Flow pattern formed over steps at a low-head dam: Salmon River Dam

ABSTRACT Building Stepping way over the low-head dams has been proposed to reduce the risk of drowning in submerged flow conditions. In the Salmon Barrier, adding two steps downstream of weir was previously suggested as an effective method for reducing the risk of drowning downstream of this dam. Given the effect of steps on the flow pattern downstream of the weir, the occurrence of dangerous submerged flows at different tailwaters was predicted, and the flow pattern expected at variable tailwaters was studied in comparison with the flow pattern passing over the sharp crested weir. According to the results, the submerged flow pattern downstream of the dam is a combination of the submerged flow pattern over the abrupt drop at low tailwaters and that over the sharp crested weir at high water levels. Consequently, similar correlations for predicting the flow regime downstream of these structures can be presented to predict the flow pattern at different tailwaters. The flow pattern over the step was compared with that over the abrupt drop, and a few diagrams were presented for predicting the occurrence of different flow patterns based on the submergence ratio of steps. Given the similarity of the flow pattern with those in the literature for sharp crested weirs, the flow pattern was also investigated at higher tailwaters. The comparison of correlations developed in this study indicated that the flow pattern approaches that reported in the literature for sharp crested weirs with increasing discharge.


Introduction
Although an exact definition of short dams is not given, short dams are typically defined as dams that are less than 25-ft.tall.Recent investigations have demonstrated that a certain flow regime that occurs more frequently in these dams is to blame for several drowning occurrences that have been documented downstream of these dams.For instance, 527 drowning deaths have been attributed to deadly aquatic eddies downstream of these dams in the United States alone (Kern, 2014).
Due to their small size, these sorts of dams typically have an overflow that is constructed as a sharp or broad edge without a control structure across the river.Although the flow appears to be rather quiet, the hydraulic leap downstream of the spillway is submerged when the downstream water level is high.The underwater submerged eddy is composed of an upstream flow, followed by an overflowing downstream flow and an underwater jet.A lot of air bubbles are introduced into the water by this current, and the combination of this strong rotating current and the weakening of the lifting force brought on by the reduced density of the water as a result of the air's entry into the water creates hazardous situations that trap swimmers or those who have fallen into the water and prevent them from escaping (Leutheusser & Birk, 1991).Even experienced swimmers may not be able to escape this eddy current, which is sometimes referred to as a submerged hydraulic jump or just hydraulic (Leutheusser & Birk, 1991).
Due to the risks associated with these types of dams, a lot of research has been done in recent years to understand the flow downstream of these types of dams, many of them have been demolished, and research has also been done on ways to lower the risk of eddy currents in the downstream of the overflow of these dams (Hotchkiss & Brik, 2001).Making a staircase downstream of the overflow of these dams is one of the methods suggested to correct the eddy flow, and reducing the size and strength of the vortex or changing the flow pattern are two methods that have been investigated in previous studies to reduce the risk of this type of overflows (Garcia, Marr, & Rogacs, 2005;Hotchkiss & Brik, 2001;Mazurek et al., 2008).The designed steps in this type of dam are submerged when the water level downstream of the spillway is high, even though stepped dams are typically designed for free-flow conditions.Depending on the height of the water level downstream of the spillway, different flow conditions may be observed downstream or on each step (Mazurek et al., 2008).It is necessary to investigate the behavior of the flow on the stairs in a wide range of downstream water-level conditions and to identify the types of possible flows at various heights of the downstream water level to assess whether these steps have eliminated the risk of drowning in a dam.
The stepped spillway of the Salmon Dam in Ontario, Canada's Salmon River is examined in this research, and the flow pattern downstream of this dam's spillway is assessed at various heights of the downstream water level.It was suggested to create the spillway of this dam as a technique to enhance the flow pattern since this dam's unique design has caused the condition of dangerous submerged eddy flow to exist in a wide range of its operating period.This spillway was built as a part of the lampari control system.This dam's spillway is 48.6 m wide, with the crest of the spillway rising approximately 1.94 m above the river's bed. Figure 1 (Mazurek et al., 2008) illustrates further information on the dam's design and placement.Due to the dam's role as a sea eel control dam, a height difference of 300 mm between the dam's crest and the water level downstream is maintained when fish migrate, and this unique design has led to the occurrence of submerged hydraulic jump flow in a variety of situations throughout the dam's operation.
In previous studies, it has been suggested to add two steps to the downstream portion of the dam, which is intended to generate a submerged plunging flow regime with a supercritical flow regime on the steps during operation during fish migration, to lessen the possibility of underwater eddies.This strategy is the greatest way to completely remove the risk of drowning in certain circumstances.According to Mazurek, the supercritical flow regime creates this plunging flow under the unique parameters of the site's downstream water level (Mazurek et al., 2008).
The flow pattern downstream of the stepped spillway will not be continuous if the water level in the downstream area is fluctuating, though.The type of flow pattern downstream of this weir, which is a stepped weir, and comparison of the patterns observed in this weir with the patterns presented in previous studies downstream of a step as well as the patterns presented in the broad edge or sharp edge weirs, can be used to predict the flow conditions in various operations of these types of dams for the various heights of the downstream water level.This article evaluates the flow pattern seen in the steps under various downstream water-level levels and compares the flow regime downstream of this type of weir to other types of weirs, including sharp-edge weirs.Physical modeling of the Salmon River short dam's overflow section has been used in this work.

