Scour due to plunging water jets is a key topic in hydraulic engineering. This study presents the topography of the scour hole formed by water jets from circular nozzles in the downstream pool. Experimental studies for topographic changes in the downstream pool are limited in the literature. Several experiments were carried out to determine scour morphology due to turbulent water jets obliquely impinged on the downstream pool. The topographic changes, maximum scour depths, volume of scour hole and upstream and downstream slopes of scour hole at the equilibrium time were studied in detail. The densimetric Froude number and impingement angle affect the scour morphology. The volume of scour hole increases with the increase of densimetric Froude number and the decrease of impingement length. The topographic maps clearly show that the water jet affected the scour with the decrease of its velocity due to contact with the air at large impingement distances. Moreover, the upstream scour hole angle is approximately equal to the downstream scour hole angle. The findings will be useful in hydraulic applications such as design of plunge pool of an impinging jet spillway, bottom outlet structures, pond water aeration systems, outlets of culverts and storm drainage pipes.

  • Scour topography in detail.

  • High densimetric Froude number.

  • Different impingement angles.

Graphical Abstract

Graphical Abstract
Graphical Abstract
d

is nozzle diameter (L)

D

is bed-sediment diameter (L)

Dc

is diameter of reduction pipe (L)

D50

is 50% of sediment diameter (L)

F0

is densimetric Froude number (−) (U0/()

g

is acceleration due to gravity (LT−2)

H

is impingement length (L)

hk

is thickness of sediment layer (L)

ls

is maximum scour hole length (L)

M0

is momentum of flux

Re

Reynolds number (–)

t

is time (T)

T

is temperature (°C)

td

is tailwater depth (L)

U0

is exit velocity of water jet (LT−1)

Qw

is water discharge (L3T−1)

ws

is maximum width of scour hole (L)

qj

is unit discharge of jet (L2T−1)

x1

is distance from maximum scour to back side edge of scour hole (L)

x2

is distance from maximum scour to location where ε = 0 (L)

x3

is distance from location of maximum scour to top of ridge (L)

εm

is maximum scour depth (L)

β1

is upstream scour angle (L)

β2

is downstream scour angle (°)

λ

is scour width ratio (ws/B)

θ

is impingement angle (°)

Δm

is maximum ridge height (L)

μ

is dynamic viscosity of water (ML−1T−1)

υ

is kinematic viscosity of water (L2T−1)

ρ

is mass density of water (ML−3)

ρs

is mass density of sediment (ML−3)

Δρ

is relative mass density (−)

σg

is geometric standard deviation of sediment (−)

Outlet flow through hydraulic structures usually occurs in the form of a jet. Jet is the fluid flow of higher velocity emitting into the surrounding fluid of lower or no velocity (Rajaratnam 1976). The properties of a jet can be affected by the nozzle design, namely its geometry (nozzle type, nozzle shape, nozzle exit cross-section) and the environmental conditions surrounding the jet. When a water jet impinges on the downstream pool, a large number of air bubbles may be entrained into the water. Interactions between water and water structures, and occurring scour are complex phenomena that involve three phases: water, air, and sediment (Lee et al. 2019). Thus, the water jet creates a three-phase flow in the downstream pool with a sediment layer. The air bubbles are entrained by the water jet to a certain depth in the plunge pool.

The depth of the air bubbles in the pool is called the penetration depth. Moreover, Tastan et al. (2016) found that the air bubbles penetrate into the bed-sediment layer. Emiroglu (2010) found that the scour depth in the downstream pool decreases with increasing the jet expansion because the bubble penetration depth decreases. Water jets are originating from water structures used in civil, environmental, and other engineering disciplines. Water jets are commonly used in aeration, oxygen transfer, and flotation processes (Bagatur & Onen 2014). Novak (1994) stated that air–water flows have three distinct regions: air entrainment, detrainment, and air transport by flowing water. Laursen (1952) defined scour as the flow expansion with the transportation of the material forming the flow boundary as a result of the flow movement. The interaction of the jet with a sediment bed of sand, gravel, or weak rock generally results in scour.

The scour is of great concern in terms of safety since excessive flood water is transferred downstream of the dam by auxiliary structures. Therefore, water jets have been a topic of interest to researchers for many years. Theoretically study of the scour problem is complex. Since complete protection against scour is too expensive, the maximum scour depth and the upstream slope of the scour hole must be predicted to minimize the risk of failure (Karim & Ali 2000). Therefore, many researchers have studied the scour phenomena through experiments. Several investigations have been performed on this subject in the past few decades. Horizontal wall jets were analyzed by Laursen (1952). Scour due to jets with circular nozzles impinging a downstream pool was investigated by Rajaratnam & Beltaos (1977), Aderibigbe & Rajaratnam (1996), Pagliara et al. (2006, 2008a), Kartal (2018), Bombardelli et al. (2018), Kartal & Emiroglu (2021). Additionally, Rajaratnam & Mazurek (2002) experimentally studied the erosion of cohesionless bed material due to an inclined circular air jet. Pagliara et al. (2006) researched the effect of jet velocity, jet air content, discharge, tailwater depth, and jet shapes on the scour from impinging jets.

