In October 2011, heavy rains from Tropical Depression 12-E caused the levees on the Lempa River to burst in multiple places (see red dots on map), forcing the evacuation of 40 of our partner communities. Six weeks later, EcoViva worked with our local partners at the Mangrove Association and allies at Engineers without Borders to organize a delegation of six engineers split into two teams: water systems and flood management.
The water systems team conducted an assessment of the extent of contamination of local potable water systems, made recommendations for clean water delivery, and trained local water committees to conduct their own water quality testing in the wake of significant bacterial contamination in virtually all water supplies. The results of their work will be highlighted in a future blog post.
The flood management team conducted an assessment of the human causes of the disaster. Their report, written in Spanish with the support of EcoViva and Mangrove Association staff, contains recommendations to the Salvadoran government and the international community about how to prevent future disasters. A key recommendation focuses on improvements that can be made to the levee structures to prevent breeches during future storms.
An executive summary of the report is below. With the backing of the mayor’s office of the municipality of Jiquilisco, the full nine-page report was presented this month to officials from five federal ministries in El Salvador and the Lempa Hydroelectric Commission (CEL). As a result, the CEL has expressed interest in investing in improvements to the existing levee system, as per the engineers’ recommendations. Last week, they requested quotes from private contractors to carry out the suggested improvements to the levees in coordination with local government officials and our local partners.
It is unprecedented that the CEL is responsive to a community-based organization. It is thanks to our local partners at La Coordinadora/Mangrove Association and their highly effective advocacy work at the municipal and national level that we have been able to make an impact on the government’s planning around flood mitigation.
Many thanks are also due to Engineers without Borders – Iowa City Professional Chapter and the specific engineers who donated their own funds and much valuable time to this important project. They are: Mike Saeugling, Richard Weller, Cindy Quast, Jim Chamberlain, Dave Werth and Matthew Havice.
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PRELIMINARY FLOOD PROTECTION ASSESSMENT
BAJO LEMPA REGION, MUNICIPALITY OF JIQUILISCO, USULUTÁN
EL SALVADOR, DECEMBER 1-8, 2011
EcoViva, Mangrove Association and Engineers without Borders (Iowa City)
Project Coordinators: Nathan Weller (EcoViva) and Walberto Gallegos (Mangrove Association)
EXECUTIVE SUMMARY
Introduction
In the wake of the extraordinary flooding event caused by Tropical Depression 12-E, EcoViva and its local partners hosted a delegation of civil and environmental engineers in December 2011 in El Salvador, affiliated with Engineers without Borders. One of the objectives of this emergency delegation was to analyze the current state of flood protection infrastructure in the Lower Lempa River Basin.
The following is an executive summary of the document produced with observations and recommendations on flood management, dam operation and levee design in the Lower Lempa. EcoViva is currently collaborating with pertinent local and national authorities, and responsible agencies, to improve levee structures in the most vulnerable flood plain in Central America.
Effects of Tropical Depression 12-E
From October 10 to 19, 2011, Tropical Depression 12-E unleashed 54.5 inches of precipitation –a record high for any storm event ever observed in El Salvador. In the wake of the event, government estimates of damages and direct losses varied between $650 million and $1.2 billion, or a minimum of 3% of El Salvador’s annual GDP. Due to such excessive rainfall, rivers across the country, including the Lempa River, exceeded their capacity to manage high flows. Dams along the Lempa River, managed by the Lempa River Hydroelectric Commission (CEL), became quickly overwhelmed, causing the flooding of an area of 2,000 km2 (or 10% of the country) and leaving more than 50,000 people temporarily homeless.
In the Lower Lempa River Basin, record flooding exacerbated by levee failures at 19 locations displaced over 5,000 rural families, the majority of which live in conditions of extreme poverty. At the height of Tropical Depression 12-E, dam operators at the CEL’s facilities discharged over 9,000 cubic meters of water per second downstream, overwhelming the earthen levee structures built to withstand between 2,500 and 3,000 cubic meters per second. Due to these failures, over 35 communities were severely affected, including damages to 1,895 homes. Severe flooding of 2,135 hectares in the local agriculture sector led to 90% to 100% crop loss on that land.
Lempa River Dams: Observations and Recommendations
There are four dams along El Salvador’s Lempa River system in El Salvador. Together these facilities meet over 40% of the region’s demand for electricity. Three of these dams–including the 15 de Septiembre site which is directly upstream from the Lower Lempa River Basin–have no available flood storage capacity in their reservoirs. During extreme flood events, such as Tropical Depression 12-E, these dam facilities must drain their reservoirs at nearly the same rate as the amount of upstream waters which enter them. This system provides for little flexibility during high flow events, or floods, and contributes to the high levels of risk and vulnerability for rural inhabitants in the Lower Lempa.
Further upstream, the dam and reservoir at Cerrón Grande has the largest reservoir, and likewise contains the greatest storage capacity for flood control throughout this system. Given the constraints in managing the hydroelectric operations and river flows, as well as the difficulties of forecasting extreme precipitation events, the volume of water stored in Cerrón Grande should be reduced at the end of the dry season in order to allow for storage of greater volumes of flood water during El Salvador’s wet season. Maintaining a lower level in the Cerrón Grande reservoir would, however, correspond to less hydropower potential for El Salvador.
In the case that flood storage is given higher priority at this facility, the possibility of increasing the energy production capacity at Cerrón Grande must be examined to meet the region’s demand for electricity. By installing a third powerhouse, additional power could be generated at Cerrón Grande during the rainy season, and thus assist to offset the loss of potential energy reduced by lower reservoir volumes. However, if a third powerhouse is not financially feasible, and if the production of energy is deemed more important than maximizing the dam’s flood control potential, then the downstream levees in the Lower Lempa should be built higher and stronger to withstand extreme flood events.
Lempa River Levee Systems: Observations and Recommendations
Independent of the dam authority’s operational priorities and policies, the levee systems in the Lower Lempa must be reconstructed to a higher standard. This standard should adequately respond to the maximum length of time, frequency and height of flood events, high-water flows in the river, and the CEL’s discharge regime.
During the 12-E flood event, levees in the area failed either from “over-topping” or from seepage through the structure itself. This seepage destabilized the interior of the compacted levee material, and ultimately caused the collapse of key points along the structure. Levee designs in the Lower Lempa should consider stabilizing this foundation on the land side with permeable, yet stable, materials such as rock, gravel and sand, in order to allow for seepage water to exit the earthen structure without compromising its integrity. A parallel drainage ditch can be incorporated into the existing drainage system, so that seepage water would eventually flow into existing drainage systems.