Salmon River Dam model
As previously noted, the Salmon River Dam was chosen as the investigated stepped dam since it is one of the dams constructed with a lamprey pest management system.The part chosen for modeling was section C -C, which receives the majority of the flow when the dam is operating.This dam has three separate sections throughout its breadth.According to earlier research, there was submerged hydraulic jump flow for 98% of the operation period, while the spillway was in operation, and the flow rate of the flow going over the spillway during operation ranged between 1.89 and 66.9 cubic meters per second (Mazurek et al., 2008).The design discharge during dam operation has been explored in the range of 13-35 L per second (2.81 to 7.56 cubic meters per second at the site), which has existed in 80% of the time this dam has been used.This is in accordance with laboratory constraints.In earlier studies, two steps were added downstream of the spillway owing to the possibility of submerged eddy current downstream of this dam and to lower its risk.The design calls for a drop flow with a supercritical flow regime on the stairs when the flow is free and the downstream water level is low.These steps have significantly lessened the Figure 1.The full-scale geometry for step configuration on Salmon River barrier dam (Mazurek et al., 2008).
development of harmful underwater circulation in this dam, according to the findings of a study (Mazurek et al., 2008).

Dimensional analysis
To develop the mathematical relationships between the flow rate and depths at the Salmon River Dam and the studied smaller scale sections of that, first a theoretical relationship between the flow depth just upstream of the dam and the flow depth at the crest was developed.This was done using the theory for compound weirs (Salmon River dam has several notches).The flow rate at the weir was then related to the flow depth just upstream of the weir.The associated flow depth downstream of the weir was then related to this flow rate.Next, a trial length scale for the model was chosen as L r = 6, where L r is the ratio of a length in the full-scale dam to the same dimension in the scale model.
For this scale, since the laboratory flume where the tests were to be run is 0.8 m wide, and the ratio of the cross-sectional area of flow at the dam crest in the modeled section of the weir to that of the total flow was assessed.To find the flow rate in the modeled section of the barrier at the scaled-down size, for Froude models it is known that: where Q f is the flow rate at full-scale and Q m is the flow rate through the same section of the scaled-down model.Similarly, the distance to the water surface as compared to the crest of the weir, y t , in the full-scale barrier and in the scale model is given by: According to hydrological studies, the daily discharge of Salmon Dam ranges from 1.89 to 66.9 m3/s (Mazurek et al., 2008).The gravitational force is dominant according to flow characteristics, and the similarity of the Froude number in the model and the original sample should be considered.To this end, the experimental discharge should be in the range of 6.46-79.5 L/s for hydraulic simulation, considering the model scale.Given experimental limitations, the discharge was considered in the range of 13.82-45.38 (11.90-38.29 m3/s in the site), which had respective exceedance probabilities of 31.5% and 85% in Salmon Dam site (Mazurek et al., 2008).
The area where submerged jump takes place downstream of the dam (SI) can be theoretically assumed as a function of a set of flow hydraulic parameters, as shown by: where Q is the flow rate, g is the acceleration of gravity, r w is the water viscosity, P is the dam height, Y t is the tailwater depth.t 1 and t 2 are the water depth above the steps.The rest of the parameters are specified using Figure 2. According to the literature on the flow on stepped weirs under a falling regime and the studies on the submerged hydraulic jump, the dimensionless parameters can be defined as follows: Figure 2. The experimental setup.
It should be noted that the fixed dimensionless parameters in these tests and the change of water viscosity due to air entrainment were ignored.

