The Rio Lempa is a significant river in El Salvador, with several tributaries that receive rainfall from a large part of the country as well as areas of Honduras and Guatemala. The river flows in the lower reaches of the Rio Lempa in the Bajo Lempa region are the result of the combined effects of all of the inflow and dam and reservoir controls that occur along the river.

Since the devastating flooding that occurred in late October and November 1998 due to Hurricane Mitch, the Bajo Lempa communities and involved agencies have been evaluating the infrastructure in the area and implementing measures to reduce the impacts of future floods. Projects including larger river levees and improved drainage structures have been designed and are in the process of being built in the region. A key element of flood control which to date has received less attention is the operation of the three dam and reservoir facilities on the Rio Lempa. While the dam facilities have been operated primarily for the purpose of hydroelectric power generation, they clearly also play a role in regulating river flows and potentially mitigating floods.

This report presents an overview of the Rio Lempa river system and the operations of the three dams and reservoirs. The most upstream Cerron Grande dam has the largest reservoir, which is usually emptied prior to the rainy season in order to have available volume to hold back high inflow and minimize flooding below the dam. The middle dam, 5 de Noviembre, has a smaller reservoir which is normally kept near full to maximize capacity for hydropower generation. The lower 15 de Septiembre reservoir has been periodically drawn down, but not in a consistent manner.

The optimum dam and reservoir operation for flood control is to allow reservoir water levels to drop before the rainy season, then gradually refill as rainstorms occur. This mode of operation is dependent upon relatively accurate predictions of the upcoming season's weather, as well as reliable data regarding the interaction of the river inflow and the complex operations of the three dams.

The following observations have been made regarding the Rio Lempa dam facilities:

• The three dams provide a significant amount of El Salvador's electricity, so hydropower generation is an important priority.
  
  
• While the upper dam is operated to provide best available flood control, since it is the most upstream facility it cannot directly affect high inflows that occur in the rest of the Rio Lempa watershed.

In order to provide the best available flood control in conjunction with the other beneficial uses of the dams and reservoirs, the following alternatives have been identified:

• More advanced weather data should be an integral part of the facility operations. Based on most probable precipitation forecasts, the reservoir levels can be maintained at optimum levels for hydropower generation as well as flood control. Since the reservoirs hold large volumes of water, planning and releases must occur during the months before the seasons change.
  
  
• The operations of the dams and reservoirs have been coordinated with a variety of agencies, and this effort should be extended to the affected communities, particularly in the Bajo Lempa region. The facility operators should discuss planned measures, emergency plans, and other issues on a regular basis with community representatives.
  
  
• An ordinance should be adopted which codifies the main elements of the dam and reservoir operations. This will help to make operational decisions more consistent year to year, and provide a common understanding of the roles and responsibilities of all of the parties associated with the river.
  
  
• Alternative methods of operation and sources of electricity should be explored for use during periods of high flood risk, to determine whether other sources or more efficient hydropower technologies would increase operational flexibility.
  
  
• All releases from the dams should be conducted with non-damaging flowrates whenever possible. The acceptable release rates are determined by physical data of the river channel and other concurrent flow and water levels. Non-damaging releases should be achievable throughout the dry season.
  

The findings of this report are based on available information and standard hydrologic practices. Further evaluation of the suggested alternatives would require a compilation of detailed dam and reservoir operational data, and close coordination with the involved agencies.

Additional information:

• "Review of Dam Operations on the Rio Lempa" October 2000
• "Repaso de Operaciones de las Presas del Rio Lempa" octubre 2000

Further information regarding this project, including reports and memorandums, is available upon request. Info@gvengineers.org