Drying Heights and Tidal Level Tables
Drying heights above Chart Datum are also indicated on the Tidal Level Tables and measured above chart datum. So we need a certain Height of Tide above chart datum to cover any drying feature on the chart. We can use the Tidal Levels information on the chart to quickly asses the situation.
A fish-farm barge broke free from its mooring in a storm and has gone aground immediately north of Airy Point in Dawson Harbour (Fig 5.14). The barge has a draft of 1.7m and is resting on a drying patch that is 5.3m above chart datum. The barge needs an extra 50 cm clearance for it to float free from the drying patch. Can we float it off or do we need to crane it off?
Firstly we need to look at the chart and can see that the dying height of 5.3m tells us that we need at least this much Height of Tide above Chart Datum before the bottom of the barge starts to get wet.
We need to calculate the Height of Tide needed to float the barge off. Height of Tide required = Drying Height + Draft + Clearance needed = 5.3m + 1.7m + 50cm = 7.5m. So we need a tidal height of 7.5m above chart datum.
We can see from the Tidal Levels table on RYA Charts 4 that at MHWS there is a Height off Tide of 7.6m. So we have a small window of opportunity around MHWS to float the barge off.
The total clearance under the barge at MHWS will be Clearance = Height of Tide – Drying Height + Draft = 7.6m – 5.3m + 1.7m = 0.6m.
We need 50cm of clearance to get the barge clear of the reef which leaves 10cm to play with. It is going to be tight and will have to work fast but should be able to float the barge off the reef on a Mean High Water Spring tide.
Again there are limitations on what information the Tidal Levels information on the training charts can provide. In reality everyone and a JCB would be standing by and waiting for the tide to lift the barge. However if you remember from above, the high tides each day are at different levels 12 hours apart. We might need to make sure we get the highest Spring Tide of the day to successfully move the barge. The tidal levels chart information does not supply us with this information, or a what time the tide arrives but is a good rough guide to what is happening with the tides in certain areas.
Later in this module well be considering intermediate points between high and low water to determine when there will be sufficient height of tide. Using tide tables we will also be examining the daily variations in tidal level. For now the objective is just to understand what information is provided by the tidal levels table on a chart and how it can be applied.