The small community of Good Hope, on the east coast of Dominica, West Indies, nestles along a narrow valley bottom by Grand Marigot Bay. By nightfall of November 11, 1986, the community and surrounding area had experienced several days of heavy rainfall. Earlier that day, a small landslide developed in the cutslope of the Castle Bruce-Petit Soufriere road where it crosses the steep hillslope above the town. Except for this landslide, the rains had produced no other unusual events.
Shortly after 3:00 AM on November 12, 1986, nearly 17,000 cubic meters of soil and weathered rock slid from the hillslope above the Castle Bruce-Petit Soufriere road. Bananas, coconut, and bay trees growing on the hillslope were swept away. The rapidly moving slide mass destroyed citrus trees and vegetable crops growing below the road. The slide engulfed the health clinic near the base of the slope, seriously injuring the nurse and killing her ten-year-old daughter in their sleeping quarters. Several meters farther downslope, the impact of the slide shoved the primary school partly off its foundation, collapsed the back wall, and buried the upslope side to the roof line. As the debris came to rest, the toe of the slide deposited material 1 to 4 meters deep along a 15-meter length of the principal street through Good Hope. On the slope above, a 90-meter section of the Castle Bruce-Petit Soufriere road lay blocked by the upper slide mass (Fig. 1).
Taken from: DeGraff, J.V., Bryce, R., Jibson, R.W., Mora, S., and Rogers, C.T. 1989. Landslides: Their extent and significance in the Caribbean. In E.E. Brabb and B.L. Harrod (eds), Landslides: Extent and Economic Significance. p. 51-80. Rotterdam: A.A. Balkema.
Landslide Dams in Dominica
The island nation of Dominica in the Windward Islands is known to tourists as the "Nature Isle". This term normally refers to the extensive remaining rainforest, beautiful waterfalls, and steep volcanic terrain which attracts visitors interested in eco-tourism. In November, the country experienced a different side of nature as two landslide dams formed and failed in the Layou river valley. The Layou river is the longest river on this 290 square mile island in the eastern Caribbean. About five miles from its mouth on the Caribbean Sea, the Layou river is joined by one of its tributaries, the Matthieu river. At their junction, the rivers flow through a gorge with 300-foot high cliffs carved in welded tuff. This was the site of the landslide dams.
An area subject to periodic landslide activity is present within the Matthieu river valley just upstream from its junction with the Layou river. Small landslides occurred at this location as far back as 1988. Several large landslides were triggered in this area by storms during 1995.
On November 18, 1997, a large debris flow from the unstable valley wall within the Matthieu river passed down the gorge and into the Layou river channel to form a landslide dam. The landslide dam was about 50 feet high at its highest point and formed a solid plug in the river for about 200 feet upstream and 300 feet downstream from the Matthieu river junction. Water rose quickly to form a lake behind the dam. By early morning on Friday, November 21, the dam was breached and sent an estimated 300 million gallons of water down the valley.
The following week on November 25, another landslide took place at the same general location in the Matthieu river valley. A debris flow again passed through the gorge and deposited on the remnants of the earlier landslide dam. The Layou river was again dammed. Additionally, most of the landslide came to rest as a solid mass wedged between the cliffs forming the gorge and blocked the Matthieu river. The landslide dam on the Layou river was estimated to be 60 feet high at its highest point. The Matthieu landslide dam is about 200 feet high as measured on its downstream face.
By Friday, November 28, the water impounded by the second landslide dam on the Layou river overtopped the dam at about 2:00 p.m. An estimated 350 million gallons of water flooded the Layou river valley. Both flooding events required evacuation of several hundred residents of the community of Layou at the river's mouth. While few structures were damaged by flood waters, significant crop loss, road damage, and deposition of sediments occurred along the river.
The landslide dam on the Matthieu river continues to fill. The smaller drainage area upstream means several weeks to months will pass before it is filled to capacity. If it completely fills before breaching, it is estimated that the water impounded behind this dam will be 2 to 5 times greater in volume than the water impounded by either of the landslide dams on the Layou river.
The steep terrain and rivers make taking action to prevent complete filling of the valley behind the Matthieu landslide dam virtually impossible. There is too little time and too few funds to accomplish an engineering control or stabilization of the landslide dam. Efforts are being focused on monitoring to ensure safety of inhabitants down stream and permanent relocation of people from the path of the future floods. The river channel has lost much of its water carrying capacity from sediment deposited from earlier landslides and the material carried downstream from the two failed landslide dams.
From: International Landslide Research Group (ILRG) Newsletter Vol. 12, No. 1, 1998
