Nappe Flow Research Articles

The effect of gabion steps on the hydraulic jump characteristics downstream of stepped spillways

ABSTRACT Gabion steps on stepped spillways can be a suitable alternative to solid impermeable steps due to their economic advantages, ease of implementation, stability through water pressure reduction, and energy dissipation. This study explores hydraulic jumps and the rate of energy dissipation downstream of gabion stepped spillways under different flow regimes. Six stepped spillway models, including a solid spillway with solid steps and five models with gabion steps, were made with different step arrangements in a 26.6° slope (1 V:2 H) facility. The results showed that in stepped spillways with gabion steps and in the nappe flow regime, as a result of seepage flow through the pores of the aggregate and overflow passing the gabion step, there is an increased energy dissipation associated with a smaller sequent depth ratio, the relative length of jumps, and roller length compared to solid and impermeable steps. The reduction in the hydraulic jump depends on the flow regime and the arrangement of the gabion steps. For skimming flows, smaller energy dissipation rates were measured downstream of the gabion step. On average, when gabion steps are placed on all steps, the sequent depth ratio, jump length, and roller length decrease by 3.34%, 8.61%, and 8.14%, respectively, compared to solid steps. The maximum energy dissipation was measured at the downstream end of the gabion steps with a value of 74.4%, which is 5.03% more than that with the solid steps. On the solid stepped spillway, the nondimensional residual head was 4.65, and for the same flow conditions, the average dimensionless residual head on the gabion steps was 4.21.

Characteristics of hydraulic jump and energy dissipation in the downstream of stepped spillways with rough steps

Stepped spillways are used to improve the hydraulic performance of a water-flow structure. This research describes the study of two types of physical models to investigate the impacts of rough steps on stepped spillways in terms of energy dissipation and hydraulic jumps. To this end, six stepped spillway models, including a flat spillway with smooth steps and five models with rough steps using gravel with a given grain size (i.e., d50 = 18.9 mm), were made with different step arrangements. The results showed that stepped spillways with rough steps and in the nappe flow regime show a better performance regarding energy dissipation of about 20.97 % compared to smooth steps. On the smooth stepped spillway, the non-dimensional residual head was 4.65, and for similar operating conditions, the average dimensionless residual head on the rough steps was 2.92. Furthermore, when rough steps are placed on all steps, the hydraulic jump characteristics, including sequent depth ratio, jump length, and roller length, decrease by 3.34, 8.61, and 8.14 %, respectively, compared to smooth steps.

Investigating pressure changes at the bottom of the stilling basin after the stepped spillway using a numerical model

Stepped spillways are hydraulic components of dams used to dissipate energy downstream of the dams. Numerous studies have been conducted to understand the hydraulics of flow on stepped spillways, primarily in laboratory settings. The flow passing over stepped spillways can be categorized into two types of flow regimes: Nappe Flow and non-Nappe Flow. In this research, the FLOW-3D software was utilized to model the Cardan configuration of the downstream basin of the stepped overflow based on laboratory data. In this investigation, we compared the distribution profile of maximum, minimum, and average pressures in three areas: the center, right, and left, with the laboratory data in the numerical model using graphical methods and statistical indicators. The results revealed that 08% of the pressures obtained from the model closely matched the laboratory data with an error rate of only 5.1%.

Investigation of labyrinth weirs discharge coefficient with the same length

One of the approaches to reduce flood depth in dams and rivers is the application labyrinth weirs instead of simple linear weirs. Labyrinth weirs have a longer length than linear weirs; therefore, the flood will pass through it with a lower depth. In this study, two types of triangular and semicircular labyrinth weirs were experimentally investigated. Experiments were performed in a flume with a width of 0.8 m and a flow discharge of up to 70 L/s. Eight weir models with a thickness of 4 mm were made, of which four models are triangular weirs and four models are semicircular weirs. Weirs were used in two types, one-cycle and two-cycle. The height of the weirs is 15 cm and the ratio of the weir length to the flume width (L/W) is equal to 1.57. The results showed that the increase in the total water head ratio (HT/P) decreases the discharge coefficient in all models, which is due to the increase in the Nappe interference and the local submergence. Also, increasing the number of cycles in labyrinth weirs has reduced its efficiency. The reason for this decrease is the impact of the issuing jets at the junction of the cycles and the flow disturbance over the weir. The comparison between triangular and circular weirs showed the better efficiency of triangular weirs. In addition, the results showed that the placement of the top of the cycle towards the upstream will cause a better performance in the weir. It seems that the low angle of the weir wall in the downstream with the channel wall causes the Nappe flow to collide with the channel wall and consequent local submergence will ultimately reduce the discharge coefficient.

