TIMING OF FLOOD STREAMS
Shoals can play other tricks with the flow of water. Diagram 11 shows a curve plotted from the tide gauge at Sharpness Harbour, in the upper Severn Estuary (Diagram 12). The mighty Severn flood is slowed by the extensive sand banks and when it arrives it does so in a surge, with the water level rising fast enough to be followed by eye. An up-river passage plan must anticipate that there will be little rise until a couple of hours before high water, when everything will happen in a rush. A few miles further upstream, the flood curve becomes even steeper and sometimes creates the famous Severn Bore.
Across the river from Sharpness, at Lydney’s historic gated harbour (Diagram 13), the tidal regime demands particularly good seamanship, because the Severn is another long, shallow estuary where the tide moves inland as a wave. As in all waves, the water at the crest is surging forward and reversal of flow does not occur until some time after the crest has passed, when the water level is already dropping (see Diagram 7). However, Lydney’s gates have to be shut before the tide begins to drop, to avoid damage to the primitive closing mechanism. This means that an in-coming boat must enter no later than dead high water, when the flood tide will still be sluicing across the narrow harbour mouth – at possibly 4 knots. In the old days, coasters would have been guided by local pilots and assisted into the dock by windlasses and warping sheaves. The modern yacht skipper will probably have none of those aids but must rely on careful timing and faultless boat-handling. Tides do not merely rise and fall: the vertical movements are caused by horizontal flows and vice versa. This should always be taken into account for pilotage within a shallow estuary, and it may be necessary to make adjustments to published data. Wherever you are sailing, and whether or not you are nosing up shallow rivers or not, it pays to pay close attention to the shape of the tidal curve. A perfect sine wave between single low-water and high-water points is in reality very rare. Double high or low tides, long stands at either, and sudden changes in height are common all around the coast of our islands, and trusting what a web prediction churns out could land you aground.
BARNSTAPLE – A CAUTION
Barnstaple is featured in this article because its tidal patterns are interesting, and not because I am recommending a visit. A passage up the River Taw will involve difficulties, including sand banks, strong currents, rock ledges, missing pole beacons and very few other navigation aids. Then comes confusion at Barnstaple’s high-level Western Bypass Bridge. Current charts have an adjacent note referring to a swing bridge with 1.8m headroom, but that is a different bridge, over the River Yeo tributary, and there is no air draught note for the high-level bridge. It is reported to have a centre-span air draught of 12m above MHWS but there is a high sand bank under that span; the principal channel is under the northof-centre span but there appears to be no air draught figure published for that span. So take great care if you do visit or stay downstream!