Increase In Flow Depth Research Articles

Hydraulic characteristics of flow through angled baffle-plates in an open channel

ABSTRACT This paper aims to study the flow characteristics through angled baffle-plates in an open channel. The experiments were conducted in a self-circulating hydraulic flume with three designated baffle-plates’ angles (30°, 60°, and 90°) and three flow conditions (high, medium, and low). Results showed that all the designed angles for the different flow conditions do retard the flow resulting in reduced downstream mean discharge as compared to without the baffle-plates. The baffle-plates installed at 90° showed the greatest percentage of mean discharge reduction for all the flow conditions (ranged between 5.32% and 11.67%) as compared to 60° (ranged between 3.96% and 8.3%) and 30° (ranged between 3.55% and 7.07%). An equation to predict the relative flow depth increase due to the retarding behaviour of the angled baffle-plates structure was also developed in this study using multiple linear regression. The proposed equation has an R 2 value of 0.69, value of 0.62, Mallow’s Cp value of 2 and a mean square error (MSE) value of 0.4548, which is the best among all the possible regression models for this study. The equation can be used by the designer in determining the desired angle for the baffle-plates structure to prevent overflow at upstream of the structure.

Sudden pollutant discharge in vegetated compound meandering rivers

In this study, the effects of different vegetation densities and different relative flow depths on the longitudinal dispersion coefficient are investigated in a compound meandering channel. Simulated vegetation with three different densities was placed over the floodplain and tracer was released in line simultaneously and equally in the main channel and flood plain. Digital image processing technique was used to measure the tracer concentration along the channel by analyzing a series of sequential images of the tracer cloud. Acoustic Doppler Velocimeter was used to measure 3-D velocity components. The results showed that the depth-averaged longitudinal velocity increases in the main channel due to the presence of vegetation but it declines in the floodplain. The maximum amount of the turbulent kinetic energy and the dimensionless longitudinal dispersion coefficient (K/U⁎H) were observed at the bend apex. Moreover, as the relative flow depth increases, K/U⁎H declines in the compound meandering channel for all the vegetated cases. Additionally, the longitudinal dispersion coefficient increases up to 59% in the main channel and decreases up to 42% in the floodplain by increasing the vegetation density in a specific relative flow depth.

Shockwaves in spillways with the particle finite element method

Changes in direction and cross section in supercritical hydraulic channels generate shockwaves which result in an increase in flow depth with regard to that for uniform regime. These disturbances are propagated downstream and need to be considered in the design of the chute walls. In dam spillways, where flow rates are often high, this phenomenon can have significant implications for the cost and complexity of the solution. It has been traditionally analysed by means of reduced-scale experimental tests, as it has a clear three-dimensional character and therefore cannot be approached with two-dimensional numerical models. In this work, the ability of the particle finite element method (PFEM) to reproduce this phenomenon is analysed. PFEM has been successfully applied in previous works to problems involving high irregularities in free surface. First, simple test cases available in the technical bibliography were selected to be reproduced with PFEM. Subsequently, the method was applied in two spillways of real dams. The results show that PFEM is capable of capturing the shockwave fronts generated both in the contractions and in the expansions that occur behind the spillway piers. This suggests that the method may be useful as a complement to laboratory test campaigns for the design and hydraulic analysis of dam spillways with complex geometries.

Modeling the Infiltration Capacity of Permeable Stormwater Channels with a Check Dam System

The use of permeable stormwater channels has introduced concerns over the effects of infiltration on the hydraulic behavior of their flow and the effects of flow hydraulic conditions (e.g., the water level, channel section, flow velocity, and vegetation) on the channel infiltration capacity. A check dam system provides backwater ponding, which increases the flow water depth along a channel. In this study, a channel model was used to investigate the variation in the infiltration capacity of permeable stormwater channels under different flow hydraulic conditions. Increasing the downstream check dam height and using a grass cover increased the infiltration rate and cumulative infiltration because of the decreased velocity and increased flow depth. The presence of subsurface water did not affect the hydraulic characteristics of the channel flow but decreased the cumulative infiltration because of the fast saturation of the soil. An empirical equation was developed for predicting the infiltration capacity of grassed channels in which four hydraulic parameters (i.e., the water depth, base width, side slope, and velocity) are introduced to the modified Kostiakov model. The developed model was used to calculate the runoff reduction due to infiltration along a grassed channel with and without a check dam system. The percentage of infiltrated water increased from 8 to 14% with the check dam system. The developed model can be used to predict the infiltration capacity of permeable channels for improved stormwater management and provides a valuable decision support tool for permeable channel design.

