Showing papers on "Groyne published in 2014"

Hydrodynamics of groyne fields in a straight river reach: insight from field experiments

Abstract: This study explores the three-dimensional structure of turbulent flow around a sequence of groynes in a river reach under semi-controlled conditions. Detailed measurements were carried out in groyne fields with aspect ratios (length of groynes/distance between successive groynes) of 0.75 and 0.35 for emerged and submerged conditions and provided detailed spatial patterns of mean and turbulent fluxes of momentum. The results indicate that in mixing interfaces the vertical fluxes of turbulent momentum are dominant near the riverbed whereas horizontal fluxes dominate near the surface. Strong three-dimensional effects are reported for submerged groynes when a wake is formed at the leeward side. Groyne sequences increase flow resistance mainly by flow constriction that in turn generates an increase in velocity and bed shear stress in the main channel, rather than by production of excessive turbulence near the groynes.

Fluvial sediment budget of a modern, restrained river: The lower reach of the Rhine in Germany

Abstract: The Rhine River is a restrained river which is intensely used for navigation. Its river bed is subject to human-induced erosion and sedimentation processes. For river management, information on the amount, type, source, transport mode and fate of the sediments moving through the Rhine is indispensable. The objective of this study was to quantify the downstream fluxes of clay, silt, sand, gravel and cobbles through the Rhine between 1991 and 2010 and to identify the sources and sinks of these sediments. This was done by analysing a unique dataset containing thousands of sediment transport measurements and by evaluating the sediment budget. The river bed of the Rhine was found to be subject to a net bed degradation of 3 mm/a between 1991 and 2010. Bed degradation has been induced by 18th–20th century river training works and nowadays is concentrated in areas with Tertiary sands close to the bed surface, in areas with mining-induced subsidence and in the gravel–sand transition zone. Sediment transport was found to be dominated by suspended clay and silt. Morphologically relevant, however, are only the sand, gravel and cobble fractions. Despite the armoured, gravely river bed, sand is the main morphological agent. Sediment loads change in the downstream direction: sand and fine gravel loads increase due to erosion of the bed, whereas coarse gravel and cobble loads decrease due to a reduced sediment mobility caused by the downstream decreasing bed slope. Approximately one third of the sand and gravel load comes from upstream (Rhenish Massif), one third is supplied by bed degradation and one third is supplied artificially by humans for bed stabilisation purposes or as substitute for natural bed-load. Slightly more than one half of the sediment was transported downstream into the North Sea Basin (Rhine Delta), a small amount was lost by abrasion, and the remainder must have been deposited in groyne fields, on floodplains or in ports. The transfer of sand, gravel and cobbles from the hinterland towards the Rhine delta equalled 0.66 Mt/a ± 26%. Despite the long history of human impact, this rate does not differ significantly from the Holocene rate of sediment transfer to the Rhine delta.

Shoreline configurations affect dispersal patterns of fish larvae in a large river

Abstract: The dispersal patterns of marked larvae of the nase carp (Chondrostoma nasus L.) were observed alongside dissimilar shoreline configurations in the main channel of the free-flowing Austrian Danube and compared with those of floating particles to investigate the mode of dispersal (active– passive). Individuals of different larval stages and floats at similar densities were released at an artificial rip-rap with groynes and a rehabilitated gravel bar. In both habitats, marked individuals were recaptured during the sampling period of 4 d after release. Relevant shoreline attributes for larval dispersal, such as the accessibility of nursery habitats, connectivity between adjacent habitats, and retention potential, were more pronounced at the gravel bar than at the rip-rap. At the gravel bar, larvae moved upstream and downstream within the connected bankside nurseries and displayed longer residence times. Larvae settled in groyne fields along the rip-rap as well; however, longitudinal dispersal was disrupted by groynes, forcing larvae to enter the main channel. Rather than settling in subsequent groyne fields, we assume that these larvae are displaced downstream and potentially lost from the local population.

