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J. Korean Soc. Hazard Mitig. > Volume 17(3); 2017 > Article
Song, Lee, Joo, and Park: A Study on Hydraulic Experiment of Capture Efficiency for Drainage Reduction Facilities in Mountain and Urban Areas

Abstract

배수로 월류에 따른 토양의 함수량 증가는 토사재해의 가능성을 증가시켜 이러한 피해를 대비하기 위한 배수로 저감시설이나 설계기준이 마련되어야 한다. 그러나, 국내외의 배수로 퇴적을 저감하는 연구나 시설은 미미한 실정으로 배수로 월류에 따른 재해 발생가능성을 저감하기 위한 저감시설의 개발이 시급한 실정이다. 본 연구에서는 수리실험을 통해 배수로의 저감시설별 유송잡물의 포착효율을 산정하고 산지지역 및 도시지역의 설계유량에 따른 목표 포착효율별 저감시설 설치방안을 제안하였다.

요지

An increase in soil moisture content due to drainage overflow raises the possibility of sediment disaster. Thus, it is necessary to build reduction facilities for drainage or to establish design criteria to prevent such damage. However, it is a fact that there is an urgent need to develop facilities to reduce the probability of disaster caused by drainage overflow because there has been little research or facility to reduce domestic and international drainage sediments. This study proposed a method for setting up reduction facilities for each target capture efficiency according to design flow in mountain and urban areas by calculating the debris capture efficiency for each reduction facility for drainage by hydraulic experiment.

1. Introduction

In mountain and urban areas, drainage is installed to facilitate outflow in rainfall. The movement of debris according to the drainage flow is deposited in drainage facilities and increases the possibility of sediment disaster caused by the reduction of discharge capacity of drainage facilities. Sediment disaster occurring near drainage is caused by debris sedimentation and occurs mainly in drainage located in general roads, urban areas, and mountain areas. In rainfall beyond the design frequency, debris deposited in drainage causes overflow damage due to the lack of discharge capacity, and soil super saturation caused by overflow raise the possibility of sediment disaster. However, design criteria for drainage installed in domestic and international mountain and urban areas consider only excess cross-section for sedimentation. Therefore, this study aims to develop reduction facilities to capture debris in the drainage installed in mountain and urban areas by hydraulic experiment.

2. Literature Survey

FHWA. (1971), AASHTO. (1975), AASHTO. (1999), FHWA. (2005) is the debris classification and control facilities to secure the discharge capacity that designers and engineers can select and refer in designing debris control facilities (Table 1).
Table 1
Classification Debris According to Reduction Facilities
Debris Facilities
Deflector Rack Riser Crib Fin Dam
LFD
MFD
HFD
FD1
FD2
CD
Bardou et al. (2007), Kim et al. (2012) evaluated the performance of debris reduction facilities by changing the size, distance, and diameter of rockfall protection nets to reduce sediment disaster. A slit-check dam was designed to carry out hydraulic model experiment on the position and direction of deposition by driftwood in times of sediment disaster(Badri et al. (2012)). Based on the results, this study examined only the performance evaluation of debris reduction facilities to reduce the damage of large-scale sediment disaster, and there were difficulties in applying the results to small-scale drainage facilities.
This structural changes in the slit dam and H-shaped facility for check dam and experimented the capture effects of driftwood, rocks, and stones by hydraulic model experiment. A-shaped and D-shaped slit dams showed the capture rate of more than 80% and 90%, respectively (Lim et al. (2008), Chun et al. (2010)). In addition, the H-shaped facility showed the highest capture rate of 96% depending on structural changes. kim et al. (2013) study examined the capture effect of driftwood and debris flow with the bottom screen at intervals of 3, 6, and 9 mm in order to capture driftwood and debris flow by hydraulic model experiment. Debris reduction facilities are built at the entrance of cross-drainage culverts located in the mountain slope valley. It is difficult to find an alternative to debris reduction facilities because sediment disaster occurs with a mixture of more than two types of debris. According to the results, it is unreasonable to reproduce the accurate performance of debris reduction facilities with hydraulic experiment using a scale model of debris reduction facilities for large-scale sediment disaster.

