Showing papers on "Groyne published in 2012"

Long term performance of a submerged Coastal Control Structure: A Case Study of the Narrowneck Multi-Functional Artificial Reef

Abstract: In response to the increasing occurrences of beach erosion along Surfers Paradise and Main Beaches - Gold Coast, Australia, the Northern Gold Coast Beach Protection Strategy [NGCBPS] was developed to widen the beach by 20- 30m as well as improving surfing conditions as a secondary objective. The strategy, implemented in 1999- 2000, involved large-scale beach nourishment and construction of a submerged breakwater "reef" to act as a control point at Narrowneck. Construction of the reef involved innovative filling and placement methods using very large sand filled geotextile containers coupled with significant advances with regards to design of the geotextile material and containers. In the 11 years since construction, there has been substantial monitoring of the project since its completion in late 2000 including: - video imaging using webcams; hydrographic and beach surveys; aerial and oblique photography; surf and surf safety observations and GPS surfing track plots; and geotextile container condition and stability. This paper presents an update on the performance of the reef over the last four years. In particular, the response of the structure and the shoreline to a series of major storm events in 2009 has been examined. The results have shown that the erosion caused by these major events was accommodated within the wider beach created in 1999. Over the next 2 years there was a gradual recovery in the lee of the reef with a subtle groyne effect resulting in an even larger increase in the width of the updrift beach. A detailed underwater condition survey was also undertaken in 2011, to determine changes in the condition of the geotextile containers. This revealed a number of containers missing or damaged, and that seaward containers were covered by sand. The marine habitat which has been a feature of the reef has been impacted by the increased coverage of sand, but still shows high abundance and biodiversity.

Long term performance of a submerged coastal control structure: a case study of the narrowneck multi-functional artificial reef

Abstract: In response to the increasing occurrences of beach erosion along Surfers Paradise and Main Beaches - Gold Coast, Australia, the Northern Gold Coast Beach Protection Strategy [NGCBPS] was developed to widen the beach by 20-30m as well as improving surfing conditions as a secondary objective The strategy, implemented in 1999- 2000, involved large-scale beach nourishment and construction of a submerged breakwater “reef” to act as a control point at Narrowneck Construction of the reef involved innovative filling and placement methods using very large sand filled geotextile containers coupled with significant advances with regards to design of the geotextile material and containers In the 11 years since construction, there has been substantial monitoring of the project since its completion in late 2000 including: - video imaging using webcams; hydrographic and beach surveys; aerial and oblique photography; surf and surf safety observations and GPS surfing track plots; and geotextile container condition and stability This paper presents an update on the performance of the reef over the last four years In particular, the response of the structure and the shoreline to a series of major storm events in 2009 has been examined The results have shown that the erosion caused by these major events was accommodated within the wider beach created in 1999 Over the next 2 years there was a gradual recovery in the lee of the reef with a subtle groyne effect resulting in an even larger increase in the width of the updrift beach A detailed underwater condition survey was also undertaken in 2011, to determine changes in the condition of the geotextile containers This revealed a number of containers missing or damaged, and that seaward containers were covered by sand The marine habitat which has been a feature of the reef has been impacted by the increased coverage of sand, but still shows high abundance and biodiversity

Two-dimensional simulation of flow pattern around a groyne using semi-implicit semi-Lagrangian method

Abstract: In this article, a numerical model was obtained in a two dimensional flow pattern simulation around a groyne. The model approximates the depth-averaged, shallow water equations with a finite volume, semi-implicit and semi-Lagrangian representation. Runge-Kutta scheme was used for departure point determination. The results showed that numerical model has high stability and efficiency. Also, numerical representation has the ability to recognize land boundaries; therefore, closed boundaries do not need specification. The comparison between numerical formulation results and experimental results show that except in areas with strong downward flow, there are good agreements between experimental and calculated results. Key words: Semi-Lagrangian, semi-implicit, shallow water, flow pattern, groyne.

Human interference on soft cliff retreat: examples from Christchurch Bay, UK

Abstract: Human interference in soft coastal cliff retreat causes problems worldwide. Building defences alters the sediment budget, frequently causing a sediment deficit down-drift resulting in increased retreat rates. Subsequently, undefended shorelines become set back from protection works, often causing excessive and unexpected land and infrastructure loss, prompting defence extensions. Down-drift of groynes, this is known as the terminal groyne effect. From case studies, this paper determines the effects of human interference on soft cliffs, and investigates excessive land loss and likely future coastal response. Using 10.5 km of soft cliffs in Christchurch Bay, UK as a study region, a historical shoreline analysis from the mid 19th century to the present day was undertaken. Detailed analysis was conducted at three case study sites to determine whether retreat rates had increased down-drift after the construction of protection works. After defence construction, increased retreat occurred at all three sites, albeit for only a few hundred metres down-drift, as propagation was limited owing to a headland or large sediment volumes. Set-backs can lead to artificial headland formation, making the coast more challenging and costly to defend. Shoreline management plans advocating protection or realignment should take account of natural features to enhance engineering design and reduce excess land loss.

