Bembridge Tides

[GuyC]

Hopefully there will be someone on here who can answer a question on Bembridge Harbour tides.

On EasyTide which is the UKHO tide times website, Bembridge has a couple of tide locations - Bembridge Harbour and Bembridge Approaches.

Today (19th June) at 1600, the tide height at Bembridge Harbour is 3.0m and the tide height at Bembridge Approaches is 4.6m.

On the assumption that Chart Datum is on the same horizontal plane, there is apparently a 1.6m difference in height between the water in Bembridge Harbour and the water in Bembridge Approaches.

I could get my head around this if there was some Saltstraumen type tidal flow into Bembridge Harbour at high tide but there isn't.

Possibly more oddly at 1000 today the tide height in Bembridge Harbour is 0.0m and the tide height in Bembridge Approaches is 1.3m. As Bembridge Harbour is flooded / emptied by the Solent, how can the level in Bembridge Harbour be lower than the lowest Solent tide? It can't empty itself up hill, can it?

A possible explanation is CD in Bembridge Harbour is different to that in Bembridge Approaches. Whilst I know that the Ordnance Survey datum changes dependent on location, I thought CD was always at the same level.

Are there any Bembridge locals that can explain please?

Incidentally, it is probably my new favourite place to go in the Solent. Whilst it is tide dependent, the double high means there is no rush. There are good places to berth with a decent water taxi that will take you to a number of locations, it is reasonably priced - £10 and there is good food.

[GuyC]

I've answered my own question - it is the Ordnance Datum (OD) that is common everywhere. I'd got them the worng way round.

The Chart Datum varies dependent on area.

Chart Datum in Bembridge Harbour is 1.34m below OD and CD in Bembridge Approaches is 2.74m below OD.

[willk]

Originally Posted by GuyC

I've answered my own question...

That's cheating - no wonder this forum is going down the s#1tt3r.

Ask yourself another one

[Pikey Dave]

Originally Posted by GuyC

I've answered my own question - it is the Ordnance Datum (OD) that is common everywhere. I'd got them the worng way round.

The Chart Datum varies dependent on area.

Chart Datum in Bembridge Harbour is 1.34m below OD and CD in Bembridge Approaches is 2.74m below OD.

I thought it was the other way around. There are several OS datums & only one chart datum.

[GuyC]

I have to admit being slightly confused by it.

The Ordnance Datum was apparently the mean sea level recorded at Newlyn between 1915 and 1921. This was then projected across the UK so for OD heights it is all relative to the OD at Newlyn (ODN). There are a couple of other datums in Belfast and Dublin and I believe some of the islands have their own OD but for mainland Britain, it is all based on ODN.

Chart Datums are related to the lowest astronomical tide which is the lowest predicted tide height excluding meteorological effects i.e. wind and pressure.

In Bembridge Harbour the lowest astronomical tide is predicted to be 1.34m below ODN therefore the CD in Bembridge Harbour is 1.34m below ODN.

In the approach to Bembridge, the lowest astronomical tide is predicted to be 2.74m below ODN and therefore CD at Bembridge Approaches is 2.74m below ODN.

I can sort of keep up with this idea as the restricted entrance will throttle the flow into and out of the harbour keeping the range of the tide inside the harbour less than outside the harbour. If I translate the Bembridge figures relative to ODN, the height in the harbour lags about 0.2m behind the approaches during the flood which makes sense.

Does all this make any difference at all?

If I applied the Bembridge Approaches tide height to the soundings on the Bembridge Harbour chart, I could be out on my water depth estimate by 1.4m, which in the Solent is often the difference between being aground and not being aground.

The lesson identified is that you need to look at the tide heights for the specific location and apply that to the soundings in that location.

I had a quick scout around the Solent area and the Bembridge example seems reasonably unique. I thought there might have been differences at Poole Harbour or Christchurch Harbour but the CD's are the same relative to ODN.

Every day is a school day.

Apologies for the insipid post and speaking of insipid - happy father's day to all those unfortunate souls.

[lakelandterrier]

From memory, the approach to Bembridge and most of the harbour dries at low tide whatever the datum.- I'd probably not want to be in either after half tide, be it springs or neaps.

Ducked in there once and was underwhelmed.

[Matt]

I’m sitting just inside the harbour entrance right now. Lovely little harbour.

Anyway, to gauge access to the harbour, your best bet is to use the harbours bar ride height gauge : https://www.bembridgeharbour.co.uk/weather-and-tides

So right now, on an ebb tide, 3 hours after HW there’s still a couple of metres of water over the bar according to their gauge. There’s still water round the marina at low water, but it does get quite dry elsewhere, so if you plan on being in harbour through low water, choose your mooring carefully

[Voodoo]

We have been over to Bembridge and Priory Bay a lot recently....lovely over there.