BOURNEMOUTH BEACH MANAGEMENT

Transcription

1 BOURNEMOUTH BEACH MANAGEMENT Information pack for SCOPAC visit, 20 th May 2016 Plan A Good weather 09:45 Delegates board for complimentary tea and coffee 10:00 Depart for Bournemouth scheme, observing landmarks and landslide on the way. Talks by Dr Dave Harlow, Tony Flux and Stuart Terry 12:30 Lunch provided by Dorset Catering Services 14:00 - Arrive back at quay and disembark Plan B Bad weather 09:45 Delegates board for complimentary tea and coffee 10:00 Depart for Sandbanks and Brownsea Island Talks by Tony Flux and Stuart Terry Travel to Wareham Talks by Dr Dave Harlow and Neil Watson Lunch provided by Dorset Catering Services 14:00 - Arrive back at quay and disembark Introduction Bournemouth Beach Management is a 17-year project by Bournemouth Borough Council extending over 3 phases of the Environment Agency s Medium Term Plan (MTP). Year MTP Groynes Groynes Replenishment Other 2014/15 0 PAR 2015/ BIS5 2016/ / ,59, / / ,10, /21 6 BIS6 2021/ ,46,48,49 HHLG 2022/ ,51 HHLG 2023/ ,6 2024/ ,7,8,9 2025/ ,26 BIS7 2026/ / ,16 Solent Beach 2028/ / / ,57,58 Totals except 11 3 campaigns

2 Information pack for SCOPAC visit, 20 th May Timber Groyne Construction Bournemouth Borough Council has just completed the first phase of the 17-year Bournemouth Beach Management (BBM) scheme. The 17 year scheme will see the replacement of all 53 groynes in multiple short phases. The new groynes are the third series (C-series) of groynes to be constructed on Bournemouth s beach. The groynes follow a common design, similar to the old groynes and require approximately 80,000 of timber per groyne, although each groyne is carefully tailored to suit its location in relation to the beach and seawall. The positions of the new groynes have also been redesigned to adopt a more effective spacing throughout the bay. Over the winter 2015/16 the first 10 (C27 to C36) groynes have been constructed by Bournemouth Borough Council appointed contractor Mackley. Work commenced November 2015 and work finished on site May The programme of work was restricted by tides as the outer ends of the groynes could only be worked on during the low spring tides. The piles for all 10 groynes were driven into position and planking commenced shortly after. The existing (B-series) timber groynes were removed as part of the scheme, the components carefully labelled and retained for inspection, with a view to investigating the condition of the timber after 29 to 31 years of service.

3 1.1. Timber Store Site compound with delivery of new timber piles. Timber marked with BBC ownership. Pile length 10m, beacon (king) pile length 13m.

4 1.2. Existing Timber Groynes Life-expired greenheart timber piles Timber is exposed to abrasion from moving sand. Biological attack from marine borers (Gribble predation) occurs in the sapwood areas. Gribble leave approx 1mm burrows in the surface. Where burrows are numerous, the wood is weakened and more vulnerable to sand abrasion. Gribble also affects timber where there are splits, gaps and joints. This causes gaps to form between planks and also weakens the groynes by loosening the timber around the connecting bolts. This in turn provides further sheltered environments for gribble to inhabit. Pile rings on top of piles provide added strength to the ends of the piles and prevent the timber from splitting. These have only been installed on the 10 piles of landward end of the new groynes, where machinery may track back and forth. It was felt that these were not necessary on the seaward end.

5 1.3. Forming the sand bund at Low tide The small tidal range makes it necessary to form a sand bund to fit planking at the seaward end of the groyne.

6 1.4. Auger Pilot holes are created with an auger to loosen the ground in the location of the piles being driven. Photo below shows clay substrate on auger screw.

7 1.5. Piling Movax in combination with water jetting Movax making fine adjustments to pile position The method of pile driving changed in the first stages of the scheme. Piles were not able to be driven to full depth by simple hampering alone due to the dense clay substrate. A combination of pile driving methods was carried out including hammering, jetting and vibrating piles with a vibratory piling tool (Movax). However, the contractor found that the Movax could drive the piles to depth where the ground had been loosened by auger shortly before. The Movax grasps the piles and holds them in place while vibrating them in. This method allows fine adjustment of piles during the driving and eliminates the need for any support structures to position piles. This method is considerably quieter and less intrusive than hammering, which is important when working throughout the night.

