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raj

Calculating Bridge Clearance From Tide Tables

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Is is possible to use published bridge heights and online tide heights to calculate bridge clearance? For example, Beccles Road Bridge is published at 6'6" at Average High Water. If the current tide height (WillyWeather) shows 5ft (I assume this is above average low water) and predicted low is 0.95 feet, can I use those figures to calculate bridge clearance now and at low water, and from the graph approximately what time I would have sufficient clearance? I know barometric pressure will have an effect but I could correct for that. I don't understand the relationship between published bridge clearance at AHW and the tide height above ALW at any given time. Surely I would need to know the bridge clearance at ALW for online tide heights to be of any use?

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You can calculate all you like, there are just too many variables to get an accurate result. If you get a result accurate to plus or minus 3 inches, will that be enough to say yay or nay to making a journey? I've often arrived at supposed slack water at Yarmouth, to find a raging current flowing, and continuing for a good 20 minutes after supposed slack. We've all experienced the way the wind in the North Sea can hold back the tides, so there's that, and rainfall upstream to deal with, too. With enough data available, I suppose it's possible to do those calculations, but even the Met Office with their SuperDuperHyper computers and bazillions of input data get it wrong!

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I would tend to agree, the published figures for bridge clearance are for guidance really and trying to calculate variables particularly where the weather is concerned is very difficult to say the least. To add to the conundrum, bridge height gauges (and boat height plaques for that matter) are usually pessimistic for obvious reasons.

Beccles is often quoted, probably as the high water measurements may seem daunting. In practice the rise and fall of tide at Beccles is quite considerable and 8 feet or more is quite achievable, its just a matter of avoiding the high tide and be prepared, if necessary to moor up and wait for an hour (been there, got the T shirt)

Best bet is always have a plan B.

Many years ago we were moored at Geldeston and torrential rain fell overnight. The Waveney was in full flood and we couldn't get back through the bridge. Eventually after waiting all day Astons got us through, only be reversing and still scraped the roof. Fortunately we had hired from their sister yard at Loddon.

There's nothing so unpredictable as the British weather!

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Welcome raj,bridge heights can be confusing. Hire companies often add the odd inch to there heights. As said the north sea has an affect on tides at times raising the water.Best to check with loca,,such as YS harbour masters.At Beccles Tim is your man.If I am in places such as Reedham I check with caution and approach slowly.

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Hello Raj,

Welcome to the forum. I always use the Broads Authorities tide tables because they give me estimated height of low water and the tide variation of the various locations on the Southern & Northern Broads. Because we have 9 foot 8 inch airdraft we have to plan if passage through Yarmouth is possible during our allocations.

Regards

Alan

 

 

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1 hour ago, Hockham Admiral said:

A warm "Welcome Aboard" the good ship NBN, Raj.  

Good to see one of your warm welcomes again!:default_beerchug:

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I've done some work in this area that I can't really talk about, and yes it is incredibly complicated.

For the Broads, you could derive a forecast from the data but if it has rained hard all week and there's a strong onshore wind at Yarmouth then it'll be completely wrong.

I wouldn't say it's impossible, but it's incredibly difficult.

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Thanks all for your contributions. First post on here but have been a regular broads visitor for many years. Always apprehensive about bridges when planning trips, whether l would be able to get under X when I get there, or whether there's any point heading for certain destinations (Geldeston, Dillham, Coltishall, etc). It seems from your posts that it's effectively impossible to predict with sufficient accuracy based on tide tables. Thanks for ending my uncertainty.  

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I think there is often a perception that when asking this type of question, that the questioner wants a certain answer in an uncertain world. Often what is needed is an understanding of the facts of a situation and then and understanding of the variables, the likelihood of them occurring and the likely impact. 

So, bridge heights are quoted as at Average High Water. Whilst this is a notional value it must be based on an actual value. So it would help to understand that Average High Water is x.xm AOD at y location. Assuming that y location has data published) or better still live values available on line you can very easily have information as accurate as the tide tables themselves assuming the location is close to the bridge. We happily accept the variability of the tide tables, so why not this information. Where bridges are further inland, would it not be better to publish bridge heights, not in relation to Average High Water, but in relation to specific values on the nearest gauge?

