Drainage Pipes Research Articles

Experimental and numerical investigation on the mechanism of ground collapse induced by underground drainage pipe leakage

Experimental and numerical investigation on the mechanism of ground collapse induced by underground drainage pipe leakage

Determination of Winter Irrigation Quotas for Corn and Oil Sunflower Considering Crop Salt Tolerance Threshold under Subsurface Pipe Drainage Technology

Subsurface pipe drainage (SPD) is an important technique for the improvement of saline–alkali lands in China. Winter irrigation after crop harvest is a key measure used in the Yellow River irrigation area in northwest China to reduce soil salinity in the root zone of crops. To optimize winter irrigation under SPD, the calibrated HYDRUS-2D model was utilized in this study to investigate the effects of soil texture (clay loam, silt loam, loam, and sandy loam), initial soil salinity (1, 3, 5 g/kg), and the winter irrigation quotas (80, 100, 120, 150, 180 mm) on the rate of soil desalination. In this study, soil salinity levels during the stable production of common crops such as sunflower and corn in the Yinbei Irrigation District in Ningxia, China, were taken as the thresholds, efficient utilization of irrigation water was considered, and suitable crops and appropriate winter irrigation quotas for different soil textures and levels of soil salinity were proposed. Soil with a salt content of 1~3 g/kg is suitable for the planting of corn with 80 mm of irrigation water. Sandy loam soil with a salt content of 3~5 g/kg is suitable for sunflower–corn intercropping with 120 mm of irrigation water. Sandy loam soil with a salt content exceeding 5 g/kg is suitable for the planting of sunflower with 80 mm of irrigation water. Other types of soils need to be improved by reducing the spacing between subsurface pipes, using desulfurized gypsum, biochar, and other additives. People engaged in agriculture can utilize this research to determine the appropriate volumes of irrigation water, crop types, planting systems, and subsurface pipe parameters based on local conditions.

DETERMINATION OF EFFECTIVE STRUCTURAL CHARACTERISTICS OF COLLECTIVE DRAINAGE PIPELINES FOR MELIORATIVE SYSTEMS

Methodology for calculation the effective structural characteristics of pressure collecting drainage pipelines is proposed in the paper. It is based on the analysis of the differential equations describing the movement of liquid with variableflow rate in such pipes. It is assumed that the flow rate variation along the length of the collector is taking place continuously. The studied system of differential equations consists of the variable mass hydraulics equation and the modified filtration equation., the original system is reduced to a dimensionless form by introducing new original variables.The effective characteristics of the drainage pipeline are meant as the parameters at which groundwater enters the pipe along its entire length with an intensity no less than specified by technological requirements.It is shown that the value of the effective parameters mainly depends on the value of three important factors: the resistance coefficient of the collectivedrainage pipeline «ζl»; distances between drainage pipes «E»; the generalized parameter «A», which comprehensively takes into account the structuraland filtering characteristics of the considered stream. The concept of a fictitious drainage pipeline of infinite length or a pipeline with an infinite filtering capacity of its side surface was used in the analysis. At the same time, the necessary pressure difference between the groundwater level and the piezometric pressure inside the real and fictitious pipelines is ensured. The resulting calculation formulasare quite simple and convenient to use.The example of calculating the effective characteristics of a real pressure collecting drainage pipeline is given. The presented calculation method will certainly be useful in the actual design of prefabricated drainage pipes of reclamation systems, since its use will allow to rationally calculate their parameters and, due to this, significantly reduce the cost of construction and ensure optimal conditions for the operation of reclamation systems.

Surface Subsidence over a Coastal City Using SBAS-InSAR with Sentinel-1A Data: A Case of Nansha District, China

