Rebuilding of Marine Cavernous Boulder Reefs in Kattegat BLUE REEF

Transkript

1 The Danish Forest and Nature Agency Rebuilding of Marine Cavernous Boulder Reefs in Kattegat BLUE REEF August 2009

2 NIRAS i The Danish Forest and Nature Agency Rebuilding of Marine Cavernous Boulder Reefs in Kattegat BLUE REEF August INTRODUCTION THE QUARRY SEA TRANSPORT PLACING OF BOULDERS CONSULTING / SURVEY COMMENTARY MAPS DANISH SUMMERY / SAMMENFATNING Rapport Blue Reef JOS AHA JOS 0,0 Rapport Blue Reef JOS GBJ JOS Udgave Betegnelse/Revision Dato Udført Kontrol Godkendt NIRAS A/S Sortemosevej 2 Telefon \\allkfs01\data\sag\13\177.00\project DK-3450 Allerødocumentation\rapport\blue Fax reef rapport 1.0.doc Tilsluttet F.R.I niras@niras.dk

3 NIRAS 1 1. INTRODUCTION The objective of this project was to restore the Blue Reef- a boulder reef about 8 nm north-east of the Island of Læsø. The project will restore 6,5 ha of maritime cavernous boulder reef and stabilise 6 ha of existing reef area. In total tonnes / m³ of boulders imported from Norway, were used in the project. Blue Reef is the first large marine nature restoration project in Denmark as well as the first offshore restoration project receiving 50% co-financing from the European Commission Life Fund. The project commenced on 1 st of August 2006 and will be completed in the spring of The Danish Forest and Nature Agency (DFNA) and Agency for Spatial and Environmental Planning (ASEP) selected two prequalified companies to carry out the task: NIRAS is a leading consultant on civil works in Denmark in general and marine works in particular. NIRAS was therefore a natural choice when DFNA/ASEP were looking for a company to supervise the contractor/works. HOFFMANN A/S is a contractor based in Denmark and the company has many years of experience in maritime civil works including harbours, piers, dredging etc. HOFFMANN A/S had the right equipment and manpower to construct the Blue Reef.

4 NIRAS 2 2. THE QUARRY The boulders used for building the reconstructed reef at Læsø Trindel Blue Reef originate from a quarry in the southern part of Norway just outside the town of Krageø. The bedrock in this area is Norit / Hyporit and was found suitable for use in the Blue Reef project. The quarry s main product is aggregate for asphalt and tarmac production. Denmark is by the way its primary market. In the production, while blasting, there is a surplus of boulders which are so large that they can t fit in the crusher. The large boulders are normally blasted down to handling size but in this case they were used for reconstruction of the Blue Reef. The large amount of surplus boulders came in various sizes from 0.5 to 10 tons so it was important to sort out the right sizes for the project. As specified in the project two fractions were needed, one with a mean weight of W=1600 kg and one with a mean weight of W=3300 kg. At the The reference pile by the excavator. Sammenlignings stak ved gravemaskine.

5 NIRAS 3 same time all the boulder fractions should be in the range of 600 to 6000 kg. In the quarry the surplus boulders were sorted in two different piles according to mean weight as a standard of reference. The piles consisted of approximately units. Each pile was used as standard reference for production of boulders for the project. The weight was painted on the face of each boulder in the reference piles. NIRAS visited the quarry to ensure that quality and size of the boulders were correct. It was also important to inspect that handling/ loading and the sorting were done according to specifications. The digits on the boulders were so large that the wheel loader and excavator operators were able to see them from their cockpits. This was important so they were able to grade the boulders correctly. 80 % of the boulders for use in the project were in stock even before the first load was sent to Læsø Trindel. Dumper loaded by an excavator. Dumper læsset af gravemaskine. When the barge arrived at the quay near the quarry excavators loaded the boulders from the different piles onto wheel dumpers. The wheel dumpers drove the short distance from the quarry to the barge and unloaded the boulders directly on the deck of the barge. Then a wheel loader stacked the boulders op to a height of approximately 5 metres. This process continued for about a day until the barge was fully loaded. ( tonnes)

6 NIRAS 4 3. SEA TRANSPORT A large tug boat towed the barge from Krageø to the project area at Læsø Trindel, a distance of about 110 nm. The barge was able to leave the quay at high tide only, due to the shallow waters in the fjord leading to the quarry. Onboard the barge along with the boulders there were two excavators and one wheel loader, equipment to be used for unloading the barge. Barged with boulders and unloading equipment. Flåde med dæksten og entreprenør grej. The round trip was done eight times and the turnaround time was about one week. The working area at Blue Reef was marked with 6 large yellow buoys. The buoys were placed by the Danish Maritime Authorities. They were placed to indicate work area and shallow waters. They also helped keeping unauthorized persons off the site.

