Hurricane Pauline - its impact on Acapulco and subsequent debris-flow prevention measures

The city of Acapulco is situated at the vein of the Pacific Ocean, around the Acapulco Bay, at the foot of the Sierra Madre del Sur. From a small beach area, steep slopes are climbing to an elevation of up to 900 m asl. The upper third of the slopes is densely forested; the lower two thirds are housing areas. Thick soil layers cover the bedrock. Small ravines are carved into the slopes forming a drainage pattern with many small side branches, starting with natural beds from near the crest of the hills, but being channeled within the lower populated areas. Usually these gullies are almost dry. In the early morning of October 9, 1997, hurricane Pauline hit the city of Acapulco with an excessive precipitation of 411 mm within 24 hours. The resulting floods, debris flows, mudflows and landslides caused the loss of about 160 lives, the disappearance of another 260 people, and heavily damaged urban infrastructure. One creek reached a maximum discharge of 107 m(3)/s. Boulders with volumes up to 30 m(3) were displaced. Huge quantities of gravel and sand were eroded and were transported from the hills down to the urban areas blocking culverts, bridges, tunnels and roads and even filling the basement of residence buildings. Total loss was estimated to 1 billion US $. First, the topographical, geological and meteorological situation of the Bay of Acapulco is outlined. Its history of hurricanes, their duration, precipitation and return periods are analyzed. Hurricane Pauline is integrated in this context. Relevant drainage conditions before Pauline, contributing to the large extent of losses, are explained. Damages caused by the hurricane are listed. Natural boundary conditions as well as man- made reasons behind the damages are presented. A modem approach to debris flow mitigation is described. Basic mitigation measures are dealt with. Pros and cons of the different solutions are discussed from technical, economic and ecological points of view. The solutions having been finally selected are presented in general together with the reasons for their selection. Some are described in detail.