Experiments on scour around multiple piles were carried out for several current and wave conditions: pile numbers were 1, 3, and 7; and wave only, current only, and current and wave flows. A moderate wave condition was chosen as the representative wave, which produced relatively small scour depth. A depth-average current speed was chosen as the representative current, which produced relatively large scour hole. When waves were superimposed on current, scour hole depth decreased. Existing the Sumer and Fredsøe’s prediction formula of scour hole depth for current and wave flows for multiple piles describes the present experimental results reasonably well

Byoungwook Min

Hee-Suk Ryoo

Hyoseob Kim

English

Vibroengineering PROCEDIA

 210 VIBROENGINEERING PROCEDIA. SEPTEMBER 2018, VOLUME 19  
Scour depth around multiple piles for current and wave 
Hyoseob Kim1, Byoungwook Min2, Hee-Suk Ryoo3 
1, 2Kookmin University, Seoul, Korea 
3Korea Electrotechnology Research Institute, Gyeongsangnam-do, Korea 
2Corresponding author 
E-mail: 1hkim@kookmin.ac.kr, 2minbw92@kookmin.ac.kr, 3hsryoo@keri.re.kr 
Received 16 August 2018; accepted 24 August 2018 
DOI https://doi.org/10.21595/vp.2018.20164 
Copyright © 2018 Hyoseob Kim, et al. This is an open access article distributed under the Creative Commons Attribution License, which 
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 
Abstract. Experiments on scour around multiple piles were carried out for several current and 
wave conditions: pile numbers were 1, 3, and 7; and wave only, current only, and current and 
wave flows. A moderate wave condition was chosen as the representative wave, which produced 
relatively small scour depth. A depth-average current speed was chosen as the representative 
current, which produced relatively large scour hole. When waves were superimposed on current, 
scour hole depth decreased. Existing the Sumer and Fredsøe’s prediction formula of scour hole 
depth for current and wave flows for multiple piles describes the present experimental results 
reasonably well. 
Keywords: scour, group piles, experiment, scour depth, wave, current. 
1. Introduction 
Scour around vertical structures can cause several engineering problems, e. g. reducing bearing 
capacity of the structure, or increasing tensions of relevant pipelines or cables attached to the 
structure. Prediction of scour focuses on either final equilibrium state, or time-evolving feature. 
For instance, the scour starts to develop when the structure is constructed until reaches  
equilibrium. Scour could keep on changing, if the surrounding flows change continuously like 
tidal flow. 
Equilibrium scour hole depth around vertical cylindrical structures for steady flow has been 
extensively studied through laboratory experiments or numerical experiments, and empirical 
formulas for prediction have been proposed by many researchers. In study of Sumer and Fredsøe 
[1], the Keulegan-Carpenter number has proven to be the dominant variable deciding the 
equilibrium scour depth. Sumer et al. [2] studied scour around vertical pile in wave. In that study, 
the results of an experimental measuring on scour around piles were presented. Also based on 
their data, a design equation was established, relating the scour depth to the Keulegan-Carpenter 
number. When multiple piles are constructed, scour hole depths are affected by the number of the 
piles as well as the dimensions of the mono pile due to enhanced turbulence generated by many 
piles. The additional influence of scour depth has been accounted by the ratio between the gap 
width and the pile diameter (Sumer and Fredsøe [3]). However, it is presumed that the above ratio 
of the gap width over the pile diameter may not be sufficient for prediction of the overall scour 
hole depth. The number of piles or the total area of the multiple piles may influence the 
group-related scour depth, 𝑠௚. 𝑠௚ is the group-related scour depth, and 𝑠௟ is the local scour depth, 
see Fig. 1. 
 