Successively, Pagliara et al. (2008a, 2010) studied the three-dimensional (3D) plunge pool scour and proposed various analytical relationships to estimate the main geometrical parameters of the scour hole with and without protection measures. Pagliara et al. (2008a) considered scour width ratio to examine scour in the plunge pool. They stated that 3D plunge pool scour includes scour width ratio λ = ws/B in which ws is the maximum scour hole width and B is the channel width. They found that the scour plan shape is 3D for λ < 1.50, but it is nearly 2D for λ > 3.0. They classified 3D jet scour.

Pagliara et al. (2008b) investigated temporal development of scour, defined scour hole characteristics for developed phase and found that smaller impact angles led to forming a deeper and longer scour hole. Bombardelli et al. (2018) studied temporal evolution jet scour and presented results for two different phases of scour process (developing and developed phase). The developing phase is very rapid; the scour process starts when jet is impinged to sediment surface and a ridge starts to occur. After the upstream and downstream surfaces of the scour hole are completely formed, the developing phase can be assumed completed. Then, the developed phase occurs. The developed phase refers to form by proportional expansion of both the scour hole and the ridge.

Chakravarti et al. (2013) conducted experiments to study the behavior of scour in gravel material by a submerged circular water jet. In addition, Tastan et al. (2016) examined the effect of sediment thickness on scour caused by a water jet. The authors found that the thickness of the bed-sediment layer is a vital parameter in the scour phenomena. Hou et al. (2016) investigated the scour depth caused by a submerged water jet. Kartal & Emiroglu (2021) studied local scour due to water jets from circular nozzles with and without plates. They stated that the use of plates in water jets led to increased air entrainment rate. So, it reduced scour depth as the scour hole spread to a larger area. Palermo et al. (2021) studied the time-evolution process and the equilibrium configuration of scour for a wide range of hydraulic structures. The results verified the existence of two phases in scour development (developing and developed phase) similar to the results of other studies.

Rajaratnam (1976) reported that bed material forming the base of a stream or channel is carried by a jet or flow as a result of the water jet being dislodged, fragmented, or eroded by the water jet plunging or impinging on the water surface. The hole formed by the transport of sediment in the bed layer at the bottom of the stream defines the scour geometry. Because of the friction between the water jet and air, a large amount of air enters the jet along the path from the nozzle to the downstream pool (see Figure 1). The cross-section of the jet in the air increases rapidly, and the average density and velocity of the jet flow decrease. The potential energy of the water jet in the air is transformed into kinetic energy upon impact with the water surface in the downstream pool. The changes occurring along the jet in the air have an effect on the geometric shape of the scour. It is challenging to theoretically determine the properties of the jet at the point where water jet plunges into the downstream pool. There is no analytical equation defining the jet cross-section in the air, but there are empirical and semi-analytical correlations for this cross-section variation. Generally, these empirical equations are valid for the specific conditions of the experiment conducted and vary considerably from each other. The properties of a jet at the point of plunging into the downstream pool depend on the impingement distance of the jet (distance from the water surface along the axis of the jet), gravity, the impingement angle of the jet, the viscosity and density of the fluid, the exit diameter of the nozzle, and the air conditions to which jet is exposed.

Figure 1

Jet in the air.

The submerged jet section in the downstream pool has a very complex flow environment. There is water, air, and a large number of sand grains suspended owing to the scour caused by the jet in the sediment layer. Therefore, the submerged jet section is a three-phase flow with extremely high turbulence and complexity. In addition, because of friction (viscosity) and high-velocity gradients at the outer boundaries of the jet, the vortices penetrating the submerged jet cause an increase in the jet cross-section, decrease in the jet velocity, jet energy breakage, and severe instability of the jet flow. A vortex formed in the jet environment creates strong turbulence in the place of the flow and oscillation of the flow environment (instability). Therefore, sediment grains in the sediment layer are shaken, dislodged, suspended, and then dispersed and transported (see Figure 2). This consequence of a jet in the downstream pool as a major source of scour.

Figure 2

Displacement of sand grains in the sediment layer due to the movement of the air bubble.

Figure 2

Displacement of sand grains in the sediment layer due to the movement of the air bubble.

Close modal

Tastan et al. (2016) stated that the air bubbles penetrating the sediment layer from the water jet impingement on the downstream pool have the following effects on scour: (1) an air bubble has to rise through the sand grains in the water owing to buoyancy, (2) the vertical acceleration of the bubble results in an upward dynamic force on the surrounding sand grains (Figure 2). As seen in Figure 2, the air bubble causes a sand grain to rise and move from its current location.

