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Numerical Analysis of Flow Characteristics for Idealised Y-Shaped Channels
Abstract
Channel confluence and bifurcation flows are common phenomena in a natural river. Understanding the characteristics of these dividing flows can help engineers to make decisions on flood risk reductions. In this study, a numerical model has been applied for idealised Y-shaped channels in both cross-sections of rectangular and trapezoidal. InfoWorks ICM software is used to run the numerical analysis on the proposed model. The model included the main channel upstream with a bifurcated angle of 45° at each downstream channel. A series of simulations have been carried out to investigate the effects of various flow characteristics on the water depths and flow velocities. These characteristics are manning’s roughness coefficient, the width of the channel, and the shape of the channel cross-section. Results show that the increase in manning’s roughness coefficients caused a decrease in flow velocities and increased the water depths for both rectangular and trapezoidal cross-sections. Furthermore, increases in channel widths have caused decreases in flow velocities and water depths for both rectangular and trapezoidal cross-sections. In addition, the rectangular cross-section showed lower flow velocities and water depths compared to the trapezoidal cross-section. The findings from this study may provide a good understanding of the flow characteristics in a bifurcation river to mitigate floods.
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Conservative peak flood discharge estimation methods such as the rational method do not take into account the soil infiltration of the precipitation, thus leading to inaccurate estimations of peak discharges during storm events. The accuracy of estimated peak flood discharge is crucial in designing a drainage system that has the capacity to channel runoffs during a storm event, especially cloudbursts and in the analysis of flood prevention and mitigation. The aim of this study was to model the peak flood discharges of each sub-watershed in Selangor using a geographic information system (GIS). The geospatial modelling integrated the watershed terrain model, the developed Soil Conservation Service Curve Cumber (SCS-CN) and precipitation to develop an equation for estimation of peak flood discharge. Hydrological Engineering Center-Hydrological Modeling System (HEC-HMS) was used again to simulate the rainfall-runoff based on the Clark-unit hydrograph to validate the modelled estimation of peak flood discharge. The estimated peak flood discharge showed a coefficient of determination, r2 of 0.9445, when compared with the runoff simulation of the Clark-unit hydrograph. Both the results of the geospatial modelling and the developed equation suggest that the peak flood discharge of a sub-watershed during a storm event has a positive relationship with the watershed area, precipitation and Curve Number (CN), which takes into account the soil bulk density and land-use of the studied area, Selangor in Malaysia. The findings of the study present a comparable and holistic approach to the estimation of peak flood discharge in a watershed which can be in the absence of a hydrodynamic simulation model.
This study aims to compare the flow characteristics in rectangular and trapezoidal open channel by investigating the effects of Manning coefficient, channel bottom slope, channel width, and channel side slope which represented by modified Saint-Venant equation according to the channel cross section, which are rectangular and trapezoidal, and the slope of channel friction using the Manning coefficient. The equation is solved numerically by using finite difference methods with Forward Time Backward Space approach. The model is simulated by using python-based desktop application to show the flow characteristics. Computational results are displayed in the form of summaries, tables, two-dimensional graphics, and three-dimensional graphics. Validation is done by comparing computational results data with data from the other studies that have been done before and concluded that the simulation results have been valid and indicate compliance with the prior studies. The simulation results show that trapezoidal channel is better at minimizing the risk of flooding than rectangular channel in consideration of smaller discharge, smaller velocity, and lower water level.
We derive an idealized model of a gravel-sand river bifurcation and analyze its stability properties. The model requires nodal point relations that describe the ratio of the supply of gravel and sand to the two downstream branches. The model predicts changes in bed elevation and bed surface gravel content in the two bifurcates under conditions of a constant water discharge, sediment supply, base level, and channel width and under the assumption of a branch-averaged approach of the bifurcates. The stability analysis reveals more complex behavior than for unisize sediment: three to five equilibrium solutions exist rather than three. In addition, we find that under specific parameter settings the initial conditions in the bifurcates determine to which of the equilibrium states the system evolves. Our approach has limited predictive value for real bifurcations due to neglecting several effects (e.g., transverse bed slope, alternate bars, upstream flow asymmetry, and bend sorting), yet it provides a first step in addressing mixed-size sediment mechanisms in modelling the dynamics of river bifurcations.