Experiments
In the hydraulic laboratory of the Faculty of Engineering, University of Saskatchewan, experiments have been conducted in a lab flume that is 10 m long, 80 cm wide, and 70 cm high.The actual model is built of wood and is scaled at 1:6.Water height characteristics were measured during the tests at the channel's middle, upstream of the spillway, on its crest, on the stairs, and downstream (Figure 1).Additionally, in each experiment, the flow is photographed once the circumstances are set, and several necessary parameters, such as the position of the leap, the size of the vortex, the size of the wave, and so on, have been extrapolated from them.Additionally, model dolls have been used to explore the vortex's state, and researchers have looked into how someone may become trapped inside.Figure 2 displays the model's schematic and the channel that was employed.The flow rate was measured using a flowmeter with an accuracy of ± 0.5% during experiments with 3 distinct flow rates at various heights of the downstream water level.Figure 2 displays a few of the metrics used in this study.

Classification of submerged flow pattern in sharp edge spillway
Chow published the initial research on submerged hydraulic jumps.These research' findings suggest that there are four different sorts of hydraulic jump patterns that can happen based on the height of the water level downstream and the type of overflows being employed (Chow, 1959).These patterns include submerged jump, swept-out jump, ideal jump, and optimal jump.Rajaratnam and Muralidhar presented connections for the flow pattern in short weirs in the surface jet pattern mode employing correlation of laboratory data in other research by examining various forms of flow patterns downstream of sharp-edged weirs (Rajaratnam & Muralidhar, 1969).In addition, in 1991, Leutheusser and Brik analyzed the flow patterns that Chow had provided and laid out connections for different kinds of flow patterns.Following sharp edge weirs, Wu and Rajaratnam separated the submerged flow into two impinging regimes and surface flow regimes.The four flow regimes of impinging jet, surface jump, surface wave, and surface jet were defined for different amounts of flow absorption downstream of sharp edge weirs in submerged hydraulic jump mode based on the change in the speed of the surface flow relative to the main current passing over the weir as well as the direction of the eddy current created downstream of these weirs and defined correlations utilizing laboratory data correlation to categorize surface flow and impinging (Figure 3) (Wu & Rajaratnam, 1996).
The effects of various submerged flow patterns and erosion control techniques downstream of short spillways were given by Freeman and Garcia at various heights of the downstream water level (Freeman & Garcia, 1996).As one of the suggested techniques to slow the rate of erosion, these studies looked at the use of stairs downstream of the spillway and the decrease in flow power by utilizing stepped spillways.They also looked at the flow patterns in various phases of the design of the stairs.According to the importance of upstream flow speed in swimmers' ability to escape eddy currents (Leutheusser & Fan, 2001), investigated the submerged current pattern downstream of sharpedged weirs.Using the correlation between the data, they presented relations to determine the velocity of the upstream flow in the submerged hydraulic jump in two states of increasing or decreasing the downstream water level a. Mazurek et al. (2008) explored several strategies for eradicating the hazardous submerged hydraulic jump flow pattern in short dams in the Lake Ontario eel pest management system and proposed strategies to lessen the risk of short dams in this system.Salmon Dam was identified as the most hazardous dam in this system, and building two steps downstream of the dam's overflow was among the recommendations made in this study to lower the risk of drowning.Beygipoor and Mazurek's further research, however, indicated that this approach is insufficient to reduce the risk of drowning in all downstream water-level values (Beygipoor & Mazurek, 2021).
For determining the quantity of flow going over the weir in submerged flow mode for various flow rates and absorption percentages downstream of sharpedge weirs, Azimi et al. looked at the correlations given in earlier studies.Additionally, they explored the various wave characteristics in this flow regime, developed relationships to forecast the flow pattern in this condition, and concentrated on the surface wave flow downstream of these spillways (Figure 3).The correlations shown in Figure 3 may be used to compute the waveform flow parameters downstream of the sharp edge spillway based on the findings of this study.These connections display the wave's parameters as a t/h relationship (Azimi, Rajaratnam, & Zhu, 2016).
To lower the risk of drowning in the impinging jet flow mode (McGhin, Hotchkiss, & Kern, 2018), developed a solution based on the alteration in the eddy flow pattern in the spillway of short dams.Accordingly, the establishment and growth of hazardous upstream flow in this location can be stopped by adding appendages to the downstream body of the spillway and disrupting the flow pattern downstream of the spillway.Beygipoor and Mazurek (2021) looked into how the flow pattern changed in the spillway of the short dam of the Salmon Dam and demonstrated how adding wedges to the surface of the stepped weir reduces the dangers downstream of this weir by increasing turbulence and changing the flow pattern from the vertical vortex to the horizontal vortex.As previously mentioned, the majority of studies on submerged flow patterns have been carried out in conventional weirs with a sharp or wide edge.However, using these types of weirs in practice is challenging due to implementation issues, and the design of conventional Oji spillways may also be prohibitively expensive and uneconomical.Long-step spillways are a good alternative for safe current passage over short overflows (Chanson) because they have more stable conditions and have been accepted as a method to lower drowning risk in previous studies.They are also simple to install and reasonably inexpensive (Chanson, 2002).However, the majority of the relationships offered for the design of stepped spillways are presented under free flow conditions, and the presented relationships should be reexamined in the circumstances of high downstream water levels and the development of submerged flow.Freeman and Garcia (1996) have been suggested as a feasible approach to promote safety in short dams since previous studies have shown that under the conditions of downstream inundation, due to the decrease in the height of the flow drop, the power of the underwater vortex, and its dimensions are decreased.The complicated circumstances of altering the flow pattern and the simultaneous impact of the flow decrease on the step and submerged hydraulic leap, on the other hand, have received less attention in prior studies.

