East Coast Tide & Depth Model

Reading depths, levels and streams from the map

Hover over the map to read the water at that spot, and click to pin the reading in place. A pinned point also draws a small plot of the tide height, stream rate and direction for the whole tide, from six hours before high water at Harwich to six hours after, with a line at the time on the slider. The box shows the stream rate in knots, the direction the water is moving towards (degrees true), the tide height, the sea-bed height and the water depth, all for the time set on the slider. In the Felixstowe, Harwich Haven and Deben views, heights are in metres above Chart Datum (CD), with the Ordnance Datum (OD) figure in brackets. In the Southern North Sea view they are metres about mean sea level.

• Depth Layer (m CD) in the Colour menu paints the sea bed instead of the stream, from 15 m below Chart Datum to 5 m above it (60 m below to mean sea level in the Southern North Sea view), so the channels, banks and the Deben bar show up as colour. The arrows stay on.
• Water level (η) instead of stream rate paints the height of the water surface rather than the speed of the stream: the dark end is low water and the bright end is high water, in the colours chosen in the Colour menu. The scale is fixed at 2 m either side of mean tide level, so Scale top does nothing in this mode.
• Chart Datum is 2.02 m below Ordnance Datum in the Haven Area, Felixstowe and Harwich Throat views, and 1.93 m in the Deben view. One value is used for each view, so heights in Chart Datum are good for the harbour and the Deben, and approximate towards the edges of the wider views, where the true Chart Datum differs.
• The sea-bed height is the average over one model cell (60, 20, 10 or 5 m, depending on the view), not a single sounding, so a narrow channel or a sharp bank reads shallower or deeper than the survey shows. The depth reading is for interest and must not be used for navigation.

The Colour menu options are Stream Rate, Chart Speed Field, Ember Speed Field (three colour schemes for the stream speed), Red Rise / Green Fall (water level rising or falling, see below) and Depth Layer (m CD). Hover over each to see what its colours mean.

About the Southern North Sea view

Choosing Southern North Sea in the View menu switches to a different model run, built to show the whole basin from Cromer to the Dutch coast and from the Thames Estuary down the Dover Strait. It is made the same way as the Felixstowe views, but it answers a different question, so a few things are not the same. Please read the colours with these differences in mind.

Reading the red and green colours

The Colour menu has a Red Rise / Green Fall option. It paints the sea red where the water level is rising, green where it is falling, and white where the level is steady, for example at the turn of the tide. The depth of colour shows the speed of the stream, so a strong stream on a rising tide is a deep red and a weak one is pale. The hover box adds how fast the level is changing, in metres per hour.

The colour is about the water level, not the compass direction of the stream, and that is deliberate. Out in the open sea the stream and the water level do not turn together. The stream is often strongest around high or low water, when the level is barely changing, so a fast stream can look pale. The direction that goes with a rising tide is also opposite on the English and Dutch sides of the North Sea, and it turns right round the amphidromic point, so no single “flood” direction can be drawn across the whole basin without a break somewhere. “Flood” and “ebb” are best kept for the estuaries.

High water reaches different places at different times, so the colours across the sea are only roughly in step. Three hours before high water at Harwich, for example, the water is rising over most of the Southern Bight, the Channel and the Thames approaches, but is already falling north of the amphidromic point. Move the slider to watch the red and green patterns travel round.

What the model includes, and what it leaves out

• Driven only by the tide at its edges, taken from the TPXO10 global tide model across the northern edge, the Channel and the outer Thames. There is no wind, no surge, no river flow and no waves, so it shows an ordinary tide and never a storm.
• A typical spring tide. It is the main lunar (M2) and solar (S2) tides, plus two of their shallow-water overtides (M4 and MS4). It is not a particular day and carries no calendar date, so it says where the tide is large and fast, not what it will do next Tuesday. A neap view is not yet available.
• Depth-averaged currents. The model works on the average flow from top to bottom. Surface and seabed currents can differ, and eddies behind banks and headlands are only roughly represented.
• No tidal flats. Only ground below mean sea level can fill, so the Wash and the Wadden Sea are closed off at mean level and lose some of their tidal volume, and other shallow estuary heads are affected the same way. Tides in those shallows are not reliable. Use the area views for the Orwell, Stour and Deben.
• One friction value for the whole sea bed. The sandbank fields off East Anglia and the Dutch coast probably want a different value in places.

How good is it?

The model was checked against measured tides at tide gauges on both sides of the North Sea (British, Dutch and French). The main lunar tide comes out within about 3% of the measured range from Westkapelle to Boulogne and at Harwich and Lowestoft, within about 6% at Dover and Cromer, and about 9% high at Hoek van Holland. It also reproduces the quiet area off the Dutch coast where there is almost no rise and fall (the “amphidromic point”, marked on the map) without being told where it is. The tide there rises and falls by only about a quarter of a metre, because the tidal wave swings round that spot like the hand of a clock. The currents do not vanish, and they turn through every compass direction over the cycle. The marker is where the model’s own tidal range is smallest, about 2 km from where the TPXO global model puts it. Tidal times are good to a few degrees along most of the coast, but about 16–20 minutes late at Harwich. The speeds are broadly in line with the Admiralty tidal stream atlas, but individual arrows are not. They are least reliable near slack water and in the Dover Strait, where the currents are strongest and the channel is narrowest.

In short: trust the pattern, not the individual number. For the streams at a particular beach, channel or harbour entrance, switch to the Felixstowe, Harwich Haven or Deben views, which are built for that. The model must not be used for navigation.