REHABILITATION OF ST-MARC DAM EXPERIMENTAL OPTIMIZATION OF A PIANO KEY WEIR Marcelo Leite Ribeiro(1), Jean-Louis Boillat(2), Anton J. Schleiss(3), Frédéric Laugier (4) and Claude Albalat(5) (1), (2), (3)
Laboratory of Hydraulic Constructions, Ecole Polytechnique Fédérale de Lausanne, EPFL-ENACICARE-LCH, Station 18, 1015 Lausanne, Switzerland, phone: +41 21 693 2385, fax: +41 21 693 2264, e-mail: (1)
[email protected], (2)
[email protected], (3)
[email protected] (4), (5) EDF-CIH, Savoie-Technolac, 73 393 Le-Bourget-Du-Lac, France, phone: +33 4 79 60 60 60, fax: +33 4 79 60 62 31, e-mail: (4)
[email protected], (5)
[email protected]
ABSTRACT Labyrinth weirs are normally built to increase the total effective crest length for a given spillway width. They can be used to increase the discharge capacity for a given head or to decrease the head for a given discharge. As their application is sometimes difficult in rehabilitation projects due to inappropriate supporting conditions, a new concept of labyrinth spillways has been proposed with a new shape, called PK-Weir (Piano Key Weir). This innovative alternative of labyrinth spillway provides an increase in the stability of the structure which can be placed on the top of most existing or new gravity dams, unlike traditional labyrinth weirs. In this framework the results of physical modelling tests for the rehabilitation of St-Marc Dam with a PK-Weir are presented. A conceptual model for fitting the rating curve of the PKWeir of St-Marc is proposed, based on the loss of effective length with the increase of the upstream head. Original developments are presented concerning the crest geometry of the weir, the energy dissipation inside the structure itself and the nappe aeration of the overtopping jets. Keywords: dam rehabilitation, labyrinth weir, PK-Weir, effective overall crest length, rating curve 1 INTRODUCTION The Saint-Marc dam, of EDF - France is a 40 m high concrete gravity dam with an overall crest length of 170 m. It was built between 1926 and 1930 and is located 20 km upstream of Limoges in the centre of France. The update of hydrological studies according to the Gradex method has shown that the existing discharge capacity of the dam is smaller than the design flood corresponding to a 1´000 years return period. EDF-CIH undertook feasibility studies to provide additional discharge capacity to the existing spillways, as other solutions (flood routing through the reservoir or increase of the maximum water level) could not be envisaged. A technical-economical comparison of several solutions was carried out, among them the construction of a new type of labyrinth spillway called Piano Key Weir (PK-Weir) turned out to be the best solution. This also is an alternative with a high performance regarding risk analysis (reliability of automatic free flow spillway) and maintenance costs (no mechanical or electrical device). Labyrinth spillways are built to provide a longer total effective crest length for a given overall spillway width. This type of structure is used to increase the discharge capacity for a given head or to decrease the head for a given discharge. Such behaviour is useful for limiting the height of the dam or raising the normal water level. An extensive investigation of labyrinth weirs was performed by Taylor (1968) presenting a capacity ratio by comparison with sharp-crested linear weirs of same width. As a follow up, Hay and Taylor (1970) propose a design procedure for estimating the discharge over triangular and trapezoidal labyrinth weirs. Their work, also presented in Sinniger and Hager (1989), shows a trapezoidal layout of labyrinth weir as being optimum.
Darvas (1971) established a family of curves for designing labyrinth weirs, which are based on experimental results of model studies for the weirs Wonorora and Avon in Australia. Magalhães and Lorena (1989) present curves of discharge coefficients Cd for different ratios “hydraulic head over height of walls” H/P whose values are systematically lower than those obtained by Darvas (1971). Tullis et al. (1995) propose discharge coefficient values for labyrinth angles between 6° and 35°. The given equations are valid for an apex width between t and 2t, and H/P1.75 m, the overcrossing jet becomes significant, decreasing the effective length and consequently the PK-Weir capacity.