Flow pattern in stepped overflows in free and submerged state
Free flow patterns in stepped, wide-edged, and sharpedged weirs have already received extensive study.The study's findings show that the pattern of falling flow in a stair spillway's steps is quite similar to the pattern of flow over a breakwater, and the correlations offered for the construction of stair spillways are also given in light of this resemblance (Bhatta, 2019).Additionally, these connections were used to build the Salmon Dam spillway such that in the free flow mode, the flow pattern on the steps is dropping (Chanson, 2002).Rand (1995) offered relationships for the flow parameters in the downstream step in the event of a falling flow based on the upstream flow's characteristics (Figure 4).In light of these equations (Chanson, 2002): These equations apply when the flow in the upstream step is subcritical, and they may be used to compute the characteristics of the flow in the downstream step when the flow in the upstream step is supercritical (Chanson, 2002): ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi where Fr is the flow Froude number on the upstream step.
Submerged flow downstream of stepped dams has not received much attention, but studies have been conducted and the flow patterns and conditions of their formation downstream of these structures have been described to design a safe route for the movement of boats in rough paths and downstream of sudden drops in these paths, which have conditions similar to a staircase.In his description of the submerged flow pattern downstream of breakwaters, Hsu separated the flow pattern into five sections (Figure 5).He claimed that the hydraulic leap flows upstream of the step for water depths more than zone 1, while the flow moves downstream for water depths less than zone 5.In regions 2 and 4, the hydraulic jump is steady, whereas area 3 features waves that are unstable and do not shatter like the waves in areas 2 and 4 (Bhatta, 2019).Hsu et al. examined the flow in regions 2 and 4 using the momentum relation (Hsu et al., 1950).
A similar categorization for submerged flow patterns downstream of breakwaters was offered by Moore and Morgan in 1959 (Caisley, Bombardelli, & Garcia, 1999a).They divided the jump pattern into two groups, A and B, which corresponded to areas 2 and 4 of Hsu's investigations, and they categorized the wave pattern as belonging to region 3.However, their model research also included a flow that stretched horizontally, which Hsu's calculations did not take into consideration.From this point on, it is presumable that region 2 represents the A leap, region 3 represents the wave, and region 4 represents the B jump.To predict the sort of surge that would emerge at a specific Froude number and relative downstream depth, Moore and Morgan presented flow patterns as a function of dimensionless Froude number and downstream depth (Caisley, Bombardelli, & Garcia, 1999b).The flow patterns for five different modes -A-jump, Wave-jump, Wave train, B-jump, and Minimum B-jump downstream of an abrupt drop -are shown in Figure 6 (Ohtsu & Yasuda, 1991).
In 1977, Rajaratnam and Ortiz investigated the wave jump.According to this research, the downstream back pressure causes the upstream supercritical flow for this flow pattern to be redirected upward into a waveform.The jet then impacts the downstream riverbed as it dips into the water below (Figure 7) (Caisley, Bombardelli, & Garcia, 1999a).In recent years, mathematical and computational tools have been used to study the links found in the subject of leaping patterns downstream of rapid dips (Padova, Mossa, & Sibilla, 2017).Ishikawa, Takahashi, and Ohtsu (2019), by studying the jumps in the free jump, the submerged jump, and the B-jump, presented new equations to determine the length of the jump under these conditions (Ishikawa, Takahashi, & Ohtsu, 2019).Wahl and Svoboda presented a spreadsheet for predicting dangerous underwater roller at the site of low-head dams so that the dangerous conditions can be recognized by people.
Figure 5. Types of jump behavior at an abrupt drop (Hsu et al., 1950).

