A number of boat owners have recently enquired about the clearance heights of the bridges on the Hamble above Burseldon pool. The lowest of the three bridges here is the arched A27 road bridge and this is shown on charts and in nautical almanacs as having a charted vertical clearance of 3.5m. above the highest astronomical tide (HAT) Several people with larger boats have asked how to relate this to tide tables in order to time their passage through the bridge.
To explain this more fully, the HAT figure is the highest predicted tidal level within a period of years under normal meteorological conditions and is given as a height above chart datum level. Tide tables for the Southampton area give a HAT figure of 5.0m, and added to the 3.5m charted vertical clearance this indicates the clearance at Burseldon bridge to be 8.5m above datum. Consulting the tide tables will thus allow a quick calculation of the minimum clearance at high water by subtracting the predicted height from 8.5m. A clearance height at any state of the tide may be similarly calculated using the tidal curve or other methods for obtaining intermediate tidal heights. The depth of water under the bridge is 1.5m. at datum giving the total height from riverbed to the centre of the bridge arch of 10.5m. (So if your vessel has a draught of 2m and a mast standing 8m above the waterline you may have to judge this to a nicety!)
For an on-the-spot check when approaching Burseldon bridge there is an air-draft gauge on pile Z1, adjacent to the training wall. There are tide gauges at Stone Pier Yard, Pile D9, Crableck Beacon and at pile Y16, near the bridge, showing the tide height above datum (not to be confused with the depth gauges on the maintenance piles which indicate the depth of water at that point)
Be aware that the actual height of the tide may vary from the astronomically predicted level. The tides in the River Hamble are significantly influenced by the wind and the atmospheric pressure, with generally South Westerly winds and low pressure increasing the tidal height and high pressure pushing it down. For example, a low pressure weather system coming in from the Atlantic with a South Westerly gale may increase the tide by as much as 1.2m especially if its progress up the English Channel coincides with the high tide. Exceptional low tides tend to occur when calm high pressure conditions coincide with spring tides, and the level may fall to 0.5m below chart datum.
Burseldon bridge and air draft gauge showing clearance of 5 metres. Vessels proceeding to R K Marine, the Cabin, Riverside and Eastlands Boatyards or the upper Hamble river must pass through the bridge.