Experimental and numerical investigation of blockage effects on flows in a culvert

Introuduction
Culverts are common structures for runoff drainage system in the design and construction of roads and railways, in both urban and rural areas. Due to the nature of runoff flow, large amount of sediments, foliage, urban waste and debris materials may accumulate in the entrance of culverts, particularly in flood events.
Blockage in the culvert’s entrance can result in a significant increase in flood risk, through elevated flood levels and diverted flow paths through the urban or rural areas (Rigby et al., 2002). Sudden blockage in a runoff system is also one of the common problems. The study of culvert’s blockage would be useful in the prediction and prevention of flood hazard in the vicinity of drainage systems. Current study deals with this problem in box culverts. Blockage effects on the upstream water level were investigated using both experimental and numerical modeling. The FLOW-3D model was chosen, because the sufficiency of this model for such flow conditions was already reported by several studies such as Abad et al. (2008), Ayaseh (2010), Brethour and Brunham (2009) and Gunal et al. (2019).

Methodology
Experimental tests were conducted in Hydraulic Laboratory of Water Engineering Department in Bu Ali Sina University, Hamedan, Iran. The box culvert models made of glass and smooth water pipes used as circular culvert models. The experimental setup includes a glass wall flume with 10m length, 0.5m width and 0.6m deep. Rectangular plates in different sizes were used in order to make sudden blockage into the culverts. An extensive experiment tests was conducted under different flow condition and blockage scenarios, and 21 experimental data sets were provided.
The FLOW-3D model, Version 11.3, was performed on the main server of Water Institute at the University of Tehran, and adapted to the experimental data sets from this study. The stability and sensitivity of this model have been tested according to: mesh cell size, simulation time step, turbulent model, and culvert hydraulic characteristics such as wall roughness. The simulation convergence was achived with an efficient simulation time step of 80 seconds. Three different mesh blocks were used for pre-simulation cases, and a block in block with 1.0 cm and 0.5 cm mesh cell sizes were chosen as the best meshing senario. The RNG was found to be an apropriate turbulence model. The slope of culvert barrel was changed from horizontal to 0.005 in the flow direction, and the roughness coefficient modified from 0.00085 m to 0.001 m in the culvert barrel. The relative error of simulated water levels and discharge for calibrated model were to be in the order of acceptance ranges, and the simulation FLOW-3D model was adjusted as an efficient and reliable tool.
The FLOW-3D model was then calibrated and verified using the experimental data sets, and was used to simulate different flow conditions into the culverts, under different entrance-blockage scenarios.

Results and discussion
Effect of the inlet blockage on upstream water level was tested for three flow rates (the design discharge of 27.5 lit/s, and two lower discharges of 10.5 lit/s and 16.5 lit/s), in four different sizes of inlet blockage (B). Simulation results showed a good agreement in upstream backwater level in all cases. In the case of flow with 16.5 lit/s, upstream water level raised from 28.5 cm in non-blocked inlet to 31.4, 34.2 and 38.5 cm in presence of 20% ,40% and 60% blocked inlet area, respectively. The rate of the upstream water level increase (DHu) against the reduced inlet area (1-B) represents a higher rate for discharges smaller than the culvert-design discharge. The evaluated equations for upstream water level enhancement were :
DH_u=-0.48(1-B)+45.089 (1)
DH_u=-0.82(1-B)+75.663 (2)
in which, Eq. (1) is for design discharge and Eq. (2) for the smaller discharges. Blokage has been affected flow in the barrel and in the downstream of the culvert. Investigation of turbulent characteristics and shear velocity values in both the barrel and downstrem indicated the impact of blocked inlet. Turbulent enrgy of flow in the 60% blocked-inlet area was 5 times greater than that of non-blocked inlet for the design discharge. Also shear velocity in the same blockage situation increased by 2 times in downstream which results in a greater scouring power of the flow downstream. Sorourian et al. (2015) rep[orted this phenomenon with even higher scour downstream of bocked cualverts. Maximum value of shear velocity increased with the increase level of blockage in the all flow condition, however in the design discharge it seems to be constant for blockages greather than 40%.

Canclusion
The FLOW-3D model was calibrated and validated to simulate the flow into the culverts. Influence of inlet obstruction on the upstream water level and flow characteristics into the barrel and downstream of the culvert was investigated. The results show a linear increase in the upstream water level by decreasing the percentage of culvert inlet. The upstream water level for the design discharge was lower than the other tested discharges. Changes in turbulent flow properties and shear velocity inside the barrel and downstream were also investigated in the presence of obstruction. Shear velocity increased 3 times in the presence of 80% blockage for 10.5 lit/s. and for the design discharge (27.5 lit/s) with 60% inlet blockage increased 2 times. The turbulence energy for the design discharge has also increased by about 5 times. The present results confirm the previous studies on the effect of the culvert inlet obstruction on the geometry of the scour hole downstream of culverts.