Attractiveness of a lateral shelter in a channel as a refuge for juvenile brown trout during hydropeaking

Abstract: Peak power production in hydroelectric storage power plants results in frequent and intense flow variations in the rivers downstream of the plants. Fish populations can be negatively impacted when subjected to these so-called hydropeaking phenomena. In researching mitigation solutions, shelters in the riverbanks of channelized rivers have been identified as a means of protecting fish from excessive flow velocities. These shelters were studied systematically using juvenile brown trout (Salmo trutta fario) in an experimental configuration in which a straight channel was equipped with a lateral embayment. The purpose of the experiments was to generate hydrodynamic hydropeaking conditions in the channel that are undesirable for juvenile trout, thereby causing them to enter the shelter. The flow velocity distribution in the intersection plane between the main channel and the lateral shelter was found to be a significant parameter for attracting fish to the shelter. The utilization rate of trout in the shelter was used as a performance indicator. Using a basic rectangular shelter configuration without forced water exchange between the shelter and the channel, the utilization rate was only 35 %. This rate was more than doubled by introducing a deviation groyne to force water exchange between the channel and the shelter. The position and orientation angle of this groyne were systematically varied to maximize the utilization rate. Maximum utilization rates approaching 90 % were obtained for an optimum configuration in which an island-type groyne was placed in the shelter. The results of the systematic channel tests showed the potential of the shelter to attract fish. Such a shelter could be used in channelized rivers both for morphological revitalization and to improve fish habitats. As a next step in this research, prototype shelters will be built on a natural river and monitored for 2–3 years under a hydropeaking flow regime.

Effects of vessel-induced waves on the YOY-fish assemblage at two different habitat types in the main stem of a large river (Danube, Austria)

Abstract: The effects of navigation on young-of-the-year (YOY) fish were investigated for the Danube River based on an integrative approach. During YOY growing season, wave height, wave frequency and water currents were recorded. Synchronously, fish drift and fish abundance of YOY were monitored in two distinct inshore nursery habitats—a gravel bar and a groyne field. The characteristics of vessel-induced waves were correlated to YOY-drift. In both habitats, an increase in drift densities during ship passages was observed. At the same time, assemblage composition and drift densities differed between habitat types. At the gravel bar, drift densities of larval cyprinids—representing many keystone species of the Danube—were higher during ship passages. In the groyne field, drift was low and percids dominated the YOY-assemblage along the shore. Banks with gentle slopes in the main channel seem to be particularly affected by vessel-induced wave wash. Precisely such habitats are often established within the scope of modern river restoration projects because they provide suitable nursery habitats for riverine fish species. Beside the positive effects on biota, the establishment of these habitats in navigable reaches may create problems for YOY-fish due to displacement effects induced by ship-generated waves.

An Experimental Study on Flow Characteristics for Optimal Spacing Suggestion of 45° Upward Groynes

Abstract: Groyne to control the direction and velocity of flow in rivers is generally installed for the purpose of protecting riverbanks or embankments from erosion caused by running water. In particular, as interest in river restoration and natural river improvement increases, groynes are proposed as a key hydraulic structure for local flow control and riparian habitat establishment. Groynes are installed mainly in groups rather than as individual structures. In case of groynes installed as a group, flow around the groynes change according to spacing in between the groynes. Therefore, groyne spacing is regarded as the most important factor in groyne design. This study aimed at examining changes of flows around and within the area of groynes that take place according to the spacing of groynes installed in order to propose the optimal spacing for upward groynes. To examine flow characteristics around groynes, this study looked at flows in main flow area and recirculation flow area separately. In main flow area, it examined the impact of flow velocity increasing as a result of conveyance reduction that is exerted on river bed stability in relation to changes in the maximum flow velocity according to installation spacing. As a factor causing impacts on scouring and sedimentation within the area of groynes, recirculation flow in the groyne area can lead problems concerning flow within the area and stability of embankment. As for recirculation area, an analysis was conducted on the scale of rotational flow and the flow around embankment that exerts impacts on stability of the embankment. In addition, a compara- tive analysis was carried with reference to changes of the central point of rotational flow that occur within the area of groynes. As a result of compositely examining the results, the appropriate installation spacing is proposed as min. four times-max. six times considering a decrease in flow velocity according to the installation of upward groynes, river bed stability and stability of embankments against counterflow within the area of groynes.