Raindrop Size and Flow Depth Control Sediment Sorting in Shallow Flows on Steep Slopes

AbstractRaindrop size and flow depth interactions determine the energy available for erosion processes. A detailed laboratory study was conducted to investigate the influence of raindrop size on sediment sorting in shallow flows on steep slopes. Raindrops with sizes of 1.34, 2.45, and 4.04 mm were applied to three inflows (0.65, 1, and 2 L/min) with the same rainfall intensity (90 mm/hr). The De Brouckere mean diameter (D[4,3]) was enhanced with increases in the ratio between flow depth and raindrop size (h/d; p < 0.05), indicating that more raindrop energy dissipated in deeper flows. Moreover, a greater proportion of energy was dissipated from small raindrops than from large ones with an increasing flow depth. Particles >0.5 mm were slightly enriched by rolling under 1.34‐mm raindrop impact, whereas enrichment did not occur under 2.45‐ and 4.04‐mm raindrop impact, probably because the latter induced more severe aggregate disintegration, resulting in fewer coarse particles. The mean sediment concentrations under inflows of 0.65, 1, and 2 L/min were 101.65, 89.32, and 79.02 g/L, respectively. Shallow flows on steep slopes effectively carried detached particles; moreover, coarse particles could roll along the slopes, which was inconsistent with their behavior on gentle slopes. This study highlights the need to understand the influence of the interaction between raindrop size and flow depth on erosion processes associated with sediment size selectivity for better erosion modeling and water quality prediction.

Characteristics of Positive Surges in a Rectangular Channel

A positive surge is an unsteady open channel flow motion characterized by an increase of flow depth. In previous experimental studies, a positive surge was typically induced by either a sudden increase of discharge in a channel or by the rapid closure of a downstream sluice gate, thus leading to a steep initial profile. However, in many instances, the evolution of a positive surge is of a progressive manner (e.g., in the downstream navigation canal during the emptying operation of lock chambers). In the present work, the inception and development of a positive surge induced by a progressive increase of discharge was investigated in a rectangular channel with a smooth bed. Both undular and breaking surges were studied. The results demonstrate that the maximum wave height at the first wave crest of an undular surge is in very close agreement with the McCowan theory. Additionally, the wave amplitude essentially shows a linearly increasing trend with an increasing surge Froude number up to Fr0 = 1.26 to 1.28, whereas it tends to suggest a power law reduction for larger surge Froude numbers. Moreover, the dispersion of undular surges is consistent with the linear wave theory only for surge Froude numbers close to unity. Overall, the present study demonstrates the unique features of positive surges induced by a progressive increase of discharge.

Study of the Impact of Vegetation Direction and Slope on Drag Coefficient

In order to understand the influence of vegetation on resistance to overland flow, the relationship between drag coefficient and flow depth was modeled and studied for three types of vegetation distributed at four different slopes (0, 0.5,1.0, and 3.0%) based on three angles (30°, 45°, and 90°) between the vegetation row direction and flow direction. An indoor fixed-bed erosion test was used for this study, and the vegetation stem was simulated by a cylinder. The results show that at the same slope and flow depth, the drag coefficient of vegetation decreases as the angle between the vegetation row direction and the flow direction increases; at the same slope and angle, the drag coefficient of the vegetation first decreases and then stabilizes as the flow depth increases. A threshold depth was found beyond which the flow depth no longer affected the drag coefficient of vegetation. The threshold depth decreased with the increase in slope; thus, the slope affects the drag coefficient of vegetation by altering threshold depth.

