Report 47. Technical assessment of the irrigation systems of Marigot and Jacmel and preliminary observations on the Marigot Watershed

Abstract

An irrigation system located near Marigot was damaged by flooding from Hurricane Georges. A water engineer and an agronomist were asked to determine what was needed to rehabilitate the system and to protect the system from damage from future storms. The site of a second irrigation system, planned for Jacmel, was also visited and recommendations made. Irrigation Systems: The Marigot system has two components, one with intake at Rodaille, server 360 ha on the east side of the Plantil River, and one with intake at Belle Roche serving the 165 ha on the west side. The canals at the field level were in good condition, although water management within individual fields appeared inefficient. The intake structures at Rodaille and Belle Roche were severely damaged. A syphon crossing under the river from the intake at Rodaille was destroyed. The east bank of the river was eroded several meters inland at this point and the course of the river was displaced eastward. Without protection, this bank will continue to erode eastward, with the loss of cropland and sections of the primary canal. A closed conduit crossing the dry riverbed appeared to be intact but was filled with rock and debris. A portion of the primary canal running along the west bank of the river from the Belle Roche intake was destroyed when the riverbank collapsed as a result of the flooding. Because of the destruction to the upper portion of the Rodaille system, irrigators on the westside secondary canal constructed their own diversion, running water into the secondary canal and at one point passing water under the secondary canal. If this situation is allowed to persist, the system will be irreversibly damaged. Recommendations: - Relocate the intake for the Rodaille system upstream from the present site and on the east bank of the river, to avoid the need for a syphon. - Build a new upper canal section to connect up to the existing primary canal. - Install conservation structures to protect the primary canal from further erosion of the east bank of the river. - Divert the stream channel westward within the riverbed. - Clean the enclosed conduit and the canal between the syphon and the conduit. - Relocate the Belle Roche intake and the main canal. The Jacmel system will require 17 km of main conveyance canal and will server approximately 500 hectares, of which 300 ha will be in the lowland and 200 ha on hillsides. The most difficult part of the system will be the construction of the upper section of the canal along the steep river bank next to the proposed intake. The site proposed for the intake appears to be adequate. The greatest hazard to the system will be the potential for increased soil erosion on the steep slopes due to the added water. Recommendations: - Provide training in soil and water conservation along with installation of the system at Jacmel - Install conservation structures such as bench terraces on irrigated hillsides. - Implement programs for irrigation system maintenance and water use management at Jacmel and at Marigot in order to ensure efficient water use and system longevity. - Train system operators and users in system maintenance and water use management Marigot Watershed: Evidence from the deposition of large rocks on farmland and widening of streambeds suggest that the flooding that occurred with Hurricane Georges was unprecedented. It appears unlikely that the extent of damage could be attributed to conditions in the foothills. These are fairly well vegetated to the northwest, although the foothills to the northeast, draining into the intermittent stream, have more extensive clearing and erosion. The Marigot watershed drains a 12-15 km length along the south slope of the La Selle chain of mountains, which surpasses 2000 m in elevation. The Rivière Blanche and Plantil Rivers drain the western part of the watershed, while the eastern side is drained by intermittent streams originating in the high plateau that forms the buffer zone to Parc La Visite. Although the high peaks are forested, a vast area below this is almost devoid of trees. Some areas are intensively cropped with vegetables, but large areas have been used as pasture and the vegetation closely grazed. Soil and water conservation measures are virtually non-existent. Gullies and rill erosion are common. Recommendations: Soil and water conservation measures in the upper part of the watershed are required on a large scale in order to reduce the flooding hazard to the irrigation system at Marigot. These include: - ravine stabilization measures - establishment of woodlots and natural vegetation, contour plantings of trees and grasses - introduction of soil and water conservation practices into improved crop production (rock walls, contour hedgerows, alley cropping and other agroforestry practices) - improved soil fertility management - greater emphasis on perennial crops - improved livestock husbandry and pasture management - In the long term, a solution must be found to limit the free grazing of livestock that prevents the establishment of adequate ground cover.