuraate for pressure, one HPa up or down for eacn HPa from the standard pressure of 1013HPa. Modification for wind is less exact but easy to understand the type of effect.

Once we get in to rivers, then we also have a significant impact from things like rainfall, topography changes (e.g.silting) etc. None of this is well documented. I suspect that the app referred to in the OP tries to make a prediction based on some poorly defined criteria, and has got at least the range fundamentally wrong.

[wooster]

So putting aside the questions of variability of weather, data collection etc, the theoretical approach I'm using is right? I understand that predictions are uncertain and there is a need for making sure the height is adequate on arrival etc but it's my understanding of the way that to read the tables that confuses me.

What I can't get over is that I find it odd that these tables can be so far out, as on every occasion, they are predicting the rise and fall of water to be around so much more than the amount it actually is 

This is what makes me wonder if my understanding of how they work is wrong but everywhere I look the chart reading advice suggests I'm not.

[grendel]

while any predictions can be inaccurate (because the tides dont behave and do as they are predicted at best of times), relying on them for actual heights to my mind is even less sure of a thing, they will be what they are, and with wroxham or potter heigham, only the pilots with years of experience can say with any degree of certainty what the next tie may be, depending upon weather, wind direction springs / neaps etc etc and years of experience, and that jut to predict the change from one tide to the next, maybe 12 hours, based upon what they are seeing today.

It might be worth contacting the app developers (they must have a contact number or email in the app surely) and asking them what the figure is based upon, if anyone they will know, the best any of the rest of us can do is guess.

[expilot]

For ten years as a Hoseasons and Blakes bridge pilot at Potter Bridge, I can inform you that we used the tide tables published by the BA to predict high and low water times.  They were very rarely out by more than a few minutes.  We never used the published predicted heights at all.  Instead, we monitored the river levels on a daily basis and checked with the weather forecasts for changes of wind direction and air pressure.  Predicting river levels at Potter bridge is a black art.  We got caught out as often as the weather forecasters!

[wooster]

It's not so much the accuracy I'm puzzled by, but what information the numbers in the tables re water levels are supposed to convey. That's really what I'm wondering about. 

[rightsaidfred]

Following this thread it would seem this is another case of being blinded by science, for the majority it would appear that observation in conjunction with tide times is the only reliable method.

Fred

[vanessan]

The BA tide tables quote predicted heights relating to Yarmouth Bar. If you look down the tables at the heights given for high and low water times, it is very easy to see when the highest and lowest tides are occurring. Many of us mere mortals are unable to judge weather conditions, pressure etc along with these details so rely on tide times/heights alone. If the water is high at Yarmouth Bar, then it will be high further up the system, just not as much. (I find them useful in deciding whether or not to stop at Berney Arms or Burgh Castle for any length of time, neaps provide easier moorings.)

[Islander]

Tidal prediction is certainly a black art I’m still trying to master. Here’s an example of the current situation.