Source

The were no strong winds associated with the event – the powerful swell was generated by high winds in the circulation of an intense depression which was centred near Newfoundland three days earlier (Eden, 2008a). This is also recounted by Palmer et al. (2014) from assessments by Draper and Bowmass (1982). The event on 13^th February 1979 formed from a depression in the mid-North Atlantic which moved at an unusually high speed (approximately 15.5 m/s) such that it travelled at the same velocity and in the same direction as wave components with a period of 18–20 seconds. Resultantly, wind energy was favourably transferred into this longer-period part of the spectrum and generated high amplitude, long period waves which propagated into the English Channel, arriving at a time of spring tides and low barometric pressure (Palmer et al., 2014).

Tide gauge data at Newlyn and Portsmouth indicates that the sea level return period at these two sites was less than a 1 in 1 years (albeit with an approximately 0.4 m surge).

The swell event was thought to have a return period of about 1 in 50 years (e.g. Lewis, 1979) although the rarity of these events makes probabilities difficult to assess (Palmer et al., 2014).

Pathway

Overtopping was the primary flood pathway during this event, most notably at Chesil Beach owing to long period waves exceeding its 12 m shingle crest. Eden (2008a) describes that following approximately 1 hour of “relentless” wave action, the beach was breached in several places, whereas Frampton (1997) suggests there was not actual breaching or lowering of the crest but flattening of the beach face and massive overspill (of water and shingle). There were also damages at moorings and in harbours in the Isles of Scilly and Plymouth, and damage at Tenby (South Wales). At Seaton (Devon) coastal structures were “badly damaged” and there was flooding at Hayling Island (Draper and Bowmass, 1982).

Receptor and Consequence

Despite the relatively modest sea levels, the long period swell waves resulted in considerable overtopping and consequent damage during this event. Damage was recorded all along the southern English coastline (Draper and Bowmass, 1982), from Land’s End to Hayling Island (Palmer et al., 2014). The overtopping at Chesil Beach badly affected Chiswell, Isle of Portland which became isolated once the road adjoining the mainland was inundated (Lamb, 1991; Zong and Tooley, 2003; Eden, 2008a, Eden, 2008b). This area had been impacted by severe flooding only 3 months earlier (13^th December 1978).  Victoria Square was under 4 ft. [1.2 m] of water (Lewis, 1979). Parked vehicles were displaced into rows of “crumpled” wrecks, and some residential properties were flooded to a depth of 6 ft. [1.8 m] (West, 2014). Parked cars piled on top of one another (Lewis, 1979). This event caused massive trauma and stress for both residents and emergency workers.  Damage to property was estimated at £250,000 [approximately £1.3 million at 2016 prices] (Lewis, 1979). The indirect and intangible costs, however, are likely to have been higher (Frampton, 1997). The inundation temporarily isolated Portland from the mainland, which had profoundly disruptive impacts (Frampton, 1997). The absence of strong local winds and the long period swell waves allowed a warning of approximately 1 hour. As a direct response to this event (and the flooding in 1978), over £5 million was invested in defence improvements within the Chiswell area.

Summary Table

*No known sources of information available