FLOOD RISK?
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.
Figure 1 – Factors that cause the observed sea level to be different from the predicted tide height
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 – Diagram to illustrate how the Highest Astronomical Tide in 2015 compares to recent tidal events
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.•
107
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156