# Turbulence Characteristics in Mild and Steep Entrance Slopes of Pool-Riffle Sequences

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## Abstract

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## 1. Introduction

- How do velocity and Reynolds stress values change in different entrance slopes of an artificial pool-riffle sequence?
- What are the dominant events in the bursting process in mild slope (5°) and steep slope (20°) in an artificial natural pool-riffle sequence? Can the aspect ratio influence the contribution of each event?
- How much agreement exists between the results on a laboratory scale with those in a gravel-bed river?

## 2. Materials and Methods

#### 2.1. Field Measurements

_{50}and d

_{90}indicate that 50% and 90% of the particles in the study area are smaller than this, respectively; in the study area, the flow was subcritical (the Froude number was between 0.04 and 0.4). The relative submergence parameter, $\frac{h}{{d}_{50}}$, and the aspect ratio parameter, $\frac{W}{h}$, had mean and average values of 3.36 and 132.9, respectively (Table 1). The standard deviation of the particle size $\sqrt{\frac{{d}_{84}}{{d}_{16}}}$ was 1.4, showing uniform particle distribution in the selected reach.

#### 2.2. Laboratory Experiment

#### 2.3. Quadrant Analysis

## 3. Results

#### 3.1. Field Results

#### 3.1.1. The Velocity Profile in Pelasjan River

#### 3.1.2. Reynolds Stress Profiles

#### 3.1.3. Quadrant Analysis

#### 3.2. Laboratory Results

#### 3.2.1. Velocity Profiles

#### 3.2.2. Reynolds Stress Profiles

#### 3.2.3. Quadrant Analysis

## 4. Discussion

## 5. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 2.**(

**a**) The selected reach and cross-sections of the Pelasjan River, (

**b**) rectangular grid and mapping operations, (

**c**) ADV deployment in the river to collect velocity data, and (

**d**) topographic map of the Pelasjan River in the study area (in x, y, z direction, all units are meters).

**Figure 3.**(

**a**) Front view of the laboratory channel, (

**b**) side view of the laboratory channel, and (

**c**) schematic of the bed from upstream to downstream (shaded parts represent the length of the test area).

**Figure 4.**The bursting sequence of events including: (

**a**) the beginning of stream streaks separation with the low-velocity flow (ejection), (

**b**) the movement of the separated flow into the upper layer flow (except the low-velocity flow), (

**c**) the collapse of the separated flow (except for low-velocity flow), and (

**d**) moving of high-velocity stream streaks into the bed (sweep).

**Figure 6.**(

**a**) Nondimensional velocity profiles ($\frac{\mathrm{u}}{{\mathrm{u}}_{\mathrm{max}}}$) in the longitudinal direction and at 5 cm of the left bank of the river, (

**b**) nondimensional velocity profiles in the central axis of the river, (

**c**) velocity profiles for the pool entrance (CS3), and (

**d**) velocity profiles for riffle area in the central and 5 cm from the riverbank (CS4).

**Figure 7.**Nondimensional Reynolds stress profiles ($\frac{-\mathrm{U}\prime \mathrm{W}\prime}{{\mathrm{U}*}^{2}}$) in the longitudinal direction at: (

**a**) 5 cm of the left bank of the river, and (

**b**) the central axis of the river.

**Figure 8.**Reynolds stress profiles for: (

**a**) the pool area and (

**b**) the riffle area in the center and 5 cm from the riverbank.

**Figure 9.**Contribution of each event in the central axis of the river in: (

**a**) the pool and (

**b**) the riffle.

**Figure 10.**Velocity contour in the x-direction for: (

**a**) series “a” test in the central axis of the channel, with an angle of 5° and aspect ratio of less than five, (

**b**) series “b” test in the center axis of the flume, with an angle of 5° and aspect ratio of more than five, and (

**c**) series “c” test in the center axis of the flume, with an angle of 20° and an aspect ratio of less than five.

**Figure 11.**Velocity profiles for: (

**a**) series “a” test on the central axis of the channel with an angle of 5° and aspect ratio of less than five, (

**b**) series “b” test on the central axis of the flume with an angle of 5° and aspect ratio of more than five, and (

**c**) series “c” test on the central axis of the flume with an angle of 20° and aspect ratio of less than five. In the experiments with angles of 20°, in the pool area (x = 8.25 m and x = 8.75 m), the maximum velocity occurs at $\frac{Z}{H}=0.25$, and in the riffle area (x = 9.25 m), the maximum velocity was observed at $\frac{Z}{H}=0.4$.

**Figure 12.**Reynolds stress profiles for: (

**a**) series “a” test on the central axis of the channel with an angle of 5° and aspect ratio of less than five, (

**b**) series “b” test on the central axis of the flume with an angle of 5° and aspect ratio of more than five, and (

**c**) series “c” test on the central axis of the flume with an angle of 20° and an aspect ratio of less than five.

**Figure 13.**Dominant interactions using quadrant analysis for: (

**a**) series “a” test on the central axis of the channel, with an angle of 5° and aspect ratio of less than five, (

**b**) series “b” test on the central axis of the flume, with an angle of 5° and aspect ratio of more than five, and (

**c**) series “c” test on the central axis of the flume with an angle of 20° and aspect ratio of less than five.

W (m) | h (m) | d_{16} (cm) | d_{50} (cm) | d_{84} (cm) | d_{90} (cm) | $\frac{\mathit{h}}{{\mathit{d}}_{50}}$ | $\frac{\mathit{W}}{\mathit{h}}$ | $\mathit{\sigma}$ | Fr |
---|---|---|---|---|---|---|---|---|---|

49.2 | 0.37 | 10.4 | 11 | 17.5 | 21 | 3.36 | 132.9 | 1.39 | 0.19 |

Series | $\frac{\mathit{W}}{\mathit{h}}$ | h (cm) | Slope (degree) |
---|---|---|---|

A | 6 | 15 | 5 |

B | 3.6 | 25 | 5 |

C | 3 | 30 | 20 |

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**MDPI and ACS Style**

Soltani, N.; Afzalimehr, H.; Tabarestani, E.S.; Eftekhari, A.; Khabari, M.; Nazari-Sharabian, M.; Karakouzian, M.
Turbulence Characteristics in Mild and Steep Entrance Slopes of Pool-Riffle Sequences. *Water* **2023**, *15*, 720.
https://doi.org/10.3390/w15040720

**AMA Style**

Soltani N, Afzalimehr H, Tabarestani ES, Eftekhari A, Khabari M, Nazari-Sharabian M, Karakouzian M.
Turbulence Characteristics in Mild and Steep Entrance Slopes of Pool-Riffle Sequences. *Water*. 2023; 15(4):720.
https://doi.org/10.3390/w15040720

**Chicago/Turabian Style**

Soltani, Negar, Hossein Afzalimehr, Ehsan Shahiri Tabarestani, Alireza Eftekhari, Marzieh Khabari, Mohammad Nazari-Sharabian, and Moses Karakouzian.
2023. "Turbulence Characteristics in Mild and Steep Entrance Slopes of Pool-Riffle Sequences" *Water* 15, no. 4: 720.
https://doi.org/10.3390/w15040720