Seite - 537 - in Book of Full Papers - Symposium Hydro Engineering

3.1. DESIGN CHANGE HISTORY OF SAND TRAP The initial sand trap design was presented in the Feasibility Study prepared in 2007. In that study, the layout included a weir just downstream of the bend of the Kunhar River, with a Power intake in front of the weir on the left side and three underground sandtrap chambers to accomplish goal of sediment management along the headrace tunnel axis. Table 3. Specifications of the Sand Trap (Feasibility Study, 2007) Type Design Discharge Chamber Flushing Tunnel Section Length Section Length Underground sand trap 153m3/s (51×3chamber) 15×17m 180m 3.5×3.5m 407m In the basic design, however, the location of the weir axis was moved 150m upstream from the proposed location in the Feasibility Study, and a surface sand trap was designed as an alternative instead of an underground type sand trap in the Feasibility Study. This is because the geological properties of the original sand trap site don’t have sufficient supporting force for the foundation of the structure. According to a drilling investigation, a clay silt layer is distributed up to 6.0m from the surface. RQD is mostly 0%, even if the values of 10~24.0% are shown, partially. Table 4. Specifications of the Sand Trap (Basic Design, 2010) Type Design Discharge Chamber Flushing Tunnel Section Length Type Section Open Type 153㎥/s (76.5㎥×2 chamber) 23×26.7m 140m Box Culvert 1.8×2.5m Fig 3. Project Layout of the Feasibility Study(left) and Basic Design(right) A numerical analysis(HR Wallingford, 2012) was conducted to review the appropriateness of the modified project layout. For modeling the deposition, a water level of 765m asl, Normal High Water Level, was prescribed at the weir. 537