2DH Morphodynamic Time-Dependent Hindcast Modelling of a Groyne System in Ghana

Abstract: A groyne system was constructed west of the Volta river mouth at Ada (Ghana) to mitigate the local beach retreat of up to 6 m/year. During and after these coastal protection works, surprisingly severe erosion was observed downstream from the first groyne. An in-depth analysis was done to identify the causes by performing a hindcast over one year with the 2D depth-averaged time-dependent morphological model XBeach. The model appears to qualitatively reproduce the same morphological evolutions of the groyne system as measured in situ. The main causes of the severe erosion are a 2D bathymetrical feature (longshore sediment transport gradient) and differences in beach crest height east and west of the first groyne (difference in overwash occurrence). A parallel hindcast, without introducing groynes into the model, was also performed. This shows that the groynes, while contributing to the severe erosion, are not the main cause of it. The beach west of the first groyne has clearly benefitted from the construction of the groynes, since the beach erosion to the west of the groynes was less with groynes than without.

Assessment of the application of permeable pile groins as coastal protection

Abstract: Permeable wooden pile groins have a long tradition as being part of the coastal protection in the Netherlands, as well as in Germany and the United Kingdom. They are built to protect beaches from eroding by reducing the current velocity along the beach and thus its ability to transport beach material. This type of groin is most popular among the population; in Zeeland (NL), they are incorporated into the national heritage. In scientific research however, they lack attention. More focus is set on beach processes themselves and on the (supposedly) more stable impermeable rubble mound groins. As several researches stress their potential as soft methods and their advantages over impermeable groins, this thesis intends to put their usage more into focus. By means of generic numerical research and a comparison to available laboratory data and analytical approaches, the understanding of this type of groins is enhanced. This thesis explores the usage of permeable wooden pile groins in coastal waters under the influence of current and wave forcing. Both impermeable and permeable groins are able to reduce the sediment transport capacity and yield sediment accumulation. Impermeable groin are researched on reasonable well, as they are supposed to be more stable and their effects easier predictable. Important disadvantages are also originating from their impermeable structure: they deflect the approaching current causing strong offshore directed flows and large scale eddies between neighbouring groins. The paths of the sediment transportation will lead to a zig-zag profile of the shoreline, eventually causing erosion landward of the original shoreline and outflanking of the groin. Permeable groins reduce the original current velocity originating from the tide or breaking waves while not changing the general flow field. As a consequence, the sediment transport capacity reduces and dangerous rip-currents are prevented to a great extent. The thesis provides insight into the theoretical behaviour of the flow field around groins based on available laboratory experiments and theoretical considerations. This is enriched by a numerical research. For the forcing by a current, an analytical model is developed which provides the global change in the water level upstream and downstream of a stream. Further, the change in the velocity behind the groin and next to the groin can be obtained. In numerical models available for hydraulic engineering and research, the direct implementation of permeable pile groins is not explicitly provided. For most large scale hydrodynamic and morphology models the flow around a groin is on a too small spatial scale to be of interest compared to the whole flow domain. However, there are tools available to approximate the effects of permeable groins. The effect of the groins on the hydrodynamics is evaluated by means of the numerical model SWASH (Simulating WAves til SHore). This relatively new model is developed with the intention to reproduce both small and large scale flows in shallow water. In SWASH, two possible tools are available of which one on basis of a Morison equation proved to be suitable for the representation of groins on a laboratory scale. The numerical research evaluates the effect of permeable pile groins on the flow field when subject to a forcing by either a constant current originating from the tide, or the like, or from breaking waves. It is evaluated that many effects caused by the presence of groins on the flow field can be reproduced on a flat and an inclined (beach) bottom, both under a current or a wave forcing. For the wave forcing at a beach profile, a more detailed analysis is done on the effect on the flow field and bottom shear stresses under variations in the physical parameters. This study gives the range of boundaries under which permeable groins are applicable and their consequent effect on the flow field.

The Impact of Makeshift Sandbag Groynes on Coastal Geomorphology: A Case Study at Columbus Bay, Trinidad

