Analysis of flow around impermeable groynes on one side of symmetrical compound channel:An experimental study

This paper presents the results of an experimental study on the influences of floodplain impermeable groynes on flow structure, velocity, and water depth around the groyne（s）. A wooden symmetrical compound channel was used. Groyne models with three different groyne relative lengths, 0.5, 0.75, and 1.0, were used on one floodplain with single and series arrangements. Analysis of the experimental results using the measured flow velocity and water depth values showed that flow structure, velocity, and water depth mainly depend on groyne relative length and the relative distance between series groynes. The flow velocity at the main channel centerline increased by about 40%, 60%, and 85%, and in other parts on the horizontal plane at the floodplain mid-water, depth by about 75%, 125%, and 175% of its original value in eases of one-side floodplain groyne（s） with relative lengths of 0.5, 0.75, and 1.0, respectively. The effective distance between two groynes in series arrangement ranges from 3 to 4 times the groyne length. Using an impermeable groyne with a large relative length in river floodplains increases the generation of eddy and roller zones downstream of the groyne, leading to more scouring and deposition. To avoid that, the groyne relative length must be kept below half the floodplain width,

Flow Characteristic Variations on Groyne Types for Aquatic Habitats

The field formed by groyne has the function of aquatic habitats for the underwater biology. The characteristic of groyne field occurring around downstream of groyne depends on groyne type and shape. Thus to maximize the function of groyne, it needs to understand the flow characteristic around groyne. In this study, experiment model test was conducted in recirculation zone located in downstream of groyne. Groyne types for experiment are three: permeable, impermeable and inclined crest groyne. LSPIV (Large Scale Particle Image Velocimetry) is used to measure flow field around the groyne and it revealed flow characteristic in recirculation zone at each case. In order to estimate the aquatic habitats of groyne fields, critical swimming speed of major fleshwater fish in Korea was compared with the variation of velocity distribution in groyne fields. From the results, the rate of velocity decreases in groyne fields, V/Vapp were measured to be 0 to 0.5 and the results can be fundamental data which are used to estimate the aquatic habitable function of groyne.

Experimental Study on the Flow Characteristics around the Refraction Groyne

The refraction groyne is a complex structure consisted with groyne and groyne’s arm. This study conducted a experiment on the flow influences around the refraction groyne due to changes in the arm angle (θ) and length (AL). Results of experiment were analyzed on the impacts of the refraction groyne according to the projection length (L’). Velocity increase in main channel occurred greater the upward groynes than downward groynes. The vortices occurring at recirculation area of the upward and downward refraction groynes were formed in different shapes. The thalweg height did not have great impact vis-a-vis the extended arm length ratio and refraction angle change. The length of the recirculation area showed a gradual uptrend as the arm length of the groyne increased. Such area was formed at the range of 29% - 47%. For the length of the recirculation area, it was observed to be 10.2 - 14.7 times (URG), 8.4 - 12.7 times (DRG), and 10.6 - 13.8 times (right angle groyne) the projection length (L’) incensement.

Assessment of Habitats according to Groyne Types (Using Pale Chub)

A groyne zone formed by installing groynes functions as habitat or shelter for aquatic organisms during a flood. Since flow characteristics over a groyne zone are affected directly by groynes, understanding flow patterns due to shapes and types of groynes is of importance for designing groynes. In order to use groynes as an eco-friendly river structure, the ecological effects should also be considered at the design stage. In Korea, the ecological effects of groynes have rarely been examined or included for the actual design. In this study, a set of experiments were carried out to investigate flow pattern changes depending on different types of groynes. Based on the flow characteristics in the groyne zone, the scales of habitats and shelters by the groyne types were estimated. In addition, to test the applicability of a habitat evaluation model to the ecological design of groynes, River2D was used for a virtual river. For assessment of the groyne’s function of ecological habitat, the suitability index of a habitat for pale chub, one of the popular fishes in Korea, was used and the habitat areas by the groyne types were analyzed. The flow changes depending on the groyne types simulated by River2D show the applicability for the simulation of an ecological habitat to the groyne design.

Design and Site Experiment of Groyne in Bore Surging Area of Qiantang River

The groyne has been used widely in bank protection and river regulation. The plane layout and structural style of groyne are closely related to the stability of the groyne itself and the effect of bank protection. In this paper, based on the preliminary summary of the design of bank protection in the bore surging area of Qiantang River, some problems regarding the structure of groyne are pointed out and an improved plan is put forward. Site experiment was carried out combined with an emergency repair. The results of the experiment indicate that the improved project is reliable and successful.

