The effect of pressure flow conditions on bridge pier scour in compound open channels with vegetation

Document Type : Research Article


1 Department of water Eng, Agriculture faculty, Lorestan university

2 Dep. of Water Eng. Agriculture faculty, Lorestan university

3 River and coastal Engineering ,Soil Conservation and Watershed Management Research Institute, Tehran,Iran


Bridges are one of the most important structures built on rivers and are considered as a structure connecting the two parts of the road. One of the most important reasons for the destruction of bridges is the scouring of its piers. New bridge design challenges, due to climate change and human intervention, as well as uncertainties associated with maximum events, may not adequately lead to accurate hydraulically design of bridges and may therefore as a result, in some floods, the bridge deck submerged. Under these conditions, the flow can be converted to a pressurized. This pressurized flow passes at high velocity in the region of bridge piers. As a result, it can increase the erosion potential of bed materials near bridge piers. Up to now, many studies have been performed to determine the relationship between estimating the rate of scouring of bridge piers in laboratory conditions with clear water and living bed, Such as: CSU equation.
Under pressurized flow condition, researchers such as Umbrel et al., Richardson and Davis, Zehi, and Karankina et al. Have developed relationships to determine the amount of scouring of bridge piers in simple channels. Due to the difference in flow velocity in the main channel and floodplains in the compound open channels, the important changes occur in the kinetic structure of the flow near the connection line between the main channel and floodplains. These changes also cause vortices as a result of excess energy loss in the flow. In addition, the presence of vegetation on floodplains complicates the hydraulic analysis of the flow in such sections. Up to now, many studies have been performed to explain the hydraulic conditions of the flow in compound channels with and without vegetation, including Shiono knight (1991), Rameshwaran and Shiono (2007), Zarati et al. (2008), Yu-qi Shan et al. (2016), Tanino et al. (2008). and Sonnenwald et al. (2018).

In previous studies, the amount of scouring of bridge piers in the conditions of pressurized flow under the deck in compound channels with vegetation has not been investigated.
The aim of this study was to investigate the effects of vegetation density, pressurized flow under the bridge deck with different geometric and hydraulic conditions on the scour depth of bridge piers in a compound channel.
Experiments of this research was performed in a laboratory channel with a width of 1.5 meters and a length of 10 meters. The experiments in this study were performed with 3 geometric ratios of cross section (=B/b), 3 relative depths (Dr) and 3 vegetation densities (). It should be noted that the experiments are designed in such a way that in all of relative depths, the bridge deck is submerged and the flow pressurized.
The maximum depth of scouring under the flow pressurized passing under the bridge can be expressed as a simple and dimensionless equation (1):
( (1
Considering the control volume from the upstream of the bridge deck to the downstream of it, the momentum equation can be written to calculate the apparent shear stress as follows:

Results and Discussion
A: Depth averaged velocity
In vegetation densities used in this study, the average velocity on floodplains with vegetation is relatively constant in most cases. This shows that except in the interface of the main channel and floodplains, the flow distribution on floodplains can be considered two-dimensional. As the vegetation density increases, the depth averaged velocity difference between the main channel and the floodplain increases between 50%-80%.
B: Shear stress
Due to the presence of vegetation, the reduction of the average flow velocity on the floodplain occurred as a result of shear stress has also decreased. The transverse changes of shear stress downstream of the bridge, due to the behavior of the pressurized flow passing in the deck, have more fluctuations and are on average about 25% more than the average values upstream of the bridge.
C: Local friction factor
The Darcy–Weisbach friction factor in the floodplain area increases significantly due to the presence of vegetation elements. The pattern of variability of Darcy–Weisbach friction factor on the floodplain also causes a sinusoidal pattern due to the reduction of flow velocity and the presence of skin friction on the surface of the rods.
D: Apparent shear stress
Due to the resistance due to increasing vegetation density, the amount of apparent shear stress at higher densities increases. On the other hand, with increasing relative depth and decreasing of secondary current, the amount of apparent shear stress decreases. As the width of the floodplain increases and the secondary currents become stronger, it shows an average of 40% apparent shear stress.
E: Equation for predicting maximum scour depth
Based on determining the effective parameters in the amount of scour rate and using the data of this study, the following equation is presented to estimate the amount of scour of the bridge pier under pressurizes flow conditions.

- Increasing the density of vegetation increases the longitudinal velocity in the main canal and decreases it in the floodplain.
-Bridge pier scouring develops faster in pressurized flow than in free surface flow.
-With the exception of the height of the dune in the pressurized flow, the depth of scour hole on a small laboratory scale is less than 50% of the depth of the upstream of the bridge deck.
-The position of the maximum scouring depth quickly reaches its equilibrium position near the downstream edge of the bridge deck.