Experimental and Numerical Study of Free-Surface Flows in a Corrugated Pipe

A new discharge computational model is proposed on the basis of the integration of the velocity profile across the flow cross-section in an internally corrugated pipe flowing partially full. The model takes into account the velocity profiles in the pressurised pipe to predict the flow rate under free-surface flow conditions. The model was evaluated through new laboratory experiments as well as a literature datasets. The results show that flow depth and pipe slope may affect the model accuracy; nevertheless, a prediction error smaller than 20% is expected from the model. Experimental results reveal the influence of the pipe slope and flow depth on the friction factor and the stage-discharge curves: the friction factor may increase with pipe slope, while it reduces as flow depth increases. Hence, a notable change of pipe slope may lead to the variation of the stage-discharge curve. A part of this study deals with numerical simulation of the velocity profiles and the stage-discharge curves. Using the Reynolds-Averaged Navier-Stokes (RANS) equations, numerical solutions were obtained to simulate four experimental tests, obtaining enough accurate results as to velocity profiles and water depths. The results of the simulated flow velocity were used to estimate the flow discharge, confirming the potential of numerical techniques for the prediction of stage-discharge curves.

A comparative study of longitudinal dispersion models in rigid vegetated compound meandering channels

Releasing and mixing of pollutants in rivers can cause serious threats for downstream users. Longitudinal dispersion is an important factor in describing pollutant transport process in rivers. The focus of this study is on the effect of floodplain rigid vegetation characteristics including its arrangement, density and relative depth (the ratio of the flow depth in the main channel to that over floodplain) on flow and longitudinal dispersion coefficient in a meandering channel with compound cross section. Digital image processing technique was used to measure the tracer concentration. Sequential images were continuously captured at seven sections downstream of release point. PVC cylinders were installed over the floodplain to simulate rigid vegetation in tandem and random arrangements. Acoustic Doppler Velocimeter (Micro-ADV) was used to measure 3-D velocity components. The results showed that roughening the flood plain with stems increases the longitudinal flow velocity and Reynolds shear stress in the main channel, while these parameters decrease in the flood plain compared to non-vegetated cases. As a result, in the presence of rigid vegetation, the travel time decreases in the main channel up to 20% compared to non-vegetated conditions. As the relative flow depth increases, the dimensionless longitudinal dispersion coefficient, K/U∗H, decreases up to 74%. Also, the dimensionless longitudinal dispersion coefficient increases up to 38% with stem density in a specific relative flow depth of Dr = 0.11. Finally, a comparison has been made between laboratory data and some well-known previously reported models. Later, the most accurate equations are presented for predicting the longitudinal dispersion coefficient in rigid vegetated compound meandering channels. The results of the present study reveal that rigid vegetation over flood plain can be used as an alternative method in river rehabilitation projects for environmental management purposes.

Head discharge relationship for inclined non-rectangular notches

Experimental results on the discharging capacity of inclined sharp crested weirs are presented. Three non-rectangular notch shapes such as parabolic, semi-circular and inverted semicircular notches were considered in the study. It is found that the theoretical discharge increases nonlinearly with an increase in flow depth over the crest of the notch and inclination of the notch with vertical. The regression equations are developed between the non-dimensional theoretical discharge and non-dimensional flow depth over the crest. The regression coefficients are found to be the function of the angle of inclination of the notch. Further, the actual discharge varies linearly with the theoretical discharge, with coefficient of discharge being the function of angle of inclination of the notch with vertical. The average coefficient of discharge for parabolic, semi-circular and inverted semicircular is found to be 0.638, 0.796 and 0.77, respectively. In addition, the discharging index for the inclined notches are defined which increases with an increase in inclination of the notch with vertical.