Mitigation of the amplitude and celerity of dam break shock wave using combination of groyne and vertical pier

This paper aims to numerically investigate the impact of the combination of groynes and vertical piers on mitigating the amplitude and celerity of dam break wave-front.First,a robust numerical model based on the Finite Volume Method(FVM)with the Harten,Lax and van Leer(HLL)approximate Reimann’s solver’s technique is developed and validated with other research studies.Then many different scenarios including group of piers,group of groynes with different shapes and combination of groynes and piers are considered for dam break flow modeling.The final results showed that group of piers alone or group of groynes alone have no significant effects on the wave-front depth and celerity but,combination of L-head groyne with piers can decrease the amplitude and celerity of the dam break wavefront or flood wave.In this case,a blockage zone between the groynes and intensive backwater effects behind them were observed.

Experimental Study on the Flow Characteristics of ￢-Type Groyne

A hydraulic model test for ￢-type groynes (with “￢” shape) was conducted to analyze the flow characteristics around these groynes. The results of the model tests are expected to be used as fundamental information in designing the ￢-type groyne constructed in the field. Main hydraulic factors such as velocity and thalweg line changes in the main channel and separation area were analyzed in this study. The thalweg line is a stream line where the maximum velocity occurs, whereas the separation area is the boundary between the main flow and the recirculation zone. Model tests with 5 different arm lengths of the ￢-type groynes were conducted by changing the velocity. The LSPIV (Large-Scale Particle Image Velocimetry) technique was used to measure and analyze flow variation around the ￢-type groynes. The velocity in the main channel measured to be increased by 1.5 times. The velocity variation on groyne arm length is little. The width of the thalweg lines (TCL) was changed to 55 - 57% of the channel width. The Froude number did not affect the thalweg line (TCL) and separation line (Sh) changes, however.

Investigation of sediment transport pattern and beach morphology in the vicinity of submerged groyne （case study： Dahane Sar Sefidrood）

Marine structures, such as Groynes, Sea walls and Detached Breakwaters, are constructed in coast of area to improve coast stability against bed erosions due to changing wave and current pattern. Marine mechanisms and interaction with the hydraulic structures need to be intensively studied. Groynes are one of the most prominent structures that are used in shore protection and littoral sediment. The main hydraulic function of the groyne is to control the long shore current and littoral sediment transport. This structure can be submerged and provide the necessary beach protection without negative aesthetic impact. However, for submerged structures adopted for beach protection, the shoreline response to these structures is not well understood. The objective of this study is to predict sediment transport in the vicinity of submerged groyne and comparison with non-submerged groyne focusing on a part of the coast at Dahane Sar Sefidrood, Guilan Province, Iran, where serious coast erosion has been occurred. The simulations were designed using a one-line model which can be used as a first approximation of shoreline prediction in the vicinity of groyne. The results of the proposed model are compared with experimental data to determine the shape of the coast. The results of predicted beach deformation show that when submerged groyne construct in the beach, sediment accumulation will be slightly less than the non-submerged groyne; because transfer coefficient for the submerged groyne is more than non-submerged groyne. This result will cause more sediment passing on submerged groyne. Finally, the result of the present study show that using submerged groyne is an efficient way to control the sediment and beach erosion without causing severe environmental effect on the coast.

Detached-eddy simulation of turbulent coherent structures around groynes in a trapezoidal open channel

The hydrodynamics in a straight open channel with a multiple-embayment groyne field was investigated using the detached-eddy simulation(DES).A series of short groynes were included on a 1:3 side slope of the channel.This work focuses on the turbulent coherent structures around groynes on an uneven bottom.Flows around groyne fields are characterized by massive separation and highly unsteady vortices.DES can capture a wide spectrum of eddies at a lower computational cost than the large eddy simulation(LES)or direct numerical simulation(DNS).In the present work,a zonal DES model(ZDES)was used to simulate the flow around groynes.The ZDES model is a modified version of the DES designed to overcome the model-stress depletion(MSD)of the RANS/LES hybrid model.The vortex system consists of the horseshoe vortex(HV)formed at the base of the obstructions,the necklace vortex(NV)that wrapped the groyne tips near the free surface,and the shedding vortex(SV)underneath the free surface.The effects of the incident flow and local topography on the vortex evolution were investigated by analyzing the mean flow structures and the instantaneous turbulent flow fields.Some important vortices cannot be captured because of the averaging process,while some flow structures cannot be observed in the instantaneous flow.The mean flow is only a reflection of the averaging process when complex vortices are present.