The loss of life and property in economically developed areas due to geological hazards caused by intense ground subsidence is incalculable. As one of the fastest growing areas in the Guangdong-Hong Kong-Macao Greater Bay Area, the study of ground subsidence in Nansha will help to provide a scientific basis for urban planning and improve the capacity of monitoring and prevention of ground subsidence. The combination of coastal soft soil foundation and urbanization conditions creates a certain risk of land subsidence. We chose Nansha District, the geographical center of the Greater Bay Area, as the study area to analyze its surface subsidence characteristics in recent years. The 20-view Sentinel-1A data and SBAS-InSAR technique were used to monitor the ground subsidence in Nansha from 2017 to 2023. The main rate of ground subsidence in Nansha ranges from −19.4 to 7.7 mm/yr and is distributed in the urban area, along the rivers, in the construction area, and in the reclamation area. As of 4 May 2023, the average ground settlement in Nansha is 10.05 mm and the maximum settlement can be up to 142.45 mm. The 6-year total settlement at all four settlement intensities is greater than 60 mm, with the highest value exceeding 110 mm. The cumulative settlement increases with time, but inverse settlement and no settlement also occur at points where settlement is severe. For settlement caused by soft soil consolidation, it is recommended that drainage pipes be installed to accelerate drainage as a means of stabilizing settlement. For settlement caused by groundwater extraction and additional loads on the road surface, it is recommended to rationally extract groundwater and reinforce the foundation of the road surface with severe settlement.

Determination of the particularities of the hydraulic friction factor variation of collecting drainage pipelines

The object of the study in this paper is collecting perforated drainage pipelines, in particular, the determination of the hydraulic friction factor λdr, which significantly affects the parameters of the investigated pipes. Clarifying this issue will solve an important engineering problem – to develop a reliable method for hydraulic calculation of collecting perforated drainage pipes. As the main drawback of most of the available theoretical and experimental works on this topic is the insufficient consideration of the filtration characteristics of the surrounding soil and the material of the pipeline walls. Experimental studies of pressure drainage pipes with different design characteristics made it possible to find out the real picture of fluid flow in the pipe. Based on the research results, experimental dependences of λdr on the ratio of the fluid inflow velocity to the average flow velocity in the corresponding section (Uh/V), as well as on the design characteristics of the channel were obtained. The maximum possible flow that can be collected and passed by a collecting drainage pipeline with the specified design and filtration characteristics is determined. It is shown that a larger value of the specified ratio corresponds to larger values of λdr. This result can be explained by the effect of the attached flow on the main flow. Obviously, some energy is spent on the interaction of the attached and the main flow in the pipe, which leads to additional head losses. In the paper, dependencies were obtained for calculating the studied factor for collecting drainage pipelines. Using them in the calculation of drainage pipes will increase the reliability and efficiency of land reclamation systems, in which these pipelines are important structural elements

Simulating Water and Salt Migration through Soils with a Clay Layer and Subsurface Pipe Drainage System at Different Depths Using the DRAINMOD-S Model

Soil salinization affects more than 25% of land globally. Subsurface pipe drainage is known for its effectiveness in improving saline–alkali land. The red clay layer (RCL) hinders soil improvement in the Hetao Irrigation District of Inner Mongolia, China. The soil water and salt migration rules at different buried depths and RCL were studied based on the field subsurface pipe drainage test and simulation using the DRAINMOD-S model (Version 6.1). The following implications can be drawn from the results: (1) Although the RCL affected the accuracy of the model, the calibrated statistical results met the application requirements, and the DRAINMOD-S model can be used to analyze subsurface pipe drainage under different distribution conditions of the RCL. (2) The RCL can reduce the drainage efficiency of the subsurface pipe, specifically when the distribution is shallow. (3) The soil desalting rate increased with an increase in the buried depth of the subsurface pipe. The desalination effect of shallow soil was better than that of deep soil. The RCL reduced the drainage and salt removal efficiency of the subsurface pipe. Burying the subsurface pipe as far above the RCL as possible should be considered. Thus, it is feasible to apply the DRAINMOD-S model to relevant studies.

Subaqueous soil sequence in marine waters of the Strunjan Bay

Soil research focuses mainly on soils on land, but the process of soil formation also takes place in shallow waters. Subaqueous soils form below the water surface, mainly on the bottom of shallow, stagnant waters such as bogs, swamps, lakes, and shoals in the sea. Three subaquatic soil profiles were investigated in the Strunjan Bay to study them in detail. The sites for the soil investigation were selected after observing the movement of seawater and thus its possible influence on subaquatic soils. All the soils studied are permanently and year-round under water. Therefore, all soil samples were collected under water using drainage pipes and by excavation. The samples were slowly drained in the pipes to prevent mixing of the material and to keep the horizons as intact as possible. Visual observations and standard analyses were carried out in the laboratory according to the identified horizons. During the investigation, we found that apart from water properties and movement, most soil-forming factors can be considered constant, so we investigated and presented a soil hydrosequence. According to the WRB classification, the subaqueous soils in the waters of the Strunjan Bay were divided into the reference groups Gleysols and Leptosols.

