FLOOD RISK? Flood risk to Dartmouth from high tides?
SOME LOCAL RESIDENTS AND BUSINESSES ARE CONCERNED THAT THE HIGHEST ASTRONOMICAL TIDE (HAT) THAT IS PREDICTED TO OCCUR IN SEPTEMBER 2015 WILL CAUSE FLOODING IN THE TOWN. THE KINGSWEAR PARISH COUNCIL RECENTLY CONTACTED THE ENVIRONMENT AGENCY FOR ADVICE ON THIS MATTER WHO THEN SUPPLIED THE FOLLOWING ANALYSIS AND COMMENTARY.
HAt and other high tides but, for reassurance, the predicted HAt level is more than 0.5m (nearly 2 feet) below the level of the quayside wall which is the tidal flood defence for most of Dartmouth. It is important to note that flooding doesn’t always
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happen when HAt tides are predicted and similarly, flooding can happen when predicted (i.e. tide table) tides are lower than the HAt. It all depends on the weather at the time of high tides. this is because poor weather (strong winds, waves and low pressure) can raise predicted tide levels and similarly, good weather (light winds and high pressure) can lower tide levels. this ‘surge’ (the difference between actual and predicted tide levels) is key to knowing if properties are likely to flood at times of high water. For example, the HAt tide level in Dartmouth on 29
september 2015 is predicted to be approximately 5.45m above chart Datum (cD) in Dartmouth or 2.83m above Ordnance Datum (OD). this is 0.57m below the defence level of around 3.40m OD. therefore at the time of highest tide, there needs to be a surge of over 0.5m before significant flooding is likely in Dartmouth. this is possible, but not expected unless there are severe storms at that time. by way of comparison, the maximum flood level (a combination of predicted tide and surge) in Dartmouth
Flooding at bayards cove 2014 photo by Leanne tucker
ome low lying properties around the estuary at Dartmouth may be at risk from next year’s predicted
during the severe storms in October 2004 was 3.20m OD and numerous properties along bayards cove and near the lower ferry flooded. Again, during the winter storms of 2014 there was further property flooding here but less severe as the flood level on this occasion was lower. this highlights that although the risk of significant flooding is dependent on weather conditions it is important to be vigilant to the risk of flooding all year round, every year. Have a look at these frequently asked questions, to help
you understand what factors influence the sea level here on the Dart estuary, how you can check if you’re at risk of flooding and what you can do to stay safe and minimise damage.
Can you tell me a bit more about tide tables? tide tables give the high and low water times and predicted (astronomic) tide height at a location. the times are given in Greenwich mean time (Gmt), which means we need to add an hour during british summer time (bst). the levels quoted in local tide tables are relative to local chart Datum (cD).
What’s Chart Datum? chart Datum is the approximate lowest sea level that can be predicted to occur due to astronomical effects, for the local charted area. the tide will almost never fall below this level, which is why this datum is used for marine navigation and surveying purposes. chart Datum (cD) is different to Ordnance Datum (OD).
Ordnance Datum is the standard, national height system used by land surveyors, and is fixed for the whole country. chart Datum varies from place to place, because it’s dependent on the tidal range at a particular location. so, for example, chart Datum in Dartmouth is 0.6m higher than chart Datum in Plymouth. to compare tide levels that are relative to chart Datum
to levels on the mainland, you need to convert the local chart Datum level to Ordnance Datum. chart Datum in Dartmouth is 2.62m lower than Ordnance Datum. so a predicted tide level of 5.30m above chart Datum in Dartmouth will be 5.30 - 2.62 = 2.68m above Ordnance Datum.
So what influences our tides? Our tides follow regular and predictable cycles, which are principally caused by the gravitational forces of the moon and sun.
A bulge of water in the sea, follows the line of the rotating sun and moon, which results in the rise and fall of sea levels that are our tides. the gravitational pull of the moon has a greater influence than the sun but the combination of the earth, sun and moon’s orbits affects the cyclical variations in the tides, such as neap and spring tides. tide tables do not consider the effect of weather
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