7 NIRAS 5 Before the contractor commenced the placing of boulders all coordinates were double checked together jointly with the consultants. The working area at Blue Reef was marked with 6 large yellow buoys. Arbejdsområdet ved Blue Reef var markeret med seks store gule bøjer

8 NIRAS 6 4. PLACING OF BOULDERS The Barge arrives at the work site. Flåden ankommer til arbejdsområdet. The barge was towed into position and fixed to anchors in combination with a constant pull from the tug boat. The position of the barge was constantly checked by an onboard GPS tracker system. When in position the excavators started to dump boulders over the side of the barge. A separate crane with at grab was also used for placing the boulders. Some boulders were placed in an in-water stock pile. This was done in order to minimize the turnaround time for the barge and so that the crane had boulders to work with while the barge was away in Norway.

9 NIRAS 7 The excavators placing boulders at the work site. Gravemaskiner placerer sten ved arbejdsområde. To ensure that the boulders were placed correctly the contractor had mounted at GPS tracking system on each piece of equipment. By activating the GPS-tracker the operator was able to verify the position of the boulders. A GPS tracker was also installed on the crane. NIRAS visited the working area a few times while the Contractor was placing the boulders to ensure that the work was done according to the specifications. NIRAS also conducted an in-water survey and at the same time made a video film showing boulders as placed on the seabed. The design of the restored reef was divided in three sections (see map on page 11): Lobster Bank M1 : A rectangular and flat area (240 x 40 meters). This area was built up by dumping the boulders over the side of the barge while two tugs pulled the barge slowly from one end of the area to the other. Then the area was surveyed and corrections were made if necessary. Cones W1 +W2. The Cones were in two different sizes and placed in groups. Two large Cones M2 were located alongside the Lobster Bank M1. The cones were built up by dumping the

10 NIRAS 8 boulders over the side of the barge while fixed. Then the cones were surveyed and if needed adjustment were made. The adjustments were carried out by a crane mounted on a dredger. The last and the largest areas were called the stabilised area E1 + E2. Located at the fare east end of the working area. Along the east side of E1+E2 there is a steep slope. It was important to protect this slope by covering it with boulders. The largest area of the E1+E2 was covered with boulders directly from the barge as described earlier. Whereas the boulders on the slope were placed by a crane mounted on a dredger. Diver s buoy showing the location of the Blue Reef Dykker bøje viser placering af Blue Reef Top of Diver s buoy with logos. Top af dykker bøje med logoer.

11 NIRAS 9 5. CONSULTING / SURVEY While the work was in progress a number of site meetings were agreed, about every fortnight. These meetings were held at (DFNA) main office. At every meeting representatives from the Contractor (HOFFMANN), the Consultant (NIRAS) and of course the Agency for Spatial and Environmental Planning were present. The purposes of the meetings were to inform all parties about the progress of the work and whether there were any changes to the contract. The time schedule was scrutinized at all meetings. There was a good atmosphere at the meetings due to the well prepared participants. One of the site meetings was even arranged onboard the barge, and the representatives from the The Danish Forest and Nature Agency and the Agency for Spatial and Environmental Planning had a hand-on experience of the work. When the last boulder had been placed a survey company was contracted to carry out the final survey of the entire area. This was an important factor to ensure that the work had been carried out correctly. Due to the bad weather situation in the autumn of 2008 the survey company was only able to carry out the survey in January 2009, 4 months after the last boulder had been placed. In general the survey showed that the boulders were placed correctly but unfortunately the survey also showed that in a section of E1 the boulders were scattered more than acceptable. The Contractor was instructed to return to the area so he could move the wrongly placed boulders to their right position. NIRAS made a final survey one year after the project started and was satisfied with the adjustments made by the contractor as mentioned above.