Rivers have been trained for centuries by channel narrowing and straightening. This caused important damages to their ecosystems, particularly around the bank areas. We analyse here the possibility to train rivers in a new way by subdividing their channel in main and ecological channel with a longitudinal training wall. The effectiveness of longitudinal training walls in achieving this goal and their long-term effects on the river morphology have not been thoroughly investigated yet. In particular, studies that assess the stability of the two parallel channels separated by the training wall are still lacking. This work studies the long-term morphological developments of river channels subdivided by a longitudinal training wall in the presence of steady alternate bars. This type of bars, common in alluvial rivers, alters the flow field and the sediment transport direction and might affect the stability of the bifurcating system. The work comprises both laboratory experiments and numerical simulations (Delft3D). The results show that a system of parallel channels divided by a longitudinal training wall has the tendency to become unstable. An important factor is found to be the location of the upstream termination of the longitudinal wall with respect to a neighboring steady bar. The relative widths of the two parallel channels separated by the wall and variable discharge do not substantially change the final evolution of the system.
Side channel construction is a common intervention applied to increase the river's conveyance capacity and to increase its ecological value. Past modelling efforts suggest two mechanisms affecting the morphodynamic change of a side channel: 1) a difference in channel slope between the side channel and the main channel and 2) bend flow just upstream of the bifurcation. The objective of this paper is to assess the conditions under which side channels generally aggrade or degrade and to assess the characteristic time scales of the associated morphological change. We use a one-dimensional bifurcation model to predict the development of side channel systems and the characteristic time scale for a wide range of conditions. We then compare these results to multitemporal aerial images of four side channel systems. We consider the following mechanisms at the bifurcation to be important for side channel development: sediment diversion due to the bifurcation angle, sediment diversion due to the transverse bed slope, partitioning of suspended load, mixed sediment processes such as sorting at the bifurcation, bank erosion, deposition due to vegetation, and floodplain sedimentation. There are limitations to using a one-dimensional numerical model asit can only account for these mechanisms in a parameterized manner, but the model reproduces general behaviour of the natural side channels until floodplain forming processes become important. The main result is a set of stability diagrams with key model parameters that can be used to assess the development of a side channel system and the associated time scale, which will aid in the future design and maintenance of side channel systems.
Most urban rivers carry much natural debris, such as vehicles and trees, during extreme flood events, and these large debris, particularly vehicles, can block a local hydraulic structure such as a bridge. Such blockages usually increase the upstream water levels and cause more water to be diverted into adjacent urban areas. A scaled physical river model was constructed in a laboratory flume, consisting of a sketched urban reach with a compound cross-sectional geometry, and experiments were conducted on three model bridges, blocked by vehicles, with the corresponding hydrodynamic impacts of these vehicles blocking bridges being investigated. The main findings obtained from the investigation show that: (i) the upstream water depths in the urban river increased significantly due to the vehicle blockage, for the single opening arch bridge and the straight deck bridge, while the upstream depths increased slightly for the three-opening straight deck bridge; (ii) the non-uniformity in the velocity profiles varied along the channel centreline, with the exponents in the power-law velocity distribution reaching their maximum values around the first two blocked model bridges, while the exponent values changed slightly along the channel for the third model bridge due to the middle opening not being blocked; and (iii) the downstream mean velocity was slightly greater than the upstream velocity for the first two cases, with the minimum value occurring just downstream of the bridge. However, the depth-averaged velocities just downstream of the bridge were relatively higher for the third case. Therefore, the obstruction caused by vehicle blockage at bridges has significant impacts on the hydrodynamic characteristics in an urban reach, and these impacts depend on the specified bridge type and the blockage mode.
