Jurassic Coast - the scheme

Chiswell case study: The Scheme

Aerial view of Chiswell, the beach, road and Naval Base (now Portland Port and Sailing School), 1997

The scheme involved gabion baskets on the beach crest, some improvement to the existing sea walls and a sunken culvert drain within the beach linking to an open channel draining into Portland Harbour. The main road behind the beach was also raised in order to alleviate flooding

Chesil Beach from the West Weare cliffs of Portland. The beach is a Site of Special Scientific Interest

The Scheme

12. The responsibility for maintaining sea defences rested with both Wessex Water, under the Land Drainage Act 1976, Section 17, and the Weymouth and Portland Borough Council, under Section 98. Both authorities had permissive powers to maintain and improve the existing sea defences. The construction of new sea defences is eligible for grant aid if approved by the Ministry of Agriculture, Fisheries and Food (MAFF).

13. Following the two flood events, the two authorities (sponsoring authorities) instructed Dobbie and Partners, Consulting Engineers (Dobbie), to recommend options for dealing with the stability of the Beach and thus to alleviate the flooding of Chiswell. A preliminary report was received in March 1979, but from the information then available, conclusive recommendations could not be made. The sponsoring authorities, therefore, authorised further investigations of the shingle structure and offshore marine conditions, together with a Cost benefit assessment of the preferred scheme. The consultants' brief was to consider alternative methods of achieving major flood alleviation to the community at Chiswell, and proposals to protect the access along the Weymouth Road. The final report was received in July, 1980.

14. The consultants advised that it was practicable to provide a protection against a storm with an estimated return period of five years, and also that substantial reductions in depth and duration of flooding could be effected during storms of a much higher return period. In addition, the Council accepted that much could be achieved further to secure individual properties, and it recommended the approval of a General Improvement Area Scheme (GIA) for Chiswell and the adjacent Fortuneswell area, thus demonstrating confidence in the proposed sea defence scheme and hence encouraging the enhancement of the settlement.

15. The consulting engineers recommended a scheme which comprised the following

(a) a length of gabion crest protection to the Beach, extending 1600 m northwards from the existing sea wall

(b) modifications to the existing sea wall

(c) a new intercepting drain along the landward side of the Beach, from Brandy Row to the head of the Wessex Water Authority channel and modification of the existing Wessex Water Authority channel

(d) raising Weymouth Road to a level of + 3.00 m A.0.D. The estimated capital cost of the works was £5 million with an annual maintenance cost in the region of £37 000.

16. Because Chesil. Beach is a unique national heritage, the Institute of Oceanographic Science (IOS) was asked to produce a report on the environmental impact of Dobbie's scheme, and the Nature Conservancy Council (NCC) (now English Nature) was consulted in respect of the effect of the proposals on the Beach. Their comments can be summarised as follows: the IOS Report, in general welcoming and supporting the scheme, commented:

Although in most respects the scientific Interest of the site would be best maintained with the least man-made intervention, it may be argued with some justification that an element of sea defence work is necessary to enable the very existence of Chesil Beach itself as both protective bastion and scientific entity.

The NCC accepted that the interceptor drain was an essential element of the scheme. It would have some impact on the Beach and its environs, but this could be limited if it were formed as near to the road and as far from the Beach crest as possible. It would also be helpful if the excavated material could be placed on the seaward side so that it might, if necessary, be used to reinforce the crest in the future. The NCC also expressed reservations on the trial length of gabions, and stressed that it would be acceptable for foreign material to be placed in the baskets only if it could be clearly distinguished from that indigenous to the Beach. It was therefore agreed that for part of the trial length, the gabions would be filled with indigenous materials, and for the remaining part with material from the local Portland quarries.

17. The sponsoring authorities commissioned the Flood Hazard Research Centre of Middlesex Polytechnic to present a Cost benefit assessment of the project to ensure that it would be reasonably viable. This, in turn, would support an application to MAFF for grant aid. The report concluded that the cost of direct and indirect damage attributed to the storm of five-year return period totalled £525 000 for each event, and that attributed to the ocean swell of fifty-year return period totalled £923 000 for each event. It also concluded that, by discounting the capital sums, the amounts that were worthwhile allocating to prevent future flood damage were: storm surge flood £12.12 million; ocean swell flood - £14.50 million. The report also referred to the intangible benefits of flood alleviation at Chiswell, which were unusually high, and specific reference was made to the level of fear and stress in the community during the events.

18. Armed with favourable reports from the NCC and with the encouragingly positive benefit assessment, application was made for grant aid to the Ministry of Agriculture, Fisheries and Food. The Dorset County Council agreed to provide financial support; but the Ministry of Defence, despite the benefit to its property, felt unable to provide a contribution towards the scheme.

19. It should also be recorded that, in addition to the representatives of Wessex Water, the Borough Council and Dobbie, the regional engineer of MAFF, co-opted member of the Technical Working Group engaged on the detailed design of the project, rendered a valuable contribution to the ultimate design.

20. After consideration of the recommendations, the sponsoring authorities instructed Dobbie to proceed with the scheme in September 1980, beginning with the gabion crest protection.