Description of the flow patterns observed in the stepped spillway of the Salmon Dam in a submerged jump state
The sorts of patterns stated in the circumstances of hydraulic jump absorption may be seen downstream of the Salmon Dam, regardless of the quantitative study of the patterns that exist in the spillway of the stepped dam of the Salmon Dam as stepped spillway under submerged flow conditions.An effort has been made to derive relations to forecast the flow pattern downstream of a stepped spillway based on the conditions under which these patterns formed.
According to the flow hydraulics in a spillway, when the flow flows over the spillway, a drop flow is generated on the steps, and the flow regime on the steps is supercritical.Upstream of the spillway, the flow is calm in the free flow condition.As a result, the upstream flow conditions in the first step and the lower stages will differ.The downstream flow pattern analysis took these various factors into account, and the flow conditions were looked at in light of the   (Caisley, Bombardelli, & Garcia, 1999a).upstream flow conditions.The hydraulic leap at the channel bottom advances upstream as water levels rise downstream of the dam, and a submerged hydraulic jump forms at the bottom of the channel just downstream of the first step.Impinging jet flow patterns, the development of a submerged leap downstream of the lowest overflow step, and surface wave flow patterns are all seen as the water depth increases (Figure 8).Even though this flow's characteristics may be distinct from those described downstream of a sharp-edged spillway due to the presence of steps and various conditions in the flow regime upstream, the patterns are essentially the same, with the exception of the differences in the wave's shape and properties.The leap is transmitted to the upstream step when the water level in the downstream step rises and identical patterns develop on the upper step.Similar requirements apply to the flow on the top step, but the upstream flow regime is different.Figure 7 depicts the measured flow patterns with increasing water depth at constant discharge downstream of the Salmon Step Spillway.This figure shows that the flow patterns seen downstream of the stairs are similar in appearance and that the flow from the spillway has the same patterns as the flow from a spillway with a sharp edge on the highest step and immediately downstream of the spillway.In this instance, four patterns that resemble the aforementioned patterns can be seen downstream of the spillways with sharp edges (Figure 8).

