Rehabilitation of St-Marc dam: Model studies for the spillways

The Saint-Marc dam, owned by EDF-France is a concrete gravity dam built between 1926 and 1930. It is located near Limoges (France) on the Taurion River and has currently two gated spillways, one 7.50 m wide on the right bank and the other with two passes of 10.00 m wide on the centre of the dam. The crest of the spillways is located at the elevation 278.50 m NGF. Figure 1(a) shows a downstream view of the dam. According to estimations by EDF-CIH, the St-Marc dam presents a lack of capacity. To provide additional discharge to the existing spillways, a new type of labyrinth spillway called Piano Key Weir (PK-Weir) is proposed. This new shape was developed at Hydrocoop France in collaboration with the Laboratory of Hydraulic Developments and Environment of the University of Briska, in Algeria and the National Laboratory of Hydraulic and Environment of Electricité de France (EDF-LNHE Chatou). The EDF rehabilitation project is based on a 15 m wide PK-Weir installed near the right bank between the existing spillways. The crest of the PK-Weir is fixed 15 cm above the normal water level (282.15 m NGF), therefore the maximum water head of PK-Weir is 1.35 m (maximum water elevation at 283.50 m NGF). Thus, when a flood occurs, the three existing sluice gates will start first to fully operate. The PK-Weir will only start to operate when the incoming flood exceeds 380 m3/s, approximately corresponding to a 50 years return period flood. The St-Marc physical model at scale 1/30, constructed and tested at LCH has permitted the validation of the theoretical estimation. It’s also give essential information about this hydraulic scheme, which can be resumed as follows: • Determination of the maximum evacuation capacity of the existing spillways and definition of the rating curves, including with different gate operations. Experimental results show that the maximum capacity of the existing spillways at the elevation 283.50 m NGF is 623 m3/s. As this discharge is smaller than the design flood of 750 m3/s, corresponding to a 1´000 years return period, improving the discharge capacity of the existing spillways is required for the dam security. • The model study has allowed the optimization of the PK-Weir initial design by EDF-CIH. The maximum capacity of 134 m3/s at 283.50 m NGF and the rating curve of the PK-Weir were established after the model experiments. PK-Weir allowed to an increase about 3 times the discharge capacity at the maximum water elevation comparing to a linear crested spillway 15 m wide. This result demonstrates its efficiency for low heads. • The maximum capacity of the future scheme is 757 m3/s, therefore higher than the 750 m3/s required for the transit of the designed flood. • Concerning the energy dissipation from the PK-Weir, different solutions have being proposed and analysed in the model. The adopted alternative is a leaned “ski-jump” gutter placed at the contact line between the downstream face of the dam and the natural rock. This variant consists in a cylindrical profile with a constant diameter placed after an inclined axe with a horizontal reach at the end of the structure (Figure 1b). The aim of this solution is to guide the flow from the PK-Weir to the stilling basin of the left existing spillway. Pressure measurements have been performed in the inlet of the PK-Weir, in the downstream face of the dam, in the gutter, in the right guide wall of the left spillway and in the stilling basin of the left spillway. The given results do not highlight major problems concerning the operation of the entire evacuation system of the St-Marc dam. • In order to improve the energy dissipation and to mitigate impact length of the flow on the downstream face of the dam, steps have been added in the outlets of the PK-Weir. They have proved to be quite efficient and improved the operation of the ski-jump gutter.