12 NIRAS COMMENTARY There will always be bumps on the road with contracts of this size. We had our problems too. The weather was not our best friend all the time; it was a very windy summer of So windy that, one time, the barge loaded with tons of boulders and equipment broke loose and went on its way towards Sweden. The barge stopped after app. 10 nm when its anchor caught a major power cable. This was of course inconvenient. Our biggest problem was however the strong current. Due to the current it was quite a task to hold the barge in place while unloading. The original plan to hold the barge with two anchors and one tug boat, had to be changed to two tug boats only. It is interesting to see the impact from the reef on the environment. After one year all the boulders had been covered with sea grass and seaweed. It s now hard to see the surface of the boulders. It looks like a long haired carpet when you view over the area. While diving to the boulders I passed a school of cod and in the distance I saw the blink from a school of herring. Further down and in between the boulders I saw cuokoowrasse and ballanwasse in the hundreds. On almost every boulder there was a starfish and Crab on the seabed. I saw shrimp swimming all over the place. I didn t see any lobster or Norway lobster, but one thing is for sure: The environment at Blue Reef is ready for them

13 NIRAS MAPS Ekstra stabilised area Boulder cones. W1 and W2 Lobster Bank and cones. M1 and M2 Stabilised area E1 and E2 Drawing made by HOFFMANN A/S

14 NIRAS 12 Stabilised area Lobster Bank and cones. Scan from the plotter showing by flags where the boulders were placed. One map of every load of boulders was made, eight in total. Red flag: Corners of the Lobster Bank. Green circle: Cones. Green flag: Borders of the E1 and E2 area. Blue flag: Position of 5-10 boulders. Skærmbillede af plot der med flag viser placering af udlagte dæksten. Der blev udarbejdet et skærmbillede pr. last. 8 i alt. Røde flag: Hjørner af Hummer banke. Grønne cirkler: Kegler. Grønne flag: Afgrænsning af E1 og E2. Blå flag: Placering af 5 10 dæksten.

15 NIRAS DANISH SUMMERY / SAMMENFATNING INTRODUKTION. Formålet med dette projekt er at genoprette dele af stenrevet ved Læsø Trindel kaldet Blue Reef (BR). 12,6 ha er genoprettet hvor den ene halvdel er huledannende sten og resten er et lag stabiliserende sten. Der er udlagt tons. Projektet er det største af sin art i Danmark og samtidig det første der er udført på hav bunden. EU (Life Fund) deltager i finansiering med 50 %. Projektet strækker sig over perioden august 2006 frem til foråret Skov- og Naturstyrelsen (SNS) og By og Landskabsstyrelsen (BLST) er bygherre på projektet og følgende selskaber blev udvalgt til at udfører: NIRAS er et af de førende rådgivende ingeniørfirmaer i Danmark i almindelighed og inden for maritime arbejder i særdeleshed, og et naturlige valg til dette projekt. HOFFMANN A/S er et dansk entreprenørfirma med mange års erfaring inden for maritimt arbejde bl.a. uddybning, havne, moler m.m. STENBRUDDET. Sten der blev brugt til projekt blev brudt i et stenbrud i det sydlige Norge. Stenbruddet ved Krageø producerer normalt skærver til asfaltproduktion, bl.a. til Danmark. Ved udsprængning af fjeld er der et overskud af sten, der er for store til knusning, disse sten sprænges normalt ned i størrelse. men blev nu lagt til side for brug ved BR. Der skulle bruges 2 fraktioner ved BR, henholdsvis middel vægt på W=1600 kg og W=3300 kg. Alle sten skulle veje mellem 600 kg og 6000 kg. Der blev lavet to sammenlignings stakke. Vægten af de enkelte sten blev malet på siden, så maskinfører kunne sortere korrekt. Når stenprammen var ankommet blev stenene læsset på dumpere, der kørte direkte fra stenbrud ombord på stenprammen og læsset af. Ombord på flåden blev sten stakket med en gummihjulslæsser op til en højde på ca. 5 meter, for at udnytte pladsen bedst. Lastekapaciteten på flåden var ca tons. SØTRANSPORT: Stenprammen blev trukket at en slæbebåd de ca. 110 sømil til projektområdet. Stenprammen var så tungt læsset, at den kun kunne forlade Krageø ved højvande. Ombord på stenprammen var udover sten, materiel til losning. Der blev sejlet 8 læs fra Norge til Læsø trindel, frekvensen var ca. en uge. Projektområdet ved BR var afgrænset af 6 stk. gule bøjer, der var sat at Farvandsdirektoratet. Før opstart af arbejdet blev alle koordinater kontrolleret af entreprenør og tilsyn. UDLÆGNING AF STEN:

