The effects of vegetation Manning roughness coefficient on the hillslope in Aghghala rangelands

Vegetation as one of the main effective factors in the flow resistance directly influence the flow velocity by increasing the surface roughness. The calculation of flow velocity in hydraulic, hydrology science and all water projects, particularly river engineering, flood control, also for hydraulic and hydrologic modeling is necessary. One of the methods used for determing the velocity of flow is using the manning equation. In this research, some field experiments have been carried out to simulate runoff flow on two slops in order to determine the effects of vegetation on manning roughness coefficient on the hillslope of Aghghala in Golestan province. These experiments have been done on two rectangular plots, with a width of 2 meters, length of 18 meters, on two different slops i.e. 4.1, 12.9 percent. The runoff was generated by pumping water out of a reservoir. The experiments were done firstly with vegetation and then the vegetation was removed from the plots and the experiments were repeated. Totally, these experiments were repeated 20 times. The slope, vegetation percentage, flow discharge rate for the hillslope were measured. Then, in each repetition, the flow velocity was calculated and also the hydraulic radius in 6 sections were determined. The manning roughness coefficient, with vegetation and without vegetation, determined using the manning equation for the two hillslopes was 0.0557, 0.0510 for low slope hill and 0.0652, 0.0564 for the high slope hill. After the calculation of manning coefficient of all repetitions, the T test results showed that the vegetation can significantly increase the roughness coefficient. The structure of the field experiments as well as the fact that sheet flow runoff and unsubmerged vegetation being the focus of the study were new in this research and there is no enough knowledge on them.