Introduction Rivers are known as the main sources of surface water in the world, which experience seasonal fluctuations in water level. These resources have severe damage to human societies and nature in flood conditions and have irreparable consequences in the drought seasons. Optimal utilization of these resources with maintaining the environmental conditions of the waterway and minimizing flood damage is considered one of the river engineering goals. Since the conventional methods of river management are imposed serious environmental threats on waterways and wetlands, consideration to these water resources requires attention to issues related to plant ecosystems, solving challenges of coastal bed erosion and predict the condition and management of the river in the future (Callow, 2012; Dawson and Haslam, 1983; Fan et al., 2013; Rose et al., 2010; Rowinski et al., 2018). One of the strategies that cause loss of flow energy in the river improves the hydrological system and river ecosystem is the presence of vegetation in the river banks and floodplains. Native vegetation in floodplains and coastal forests plays an important role in conserving waterway ecosystems, flood management, coastal protection in urban lands and agriculture adjacent to the river (Fathi-Moghadam, 1996). Vegetation will also control the width of the river and increase the stability of the shores by absorbing and settling suspended sediments in river banks. The plant species along rivers and waterways are composed of various vegetative components, mainly affected by the environmental conditions of their habitat, including the distance from the waterway bed, hydrological characteristics of the river, climatic and soil conditions. Obviously, the effect of each plant species in the ecosystem cycle varies and for each section of the river, a specific combination of plants will create optimal conditions. Methodology In this paper, effective parameters have been identified to investigate the effect of vegetation properties on drag force and the final functional relationship between dimensionless parameters for estimation of resistance to flow and drag coefficient in vegetated rivers will be: CD = f [yn/H, DI, ρV²yn4/EI] (1) Where CD is the drag coefficient, yn/H is the relative depth, DI is the vegetation density index and last parameter is the dimensionless number of the ratio of the flow velocity to the bending stiffness, which is used to investigate the effect of the vegetation flexibility. The experiments were conducted in an 8.30×0.80×0.55m flume with Plexiglas sidewalls and metal structure located in the Water and Environmental Engineering Faculty at Shahid Chamran University of Ahvaz, Ahvaz, Iran. The knife-edge part of the flume, which can move freely in a limited range, was used to measure the exerted force to the moveable part of the flume (which supports the vegetated model) by means of a dynamic load cell installed between the movable and fixed parts of the flume. the artificial vegetation models used in this study were similar to the natural sample of trees. Galvanized and polyethylene sheets have been used to make rigid and flexible artificial specimens with the same geometric shape and form to allow more accurate comparisons. Results and Discussion In this section, with the aim of practical use of the results, the independent variables are defined as dimensionless numbers based on the drag coefficient. Investigating the effect of the vegetation density index, the results showed that in both rigid and flexible models, increase of the vegetation density index leads to a decrease in the drag coefficient. By increasing the vegetation density index in the flexible model at yn/H = 1, the drag coefficient decreased by 19.8%, which is the highest rate of decrease in the drag coefficient. Also, the lowest rate for reduction of the drag coefficient with increase of the vegetation density index occurred in the rigid model and at yn/H = 0.6, which is equivalent to 10.3%. In fact, in the case of the maximum decrease in the drag coefficient, for an 80% increase in the vegetation density index, the drag force increased by 43.8%, and in the case of the minimum decrease in the drag coefficient, the drag force increased by 66.5%. By comparing the effect of flexibility of vegetation in rigid and flexible models, the results showed that in both cases, the drag coefficient decreases with increasing in velocity. In the flexible models, the slope of the drag coefficient curve has a decreasing trend faster than the rigid case. For the effect of the relative depth of the flow, the results showed that the drag coefficient in both models decreases with the increase in the relative depth, so that in the rigid and flexible models, with the increase in the relative depth from 0.6 to 0.8 and from 0.8 to 1, the drag coefficient decreases by 12.81% and 10.43%, respectively. Correlating the results, following relationships were obtained for the estimation of the drag coefficient for the rigid and flexible models, respectively: CD = 3.26 [yn/H]0.13 - 2.65(DI)-0.12 + 0.28[ρV²yn4/EI]-0.29 R² = 0.87 (2) CD = [yn/H]1.2 + (DI)0.16 - 0.32[ρV²yn4/EI]0.21 R² = 0.89 (3) Conclusions In the present study, the drag force absorption rate and the resistance coefficient were investigated and equations were correlated to estimate the drag coefficient. All experiments were performed in steady, uniform and turbulent flow. Rigid and flexible models were tested by different hydraulic parameters according to the flow conditions. A new index is produced to account for the effect of vegetation density on the drag coefficient. Investigating the effect of the vegetation density index showed that the increase in this index has a significant effect on reducing the drag coefficient, so that at the maximum increase of the density index, a decrease of 19.8% in the drag coefficient was measured. The results showed an increase in drag force absorption as a result of increase in vegetation density index and relative depth. In general, the increase in vegetation density index ranges 0.4 < DI ≤ 0.8, 0.8 < DI ≤ 1.1, 1.1 < DI ≤ 2.2 and 2.2 < DI ≤ 3.8 in comparison to the range of 0.2 ≤ DI ≤ 0.4; the drag force absorbed increases by 1.63, 2.46, 4.12 and 6.33 times, respectively. Also, increase of the relative depth from 0.6 to 0.8 and 1, the drag force increased by 49.7% and 46.9%, respectively. The index introduced in this study can be a reference index for monitoring the types of vegetation in floodplains and aquatic plants. The results of this study can be used in the numerical modeling for estimation water level during flood events and river engineering management. [ABSTRACT FROM AUTHOR]