Tide words, explained

WalkHilbre labels a day a spring tide when its highest predicted high water at Hilbre is 9.0 m above Chart Datum or more, and a neap tide when it is below 7.7 m.

High water

The highest level the tide reaches in each cycle. It happens about twice a day at Hilbre, roughly 12 to 13 hours apart. Times and heights are predictions, and the real water level can differ because of weather and surge. The calculated window starts 3 hours after a predicted high water and ends 3½ hours before the next one.See also: Low water, Tidal range, Calculated window

Low water

The lowest level the tide reaches in each cycle, roughly six hours after high water, when the most sand is exposed. The tide turns here and the flood begins, so low water is not a reason to delay your return. Times and heights are predictions, and weather can change the actual level.See also: High water, Tidal range, Flood and ebb

Spring tide

A tide with the biggest range of the monthly cycle, with higher high waters and lower low waters. At most ports spring tides come about two days after the new and full moon. The name has nothing to do with the season. The water rises through a greater height, so it reaches further up the sands, and wading birds use the islands to roost when other places are covered.See also: Neap tide, Tidal range, Chart datum

Neap tide

A tide with a smaller range than usual, around the half moon. High water is lower and low water is higher than on a spring tide, so less sand is uncovered. The route still floods and the channels still fill, so the calculated window applies on every tide, whatever its size.See also: Spring tide, Tidal range

Tidal range

The height difference between a high water and the next low water. The range changes over the month, from large at spring tides to small at neap tides. A big range means more sand is uncovered at low water. Tide height predictions are measured above chart datum.See also: Spring tide, Neap tide, High water

Flood and ebb

The flood is the rising tide, between low water and the next high water. The ebb is the falling tide, between high water and low water. The calculated window opens during the ebb and runs through low water into the early flood, so the water is coming back towards the route for part of the walk home. The tide turns at high water and low water.See also: High water, Low water, Gutters and channels

Calculated window

WalkHilbre’s planning estimate of when to be on the sands between Dee Lane and Hilbre. It opens 3 hours after a predicted high water and closes 3½ hours before the next. WalkHilbre treats that close as the latest time to be back on the mainland at Dee Lane, a cautious reading of RNLI advice. It cannot account for surge, weather or your own pace, so it is an estimate, not permission to cross.See also: Calculated leave-by time, High water, Surge

Calculated leave-by time

The latest time WalkHilbre suggests you start back from Hilbre so you can reach Dee Lane by the end of the calculated window. It is 60 minutes before the window closes, which allows for the longer end of the 45 to 60 minute walk back. It is a limit, not a target: start earlier if you are slow, have children or dogs, or the weather is poor. Staying on the islands over high water is not recommended.See also: Calculated window, Low water

Gutters and channels

Low channels across the sands that hold water longest and fill first as the tide returns. They can be deep and fast-flowing while the sand beside them looks dry, and they can lie behind you on the way home. Wirral Council warns of deep tidal channels on the east side of Hilbre and Middle Eye, and storms can move them, so use only the recognised route.See also: Flood and ebb, Hilbre Swash

Surge

The difference between the water level actually observed and the level predicted for the tide, caused by weather. Strong wind pushes water towards the coast and low air pressure lets the sea surface rise. A positive surge means water higher than predicted. WalkHilbre’s window cannot account for it, and the Friends of Hilbre warn that wind and low pressure can cut the route off quickly and unexpectedly.See also: Chart datum, High water, Calculated window

Chart datum

The level from which tide heights and charted depths are measured. In UK waters it is set at roughly the lowest astronomical tide, so low that the tide rarely falls below it. A height of 8.6 m means 8.6 m above chart datum. At Hilbre, chart datum is 4.93 m below Ordnance Datum Newlyn, the national datum for land heights, so the two scales differ.See also: High water, Spring tide, Surge

Hilbre Swash

A buoyed navigation channel into the Dee Estuary that runs close to Hilbre Island. It is used by boats, not walkers, and is not part of the walking route. Trinity House describes Hilbre lighthouse as a port landmark for the Hilbre Swash, and early sailing markers on Hilbre, Little Eye and the Hoylake shore were lined up to guide ships into it.See also: Gutters and channels