This study is an experimental study to analyze the dissipation effect of a baffle shape installed to reduce the scour problem downstream of a weir. The hydraulic experiment on the flow dissipation effect created by baffle installation was an experiment under a fixed bed condition to investigate the flow dissipa tion effect based on changes in flow in the apron downstream caused by changes in baffle shape. The experimental analysis was conducted by measuring the flow rate at the apron downstream with and without the installation of a baffle on the basis of the flow dissipation effect. This experiment compared the flow dissipation effect achieved with five baffle shapes to analyze the effect of the baffle shape. Two conditions of flow rate were considered: when a water level in the area of super-critical flow was lower than the baffle height, and when it flowed over the baffle. Overall, the flow dissipation effect was found to be good when a square baffle with a large flow blocking area was used. The analysis also found that hydraulic jump was facilitated when the front part of the baffle was inclined, as flow was induced to the upper direction. The result of the experiment showed that when performing flow dissipation through the use of a baffle, the effect of flow blocking and flow duration alteration should be considered.
A weir structure is established to enable water use and flood control, and consists of the main weir body wall and downstream apron. Since the River Design Criteria (RDC) were suggested based on general weirs constructed previously, there were technical limitations in the RDC with regard to designs of mid-sized weir structures (fixed and movable weirs) [
Since there have been no previous studies conducted in Korea on the design of large-scale weirs, overseas scour protection and flow dissipation techniques were adopted without modification, and the technical level of technique development is relatively low. In addition, while a variety of studies on flow dissipation work, size of weir downstream, and riverbed protection work are needed to maintain the riverbed stability in the weir downstream, most studies in Korea have concentrated on the size of scour and riverbed protection work. The Ministry of construction & transportation national construction research institute performed an experimental study on size calculation of scour reinforcement in weir stream and proposed a calculation equation, but this was limited to a fixed weir related study [
Looking at overseas cases, various techniques on scour protection and flow dissipation in structures downstream have been proposed, but most of them relate to dams downstream. Overseas studies on scour and riverbed protection work are as follows: McLaughlin Water Engineers investigated the advantages and disadvantages of various cross structures and proposed improved structures suitable for Denver City [
This study aims to analyze the effect of flow dissipation due to various baffle shapes for flow control in the weir downstream. To do this, the researchers conducted a hydraulic experiment with regard to the shape of flow dissipation work in the fixed weir downstream, measured the change in the flow rate and presented the effect based on the analysis.
The experiment device used in the hydraulic experiment can be divided into a channel and flow supply unit. The flow supply unit was composed of underground storm water storage and pump, and the experimental device water channel consisted of baffle, model water channel, and downstream collecting well. The open channel used in the experiment device was installed to flow an amount of water up to 0.3 m3/s. The experiment water channel was designed in the River Test Center, and the specifications of the experiment water channel are as follows: flow supply capability was 0.3 m3/s, channel width was 1.5 m, channel length was 30 m and channel height was 1.2 m. The main sections of the experiment channel were the weir, baffle installation area, experiment measurement area (aluminum angle) and flow stabilization area (pebbles) (
experiment was a basic shape whose width, height, and length were 1.5 m, 0.3 m, and 0.37 m, respectively (
To measure the change in flow that occurred due to the baffle installation, an experiment measurement area whose width and channel length were 2.0 m and 1.0m, respectively, was set up at the downstream of the baffle installation area. The measurement gap was marked every 10 cm in the horizontal and vertical directions. There were 190 measurement points in total. The weir in the experiment structure was installed at a place where flow in the straight water channel was sufficiently uniform.
A flow in the fixed weir downstream forms a super-critical flow due to a weir with a head drop, and hydraulic jump occurs in a certain area based on downstream
water level and bed roughness. The location in the weir downstream where hydraulic jump occurs is affected by the water level in the downstream. That is, when the water level in the downstream is lower, the location where hydraulic jump occurs becomes farther from the weir. When the location where hydraulic jump occurs becomes farther away, scour is more likely to occur at the apron downstream, thereby affecting the weir’s safety. Thus, it is advantageous for riverbed safety to have hydraulic jump in the weir downstream occur inside the apron as much as possible. To determine this, in this experiment an energy dissipator (baffle) was installed at the weir downstream, forcibly generating hydraulic jump inside the apron to analyze the effect in terms of maintaining the riverbed. To do this, flow rate, which was one of the main factors in this experiment, was measured at the downstream of the energy dissipator to verify the effect of the baffle’s shape. To compare the flow rate with and without the installation of a baffle, an experiment measurement area whose width and channel length were 1.0 m and 1.0 m was set up at the downstream of the baffle installation area.
Two flow rate conditions were set to analyze the effect of flow dissipation due to the baffle arrangement in the weir downstream. The inflow rate conditions at the weir upstream were 0.140 m3/s and 0.325 m3/s, and the downstream water level conditions were 0.085 m and 0.140 m (
Category | Inflow rate (m3/s) | Upstream water level (m) | Downstream water level (m) | Baffle height |
---|---|---|---|---|
Case 1 | 0.140 | 0.390 | 0.085 | Not installed |
Case 2 | Square shape | |||
Case 3 | Round shape | |||
Case 4 | Equilateral triangle | |||
Case 5 | Trapezoid | |||
Case 6 | Stepped shape | |||
Case 7 | 0.325 | 0.440 | 0.140 | Not installed |
Case 8 | Square shape | |||
Case 9 | Round shape | |||
Case 10 | Equilateral triangle | |||
Case 11 | Trapezoid | |||
Case 12 | Stepped shape |
baffle shape difference. Flow in the weir downstream without baffle installation maintained a super-critical flow where a hydraulic jump did not occur within the experiment area. A hydraulic jump at the weir downstream occurred within the apron in all cases of baffle installation. This experiment measured a flow rate at the hydraulic jump downstream and compared the effect of flow dissipation due to baffle shape.