Water infiltration in drained circular tunnels: an analytical solution and its simplification

Underwater tunnels are often subject to groundwater infiltration, which can pose risks both to the tunnel's safety and the groundwater environment. Analytical solutions are commonly used to estimate the rate of water infiltration in tunnels. However, there is a lack of research on the analytical solution of tunnels with drainage systems. The aim of this study was to investigate groundwater seepage in tunnels with circumferentially arranged drainage pipes. The analytical formula for estimating the inflow rate takes into account the seepage processes in the ground, primary lining and geotextile. The accuracy of the analytical solution is confirmed by comparing it with numerical results. The analytical solution's general form is simplified for tunnels with varying permeabilities of the surrounding strata and geotextiles, and a query chart is created to facilitate engineering applications. The research findings can serve as a theoretical basis and reference for the establishment of relevant specifications and the design of tunnel drainage systems.

The effect of underground drainage on peat meadows and inactivation of the drainage in an attempt to restore these meadows, which failed as it reduced the ability of soils to retain water

Drainage is often used to increase agriculture production, but it has adverse effects on biodiversity and water retention. Here, the effect of subsurface pipe drainage on peat meadows near Senotín (Czechia), which were drained from the mid-1980s to 1990s, was studied. Attempts were made to restore the peat meadows by damming drainage pipes using clay-filled trenches in 1996. In this case study, the effect on the depth of the water table, soil water retention, infiltration and soil temperature were recorded. Measurements of the original peat meadow (undrained site), drained meadow (drained site) and restored meadow (restored site) before restoration and two decades after restoration were recorded. The water table in undrained areas was higher than at drained and restored sites, indicating that drainage had lasting effect on drained and restored sites. Infiltration was lowest at the undrained site, greater at the drained site, and highest at the restored sites. Field water capacity was lowest at the restored site, greater at the drained site and highest at the undrained site. Soil water content at maximum saturation was lowest at the restored site, greater at the drained site and highest at the undrained site. Soil temperature was highest at the restored site with no significant difference between the undrained and drained sites. Soil moisture levels were highest at the undrained site and lowest at the drained site. In addition, the undrained and restored sites did not differ significantly in soil moisture content. In conclusion, restoration did not have a significant effect on the level of the water table, initiation of peat formation or ability of soil to hold water.

Investigation of individual problems of calculation of drainage pipes in complex geotechnical conditions

The article notes that changes in the loads acting on the drainage pipe (pipeline) in accordance with the relative ability of the drainage pipe and the ground filling to deform are essential. To determine the magnitude of the loads on the drainage pipeline laid in the trench, the formula proposed by A. Marston most accurately corresponds to the results of experiments and is convenient for engineering practice. The perforation of the walls of the drainage pipe affects their strength not only when the pipes are crushed, but also for the conditions of transportation and installation of pipes. The contact interaction of the cross-section of the drainage pipe with the soil base is investigated, in this case, the interaction of cylindrical (having a semicircular contact) or reloid (having a contact of the circle sector) shells lying on an elastic soil base is considered. A differential equation is considered to describe the deformation of an elastic shell, which includes the stiffness parameter, linear differential operators, components of displacement of the neutral axis of the drainage pipe, external load and dimensionless coordinates. For the elastic base models of E.Winkler and V.Z.Vlasov, a general solution of the differential equation describing the deformation of the drainage pipe is proposed. The solution of this equation and the external load is presented in the form of double or single trigonometric series, depending on the task (two-dimensional or one-dimensional). A horizontally lying cylindrical (or reloid) shell (drainage pipe) supported on a ground base is considered. The contact pressure is determined by summing the Fourier coefficients. This solution additionally takes into account the distribution capacity of the ground base of the drainage pipe.

