Showing papers on "Groyne published in 2021"

Coastal Structures as Beach Erosion Control and Sea Level Rise Adaptation in Malaysia: A Review

Abstract: The shoreline of Malaysia is exposed to threats of coastal erosion and a rise of sea level. The National Coastal Erosion Study, 2015 reported that 15% of an 8840 km shoreline is currently eroding, where one-third of those falls under the critical and significant categories that require structural protection. The Study of Sea Level Rise in Malaysia, 2017 presented a sea-level increase of 0.67–0.74 mm on average yearly. This study reviewed selected coastal protection structures along the shoreline of Malaysia as an erosion control and sea-level rise adaptation based on coastal management strategies. Hard structures such as rock revetment and breakwater are commonly used as erosion protection systems in the “hold the line” strategy. Increased platform level of seawalls and earth bunds, considered as an “adaptation” approach, are effective in erosion protection and are adaptive to sea-level rise. Mangrove replanting is suitable as a “limited intervention” approach in minimizing the long-term impact of both threats. However, offshore breakwater, groyne, and geotextile tubes are solely for protection purposes and are not as effective for sea-level rise adaptation. As the sea level is continuously increasing, their function as coastal protection will also become less effective. In summary, this comprehensive review on coastal protection in Malaysia will benefit the related agencies on the future assessment.

Study of Sediment Transport in a Tidal Channel-Shoal System : Lateral Effects and Slack-Water Dynamics

Abstract: Lateral flows redistribute sediment and influence the morphodynamics of channel-shoal systems. However, our understanding of lateral transport of suspended sediment during high and low water slack is still fairly limited, especially in engineered estuaries. Human interventions such as dike-groyne structures influence lateral exchange mechanisms. The present study aims to unravel these mechanisms in a heavily engineered, turbid channel-shoal system in the Changjiang Estuary, using a high-resolution unstructured-grid three-dimensional model and in situ observations. Analysis of model results reveals two typical transport patterns during slack-water conditions, that is, shoal-to-channel transport during low water slack and channel-to-shoal transport during high water slack. A momentum balance analysis is carried out to explain mechanisms driving the lateral transport of suspended sediment during high water slack, revealing the importance of lateral pressure gradients, Coriolis force, and the curvature-induced term. Groyne fields play a crucial role in sediment transport, especially during low water slack. A model scenario in which one groyne is removed reveals that groyne fields strongly influence lateral sediment transport. The decomposition of the sediment transport flux reveals that the turbidity maximum is shaped by a balance between seaward advection by residual flows, and landward transport by tidal pumping and gravitational circulation. Within the turbidity maximum, sediment is laterally redistributed by lateral flows during slack-water conditions, greatly influencing estuarine channel morphology.

Cross-Shore Profile Evolution after an Extreme Erosion Event—Palanga, Lithuania

Abstract: We report cross-shore profile evolution at Palanga, eastern Baltic Sea, where short period waves dominate. Cross-shore profile studies began directly after a significant coastal erosion event caused by storm “Anatol”, in December of 1999, and continued for a year. Further measurements were undertaken sixteen years later. Cross-shore profile changes were described, and cross-shore transport rates were calculated. A K-means clustering technique was applied to determine sections of the profile with the same development tendencies. Profile evolution was strongly influenced by the depth of closure which is constrained by a moraine layer, and the presence of a groyne. The method used divided the profile into four clusters: the first cluster in the deepest water represents profile evolution limited by the depth of closure, and the second and third are mainly affected by processes induced by wind, wave and water level changes. The most intensive sediment volume changes were observed directly after the coastal erosion event. The largest sand accumulation was in the fourth profile cluster, which includes the upper beach and dunes. Seaward extension of the dune system caused a narrowing of the visible beach, which has led to an increased sand volume (accretion) being misinterpreted as erosion

Evaluation of optimised groyne designs in response to long-period ship wave loads at Juelssand in the Lower Elbe Estuary

Abstract: In the past 15 years, increased severity of damages to rock structures such as groynes, training walls and revetments have been observed across the major German estuaries. Investigations into the cause of the damages concluded that structure deterioration could be attributed to long-period primary ship wave loading. Owing to changes in the shipping fleet, in particular the ever-increasing dimensions of container vessels, the intensity of ship-induced loads has increased significantly. As a result, existing rock structures are, in certain parts, under-dimensioned for the presently prevalent load intensities. To evaluate the performance of two new groyne designs, optimised with a view to increase structure resistance to long-period wave loads, a prototype study was undertaken. Two groynes in the tidal Lower Elbe were rebuilt, one with a large-radius root and one with a recessed root. During the field experiment the incident ship-induced wave loading and the armour layer deformation was recorded. The field experiment and some fundamental findings with regard to the wave-structure-interaction at prototype scale and the performance of the optimised groyne designs are documented here. Furthermore, insights into mitigation measures are given, based on analyses of the collected data.

