Monitoring Mitigation Management
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MONITORING MITIGATION MANAGEMENT THE GROUNDWATER PROJECT – SAFEGUARDING THE WORLD HERITAGE SITE OF BRYGGEN IN BERGEN 1 2 3 RIKSANTIKVAREN 2015 Tittel: Monitoring, Mitigation, Management, The Groundwater Project – Safeguarding the World Heritage Site of Bryggen in Bergen Redaksjon: Jens Rytter og Iver Schonhowd Opplag: 625 Design: Alkymi Design Trykkeri: A7 Print Trykkeår: 2015 Skriftsnitt: DIN Gramvekt 300/100 Papirtype: Brownboard ® Riksantikvaren 2015 www.ra.no ISBN 978-82-7574-085-2 (trykt) ISBN 978-82-7574-087-6 (pdf) henvendelse kan rettes til RIKSANTIKVAREN Postboks 8196 Dep 0034 Oslo E-post: [email protected] 4 5 DEDICATION, ANN CHRISTENSSON In loving memory of our Ann who thought of Bryggen as a bunch of delicate blossoms and knew that its continued well-being depends on getting the right mix of soil and water. This project and this publication owe a great deal to her labours and love, and we are very glad and grateful that she got to hear that the initial results were encouraging. We promise to keep tending those flowers for you, Ann. 6 7 THE HISTORY OF BRYGGEN MANAGEMENT HISTORY ARCHAEOLOGICAL ASPECTS CONTENTS 01 UNTIL C. 1900 03 05 13 Bryggen as part of the present-day townscape 40 Bryggen: World Heritage site 62 Assessment of state of preservation of 14 Bryggen as part of the oldest town nucleus 41 Management organic contexts 14 Property patterns and ownership 43 Organization 64 Practical applications 15 The Bryggen tenements 44 Acquiring knowledge 68 Review of state of preservation situation 17 Bryggen and the Hanseatic Kontor 46 Knowledge transfer at Bryggen 18 The topographical development of Bryggen 71 Schøtstuene & Øvregaten (the High Street) 73 Conclusion & future perspectives WHAT THE GROUND REVEALED – OPERATION GROUNDWATER PRESERVATION CONDITIONS 02 THE ARCHAEOLOGICAL PERSPECTIVE 04 RESCUE 06 AND DECAY RATES 1 Message from Minister of Climate and Environment 26 The magnificent archaeological deposits 50 Subsidence 76 Introduction 1 Message from UNESCO 28 Documenting and excavating Bryggen - from 54 Objectives and solutions 80 Preservation conditions above the groundwater level 3 New knowledge saves Bryggen in Bergen monuments to scraps of leather 55 Funding 87 Preservation conditions below the groundwater level 5 Introduction 29 The archaeological material 56 Measures 97 Conclusions 31 An international town arises 58 Sustainable infiltration system 31 Faces of Grimston - a tale told by pottery shards 59 Maintenance and Monitoring 32 Voices from the past 34 Concluding remarks TEMPERATURE, REDUCING AND GROUNDWATER LEVEL RESTORATION SUBSIDENCE AT BRYGGEN: LESSONS FROM THE OXIDIZING CONDITIONS THE BRYGGEN SITE PAST, MEASURES IN THE PRESENT, 07 09 11 13 CHALLENGES FOR THE FUTURE 94 What is meant by ‘redox potential’? 126 Introduction 152 Subsidence rates and monitoring 176 Introduction 95 Redox potential values at Bryggen 127 Local subsurface conditions influencing groundwater 154 Monitoring results - situation before and 178 The Historical-Archaeological Background 97 Monitoring results near sheet piling flow and level after mitigation 179 Site Management: the Problem 99 Monitoring results near swales in Bredsgården 127 Field surveys investigating groundwater flow directions 164 Conclusions and recommendations 179 Heritage Management in a Sustainable Society 100 Soil temperatures 128 Groundwater replenishment 180 From Action Research to Knowledge Transfer 102 Conclusions and recommendations 132 Recommendations for further mitigation 181 Towards a Management Plan: a Risk Assessment 183 The Management Plan: Methodology and Methods 183 Agenda for the Future GROUNDWATER BALANCE STORMWATER QUALITY AND MAINTENANCE AND MONITORING SUSTAINABLE URBAN DRAINAGE 08 10 MANAGEMENT 12 106 Urban geology and hydrogeology 137 Introduction 168 Introduction 189 Endnotes 109 Groundwater levels 137 Monitoring (storm-)water 168 Maintenance of the water management system 195 Glossary 118 Groundwater temperatures 139 Sustainable Urban Drainage Systems (SUDS 199 References 120 Groundwater modelling 143 Rainwater garden 211 Biographies 122 Conclusions and recommendations 144 Swale 147 Permeable pavement 148 Conclusions MESSAGE FROM MINISTER OF CLIMATE AND ENVIRONMENT World Heritage Site Bryggen in Bergen is one of our most important historical monuments and one of the country’s most popular tourist destinations. The Ministry of Climate and Environment has overall responsibility for the upkeep of Norway’s world heritage sites and has kept a close eye on the work at Bryggen in Bergen for a number of years. In 2011 a sum of NOK 45 million was allocated for planning and implementing measures to tackle the problems at Bryggen. Since then we have received regular reports documenting