Case study of spillway modifications——a means to a higher level of dam safety

New Swedish design-flood and dam-safety guidelines require a higher level of structural and hydraulic safety of the existing dams in the country.The guidelines initiated a review of each river to find good solutions-combinations of retention of water and increased spillway capacity.To comply with the guidelines,a dam-safety program undergoes in the country to rebuild a large number of dams to improve their safety-for higher design floods and also in many other respects.Technical solutions include dam reinforcement,spillway capacity increase and temporary storage of water in large reservoirs.Stenkullafors has a spillway with two gated overflow openings.Its revised design flood is 40 % higher than its existing spillway capacity.Based on a cost-benefit analysis,modifying one spillway opening to a lower crest was chosen as the main measure to cope with the flow.Through hydraulic model tests,the spillway crest profile was optimized so as to achieve the required capacity on one hand and to minimize the cost of the spillway crest and the new gate on the other.The sill elevation in the right spillway opening was lowered by 4.3 m and a new higher tainter gate was installed.The dam has now been rebuilt to higher safety level.This paper addresses hydraulic model tests and the risk of flood occurrence during the construction time.Also,the construction process and difficulties that were encountered are described.Other engineering measures are strengthening of the dam body,new erosion protection on upstream slope,energy dissipater and channel downstream.