a)
b)
c)
d) Figure 8: Top view of the PK-Weir of St-Marc Dam at different heads a) H= 0.80 m b) H= 1.25 m c) H= 1.75 m d) H= 2.25 m
3.3.3 Crest geometry The initial project of the PK-Weir of St-Marc considered a quart-rounded crest with the round face on the downstream part of the lateral wall, considering the flow direction. After preliminary tests, it was remarkable that the jet stuck to the downstream lateral wall due to non-aeration of the flow over an important range of water heads. This behaviour impeaches the jet’s crossing and the consecutive energy dissipation due to their interaction. In order to reduce the head up to which the jet remains attached to the wall, two other types of crest have been considered, a flat and a quart-rounded crest with the rounded face on the upstream part of the wall. Figure 9 illustrates the behaviour of the jet with different crest configurations. With a flat crest, the jet starts to detach for head values of about 0.55 m while with the quart-rounded crest, the jet separates from a 0.35 m head. upstream quartrounded crest
flat crest
FLAT CREST
UPSTREAM QUARTROUNDED CREST
detached jet DOWNSTREAM QUARTROUNDED CREST
Figure 9: Downstream view of the PK-Weir operating under a 0.35 m water head, with quartrounded crest on the upstream part (left) and a flat crest (right)
Both configurations presented higher efficiency in terms of energy dissipation than the downstream quart-rounded crest considered initially. However, the maximum capacity of the PK-Weir for the 1.35 m design head (elevation 283.50 m NGF) is identical for all considered crest forms and equal to 134 m3/s. 3.3.4 Downstream alveoli During the optimization procedure, the length of the original downstream alveoli had to be shortened in order to modify the jet trajectory issued from these alveoli with the purpose of increasing the efficiency of the energy dissipation structure downstream. For this new structure some steps were constructed in the downstream alveoli. These steps contribute to raising the partial energy dissipation and consequently to shortening the jet’s trajectories. Figure 10 illustrates the PK-Weir with the changes mentioned above. The design of the steps was done taking care not to reduce the hydraulic capacity of the PK-Weir over the operating range. The same maximum discharge of 134 m3/s could be maintained at 1.35 m head.
HORIZONTAL STEPS
REMOVED
Figure 10: PK-Weir optimization measures: Steps insertion and length shortening of the downstream alveoli. Schematic profile (left) and downstream view of the model (right) 3.3.5 Nappe aeration In order to analyze the behaviour of the PK-Weir related to nappe aeration, dynamic pressures were measured inside one of the upstream alveoli of the PK-Weir and on the downstream face of the dam, under the tin flow screen issued from the upstream alveolus. The measurements were performed for the maximum water level in the reservoir (283.50 m NGF) corresponding to a 1.35 m head. The signal registered with a 100 Hz sampling frequency over a time period of 90 sec was submitted to a classical statistical analysis including the Energy Spectrum Density and the Distribution function curve. A simple outline of the Energy Spectrum inside the upstream alveoli put in evidence that vibrations occur on the structure with frequencies at 0.8 Hz (prototype scale). This vibration disappears when the nappe of the flow is aerated artificially (Figure 11). Spectrum amplitude
1E+00 point24 point 24_aerated
1E-02 1E-04 1E-06 1E-08 1E-10 1E-12
0.1
1.0
10.0
Frequence (Hz)
Figure 11: Energy Spectrum Density inside the upstream alveoli, with and without aeration
4 CONCLUSIONS AND RECOMMENDATIONS
The EDF rehabilitation project of St-Marc Dam requires an increase of the spillway capacity. Constrained by the available space on the dam crest, a labyrinth PK-Weir is designed whose hydraulic behaviour was investigated experimentally on a 1:30 scaled model. The rating curve could be adjusted using a conceptual model based on the classical equation of a linear crest spillway. This model considers a progressive diminution of the effective length and of the effective height of the lateral walls with increasing hydraulic head. Experimental tests of the PK-Weir considered three different crest profiles. Among them, the quart-rounded crest with the rounded face on the upstream part of the wall was found to be the most efficient regarding the energy dissipation resulting from jets crossing. In order to produce an additional raise of the partial energy dissipation and consequently to shorten the jets trajectory issued from these alveoli, some steps were installed inside. Finally, dynamic pressure measurements confirmed the necessity of the jets aeration in order to avoid undesirable vibrations. NOTATIONS a b B c Cd d f g H L
Width of the upstream alveolus Width of the downstream alveolus Length of the side weir Length of the upstream overhang Discharge coefficient Length of the downstream overhang Base width of the PK-Weir Acceleration due to gravity Upstream pressure head Total developed length of the weir
m m m m m m m/s2 m m
Leff P Q R t W z α ν ρ
Effective length of the weir Maximum height of the wall Discharge Radius of crest curvature Wall thickness Width of the weir Height of the wall Labyrinth angle Kinematic viscosity of water Density of water
m m m3/s m m m m ° m2/s kg/m3
REFERENCES
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