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		Chiswell case study: The Scheme
Aerial view of Chiswell, the beach, road and Naval Base (now Portland Port and Sailing School), 1997 The scheme involved gabion baskets on the beach crest, some improvement to the existing sea walls and a sunken culvert drain within the beach linking to an open channel draining into Portland Harbour. The main road behind the beach was also raised in order to alleviate flooding Chesil Beach from the West Weare cliffs of Portland. The beach is a Site of Special Scientific Interest		The Scheme 12. The responsibility for maintaining sea defences rested with both Wessex Water, under the Land Drainage Act 1976, Section 17, and the Weymouth and Portland Borough Council, under Section 98. Both authorities had permissive powers to maintain and improve the existing sea defences. The construction of new sea defences is eligible for grant aid if approved by the Ministry of Agriculture, Fisheries and Food (MAFF). 13. Following the two flood events, the two authorities (sponsoring authorities) instructed Dobbie and Partners, Consulting Engineers (Dobbie), to recommend options for dealing with the stability of the Beach and thus to alleviate the flooding of Chiswell. A preliminary report was received in March 1979, but from the information then available, conclusive recommendations could not be made. The sponsoring authorities, therefore, authorised further investigations of the shingle structure and offshore marine conditions, together with a Cost benefit assessment of the preferred scheme. The consultants' brief was to consider alternative methods of achieving major flood alleviation to the community at Chiswell, and proposals to protect the access along the Weymouth Road. The final report was received in July, 1980. 14. The consultants advised that it was practicable to provide a protection against a storm with an estimated return period of five years, and also that substantial reductions in depth and duration of flooding could be effected during storms of a much higher return period. In addition, the Council accepted that much could be achieved further to secure individual properties, and it recommended the approval of a General Improvement Area Scheme (GIA) for Chiswell and the adjacent Fortuneswell area, thus demonstrating confidence in the proposed sea defence scheme and hence encouraging the enhancement of the settlement. 15. The consulting engineers recommended a scheme which comprised the following (a) a length of gabion crest protection to the Beach, extending 1600 m northwards from the existing sea wall (b) modifications to the existing sea wall (c) a new intercepting drain along the landward side of the Beach, from Brandy Row to the head of the Wessex Water Authority channel and modification of the existing Wessex Water Authority channel (d) raising Weymouth Road to a level of + 3.00 m A.0.D. The estimated capital cost of the works was £5 million with an annual maintenance cost in the region of £37 000. 16. Because Chesil. Beach is a unique national heritage, the Institute of Oceanographic Science (IOS) was asked to produce a report on the environmental impact of Dobbie's scheme, and the Nature Conservancy Council (NCC) (now English Nature) was consulted in respect of the effect of the proposals on the Beach. Their comments can be summarised as follows: the IOS Report, in general welcoming and supporting the scheme, commented: Although in most respects the scientific Interest of the site would be best maintained with the least man-made intervention, it may be argued with some justification that an element of sea defence work is necessary to enable the very existence of Chesil Beach itself as both protective bastion and scientific entity. The NCC accepted that the interceptor drain was an essential element of the scheme. It would have some impact on the Beach and its environs, but this could be limited if it were formed as near to the road and as far from the Beach crest as possible. It would also be helpful if the excavated material could be placed on the seaward side so that it might, if necessary, be used to reinforce the crest in the future. The NCC also expressed reservations on the trial length of gabions, and stressed that it would be acceptable for foreign material to be placed in the baskets only if it could be clearly distinguished from that indigenous to the Beach. It was therefore agreed that for part of the trial length, the gabions would be filled with indigenous materials, and for the remaining part with material from the local Portland quarries. 17. The sponsoring authorities commissioned the Flood Hazard Research Centre of Middlesex Polytechnic to present a Cost benefit assessment of the project to ensure that it would be reasonably viable. This, in turn, would support an application to MAFF for grant aid. The report concluded that the cost of direct and indirect damage attributed to the storm of five-year return period totalled £525 000 for each event, and that attributed to the ocean swell of fifty-year return period totalled £923 000 for each event. It also concluded that, by discounting the capital sums, the amounts that were worthwhile allocating to prevent future flood damage were: storm surge flood £12.12 million; ocean swell flood - £14.50 million. The report also referred to the intangible benefits of flood alleviation at Chiswell, which were unusually high, and specific reference was made to the level of fear and stress in the community during the events. 18. Armed with favourable reports from the NCC and with the encouragingly positive benefit assessment, application was made for grant aid to the Ministry of Agriculture, Fisheries and Food. The Dorset County Council agreed to provide financial support; but the Ministry of Defence, despite the benefit to its property, felt unable to provide a contribution towards the scheme. 19. It should also be recorded that, in addition to the representatives of Wessex Water, the Borough Council and Dobbie, the regional engineer of MAFF, co-opted member of the Technical Working Group engaged on the detailed design of the project, rendered a valuable contribution to the ultimate design. 20. After consideration of the recommendations, the sponsoring authorities instructed Dobbie to proceed with the scheme in September 1980, beginning with the gabion crest protection.

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