Fig. 1. Scour depth group-related and local scour depth 
SCOUR DEPTH AROUND MULTIPLE PILES FOR CURRENT AND WAVE.  
HYOSEOB KIM, BYOUNGWOOK MIN, HEE-SUK RYOO 
 ISSN PRINT 2345-0533, ISSN ONLINE 2538-8479, KAUNAS, LITHUANIA 211 
It may be meaningful to investigate the effect of the number of piles with the same ratio of the 
gap over the pile diameter, while keeping all the other parameters identical. 
2. Experiment design 
In order to distinguish the effect of multiplicity of piles explicitly, experimental cases were 
carefully designed for the present study. We intend to extract the effect of pile numbers, while 
keeping all the other variables steady.  
Typical three flow conditions were chosen as representative flow environments: wave only, 
current only, and current and wave coexisting flows. It could be expected that these cases may not 
cover wide range of flow situation, but reveal the characteristics of the scour with multiple piles. 
The two-dimensional flume at Kookmin University, Seoul, Korea was used: 0.8 m wide, 
1.0 deep, and 23 m long. The flume includes panel-type wave generator, and reversible current 
generating pump, see Fig. 2. 
The water depth was maintained 0.5m at centre of sandy bed, and the circular vertical pile 
diameter was 10 cm. the gap width ratio for both 3 and 7 piles were 2, see Fig. 3.  
 
