Cowes Breakwater causes new tidal flow

by RYA on 7 Sep 2014 Its not an uncommon experience around the world that new breakwaters can have a significant impact on tidal flows.

Here is an example. Since the start of the construction of Cowes Breakwater some of the tidal flows have changed. The Local Notice to Mariners contains useful information.

Local notice to mariners dated No 23 of 2014 refers to Change to Tidal Flows in Cowes Harbour as result of the continuing construction of the new Breakwater.

The construction of the Cowes breakwater has reached a level of 3.5 metres above chart datum rising to five meter above chart datum by October 2014.

As predicted prior to construction the breakwater has had an effect on the tidal flows in the Inner Fairway to the west and south of the breakwater as follows: Inner Fairway west of the breakwater.

When the tidal flow at Nos. 1 and 2 fairway buoys is running in a westerly direction, (expected from HW -3hrs to HW +3hrs) a tidal shadow with significantly reduced flow will be experienced immediately west of the breakwater in the Inner Fairway and extending along the foreshore in front of the Royal Yacht Squadron.

Between HW-3hrs and HW vessels transiting the Inner Fairway between the Island Sailing Club and the Small Craft Channel will experience a westerly setting tidal flow of up to 1.25 knots which will have a tendency to set vessels down towards the western shore.

Large vessels entering or leaving the harbour during this period must be aware that due to the westerly going tide at Nos. 1 and 2 buoys, the significant reduction in tidal flow in the shadow of the breakwater and a possible westerly going tide in the Inner Fairway between the Island Sailing Club and the Small Craft Channel, that when the vessel crosses the boundaries of these areas the differing tidal flows may induce a turning moment.

All vessels will have a tendency to be set down towards the western shore when within the westerly tidal streams. All mariners are instructed to navigate with particular caution in the vicinity of the harbour entrance and Inner Fairway.
A full set of actual tidal flows will be recorded in November once the 2014 works have been completed and the breakwater reaches its 2014 design height of +5m above chart datum. This data will be published as soon as it is available.