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The nonlinear effects of vegetation section length on energy dissipation in open channel flow are experimentally investigated in a laboratory flume with woody cylinders simulating the emergent vegetation in rivers.The results of the research reveal that: the water depths in the vegetation sections decrease along the flow direction and increase with flow discharge and vegetation section length.A nonlinear empirical equation is developed for describing the variations of total energy in the flow.The total energy of water flow in vegetation section seems increases with water depth although the governing equation is a quadratic equation.In the total energy,the proportion of kinetic energy decrease with the length of vegetation section.The total energy for a certain water depth is higher for the shorter vegetation section length.The total energy losses within the vegetation section noticeably increase with the lengths of vegetation section but seem not sensitive to the flow discharge.