Tides and Storms – Stromness – 9 Feb 2016 – Tide height 3.9m (by tide table), pressure from BBC weather 973mb, all measurements taken on pier adjacent to Double Houses, Stromness.
The atmospheric pressure is measured in millibars (mb). The standard pressure assumed by tide tables is 1013 mb. The lowest pressure ever recorded was 925db.
As more pressure is exerted per mb the sea height decreases by about 10mm and of course the reverse is true. Generally the maximum effect pressure has is 300mm.
By measurement the tide maximum height today was 400mm below the top of the pier. The pressure was 973mb or 40mb below the presumed standard pressure. Therefore the tide was 300mm higher than the standard pressure and of course the maximum tide height (4.2m) will be 100mm above the top of the pier when the pressure is at the lowest level ever recorded.
From the above measurements can be calculated the height of each pier/shore front property in Stromness and whether or not they are likely to flood when only affected by pressure (by comparable measurement or by utilising Ordnance Survey data).
Tides are caused by the effect of gravity on the seas by the moon and the sun, the amphidromic systems of the oceans and the shape of the coastline etc. As air warms, it rises leading to low pressure at the surface. As it cools it descends leading to high pressure at the surface. In general, the lower the pressure the higher the winds and unsettled weather (rain etc) and the higher the pressure the lower the winds (sunshine etc).
High winds in combination with high tides cause storm surges generally in the direction of the wind – this raises the tide level. The tide damage is caused by the waves which store energy and then release this energy in combination with the winds.
The perfect storm conditions for a sea front property in Stromness are when the moon and sun align and are at their closest to the earth, when pressure is low and when the high winds and storm surges come from the South East.
Rarely, a transient or a temporary oscillation may be experienced causing a tide which continues beyond the tide table predictions – this is not correctly understood – this condition may be known as a tidal strand and also a double high tide. The conditions required are hypothesised as a sudden change in force, an entrance protected by island(s), pressure, winds etc. etc.
Importantly extreme and unusual conditions will occur periodically which will seriously affect water side properties (for example the great storm of 1953 etc).
A pier or shore front property owner should ensure they are correctly insured or have adequate funds to cover any maintenance, repairs or storm damage. Communal owned piers would be well advised to put in place a sinking fund managed by a company. It may well suit burdened owners who are not at risk to give up their rights to a pier to an at risk owner who in exchange for the ownership would take on the maintainence costs. When purchasing a pier or shore front property they should ensure they commission an adequate survey. Should damage or suspected damage occur they should advise their insurance companies and utilise the professional services of an appropriate Chartered Construction Professional.
The Chartered Construction Professional, when assessing any damage, will be seeking data and analysing it as follows:
- Construction of the pier, its mass, its bonding, its foundations and signs of damage.
- Historical storms, their strength and damage caused.
- Local conditions ie it has been suggested that the newest Ferry may be undermining the piers.
The Professional will be able to advise how the piers/structures should be repaired and maintained.
In extreme conditions Local Government etc assistance may be required.