Author(s): Takeshi Takemura; Norio Tanaka
Keywords: Typha angustifolia; Drag coefficient; Breaking strength; Expansion threshold
Abstract: To elucidate the expansion threshold of Typha angustifolia, an emergent macrophyte, in a river, the roughness characteristics and breaking strength were investigated using a water flume and field observation, respectively. In the experiment, real plant parts were used to create three roughness models: shoot-base, dense-leaf and sparse-leaf. The drag coefficient, Cd, for each model was measured with changing roughness inclination toward flow direction. The drag coefficient varied from 0. 2 to 1. 6 with the change of parts and inclination. The Cd of the shoot base was around 1, but it increased with changing water depth and peaked at the height of the dense leaves because of the complexity of the leaves just above the shoot base. However, it decreased from the peak with increasing water depth due to the large inclination of the leaves. By evaluating the real Cd of T. angustifolia, the breaking moment of the shoot base and the velocity in the primary shoot and secondary shoot zones, a threshold line for their enlargement could be projected. The velocity and breaking moment were calculated with a depth-averaged Reynolds equation in two rivers, the Touemon River, in which the vegetation width changed seasonally and remained narrow, and the Denu River, in which the stream width was almost completely covered by plants. Taking into account the complex changes of Cd with water depth, the threshold line can represent the actual enlargement potential. If the hydraulic conditions of a river do not exceed the threshold line, some artificial control is required.