Mechanical and piezoresistive properties of multi-walled carbon nanotubes reinforced epoxy matrix composites for pipeline monitoring

Pipeline deformation monitoring is of vital importance for pipeline maintenance. However, no durable and effective methods have been seen for deformation monitoring of buried pipes, especially drainage pipes. In this paper, an embedded multi-walled carbon nanotubes (MWCNTs) reinforced epoxy (EP) matrix composite for pipeline monitoring was developed. The bending properties, piezoresistive properties and stability of MWCNTs/EP composites were investigated using three-point bending tests, cyclic tests, resistivity tests, and scanning electron microscope (SEM) observations. The results indicated that when the MWCNTs content was at 0.1 wt%, the bending strength, bending modulus and bending strain increased by 12.3 %, 8.7 % and 25.1 % respectively. The conductive percolation threshold was at 0.7 wt%, and the resistance changed in three stages under bending. In addition, the MWCNTs/EP composites had stable piezoresistive properties under different loading rates and small strain cyclic (ε < 1.5 %). The MWCNTs content and dispersion effect were the main factors affecting the bending properties of MWCNTs/EP composites, while the resistance change mainly originated from the change of the MWCNTs conductive network. This work provided a new idea for pipeline deformation monitoring.

Prospects in the application of a frontally curable epoxy resin for cured‐in‐place‐pipe rehabilitation

AbstractPhotocurable acrylates and vinyl esters are among the most commonly used resins in cured‐in‐place‐pipe (CIPP) rehabilitation technology, as they impart excellent thermomechanical properties in composite pipes. In the quest for achieving a higher energy efficiency of the photo‐curing process in CIPP, the frontal polymerization technique is a viable alternative that requires a lower irradiation dosage coupled with exceptionally high curing speeds and depths. Herein, for the first time, we report the application of frontal polymerization in the rehabilitation of underground pipes using a newly developed frontally curable epoxy‐based resin (Trelleborg Self‐Curing*). The neat resin is characterized for degree of cure, glass transition temperature, and mechanical properties via FTIR, DMA, and tensile tests, respectively. In a comprehensive way, the properties are benchmarked against commercially available acrylate (Trelleborg Light Cure*) and vinyl ester (Trelleborg Rapid Cure*) resins to evaluate their applicability for CIPP. The results show a higher glass transition temperature and final monomer conversion for the frontally cured resin, which cures significantly faster than the reference resins under the same irradiation conditions. In proof‐of‐concept trials, the newly developed resin successfully cures polymeric liners in a PVC host pipe with 100% water tightness and without losing its structural integrity. Results from ring stiffness tests for cured composite pipes additionally show that liners cured with Trelleborg Self‐Curing* resin pass the minimum required Young's modulus for non‐pressure drainage pipes as per ASTM F1216. Thus, frontally curable epoxy‐based resins are a promising and competitive alternative to acrylates and vinyl esters in CIPP.

Mechanical response of concrete pipe rehabilitated by liner under external load: Analytical solution

AbstractUsing pipe lines to rehabilitate concrete drainage pipes is a popular trenchless technique. In this paper, the concrete pipe and liner are equivalent to a symmetrical structural mechanics problem, and the mechanical solution of the composite structure is obtained using the force method. Subsequently, the analytical solutions are compared with the test and finite element results. The distribution law of axial force, bending moment, and shear force of the composite structure are obtained, and the circumferential strain law of the inner and outer surfaces of the host pipe is analyzed. Lastly, the influence of pipe diameter and liner thickness on the bearing capacity of the host pipe is examined. The results indicate that the axial and shear force distribution of the composite structure remains the same for different pipe diameters, as the distribution diagrams of shear and axial force along the circumferential direction coincide. While the elastic modulus and thickness of the liner have no effect on the internal force distribution of the composite structure, they do directly affect the stress state of the host pipe. Additionally, the bottom support causes abrupt changes in the bending moments and strains of the host pipe.

A laboratory ice machine as a cold oligotrophic artificial microbial niche for biodiscovery