Tracer advection in a pair of adjacent side-wall cavities, and in a rectangular channel containing two groynes in series

A model is presented of particle advection near groynes in an open channel. Open channel hydrodynamics is modelled using the shallow water equations, obtained as the depth-averaged form of Reynolds-averaged continuity and Navier-Stokes momentum equations. A Lagrangian particle-tracking model is used to predict trajectories of tracer particles advected by the flow field, with bilinear interpolation representing the continuous flow field. The particle-tracking model is verified for chaotic advection in an alternating flow field of a pair of blinking vortices. The combined shallow flow and Lagrangian particle-tracking model is applied to the simulation of tracer advection in flow past a pair of side-wall cavities separated by a groyne, and in an open rectangular channel containing a pair of parallel groynes oriented normal to the channel wall. The study is potentially useful in understanding mixing processes in shallow flow fields near hydraulic structures in wide rivers.

Tracer advection in an idealised river bend with groynes

This paper presents numerical simulations of particle advection in the bend of an open channel containing groynes, which is an idealised form of a shallow river bend in a wide river. The flow field is computed using a boundary-fitted solver of the non-orthogonal, curvilinear shallow water equations. The computational grid is generated by solving Poisson-type elliptic partial differential equations using an iterative multi-grid scheme for prescribed boundary coordinates. The shallow water equations are discretised with finite differences in space, and 4 t h order Runge-Kutta integration in time. Tracers introduced at specific initial locations have their trajectories computed using Lagrangian particle tracking. The numerical shallow flow model is verified by comparison to the analytical solution of fully developed flow in an open channel. The combined shallow flow and Lagrangian particle-tracking model is then used to simulate the advection of tracer particles in a rectangular channel containing a pair of parallel groynes, and tracer particles in a curved open channel containing groynes, of dimensions roughly equivalent to a Danube River bend.

Shoreline Instability Study on Fun Island Resorts, Maldives

Shoreline changes induced by erosion and accretion are natural processes that take place over a range of time scales. They may occur in response to smaller-scale events, such as storms, regular wave action, tides and winds, or in response to large-scale events such as glaciation that significantly alter sea levels (rise/fall) and tectonic activities that cause coastal land subsidence or emergence. Environmental related issues are one of the major problems which the native islanders of the Maldives are facing throughout the history. The global warming and sea level rise have been a threat for the people of these islands. The scope of this paper is to find the shoreline instability of the coastal area along Fun Island Resort Maldives and aim is to find the abrasion, erosion on the coastal area of the island throughout the monsoon season. The coastal structures adopted as a protection against erosion has been damaged due to the 2004 Tsunami and the groynes are still to be repaired. A map was generated with the data collected on the topographic survey which shows that the maximum height of the vegetation line was found to be just +2.15 m above Mean Sea Level (MSL). Beach profiles around the island were taken to access the damage to the shoreline of the island.

Influence of near-shore marine structures in a beach nourishment project on tidal currents in Haitan Bay, facing the Taiwan Strait

The Longfengtou Beach in the Haitan Bay, located in Fujian Province of China and facing the Haitan Strait, has been suffering severe erosion due to the construction of seawalls. A simple beach nourishment project implemented has not achieved the anticipated beach restoration. Thus a practical solution has to rely on a combination with near-shore marine structures. In this study, a 2-D calibrated flow model is set up to investigate the effects of three different layouts of near-shore marine structures on the tidal current. It is shown that the breakwaters situated in both the north and south ends play a vital part in the protection against erosion. The offshore breakwaters can serve as a barrier to obstruct the current circulation then reduce the current velocity. The groyne linking the Guimo islet and the coast significantly reduces the south-to-north water exchange through the channel and redirects the current direction nearly perpendicular to the north breakwater, which cuts off the longshore transport that may have a negative influence on the beach, especially, the northern part. It is also noted that the monsoon at the site with different directions increases the current velocity. In general, proper layouts of marine structures can reduce the current velocity thus lead to less intense sand transport near the beach.