steadily rising groundwater levels and declining rates of subsidence. Sustainable systems of water management ensure that all the available surface water is channelled to where it is needed for infiltration purposes. The methods that have been employed are designed to be resilient in the face of climate change, and can easily be modified for implementation in other Norwegian centres. Our grateful thanks to all who, through the application of modern technology and innovative solutions, have contributed to the present efforts to ensure the survival of this iconic monument. Tine Sundtoft Minister of Climate and Environment MESSAGE FROM UNESCO The Bryggen World Heritage Site has been suffering from severe, long-term subsidence caused by groundwater drainage. By combining geological mapping, groundwater monitoring and modelling, along with geotechnical, geochemical and archaeological investigations, the systematic and interdisciplinary Groundwater Project has been instrumental in re- establishing groundwater levels and combating the insidious threats of subsidence and decay. This publication presents the results from this important work, which will without doubt contribute significantly to the conservation of other UNESCO heritage sites throughout the world. Kishore Rao Director, Division for Heritage & World Heritage Centre Culture Sector 12 1 NEW KNOWLEDGE SAVES BRYGGEN IN BERGEN In 2011 we started a rescue mission for Bryggen, the old wharf, in Bergen. The work was financed by the Norwegian Government, following a recommendation from the Ministry of Climate and Environment. The preparations and the work itself have been carried out by several Norwegian and international institutions. All of which have been involved in a groundbreaking project that has been crowned with success. The UNESCO cultural heritage site, Bryggen in Bergen, consists of much more than what meets the eye. Beneath the rows of wooden buildings archaeological deposits tell the tale of Bryggen’s 1000-year long history. Today, Bryggen’s appearance stems from the time after a devastating fire in 1702. But Bryggen was ruined by at least seven large fires throughout the centuries. Each time Bryggen was rebuilt on top of its old foundations and refuse. This has left us with, in some places, 10 meters thick archaeological deposits under Bryggen. By the turn of the 21st century we could observe that the old wooden buildings were suffering from severe subsidence. After initial monitoring and research we could conclude that a leaking sheet pile wall at a neighbouring hotel site was the cause of the problem. Groundwater was leaking away from Bryggen and into the hotel site, resulting in decomposition of the archaeological layers underneath Bryggen. Repairing the sheet pile wall would only be part of the cure for Bryggen. More important, the levels of groundwater had to be raised and reestablished. And new methods had to be established to secure a sustainable supply of water to the ground beneath Bryggen. The damage to the archaeological deposits can never be repaired. But today we may conclude that the groundwater beneath Bryggen has been raised and the subsidence has been reduced to a natural rate. And through new and groundbreaking methods we have secured the site’s groundwater level for the foreseeable future. Bryggen in Bergen is amongst Norway’s most important cultural heritage sites, and I would like to thank all who have contributed to the work for the past four years. This book is a testament to your knowledge and inventiveness. Congratulations all. Jørn Holme director general Directorate for Cultural Heritage 2 3 INTRODUCTION Jens Rytter & Iver Schonhowd, Directorate for Cultural Heritage Norway What we see standing at Bryggen today is the long rows of wooden buildings (tenements) running perpendicularly to the waterfront. These were features typical of most Northern European harbour towns, but the fact that it has survived the ravages of time makes Bryggen in Bergen virtually unique. It is the best preserved monument to Hanseatic trading activities in the North Sea and Baltic regions, activities and connections that have been well documented through extensive archaeological excavations and detailed historical and architectural research. The fact that Bryggen was the seat of one of the major Hanseatic overseas settlements along the North and Baltic Sea, and outlasting by far all the others, is a measure of its importance. Bergen Novgorod Lübeck Hamburg London Bruges Figure 1 Figure 1 Bryggen in Bergen and the other towns with Hanseatic privileged enclaves: London, Bruges and Novgorod. Map: O.M. Hansen, Alkymi Design. 4 5 Figure 2 Fig.2. The standing buildings, together with the deposits The ground beneath the wooden tenements, which were For a wooden settlement like Bryggen, fire has always By