The effect of ratio between rigid plant height and water depth on the manning's coefficient in open channel

\One of the important factors in channel dimension is the Manning's coefficient (n). This coefficient is influenced not only by the channel roughness but also by the presence of plants in the channel. The aim of the study is to see the effect of the ratio between the height of the rigid plant and water depth on the Manning's coefficient (n) in open channel. The study was conducted in open channel with 15.5 m long, 0.5 m wide and 1.0 m high, in which at the center of the channel is planted with the rigid plants with a density of 42 plants/m(2). The flow was run with a discharge of 0.013 m(3)/s at 6 ratios of H-plants/H-water, namely: 0; 0.2; 0.6; 0.8; 1,0 and 1,2, to obtain the velocity and water profiles. Then the value of n is analyzed using Manning's equation. The results showed that the mean velocity becomes decrease 17.81-34.01% as increase the ratio of H-plants/H-water. This results in increasing n value to become 1.221.52 times compared to the unplanted channel (n(o)=0.038). So, it can be concluded that the ratio between the rigid plant's height and water depth in the open channel can affect the value of Manning coefficient.