Microorganisms are ubiquitously distributed in nature and usually appear as biofilms attached to a variety of surfaces. Here, we report the development of a thick biofilm in the drain pipe of several standard laboratory ice machines, and we describe and characterise, through culture-dependent and -independent techniques, the composition of this oligotrophic microbial community. By using culturomics, 25 different microbial strains were isolated and taxonomically identified. The 16S rRNA high-throughput sequencing analysis revealed that Bacteroidota and Proteobacteria were the most abundant bacterial phyla in the sample, followed by Acidobacteriota and Planctomycetota, while ITS high-throughput sequencing uncovered the fungal community was clearly dominated by the presence of a yet-unidentified genus from the Didymellaceae family. Alpha and beta diversity comparisons of the ice machine microbial community against that of other similar cold oligotrophic and/or artificial environments revealed a low similarity between samples, highlighting the ice machine could be considered a cold and oligotrophic niche with a unique selective pressure for colonisation of particular microorganisms. The recovery and analysis of high-quality metagenome-assembled genomes (MAGs) yielded a strikingly high rate of new species. The functional profiling of the metagenome sequences uncovered the presence of proteins involved in extracellular polymeric substance (EPS) and fimbriae biosynthesis and also allowed us to detect the key proteins involved in the cold adaptation mechanisms and oligotrophic metabolic pathways. The metabolic functions in the recovered MAGs confirmed that all MAGs have the genes involved in psychrophilic protein biosynthesis. In addition, the highest number of genes for EPS biosynthesis was presented in MAGs associated with the genus Sphingomonas, which was also recovered by culture-based method. Further, the MAGs with the highest potential gene number for oligotrophic protein production were closely affiliated with the genera Chryseoglobus and Mycobacterium. Our results reveal the surprising potential of a cold oligotrophic microecosystem within a machine as a source of new microbial taxa and provide the scientific community with clues about which microorganisms are able to colonise this ecological niche and what physiological mechanisms they develop. These results pave the way to understand how and why certain microorganisms can colonise similar anthropogenic environments.

SILKWORM COCOONS WASTE AS AN INNOVATIVE ADSORBENT FOR RAINWATER PURIFICATION

Heavy metal pollution of water bodies is a serious environmental problem, especially in developingcountries. The search for environmentally friendly and efficient adsorbents for water purificationhas been the subject of extensive research. The research examined the possibility of using cheapand renewable silkworm cocoon waste as potential adsorbents for rainwater pollution. The wastecocoons of the mulberry silkworm (Bombyx mori L.) came from the Institute of Natural Fibres andMedicinal Plants - National Research Institute (IWNiRZ-PIB) in Poznań. Rainwater was collected froma galvanised drain pipe that drains water from a felt roof, located in the city of Tychy. It was shownthat rainwater contained many micropollutants including alkanes, esters and carboxylic acids. Theadsorbent was characterised by high adsorption capacity of the micropollutants under study. Therewas a 70% removal of neadecanoic acids and a 90% removal of esters. The silkworm cocoon was alsofound to have considerable efficiency in the removal of zinc from rainwater at a level of 61–90% (dosefrom 2g/L to 10g/L). The lowest dose allows zinc removal below the value standardised according to theregulation. The adsorption time needed to establish equilibrium was short and amounted to 20 min.

Evaluation of the Use of Permeable Interlocking Concrete Pavement in Chile: Urban Infrastructure Solution for Adaptation and Mitigation against Climate Change

There is currently a context of climate change due to the way modern cities are developed, and they are made up mainly of impermeable surfaces and concrete buildings that change the hydrological cycle, causing (i) an increase in temperatures, (ii) the accumulation of stormwater on different surfaces, (iii) overflow in drainage systems, and (iv) the alteration of ventilation patterns, among others. This article presents a case study on the implementation of a permeable interlocking concrete paving (PICP) system, and it develops physical–mathematical modeling using software for the design of a parking lot that currently does not have adequate paving and urban drainage, resulting in sporadic flooding due to heavy rainfall in the city of Temuco, La Araucanía region, Chile. This article’s contribution highlights the application of new technology in Chile, discussing road infrastructure solutions based on sustainable urban drainage systems (SUDSs), which seek to implement feasible alternatives in urban sectors to improve human livelihood. The factors studied include structural and hydrological properties, along with the infiltration analysis of the system according to historical rainfall records in the area. This research concludes that the permeable pavement system with a drainage pipe and smooth roughness coefficient performs satisfactorily for an extreme hydrometeorological event corresponding to 140 mm considering 24 h of rainfall with a return period of 100 years equivalent to an inflow of 673 m3/day. Finally, the results indicate that, at least in the conditions of the city of Temuco, the use of permeable interlocking concrete pavement (PICP) proves to be a sustainable and feasible alternative to implementing measures of adaptation and mitigation against climate change, reducing the city’s flooding zones and allowing the irrigation of urban green areas.

