Page - 1037 - in Book of Full Papers - Symposium Hydro Engineering

Based on these two distinctions Castillo [4] classified four types of jet conditions realized before and after entering the plunge pool: a) undeveloped jet H/Lb <1, shallow pool Y/Bj < 4 b) developed jet H/Lb >1, shallow pool Y/Bj < 4 c) undeveloped jet H/Lb <1, deep pool Y/Bj > 4 d) developed jet H/Lb >1, deep pool Y/Bj > 4 Applying Castillo’s approach [3], the jet break-up length was calculated and it was found that the disintegration occurs before the impact on the plunge pool water surface. Thus, a certain amount of energy will be dissipated in the air. The existence of dents on the buckets shall improve and guarantee the degree of jets’ aeration and disintegration. Considering the jet’s issuance characteristics, travelling length, falling heights and the tailwater levels at the plunge pool for NT1 case (Table 1), the jets can be classified as type (d) which implies developed (disintegrated) jets condition in the air and a deep pool situation. Table 1 NT1 spillway design flood jet characteristics at issuance section and before/after impacting the plunge pool surface Flood q [m2/s] Y [m] Tu Lb [m] H [m] H/Lb Bj [m] Y/Bj 100-year 134.9 55 0.035 95 125 1.3 7.1 7.8 7. PRESSURES REGISTERS AND INTERPRETATION The pressures at the invert and berms of the pre-excavated plunge pool have been measured by means of low range industrial pressure sensors (10 Hz). The impact pressures time series on the pre-excavated plunge pool bed and berms for Q100=14,000 m3/s are plotted in Fig.6. The flow pattern along the plunge pool area during model test is shown in Fig. 7. According to Bollaert and Schleiss [5], the scouring process in plunge pools is the result of the interactions of the three phases water, rock and air. Moreover, the highly turbulent nature of the flow and the resulting pressure fluctuations on the water-rock interface and inside rock fissures make appropriate scaling very difficult in hydraulic modelling. Thus, the applicability of Froude-based reduced- scale models is limited and shall only be used as qualitative indication. The impact of the plunging jets on the proposed pre-excavated plunge pool invert and berms are evaluated using a fixed bed, by measuring the pressures induced by the water jets through electronic transducers. Then, the parameters of the dynamic pressure distribution on the plunge pool surfaces such as mean dynamic pressure (Cp), RMS (root of mean square) (Cp’) fluctuation, maximum and minimum peak fluctuations (Cp+ and Cp-) were analysed and compared to the values available in literature. 1037