16 NIRAS 14 Stenprammen blev trukket i korrekt position af slæbebåd, hvis position blev kontrolleret gentagne gange med GPS. Når stenprammen var i korrekt position begyndte gravemaskiner at lægge sten udover siden. Ca. ved hver femte sten blev position registeret på GPS- trekker. Der foregik også udlægning af sten fra stenfisker, disse blev også registret på GPS- trekker. For at udnytte stenprammen bedst muligt blev nogle sten lagt i et undersøisk depot, så flåden kunne returnerer til Norge. Stenfisker arbejdede videre med sten fra depot mens pram hentede flere sten. Sten blev lagt ud i tre forskellige designs (se kort på side 11): Hummerbanke (M1); et 240 x 40 meter område hvor dæksten blev lagt ud i et lag. Ved to af siderne blev sat stenkegler (M2). Kegler (W1-W2) i to forskellige størrelser der blev sat i grupper mod vest i projektområdet. Stabiliseret område (E1+E2); det største område med et bundstabiliserende lag. Langs den østlige kant af området er en skråning, et fald på 4-6 meter, der blev stabiliseret med sten. TILSYN / OPMÅLINGER. Under hele arbejdsperioden førte NIRAS tilsyn med arbejdet, både stenbrud og projektområdet ved BF blev besøgt gentagne gange. Udover besøg blev der foretaget dykkerinspektioner og optaget video-film af sten på bunden. For at orienterer alle parter blev der ca. hver fjortende dag afholdt byggemøde hos SNS, BLST, med deltagelse af bygherre, tilsyn og entreprenør. Et af møderne blev afholdt ombord på stenprammen så repræsentanter fra SNS og BLST kunne få et førstehånds indtryk af arbejdet. Da den sidste sten var lagt blev der foretaget en opmåling af hele området, dels for at se om sten var lagt korrekt og for at sende resultater til Farvandsvæsnet med tegning af søkort for øje. Grundet meget dårlige vejrforhold, især vind, blev dette arbejde først fortaget i januar 2009, 4 måneder efter den sidste sten var lagt. Generelt viste opmåling at stenene lå acceptabelt, dog med undtagelse af et område ved østlig side af E1 og E2. Entreprenør vendte tilbage til BR og fik flyttet sten til ret placering. Et år efter projektstart foretog NIRAS det sidst tilsyn med dykker. Inspektionen viste at det område ved den østlige side af E1 og E2, hvor der manglede sten, var bragt i orden. Den sidste gerning på projektområder BLUE REEF var at placerer en Dykkerbøje som identifikation og som udgangs punkt for sportsdykkere. KOMMENTAR. Med projekter af denne størrelse vil der altid være ekstra udfordringer i forbindelse med udførelse. Vi havde vores. Det blæste meget i kontrakt perioden, så meget at stenprammen med tons sten og entreprenørmaskiner rev sig løs og var på vej mod Sverige. Prammen blev stoppet da dens anker greb fat i et undersøisk el-kable, efter ca. 10 sømil. Prammen blev bragt tilbage og arbejdet kunne fortsætte. Vores største problem var dog den kraftige og konstante strøm. Det viste sig at være et problem at holde prammen på plads med ankre og en slæbebåd. Entreprenør valgte at bruge en ekstra slæbebåd i stedet for ankre. Der er interessant at se den indflydelse stenene har haft på miljøet efter bare et år.

17 NIRAS 15 Efter et år er bunden dækket af søgræs, og der vokser tang på stort set alle sten. Når man ser ud over området ligner det et meget langhåret tæppe, der står og svajer for strømmen. Da jeg dykkede ned mod stenene passerede jeg stimer af torsk, idet fjerne var der blink fra stimer af sild. Længere ned mellem sten var der mange, mange blåstak/rødnæb og berggylte. På næsten alle sten sad der søstjerner. Krabber kravlede rundt ned på bunden ind og ud af huler, rejer svømmede rundt over der hele. Jeg så ikke nogle hummere eller jomfruhummer, men en ting er sikkert: Miljøet ved Blue Reef er indflytnings klar.