Modification of spillways for higher discharge Capacity

In Sweden, many dams are to be refurbished in light of the new dam-safety guidelines that lead to higher design floods. To modify the spillway of a dam is often one of the alternatives proposed to increase its discharge capacity. Spillway rebuilding proposals for Ajaure, Bergeforsen, Midskog and Stenkullafors have been examined in hydraulic models. This paper deals with modification of the bottom outlet at Ajaure and proposed crest lowering at the other three overflow spillways. The hydraulic characteristics after the modifications are summarized and compared with the existing situation. The results are shown mainly in terms of water stage-discharge relationship and discharge coefficient, reasons for discrepancy are discussed and explained for each case.