Hager et al. (2019) stated that the scour process consists of two stages. In the first stage, the disintegration phase, the matrix is fractured from dynamic pressure action. In the second stage, the transportation phase, lifted rock fragments or granular material are entrained by the flow and deposited at the rims of the scour area. The high velocity of the turbulent jet flow when in contact with both the air and the water in the downstream pool causes the jet flow to create a large number of strong vortices in the downstream pool. Since the turbulent jet flow creates very strong vortices in the downstream pool, it entrains large amounts of air into the water and increases residence time of this air in the downstream pool, the turbulent jet removes, suspends and carries too much sediment particles from the sediment layer in the pool. The air bubbles, which are entrained by the water jet and rapidly plunged into the downstream pool, reach the sediment layer and enter the spaces between the sediment grains and lift due to the buoyancy, while they lift and raise the sediment grains on them so that these grains can be easily dislodged and moved, suspended or cause them to be moved and taken away. Due to this effect of the air bubbles, the scour phenomena occur much more easily. Along the entire submerged length of the jet, the ambient water, vortices, and air bubbles are re-entrained into the jet flow, causing reductions in the velocity and energy of the jet flow and strong oscillations in the jet and ambient flow field in the pool. With the strong and complex turbulent flow, the dynamic pressure effects of the submerged impinging jet on the bed-sediment layer, oscillation of the entire flow field, and rising air bubbles, within the ambient flow field and scour in the bed-sediment layer, cause the sediment to be highly unstable. The sediment particles vibrate and are dislodged, dispersed, suspended, and transported by the flow (Figure 2).

Most previous studies have focused on maximum scour depth and scour geometry in water jets. However, this paper presents the topographic changes in the downstream pool, maximum scour characteristics, time-dependent changes of scour, volume of scour hole and upstream scour hole angles at the equilibrium time in the downstream pool due to water jets from circular nozzles in detail. The study aims to investigate experimentally water jets from circular nozzles and analyze scour morphology and topographic changes in the downstream pool due to water jets obliquely impinged downstream pool at different impingement angles, nozzle diameters, densimetric Froude numbers and impingement lengths under unsubmerged conditions.

Dimensional analysis

The functional relation between the maximum scour depth (εm) and the influencing variables can be written as:
(1)
where U0 is the jet exit velocity (m/s), g is the acceleration due to gravity (m/s2), ρ is the mass density of water (kg/m3), ρs is the mass density of sediment (kg/m3), (= ρsρ) is the buoyant sediment density, μ is the dynamic viscosity of water (kg/m s), D50 is the mean size of particles of bed material (m), d is the diameter of the nozzle (m), td is the tailwater depth (m), H is the jet impingement length (m), B is the width of downstream pool (m), θ is the angle of the axis of the water jet with the horizontal axis (°), t is the time (s), and σg is the sediment uniformity coefficient (σg = (D84.1/D15.9)1/2). Application of the Buckingham Π-theorem to Equation (1) by choosing ρ, U0, and H as repeating variables, leads to a functional relationship in terms of the dimensionless groups shown in Equation (2):
(2)
Equation (2) is written as dimensionless groups:
(3)
For Re greater than about 104, it is reasonable to neglect the effect of the Reynolds number on the turbulent jet flow (Rajaratnam 1976). Since the main purpose of this study was to investigate the effect of the scour morphology for different diameter of circular nozzles, the parameters of median sediment size, tailwater depth, and thickness of the sediment layer were taken as constant in the study. Scour depth changed rapidly at small values of time (t), but remained approximately constant (asymptote) when t was large enough. So, was neglected. After removing the above-mentioned parameters from Equation (3), then Equation (3) can be written as follows.
(4)
Densimetric Froude number
(5)
Jet Reynolds number
(6)
in which ν is the kinematic viscosity of water (m2/s), qj is the unit discharge (m3/s/m). Where . Plan and longitudinal section of scour geometry is given in Figure 3. where β1 and β2 are the upstream and downstream angles of scour (°), respectively. Δm is the ridge height (m), x1 is distance from maximum scour to back side edge of scour hole (m), x2 is distance from maximum scour to location where ε = 0 (m) and x3 is distance from location of maximum scour to top of ridge (m)
Figure 3

Plan and longitudinal section of scour geometry: (a) plan, (b) longitudinal section of scour geometry.

Figure 3

Plan and longitudinal section of scour geometry: (a) plan, (b) longitudinal section of scour geometry.

Close modal

The experiment setup for the present study consists of three adjoining compartments: a downstream pool, a sand-trapped region, and a feeding tank. Downstream pool width, height and length are 1.50, 1.55 and 3.00 m, respectively. The schematic view and cross-section of the experimental setup are given in Figure 4(a) and 4(b), respectively. In addition, the details of the circular nozzles used in the experimental study are shown in Figure 4(c). The geometric properties of the nozzles used in the experiments are listed in Table 1.