Effect of longitudinal block on flow structures around box groyne

Abstract: A box groyne is composed of various types of single groyne to satisfy the different river ecological requirements. Fewer studies are available for the flow structure around box groyne with different single shaped groyne. The flow velocity near typical box groynes with a longitudinal block, which is composed by straight and L-shape groyne, are measured by PIV (Particle Image Velocimetry) under emerged flow in horizontal plane. The paper shows experimental results of flow patterns for this box groyne. Due to the longitudinal block of box groyne limits the generation and development of the large coherent structure in the groyne field, flow periodicity and the capacity of mass exchange with main stream are significantly different between these types of box groyne. The placement of block obviously influences the number and scale of circulating flow inside the box groyne field, because of the distribution difference of vortex structure in longitudinal profile of mixing layer.

The effect of navigation on the flow in a river trained with groynes

Abstract: The study deals with the investigation of flow structures in groyne fields interacting with the flow generated by navigation. Groynes are traditionally constructed to ensure adequate water depth for navigation, so the interaction of navigation and groyne fields is nearly guaranteed. The Hungarian Danube section – which is part of the 7th European transport corridor – has a great importance, however, there are several places where the minimum 25 dm navigational water depth is not available in low water regime. As the River Waal (a branch of River Rhine), which falls in the same category of navigable waterways as River Danube, in my TU Delft based research I used that particular Dutch river as case study. The goal of this graduation project is formulated as follows: What kind of effects does navigation have on river morphology? This was investigated on physical basis by means of a numerical model. First, the influence of push towing on morphology is investigated by analyzing the bed shear stress distributions, then the impacts on groyne fields. It could be concluded that primary waves caused by push tows increase the bed shear stresses around the ship, moreover, groyne fields are also affected, especially their zone close to the main stream. The impact of different ship sets on shear stresses was also studied. It was found that the most significant effect was caused by the ship velocity, the sailing distance between ship and groynes, a moderate one by the ship’s width and a minor one by the ship’s length. A numerical morphological model could offer a more detailed insight in the morphological changes than analyzing the bed shear-stress distribution, only, a recommendation for future studies is therefore the implementation of such a model. FINEL2D’s capability of predicting the flow changes at groyne fields caused by navigation have been shown, although FINEL2D only uses advective momentum transport (but suffering from some numerical viscosity) to predict the flow, while in the groyne fields inhomogeneous turbulent processes are supposed to be dominating. Nevertheless, FINEL2D showed a sufficient match with physical model results.

Detecting the chaotic nature of advection in complex river flows

Abstract: In order to detect signatures of chaotic advection in river surface motion, surface buoys equipped with GPS were deployed in a field experiment in River Danube, Hungary. The buoys were released in the vicinity of groynes where complex mixing processes occur. A detailed analysis of the trajectories was carried out, focusing on the time evolution of the distance between buoy pairs. The analysis included the determination and comparison of local Lyapunov exponents and prediction times of finite-time hyperbolic behaviour, which is related to strong mixing. Despite of the small number of applied buoys we found evidence on Lagrangian chaos in the wake of a groyne field. In order to supplement the field data obtained by this, inherently Lagrangian, approach, experiments in a small-scale laboratory model were also carried out, in which the Lagrangian surface dynamics was detected by following the motion of numerous floaters using particle tracking velocimetry (PTV).

Numerieke simulatie van hydraulische en morfodynamische processen in nevengeul/hoofdgeul-systemen: inzicht in effecten van ontwerpparameters nevengeulen

