Hydraulic Studies of The Spillway of The Karnafuli Hydroelectric Project East Pakistan

In August, 1961, while still under construction, the spillway chute of the Karnafuli Hydroelectric Project was severely damaged by flows up to a maximum of 123,000 cfs. A Special Board of Consultants reviewed the damage and considered three possible causes of the damage: (1) seepage uplift pressures, (2) failwater uplift, and (3) impact of logs. An inspection of the site and piezometer records indicated that seepage was not the primary cause. Opinions were divided on the other two items. A revised design of the chute was recommended for construction before the next high water, and recommendations were made for model and prototype studies to assist in an evaluation of the cause of the damage and the adequacy of repairs. A 1:28 scale section model was constructed and tested in the Hydraulic Laboratory of the East Pakistan Water and Power Development Authority. A report of October, 1962, presented the results of these studies together with prototype flow measurements on the reconstructed spillway. The present study was authorized effective October 2, 1962. A comprehensive model of the spillway and associated area was constructed to a scale of 1:132. A section model of one full bay and two half bays was constructed to a scale of 1:60. The model studies involved (1) measurements of flow pattern, log retention, and scour in the comprehensive model , and (2) measurements of temporal mean pressures, fluctuating pressures, log velocities and accelerations, and movement of model chute slabs in the section model. In addition, exploratory data were obtained on full-scale log impacts on a section of slab similar to the original chute. It was concluded that: (1) logs could have caused appreciable damage to the initial chute design, but some other mechanism was needed to remove the slabs; (2) fluctuating pressures associated with the hydraulic jump have caused sufficient uplift to remove the slabs with or without damage logs; (3) the increased thickness and new design of the drainage system the revised chute should provide adequate protection against fluctuating pressures; and (4) consideration should be given to doweling or otherwise holding down the floor of the stilling basin to avoid possible uplift of the basin slabs.