Category | Type of baffle arrangement | Photo of baffle arrangement |
---|---|---|
Case 1 | ||
Case 2 | ||
Case 3 | ||
Case 4 | ||
Case 5 | ||
Case 6 |
Category | Flow characteristics (downstream) | Flow characteristics (side) |
---|---|---|
Case 1 | ||
Case 2 | ||
Case 3 | ||
Case 4 | ||
Case 5 | ||
Case 6 |
Category | Overflow height (m) | Baffle shape | Baffle arrangement | Mean flow rate (m/s) | Maximum flow rate (m/s) | Hydraulic jump distance (m) |
---|---|---|---|---|---|---|
Case 1 | 0.09 | Not installed | Two-row straight arrangement | 1.855 | 2.340 | 1.348 |
Case 2 | 0.09 | Square shape | Two-row straight arrangement | 0.629 | 0.833 | 0.200 |
Case 3 | 0.09 | Round shape | Two-row straight arrangement | 1.177 | 1.607 | 0.200 |
Case 4 | 0.09 | Equilateral triangle | Two-row straight arrangement | 0.867 | 1.238 | 0.200 |
Case 5 | 0.09 | Trapezoid | Two-row straight arrangement | 0.801 | 1.004 | 0.200 |
Case 6 | 0.09 | Stepped shape | Two-row straight arrangement | 0.957 | 1.321 | 0.200 |
Category | Overflow height (m) | Baffle shape | Baffle arrangement | Mean flow rate (m/s) | Maximum flow rate (m/s) | Hydraulic jump distance (m) |
---|---|---|---|---|---|---|
Case 7 | 0.14 | Not installed | Two-row straight arrangement | 2.371 | 2.650 | 1.714 |
Case 8 | 0.14 | Square shape | Two-row straight arrangement | 1.228 | 1.550 | 0.200 |
Case 9 | 0.14 | Round shape | Two-row straight arrangement | 1.464 | 1.650 | 0.200 |
Case 10 | 0.14 | Equilateral triangle | Two-row straight arrangement | 1.364 | 1.745 | 0.200 |
Case 11 | 0.14 | Trapezoid | Two-row straight arrangement | 1.122 | 1.497 | 0.200 |
Case 12 | 0.14 | Stepped shape | Two-row straight arrangement | 1.077 | 1.686 | 0.200 |
square baffle that blocked the flow, as water depth in the super-critical flow area was lower than the baffle height at the small flow rate condition. When flow rate was large, the flow in the super-critical flow area overflowed the baffle. Here, the front sides of the trapezoid and stepped shapes induced a flow to the upper direction, facilitating the occurrence of hydraulic jump, which resulted in large flow dissipation in the downstream.
The flow dissipation effect could be verified in the riverbed at the apron downstream by installing a baffle in all baffle shape conditions in this experiment. The result of the experiment showed that the square baffle had the best flow dissipation effect considering the flow rate conditions applied. In the trapezoidal and stepped shapes, a phenomenon that induced the upstream flow into the upper direction when the flow was large was verified. Consequently, it facilitated a hydraulic jump, thereby causing a large reduction in the flow rate at the riverbed in the downstream. This means that they can be highly applicable to weirs whose flow rate is large. The experiment result showed that flow dissipation through the baffle shape should be applied by considering the effect of flow blocking and flow duration alteration.
This study is an experimental study that analyzes the flow dissipation effect produced by the use of a baffle as a measure to minimize changes in the riverbed in the downstream of weir apron that result from scour. Five baffle shapes were selected, and the flow dissipation effect was analyzed through experimental measurement for each baffle shape.
The experiments were conducted under two flow rate conditions: 0.140 m3/s and 0.325 m3/s. The 0.140 m3/s flow rate condition involved a shallow water depth in the super-critical flow area in the baffle upstream. Here, the largest flow dissipation effect was exhibited with the square baffles (approximately 65% flow dissipation). At 0.325 m3/s flow rate with stepped shape condition, flow dissipation of approximately 60% was revealed a good flow dissipation effect. The square baffle had large flow dissipation due to a large flow block effect because its flow blocking area was relatively larger than those of other shapes. In the trapezoidal and stepped shapes, a phenomenon that induced the upstream flow into the upper direction was verified at the condition where water depth in the super-critical area was deeper than the baffle height. Consequently, it facilitated the occurrence of a hydraulic jump, thereby achieving a large reduction in flow rate at the riverbed. The experiment results showed that the square baffle provided a good flow dissipation effect considering the flow rate condition. However, flow dissipation based on baffle shape must consider the effect of flow block and flow duration change when flow rate condition and size of weirs are taken into consideration.
This research was supported by the Korea Institute of Civil Engineering and Building Technology (Project title: Development of maintenance technology for enhancement of river water front and environmental values, Project number: 20170100).
Kang, J.-G. (2017) An Experimental Study on the Dissipation Effect of a Baffle Downstream of a Weir. Engineering, 9, 937-949. https://doi.org/10.4236/eng.2017.911056