Join WILLIAM THOMSON on his journey each month exploring a Shipping Forecast region
In last month’s column I focused on Plymouth and the way the River Tamar creates a natural threshold to the wonders of Cornwall. Following this theme of geological features creating the transition from one place to the next, few are more notable than the iconic Isle of Portland, connected to the mainland by a narrow strip of shingle at the end of Chesil Beach. Protruding deep into the English Channel, Portland creates a natural barrier and the coastline on either side of the isle is strikingly different.
To the east is the vast sweep of Lyme Bay, a pebbly beach which flows in a smooth arc all the way from Lyme Regis to the cliffs of Portland. The shoreline is characterised by crumbly cliffs around Golden Cap and West Bay, with a steeply sloping beach that creates dangerously steep shore breaks. In contrast, to the west of Portland the coast is punctuated with coves and sea arches, immortalised in the picture-postcard beautiful Durdle Door and Lulworth Cove.
Regardless of which side of Portland you see it from – the crumbly Lyme Bay or hard Purbeck Coast – there is something about it that looks ‘back to front’. Rising steeply up from Portland Harbour with near vertical cliffs, it then gradually slopes down to the sea at the infamous Bill. This is opposite to most cliffs, which gently rise up as they get close to the sea, then drop precipitously down to the waterline. This backto-front geology is a reminder that different rules apply here, that you should expect the unexpected, a quirk of nature.
Portland’s uniqueness is forewarned in nautical charts with a series of squiggly lines. These are the symbols for overfalls, defined by the International Hydrographic Dictionary as ‘short breaking waves occurring when a strong current passes over a shoal or other submarine obstruction or meets a contrary wind or current’.
In the case of Portland, the fearsome overfalls that constitute the world-renowned race are made by fast tidal streams being accelerated down the isle before they collide with an underwater reef that extends out from Portland.
For anyone navigating around the Bill, keeping a close ear out for the Shipping Forecast is vitally important. The final sentence of the definition gives a clue as to what for look out for, in particular when the wind or swell is contrary to the direction of the tidal streams.
When this happens, the race gets supercharged and it is advisable to stay at least ten nautical miles (18km) offshore to avoid the steep breaking waves. Nobody should ever contemplate swimming, kayaking