Fig. 2. Experimental model and current and wave direction 
 
a) Single pile 
 
b) 3 piles 
 
c) 7 pile 
Fig. 3. Floor plan of experimental case for the number of piles 
Bed material should be chosen to represent field conditions. Considering west coast of Korea, 
where bed material is mainly sand along main tidal channels, and mixed sand and silt around 
shaded zones. Median sand diameter is between 100-300 μm on the west coast of Korea. Applying 
the length scale of model over prototype of 1/40, and Froude similarity, could be applied to the 
settling velocity. Then, the adequate sand median diameter should be 80 μm, which was used for 
the present experiments.  
The sandy bed was prepared within 2.0 m length between two transitional steps, see Fig. 2. 
The sand depth was 20 cm, which may be sufficiently deep for possible scour due to waves and 
currents. 
In order to distinguish the effect of the total pile number on the total scour depth, a typical 
wave condition needs to be chosen, so that it is applied to most experimental cases. 
In study of Sumer and Fredsøe [1], the Keulegan-Carpenter number has proven to be the 
SCOUR DEPTH AROUND MULTIPLE PILES FOR CURRENT AND WAVE.  
HYOSEOB KIM, BYOUNGWOOK MIN, HEE-SUK RYOO 
212 VIBROENGINEERING PROCEDIA. SEPTEMBER 2018, VOLUME 19  
variable dominating the equilibrium scour depth: 
𝐾𝐶 = 𝑈௠𝑇𝐷 , (1) 
where 𝑈௠ is the undisturbed linear near-bed orbital velocity amplitude, 𝑇 is the wave period, and 
𝐷 is the pile diameter. 
Keulegan-Carpenter number is known as a dimensionless quantity describing the relative 
importance of the drag forces over inertia forces for bluff objects in an oscillatory fluid flow.  
We choose a monochromatic wave train of period 1.5 s, and 10 cm amplitude on the water 
depth of 0.5 m, which results in KC of 2.5, which implies waves are large to produce scour around 
piles. The fact that the KC number does not reflect information on the bed material like median 
bed size, implies that the scour hole depths are strongly controlled by the structural dimension, if 
the flow speed is above a threshold value. 
Preliminary experiments demonstrate bedform existence for a range of current speeds. 
Depth-average current velocity of 50 cm/s produces large bed shear stress, and consequent scour 
holes. Depth-average current velocity was kept on 0.5 cm/s by adjusting the flow rate meter, for 
current only cases and current and wave cases. 
The experimental cases were designed as combinations of 3 flow cases and 3 pile alignments 
cases, i.e. current, wave, current/wave flows, multiplied by single, 3, and 7 piles cases; i.e. 9 cases 
altogether. 
3. Results 
The experiment results of the scour depths are shown in the following tables. 
Table 1. Measured scour depth for single pile 
Case number Condition Scour depth (cm) 
1 Current only 7.2 
2 Wave only 0.3 
3 Current and Wave 6.9 
Table 2. Measured scour depth for 3 group piles 
Case number Condition Pile Scour depth (cm) 
4 Current only 
1 8.5 
2 9.6 
3 10.3 
Average 9.5 
5 Wave only 
1 1.7 
2 1.9 
3 0.9 
Average 1.5 
6 Current and Wave 
1 7.3 
2 8.7 
3 8.9 
Average 8.3 
First, the scour depth of Case 1 for current only and single pile was 7.2 cm, which is 0.72 times 
the pile diameter. 
The scour depth of Case 2 for wave only and single pile was 0.3 cm, which is much smaller 
than that of current only case (Case 1). The wave orbital velocity near bed is 17.7 cm/s, and is 
thought to be comparable with the depth-average current speed of 0.5 m/s. small scour depth for 
their case could be understood as periodically reversing flow behaviors.  
SCOUR DEPTH AROUND MULTIPLE PILES FOR CURRENT AND WAVE.  
HYOSEOB KIM, BYOUNGWOOK MIN, HEE-SUK RYOO 
 ISSN PRINT 2345-0533, ISSN ONLINE 2538-8479, KAUNAS, LITHUANIA 213 
Table 3. Measured scour depth for 7 group piles 
Case number Condition Pile Scour depth (cm) 
7 Current only 
1 11.9 
2 12.2 
3 10.7 
4 12.8 
5 11.9 
6 13.3 
7 13.2 
Average 12.3 
8 Wave only 
1 2.5 
2 2 
3 1.9 
4 2.4 
5 2.7 
6 2.3 
7 2.2 
Average 2.3 
9 Current and Wave 
1 11.7 
2 11.9 
3 9.4 
4 12.5 
5 12.4 
6 12.9 
7 12.8 
Average 11.9 
The scour depth of Case 3 for current and wave flow and single pile was 6.9 cm, which is 
slightly smaller than that of for current only case. It is thought that the instantaneous friction bed 
shear stress around scour hole may exceed a limiting value dividing the normal flow and sheet 
flow.  
The scour depth of Case 4 current only flow and 3 piles was 9.5 cm on average, larger than 
those for single pile case, Case 1. The scour depth of Case 7, current only flow and 7 piles was 
12.3 cm on average, larger than those for single pile case, and 3 piles case. 
The scour depth of Case 5 wave only and 3 piles was 1.5 cm on average, larger than those for 
single pile case, Case 1. The scour depth of Case 8 wave only and 7 piles was 2.3 cm on average, 
larger than those for single pile case, and 3 piles case. 
The scour depth of Case 6 current and wave and 3 piles was 8.3 cm on average, larger than 
those for single pile case, Case 3. The scour depth of Case 9 current and wave and 7piles was 
11.9 cm average, larger than those for single pile case and 3 piles case, see Figs. 4-6. 
 
a) Wave 
 
b) Current 
 
c) Current and wave 
Fig. 4. Photo of single pile experiment result 
This paper in case of group pile, the scour depth for each pile was averaged and compared. 
SCOUR DEPTH AROUND MULTIPLE PILES FOR CURRENT AND WAVE.  
HYOSEOB KIM, BYOUNGWOOK MIN, HEE-SUK RYOO 
214 VIBROENGINEERING PROCEDIA. SEPTEMBER 2018, VOLUME 19  
Sumer et al. [2] and Sumer and Fredsøe’s parameter [4] used for compared experimental scour 
depth with the theoretical scour depth.  
Sumer et al. [2]: 
𝑆
𝐷 = 1.3ሾ1 െ exp ሺെ0.03ሺ𝐾𝐶 െ 6ሻሻሿ,   𝐾𝐶 ൒ 6. (2) 
In case of current and wave case, the scour depth is a function of the 𝐾𝐶 number and the 
parameter 𝑈௖௪ = 𝑈௖ ሺ𝑈௖ ൅ 𝑈௠ሻ⁄ . Sumer and Fredsøe introduced 𝑈௖௪ defined as: 
Sumer and Fredsøe’s parameter 𝑈௖௪ [4]: 
𝑈௖௪ =
𝑈௖
𝑈௖ ൅ 𝑈௠, (3) 
where 𝑈௠ is the maximum value of orbital velocity of water particles at bed, and 𝑈௖ is the current 
velocity at distance of D/2 from bed. 
For comparison, the scour depth was non-dimensionalized by 𝑆 𝐷⁄ . Compare Sumer and 
Fredsøe formula with experimental result. The results are as follows, see Fig. 4. 
 