New Equations for Rate of Energy Dissipation of a Stepped Spillway with Slope less than Critical and Specific Step Height

Stepped spillways disperse energy in floodwater by utilizing their stepping nature. Several researchers investigated the hydraulic and geometric relationships of the stepped spillway with a dam slope above 26.6o, which resulted in energy dissipation. But a few studied stepped spillways with a dam slope of 26.6o with a step height of 0.61 m. This move led to a lack of knowledge and rules for the designers of stepped spillways with slope of 26.6o and a step height of 0.61 m. The authors reviewed the researchers' publications on horizontal stepped spillways with dam slopes of 26.6o with step height of 0.61 m conducted in transition and skimming flows in large-size facilities with phase-detection intrusive probes. They retrieved data sets from them, which they took into account to create two novel equations for energy dissipation in stepped spillways: one for nappe flow and one for skimming flow. The results showed that the measured data sets compared well with the data sets calculated from the two empirical models.

Experimental Study of Flow Regimes of Stepped Weir


 Depending on the discharge and the stepped weir's dimensions, flow over a stepped weir can be primarily divided into three different flow regimes: nappe, transition, and skimming. The different properties for each flow regime make the decision of the flow regime an important component in the design of a stepped weir. Five stepped weir models have been used in experiments to determine the flow regime limits for these weirs. The models were made with various numbers of steps (3, 5, 7, 10, and 14) and downstream slope angle of 30˚. The configurations of steps that tested were flat step, fully pooled step, and zig zag pooled step . The results showed the end sills have a large impact on the type of flow regime. Based on the experimental data for flow regimes (nappe flow , transition flow and skimming flow ), a new set of empirical equations was proposed, with a determination coefficient〖 R〗^2 for these regimes (0.87, 0.71 ,0.7 ) respectively.
 
 

Numerical simulation of flow over a coastal embankment and validation of the nappe flow impinging jet

Numerical simulation of flow over a coastal embankment and validation of the nappe flow impinging jet

Experimental survey of static pressure in stepped chutes with inclined and horizontal steps equipped with end sills in nappe and skimming flow regimes

A spillway is one of the most important parts of a dam for controlling floods. Among different types of spillways, stepped spillways are one of the best energy dissipaters. Due to technological advances and the satisfaction of two elements for safe and low-cost construction, the use of stepped spillways has increased widely. Due to this, more studies are focused on stepped spillways. Researchers have made some efforts and proposed different methods to improve structural efficiency to dissipate energy. Modifications on step geometry, regarding flow regime type, are one of these efforts. Flow pressure and its fluctuations on the steps of the stepped spillways is one of the main factors affecting structural design and safety. In this experimental research, reverse inclined steps combined with the end sills have been applied in four degrees [0o (horizontal), 5o, 8o, and 11o] to obtain static pressure in both the nappe flow and skimming flow regimes of stepped spillways. Static pressure obtained from reverse inclined steps with end sills have been compared to the amount in the horizontal step. Results indicate a slight increase in the energy loss rate when reverse inclined steps have been applied in the nappe flow regime of stepped spillways.

Spatial distribution characteristics of splashing rainfall caused by nappe flow impingement on a plunge pool

Splashing is the main atomization source for a jet impinging on a liquid layer. Increased understanding of splashing rainfall characteristics can help to reduce the hazard of flood discharge atomization in hydraulic engineering. In this study, the spatial distribution of splashing rainfall caused by a nappe flow impinging on a downstream water cushion was experimentally investigated. Effects of the main hydraulic factors of impingement velocity, unit discharge, and water-cushion depth on splashing were investigated. The shape of splashing rainfall contours was approximately elliptical in horizontal planes. Maximum rainfall intensity was in the surrounding impingement region, and rainfall intensity decreased with an increase in the distance between the impingement center point and measurement points. Splashing rainfall intensity increased with increases in impingement velocity and unit discharge, whereas the opposite was observed with an increase in plunge pool depth. A gamma distribution described rainfall intensity distribution in the longitudinal and vertical direction, whereas a Gaussian distribution described intensity in the transverse direction. A series of empirical relations were proposed.