3. Description of Experiments

3.1 Specifications for hydraulic laboratory

The hydraulic laboratory was established to develop debris reduction facilities for reducing overflow damage caused by debris deposition in drainage. The hydraulic experiment were performed to examine the topographic and hydraulic characteristics of structures to be actually built. The most desirable hydraulic experiment is about the actual size. This study established the hydraulic laboratory to develop reduction facilities for drainage and applied a 1:1 scale not to the scale model but to the actual drainage size (Fig. 1).
Fig. 1
Hydraulic Laboratory for Drainage
KOSHAM_17_03_339_fig_1.jpg
The hydraulic laboratory has a total area of approximately 306.25 m2 (17.5 m wide and long). For loop supply of water, this study designed the watertank → pump → head tank → rectifying tank → drainage → return tank as shown in Fig. 1. The specifications of hydraulic experiment for each facility are shown in Table 2.
Table 2
Specifications for Hydraulic Laboratory
No Facilities Standard (Vertically×Horizontally×Height) Capacity
Water Tank 11×6.85×0.7 m 52.8 m3
Pump Facilities 15×2,10×1 HP 0.33 m3/s
Head Tank 4×6×0.8 m 19.2 m3
Rectifying Tank 5×4×0.7 m 14 m3
Drainage 0.3×8.75×0.3 m 0.8 m3
Return Tank 14×1.5×0.2 m 4.2 m3

3.2 Conditions of reduction facilities for drainage

No study has been conducted to develop reduction facilities for debris into drainage in Korea. This study aims to transform the debris reduction facility presented FHWA. (2005) and then apply it to fit the drainage in order to develop reduction facilities for drainage. The height of the opening and overflow were set to 80% and 20% of the drainage width, respectively. The height of the opening forms a grid net capable of capturing debris, and one horizontal and vertical net distance was set as an effective cross sectional area by evenly applying the horizontal and vertical net distance to the entire transmission part. The parameters of reduction facility for drainage are as follows: drainage width (b), opening height (h), overflow height (y), horizontal net distance (d1), and vertical net distance (d2), With regard to the grid net in the transmission part, around bar with 5mm in diameter was selected as a material for the grid (Fig. 2).
Fig. 2
Conditions of Reduction Facilities for Drainage
KOSHAM_17_03_339_fig_2.jpg
This study presented a total of seven reduction facilities for each grid size on the basis of 3x3 reduction facility with slight influence of its capture efficiency because of a large effective cross sectional area for each grid and 9x9 reduction facility with high capture efficiency because of a small effective cross sectional area (Fig. 3).
Fig. 3
Reduction Facility for 300 mm Drainage
KOSHAM_17_03_339_fig_3.jpg
The characteristics of the horizontal and vertical net distances and effective cross sectional area for each reduction facility are designed as shown in Table 3.
Table 3
Characteristics of Reduction Facility for 300 mm Drainage
Drainage Width (mm) Overflow Height (mm) Reduction Facilities Horizontal Net Distance (mm) Vertical Net Distance (mm) Effective Cross Sectional Area (mm2)
300 60 3×3 97 74 7,178
4×4 71 54 3,834
5×5 56 42 2,352
6×6 46 35 1,610
7×7 39 29 1,131
8×8 33 25 825
9×9 29 21 609

3.3 Conditions of reduction facilities for drainage

This study selected twigs and pieces of wood as the type of Light Floating Debris (LFD) proposed in drainage (Table 4). In addition, this study aims to establish a definition of debris by collecting it near drainage actually located mountain and urban areas because the length and diameter of twigs and pieces of wood as LFD are not clear. A total of 350 pieces of debris diameter from 1 mm to more than 20 mm was collected from mountain and urban areas near drainage (Fig. 4).
Table 4
Classification of Debris
Debris Type
Light Floating Debris (LFD) twigs, Pieces of wood, Material
Fig. 4
Debris by Mountain and Urban Areas near Drainage
KOSHAM_17_03_339_fig_4.jpg
Debris ranging from 1 mm to 12 mm and more than 12 mm accounted for 91.6% (320 pieces) and 8.6 % (30 pieces), respectively (Table 5).
Table 5
Debris Classification of Diameter for Debris
Diameter (mm) Count Ration (%) Diameter (mm) Count Ration (%)
1~2mm 16 4.6 12~13 mm 6 1.7
2~3mm 46 13.1 13~14 mm 5 1.4
3~4mm 65 18.6 14~15 mm 6 1.7
4~5mm 59 16.9 15~16 mm 2 0.6
5~6mm 44 12.6 16~17 mm 1 0.3
6~7mm 34 9.7 17~18 mm 3 0.9
7~8mm 16 4.6 18~19 mm 2 0.6
8~9mm 16 4.6 19~20 mm 1 0.3
9~10mm 9 2.6 Over 20 mm 4 1.1
10~11mm 7 2.0 Sum 350 100
11~12mm 8 2.3
Therefore, this study aims to define the diameter of debris that can be flown into drainage to be deposited as 1 mm to 12 mm in diameter equivalent to 90% of the collected debris. Although it is necessary to use twigs collected in the mountain and urban areas in order to examine the capture efficiency of debris for each reduction facility for drainage, a wooden round bar was substituted to maintain the consistency of debris. The length of debris was set to 120 mm equal to 50% of the opening height of reduction facility, and the amount of debris for each session by diameter is shown in Table 6.
Table 6
Hydraulic Experiment of Debris
Debris for Wooden Round Bar Length (mm) Width (mm) Count Effective Cross Sectional Area (mm2)
KOSHAM_17_03_339_fig_5.jpg 120 3 9 3,240
5 8 4,800
7 8 6,720
10 8 9,600
12 8 11,520
Sum 41 35,880