A further thought is what do you want to use the information for? If it is to see if your boat will go under the bridge, well there is no substitute for physically comparing the boat to the bridge height in real time (carefully, or with a tape measure of course). However, many people, especially hirers will be booking a boat sometimes 12m in advance and want at least some idea of the likelihood of navigating a bridge.

So the boat I've hired is Fair Executive. When I first booked it my knowledge of the broads was such that I hadn't heard of Ludham Bridge. With a quoted air draft of 8'10" once I did become aware of Ludham I was unsure whether we would get under. A quick google and I discovered both bridge gauges and quoted boat height are widely unreliable. Not very helpful! The best info I found was a video by Kevnheidi taking a similar boat (Fair Jubilee) through with the gauge on 8'.

Having done the bridge, I set about some research. I measured the actual height of the boat, I know the clearance on the day we went through. Also there are online river level gauges at Ranworth Broad and Barton Broad. It is possible to look at current (last 5 days) data on line and also down load the last few years worth of data for these sites. From this I know that I could get Fair Executive through Ludham Bridge when these gauges are below roughly 0.5m. Analyse the data and if you are hiring between 1 April and 31 October, there is a 90% chance you will get this boat through Ludham. For me I'm prepared to take the chance at those risks of hiring that boat. Look at the last five days date just before and once I'm aboard and I can plan which day to head up the Ant.

To have similar information on other bridges. e.g. when the nearest gauge is reading x.xm there is y.ym clearance on the bridge should be relatively simple. Boatyards could then quote a % likelihood of getting under certain bridges - particularly Potter Heigham. This would allow people to make much more informed decisions rather than the vague 'will only pass at extreme low tide...'

 

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I think the best bet is to phone the bridge pilots on those bridges that have them, and for great Yarmouth the yacht station rangers, who will know what the water levels are doing. I tend to go one better and hire from Martham boats, whose boats will pass Potter heigham most of the time (with their staff as bridge pilots) if the boat can pass Potter heigham, it will certainly pass all the other bridges, I went through Wroxham bridge in may 4 times, with clearances up to 18" all round, even at worst I had a good clear 6"

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Good point about hiring, unfortunately (or maybe fortunately?) I have my own, so I know I need 8' 4" or 7' 4" depending on whether canopy is up or down. I do now accept it's not possible to get accurate values in advance of arrival, but RS2021's post and the concept of % chance is quite interesting. If I know published bridge height, and the current river level below the next and previous high tide levels, I ought to be able to guesstimate current clearance on an *average* tide. That doesn't give me % chance of course, and all other factors (wind, pressure, rain, etc) notwithstanding, if it's a matter of a few inches either side of clearance on a descending tide I might be inclined to chance it. And the further away from my required clearance the guesstimate is the less chance I have of a successful transit. An exercise to try for next time perhaps.

 

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1 hour ago, raj said:

I have my own, so I know I need 8' 4" or 7' 4" depending on whether canopy is up or down.

Next time you go through the bridge, note the clearance and time. Then the next day check what the gauge said https://flood-warning-information.service.gov.uk/station/6204?direction=u (there is usually 5 days worth data - but sometimes its missing). You can then work out what level on the gauge you need to get through. If you are thinking of going through its then easy to look at what the river has actually been doing over the last five days. Allow for whether you are heading for springs or neaps and what rainfall is doing and you can have a good guess on what will happen over the next few days.

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As others have said the Environment Agencies Tide Gauging Stations are all over the Broads and most are reliable and in bad times are updated more than once a day but can certainly show a trend over the 5 days as the tide change from Springs to Neaps not withstanding wind rain and North Sea conditions of course.

For example the rains of recent weeks have had little effect  on the height of tides in this area

There is also a tide prediction and confirmation for the sea at Lowestoft  https://www.ntslf.org/data/realtime?port=Lowestoft this is quite interesting to follow.

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Simple answer: No.

Tide tables tell you when to expect low tide, but even so extraneous circumstances can mean they are inaccurate. 

If you find an app that tells you it can don't believe it, it can't. There is simply too much data needed which cannot be accurately predicted.

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Sadly tides cannot read predicted tidal heights and the predicted rise and falls and as everyone says, there are just too many variables!

The best, and only way, is to approach with caution, read the gauge and make a decision on the spot!

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