Investigation of flow characteristics in gabion stepped weirs

Abstract Weirs are used to control and regulate the flow in open channels. In gabion structures, the flow conditions are more complex due to the complexity of flow through the porous body of a gabion. The present study aims to investigate the water surface profile, the overflow velocity profile, and both the through-flow and overflow ratios. Six physical models of the three downstream slopes (V:H 1:1, 1:2, and 1:3) and two types of rockfill (crushed stone and rounded gravel) were investigated. Results show that for the same discharge, the milder slope model (1:3) shows higher water surface and higher velocity than the steeper slope (1:1) with about 9 and 8% on average respectively. The water surface was 60% higher on the lower steps than on the upper steps at the nappe flow regime. Moreover, the low porosity models show a slightly higher velocity and flow depth than higher porosity models for all sections. Furthermore, increasing the porosity from 0.38 to 0.42 led to about a 27% increment in the through-flow ratio. Finally, four relationships were developed to estimate the through-flow and overflow ratios at the upstream and inner sections of the gabion weir. The suggested relationships can be considered novel relationships.

Standing waves of the stepped dropshaft in a deep tunnel stormwater system.

The deep tunnel stormwater system, consisting of dropshafts and underground tunnels, is used to alleviate or prevent urban water problems associated with extreme rainfall events. The stepped dropshaft can transport surface runoff to the tunnels with high energy dissipation, low risk of cavitation and good exhaust performance, which well meets the requirement of the deep tunnel stormwater system. In the present study, the characteristics of the standing wave were investigated by experiments and numerical simulations, including the peak, trough and length of the standing wave. The flow regimes were divided into the nappe flow, the transition flow and the skimming flow with the increase of discharge, in which the standing wave mainly occurs on the external wall under the nappe flow and the transition flow. Influences of inflow discharge and dropshaft geometries were analyzed, including step rotation angle, relative step height and dropshaft curvature. The relations of the characteristics of standing wave with these effect factors were obtained. The maximum discharge capacity of the stepped dropshaft was established by considering that the standing wave just reached the above steps, which could be useful for the design and safe operation of the stepped dropshaft.

Effect of Orientation and Vegetation over the Embankment Crest for Energy Reduction at Downstream

Coastal embankments often collapse due to the tremendous destructive energy of an overtopping tsunami flow due to a deep scour by nappe flow. Hence, to clarify the nappe flow formation condition due to the overtopping, a series of tests were carried out within a laboratory flume with immobile settings by lowering the downstream surface angle of an embankment model while keeping the upstream surface slope constant (1:1) with five non-dimensional overtopping depths and six different crest conditions. The conditions imposed on the embankment crest in the flow direction were without vegetation; horizontal crest, (−)4% descending crest slope, (+)4% ascending crest slope, and adding vegetation model with three different densities across the horizontal crest to improve resistance to the flow. The increased resistance provided by the vegetation models were categorized based on the spacing ratio between cylinders to diameter: sparse, intermediate, and dense. Increased vegetation density above the crest results in a significant reduction of flow energy by approximately 30–50% at the downstream brink edge and 40–60% at the downstream plunge basin. In contrast, the maximum energy reduction was found to be by the dense vegetation model. Additionally, owing to the steep slope of the water surface profile and the increasing vegetation density, the impinging jet’s impact point moved closer to the toe of an embankment. This implies that vegetation covers a smaller area while increasing density to mitigate the destructive intensity of flood/tsunami movement. Meanwhile, the descending crest scenario results in a faster nappe flow formation. In contrast, the ascending crest scenario delays the nappe formation while reducing the downstream slope angle. It maintains the sub-critical flow at the crest, except near the downstream brink edge.

