Soil loss and sediment transport during the storms and landslides of May 1988 in Ruhengeri prefecture,Rwanda

In early May 1988, five prefectures in western Rwanda experienced catastrophic levels of precipitation, landslide, and flooding activity that resulted in a severe loss of life, property, and livelihood. Using data from runoff plot and hydrological monitoring stations of the Ruhengeri Resource Analysis and Management Project, the events and circumstances leading to these phenomena are reconstructed. These data show that mass wasting processes were preceded by more than 140 mm of precipitation during 4–7 May, which may have saturated local soils. A small earth tremor on 7 May, (Richter scale of 3) contributed to the onset of the catastrophic debris avalanche, torrent, and earthflow activity that commenced 24 h later. The more than 50 mm of precipitation that fell during 9 May, including a maximum 30 min intensity of 24 mm, resulted in continued surficial soil loss that averaged 34 t/ha on seven cropped, Wischmeier-type runoff plots with biological erosion control contours. The Nyamutera River, which drains the impacted area, delivered 567000 tons of suspended sediment to its mouth between 7 and 13 May. This corresponds to a basin-wide lowering of 12600 t/km², or more than half of the basin's annual suspended sediment yield. Theoretical distributions of maximum 24 h precipitation events suggest that Nyakinama and other regions in Ruhengeri are particularly prone to similar high volume events, exacerbating an already serious soil loss problem throughout the prefecture. Because contemporary land use practices directly contributed to the severity of the 1988 event, further applied research that identifies technologies capable of reducing soil loss, augmenting soil fertility, and minimizing the impacts of high magnitude and high volume rainfall is greatly needed.