Features of the flow downstream of the Salmon Dam overflow in free flow mode
As previously indicated, previous research on submerged hydraulic jump has primarily been done in sharp-edge weirs, even though, under some circumstances (for instance, when h 0 /L > 2), the flow behavior in finite-crest weirs is identical to that of sharpedge weirs (Azimi and Rajaratnam, 2009).However, the Salmon Dam spillway is categorized as a broadedge spillway in the analyzed flow range.As a result, the accuracy of the connections offered in the sharp edge spillways as well as the likelihood of making a mistake when applying them to the broad salmon spillway has first been assessed.The following equation may be used to determine the flow passing over a weir in free flow downstream in both sharp-edged and wide-edged weirs (Hager, 1986).
In this relation, C d is the coefficient of flow passing over the spillway, B is the width of the spillway, g is the acceleration of gravity, and h 0 is the hydraulic load on the spillway, which is measured at a distance of 3 h 0 -4 h 0 upstream of the spillway, and C d is the discharge coefficient, which is calculated from the Rehbock equations in sharp edge overflows (Hager, 1986).
Also, the following equation may be used to compute the water depth parameter at the overflow crest's sharp edge (Rouse, 1936).
Although the relationship of sharp-edged weirs can be employed with excellent precision for narrowedged weirs when (h 0 /L)>2, such circumstances do not occur for the spillway under study at the examined flow rates, and the steep upstream of the spillway will also have an impact on the flow rate coefficient.The connection of the discharge coefficient is derived in the Salmon Dam spillway for the examined discharges for the free flow on the spillway.
Furthermore, the critical depth and the depth of water on the edge of the spillway crest have the following relationships since the wide spillway edge causes less water to accumulate there than the sharp edge does.
Figure 9.The relationship between the discharge coefficient and the relative height on the weir (h0/P) and the depth of the water on the crest compared to wide-edge weirs.The connection between the flow coefficient and h 0 /P in the Salmon Dam spillway for various flow rates is shown in Figure 9.This graph shows that the Salmon Dam spillway's flow rate coefficient in the free flow mode was lower than the relationship seen for the sharp edge spillways.
In the free flow mode, as the flow crosses the overflow jet and reaches the first step, the water carries on to the lower step without generating a hydraulic leap thanks to the stairs' unique design.This procedure continues until the channel's bottom.According to how closely the flow resembles the flow coming from the breakwater, the characteristics of the flow on the highest step may be estimated (Chanson, 2002).The flow depth measured in the first step is compared to the relations provided by Chanson in Figure 10.This appears to be due to shallow water depth, disturbance of the water surface, and the impact of transverse currents on the water depth in the center of the channel, which has resulted in an error in accurate depth measurement.In the shallow discharges measured, it was lower than the value estimated by the Chanson relation.
According to a study of the water depths recorded on the highest step, a breakwater's average divergence from the relationships described in earlier research was a maximum of 14%.

Submerged flow pattern in Salmon Dam spillway
The hydraulic leap is submerged when the water level rises downstream of the dam.With rising water depth in this instance, the sorts of patterns indicated downstream of the spillway may be seen, and they are evaluated as follows.After the flow has stabilized at a consistent water-level depth downstream of the spillway in each experiment, the necessary parameters have been monitored and images have been taken.
Investigations in these studies indicate that with the increase of the water depth downstream of the stepped dam, the hydraulic jump in the downstream position of the first step is submerged and a flow formed similar to the flow pattern downstream of a sudden abrupt drop.With the further increase of the water depth downstream, the jump moves to the upstream side and the submerged flow patterns of A, wave pattern and B pattern will be formed at the bottom of this step, and transported to the higher step under comparable circumstances.Figure 11 shows the submerged flow patterns downstream of steps 1 and 2 for an approximate flow rate of 13.8 lit/s and different ratios of downstream extraction.Based on this event, two submerged flow patterns A and a wave pattern will be formed downstream of each step, respectively, and the flow will be transferred to the higher step.In other words, the B flow pattern formed downstream of each step is the start of the turbulent flow (jump) in the higher step.
Salmon Dam is built in two steps, and the same supercritical flow conditions that exist on the upstream steps also exist on the downstream side of the two steps.The flow pattern downstream of the stairs is similar to the patterns described downstream of a breakwater, according to laboratory experiments, and four patterns that resemble the patterns indicated downstream of breakwater are generated in this section.Similar patterns develop at various heights of the water level downstream of the overflow with an increase in the outflow (Figure 12).Dimensionless numbers were utilized to examine the outcomes of these tests, and the findings were compared to those of earlier studies.

Jump patterns at the bottom of the stairs
With the rise in water level downstream of the Salmon Dam's steps, the jump patterns shift from A-jump to B-jump, and the leap is moved to a higher step, much like the flow pattern downstream of a breakwater.Figure 11 depicts the patterns that were noticed at various levels of the water level downstream of the Salmon Dam steps.Although the dimensions of the vortex are less than the non-step overflow state, the B-jump pattern's vortex flow resembles the submerged hydraulic leap due to its upstream and downward pattern and direction of flow.However, if the vortex is large enough, it still poses a risk of drowning.The mannequin stays in the vortex as predicted, as demonstrated by laboratory experiments with mannequins, and the flow can still be hazardous.Figure 12 depicts several patterns, downstream of stairs No. 1 and 2, according to Froude number and relative depth of absorption.A dividing line between these two flow patterns can be seen in this picture, and the flow pattern and potentially hazardous flow conditions downstream of the stairs may be projected using the flow depth and Froude number.Although the spread of points is higher due to the turbulence of the flow, waveform patterns are between these two modes.
Water to the weir height at a discharge of 13.8 L/s in the laboratory model.