Utilization of Geogrid and Water Cushion to Reduce the Impact of Nappe Flow and Scouring on the Downstream Side of a Levee

Water overflowing from a levee generates scour holes on the toe, which progresses towards the backward crest of the levee and results in nappe flow generation. The direct collision of nappe flow on the downstream area causes levee failure. It is important to introduce a novel countermeasure against scouring caused by nappe flow. Hence, the present study utilized a new technique to reduce scouring due to nappe flow by introducing a combination of pooled water and geogrids. Herein, laboratory experiments were conducted with the three cases for rigid bed (R), named as NR, G1R, G2R (N, G1 and G2 represent no geogrid, geogrid 1 and geogrid 2, respectively), and moveable bed (M), named as NM (nothing moveable), G1M (geogrid 1 moveable), G2M (geogrid 2 moveable), to elucidate the effect of dimensionless pooled water depth (DP*), overtopping depth (DC*) and the aperture size of geogrids (d*) on flow structure and scouring. The results showed that the scour depth was reduced by around 17–31% during the NM cases, 57–78% during the G1M cases and 100% during the G2M cases by increasing the DP* from 0.3 to 0.45. Hence, the combination of geogrids with pooled water (G1M, G2M) performed a vital role in suppressing the scouring, but the results of G2M were more advantageous in terms of scouring countermeasures.

Dam-break flow dynamics over a stepped channel with vegetation

In spite of the insistence of a variety of studies on floods triggered by a dam failure, the effects of channel unevenness and vegetation have not been fully explored. Some hydrological aspects, such as the sudden change in the topography of a river section, the density of vegetation, and its influence on flood development, need to be further addressed. Therefore, the present work investigates the complex effects of vegetation and channel’s step on wave propagation during a dam break across a dry downstream channel. The Flow-3d Computational Fluid Dynamics package was used to simulate the flow phenomena during a dam break, adopting different geometric conditions and a densely vegetated area of the downstream channel in the far-field.Three-dimensional flow characteristics were reproduced by solutions of Navier-Stokes equations coupled with the standard volume of fluid to track the evolution of the free surface. The turbulent flow characteristics were represented by different approaches frequently used in the scientific literature.The computational model was optimized and validated using experimental data published in the literature. The results showed that the model had high accuracy in predicting the evolution of the free surface, moving hydraulic jump, flow velocity, and flow regime. In addition, the model was able to predict the formation and development of transitional, rotational, and transverse flows, jet flow, nappe flow, wave breaking, the bore evolution in different directions, and the change of the flow regime under the influence of the channel step and vegetation. Accordingly, the flow fluctuations during dam break, wave attenuation, flow separation, and turbulence structure evolution in the vegetated area were predicted.

Improving the performance of the compound labyrinth weirs using an artificial ventilation approach

Abstract Compound labyrinth weirs are a new shape that is used to increase discharge. The air pockets behind the nappe flow are a big problem for the hydraulic performance of the labyrinth weirs. The study aims to use the artificial ventilation approach to improve the performance of the compound labyrinth weirs. The current study proposes artificial ventilation techniques to mitigate the pressure behind nappe flow and to improve the coefficient of discharge. The data have been collected for three cases (without vented (WV), with vented (V), and with suction (S)). The results indicated that the pressure distribution behind nappe flow was not similar for all tested points. Also, a slight negative pressure was observed when H′t/P′ was measured between 0.1 and 0.2. Artificial ventilation with a suction pump was the best technique to reduce pressure behind the nappe flow when compared with a vent pipe device. Furthermore, the compound coefficient of discharge (Cdc) using the suction and vent devices was greater than the compound coefficient of discharge (Cdc) without the vent devices by 10% and 4.5%, respectively. The empirical equations were provided to predict the compound coefficient of discharge when pressure data behind nappe flow is available.

Investigations of velocity and pressure fluctuations over a stepped spillway with new step configuration

AbstractThe effect of the end sill shape of steps for two-phase flow over stepped spillways was investigated. A quantitative assessment approach has been applied. Experimental and numerical tests were used for stepped spillways with different step configurations. The computational fluid dynamics (CFD) method with a two-dimensional (2D) flow model, including volume of fluid (VOF) and k-ε models was applied to conduct a critical analysis in different flow conditions. Step heights hs were 0.03 m and 0.05 m, and the number of steps Ns were 10 and 6, respectively. The step configurations assessed were Step models, Sill models, and Curve models with angle θ = 26.6°. The developed 2D model investigated the flow patterns, velocity distribution, and pressure for several step configurations. For validation, the correlation coefficient (R), mean absolute percentage error (MAPE), and root mean squared error (RMSE) were tested. The results showed negative pressure at the flat steps at Nappe flow, whilst negative pressure did not appear in the Curve model. MAPE and RMSE were 4.32 and 0.21 respectively, with R =0.996. In the CFD results, velocity and the pressure distributions were compared with the experiments and showed a correlation within a (±6%) tolerance. The Curve models were the best in terms of the flow characteristics.

