Manual for Assessing Hydraulic Safety of Existing Dams - Volume I Doc. No. CDSO_MAN_DS_04_v1.0 June 2021 Central Water Commission Ministry of Jal Shakti Department of Water Resources, River Development & Ganga Rejuvenation Government of India Front Cover Photograph: Sardar Sarovar Dam (India) during the monsoon flood. Copyright © 2020 Central Water Commission. All rights reserved. This publication is copyright and may not be resold or reproduced in any manner without the prior consent of Central Water Commis- sion. Government of India Central Water Commission Central Dam Safety Organization Manual for Assessing Hydraulic Safety of Existing Dams June 2021 Dam Safety Rehabilitation Directorate 3rd Floor, New Library Building R. K. Puram New Delhi - 110066 Doc. No. CDSO_MAN_DS_04_v1.0 Page i Government of India Central Water Commission Central Dam Safety Organization Disclaimer The Central Water Commission under the Dam Safety and Improvement Project has undertaken to prepare this Manual for Assessing Hydraulic Safety of Existing Dams to provide necessary guidance for ensuring the safety of existing dams against adverse hydrologic and hydraulic events. The design studies and measures required will vary from dam to dam depending on the type of prob- lems encountered. While every effort has been taken to incorporate all basic details as per the latest state of the art, yet it is not possible to cover all the conditions/problems which may be faced in the field. CWC absolves itself from any responsibility in this regard and dam owners and others involved with the dam rehabilitation activity should use their discretion in imple- menting the guidelines contained in this Manual. For any information, please contact: The Director Dam Safety Rehabilitation Directorate Central Dam Safety Organization Central Water Commission 3rd Floor, New Library Building (Near Sewa Bhawan) R. K. Puram, New Delhi – 110066. Email: [email protected] Doc. No. CDSO_MAN_DS_04_v1.0 Page ii MESSAGE The Central Water Commission has been publishing various Guidelines and Manuals in the area of dam safety under the World Bank assisted Dam Rehabilitation and Improvement Project. This Manual titled ‘Manual for Assessing Hydraulic Safety of Existing Dams’ is intended to provide dam safety professionals with wealth of information and references on various aspects related to the hydraulic safety of existing dams, reservoirs and their appurtenant works, aiming to ensure safety of the dam and minimizing the risk to which the downstream population may be subject to, due to any dam incidents or failures. Hydraulic Safety of existing dams is an important concern amongst the dam owning agencies. This publication presents detailed aspects regarding assessment, evaluation and rehabilitation of hydraulic structures including spillways, outlet works and energy dissipation arrangements in existing dams. The document provides information for identifying and dealing with typical haz- ards related to hydraulic functioning. A number of case studies and failure modes catalogue are included in order to facilitate in a better understanding of the mechanics associated with various hydraulic problems/issues. This Manual is expected to assist dam engineering community for managing the safety of dam- reservoir systems during their operative life. The procedures, techniques and measures prescribed through various chapters of this document follow contemporary global best practices related to review of hydraulic safety of an existing dam and associated appurtenant structures both for the existing design flood as well as for the revised design flood. I hope, this Manual can be used as an excellent reference material by our engineers/dam owners while carrying out comprehensive hydraulic safety review and appropriate planning for rehabilitation of their dams. The contents of this publication is well organized and would provide readers with clear understanding of hydraulic problems in dams and the possible remedial measures for improving the health and safety of dams in a systematic manner. (S K Haldar) New Delhi Chairman June 2021 Central Water Commission Doc. No. CDSO_MAN_DS_04_v1.0 Page iii This page has been left blank intentionally. Doc. No. CDSO_MAN_DS_04_v1.0 Page iv FOREWORD The existing dams need a sound hydraulic safety assessment in order to work out rehabilitation measures. The hydraulic design and performance is required to be reviewed for the existing dams in case revised design flood has increased significantly as a result of periodic safety reviews. It is often seen that there is a considerable upward revision in design flood in a large number of dams. Some dams may get overtopped owing to substantial increase in revised design floods. This Manual titled ‘Manual for