Tidal streams vary in strength according to the shape of the coastline and the seabed topography. With practice, it is possible to guess where the strong streams will occur and where the streams will be less strong, or eddies will form.
Flow tends to be weak close to the shore and stronger offshore. Flow at headlands that project into the tidal stream and squeeze it can be strong. Especially strong flows can occur in narrow channels between islands.
The flow of tidal streams is similar to that of a river, but on a much larger scale. Tidal flows tend to go in straight lines over long distances, taking lots of space to turn corners. An abrupt corner in a coastline will hence usually have an eddy behind it, where the flow is weak and may even be in the opposite direction to the main stream.
The map below show some of the local tidal stream effects that occur on the North Coast of Anglesey:
Anglesey sits at the NW corner of Wales. The tidal flow is concentrated here, as the water rushes around this corner towards Liverpool.
Zooming out and looking at Anglesey with a flood tide, it should be no surprise that the fastest flows are found at the major headlands on the N and NW side of the island and in the narrow part of the straits that separate the island from Wales.
A range of information sources about tidal streams exist. The main sources are:
- Tidal diamonds (and the associated tables) on charts
- Tidal stream atlases
- Pilots and guidebooks
Each type of source presents the information in a different way, so we’ll consider them one by one in the sections that follow.