Experimental study on the energy dissipation characteristics of debris flow deceleration baffles

Debris flow can cause serious damages to roads, bridges, buildings and other infrastructures. Arranging several rows of deceleration baffles in the flow path can reduce the flow velocity and ensure better protection of life and property. In debris flow prevention projects, deceleration baffles can effectively reduce the erosion of the debris flow and prolong the running time of the drainage channel. This study investigated the degree to which a 6 m long flume and three rows of deceleration baffles reduce the debris flow velocity and affect the energy dissipation characteristics. The influential variables include channel slope, debris flow density, and spacing between baffle rows. The experimental results demonstrated that the typical flow pattern was a sudden increase in flow depth and vertical proliferation when debris flow flows through the baffles. Strong turbulence between debris flow and baffles can contribute to energy dissipation and decrease the kinematic velocity considerably. The results showed that the reduction ratio of velocity increased with the increase in debris flow density, channel slope and spacing between rows. Tests phenomena also indicated that debris flow density has significant influence on the mobility and deposition characteristic of debris flow. The deposit amount first increased then decreased when the flow density rises, and the deposit amount of debris flow density of 1500 kg/m3 reached the maximum when the experimental flume slope is 12°.

Formulating the shear stress distribution in circular open channels based on the Renyi entropy

The principle of maximum entropy is employed to derive the shear stress distribution by maximizing the Renyi entropy subject to some constraints and by assuming that dimensionless shear stress is a random variable. A Renyi entropy-based equation can be used to model the shear stress distribution along the entire wetted perimeter of circular channels and circular channels with flat beds and deposited sediments. A wide range of experimental results for 12 hydraulic conditions with different Froude numbers (0.375 to 1.71) and flow depths (20.3 to 201.5 mm) were used to validate the derived shear stress distribution. For circular channels, model performance enhanced with increasing flow depth (mean relative error (RE) of 0.0414) and only deteriorated slightly at the greatest flow depth (RE of 0.0573). For circular channels with flat beds, the Renyi entropy model predicted the shear stress distribution well at lower sediment depth. The Renyi entropy model results were also compared with Shannon entropy model results. Both models performed well for circular channels, but for circular channels with flat beds the Renyi entropy model displayed superior performance in estimating the shear stress distribution. The Renyi entropy model was highly precise and predicted the shear stress distribution in a circular channel with RE of 0.0480 and in a circular channel with a flat bed with RE of 0.0488.

Scour at wind turbine tripod foundation under steady flow

Results of a physical modelling study of clear-water scour at a tripod obstacle in uniform bed sediment under steady flow conditions are presented. 4 flow depths, 2 flow velocities and 3 angles of attack were considered. Maximum scour depth increased with increase in flow depth or velocity. For deep flows and high flow rate, the maximum value of the ratio of the equilibrium scour depth to the diameter of the tripod’s leg was 3.5, which is significantly greater than values of 1.3 or 2.5 suggested by guidelines for offshore wind turbine monopiles design. This ratio may be even larger because of the increased shear stress when waves are superimposed on current. All maximum equilibrium scour depths occurred at the upstream surface of a tripod leg despite the centre pile having a larger diameter. The alignment angle had a small effect on scour depth. But it is notable that there is an increase in scour development depth at 60° alignment angle. Equations are suggested for estimating the clear-water scour depth at wind turbine tripod foundations.

Flume experiments on baffle-posts for retarding open channel flow

ABSTRACTPresented are the results of flume experiments on the flow-retarding performance of baffle-post structures in openwater flow. Such structures consist of one to two parallel rows of unsubmerged posts that retard an approach flow, reducing flow velocity, increasing flow depth, and altering lateral distribution of flow. The results relate headloss and flow depth increase at the structure to lateral and streamwise variations in post spacing, post diameter, and approach flow conditions, and represent average velocities of flow through the structure. For given approach flows, the flow-retarding performance of baffle-post structures is shown to be much more sensitive to lateral spacing of posts and approach flow Froude number, than to streamwise spacing (for double row structures) and approach-flow depth relative to post diameter. Estimates are given for the percentage increase in water depth that baffle posts will produce. This information informs the design of baffle-post structures and evaluation of the backwater effect.