Analysis on The Effect of Groyne Type Impermeable Placement on Sediment Distribution in Lariang River Bend

Abstract: Sedimentation and erosion caused by differences in discharge is a problem that often occurs at river bends. One of the functions of placing the groyne is to reduce the river flow velocity along the riverbank, accelerate sedimentation, and ensure the embankment or river bank's safety against scouring. This study aimed to determine the distribution of current velocity, distribution of riverbed shear stress, and sediment distribution to the effect of groyne placement. There are 7 (seven) simulation models used to get the most effective groyne placement. From the model simulation results by adding a groyne building on the river's outer bend for the simulation model scenario 2 to scenario 7, the largest sedimentation production is in scenario 3, namely the height of riverbed sediment deposits 230 hours of 1.094 m. Furthermore, the groyne building's effective placement is determined based on a maximum change in the riverbed scenario. In scenario 3, the placement and dimensions of the groyne length of 17.5 m; distance between groyne 24.4 m; the groyne is inclined upstream in the direction of flow 10°.

Shoreline changes due to construction of groyne field in north of Chennai Port, India.

Abstract: Chennai Port (13.099872° N, 80.297407° E), located along the southeast coast of India, has been a hub for maritime trade since the fifteenth century. An artificial harbour was initially constructed in 1881 which underwent numerous expansions in the following years. The breakwaters of the harbour intercepted the heavy sediment-laden littoral drift along the coast, resulting in the formation of the world's second-longest urban beach south of the port, i.e. on its up-drift side. Meanwhile, the coast north of the port, i.e. the down-drift side, experienced intense erosion due to a lack of sediment supply and forces induced on the coast due to waves and currents. The shoreline change study in this paper investigates a shoreline stretch of about 6 km (protected by transitional groynes), north of Chennai Port by dividing it into three segments. The rate of shoreline changes over a period of about 12 years across three different segments was assessed using statistical parameters by employing remote sensing techniques complemented with geographical information system (GIS) and digital shoreline analysis system (DSAS) tool. It is inferred that the coast has witnessed accretion and sizable growth in beach width has been observed post the construction of groyne field.

Variation of Seasonal Monsoon Wave-Driven Circulation Pattern in the Lee of Groyne in Kuala Nerus beach

Abstract: Terengganu coast tend to dual monsoonal as presenting Northeast Monsoon (NEM) as a storm season and Southwest Monsoon (SWM) as a calm season. The area of interest is a groyne in Kuala Nerus, Terengganu on east part of Peninsular Malaysia. The use of numerical model was performed on the dual monsoon seasons with MIKE 21 modules (Spectral Wave FM and Flow Model Hydrodynamic FM modules). Two numerical model were set up according to the two season. The resulting model indicates, the current circulation pattern consisted of persistent eddy re-circulation in the lee of the groyne during both season. As the longshore current directed into the groyne, it deflected offshore resulting in rip current along the groyne. The rip current and incoming wave converge at the tip of the groyne and flow parallel to coastline before being projected onshore performing a re-circulation cell. During NEM, the model shows a clockwise re-circulation cell and counter-clockwise formation during SWM. The formation of the rip current and eddy circulation is associated with erosion event as the sediment transported offshore causing sediment shortage on the adjacent shore. The importance of this study can be a reference to any development of coastal defense monsoon dominated environment.

Simulation of flow pattern around series of groynes with different orientations in meandering channels

Abstract: The mechanism of flow and turbulence around groynes in a meandering channel is very complex phenomena. To simulate the flow pattern around groynes in meanders a 3D numerical model was built with groynes of different inclinations by using ANSYS Fluent. The developed model was verified by the results of experimental study in meanders. Pressure, velocity, stream lines and turbulent kinetic energy for different groyne angles such as 15°, 30°, 45°, 90°, 120° and 150° were compared and analyzed to find the ideal angle configuration for different functions. Streamlines profiles were studied to understand more about the formation of scours and vortices around the groyne. The study of streamline profiles helped in the study of vortex and scours. The most dynamic variation in pressure and turbulent kinetic energy within the contours were found for groynes with 90° orientation. Large numbers of vortices between the groynes were observed for groynes with 15° and 90° orientation and these vortices were observed between the second and third groynes. For groynes in the meandering, the ideal angle for controlling the velocity was found out to be 45°, as it showed very small number of vortices in the groyne field. The results of the study showed that numerical modelling could be used efficiently for the study of flow and turbulence around groynes in meanders.