DualSPHysics simulations of spillway hydraulics: a comparison between single- and two-phase modelling approaches

This technical note presents the DualSPHysics application to spillway hydraulics, and an extensive comparison has been made between the single-water and two-phase water–air modelling approaches. It has been found that the DualSPHysics code based on the smoothed particle hydrodynamics method could provide a promising design tool for spillway flows in the engineering field. The model is first validated on a documented skimming flow and the numerical results are also compared with those computed from a smoothed spillway flow. Then the model is further validated by a laboratory experiment for a nappe flow, where the computed flow depth and velocity are validated against the physical measurements. It has been disclosed that for a skimming flow, the single- and two-phase SPH modelling approaches lead to small differences, while for a nappe flow with more serious free surface motion, the two-phase model clearly improved the simulation accuracy. After the model is fully validated, it is applied to investigate a practical spillway flow under different flow discharges, bed slopes and step numbers, covering two distinct spillway flow regimes. From the simulation results, the flow energy dissipation features are investigated with regards to the different flow rates and spillway configurations. It has been found that the energy dissipation efficiency of a spillway is proportional to the number of the steps but inversely proportional to the flow discharge and the spillway slope.

Using CFD modelling to study hydraulic flow over labyrinth weirs

Abstract The compound labyrinth weir is a new type of labyrinth weir that is considered a good applicable choice for increasing the capacity of discharge. The flow over a compound labyrinth weir is a complex problem because the flow behavior is three-dimensional. The present study aims to simulate the flow over the compound labyrinth weir into the critical regions that cannot be observed when using an experimental test. The computational fluid dynamics (CFD) programme was utilised to implement a sensitive analysis for this purpose and under different flow conditions. The MAPE and RMSE indices were utilised to verify the CFD results with experimental work. The statistical indices of the maximum error ME, RMSE and MAPE were 4.7%, 0.033 and 3.9 respectively. Therefore, the findings showed that there is a good matching between the experimental and CFD results. The CFD results demonstrated that the hydraulics behaviour of the compound labyrinth weir was similar to the oblique and linear weirs in high discharges. The results also confirmed that air cavities and bubbles existed behind the nappe flow in addition to the negative pressure that may occur beneath the nappe when the flow is aerated. Furthermore, the flow was divided into two parts and most streamlines were concentrated over the notches. Moreover, the flow velocity passing through the notches was greater than the flow velocity over the high crest of the compound labyrinth weir.

Hydraulic performance of helical-step dropshaft

Abstract Aiming to achieve energy dissipation and prevention of cavitation erosion, a kind of dropshaft in urban drainage systems called the helical-step dropshaft is introduced in this paper. It dissipates flow energy by means of step geometry and prevents cavitation erosion through air entrainment. To verify its availability, the hydraulic characteristics of the helical-step dropshaft were experimentally investigated, including the flow regimes, the efficiency of energy dissipation, characteristics of air entrainment and pressure distribution. The results demonstrate that, even for a large discharge, flow can be discharged smoothly and steadily, and a high energy-dissipation rate of over 87% is achieved. There are three distinct flow regimes observed in the dropshaft, namely nappe flow, mixed flow and skimming flow. Moreover, there is no less than 1.6% air concentration and a reasonable pressure distribution on the step surface. This study provides an attractive alternative for the design of drop structures.

EXPERIMENTAL STUDY OF ENERGY DISSIPATION IN A SINGLE STEP CONDITION OVER A BROAD CRESTED WEIR

This research study the experimental performance of broad-crested weir with single-step by introducing U/S and D/S round-nose and analysing the parameters that have effect on the shape of the step and their effects on the flow characteristics and energy dissipation (E%) downstream (D/S) of the weir. Furthermore, empirical relations for E% and flow rate due to the influencing factors were derived. The results showed that the weir model with = 1.000 gives a higher E% in comparison with other weir models. Flow regimes were observed i.e nappe flow for small discharges, transition flow for intermediate discharges and skimming flow for higher discharges. Two model equations were obtained, the first to dteremine the flow rate over the weir models and the second relation to estimate E% in terms of, and Froude number. The model () can be used in the design of prototype weirs in terms of energy dissipation