4. Performance Evaluation of Reduction Facilities for Drainage

4.1 Experimental conditions

The conditions of the reduction facility for 300 mm drainage include reduction facility, debris, flow rate, and water depth. The reduction facilities had a total of seven conditions from 3×3 to 9×9, and specifications of debris had five types from 3 mm to 12 mm in diameter. They were set as a drop condition for each session. As a drop method of debris, this study applied 5 times for each direction (horizontal and vertical) a total of 10 times drop conditions to one experimental condition to calculate the average of 10 times as capture efficiency. As an experimental condition of the reduction facility for 300 mm drainage, a total of 210 conditions were established as follows: 1 condition for drainage, 1 condition for debris, 7 conditions for reduction facility, 6 conditions for velocity, and 5 conditions for water depth. According to drop methods of debris, a total of 2,100 experiments with 5 times for each method were conducted as shown in Table 7.
Table 7
Experimental Condition of the Reduction Facility for 300 mm Drainage
Width (mm) Condition of Debris Reduction Facility Velocity (m/s) Water Depth (mm)
300 Lenght = 120mm, Diameter = 3~12mm Drop methods: horizontal and vertical (Each 5 times) 3×3 0.3 60
4×4 0.6 90
5×5 0.9 120
6×6 1.2 150
7×7 1.5 180
8×8 1.8 -
9×9 - -
condition 2 condition (Each 5 Times) 7 condition 6 condition 5 condition
Sum Total 210 conditions for 2,100 times

4.2 Capture efficiency characteristics by reduction facility according to discharge

The capture efficiency by discharge of the reduction facility for 300 mm drainage was observed as shown in Fig. 5. The capture efficiency decreased with increasing discharge according to reduction facilities, and minor or major changes in the capture efficiency were observed depending on the effective cross sectional area for one grid by reduction facility.
Fig. 5
Capture Efficiency by Discharge of the Reduction Facility
KOSHAM_17_03_339_fig_6.jpg
It is not possible to use the function of the capture efficiency of the 3×3 reduction facility of 300 mm drainage because it is close to 0%. The 8×8 and 9×9 reduction facilities are more suitable for booms or screens than for reduction facilities because the change in the capture efficiency according to changes in discharge was less than 10% and these reduction facilities capture all debris flown into drainage. This study aims to develop reduction facilities without blockage or overflow even if debris is captured by drainage as a reduction facility for drainage in mountain and urban areas. When capture efficiency is too high or too low, it is not appropriate for reduction facilities for drainage.
Therefore, with regard to 4×4 and 8x8 reduction facilities (effective cross sectional area for each grid of 3,834 mm2 and 825mm2) with 300mm drainage, this study proposed a method for installing reduction facilities for each target capture efficiency according to the designed is charge. The calculation formula of capture efficiency by discharge for each reduction facility is shown in Table 8.
Table 8
Calculation Formula of Capture Efficiency by Discharge for Each Reduction Facility
Reduction Facility Calculation Formula R
3×3 Y = 54.4 – 3204.5 × X + 62288.9 × X2– 384652.1 × X3 0.38
4×4 Y = 109.8 – 3415.5 × X + 47778.2 × X2– 220247.1 × X3 0.60
5×5 Y = 99.8 – 826.9 × X + 6261.2 × X2– 97223.0 × X3 0.80
6×6 Y = 92.2 – 370.8 × X + 31712.1 × X2– 277568.4 × X3 0.75
7×7 Y = 103.3 – 958.9 × X + 20717.6 × X2– 183504.2 × X3 0.72
8×8 Y = 105.6 – 933.5 × X + 23529.2 × X2– 194014.5 × X3 0.60
9×9 Y = 102.7 – 575.1 × X + 15715.3 × X2– 134500.3 × X3 0.52

