Iranian Water Research Journal

Iranian Water Research Journal

Experimental Investigation of the Effect of Non-Submerged Rigid Vegetation in Floodplain of Rectangular Compound Channel on Friction Factors and Flow Force

Document Type : Original Article

Authors
Foad Maturi 1
مهدی behdarvandi askar 2
1 Graduated of Hydraulic Structures, Department of Offshore Structures, Faculty of Marine Engineering, Khorramshahr University of Marine Science and Technology,Master Degree of Water Engineering and Hydraulic Structures, Responsible for the Measurement
2 Associate Professor, Department of Offshore Structures, Faculty of Marine Engineering, Khorramshahr University of Marine Science and Technology
10.22034/iwrj.2026.14847.2621
Abstract
Introduction:
A compound channel is a channel that its cross-section has several sub-sections with different flow profiles and distinct characteristics such as roughness, depth, etc. A natural river that flows over the land on its banks during a flood is a simple example of a compound channel. Flooding accounts for approximately one-third of the financial losses associated with natural disasters worldwide and more than half of the fatalities from these disasters. This research was conducted to physically simulate rigid vegetation (made of PVC) in the floodplain of a compound channel and to investigate its effects on the drag coefficient, friction factor, and flow force ratio. The direct measurements carried out in this research were much more accurate than energy methods and conventional methods of force investigation which were discussed in previous researches. In energy methods, the force value usually differs from the actual values, due to existing errors. The errors were partially compensated by using direct measurement.This research led to development of experimental relationships through which the drag coefficient and friction factor were directly calculated based on the vegetation parameter.

Materials and Methods:
This research investigated the effect of vegetation cover on the flow force ratio, drag coefficient, and friction factor using physical modeling of the flow in a knife-edge hydraulic flume with a length of 14 meters, a width of 107 centimeters, and a longitudinal slope of 10-3The experimental setup was located in the Hydraulic and Sediment Laboratory of the Basic Studies Center for Khuzestan Water and Power Authority, Iran. Five diameters of vegetation cover (20, 25, 30, 40 and 50 mm) made of PVC were considered as rigid non-submerged vegetation covers. The height of this cover was determined to be non-submerged in all cases. In order to use these covers, plexiglass sheets with a length of 1 meter (the length of the knife edge part) and the width equal to that of the floodplain were installed in the flume, , according to different arrangement of vegetation cover and its placement within the flow. Five longitudinal distances (8, 10, 12, 15 and 20 cm), three transverse distances (6, 8 and 10 cm, and two types of arrangement (regular and irregular) were implemented to investigate different cases. Due to the different variables, the total number of runs in this study was 151 different experiments. In this study, for data analysis, a dimensionless parameter that included variables such as distances and number of vegetation, cross-section changes and depth difference between the main channel and floodplain (relative depth) was used as the vegetation parameter.

Results and Discussions:
The results of the experiments were presented in the form of various graphs which illustrated the changes in the flow force ratio, friction coefficient, and friction factor based on the vegetation parameter. The ratio of flow force in the case of vegetated conditions to the flow force without vegetation in all experimental conditions was graphed with respect to the vegetation parameter. An equation with a suitable correlation coefficient (R2=0.9198) was fitted to the data trend. According the results of this study, the decreasing trend of the drag coefficient with increasing vegetation parameter was clear. The main difference between the two arrengment types (regular and irregular) was the range of the drag coefficient. The changes in the drag coefficient in the range of 0-0.5 of the vegetation parameter were between 2 and 10, which was relatively higher compared with the regular vegetation arrangement. The drag coefficient obtained in this study was also compared with the results of research conducted by Tanino and Neff (2008) and Tinoco and Cowen (2013) and their equilibrium equations. It was revealed that the drag coefficient values in the current study had a relatively similar trend to the drag coefficient obtained from the force equilibrium equation, which indicated the suitability of the proposed method and equations for calculating the drag coefficient in the studied area. To investigate the effect of vegetation on changes in the total friction factor calculated based on presented equation, two graphs were presented for regular and irregular vegetation arrangements. Trend lines with relatively good correlation coefficients were also obtained. Accordingly, related equations were presented to calculate these parameters. Considering the change in the type of arrangement, a ratio between the drag coefficient and the vegetation parameter was also defined.

Conclusion:
In this study, the flow force was measured using the direct measurement method and the drag coefficient, and total friction factor were calculated through the provided equations and presented in different diagrams based on the vegetation parameter in the compound channel with the presence of vegetation in the floodplain. According to the results of the diagrams, it was determined that the flow force ratio had a decreasing trend with increasing vegetation parameter, which was consistent with the drag coefficient regardless of the type of arrangement. It is suggested that the investigation of non-rigid (flexible) vegetation cover in compound channels be conducted using this type of flume and measurement method, and the results be compared with other researches.
Keywords
Compound Channel Direct Measurement
Drag Coefficient
Friction Factor
Knife Edge Flume
Subjects
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Iranian Water Research Journal
Volume 20, Issue 2 - Serial Number 61
(Is completing ...)
Summer 2026
Pages 49-60

  • Receive Date 30 December 2025
  • Revise Date 02 February 2026
  • Accept Date 08 February 2026
  • Publish Date 22 June 2026