Analysis of Protection Layer D/S Spillway of High Dam Against Overtopping Flow Impact
Abstract
The Ethiopian Renaissance Dam, which has been designed and implemented with a full storage capacity for the regional water supply that reaches the downstream countries. Where storage reaches to the level 640 meter a.m.s.l with a capacity of 74 billion cubic meter annually. Also after the process of filling reservoir, the storage capacity of water passes through uncompleted portions of dam body with low sill of dam. This allows water to fall from different heights depending on the operational stage. the fallen water will perform to deformation on riverbed called (scour hole) through the soil layers at downstream side, which requires study to make a protective layer at downstream side to protect against the resulting corrosion.
During this study, the resulting effect of falling water on the soil layers behind the dam is calculated to determine the appropriate protective layer.
A physics model has been made for the dam body sector with a different crest level that acts as a sill to pass water through it, as well as a physical model of the protection layer and using special measurements to get stresses and deformations, which allow the expectation of the protective layer resistance to this effect.
As a conclusion that length of protected area in the flow direction, reaches to be at least 150 m perpendicular to spillway downstream face (approximately same as jet height), while its width is similar to that of temporary spillway crest width during construction, that assumed to be 275 m, which it will be subjected to the impact of flood flow overtopping the spillway crest from start of job to its end. the resulted local pressure distribution of the impacted water jet became similar to the impact action of jet after (D. L. Vischer), which uses to be as a uniform load on the suggested protection layer with different cases of structural system supports, that can be solved to reduce the chances of a scour hole formation from the high pressure which deduced on downstream side , the maximum deflection value reaches to 0.113 m for the model and 3.4 m for the prototype with engineering scale Lr= 30, at maximum height 510 cm in model and 649 m.a.s.l in prototype, which means totally damaged of the protection layer under this effect, and actually the underlining soil will not be exist. to decrease the dimensions of produced scour hole on downstream side, the protection layer case 4 (with 4 cutoff walls) can be used , it can reduce this value from 3.4 m in prototype to approximately 0.0051m. Approximately the scour hole can be protected from high depths and it makes the river bed saved from catastrophic failure and reducing in uplift pressure by the cutoff walls which makes the protection layer firmly attached with underlining soil with a sufficient depth beneath the raft thickness. this system of case 4, will be only suitable if there is no any formation of scour occur downstream dam body, otherwise it is not the right system and there is a necessary need to reduce the reservoir water level to another safe one.
