Page - 1062 - in Book of Full Papers - Symposium Hydro Engineering

sufficient downstream riprap to avoid backreaching erosion. 2.1. DISCHARGE CAPACITY Prior to the dam safety upgrades, the capacity of the spillway at Långströmmen was 1670 m3/s, while the updated design flood was close to 2500 m3/s, an additional 830 m3/s (50 % increase). Due to the very limited active reservoir volume, this leads to a water level increase of 1.8 m above the FRRL, which is 1.45 m above the central moraine core of the embankment dam. For an embankment dam, this is unacceptable. When the dam was built, in the 1950s, the design flood was about the same as the 100 year-flood, but with today’s hydrological methodology the design flood has a return period of about 10 000 years, hence the 50 % increase. For Ramsele, its existing spillway with a capacity of 800 m3/s, was able to safely handle the updated design flood of 910 m3/s. The additional 110 m3/s results in water levels 50 cm higher than the FRRL to discharge the flood, which still is under maximum design water level as the central moraine core of the embankment dam extends another 1,6 meters. This gives the spillway sufficient capacity to evacuate the full volume of the floods, even accounting for wind drag buildup and waves. 2.2. EROSION DOWNSTREAM OF SPILLWAY At Långströmmen dam, inspections have uncovered damages in the bedrock and concrete adjacent to the rock downstream of the spillway. Diving inspection uncovered that bedrock had been jacked up and washed away during flood episodes downstream the ski jump. Repairs were instigated, but rock erosion at the existing spillway was not handled in the project presented– the focus was on increasing the discharge capacity by adding the new spillway and designing it for safe passage of the design flood. Downstream of Ramsele’s left spillway chute, extensive rock erosion has occurred even though the spillways are rarely used. Before being re-built in 2017, the spillway had no engineered energy-dissipation at all, and the hydraulic conditions were non-favorable. There were reasons for apprehension concerning backward erosion, under the existing spillway chute. As no redundancy exists regarding discharge alternatives, it is of utmost importance that all three gates can be used without limitations regarding rock erosion, at a design flood event. One outcome from the latest dam safety evaluation was to introduce restrictions regarding usage of the left spillway, since discharge could possibly 1062