Abed, L.M. (1991). Local scour around bridge piers in pressure flow, Ph.D. Thesis, Colorado State University.
Arneson, L.A., Zevenbergen, L.W., Lagasse, P.F., and Clopper, P.E. (2012). Evaluating Scour at Bridges, Hydraulic Engineering Circular No. 18, Publication No. FHWA-HIF-12-003, 5th ed.
Arneson, L.A. (1997). The effect of pressure‐flow on local scour in bridge openings, Ph.D. Thesis, Colorado State University.
Arneson, L.A. and Abt, S.R. (1999). Vertical Contraction Scour At Bridges With Water Flowing Under Pressure Conditions, Paper presented at the ASCE Compendium, Stream Stability and Scour at Highway Bridges, Reston, VA.
Arneson, L. and Abt, S. (1999). Vertical Contraction Scour at Bridges with Water Flowing Under Pressure Conditions, Transportation Research Report, 98, 10–17.
Cook, W., Barr, P.J., and Halling, M.W. (2015). Bridge failure rate, Journal of Performance of Constructed Facilities, 29(3), 1061/(ASCE)CF.1943-5509.0000571,04014080.
Sonnenwald, F., Stovin, V. and Guymer, I. (2019). Estimating drag coefficient for arrays of rigid cylinders representing emergent vegetation, 57(4), 591-597.
Guo, J., Kerenyi, K., Pagan-Ortiz, J.E. and Flora, K. (2009). Bridge pressure flow scour at clear water threshold condition. Trans. Tianjin Univ., 15(2), 79-94.
Hamidifar, H., Omid, M.H., Keshavarzi, A. (2013). Mean Flow and Turbulence in Compound Channels with Vegetated­ Floodplains. Journal of Agricultural Engineering Research, 14(3), 51-66.
Kang, H. and Choi, S.U. (2006). Turbulence modeling of compound open-channel flows
with and without vegetation on the floodplain using
the Reynolds stress model. Journal of Advances in Water Resources, 29, 1650–1664.
Kumcu, S.Y. (2016). Steady and Unsteady Pressure Scour under Bridges at Clear-Water Conditions, Canadian Journal of Civil Engineering: cjce-2015-0385.R2.
Mohseni, M. (2017). Velocity Distribution and Boundary Shear Stress in a Compound Channel with Emergent, Rigid Vegetation on Floodplain, 8th National Conference on Watershed and Soil and Water Resources Management.
Musleh, F.A. and Cruise, J.F. (2006). Functional relationships of resistance in wide flood plains with rigid unsubmerged vegetation. Journal of hydraulic engineering, 132(2), 163-171.
Nepf, H.M. (1999). Drag turbulence and diffusion in flow through emergent vegetation, Water Resources Research, 35(2), 479-489.
Rameshwaran, P. and Shiono, K. (2007). Quasi two-dimensional model for straight overbank flows through emergent. Journal of Hydraulic Research, 45(3), 302-315.
Rameshwaran, P. and Naden, P.S. (2003). Three-dimensional numerical simulation of compound channel flows, J. Hydraul. Eng., 129(8), 645–652.
Richardson, A. and Davis, S. R. (2001). Evaluating scour at bridges, Retrieved from Hydraulic Engineering Circular No. 18, Publication No. FHWA NHI 01-001, 4th ed.
Richardson, E.V., Simons, D.B. and Lagasse, P.F. (2001) River Engineering for Highway Encroachments - Highways in the River Environment, FHWA NHI 01-004, Federal Highway Administration, Hydraulic Series No. 6, Washington, D.C.
Samadi Rahim, A., Yonesi, H.A., Shahinejad, B. and Torabipoudeh, H. (2021), Experimental Investigation of Floodplain Vegetation Density Effect on Flow Hydraulic in Divergent Compound Channels, Journal of Hydraulics, 16(1), 111-130.
Shiono, K. and Knight, D.W. (1991). Turbulent open-channel flows with variable depth across the channel, Journal of Fluid Mech., 222, 617-646
Tang, X. and Knight, D. W. (2009). Lateral Distributions of Streamwise Velocity in Compound Channels with Partially Vegetated Floodplains, Journal of Science in China Series E: Technological Sciences, 52, 3357-3362.
Tanino, Y. and Nepf, H.M. (2008). Laboratory investigation of mean drag in a random array of rigid, emergent cylinders, Journal of Hydraulic Engineering, 134(1), 34–41.
Umbrell, E.R., Young, G.K., Stein, S.M. and Jones, J.S. (1998). Clear-Water Contraction Score Under Bridges in Pressure Flow, Journal of Hydraulic Engineering, 124(2), 236–240.
Shan, Y.Q., Liu, C., Luo, M.-K. and Yang, K.-J. (2016). A simple method for estimating bed shear stress in smooth and vegetated compound channels, Journal of Hydrodynamics, 28(3), 497-505.
Zarrati, A.R, Jin, Y.C. and Karimpour, S. (2008). Semianalytical Model for Shear Stress Distribution in Simple and Compound Open Channels, Journal of Hydraulic Engineering, 134(2), 205-215.
Zhai, Y. (2010). Time-dependent scour depth under bridge-submerged flow, Thesis presented in partial fulfillment of requirements for MS degree, the Graduate Collage at the University of Nebraska.
  • Receive Date: 10 September 2021
  • Revise Date: 16 September 2021
  • Accept Date: 19 September 2021
  • First Publish Date: 19 September 2021