PRELIMINARY INVESTIGATION AND DESIGN CONSIDERATIONS FOR THE REHABILITATION OF TRASH SCREEN AT JOR RESERVOIR LOW LEVEL OUTLET Jansen Luis 1 , Lariyah Mohd Sidek 2 , Pierre Y. Julien 3 1 Post Graduate Student, UNITEN; 2 ,UNITEN, Jalan IKRAM-UNITEN, 43000 Kajang, Selangor; 3 Department of Civil Engineering, Colorado State University, Fort Collins 1 jansenla@tnb.com.my; 2 lariyah@uniten.edu.my; 3 pierre@engr.colostate.edu Keywords: Reservoir drawdown, reservoir sedimentation, rehabilitation of low level outlet. Abstract. Jor reservoir was constructed to assure flow regulation for the Woh hydroelectric power station, as well as to provide storage for flood control. The reservoir outlet is frequently operated once every three months to flush the deposited sediments accumulated within the reservoir. Preliminary testing and inspection on the reservoir outlet revealed that the trash screen located at upstream of reservoir outlet had been damaged and dislodged creating a large opening at the trash screen thus permitting entry of larger logs which blocks the passage of water through the reservoir outlet. Bathymetric survey showed the total height of sediment deposition above the reservoir outlet covering the trash screen was estimated at approximately 13 m. The sediment volume estimated to be cleared from above the reservoir outlet area is 15,000 m 3 . Soil investigation conducted shows that the sediments deposited within the reservoir exhibits various different profile and characteristics. The average sediment distribution at deeper depths closer to the trash screen was found to be of 27% clay, 26% silt, 43% sand and 4% gravel. The rehabilitation works on structure as the low level outlet serves as an important feature for reservoir sedimentation management as well as for safe flood discharge release as designed for the dam. It is important that the function of the reservoir outlet structure is maintained in accordance to the International Dam Safety Standards. 1. Description of Jor Reservoir Jor reservoir was constructed to assure flow regulation for the Woh hydroelectric power station, as well as to provide storage for flood control [1]. The reservoir is confined by two earthfill dams namely Jor Dam and Jor Saddle Dam which forms part of the Batang Padang Hydroelectric Scheme. Jor Dam is situated in a narrow section of the valley of the Sungai Batang Padang, together with the saddle dam on the low ridge on the right bank of the river. Jor reservoir covers a total area of 80 acres (350,000 m 2 ) and has an approximate total storage volume of 3.8 million m 3 with approximately 3.0 million m 3 as live storage for hydropower power generation. During the preliminary stage of construction, a 14 ft (4.3 m) diameter concrete lined diversion was driven from the outlet structure to connect with the spillway tunnel. The Batang Padang River was diverted through the diversion tunnel and spillway tunnel on October 1965 and the diversion tunnel was closed on July 1967 when the impounding of the reservoir commenced. 2. Description of Jor reservoir outlet The Jor reservoir low-level outlet is equipped with a hollow jet valve of 72 in diameter [1]. The sill level of the outlet intake is 463.30 m (1520 ft). The hydraulic capacity of the valve is 1100 ft³/s (31 m³/s) when the reservoir is draw down just below the sill level of Menglang tunnel intake that is 468.78 m (1538.5 ft). This flow of 31 m³/s corresponds to the average natural inflows to Jor Applied Mechanics and Materials Vol. 567 (2014) pp 128-132 Online available since 2014/Jun/06 at www.scientific.net © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.567.128 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 129.82.232.219-22/08/14,19:57:19)