Investigations on the Impact of Using Elliptic Groynes on the Flow in Open Channels

Abstract: This paper presents a numerical simulation of the flow around elliptic groynes by using CFD ‎software The flow was simulated in a flume with 4m long, 04m wide, ‎and 0175m ‎high ‎‎with a constant bed slope Moreover, the first Groyne placed at 1m from the flow ‎‎inlet with a ‎constant the Groyne height of 10cm and a 1cm thickness, and the ‎width of Groynes equals ‎7cm‎ A submergence ratio of the elliptic Groynes of 75% was assumed, corresponding to a discharge of ‎00057‎m3/sec The CFD ‎model showed a good ability to simulate the flow ‎around ‎Groynes with ‎ good accuracy The results of ‎CFD software showed that when using double elliptic Groynes, the bed shear stress is decreased with the ‎increase in the spacing between Groynes, as well as and the best spacing between the double ‎elliptic Groynes is twice of the Groyne width Moreover, the used number of Groynes has no ‎much ‎impact ‎on velocity and shear ‎stress values

Physical modelling of submerged groynes

Abstract: Currently multiple projects investigate the effect of lowering the crest of groynes in the Dutch rivers with the aim to lower the backwater effect of groynes under high water levels. There is a major uncertainty however in the modelling of groynes, and therefore in the prediction of the effect of groyne lowering. Currently submerged groynes are modelled subgrid as weirs. The flow processes differ however between groynes and weirs as discharge over a groyne is not purely defined by the contraction and expansion over the groyne, but also by lateral shear and interaction with the surrounding main channel and floodplain. The result is a lower resistance of the cross section than expected from weir modelling. For this reason a numerical investigation was started, comparing the modelling of groynes as different subgrid weirs in a 2D model and the modelling of groynes included in the bed topography in a 3D (non-)hydrostatic model. There were large differences between all different methods of groyne modelling. Furthermore the available experimental data did not suffice to explain the differences. For that reason a new physical model has been set up in the Delft University of Technology, at the hydraulic laboratory of the Faculty of Civil Engineering and Geoscience. The experiment includes a 1:30 scale model of a representative transect of the Waal river. The model is 5 m wide and 30 m long and includes 6 groynes and 5 groyne fields. It further includes a part of the main channel at one side and a part of the floodplain at the other side of the groyne fields. Gravel is fixed to the bed to ensure hydraulically rough flow conditions. In the experiment many measurements were done. The first aim of the measurements was to obtain a good spread of points to validate numerical models on. The second aim was to gain insight in the different flow processes in order to describe the differences between groyne and weir flow and quantify the resistance groynes have on a flow. An important observation was the existence of a region of low flow, at the tip of the groyne. The observation indicates a complex three-dimensional flow which effectively redistributes discharge over the transverse. The contribution of this flow to the two-dimensional momentum balance is then directly as a secondary circulation and indirectly by alternating the distribution of discharge. Complex flow furthermore invalidates the hydrostatic pressure assumption. The observed flow needs further investigation with either more accurate measurement devices or in a 3D non-hydrostatic model. Another observation includes the contraction and expansion of flow over the groyne crest. The expansion of flow over the main body of the groyne seems comparable to the expansion of flow over a weir, which can be predicted as Carnot head losses. The observation of expansion losses is however clouded by other flow processes, such as bed shear stress, the lateral flow around the groyne and possible three-dimensional processes induced by the groyne tip, so that the observed head loss over the groyne is only partially explained by weir-like expansion losses. The comparability between groyne flow and weir flow does not seem to hold at the groyne tip, where no separation of flow is observed. Measurements were performed 10 cm away from the transition between the groyne tip and main body of the groyne. This means that at full scale there is a region of 3 m on either side of this geometrical transition where the transition lies between weir-like overflow and non-weir-like overflow. With the here performed simulations it is possible to adapt weir head loss formulas on the observed head losses over groyne. It should be possible to validate a three-dimensional non-hydrostatic model, which can further quantify the effect of three-dimensional flow and differentiate the different contributions to groyne head loss. Based on these models proper tuning parameters can be chosen to represent the complex flow in a two-dimensional model and separately model the weir-like expansion.

Stability of Groynes Built Using Geo Systems

Abstract: In general, the groyne sections are made of rubble stones of different sizes. To conserve the ecosystem and also due to the scarcity of natural rocks around certain locations, the reliance on these hard materials for the construction of groyne sections becomes questionable forcing us to look for alternate eco-friendly soft materials, for example, geo systems. In order to evaluate the feasibility and advantages of geo systems for groyne construction and to test their hydrodynamic stability, the core section of rubble stones was replaced by geo-bags filled with sand. The objective of this study is to assess the wave transmission, hydrodynamic stability and wave overtopping of a groyne section with a geo-bags filled core section through physical model studies. The tests were conducted in a wave flume in Department of Ocean Engineering, Indian Institute of Technology Madras, India. A model scale of 1:15 is adopted based on Froude’s scaling law and dimensional limitations of the testing facility. The aforementioned cross sections were tested simultaneously in a wave flume that was partitioned longitudinally. The transmission and hydrodynamic stability of both the sections have been analysed by subjecting the corresponding sections to the action of random waves defined by JONSWAP spectrum through a well-controlled experimental programme. The details of test set-up, testing facility, parameters of model, test procedure, analysis of results and discussion will be reported in this chapter.

Best groyne series as countermeasure against river bend scour

Abstract: A new approach for finding a cost-effective combination of groynes in a channel bend for the prevention of bend scour is presented. A linked simulation–optimisation model is proposed for this purpo...