Table 1

The geometric properties of the nozzles used in the experiments

The geometric properties of the nozzles
d (mm)α1 (°)L1 (mm)L2 (mm)L3 (mm)
Circular nozzle 28 21 28 61.51 28 
34 34 45.32 34 
40 40 29.14 40 
The geometric properties of the nozzles
d (mm)α1 (°)L1 (mm)L2 (mm)L3 (mm)
Circular nozzle 28 21 28 61.51 28 
34 34 45.32 34 
40 40 29.14 40 
Figure 4

Experimental setup: (a) The schematic view of the experimental setup. (b) A-A cross-section. (c) Details of the circular nozzles used in the experiments.

Figure 4

Experimental setup: (a) The schematic view of the experimental setup. (b) A-A cross-section. (c) Details of the circular nozzles used in the experiments.

Close modal

In the present study, quartz sand was used as the material for the sediment layer with the following granulometric characteristics: D10= 0.84 mm, D15.9= 1.08 mm, D50= 1.28 mm, D60= 1.47 mm, D84.1= 1.92 mm, and ρs = 2,650 kg/m3, σg = (D84.1/D15.9)1/2 = 1.33, and ρs = 2,650 kg/m3. ρs is the mass density of sediment, D50 is the median size of the sediment, σg is the geometric standard deviation of sediment (the non-uniformity parameter) which is less than 1.40 for uniform sediments (Dey et al. 1995), where D84.1 and D15.9 are the sieve opening sizes through which 84.1% and 15.9% of the sediment particles pass, respectively. Grain size distribution curve of sand used in the experiments is shown in Figure 5. The quartz sand used in this study was ensured to be uniform.

Figure 5

Grain size distribution curve of sand used in the experiments.

Figure 5

Grain size distribution curve of sand used in the experiments.

Close modal

The water was recirculated by a pump, and an electromagnetic flowmeter (Krohne brand, ±0.20% accuracy) measured the discharge. Final topography measurements for the scour dimensions were performed with a gauge car that could move back and forth along a rail. The depths and heights were measured using a digital limnimeter (Mititoya brand, ±0.01% accuracy) mounted gauge car when the jet was turned off (static scour). The static scour refers to conditions when the jet flow is stopped after experiments (when pump is off), whereas the dynamic scour hole refers to conditions with the jet flow turned on.

In the study, three diameters of circular nozzles (d = 28, 34, and 40 mm) were used for the water jets (Figure 4). As shown in Figure 4, the impingement angle (θ = 30, 45, and 60°) was adjusted for the water jet using a 360° rotatable horizontal shaft mounted at the head of the downstream pool. The tailwater depth was kept constant in all experiments by means of valves and pipes placed on the experimental setup. The jet pipe was aligned with the central axis of the downstream pool. As a result of the preliminary experiments, the thickness of the sediment layer (hk = 71 cm) in the downstream pool and the tailwater depth (td = 41.2 cm) were determined through trial and error. Tastan et al. (2016) found that the maximum scour depth decreases with decreasing the thickness of the bed-sediment layer. Therefore, a sufficient thickness of the bed-sediment layer (hk = 71 cm) was ensured and then maintained in all experiments. The jet impingement distance (the distance from nozzle to the water surface along the jet centerline) H was adjusted between 15 and 30 cm by moving the pipes back and forth using special screws. When the pump (11 kW) was turned on, the water jet began to scour in the sediment layer in the plunge pool. After the water jet impinged the plunge pool (Region I), it was discharged towards the sand-trapped region (Region II) and the tank (Region III) by the effect of gravity. In this way, the water jet was pumped to the downstream pool between nine and thirteen hours until equilibrium scour occurred in each experiment under the constant flow conditions. At the end of each experiment, the release valve in the plunge pool was opened to drain the pool, and the water in the downstream pool (Region I) flowed slowly into the tank (Region III). It was ensured that the water was drained completely after each experiment. The slow drain allowed the sediment to remain in its location and not get transported to the scour hole by the flow. In this way, it was ensured that the maximum scour depth, the geometric shape, and the location of the scour hole was not disturbed while the water in the downstream pool drained. The static scour depths (when the pump is off) at the 15, 30, 60, 120, 240, 480, 540, 600th, etc. minutes were measured with the pump turned off. If the difference is close to zero according to the measured maximum scour depth at these minutes, it is assumed that the equilibrium scour conditions have been reached. So, the time when it reached the equilibrium scour depth was determined. Although there are differences between the times when it reaches the equilibrium scour, it varied between nine and thirteen hours. The depth when the relative error was nearly zero was accepted as the maximum scour depth at the equilibrium time. The depths and heights of sections in both the x- and y-directions were measured at every five centimeters of the scour hole using the digital limnimeter. Thus, the topography of the scour hole was obtained and mapped at the end of each experimental run. The three-dimensional (3D) wireframe, 3D surface, contour map, and vector map were determined using the data obtained from the measurements. Further, the volume of the scour hole (s) and upstream and downstream slopes of the scour hole under static jet flow conditions were measured in this study.