River bifurcations are critical but poorly understood elements of many geomorphological systems. They are integral elements of alluvial fans, braided rivers, fluvial lowland plains, and deltas and control the partitioning of water and sediment through these systems. Bifurcations are commonly unstable but their lifespan varies greatly. In braided rivers bars and channels migrate, split and merge at annual or shorter timescales, thereby creating and abandoning bifurcations. This behaviour has been studied mainly by geomorphologists and fluid dynamicists. Bifurcations also exist during avulsion, the process of a river changing course on a floodplain or in a delta, which may take 102–103 years and has been studied mainly by sedimentologists. This review synthesizes our current understanding of bifurcations and brings together insights from different research communities and different environmental settings. We consider the causes and initiation of bifurcations and avulsion, the physical mechanisms controlling bifurcation and avulsion evolution, mathematical and numerical modelling of these processes, and the possibility of stable bifurcations. We end the review with some open questions. Copyright © 2012 John Wiley & Sons, Ltd.
The channel junctions are an integral part of any channel network used in practice.
There are several examples of such flows in channels such as water conveyance systems in
hydropower plants, canals, natural and manmade waterways, sewers and drains etc. The water
levels in different channels before and after a junction need to be monitored effectively to
avoid problems such as over flow and skewed flow distribution. This paper analytically
investigates the complex flow features of combining and dividing flows at rectangular and
trapezoidal channel junctions. Furthermore parametric investigations have also been carried out
to establish dependence of depth ratio on various parameters for rectangular and trapezoidal
channel sections.
Floodwater flows through urban floodplains with storm water systems are often inadequate during extreme storm events and/ or when the river flood inundation extent becomes extreme. Such flows may cause potential hazard risks to humans and their properties along the floodplains. Recently, flood hazards relating to vehicles have become more noticeable and it is vital to investigate the hydraulic behaviour of vehicles on urban floodplains. Therefore, this paper outlines a study of the theoretical and experimental aspects of the hydrodynamics of floodwater flows over urban floodplains with vehicles. A theoretical background study is discussed to establish an understanding of the hydrodynamics of floodwater flows over urban floodplains with vehicles; a condition which can be very important for extreme storm events, or even moderate storm events, when the storm water system is insufficient to drain away the surface runoff. Extensive investigations have been undertaken on stationary scaled die cast model vehicles in laboratory hydraulics flumes by conducting a series of physical experimental studies on: (i) the threshold of vehicle instability, (ii) the effects of vehicle orientation, (iii) the effects of ground surface gradient, (iv) the vehicle stability on urban floodplains, and (v) the influence of vehicles on floodwater flows. The results for all the test cases have been analysed to investigate the effects of vehicles on floodwater flow propagation over urban floodplains and, on the other hand, the influence of the floodwater flows on the stability of model vehicles. The two principal factors of hazards (i.e. the floodwater depth and flow velocity) that affect the stability of model vehicles in urban floodplains have been identified to confirm the significant impact of hydrodynamic processes in urban floodplains with vehicles. All experiments undertaken so far have only looked into the conditions under which the model vehicles begin to be moved. Observations have been made from the theory studied and experiments conducted to systematically look into the hydraulic behaviour of vehicles in urban floodplains. The main findings have highlighted that: (i) the model vehicles had a significant impact on the floodwater flow propagation and the hydrodynamic processes in the flooded area, (ii) if the incoming flow depth was less than the vehicle height, then the threshold velocity increased for a decease in the depth of flow; (iii) if the incoming flow depth was greater than the vehicle height, then the threshold velocity would rise with an increase in the depth of flow, and (iv) a flooded vehicle was more likely to move if the incoming depth just approached the vehicle chassis height due to the buoyancy effects. Based on these findings, an innovative approach of a straightforward three colour zone envelope curve has been developed, and first introduced herein, which has been defined as the Traffic Light of Hydraulic Stability (TLHS) system. This novel approach can be readily used to evaluate the degree of hydraulic stability for model vehicles, and it is also invaluable for assessing the vehicle hazard conditions in urban floodplains.
