El Salvador – Flood Inundation Mapping for the Rio Lempa

In 1998, Hurricane Mitch caused a large amount of water to back up behind El Salvador’s 15 de Septiembre Dam, fed by the Río Lempa. Water releases from the dam during the hurricane caused large-scale flooding downstream of the dam. It became apparent that it was critical for El Salvador to know what downstream areas would be inundated with floodwater under different water release scenarios from the dam.

Río Lempa Basin

Working with Sistema de la Integracion Centroamericana (SICA), Riverside Technology, inc. (RTi) developed a model of the lower Río Lempa using the National Weather Service’s FLDWAV dynamic routing model. The model simulated river elevations resulting from releases from the 15 de Septiembre Dam. SICA provided detailed cross-sections along the reach from the dam to the Pacific Ocean. The FLDWAV model was calibrated using actual releases from the Hurricane Mitch event. The model accounts for tidal impacts at the lower end of the reach.

Using the FLDWAV model results, RTi determined flood inundation boundaries for several large releases from the dam under various tidal conditions. The boundaries for each combination of release and tide level are shown on maps of the river, allowing operators of the dam, prior to making releases, to identify areas of flooding risk. The maps were also provided to emergency management agencies in the area.

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