Ebb Flow Surges And Other Nearshore Tidal Stream Effects

When planning sea kayaking journeys, we often collect forecast environmental information to help us predict likely sea conditions. Wind, swell, tidal current/height can all have profound affects on the sea’s condition. Our ability to translate environmental forecasts into reasonable sea predictions, is a skill that underpins our ability to make sound journeying decisions. In this article I’m going to share some thoughts on nearshore tidal currents.

The 50/90 rule/pattern of flow

Offshore tidal currents, in semi-diurnal tidal areas like the Irish Sea, display lovely regular patterns of tidal flow. When Liverpool Bay is at low tide, water begins to enter the Irish Sea from both its northern and southern portals with the Atlantic. Slack water (minimal flow) builds to peak (fast) flow before subsiding in velocity to another slack, when the tide is high. This process is repeated on the ebb in a dependable pattern. All of this movement is, of course, driven by the Earth’s rotation under the Moon’s Lunar orbit.

At the large scale of the Irish Sea, the 50/90 rule/pattern is awesome. Slack water is represented by 0% flow, which is proceeded by flow rates at hourly intervals of circa: 50%, 90%, 100%, 90%, 50% and 0% flows. In reality this cycle is a little more than 6 hours, taking account of the Moon’s orbit. For sea kayak planning I like to make a note of the slack water and peak flow times.

Tidal atlases, pilots, etc, help us to calculate the times of tidal streams and their intensity. The flow velocity is very much dependent on whether the gravitational effects of the Moon and Sun are producing large or small tidal ocean bulges (known as spring and neap tides, respectively).

The 50/90 rule/pattern of flow is also very clear along coastlines, where nearshore effects do not overly interfere.

Nearshore effects

1. Accelerating the flow

Tidal flow is often dramatically accelerated at headlands, through narrows and at overfalls to produce tideraces. These areas behave like a tap with a thumb placed over it. Often the accelerated flow takes the faster current offshore, see the following embedded video for many examples of this.

2. Countercurrent/eddy currents

Areas either side of headlands and bays along tidal coastlines often produce countercurrent/eddy currents. Check out the following embedded video for examples of huge countercurrent/eddies either side of Portland Bill.

2. Ebb current outflows (ebb flow surges)

Constrained bodies of water (e.g. Menai Straits, estuaries, fjords, inlets, bays etc) often do not follow the standard 50/90 rule/pattern of tidal current. Frequently, they ebb releasing their water in sudden surges.

The below anecdotal tidal flow rate graph (for the Swellies, Menai Straits) demonstrates the peak ebb flow preceding the half-way point between slacks, and the local high water.

Taken from Cruising Anglesey and Adjoining Waters - Ralph Morris

The following embedded numerical model simulation of the Menai Straits (from SEACAMS, Menai Bridge), supports the above diagram. Dramatic ebb flow surges are visible in this animation.

3. Ebb flow surges, eddy’s and accelerated current are frequently apparent in the same coastal areas.

In this final embedded video (from SEACAMS, Menai Bridge) you can easily see areas of accelerated current at Holy Island’s headlands and overfalls. The headlands’ currents migrate offshore as the tidal flow increases. The times of the main tidal stream direction change coincides with the times of high and low water in the area (8 minutes before high and low water at Holyhead).

Countercurrent and eddy effects are also visible in all but the most sheltered of the bays.

If you look closely, you may also notice ebb flow surges of current departing Abraham’s Bosom and Penrhos Bay in southerly directions, in addition to the more constrained Cymyran Straits. They all display their strongest flows well in advance of the midway point between offshore slack waters.

In conclusion, when rationalising sea state, it is well worth knowing when the local offshore slack and peak flow times are. And how the main tidal flow may locally: 1) accelerate, 2) set up counter currents/eddys and 3) combine with ebb flow surges. It is certainly worth being aware of the early power of ebb flow surges from: straits, estuaries, fjords, inlets and bays.

Generally, on the West Coast of Anglesey, I am more relaxed about current in the second half of the south-going tidal cycle than the first half.

Geth Roberts