Haiti – Dominican Republic - Disasters and Conflicts - UNEP

Signs of the destruction wrought by the flashflood of 2004 can still be seen along the riverbed of the Soliette/Arroyo Blanco River where it passes the Haitian town of Fonds Verrettes.© UNEPThe issue is most critical however for the Soliette/Arroyo Blanco and Pedernales Rivers, draining intoLake Enriquillo and the Caribbean Sea respectively.Both of these rivers have very heavily degraded,steep catchments in the Haitian southeastmountains – a region prone to heavy rainfall dueto prevailing winds bringing moisture laden airfrom the Caribbean Sea. Both rivers also haveDominican towns located next to the riverbeds, onalluvial fans at the base of the catchments: Jimanifor Soliette/Arroyo Blanco and Pedernales for thePedernales River (with Anse-à-Pitres on the Haitianside). Both sites have experienced damagingfloods in the past.The forecast for these three towns is very negative:major flooding can be expected to continue forthe foreseeable future, even if there is investment inmitigation. The technical rationale for this forecastis that both of the potentially viable flood riskreduction measures (reforestation/soil conservationand micro-dams) will not fully work in these specificcases, and the flood control dams of the size thatwould be needed to make a difference are simplynot economically viable.First, reforestation and upper catchment soilconservation investments may help cap but willnot reduce the current flood risk levels. At best, theywill retain the remaining soil and flood risk levelswill not increase. This is because it is soil, not trees,which retain the bulk of the rainfall and encourageinfiltration in high rainfall events. Trees simplyprotect the soil from rain damage. However for theSoliette/Arroyo Blanco and Pedernales watersheds,much of the soil is already lost and transported assediments by the rivers either to Lake Azuei, to LakeEnriquillo, or to the Caribbean Sea.Second, micro-dams in the upper catchments, ifvery widespread and well built, will mitigate generalflash flooding, but will not mitigate hurricane ormajor storm related flooding risks. Micro-dams aresmall semi-permeable dams built across tributariesin the upper part of catchments. Generally theyare either dry or have a low flow leaking throughthe wall via drainpipes. In the event of short-termheavy rainfall, they do rapidly fill up and retainwater for some time, thereby extending thedrainage time and reducing the flood peak. As aresult micro-dams can reduce the flash floodingrisk – for some rainfall events. However hurricanesand major storms tend to bring heavy rain forextended periods and this both overwhelms themicro dam storage capacities and damages thestructures through very heavy and fast flows.Finally, localized flood channel containmentmeasures (gabionage and channel excavation)will work only in part. These types of very localHaiti – Dominican Republic: Environmental challenges in the border zone73