8 1.6. Planking New C-series groyne being constructed close to an old B-series groyne. The photo shows the planks being attached once all of the piles have been driven into position. Planks are attached individually with coach screws, working from the top downwards. Coach screws allow tighter fixings than the nuts and bolts used on some of the older groynes. Where coach screws have been used on old groynes, the screws are in good condition and have been reused.

9 1.7. Planking to substrate 12 planks have been attached in the photograph. Planking continues for up to 17 planks or until the substrate is reached. There is no excavation of substrate. Excavators continuously maintain a sand bund around the groyne to keep the sea from flooding into the excavation.

10 1.8. Closure of groyne to seawall The groyne closes up to the seawall but is not built into it. This means the seawall is not penetrated and reduces the risk of the wall being compromised. In the past, a new cast concrete staircase was built at the site of most groyne closures.

11 1.9. B Series groyne design (Contract Drawing of 1986) The B-series groynes were raised from the original (1986) level in 2006 using pile extensions. Horizontal planking with a sloping top plank cut to shape.

12 1.10. C Series Groyne design (Contract Drawing) The new C-series groynes of 2015/16 are built one plank (0.3m) higher than the B-series to accommodate sea level rise. Planks and piles follow a sloping profile. A combination of timbers has been used to provide optimum performance.

13 1.11. Timber The new design for the 2015/16 C Series groyne uses a combination of Greenheart and Ekki timber as shown in the table. One groyne has been entirely constructed from an alternative hardwood, Basralocus. Regulations on use of certain species of hardwood timbers make procurement of structurally suitable timbers difficult. New Groyne Piles Planks C27 Greenheart 50% Greenheart 50% Ekki C28 Greenheart 50% Greenheart 50% Ekki C29 Greenheart 50% Greenheart 50% Ekki C30 Ekki 50% Greenheart 50% Ekki C31 Ekki 100% Ekki C32 Ekki 100% Ekki C33 Ekki 100% Ekki C34 Ekki 100% Ekki C35 Ekki 100% Ekki C36 Basralocus 100% Basralocus

14 1.12. Planking plan from B Series Groyne 31 built 1986, removed 2016 These record drawings provide the actual date every pile was driven and each plank attached.

15 BOURNEMOUTH BEACH MANAGEMENT 2015/16 Information pack for SCOPAC visit, 20 th May Bournemouth beach replenishment BIS5 Bournemouth Borough Council undertook beach renourishment works in A volume of 320,000m³ of dredged material was tendered to be distributed on the beach between Bournemouth Pier and Solent Beach (existing groyne No. B16 in the West to groyne No. B36 in the East). A final volume of 354,000 m³ was instructed, with the extra material being pumped into a particularly low area of beach, which was initially inaccessible due to groyne construction. A total of three submerged pipeline locations provide the route for dredged material to be deposited on the beach. Dredged material was distributed across the beach by way of a shore pipeline and bulldozers and was retained behind a temporary sand bund at the point of deposition. Van Oord UK Ltd was the appointed Contractor to supply Bournemouth with beach material of a strictly specified Particle Size Distribution (PSD). Beach renourishment material was sourced from CEMEX s Crown Estate Licenced Dredging Area 137 to the west of the Isle of Wight. The trailing suction hopper dredger Volvox Olympia was used to dredge the material from the seabed. The suction (drag) head incorporated a 65mm screen to prevent the accidental loading of unexploded or remnant ordnance into the vessel s hopper. The hopper capacity ranged between 2,500 to 3,200 m³ dependent on fuel volume on board. The Volvox Olympia sailed between Area 137 and each Location 1 to 3 via a pre-determined route, as agreed between stakeholders. Liaison between the Council, Contractor, 2 appointed Fisheries Liaison Officers and the fishing community was crucial to ensuring smooth passage for the dredge vessel to site. The dredger discharged by attaching to a floating pipeline (approx. 150m long) that was located at the offshore end of a the sinker pipeline. The floating pipeline was connected to the 650m long sinker (seabed) pipeline that pumped the material to shore. A final section of 200m on-shore pipeline distributed the material directly along the beach. At one point, this was increased to 300m but it was highly dependent on pumping capability. 2.1 SUMMARY INFORMATION Date of Load #1: Mon 14 th March-16 at 13:30 Date of last Load #134: Sun 24 th April-16 at 06:10 Average cycle time: 5.5 to 6.0 hours Final volume: 355,500m 3 Final cost: 3,900,000 approx Cost per cubic metre placed: TABLE OF VOLUMES DELIVERED Volumes Gross volume m 3 Net volume m 3 Recovery ratio (on board dredger) (measured on beach) BIS 5.1 (Location 1) NYA 96, NYA BIS 5.2 (Location 2) NYA 191, NYA BIS 5.3 (Location 3) NYA 69, NYA BIS 5 totals NYA 358, NYA Note that the final net volume measured on the beach (agreed between BBC & VO) was reduced for payment because the contractor placed too much material.

