Typha control efficiency of a weed-cutting boat in the Lac de Guiers in Senegal : a preliminary study on mowing speed and re-growth capacity

Prolific growth of Typha australis in the lower part of the Senegal River and the Lac de Guiers resulted from changed ecological conditions following the construction of two high dams in the Senegal River. Fluctuation of the water level has decreased markedly and the water has changed from brackish to fresh as the inflow of salt water from the ocean is prevented. The efficiency of a hydraulic weed cutting boat (model Conver 480 H) has been tested in various plots in the Lac de Guiers. This lake, connected with the Senegal River, is economically very important as it is the city of Dakar's major drinking water supply and also provides irrigation water for vast agricultural areas. Cutting, performed at 20 cm and 50 cm below the water surface, was very effective in removing Typha stands. However, due to the excessively thick stems of the Typha plants, cutting took much more time compared to similar operations in Europe. It was estimated that approximately 35 h were required to clear one ha of dense Typha stands. Directly after cutting re-growth was observed during a relatively brief period (approximately three months), however, subsequently all newly formed growth tips in the plots had suddenly disappeared. It was concluded that this sudden collapse was linked to a destruction of the root zone due to anoxic conditions. Eventual re-growth was relatively slow as after one year the plots were still largely free from Typha plants. Efficiency of mowing was not related significantly to depth of cutting.