Sustainable reservoir sedimentation management for an effective sediment flushing bottom outlet system for Poechos Dam reservoir, River Chira, Piura, Peru

Poechos Dam is a 55-m-high clay-core rockfill dam with an 11-km-long crest, constructed between 1972 and 1978. It forms one of the largest reservoirs on the Peruvian coast, with an original storage capacity of approximately 1,000 million m³. Since its commissioning in 1976, the reservoir has experienced severe sedimentation, which was largely underestimated during the design stage. By 1990, after less than 15 years of operation, nearly 50% of the live storage capacity had already been lost. The accelerated aggradation is mainly attributed to the absence of an effective bottom outlet for sediment flushing, despite the dam being designed for high peak floods with large suspended sediment loads and despite the long-known occurrence of El Niño events. Although an initial bottom outlet with a discharge capacity of 1,045 m³/s was considered, it was later plugged, leaving only a small conduit with a capacity of approximately 100 m³/s for water transfer. As noted by the author Alvarado Ancieta (2007a, b, 2018, 2022, 2023a, b, 2024, 2025), bottom outlets are essential for effective sediment management in reservoirs in the Andes and the Himalayas. Nevertheless, by 2023, sedimentation in Poechos Reservoir had reached nearly two-thirds of its original capacity, without the application of any effective countermeasure. Increasing reservoir volume by raising the dam crest with a parapet wall did not address the underlying sedimentation problem. This research presents an engineering evaluation of the dam layout, identifying that the left portion of the structure, referred to as the left embankment, is founded on soil rather than rock and is integrated with the dam body. This condition allows for the implementation of a sediment flushing system without intrusive interventions through the dam or embankment. A bottom outlet scheme located beneath the left embankment is proposed, adapted to local geological conditions. The proposed countermeasure consists of a sediment flushing complex comprising three tunnels with appropriate diameters and slopes to ensure continuous sediment transport toward the Chira River. Implementation does not disturb the dam body, although an upstream cofferdam is required. A 20-m-deep sloped trench excavation toward the outlet is proposed based on numerical hydraulic modeling. Prognostic evaluations indicate that the proposed flushing complex could significantly reduce existing reservoir sedimentation.