WELLS, NORFOLK
Wells, in Norfolk, is a good example of a potentially hazardous estuary.
Diagram 3 (right) shows a plan of the entrance channel, which dries for most of its length, and Diagram 4 (below) is a longitudinal profile. North Sea swell often breaks as it runs over the outer sands, at the point known as Wells Bar, but the shallowest part of the entrance is further inshore, where the drying channel acts as a natural sill, retaining a low-tide pool by the town quay. For this inner harbour, a hand-drawn tidal curve can be produced as follows. (NB. In this example I have used figures for mean spring tides.)
1 Identify the tidal parameters for the mouth of the estuary
Diagram 5 (overleaf) shows a tidal curve for Wells Bar, based on the standard port of Immingham, and the usual data block with height and time differences from Immingham. The red curve has been extended to include the start of the next flood.
Using the height differences, the height of MHWS at Wells Bar will be:
MHWS Immingham minus the difference for Wells Bar: 7.3 – 1.3 = 6.0m.
As there is no data for MLWS at Wells Bar, I have assumed 0.9m, which is the value at the nearby coastal tidal stations of Cromer and Hunstanton. In the left-hand part of the diagram, those figures of 6.0m and 0.9m have been used to draw the usual diagonal line that relates tidal heights to the tidal curve.
2 Work out the drying height of the channel From the height differences again, MHWS in Wells Harbour is 7.3 – 3.8 = 3.5m.
That is 2.5m less than MHWS at the bar and indicates that the chart datum has been stepped up by 2.5m, to the height of the natural sill, as shown in Diagram 4. On the left-hand part of Diagram 5, I have inserted a vertical hatched line at a tide height of 2.5m, and from where it meets the diagonal it carries on horizontally across the diagram, chopping through the lower parts of the tidal curve.
3 Sketch in a modified tidal curve
Now comes the artistic bit. Wells Harbour with its many creeks takes longer to fill than a marina basin and the data block shows that high water in the harbour is 25 minutes later than on the bar, so I have sketched in a freehand curve, shown as a dotted line, starting from where the red line of the rising tide overtops the hatched line