16 2.3 PSD SPECIFICATION AS AMENDED

17 2.4 PSD SPECIFICATION Revised Specification Sieve size (mm) Min % Passing Max % Passing PSD SPECIFICATION AS FINALLY DELIVERED (NOT YET AVAILABLE) Approximately 500 samples were taken on the dredger and on the beach, and all were sieved to compare with the specification. One sample only contained any silt-grade material. All the samples taken from the beach will be combined to form one aggregate sample to be compared with the specification.

18 2.6 PLAN OF THE 3 REPLENISHMENT LOCATIONS Location 1 Location 2 Location 3

19 2.7 CHART SUPPLIED WITH LNTM POOLE BAY

20 2.8 CHART SUPPLIED WITH LNTM CLOSE-UP OF BOURNEMOUTH

21 2.9 CHART OF DREDGING LICENSE AREA 137 Area 137

22 2.10 BEACH CROSS-SECTIONS, SHOWING GROYNE PROFILE FOR COMPARISON Location 1: 2.8m high crest and 80m berm Location 2: 3.2m high crest and 75m berm Location 3: 3.2m high crest and 75m berm

23 2.11 A PANORAMIC VIEW OF THE REPLENISHMENT 2.12 A PANORAMIC VIEW OF THE REPLENISHMENT

24 2.13 MULTICAT NERO LIFTING THE FLOATING PIPELINE TO HAND IT TO VOLVOX OLYMPIA 2.14 VOLVOX OLYMPIA PUMPING ASHORE

25 2.15 PREPARING THE LANDWARD END OF THE SINKER PIPELINE 2.16 PREPARING THE BUND BEFORE NEXT LOAD IS PUMPED ASHORE

26 2.17 CLIFFING OF THE NEWLY PLACED FILL 2.18 CLIFFING ELIMINATED AT LOCATION 1

27 2.19 THE END RAINBOW OVER HENGISTBURY HEAD.

28 Information pack for SCOPAC visit, 20 th May EAST CLIFF LIFT The East Cliff Lift was built in 1908 at a cost of 3,500. The fall occurred at 05:00 on Sunday 24 th April CONSTRUCTION OF THE EAST CLIFF LIFT BEGINS, 1908.

29 3.2 CONSTRUCTION WELL UNDERWAY, NOTE EROSION ON EASTERN SIDE CAUSED BY RAINFALL BETWEEN THIS AND PRIOR PHOTOGRAPH.

30 3.3 THE OFFICIAL OPENING OF THE CLIFF LIFT BY LADY MEYRICK, 1908.

31 3.4 POSTCARD DATED APPROXIMATELY 1910 Note the low beach levels with the sea lapping at the sea wall. Efforts to deal with this saw construction of Bournemouth s first experimental groyne opposite the East Cliff Lift in 1915.

32 3.5 ONLINE IMAGE, TRACEY JONES, BBC, 25 TH APRIL 2016.

33 3.6 LASER SCAN BY CCO DATED 28 TH APRIL 2016