Flood risk to Dartmouth from High Tides?

Some low lying properties around the estuary at Dartmouth may be at risk from next year’s predicted 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 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 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 conditions but are based only on these astronomical conditions, which is why tidal heights can be predicted for many years in advance.What’s the Highest Astronomical Tide?

Every year there are particularly high spring tides, normally around March and September, known as ‘equinoctial tides.’ The predicted level of the highest equinoctial tides varies over an 18.6 year cycle, and the highest tide in this cycle is called the ‘Highest Astronomical Tide’ or HAT.The HAT is the highest sea level that can be predicted to occur under any combination of astronomical conditions, assuming average weather conditions.

When will the next maximum Highest Astronomical Tide occur?

The next HAT will be on 29 September 2015, when the predicted astronomical tide level in Dartmouth is 5.45m above Chart Datum or 2.83m above Ordnance Datum. Although this is the highest predicted high tide for many years, it is only slightly higher than many other high spring tides.

For example, the HAT predicted for 29 September 2015 is only 2cm higher than the equinoctial high tide that was predicted for 9 September 2010, and only 3 cm higher than that predicted for 10 September 2014.

Why do observed sea levels vary from predicted tide table levels?Tide tables show the predicted astronomical tidal heights on any given day. The actual levels observed at the coast, however, are also significantly influenced by atmospheric pressure, wind, currents and waves (Figure 1).

For example, low atmospheric pressure has the effect of raising sea levels, which is known as a positive surge.  High atmospheric pressure can reduce sea levels, known as a negative surge.

Waves can also temporarily raise sea level above the total water level expected for the astronomic tide and surge.

Whilst astronomic tidal levels can be calculated many years in advance, the atmospheric and weather effects on those predicted tidal levels can only be forecast a few days in advance.

Should I be concerned about the Highest Astronomical Tide in 2015?It’s too early to know whether the maximum HAT predicted for September 2015 could pose a flood risk.  Some low lying properties around Dartmouth may be at risk from this tide but this predicted HAT level is below the level of the quayside wall (3.4m OD), which is the flood defence for most of Dartmouth.

The actual tide in 2015 will be affected by weather conditions that can’t be forecast until nearer the time.  If there’s a combination of low atmospheric pressure, wind and waves, the actual tide will be higher than the predicted HAT.  If there’s high atmospheric pressure with no wind and waves, the actual tide will be lower than the predicted HAT.

The Environment Agency forecasts and monitors flood risk 24/7 and will be monitoring the situation closely, as we do on all high spring tides. Figure 2 shows the predicted HAT level against a scale drawing of the Embankment Quayside at Dartmouth.  Mean High Water Neaps (MHWN),   Mean High Water Springs (MHWS) and two historic tidal events are also plotted on Figure 2 for comparison.

Figure 2 illustrates that the very high astronomic tide in 2010 did not overtop the Embankment Quayside, because there was no surge.  By contrast in October 2004, a moderate astronomic tide was predicted but a large surge raised the height of the sea level and localised wave action resulted in 21 properties in the Lower Ferry area being affected by flooding.  This was the most significant flooding event recorded in Dartmouth in recent years.

So flooding doesn’t always happen when High Astronomical Tides are predicted and similarly flooding is possible when moderate tides are predicted, because of the influence of the weather on actual sea levels. This highlights the importance of being vigilant to flooding all year round.How can I check if flooding is possible?

The Environment Agency is here to help, providing a free flood warning service to many areas at risk of flooding from rivers and the sea.You can check your flood risk and find out if you can register for free flood warnings, by visiting the Environment Agency’s website (www.gov.uk/flood) or by calling Floodline on 0845 988 1188.

Other places to get the latest flood update include local weather, news and travel bulletins.The Environment Agency’s website also contains advice on what to do before, during and after a flood.

First published By the Dart October 2014

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