The accurate predictions of scour hole geometry are necessary for hydraulic design. The comprehensive experiments are conducted to determine the scour hole geometry for different impingement lengths (H = 0.30 and 0.15 m), impingement angles (θ = 30°, 45°, and 60°), nozzle diameters (d = 28, 34 and 40 mm), and exit velocity of water jets (U0 = 5, 7.5, and 10 m/s). Since the geometric dimensions of the downstream pool were sufficient (), the wall effect did not occur. The scour hole and ridge are not limited. So, they freely formed in the downstream pool. In order to examine the scour topography and to discuss the results in detail, fourteen figures were plotted using the experimental data.

It is seen that the maximum scour depths produced by the obliquely impinging jets were larger than would be produced by a perpendicularly impinging jet. For perpendicularly impinging jets, much of the sand stays suspended in a recirculating flow within the scour hole, which settles out when the jet is stopped (Aderibigbe & Rajaratnam 1996). However, the obliquely impinging jet has a component of flow that is parallel to the bed, allowing a greater capacity for transport of particles out of the scour hole.

The dimensionless scour depth is scattered versus F0/(H/d) for different nozzle diameters (d), impact angles (θ) and dimensionless lengths (H/d) in Figure 6. The dimensionless maximum scour depth usually increases with increase of F0/(H/d). Water jets impinged the downstream pool move the air bubbles deeper into the pool with the increase in the jet impact velocity (Emiroglu & Baylar 2003). Therefore, the maximum scour depths are larger due to the increase in the impact velocity of the water jet, in particular for great nozzle diameters and F0/(H/d) values as shown in Figure 6. However, when the pump was off at large impact angles (θ = 60o), the suspended sand grains fell into the scour hole. So, it has been observed that there are lower depths. The momentum flux of the jet at the nozzle ( increases with increasing velocity of the water jet. Canepa & Hager (2003) state that the jet velocity has a dominant effect on plunge pool scour. Thus, the scour at equilibrium time is deeper with higher densimetric Froude numbers. The water jet impinged the downstream pool and moved the air bubbles deeper into the pool with the increase in jet impact velocity. The momentum of flux increases with increase in the impact velocity. Therefore, larger scour depth values are obtained with the increase in the momentum flux of the water jet as shown in Figure 6, especially for large nozzle diameters and large F0/(H/d) values. As the air bubbles in the spaces between the sediment particles rise, they can also merge with other air bubbles or break apart, passing through small gaps (see Figure 2). The air bubbles apply forces, which are constantly changing in direction and size, to the sand particles so that the sand particles become unstable and easy to displace (Tastan et al. 2016). The comparison of dimensionless maximum scour depth with the literature is given in Figure 6. Kartal & Emiroglu (2021) studied scour produced by jets from circular nozzles with plates for impact angle θ = 45o and smaller diameter of nozzles (d = 10,15, 20 mm) and impingement distances (H = 100 and 200 mm). However, scour due to jets was studied for impact angles θ = 30, 45, 60o and larger diameter of nozzles (d = 28, 34, 40 mm) and impingement distance (H = 150 and 300 mm) in the present study. So, the momentum flux of the present study is higher than that of the study of Kartal & Emiroglu (2021). A larger volume of scour hole was obtained than that of Kartal & Emiroglu (2021). Although there are differences between hydraulic conditions of the study and the study of Kartal & Emiroglu (2021), the results are consistent with the study of Kartal & Emiroglu (2021). As Kartal & Emiroglu (2021) stated, the dimensionless scour depth increases with the increase in F0/(H/d).

Figure 6

The dimensionless maximum scour depth versus F0/(H/d).

Figure 6

The dimensionless maximum scour depth versus F0/(H/d).

Close modal

The similarities of scour profiles for different diameters, exit velocities, and impingement lengths are given in Figure 7(a) and 7(f). As shown in Figure 7(a) and 7(f), the scour hole profiles are similar to each other under different conditions. The results are compatible with the studies of Rajaratnam et al. (1995), Rajaratnam & Mazurek (2002), and Kartal & Emiroglu (2021).

Figure 7

The similarities of scour hole profiles for different conditions: (a) θ = 30° and H = 0.30 m, (b) θ = 30° and H = 0.15 m, (c) θ = 45° and H = 0.30 m, (d) θ = 45° and H = 0.15 m, (e) θ = 60° and H = 0.30 m, (f) θ = 60° and H = 0.15 m.