Abstract: In the Netherlands, in the last few years, side channels are constructed, with the aim to create nature and/or more space for a river in times of high water (e.g. in the context of PKB Room for the River). Side channels are secondary streams (up to several kilometers) which flow more or less parallel to a river through the floodplain of that river. The construction of a secondary channel has an impact on hydrodynamic and morphological processes in the secondary channel, the floodplain and the river itself. There is a threefold reason for the research. First, Rijkswaterstaat wants to obtain advice on the construction of side channels to minimize future efforts in terms of management and maintenance. It is also desirable from the science point of view to gain more insight into the effect of currents on the floodplain morphodynamics in and around secondary channels. Thirdly, the research institute Deltares wants to know whether it is possible to implement a secondary channel in the existing Duurzame Vaardiepte Rijndelta model (DVR model; Sustainable Navigation Depth for the Rhine Delta) as the DVR model is a useful tool to predict hydro-and morphodynamic changes in the Rhine branches. This study aims to advice on modelling of secondary channels and design parameters for the construction of secondary channels, in order minimize management and maintenance efforts by preventing sedimentation/erosion problems. From a literature review it appears that there is a need to understand the morphodynamic effects of the secondary channel design parameters like bifurcation angle, the location of branching in the groyne embayment, side channel width, side channel depth, and inflow barrier. The effects on sediment movement of these four design parameters are determined in the adjusted DVR model, by modelling the extremes. It is assumed that the management and maintenance efforts are proportional with the dredging volume. With regard to the design parameters, the model simulations resulted in four interesting insights. First, a higher discharge abstraction leads to an increase in management and maintenance efforts in terms of sediment transfer. In addition, choices in bifurcation angle and side channel width have a higher impact on the amount of dredging volume, compared to the other two modelled parameters in this research. A larger bifurcation angle or a smaller side channel width, leads to a substantially smaller amount of dredging volume. Moreover, it is shown that some of the current rules of thumb regarding secondary channel dimensions are unnecessary, namely: maximum withdrawal thresholds, and maximum sedimentation heights. The basic rules on minimum navigable depth and maximum dredging volumes are conclusive. The study also showed that there is no best approach to model a discharge curve stationary. The two main disadvantages of modelling with a stationary approach are: the lack of understanding of the morphodynamic behaviour just outside the banks of the secondary channel, and lack of wash effects. Further research can focus on the influence of suspended transport, detailed three-dimensional model simulations from the in- and outflow location of the side channel, and the model method itself

Study on Groyne Effects for River-Bank Protection in a Steep River

Abstract: Several groynes were constructed about 300 years ago to protect the banks of Japanese rivers, which are characteristically steep and short. To investigate the effects of groynes on the Shigenobu River in northwestern Ehime Prefecture, a field investigation was conducted to measure the sizes of groynes and the sizes of stones on the surface of the groynes as well as the bed morphology around the groynes. In addition, particle tracking velocimetry and numerical analyses of the three-dimensional flow and bed variation around the groynes were conducted. It was found that flow patterns, such as patterns of mean velocity and Reynolds stress, and the bed variation around successive groynes were affected by the particular arrangement of groynes. The study proposes a simple arrangement of river structure for stable bed morphology.

Three-dimensional features of the turbulent flow around series of groynes with different shapes of head

Abstract: 1Member of JSCE, Graduate School of Civil and Earth Resources Engineering, Kyoto University (Kyoto Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8530, Japan) E-mail:mansoori@uh31.dpri.kyoto-u.ac.jp 2Member of JSCE, Dr. of Eng., Prof., DPRI, Kyoto University (Shimomisu, Fushimi, Kyoto 612-8235, Japan) 3Member of JSCE, Dr. of Eng., Assoc. Prof., DPRI, Kyoto University (Shimomisu, Fushimi, Kyoto 612-8235, Japan) 4Member of JSCE, Dr. of Eng., Assis. Prof., DPRI, Kyoto University (Shimomisu, Fushimi, Kyoto 612-8235, Japan) 5Assistant Professor, University of Mohaghegh Ardebili (Ardebil, Iran)

Shear-driven flushing of micro-tidal marinas

Abstract: Flushing or residence times are typically used as a first step in assessing water quality in marinas, harbours and coastal basins. Recent publications have offered guidance in relation to optimal marina basin and entrance geometries to help achieve rapid renewal. However, these guidelines have been developed for the particular case where water exchange is strongly tide-driven and are not widely applicable, particularly in micro-tidal regions. Where water renewal rates are dominated by shear-driven circulation and lateral transfer of momentum at the interface between the marina and the adjacent water body (i.e. a mixing layer), there is a strong analogy to groyne fields and other cases involving flows containing quasi-stagnant peripheral areas (dead zones). A series of numerical hydrodynamic models, developed in the TELEMAC system, were used to investigate the potential for the dead zone model of water exchange to provide a better means to guide optimization of basin and entrance geometry under such conditions. Real-world marina case studies were used to identify any constraints affecting the practical implementation of such an approach. The numerical model results demonstrate particular conditions under which the dead zone model of water exchange can be used effectively to optimize marina basin and entrance geometry.

Shoreline Stabilisation: Lessons from South Wales

Abstract: Shoreline stabilisation in South Wales was driven by historical industrial development. Coal from the hinterland was brought to the shore, especially during the nineteenth century for use in metal industries that were located on the coast, and for export. At that time coastline impacts were not understood and development legacies have proven to be environmentally and financially expensive. Even when early in the twentieth century seawall and groyne impacts were recognised, coastal defence strategies still followed previous patterns. With industrial decline, economic regeneration has been supported by coastal location but unfortunately, lessons from the past have not been learned. Case studies show damage to new developments resulted from inappropriate infrastructure location, while undermined seawalls are now being protected by rock armour. This is becoming increasingly serious as there is little available funding to undertake effective remedial measures. It is concluded that new strategies which consider prevailing coastal processes are needed for long-term management; otherwise paving the shoreline may become more common on the Welsh coastline.