Sediment Sluicing During Hurricane María At Loíza Reservoir, Puerto Rico

Sediment sluicing is a management strategy which minimizes the retention time during flood events to route sediments through a reservoir. This strategy can be particularly useful in tropical environments where large events, such as hurricanes, can deliver sediment loads equivalent to multiple years of average conditions.

Loíza water supply reservoir in Puerto Rico, operating since 1954, has lost nearly half of its gross capacity by sedimentation. The remaining capacity lies above the crest elevation of the spillway, in the portion of the pool controlled by radial crest gates. On September 20, 2017, hurricane María produced the largest flood and sediment load since reservoir construction. Gage station data are presented showing reservoir drawdown and flows associated with the sluicing operation during the hurricane. Bathymetric data and modeling of long-term reservoir performance demonstrates that the sluicing operation was effective in releasing 2 Mm3 of sediment from the reservoir, which includes the sediment load generated by the storm plus scour of previously deposited sediment. The released sediment was equivalent to approximately 7 years of sedimentation at the historical long-term average rate. Vibracore data are presented documenting the transport of sand into the vicinity of the dam by the high velocity flow, areas which would not receive sand absent reservoir drawdown, further documenting the transport and scouring potential achieved by the sluicing operation. Bathymetric data show scouring patterns in the reservoir sediments at each bend in the reservoir.

Experience at Loíza reservoir has demonstrated that, by taking advantage of a hurricane’s large discharge to perform sediment sluicing, these extreme events can be beneficially used to release sediment from a reservoir. This is the exact opposite of the traditional mode of reservoir management under which hurricanes represent catastrophic sedimentation events.