Abstract: Coastal erosion threatens coastal land which is an invaluable limited resource to Small Island Developing States (SIDS). Columbus Bay, located on the south-western peninsula of Trinidad, experiences high rates of coastal erosion which has resulted in the loss of millions of dollars to coconut estate owners. Owing to this, three makeshift sandbag groynes were installed in the northern region of Columbus Bay to arrest the coastal erosion problem. Beach profiles were conducted at eight stations from October 2009 to April 2011 to determine the change in beach widths and beach volumes along the bay. Beach width and volume changes were determined from the baseline in October 2009. Additionally, a generalized shoreline response model (GENESIS) was applied to Columbus Bay and simulated a 4 year model run. Results indicate that there was an increase in beach width and volume at five stations located within or adjacent to the groyne field. The GENESIS model confirmed accretion within the groyne field and indicated a reduction in transport rates between groynes when compared to the rest of the bay to within 77.56 % accuracy. This study revealed that the makeshift groynes though improperly engineered, has reduced coastal erosion by encouraging accretion. The GENESIS model can be a helpful tool to inform management decisions at Columbus Bay and other beaches and bays inTrinidad and Tobago.

Littoral Drift Barriers and the Problem of Proving Accelerated Recession

Abstract: Any barrier to longshore drift has the potential to cause downdrift erosion. Understanding this problem means that any new construction of a groyne field or breakwater should be accompanied by monitoring to record changes in beach volumes and profiles as well any erosion of the coastline. In the past, such care may not have happened, especially in those cases involving boundaries between authorities responsible for the coast, or where the ground liable to be eroded was not considered to be of high value. This paper proposes a way by which previous groyne construction, or other coastal works, may be deemed responsible for increasing the rate of soft cliff erosion beyond a value which could be ascribed to being the result of a natural variation in recession rate.

Exploring the reciprocal role of hydrodynamic forcing and microbial colonization to induce bed sediment patchiness in biostabilization

Abstract: The experimental project presented herein investigated feedback mechanisms in the complex flow-sediment-ecology tripartite relationship arising from the interactions between hydrodynamics, sediment transport and segregation, and microbial colonization and bed stabilization. The experiment was conducted in a groyne field constructed in the Total Environment Simulator located at “The Deep” facility at the University of Hull. Detailed measurements of flow hydrodynamics in the main channel and groyne embayments were obtained using acoustic doppler velocimeter (ADV) probes, while the redistribution and segregation of bed sediments were measured using an array of ultrasonic range sensors (URS), coupled with detailed laser bed scans and high-resolution photographic images. In terms of investigating biogrowth colonization and stabilization of these segregated sediments, a MagPI (Magnetic Particle Induction) device was used to measure biofilm adhesiveness at various bed locations and at various stages during the growth phase. Preliminary results from this study are presented herein.

Analisis hidrolika bangunan krib permeabel pada saluran tanah (uji model laboratorium)

Abstract: One of the structures to protect river bank erosion is groyne. Groyne can serve and control water flow, reducing flow velocity and scour of river bank. The purposes of this study is to analyze the changes in the river bed elevation (morphology) and the depth of scour in the upstream groyne caused by the permeable groyne installed at the river meanders. The experiment was conducted at Fluid Mechanics and Hydraulics Laboratory, Sriwijaya University. The study tested the hydraulics models, a trapezoidal channel, meanders angle of 90u, five permeable groynes at meanders, and the water flowing in the channels was clear water. The observations were carried out with a flow rate was 63,32 Lt / min, three variations of permeable groynes angle were 45u, 90u and 135u to the upstream within 1 hour, 2,5 hours and 4 hours for each angle variations . The results of this study showed that the flow velocity of meanders was decreasing to the end of the meanders, and the changes of channel only occurred at the riverbed. Maximum riverbed changes (Bt / Bo) for permeable groyne angle of 45u, 90u and 135 u were 1,376 cm, 1,346 cm dan 1,452 cm. The maximum depth of scour (ds/y) for permeable groyne angle of 45u, 90u and 135u were 1,05 cm, 0,95 cm dan 1,17 cm. Thus, permeable groyne with angle of 90 proved to be the best with the smallest riverbed changes (Bt /Bo) was 1,346 cm and the coefficient of determination (R 2 ) was 0,9384, and also the smallest scour depth (ds/y) was 0,95 cm and the coefficient of determination (R 2 ) was 0,8317 compared to other

Application of Groyne as a Sustainable Solution to Agulu-Nanka Erosion Problem

Abstract: The paper aims at highlights the natural and anthropogenic impact in gully-erosion geo-structural failures. The application of groyne model-placement as sustainable solution which minimizes the control cost and guaranttees bed load as well as suspended load sediment transport. Keywords: Erosion model sediment, groyne, sustainable-solution, control