Figure 7

The similarities of scour hole profiles for different conditions: (a) θ = 30° and H = 0.30 m, (b) θ = 30° and H = 0.15 m, (c) θ = 45° and H = 0.30 m, (d) θ = 45° and H = 0.15 m, (e) θ = 60° and H = 0.30 m, (f) θ = 60° and H = 0.15 m.

Close modal

The longitudinal sections of the scour at the equilibrium time for d = 28 mm, different impact angles, and impingement lengths are shown in Figure 8(a) and 8(b). Kartal & Emiroglu (2021) said that ridge height becomes further with the increase of impact velocity. As they said, the ridge height occurred further (in the direction of flow) with high exit velocities and small impingement lengths. The examinations of the longitudinal sections showed variation in the impact angle, exit velocity and impingement length which affected scour geometry.

Figure 8

The asymptotic scour profiles in the plane of symmetry: (a) H/d = 10.71, (b) H/d = 5.36.

Figure 8

The asymptotic scour profiles in the plane of symmetry: (a) H/d = 10.71, (b) H/d = 5.36.

Close modal

Scour topography

The scour hole formed by the water jet was mapped as a function of the various parameters according to the experimental data. As the velocity of the water jet increases, the momentum of the jet also increases. The studies in the literature show that the penetration depth increases with the jet velocity (Emiroglu 2010). Thus, the water jet plunging into the downstream pool caused more scour depth at higher jet velocity. Bagatur & Sekerdag (2003) reported that the number of air bubbles being moved to the downstream pool also increased with the velocity of the water jet. In the experiments conducted for the impingement angle of θ = 60°, the water jet plunging into the downstream pool suspended more sediment compared to other angles. Air bubbles entering the bed material lifted the sediment. Thus, air bubbles suspend the sediment similarly to a boiling state. In the case of static scour (when the pump is off), the suspended sediment falls onto the bed material due to gravity.

Scour topography due to water jets from circular nozzles under different conditions are shown in Figures 919: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of the experiment. The figures show that the scour hole formed the smallest area with the experiments conducted at θ = 60°. Larger scour widths were obtained with small impingement angles and large nozzle diameters. The reason why the scour hole width was small at low densimetric Froude number values, especially at θ = 30°, could be a decrease in kinetic energy with the longer travel of the water jet. In addition, the width of the scour hole for θ = 60° was smaller compared to other angles because the sediment was suspended and not transported. As shown from Figures 919, the topography of the scour hole varied with the jet impingement angle. Jet shape, jet expansion, jet length, and jet velocity at the plunging point were vital parameters affecting dimensions of the scour hole. A ridge formed towards the end of the downstream pool at small impingement angles, while a ridge formed closer to the plunging point at large impingement angles. Figures 919 also indicate that the downstream slope of the scour hole formed at θ = 60° was greater than the other tested jet impingement angles. The scour morphology obtained for small impingement angles was unique compared to the other experiments tested. At small impingement angles, air bubbles entrained by the flow caused the sediment to lift and the scour hole to spread over a large area. Consequently, the scour morphology is extremely influenced by the F0, F0/(H/d) and θ. Pagliara et al. (2008a) stated that a scour hole tends to the circular shape for large jet angles but it is more elliptical otherwise. Similarly, the results show that the scour hole due to the water jet for large jet angles is circular in shape as shown in Figures 919. Sediment accumulated not only downstream of the scour hole, but also around the side wall with the increase in the densimetric Froude number, as shown in Figures 9, 11, 16, 17, and 19. Therefore, the ridges were formed in the side wall on both sides of the scour hole. Pagliara & Palermo (2013) classified scour hole types. The authors stated that scour hole geometry is characterized by an approximately circular shape of the scour hole and a ridge which is not extended in length but it can either partially or totally surround the scour hole. The accumulation of sediment in the side wall was especially evident at θ = 45°, large nozzle diameters and high Froude numbers (e.g., F0 = 69.48). In addition, small scour holes were formed in the side wall at small impingement angles (e.g., θ = 30°). Although the ridges occurred in both x- and y-directions, the largest ridge heights were obtained at the downstream of the scour hole along the longitudinal section where the scour depth was maximum. Moreover, no secondary ridge was observed on either side of the main ridge in the present study.

