Large-scale experimental study on scour around offshore wind monopiles under irregular waves

The Keulegan-Carpenter(KC)number is the main dimensionless parameter that affects the local scour of offshore wind power monopile foundations.This study conducted large-scale(1:13)physical model tests to study the local scour shape,equilibrium scour depth,and local scour volume of offshore wind power monopiles under the action of irregular waves with different KC numbers.Systematic experiments were carried out with the KC number ranging from 1.0 to 13.0.With a small KC number(KC<6),and especially when the KC number was less than 4,the scour mainly occurred on both cross-flow sides of the monopile with a low scour depth.When the KC number exceeded 4,the shape of the scour hole changed from a fan to an ellipse,and the maximum scour depth increased significantly with KC.With a large KC number(KC>6),the proposed method better predicted the equilibrium scour depth when the wave broke.In addition,according to the results of three-dimensional terrain scanning,the relationship between the local equilibrium scour volume of a single offshore wind power monopile and the KC number was derived.This provided a rational method for estimation of the riprap redundancy for monopile protection against scour.

Scour around submarine pipes due to high-amplitude transient waves

Estimation of scour dimensions below submarine pipelines is a vital step in designing offshore infrastructure.Extreme events like tsunami waves produce strong erosive forces below the underwater pipes,apt to create scour holes,jeopardizing the safety of the structure.Despite the importance of this issue,previous studies have mainly focused on steady flow cases,and the scour pattern below submarine pipes induced by high-amplitude transient waves has rarely been investigated.This paper reports the results of 40 experimental runs on transient wave-induced scour below a model pipe in a laboratory flume under a variety of initial conditions.The variables included the bed particle size and gradation,initial water depth,wave height,and slope of the bed layer.Waves were generated by a sudden release of water from a sluice gate,installed in the middle of the flume.A pressure transducer data acquisition system was used to record the wave heights at different time steps.The results indicate that,with a shallower initial depth of flow,the scour depth is relatively large.It was also found that there exists a direct correlation between the induced wave height and the size of the scour hole.It was observed that,in clear water conditions,the size of the scour hole in coarse sediments is smaller,while in live-bed conditions,larger scour holes are created in coarser sediments.It was also observed that at high wave amplitudes,the live-bed conditions are dominant,and consequently the bed elevation is altered.

Impact of an artificial chute cutoff on the river morphology and flow structure in Sipaikou area of the Upper Yellow River

Artificial chute cutoff can fundamentally eliminate the threat of flood caused by the meandering river,but it significantly changes its morphodynamic characteristics.Channel adjustments after cutoff are rapid,which makes it difficult to study the interaction between river morphology and flow structure only through field measurement.However,numerical simulations can provide insights into the hydrodynamic characteristics after artificial chute cutoffs.In this study,both field measurement and numerical simulation are employed to investigate the flow structure and bed morphology caused by an artificial chute cutoff in Sipaikou area of the Upper Yellow River in 2018.The measured hydrological data provide boundary conditions and initial values for the numerical model.The field measurement results reveal that the concave bank of the study area is severely scoured up to 270 m after the artificial cutoff,and a 20 m deep scour hole and a 2.26 km long pool are formed at the entrance and near the left bank of the chute channel.The numerical simulation results of velocity at typical cross-sections are in good agreement with the measurement results.Flow separation and stagnation zones are observed near the right bank during the low flow conditions(discharge of at least 902 m^(3)/s),but this phenomenon is not seen during larger flow conditions(discharge exceeds 2000 m^(3)/s).Interestingly,flow recirculation zones are also found near the left and right banks of the scour hole.Further,a long flux belt is formed at the scour hole and the pool.Consequently,the impact of the bed topography on the hydrodynamic characteristics is relatively prominent when the discharge is small,while the impact on the river banks and river bed is more noticeable when the water discharge is large.In addition,high shear stress is observed near the left bank at the downstream of the studied area,which indicates that the left bank at the downstream is still being scoured.These results suggest that bank protection measures along the left bank are required to maintain the effectiveness of the artificial chute cutoff.

Engineering application of submerged water jets for sediment removal in a tidal riverbed

Sediment deposition problems have attracted the interest of engineers and researchers.Several experimental studies have been conducted on scour depth using turbulent jets.However,field observation and monitoring have rarely been reported.This study aimed to eliminate sediments on a tidal riverbed using a prototype device,which consisted of a set of submerged vertical water nozzles and submerged horizontal air nozzles.The effectiveness of the water jet in sediment removal during spring and neap tides was evaluated.The quantitative relationships of dimen-sionless parameters,such as(1)the relative sediment scour volume versus the number of flows from the jet exit,(2)the relative sediment scour volume versus the relative scour depth,and(3)the relative scour size versus the relative jet intensity,were analyzed.The results showed that the freshwater flowing to the sea affected the sediment scour volume during the falling cycle of spring tides.In contrast,the rising cycle of spring tides retarded the freshwater flow,resulting in a decrease in the sediment scour volume.A steep water surface slope accelerated the river flow and further influenced the cross-flow current around the study area.As a result,a highly diffusive turbulent flow was produced,causing sus-pended sediments to be rapidly removed from the scour hole center.An increase in the number of flows from the jets led to intensified diffusion of turbulent energy into the flow.The rapidly varying water depth caused jet energy to be dissipated before approaching the riverbed,and it significantly affected the scour process during the spring-tide period.The proposed equations can be used to estimate the scour volume,scour size,and re-suspended sediments in tidal rivers within defined ranges of parameters.

Local scour by multiple slit-type energy dissipaters

Energy dissipation and scour control are all the key issues for the design of hydraulic structures.On the basis of the high energy dissipation for the multiple slit-type energy dissipaters(M-STED)developed by the authors,in this work,the characteristics of the scour hole for the M-STED were experimentally investigated through three sets of those physical models with five cases and a scour hole form index was proposed.The results show that,the M-STED results in not only the high energy dissipation but also the mild upstream slope of the scour hole thanks to the scour hole form index of larger than 0.5,which is in favor of the safety of the release works.

SCOUR OF FINE SEDIMENT BY A TURBULENT WALL JET

A total of 66 experiments were conducted to investigate the scour of fine finesediment by a turbulent wall jet. The independent variables studied were the flow velocity, the jet size, the grain sine, and the water temperature.Three Closely sized grades of bed material were used, and their median diameters were 0. 273mm, 0. 050mm, and 0. 030mm. The jet velocities varied from 0. 30m/s to 1. 10m/s for the coarse sediment (D =0. 273mm), and from 0. 30m/s to 0. 70m/s for the fine grades (D = 0. 050mm , and D= 0. 030mm). The jet size was set to 3. 18mm, 6, 35mm, and 9. 53mm for each grade size, and the water temperature varied from about 60 degrees Fahrenheit to about 85 degrees Fahrenheit.The independent variables were analyzed using dimensional analysis. Three dimentsionless Parameters, namely U(=pu2/ΔpgD), B(b/D), and G(=ΔpgD3/pv2), were obtained. These parameters enabled a close correlation of all experimental results. Other studies were also found to correlate well with these parameters.