1 INTRODUCTION Surge phenomena represent a powerful change in normal flow conditions caused by interaction with hydraulic structures in a channel system. Previous investigations in the Mühlbach channel indicated the occurrence of surge phenomena, especially by an abrupt failure or incorrect operation of one or more components of the two diversion hydropow- er plants. In order to evaluate the hydraulic security of the existing channel, the surge generated during operation of the hydropower plants Mühlbach and Maiermühle in Landsberg on the Lech River was analyzed by means of computer modeling tools. An additional comparison between the results of one and three-dimensional modeling aims to find an optimum and effective modeling solution con- sidering criteria such as the accuracy of results, and time and effort required for analysis. 2 SITUATION The Mühlbach channel is a bypass channel, si- tuated in Landsberg on the River Lech in Bavaria. The Mühlbach channel is deviates from the right of the Lech River, just upstream of the Karolinen Weir. The course of the channel is characterized by close interaction with the town within very li- mited space and it supplies two hydroelectric power plants: Maiermühle hydroelectric power plant (MMHPP) and Mühlbach hydroelectric power plant (MBHPP) as shown in Figure 1. One- and three-dimensional modeling of surge generated during operation of the hydropower plants Mühlbach and Maiermühle in Landsberg on the Lech River Evelyn Carvajal, Richard Huber, Franz Geiger and Peter Rutschmann Institute for Water and Environment, Technische Universität München, Munich, Germany ABSTRACT: Part of the Lech River water, upstream of Karolinen weir is deviated on the right side to the Mühlbach channel in Landsberg in the south of Bavaria. From the intake structure to the Mühlbach´s con- fluence two diversion hydropower plants are situated on the River Lech: HPP Maiermühle on the upper reach und HPP Mühlbach on the lower reach.Flow fluctuations such as surge might originate, among things, from natural processes such as floods and mudflows or as an effect of anthropogenic changes. Surge phenomena due to failure or incorrect operation of one or more components of the two diversion hydropower plants in the Mühlbach channel belong to the latter category and are the present object of this study. In the first part of the study, the surge generated by turbine shutdown (HPP Mühlbach) was calcu- lated by means of one-dimensional and unsteady modeling with HEC-RAS. After considering many sce- narios with different discharges, variable turbine shutdown times (HPP Mühlbach) and different flap gate opening times on the surge bypass unit in the event of turbine close, the recommendation was to use a maximum discharge of 11 m³/s. An hydraulic model and analysis of the surge in the upper part of the channel based on flow interruption or, respectively, closing of both HPP Maiermühle and the intake sluice gate provides a basis for more efficient improvement measures and optimized operation strategies. In the second part of the study, additional approaches are considered, focusing mainly on the interaction and in- fluence of HPP Maiermühle. Because of the strong 3D characteristics of the upper reach, from the intake structure to HPP Maiermühle, this channel section was simulated using the three-dimensional program FLOW-3D. The results presented in this publication are drawn from a project commissioned by the Baye- rische Elektrizitätswerke GmbH. Keywords: Hydroelectric Power plants, Surge, One-dimensional model, Three-dimensional model, Fail- ure analysis, Mühlbach channel River Flow 2010 - Dittrich, Koll, Aberle & Geisenhainer (eds) - © 2010 Bundesanstalt für Wasserbau ISBN 978-3-939230-00-7 699