Analysis of GPR Data Characteristics of Drainage Pipe and Cavity Coexisting

Drainage pipe leakage usually causes roadbed loosening and even collapse. Therefore it is of great significance to accurately locate and repair the leakage area. This paper studies the use of GPR to carry out the detection of drainage pipe leakage area, aiming to solve the problem of unclear electromagnetic wave field response characteristics when drainage pipes and cavities coexist. First, we used gprMax to carry out numerical simulation experiments to obtain the electromagnetic wave field response characteristics when a square cavity exists around a circular drainage pipe. Subsequently, similar data characteristics to the numerical simulation were obtained through field detection, and the area was confirmed to be a cavity caused by drainage pipe leakage through drilling. This study shows that the use of numerical simulations can provide a basis for the identification of cavities caused by drainage pipe leakage in GPR data.

Tracking the source of antibiotic resistome in the stormwater network drainage in the presence of sewage illicit connections

Stormwater pipes are illicitly connected with sewage in many countries, which means that sewage enters stormwater pipes and the drainage is discharged to surface water without any treatment. Sewage contains more pathogens and highly risky antibiotic resistance genes (ARGs) than surface runoff. Therefore, sewage may alter the microbial and ARG compositions in stormwater pipe drainage, which in turn leads to an increased risk of resistance in surface water. However, the effects of sewage on ARGs in the drainage of stormwater networks have not been systematically studied. This study characterized the microbial and ARG composition of several environmental compartments of a typical stormwater network and quantified their contributions to those in the drainage. This network transported ARGs and microorganisms from sewage, sediments in stormwater pipes, and surface runoff into the drainage and thus into the river. According to metagenomic analysis, multidrug resistance genes were most abundant in all samples and the numbers and relative abundance of ARGs in the drainage collected during wet weather were comparable to that of sewage. The results of SourceTracker showed that the relative contribution of sewage was double that of rainwater and surface runoff in the drainage during wet weather for both microorganisms and ARGs. Desulfovibrio, Azoarcus, and Sulfuritalea were connected with the greatest number of ARGs and were most abundant in the sediments of stormwater pipes. Furthermore, stochastic processes were found to dominate ARG and microbial assembly, as the effects of high hydrodynamic intensity outweighed the effects of environmental filtration and species interactions. The findings of this study can increase our understanding of ARGs in stormwater pipe drainage, a crucial medium linking ARGs in sewage to environmental ARGs.

Soil Water and Salt Transport in Severe Saline–Alkali Soil after Ditching under Subsurface Pipe Drainage Conditions

The subsurface pipe drainage project is essential in farmland drainage operations and is globally recognized as an effective saline–alkali land improvement measure owing to its efficient drainage capacity and low land occupation rate. This study aimed to establish enhanced methods for improving saline–alkali land by combining ditching with subsurface pipe drainage. The ditching was conducted at a depth of 60 cm based on the existing subsurface pipe arrangement. The calibrated DRAINMOD-S model was employed to simulate the test area with different ditching depths and subsurface pipe arrangement parameters. Furthermore, the law of soil water and salt transport in the subsurface pipe drainage system at different ditching depths was investigated. After ditching, the total unit drainage volume of leaching increased by an average of 14.65% over two years and the water storage of different soil layers in the different plots decreased by 1.37–1.48 mm on average. Ditching demonstrated a superior salt-leaching effect in areas with subsurface pipe layouts. The soil desalination rate of different soil layers increased by 6.40–13.40% on average, with a more significant impact on the surface soil desalination rate. The effect of the increased desalination rate was more apparent as the ditching depth increased. However, as the buried depth of the subsurface pipe increased, the relationship between the ditching depth and soil desalination rate became insignificant. Ditching improved the salt-leaching effect of subsurface pipe drainage projects, which can effectively reduce the cost of subsurface pipe burial, consequently promoting subsurface pipe use.

Flow structure of the confluence between an open channel and a pipe.

It is common that the sewage is discharged into rivers through a municipal drainage network. The hydraulic properties of the confluence between a lateral drainage pipe and a rectangular open channel are investigated using numerical simulation. The simulation results show that the recirculation zone, the separation area, and the secondary flow are highly influenced by the tailgate height and flow rate ratios. Both the recirculation zone and the separation zone enlarge with the increasing flow rate ratios. The recirculation zone also enlarges with the tailgate height within a certain range. It is worth noting that as the tailgate height increases, two vortices rotating in opposite directions at the same level initially merge into a counter-clockwise rotating vortex, and then one vortex splits into three. Ultimately, one of them dissipates and there are two vortices left on a vertical line. The tailgate can reduce the vertical velocity gradient and the streamwise velocity to moderate the erosion of the channel bed caused by the pipe flow inlet.