Assessing Hydraulic Safety of Existing Dams’ deals with the hydraulic safety of all the components of the system viz. dam and reservoir, approach channel, different types of spillways, their control and conveyance structures, outlet works in earthen/masonry/concrete dams, their intake and conveyance structure, hydro-mechanical equipment, different types of energy dissipation arrangements, plunge pool and exit channel. Various hazards and their adverse response/effects on different components have been discussed under this document. The hazards are in general due to increase in design flood resulting in increase in MWL, reservoir sedimentation, obstructions due to floating debris/ice, inoperative gates, condition of concrete surface in conveyance structure and energy dissipater, effects of increase in discharge over spillway and energy dissipater, limitations in exit channel capacity, erosion of bed and banks of exit channel, etc. The adverse response/effects include lack of spillway capacity, dam overtopping, temporal loss of gates, lack of conveyance capacity, overtopping of chute/energy dissipater walls, damages due to abrasion/cavitation, increase in hydraulic and hydrodynamic loads in spillway and energy dissipater, malfunctioning of energy dissipater, instability of hydraulic jumps etc. For increasing the spillway capacity in addition to additional conventional spillways, the comparatively recent options of unconventional spillways which include fuse plug, labyrinth spillway, piano key spillway, fuse gates, fuse plug (concrete blocks), open channels (flush bars), overtopping of dam with protected section and stepped spillways have been discussed. Under energy dissipation various rehabilitation measures for increasing the capacity of terminal structures in spillways have been comprehensively covered. I hope that professionals engaged in the comprehensive safety review of dams will find this Manual very useful in managing the hydraulic safety aspects. I thank and compliment all the individuals who have contributed to the preparation of this Manual and hope that the efforts will go a long way in improving the dam safety environment in the country. Central Water Commission also acknowledges the special support given by all members of Review Committee in finalizing this Manual. ( Dr. R K Gupta ) New Delhi Member (D&R) June 2021 Central Water Commission Doc. No. CDSO_MAN_DS_04_v1.0 Page v This page has been left blank intentionally. Doc. No. CDSO_MAN_DS_04_v1.0 Page vi PREFACE This Manual provides access to the state-of-the-art information and references on various aspects related to the hydraulic safety of existing dams. A number of case studies and failure modes catalogue are included in order to facilitate in a better understanding of the various hydraulic problems which may lead to either partial or total failure of the works. All the documentation included is expected to be helpful in evaluation of the safety levels of the dam, spillway, outlet works, energy dissipation arrangements and other appurtenance works and in working out options for rehabilitation from the view point of Hydraulic safety. The Volume 1 of the Manual contains five chapters namely; Introduction, Dam and its Reservoir, Spillway, Outlet Works and Energy Dissipators. Each chapter contains an overview of the subject/component, description of the component, assessment of hydraulic safety of that component on account of various possible hazards/defects, rehabilitation measures which can be undertaken and lessons learnt along with some case studies for illustration. Some recent developments covered in the Manual are Overtopping protection for dams and different types of unconventional spillways. Introduction to risk analysis by identification of possible failure modes is also discussed. Various examples/case studies of documented cases included in the Manual are expected to help/contribute in understanding of the issues/problems related to Hydraulic Safety of a Dam-Reservoir System and its elements in right context. The Volume 2 of the Manual has six appendices. Appendix A represents an approach to the process of Risk Assessment and Management, which envisages identification of failure modes. More and less likely factors which could lead to development of a particular failure mode have been brought out. Also, an array of suggested actions has been included. Appendix B contains few case studies involving various types of incidents/failures in dams. This appendix contains failure of Upper dam (CFRD) of Taum Sauk Pumped Storage Hydro-Electric project, USA which was due to excessive pumping from the lower reservoir and failure of instrumentation systems leading to overtopping. The case study on Spencer Dam, USA is perhaps the first of a dam to fail