FLOOD FLOW CHARACTERISTICS AND BED LOAD TRANSPORT IN NON-VEGETATED COMPOUND STRAIGHT CHANNELS

Floods are the most common natural disasters in Malaysia and have damaged structures, infrastructures, crops and even causes fatalities. It may also lead to erosion and sedimentation in rivers and this will result to complex river behaviour. A hydraulic laboratory experimental study was carried out. Also, flood flow and sediment transport in straight compound channels involving flow resistance, distribution of depth-averaged velocity, stream-wise vorticity patterns, channel bed morphology and bed load transport rate in non-vegetated compound straight mobile bed channels were investigated. The finding showed that the Darcy Weisbach friction factor f increased by 40% and 54% for floodplain and main channel, respectively when relative flood flow depth increase from 0.30 to 0.50. The small bed load transport rates of 0.09 g/s and 0.03 g/s for shallow and deep overbank flows, respectively were measured due to effect of very gentle or mild channel bed slope which was fixed at a gradient of 0.1%.

The effect of floodplain grass on the flow characteristics of meandering compound channels

Laboratory experiments were conducted in a large-scale meandering compound channel to investigate the effect of floodplain grass on the main flow field in the channel. Three-dimensional velocity fields, turbulences, and Reynolds shear stresses were measured along half a meander. The experiments revealed that flexible artificial grass planted on a floodplain can significantly reduce the conveyance capability of the entire channel. Two parallel stage-discharge curves increased with increasing flow depth. The additional resistance of the floodplain grass increased the streamwise velocity and conveyance in the main channel along a meander. An analysis of the generation mechanism of secondary flows in the main channel indicated that the secondary current consisted of an enhanced original secondary cell that was strengthened by the centrifugal force and a component of the upstream floodplain flow. The relative dominance of these two components in the secondary flows was primarily determined by the angle between the floodplain flow and the main channel ridge, and also the floodplain roughness. At the same flow depth, the secondary flow in cases with grass on the floodplain was generally stronger than that in the case of a smooth meander bend, although it was weaker near the middle cross-over section. Floodplain grass enhanced the intensity of the lateral turbulence above the bankfull level and significantly modified the turbulence structure, although it had a negligible effect on the vertical turbulence except at the bend entrance. Floodplain grass also affected the Reynolds shear stresses in the main channel, generating stronger lateral shear stresses at a low flow depth. In contrast, at a high flow depth, the distribution of the interface shear stresses changed entirely while its magnitude remained the same. When the floodplains were grassed, the vertical shear stress that was induced by secondary flows was greater at the apexes but reduced at the cross-over sections. These observations highlight the important effects of floodplain grass on the main channel flow.

Composition and Architecture of Braided and Sheetflood-Dominated Ephemeral Fluvial Strata In the Cambrian–Ordovician Potsdam Group: A Case Example of the Morphodynamics of Early Phanerozoic Fluvial Systems and Climate Change