a) Wave 
 
b) Current 
 
c) Current and wave 
Fig. 5. Photo of 3 group piles experiment result 
 
a) Wave 
 
b) Current 
 
c) Current and wave 
Fig. 6. Photo of 7 group piles experiment result 
 
Fig. 7. Experimental scour depth and theoretical scour depth (𝑆 𝐷⁄ ) 
SCOUR DEPTH AROUND MULTIPLE PILES FOR CURRENT AND WAVE.  
HYOSEOB KIM, BYOUNGWOOK MIN, HEE-SUK RYOO 
 ISSN PRINT 2345-0533, ISSN ONLINE 2538-8479, KAUNAS, LITHUANIA 215 
In cases of wave alone, 𝑆 𝐷⁄  was not much difference compared to result. Therefore, current 
cases and current and wave cases were compared. The Fig. 8(a), (b) are 𝑆 𝐷⁄  of Sumer and Fredsøe 
result and experimental result in current only flow and current and wave, see Fig. 8. 
a) Current only b) Current and wave 
Fig. 8. Scour depth of according to number of piles 
4. Conclusions 
Experiment exhibited that the pile number increase the total scour depth, for the same gap over 
diameter ratio. 
Experimental results demonstrate that the scour depths were small for wave only flows. Results 
also demonstrate that the scour depths for current and wave flows are slightly smaller or equal to 
those for current only flows. These results imply that the experimental conditions adopted include 
phases which may result in the sheer flow condition due to added reversing wave flows. 
The conditions in the present experiments don’t cover all field environment. However, it is 
hoped that the flow and sediment conditions of their experiment represent a typical environment 
of the west coast of Korea. Wider range of conditions are recommended for real field design 
purposes.  
For single pile cases, the scour occurred just around pile. However, for group piles cases, scour 
developed deeper than single pile, and scour was wider. Moreover, the larger the number of piles, 
the deeper the total depth of scour. 
Acknowledgements 
This work has been funded as a Research Project, “Installation and Corroborative Study for 
Internal Power Network of Ocean Wind Power Plant”, by Korea institute of Energy Technology 
Evaluation and Planning (KETEP) since 2015. 
References 
[1] Sumer B. M., Fredsøe J. Scour below pipelines in waves. Journal of Waterway, Port, Coastal, Ocean 
Engineering, ASCE, Vol. 116, Issue 3, 1990, p. 307-323. 
[2] Sumer B. M., Fredsøe J., Christiansen N. Scour around vertical pile in waves. Journal of Waterway, 
Port, Coastal, Ocean Engineering, ASCE, Vol. 118, Issue 1, 1992, p. 15-31. 
[3] Sumer B. M., Fredsøe J. Wave scour around group of vertical piles. Journal of Waterway, Port, 
Coastal, Ocean Engineering, ASCE, Vol. 124, Issue 5, 1998, p. 248-256. 
[4] Sumer B. M., Fredsøe J. Scour around pile in combined waves and currents. Journal of Hydraulic 
Engineering, ASCE, Vol. 127, Issue 5, 2001, p. 403-411. 


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