In this work we have investigated the equilibrium configurations of a Y-shaped fluvial bifurcation through a laboratory analysis. Three series of experimental runs have been performed in a wide flume, where a symmetrical bifurcation has been constructed joining three branches with fixed banks and movable bed made of a well sorted quartz sand; the angle between the two downstream distributaries was equal to 30 degrees. The experiments have been carried out with different values of longitudinal bed slope and water discharge, in order to investigate a range of the relevant morphodynamic parameters typical of gravel bed braided rivers. The equilibrium configuration of the bifurcation has been characterized through the measure of the discharge partition in downstream branches and of the local bed structure at the node. The existence of unbalanced equilibrium configurations has been observed and the role of migrating alternate bars has been pointed out. The experimental results confirm the theoretical predictions which have been recently obtained through the simple model of Bolla Pittaluga et al. (2003). Moreover, interpreting the measured data in the light of the concept of morphodynamic influence provides a new perspective in the analysis of the equilibrium configurations of a bifurcation.
This study investigates the suitability of trapezoidal cross-section with segment base in drainage system design. The study has considered steady uniform open channel flow. The saint-Venant partial differential equations of continuity and momentum governing free surface flow in open channels have been solved using finite difference approximation method. We investigate the effects of the channel radius, area of the cross section, the flow depth and the manning coefficient on the flow velocity. The flow variables are velocity and the flow depth while the flow parameters are cross section area of flow, channel radius, slope of the channel and manning coefficient. The study has established that increase in cross section area of flow leads to a decrease in flow velocity. Further, increase in channel radius and cross section area of flow leads to a decrease in flow velocity and increase in roughness coefficient cause flow velocity to decrease. Additionally, increase in flow depth increases velocity. The physical conditions of the flow channel have been applied to conservation equations to arrive at specific governing equations. The results of the study have been presented graphically.
Recognition that unplanned and uncontrolled development can increase the risk to life and damage to property is fundamental to successful floodplain management. Awareness of this issue is not just the responsibility of local authorities but all stakeholders, covering both public and private sectors. Whilst the land developer has the leading social responsibility for flood compatible development, the approving agencies share that responsibility through provision of relevant guidelines and information on floods, and adequate assessment of each development application in a transparent, impartial manner. In rural areas where informal approval processes are common, it is the combined responsibility of the community, respective community leaders and the local council to ensure that dwellings are not constructed on unsuitable land or in an unsuitable manner. The traditional approach to flood mitigation has primarily involved a structural approach to modify flood characteristics. Contingency measures, such as flood sirens, are also used to alert communities of impending floods. Whilst structural mitigation measures can reduce flood levels and extents, without adequate floodplain planning, the benefit from the structural works is lost due to increased flooding from unplanned development. The result is no net benefit, at a substantial financial cost to the community. The monetary benefit of improved planning through reduction of flood damages offers far greater cost-benefit measure than most structural approaches. This paper describes the principles used and development of a simple set of guidelines for controlling floodplain development in Malaysia. The guidelines have been prepared and recommended for implementation across three districts in Sabah. In time, it is expected that the principles used in the guidelines are incorporated into a national policy for floodplain development.
Based on model-technical as well as physical considerations a nodal-point relation at bifurcations is proposed for one-dimensional (ID) network morphodynamic models: the ratio between the sediment transports into the downstream branches is proportional to a power of the discharge ratio. The influence of the nodal-point relation on the behaviour of the morphodynamic model is analyzed theoretically. The exponent in the nodal-point relation appears to be crucial for the stability of the bifurcation in the model. For large values of the exponent, the bifurcation is stable, i.e. the downstream branches remain open. For small values of the exponent, the bifurcation is unstable: only one of the branches tends to remain open. The exponent also has a strong influence on the morphological time scales of the network. The conclusions from the analysis have been verified by numerical simulations using a package for one-dimensional network modelling.
A review of the flood hazard and risk management in the south Asian region particularly Pakistan
- Smh Shah
- Z Mustaffa
- F Y Teo
- Mah Imam
- K W Yusof
- Al-Qadami Ehh