Due to water under the spillway many catastrophic failures of spillway which leads to the whole dam failure has occurred in the world. The failures due to uplift pressure, piping, and/or erosion are the most critical aspects that should be considered during design of hydraulic structures. Appropriate defensive design measures should be considered for these aspects. Two dams have been constructed on Ribb and Megech Rivers, in Tana Sub basin, Ethiopia. The dams and appurtenant structures have been designed by same designer, both dams have similar foundation, capacity and profile. However, Ribb dam spillway was designed and constructed with drainage blanket beneath the spillway channel slab. Whereas, the design and construction of Megech Dam Spillway was without this material. This study investigates the distribution of uplift pressure and seepage at control, chute and terminal section of the spillways foundation of Megech and Ribb dam projects with and without drainage blanket using SEEP/W software and measured data. The result showed that on Ribb dam spillway up to 14% reduction of uplift pressure around control section and an average of 5% uplift pressure reduction at chute section of the spillway was found by using drainage blanket beneath the structure. However, providing drainage blanket beneath the stilling basin slab couldn’t reduce the uplift pressure rather it allows the tail water to enter and surcharge the under slab drainage system. If this drainage sand has been used on Megech dam spillway up to 12% uplift reduction around control section and an average of 3% reduction can be achieved at chute section of the spillway. Therefore, the provided drainage blanket beneath the spillway channel slab at control and chute section of Ribb dam spillway is necessary and good design. But this drainage system is not essential at the stilling basin of the spillway. Hence, the design of Ribb dam spillway follows the correct design procedure as per this study, standards and literatures which was mentioned in this research. Whereas, the design and construction of control and chute section of Megech dam spillway without drainage blanket beneath channel slab is not safe against uplift pressure. Careful attention should be considered to backfill requirements and the extent of cutoff walls to prevent the tail water from entering and surcharging the under slab drainage system at the stilling basin of the two dam spillways.