Jump patterns downstream of the overflow
The topmost step has different upstream flow characteristics than the lower steps, and the flow regime is subcritical upstream of the spillway.The flow patterns in the submerged state have been examined to look at the potential production of hazardous patterns in this area.The flow patterns in this location differ from the flow downstream of the steps and are more like the patterns described downstream of the sharp edge spillways due to the varied flow conditions upstream of the spillway.A study of the flow in this location revealed that a free jump initially forms when the water level rises over the tallest step, and it gradually gets submerged.Because the Salmon Dam spillway is a flat-topped spillway with a sloping upstream, it is anticipated that the flow characteristics at a constant height of the downstream water level will not exactly match the patterns of sharp-edge spillways.The flow patterns in this area are similar to the reported patterns in sharp-edged weirs.Experiments revealed that underwater vortex forms downstream of the Salmon dam spillway, similar to the sharp edge spillways when the absorption ratio rises.If we look at the physical properties of the vortex, as shown in Figure 8, with a constant discharge and a constant flow pattern, the length of the vortex initially grows and then reduces as the downstream water depth increases.Additionally, the vortex's height first decreases before finally being fixed.In addition, the vortex's length-to-height ratio first rises and then falls.The comparison of these tests' results with Lotisser's results (Figure 9b) is shown in Figure 9a.The comparison of these findings reveals that the power of the return flow decreases as the length of the vortex increases owing to an increase in the amount of energy lost and that the length of the leap has an inverse relationship with the speed of the return flow upstream.
Moreover, as seen in the graphs, the length and height of the vortex have reduced, and as a result, the energy loss at a fixed depth of the flow, with an increase in flow rate and, therefore, the Froude number.Additionally, prior research on sharp-edged weirs has demonstrated that when the number of landings rises, the velocity of the return flow upstream likewise increases (Figure 7b).A submerged hydraulic leap is also formed in each step, creating a condition similar to this one.However, because spillway steps have larger energy losses than basic weirs, it is anticipated that the flow velocity downstream of a step will be slower than the flow downstream of a simple weir (this issue was not investigated in this study).
Due to the low height of the vortex downstream of the steps, which was determined by measuring the vortex at several sites downstream of the spillway, it is possible to  infer that the likelihood of drowning from submerged hydraulic jump flow will be low when this flow is contained to the step.However, this flow may be hazardous at the lower steps downstream of the steps, in deeper downstream regions, and situations when the eddy length is higher than the step length.As a result, if a strong eddy current is also affecting the swimmer, it will be more challenging to swim against it.Even though model dolls have been used to test this issue, for technical reasons they have not been quantitatively examined in these tests.
Figure 13 illustrates the flow patterns downstream of the spillway for various releases and downstream waterlevel depths.The flow patterns downstream of the spillway are similar to those in sharp edge spillways, as can be seen in this diagram, and as the depth of the water downstream increases, the patterns of the free jump, impinging jet, surface jet, surface wave, and surface jet are generated, respectively.
A wave flow pattern develops when the water level in the area immediately downstream of the overflow rises.However, owing to research on this form of flow in sharpedge spillways, the features of this type of flow have also been examined in the Salmon Dam.Even though this flow pattern is not hazardous for people due to the direction of rotation of the eddies.According to the experiment's findings, this flow pattern resembles the wave-shaped flow pattern that occurs downstream of spillways with sharp edges.We may thus find comparable connections with the sharp edge weir for the wave pattern by measuring the properties of the wave downstream of this weir.Tests done on sharp-edged weirs have previously proven that it is feasible to determine the wave characteristics downstream of sharp-edged weirs by utilizing the flow characteristics (Azimi and Rajaratnam, 2009), as was indicated in a review of prior works.Similar correlations may be derived for the broad edge spillway under study, according to the measurement of the wave's properties, which was carried out by photographing the various stages of the trials.Figures 14 and 15 relate these correlations to the earlier findings.Figure 15 shows that the Salmon Dam spillway's first wave is more flattened and lower in height than a sharp edge spillway, but that the distance between the second wave and the first wave is less than it would be for a sharp edge spillway.These features resemble those seen in sharpedge weirs because heavier discharges raise the water level upstream of the weir.This issue demonstrates how, as the flow rate rises, the h 0 /L ratio rises as well, bringing the flow conditions at the weir closer to those finite-crest weirs and, consequently, the hydraulics of the flow closer to those of sharp-edged weirs.The assessment of additional wave characteristics, such as the gap between the initial wave's claw and the overflow edge in the Salmon dam's Flap Top Overflow, also demonstrates that this index was higher than the outcomes seen in the Sharp Edge Overflow.The value of Y w /h was similar to the outcome of the sharp edge overflow concerning flow rate; however, this issue was inverted in the case of X w /h.In contrast to an overflow with a sharp edge, this issue may be caused by the impact of the overflow form on the wave pattern.
Figure 16 depicts the variations in wave characteristics in the Salmon Dam spillway under study.