4.3 Capture efficiency characteristics by reduction facility according to discharge

The capture efficiency by Froude number of reduction facilities decreased with increasing Froude number, and minor or major changes in the capture efficiency were observed depending on the effective cross sectional area for one grid by reduction facility (Fig. 6).
Fig. 6
Capture Efficiency by Froude Number of the Reduction Facility
KOSHAM_17_03_339_fig_7.jpg
The Froude number shows a supercritical flow when this number becomes more than 1 and a subcritical flow when it becomes less than 1 as a function capable of representing the flow state. A decrease in the linear capture efficiency was observed when the Froude number was less than 1.5 in each reduction facility with 300 mm drainage, and constant or increase in the capture efficiency was observed when the Froude number was more than 1.5 in each reduction facility. Wavy surface flow and roller-shaped surface are formed when the Froude number is more than 1, and it is not possible to examine trends in the capture efficiency due to an increase or decrease in the capture efficiency for each reduction facility.Based on changes in the capture efficiency with changes in the Froude number, it is possible to confirm that debris is irregularly captured because a supercritical flow occurs with increasing water depth and flow rate. This study proposed a method for installing reduction facilities for A decrease in the linear capture efficiency was observed when the Froude number was less than 1.5 in each reduction facility with 300 mm drainage.

5. Conclusion

This study proposed a method for setting up reduction facilities for drainage in mountain and urban areas by hydraulic experiment and analyzed the capture efficiency of conditions for water depth and discharge regarding 3×3 to 9×9 reduction facilities with 300 mm drainage. In addition, with regard to 4×4 and 8x8 reduction facilities (effective cross sectional area for each grid of 3,834 mm2 and 825 mm2) with 300 mm drainage, this study proposed a method for installing reduction facilities for each target capture efficiency according to the design discharge. Also, This study proposed a method for installing reduction facilities for A decrease in the linear capture efficiency was observed when the Froude number was less than 1.5 in each reduction facility with 300 mm drainage.

Acknowledgements

This research was supported by a grant (13SCIPS04) from Smart Civil Infrastructure Research Program funded by Ministry of Land, Infrastructure and Transport (MOLIT) of Korea government and Korea Agency for Infrastructure Technology Advancement (KAIA).

References

AASHTO (1975) Guidelines for the Hydraulic Design of Culverts.

AASHTO (1999) Highway Drainage Guidelines.

Badri, B.S, Nakagawa, H.J, Kawaike, K.J, Baba, Y.K, and Zhang, H (2012) Driftwood Deposition from Debris Flows at Slit-check Dams and Fans. Natural Hazards and Earth System Sciences, Vol. 61, No. No. 2, pp. 577-602.

Bardou, E, Boivin, P, and Pfeifer, H-R (2007) Properties of Debris Flow Deposits and Source Materials Compared: Implications for Debris Flow Characterization. Sedimentology, Vol. 54, No. No. 2, pp. 469-480. 10.1111/j.1365-3091.2007.00855.x.
crossref
Chun, K.W, Lim, Y.H, Nam, S.Y, Jang, S.J, Jung, Y.S, and Yong, S.Y (2010) Sediment and woody debris trap effect of h-type slit dam with model experiment. 2010 Summer Academic Presentation of Korean Forest Society, Journal of Korean Forest Society.

FHWA (1971). Debris Control Structures. Hydraulic Engineering Circular No. 9.

FHWA (2005). Debris Control Structures Evaluation and Countermeasures. Third Edition. Hydraulic Engineering Circular, No. 9.

Kim, J.H, Chu, K.W, Juung, J.S, Kwon, Y.S, and Kim, Y.G (2013) Effects of Float-Board Screen for Catching Drift Woods and Debris Flows in Urban Area. 2013 Academic Presentation of Korean Institute of Forest Recreation, Journal of the Korean Institute of Forest Recreation.

Kim, Y.J, Nakagawa, H.J, Kawaike, K.J, and Zhangn, Hao (2012) Numerical and Experimental Study on Debris-flow Breaker. Annuals of Disas Prev. Res. Inst, Vol. 55B, No. No. 24, pp. 471-481.

Lim, Y.H, Chun, K.W, Kim, M.S, Yeom, J.J, and Lee, I.H (2008) Capture Effect of Slit Dam for Debris Flow and Woody Debris with Hydraulic Model Experiment: Focusing on A and D Type. 2008 Summer Academic Presentation of Korean Forest Society. Journal of Korean Forest Society.



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