Figure 9

Scour topography due to jets for F0 = 69.48, U0 = 10 m/s, θ = 45°, d = 40 mm, H = 0.30 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Figure 9

Scour topography due to jets for F0 = 69.48, U0 = 10 m/s, θ = 45°, d = 40 mm, H = 0.30 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Close modal
Figure 10

Scour topography due to jets for F0 = 52.11, U0 = 7.5 m/s, θ = 60°, d = 40 mm, H = 0.30 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Figure 10

Scour topography due to jets for F0 = 52.11, U0 = 7.5 m/s, θ = 60°, d = 40 mm, H = 0.30 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Close modal
Figure 11

Scour topography due to jets for F0 = 34.74, U0 = 5 m/s, θ = 60°, d = 34 mm, H = 0.30 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Figure 11

Scour topography due to jets for F0 = 34.74, U0 = 5 m/s, θ = 60°, d = 34 mm, H = 0.30 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Close modal
Figure 12

Scour topography due to jets for F0 = 34.74, U0 = 5 m/s, θ = 30°, d = 34 mm, H = 0.30 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e) photograph of experiment.

Figure 12

Scour topography due to jets for F0 = 34.74, U0 = 5 m/s, θ = 30°, d = 34 mm, H = 0.30 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e) photograph of experiment.

Close modal
Figure 13

Scour topography due to jets for F0 = 52.11, U0 = 7.5 m/s, θ = 30°, d = 28 mm, H = 0.30 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Figure 13

Scour topography due to jets for F0 = 52.11, U0 = 7.5 m/s, θ = 30°, d = 28 mm, H = 0.30 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Close modal
Figure 14

Scour topography due to jets for F0 = 34.74, U0 = 5 m/s, θ = 30°, d = 34 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map.

Figure 14

Scour topography due to jets for F0 = 34.74, U0 = 5 m/s, θ = 30°, d = 34 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map.

Close modal
Figure 15

Scour topography due to jets for F0 = 34.74, U0 = 5 m/s, θ = 30°, d = 40 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Figure 15

Scour topography due to jets for F0 = 34.74, U0 = 5 m/s, θ = 30°, d = 40 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Close modal
Figure 16

Scour topography due to jets for F0 = 52.11, U0 = 7.5 m/s, θ = 45°, d = 40 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Figure 16

Scour topography due to jets for F0 = 52.11, U0 = 7.5 m/s, θ = 45°, d = 40 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Close modal
Figure 17

Scour topography due to jets for F0 = 69.48, U0 = 10 m/s, θ = 45°, d = 28 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Figure 17

Scour topography due to jets for F0 = 69.48, U0 = 10 m/s, θ = 45°, d = 28 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Close modal
Figure 18

Scour topography due to jets for F0 = 69.48, U0 = 10 m/s, θ = 60°, d = 28 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e) photograph of experiment.

Figure 18

Scour topography due to jets for F0 = 69.48, U0 = 10 m/s, θ = 60°, d = 28 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e) photograph of experiment.

Close modal
Figure 19

Scour topography due to jets for F0 = 69.48, U0 = 10 m/s, θ = 60°, d = 34 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Figure 19

Scour topography due to jets for F0 = 69.48, U0 = 10 m/s, θ = 60°, d = 34 mm, H = 0.15 m conditions: (a) 3D wireframe, (b) 3D surface, (c) contour map, (d) vector map, (e, f) photographs of experiment.

Close modal

Table 2 shows the upstream and downstream slopes. In all the experiments, the upstream of the scour hole yielded a steeper slope than the downstream section. Experimental results of Dey & Sarkar (2006) are consistent with these findings. At low densimetric Froude number values and small impingement angles (θ = 30°), the upstream and downstream slopes of the scour hole are smaller than those at larger values of these two parameters. Approximately, the upstream slope is equal to the downstream slope. Hoffmans (1998) found that the upstream scour angle β for a free overfall jet (θ > 60°) approximately equals the downstream scour angle. Therefore, it can be clearly stated that the slopes of the scour hole for water jets are compatible with those of free overfall jets. The upstream slopes for 30°, 45°, and 60° were approximately 0.25, 0.49, and 0.54, respectively. The upstream scour angle increases with an increasing impingement angle, but the trend is not linear.

Table 2

Upstream and downstream slopes of the scour hole

Figuresd (mm)H (mm)θ (°)F0 (−)Upstream slope (tgβ1)Downstream slope (tgβ2)
Figure 9  40 300 45 69.48 0.42 0.39 
Figure 10  40 300 60 52.11 0.53 0.41 
Figure 11  34 300 60 34.74 0.51 0.48 
Figure 12  34 300 30 34.74 0.22 0.21 
Figure 13  28 300 30 52.11 0.24 0.22 
Figure 14  34 150 30 34.74 0.28 0.26 
Figure 15  40 150 30 34.74 0.26 0.24 
Figure 16  40 150 45 52.11 0.51 0.43 
Figure 17  28 150 45 69.48 0.53 0.49 
Figure 18  28 150 60 69.48 0.58 0.42 
Figure 19  34 150 60 69.48 0.55 0.40 
Figuresd (mm)H (mm)θ (°)F0 (−)Upstream slope (tgβ1)Downstream slope (tgβ2)
Figure 9  40 300 45 69.48 0.42 0.39 
Figure 10  40 300 60 52.11 0.53 0.41 
Figure 11  34 300 60 34.74 0.51 0.48 
Figure 12  34 300 30 34.74 0.22 0.21 
Figure 13  28 300 30 52.11 0.24 0.22 
Figure 14  34 150 30 34.74 0.28 0.26 
Figure 15  40 150 30 34.74 0.26 0.24 
Figure 16  40 150 45 52.11 0.51 0.43 
Figure 17  28 150 45 69.48 0.53 0.49 
Figure 18  28 150 60 69.48 0.58 0.42 
Figure 19  34 150 60 69.48 0.55 0.40 

