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Project to modify Peel Harbour flapgate 31 August 2007

Preparatory work will start next week on the Peel Harbour flapgate ahead of modifications being carried out later in September

This initial work will involve a contractor removing paint from areas at the top of the gate, which will later be cut away to help improve the effectiveness and reliability of the flapgate, which was completed in July 2005.

Captain Ken Horsley, Harbour Operations Manager, explained:

‘We carried out some work on the flapgate seal earlier this summer after it had been damaged. This work was a precursor to the project we are now undertaking.
‘When the flapgate and fish pass were designed, modelling was undertaken in order to assess the likely amount of water that would flow down the River Neb during periods following high rainfall. In normal circumstances the fish pass would ensure that the level of water in the inner harbour was maintained at 2.5 metres at the gate. In severe conditions the flapgate has been designed to allow excess water to spill over it. However, in practice it has been discovered that the quantity of water flowing down the river has exceeded the modelling assumptions for peak rainfall periods.
‘Normally this would not cause any operational difficulties to vessels, but during neap tide periods, when high water is at a relatively low level, the tide does not achieve a sufficient height, compared with the water level in the inner harbour, to allow the flapgate to lower. In such circumstances this does cause operational difficulties to vessels.’

Following discussions with Ken Grubb Associates, the Mechanical and Engineering specialists who designed the gate, alterations were made to the operating software to provide a 200mm tolerance between the level of the tide and the water retained in the inner harbour. Following implementation of this modification, the number of incidents when the gate would not operate automatically has much reduced.

Captain Horsley added:

‘Whilst this modification has helped the situation, it has not cured it completely. There are still occasions during periods of high pressure and strong northerly winds, combined with heavy rainfall where the tide does still not make sufficiently to allow the flapgate to operate automatically. After further discussion with Ken Grubb Associates, the Department decided to reduce the height of the flapgate by modifying the top.
‘The work will see the flapgate modified by cutting out two four-metre slots in the top. This will allow more water to escape more quickly from the inner harbour during periods of heavy river flow.
‘The modification to the software together with the reduction in the height of the gate should ensure that it operates on every tide, although there may still be a combination of neap tides and weather conditions that would reduce the level of the tide sufficiently to prevent the gate operating.
‘Over the course of the coming week preliminary work will be undertaken that will include blasting the paint from the upper areas of the gate where cutting or welding will be required.
‘This will allow the modifications to be completed between September 19th and September 22nd, which was identified as the optimum period for the works. We want the effect on normal harbour life to be minimised, so we have waited till the end of the summer season and picked a period of neap tides. This means we will have low tides during the day, thereby allowing most of the work to be completed during daylight hours.
‘However, it is still weather dependent – if we get very heavy rainfall, that will delay the whole process, as work cannot be undertaken if there is too much water coming down the river.’

The Department’s Works Division will carry out the project, with assistance from KGA.

Notes for Editors: Tides are created due to the gravitational effects of the Moon and Sun. Neap tides occur when the gravitational forces of the Moon and Sun act at right angles to each other, shortly after the Moon is in its First and Third Quarter - i.e. about once every 14 days. This results in a lower than normal tidal range, effectively giving ‘lower’ High Waters and ‘higher’ Low Waters during these periods. These normally last for about three days at a time. In the Isle of Man, during these tides, High Waters occur in the early mornings and evenings between 5am and 7am and 5pm and 7pm, and Low Waters between 11am and 1pm and 11pm and 1am.

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