Enlarging the undersized spillway of Höljes dam

Holjes dam is one of the largest embankment dams in Sweden. Studies have shown that it suffers from several deficiencies that would jeopardize the safety of the dam during extreme floods. The major deficiencies include insufficient spillway discharge capacity in relation to the updated design flood and potential risk of erosion in the dam toe in case of large spillway discharges. To safely pass the higher design flood, four major rehabilitation options are evaluated with the help of hydraulic model tests. Option 1 refers to the use of the existing spillway in combination with extra water storage above the full reservoir retention level (FRRL). Option 2 includes construction of a new, separate tunnel spillway. In Option 3, a new gated spillway, 17 m in width, is added to the right of the existing one. Option 4 refers to construction of a new spillway in the form of a 155 m long overflow weir in the reservoir in combination with moderate storage above the FRRL. In Option 1 and 2, the width of the existing spillway channel remains the same and the stilling basin is enlarged, while in Option 3 and 4, the spillway channel is widened and the stilling basin volume is also increased. This paper addresses the advantages and disadvantages of those options. Together with other investigations, hydraulic model testing provides necessary basis for finalization of each design option in terms of spillway discharge capacity and waterway configuration. After weighing the pros and cons, the dam owner has chosen Option 3 as the final rehabilitation alternative. Further investigations are made to optimize the design.