Norwegian University of Department of Hydraulic and Science and Technology Environmental Engineering NTNU SNOW AND ICE BLOCKING OF TUNNELS By Leif Lia A dissertation Submitted to the Faculty of Civil and Environmental Engineering, the Norwegian University of Science and Technology, in partial fulfilment of the requirements for the degree of Doctor Engineer Trondheim, Norway, March 1998 IVB Report B2-1998-1 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Abstract 1 Abstract Hydropower development in cold regions causes a number of concerns about operational reliability and dam safety. Diversion tunnels in a hydropower scheme have to be operated in a cost-effective way. The reliability of spillways influences overall dam safety, consequently this thesis presents analyses of snow and ice blocking of spillways and diversion tunnels. Diversion tunnels with free water surfaces are more frequently exposed to snow and ice problems than submerged tunnels. A review of design practice agree with this statement, and the variation in spillway design practice in different cold regions is the reason why snow and ice blocking does not cause concern everywhere. As spillways are rarely in operation, experience acquired from diversion tunnel operation is used to solve problems related to spillways. Aufeis forms when a shallow flow of water freezes onto an ice cover. Aufeis build-up causes problems in civil engineering, e.g. blocking of culverts, roads, channels and tunnels. The initiation of aufeis is not sufficiently described in the literature, and thermal growth combined with declining discharge is the best description that matches the observations presented in this thesis. Growth and melting of aufeis are both predictable, and the rates depends on the heat flux and meltwater temperature respectively. Drift snow forms a compact and strong snow pack. This means that snow drifts are able to dam water. The heat balance in the snow causes freezing and creation of impermeable ice sheets when water saturates it. If the heat balance is negative this penetration stops. If the heat balance is positive, permeability in the snow determines the time required for water to penetrate the snow plug. This plug requires shear strength to withstand the water pressure. Shear strength is calculated from density. In this thesis, studies of temperature distribution in tunnels are performed by air temperature measurements in six tunnel spillways and five diversion tunnels. These measurements lasted for two consecutive winters. The air throughflow tunnels is used as it causes cooling of both rock and water. In open spillway tunnels, frost reaches the entire tunnel. In spillway tunnels with walls, the frost zones reach -100 m from the downstream end. In mildly-inclined diversion tunnels (S0< 1% and 1, > 500 m), a frost free zone is located in the middle of the tunnel, and snow and ice problems were only observed in the inlet and outlet. Severe aufeis accumulation is observed in the frost zones. Finite element models (FEM) are used to calculate the heat transfer from rock ii Abstract to air, water and ice. A prediction model for the calculation of aufeis build-up is based on the results from the calculations, combined with local field observation data. FEM is also used for calculations of the water penetration of snow plugs based on the heat balance. The required time for water to enter the blocked tunnel or channel is calculated, and in most cases this is between 20 - 50 days. Empirical observations indicated that the required time was 30 - 60 days. With T = 0°C in the snow pack the required time is -1 day. Sensitivity analyses are carried out for temperature variations in rock, snow, water and air. A set of systematic field observation were performed in order to make a modest contribution to the assessments of snow and ice blocking. Previously little has been reported on this topic. Snow and ice blocking were detected in several places, adding valuable knowledge to this area. The most frequently observed blocking event was blocking by snow drifts in the intake or outlet. Aufeis build-up was observed in both spillways and tunnels, at most it blocked two-thirds of the cross section. This amount of ice reduces the discharge capacity severely. Consequently, work in this area is of considerable interest for hydropower companies. Risk analysis (RA) has recently been developed for dam safety engineering. The method is useful for both dam safety improvements and the classification of dams. A case study for the spillway at Dravladalen is described in this thesis. Event trees for snow and ice blocking of spillways in general are presented, and the reliability of the RA method is discussed. Solutions which can reduce the snow and ice problems are proposed in this thesis. In order to eliminate aufeis build-up in the downstream end of a spillway, a solution with an insulating wall and a deep ditch in the tunnel bed is proposed. For channels exposed to drift snow, several improvements of the cross section are proposed. Measures to reduce of the amount of snow at specific points include snow fences and relocation of the intakes/outlets. An improved system of monitoring and inspections can detect the blockage problems, and adds knowledge to this field of research as well. Preface in Preface This report gives a summary of studies, field observations and measurements undertaken to understand the phenomena involved in snow and ice blocking of tunnels. I have been a doctoral student at the Norwegian University of Science and Technology (NTNU), Department of Hydraulic and Environmental Engineering, during the period 1994 to 1998, with a scholarship from NTNU. The research has been financed by support from several hydropower companies. I would like to thank in particular Lyse Kraft, Statkraft and the Norwegian Electricity Federation (EnFo) for funding and interest in my work. Without their contributions no research would have been carried out. I also thank the engineers and employees at Norwegian hydropower companies who have assisted me. They have welcomed me and helped me in all possible ways to perform countless visits to tunnels and spillways. Much of the knowledge presented in this thesis is acquired from the outdoor operators in the hydropower companies. Special thanks go to my advisor Adjunct Professor Torkild Carstens at the Department of Hydraulic and Environmental Engineering, NTNU. I thank him for his advice, discussions in physical processes, review of ideas, constructive criticism of the thesis and belief in my work. He has taught me a lot about the importance of understanding the physics involved in hydraulic and cold regions engineering. I also thank Professor Dagfinn K. Lysne for taking care of hydraulic engineering education. I would like to thank friends and colleagues at the Department of Hydraulic and Environmental Engineering for making it such a good place of work. They have created the working atmosphere which I have been looking forward to participate in every morning. In particular I want to thank Lars Jenssen for his valuable feedback in the final part of the study, and for having no mercy in his criticism. Thanks go to Knut Alfredsen for technical advice and tolerance in the office we shared. I am very grateful to Eirik Fyhn at Faculty of Civil and Environmental Engineering, who has prepared my sketches. The figures represent a valuable part of this thesis. I would like to thank Stewart Clark in the Student and Academic Division at NTNU for his valuable advice on the English language. IV Table of content Abstract ..................................................................................... i Preface ................................................................................... iii Nomenclature ..................................................................... viii 1. Introduction ......................................................................... 1 1.1 Objectives ................................................................................................... 1 1.2 The thesis..................................................................................................... 1 1.3 Background ................................................................................................ 2 1.4 Spillway design............................................................................................ 3 1.4.1 Norway ..................................................................................................... 4 1.4.2 Sweden ....................................................................................................... 4 1.4.3 Canada ..................................................................................................... 5 1.4.4 The USA ..................................................................................................... 5 1.4.5 Austria....................................................................................................... 5 1.4.6 ICOLD.......................................................................................... 6 1.5 Diversion tunnel design............................................................. 6 1.5.1 Economic optimization............................................................ '............. 6 1.5.2 Austria....................................................................................................... 7 1.5.3 Norway ....................................................................................................