Abstract: Two fluvial facies associations, braided and ephemeral fluvial, are recognized in strata of the Cambrian–Ordovician Potsdam Group in the Ottawa Embayment and Quebec Basin in northeastern North America. Braided fluvial sedimentation was dominated by the migration of coarse-grained sandy dunes and rare unit bars and local gravels that built up low-relief 0.4–5-m-thick downstream-accreting compound bars in very wide and shallow braided channels. Rare meter-scale confluence scours, incision, and filling of decimeter-thick bypass channels, and thin (≤ 7 cm) overbank deposition also occurred. Ephemeral fluvial sedimentation was dominated by the progradation of lobate terminal splays, most of which were deflated and/or reworked into thin (10–95 cm) sheets by eolian processes during the long recurrence intervals between floods. These strata were locally incised by rare, 0.3–1.9-m-thick scour-filling high-energy supercritical bedform strata and decimeter-thick bypass channels. Both braided and ephemeral fluvial deposits consist of meter-scale stacked, high-order stratal units separated by horizontal, laterally extensive surfaces recording the recurring buildup, avulsion, and finally abandonment of channel belts. In braided fluvial strata of the Potsdam the dominance of thin (≤ 2 m) elongated compound bars with low-relief (≤ 5°) bar margins and internal accretion surfaces (≤ 15°) suggest that braided channels and in-channel compound bars expanded laterally with negligible increase in flow depth during periods of increased discharge in the absence of bank-stabilizing vegetation, similar to other examples of the pre-vegetated “sheet-braided” style. However, in areas where bedrock topography inhibited the lateral expansion of channels, thicker compound braid bars (1.2–3.5 m) with higher-relief accretion surfaces (up to 35°), like those observed in post-Silurian and modern systems, preferentially formed in relatively deep channels. Ephemeral fluvial deposits here are similar to ephemeral examples throughout geologic history in that they mostly comprise poorly confined sheet-like terminal splays that accumulated in topographic lows. However, compared to several post-Silurian examples, strata in the Potsdam exhibit considerably more eolian reworking of splays and preservation of supercritical-bedform strata, a manifestation of greater rates of surface-water infiltration due to a lack of sheltering by rooted plants and propensity for unconfined flooding. Finally, in the upper part of the Potsdam two regional ephemeral fluvial units are interstratified with two braided fluvial units, providing evidence for shifts in regional climate. The sharp, areally extensive surfaces that separate these fluvial systems are interpreted to approximate time lines, and therein provide a means for relative and absolute chronostratigraphic correlation. These changes are correlated with documented Late Cambrian to Earliest Ordovician global climate fluctuations, with semiarid conditions and related ephemeral fluvial systems corresponding to global cooling events at ca. 491 and 487 Ma.

Modeling Open Channel Fluid Flow with Trapezoidal Cross Section and a Segment Base

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.

Coupling between free-surface fluctuations, velocity fluctuations and turbulent Reynolds stresses during the upstream propagation of positive surges, bores and compression waves

In open channel, canals and rivers, a rapid increase in flow depth will induce a positive surge, also called bore or compression wave. The positive surge is a translating hydraulic jump. Herein new experiments were conducted in a large-size rectangular channel to characterise the unsteady turbulent properties, including the coupling between free-surface and velocity fluctuations. Experiments were repeated 25 times and the data analyses yielded the instantaneous median and instantaneous fluctuations of free-surface elevation, velocities and turbulent Reynolds stresses. The passage of the surge front was associated with large free-surface fluctuations, comparable to those observed in stationary hydraulic jumps, coupled with large instantaneous velocity fluctuations. The bore propagation was associated with large turbulent Reynolds stresses and instantaneous shear stress fluctuations, during the passage of the surge. A broad range of shear stress levels was observed underneath the bore front, with the probability density of the tangential stresses distributed normally and the normal stresses distributed in a skewed single-mode fashion. Maxima in normal and tangential stresses were observed shortly after the passage of a breaking bore roller toe. The maximum Reynolds stresses occurred after the occurrence of the maximum free-surface fluctuations, and this time lag implied some interaction between the free-surface fluctuations and shear stress fluctuations beneath the surge front, and possibly some causal effect.

Assessment of Risk Due to Debris Flow and Its Application to a Marine Environment

The purpose of this study is to evaluate the behavior and mechanism of a debris flow on various slopes through numerical simulation. The numerical simulation consisted of using equations related to mass conservation and momentum conservation in order to consider erosion and deposition, and the Finite Difference Method was applied. As the inflow water discharge in the upstream of the channel increases, the curve of the water discharge exhibits instability and, as time passes, the fluctuation of the high water discharge continues. In regions where the mountain areas and the ocean are connected, it is deduced that the high level of sediment concentration can greatly affect the environment surrounding the ocean. The numerical model of this study was applied in Kangwon Province of South Korea. The results show that when the debris flow reaches downstream, the flow discharge and water flow depth increase. Erosion occurs more than deposition and much of the sediment runs off downstream. The result of the simulation performed at point of sediment discharge runoff is 114,216 m3. This study will provide useful information in predicting disasters caused by debris flow and in planning for various countermeasures to prevent debris-flow-related disasters.