Page - 1033 - in Book of Full Papers - Symposium Hydro Engineering

2. PLUNGE POOL DESIGN CRITERIA A pre-excavated plunge pool is proposed for NT1 project in order to prevent uncontrolled progression of the scour towards the dam foundation and the formation of a mound of scoured material at the powerhouse tailrace area. This would cause higher tailwater levels affecting the energy production. The spillway is designed to safely discharge a Probable Maximum Flood (PMF) of 30,200 m3/s without causing serious damage to the permanent works, whereas the design of the pre-excavated plunge pool focusses on the serviceability of the powerhouse, justifying a 100-year return period as a design basis. For this event the design of the pre-excavated plunge pool shall not allow for any major erosion process by the released water of the spillway and the bottom outlet. 3. SCOURING EVALUATION USING EMPIRICAL FORMULAE For the hydraulic design of NT1 a theoretical analysis has been coupled with laboratory investigations. The theoretical analysis is based on (semi-) empirical scour depth formulae, the main parameters of which are the duration of the spillway operation, the specific discharge (per meter width) and the head. Other parameters are air entrainment in the jet, the rock characteristics and the tailwater depth. Martins, Chian, Mason and Veronese equation modified by Yildiz and Üzücek have been used to estimate the ultimate scour depth. For NT1 spillway jets, average scour depths in the range of about 45 m to 80 m below tailwater level were achieved for 30-year flood to PMF. Mason and Modified Veronese yielded high range values while Chian and Martins gave lower closer values. 4. PHYSICAL MODEL QUALITATIVE SCOURING TESTS RESULTS The hydraulic studies of the spillway jets and plunge pool were carried out using 1:80 (overall dam) and 1:60 (partial of three chutes) reduced scale physical models, adopting Froude similarity, built at the Asian Institute of Technology (AIT) in Bangkok, Thailand. The 1:80 overall model reproduces a part of the reservoir and of the dam, the entire spillway (sill, gates, chute, flip buckets), the plunge pool and the powerhouse tailrace together with a downstream river reach for implementing the downstream tailwater condition. The spillway model, built in PVC and mortar, was designed to accommodate modifications and several alternatives for the flip buckets (varying the lip angle and provision of dents). Erosion in the plunge pool was studied 1033