Pagliara et al. (2008a) stated that scour hole volume depends on the densimetric Froude number F0 and jet impact angle θ. Scour hole characteristics (maximum scour depth, ridge height, scour hole length, scour hole width and volume of scour hole) due to plunging water jets are given in Table 3. It can be seen in Table 3 that densimetric Froude number, impact angle, impingement distance and dimeter of nozzle were important parameters on scour hole volume. Scour volume is increased with increase of densimetric Froude number under the same conditions and also decreased with the increase of impingement length. Scour volumes are small for large impact angles, because of that suspended sediment is not transported and when the pump is off sediment grains fall into the scour hole. Since plunging water jets are taken longer distances for small exit velocities under small impact angles and large impingement distance, the kinetic energy of jets is broken so scour volume is small.

Table 3

Values of scour hole characteristics due to plunging water jets

θ (o)d (mm)F0 (–)H (mm)εm (mm)Δm (mm)ls (mm)ws (mm)s x106 (mm3)
30 28 52.11 300 153 154 1,218 797 45.4 
30 34 34.74 150 127 145 865 666 24.8 
30 34 34.74 300 93 94 821 560 11.4 
30 40 34.74 150 155 146 1,055 877 48.83 
45 28 69.48 150 241 214 1,090 912 71.6 
45 40 52.11 150 271 216 1,160 987 89.1 
45 40 69.48 300 287 257 1,511 1,108 159.5 
60 28 69.48 150 134 114 875 694 27.1 
60 34 69.48 150 144 129 1,040 817 50.4 
60 34 34.74 300 210 154 778 732 30.6 
60 40 52.11 300 176 133 1,018 873 51.8 
θ (o)d (mm)F0 (–)H (mm)εm (mm)Δm (mm)ls (mm)ws (mm)s x106 (mm3)
30 28 52.11 300 153 154 1,218 797 45.4 
30 34 34.74 150 127 145 865 666 24.8 
30 34 34.74 300 93 94 821 560 11.4 
30 40 34.74 150 155 146 1,055 877 48.83 
45 28 69.48 150 241 214 1,090 912 71.6 
45 40 52.11 150 271 216 1,160 987 89.1 
45 40 69.48 300 287 257 1,511 1,108 159.5 
60 28 69.48 150 134 114 875 694 27.1 
60 34 69.48 150 144 129 1,040 817 50.4 
60 34 34.74 300 210 154 778 732 30.6 
60 40 52.11 300 176 133 1,018 873 51.8 

In this study, the topography of the scour formed in the downstream pool by a water jet from a circular nozzle with different flow conditions was experimentally investigated. The findings are summarized below:

  • At small densimetric Froude number and impingement angle values, the upstream and downstream slopes of the scour hole are smaller than those at the larger values of these two parameters. In all experiments, the upstream slopes are slightly greater than the downstream slope values. Approximately, the upstream scour angle is equal to the downstream scour angle. The upstream slopes for 30°, 45°, and 60° were approximately 0.25, 0.49, and 0.54, respectively. Upstream slope values increase with increasing impingement angle, but the trend is not linear.

  • Jet thickness, nozzle diameter, jet width, impingement distance, densimetric Froude number, and jet impingement angle are vital parameters for the scour morphology formed in the downstream pool.

  • The area of the scour hole is smaller at small impingement angles and F0/(H/d) values than those tested at other angles.

  • The topographic maps clearly show that the water jet affected the scour with the decrease of its velocity as a result of contact with the air at a large impingement distance.

  • As a result of the water jet entraining more air bubbles into the flow at small impingement angles, the sediment grains are lifted, causing the scour hole to spread over a wider area.

  • The greatest differences between static scour (when the pump is off) values and dynamic scour (when the pump is on) values are in the experiments performed for large impingement angles. Approximately 65% of the scour hole formed due to the water jet occurred within the first 30 minutes.

  • The volume of scour hole increased with the increase in the densimetric Froude number and decrease in the impingement length under the same conditions.

The financial support for this work was provided by Firat University Scientific Research Projects (FUBAP). Its project number was MF.17.38.

Data cannot be made publicly available; readers should contact the corresponding author for details.

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