Conclusions
This research has looked at the flow patterns that have developed downstream of the Salmon Step Dam spillway as a result of the rising water level there.The findings of this study indicate that at various heights of the downstream water level, various patterns of flow will occur downstream of a stepped dam.These patterns will include the patterns anticipated downstream of steps, which are anticipated to have lower water table heights, with the patterns anticipated downstream of overpass spillways, which are anticipated to have greater intake ratios.These patterns resemble those that have already been discovered for flows that are downstream of drops and sharp-edged weirs.To provide dimensionless relationships for predicting the flow pattern in the downstream absorption ratios, which can be used to predict some of the characteristics of the flow downstream of the Salmon Dam spillway, it has been attempted to examine the results presented in previous studies about breakwaters and sharp edge spillways in these tests.The findings of these studies demonstrated that when the flow rate is increased, the influence of overflow shape reduces, and as the h0/L ratio is increased, the results begin to resemble those for sharp edge overflows.
The flow pattern downstream of the steps is identical to the flow regime upstream of each step.B-jump and Wave-jump flows have been identified as patterns that, because of the shape and size of the vortex, can be hazardous and are located downstream of the Salmon Dam's overflow steps.Depending on the size of the vortex, the impinging jet flow pattern downstream of the overflow may endanger people, children, or animals by drowning depending on the direction the vortex rotates.The studies done using model dolls revealed that these eddies have not completely disappeared, and the mentioned patterns are still observed at some different heights of the downstream water level, even though the stepped form has greatly reduced the range of occurrence and dimensions of these eddies.Therefore, it is preferable to anticipate the type of flow that such structures would experience throughout various phases of operation based on the depth of the flow downstream of the overflow and to take into account the appropriate drowning warnings.

Figure 4 .
Figure 4. Sketch of the parameters in a drop structure in a free jump regime (Azimi, Rajaratnam, & Zhu, 2016).

Figure 8 .
Figure 8.The flow patterns observed over the upper step of Salmon barrier dam and immediately after the weir in comparison with flow patterns over sharp crested weirs.

Figure 10 .
Figure 10.The measured depth in the second step compared to the predicted depth for the breakwater in the first step.

Figure 11 .
Figure 11.The change of the flow profile downstream of the steps of the Salmon dam with the increase in the ratio of the downstream water depth to the overflow height per-flow rate of 13.8 lit/s in the laboratory model.

Figure 12 .
Figure 12.Regime chart for flow configurations in steps.The dashed line shows the approximate boundaries between a and B jump types.

Figure 13 .
Figure13.Change the flow profile downstream of the Salmon dam spillway by increasing the ratio of the downstream water depth to the overflow height for the flow rates of 13.8 lit/s, 16.7 lit/s, and 35.9 lit/s in the laboratory model.

Figure 14 .
Figure 14.The changes in the dimensions of the first wave with the change of t/h in different discharges in the surface wave flow downstream of the Salmon dam spillway compared to Azimi et al.'s results in sharp edge spillways; (a): change of a/h with t/h; (b): Variation of b/h with t/h.

Figure 15 .
Figure 15.Variations of the first wave two location in the surface wave with t/h at different discharges downstream of Salmon dam weir in comparison with the results of Azimi et al. for sharp crested weirs, (a) X w /h versus t/h and (b) Y w /h versus t/h.

Figure 16 .
Figure 16.Variations of wave characteristics and the location of the first wave toe in the surface wave flow with t/h downstream of the Salmon barrier dam weir.