Showing papers on "Groyne published in 2002"

On the correspondence between morphological and hydrodynamical patterns of groyne fields

Abstract: This paper examines a variety of recirculation flow patterns that develop in the groyne fields on rivers. A comprehensive data set was obtained from flume experiments at Delft University of Technology and field measurements performed on the Elbe River in Germany. The analysis focuses on patterns of velocity, scour and deposition, and corresponding change of riverbed morphology. The results show that velocity patterns in the groyne fields depend mainly on the aspect ratio between groyne length and length of groyne field. When the aspect ratio is greater than 0·5, a one-gyre pattern of recirculation develops, while at groyne fields with aspect ratios less than 0·5 a two-gyre recirculation pattern emerges. The spatial distribution of fine-sediment deposition between the groynes coincides with the locations of gyres. Moreover, the thickness of the fine-sediment layer decreases toward the gyre margins where recirculation velocities are greatest. Although the total concentration of suspended sediment in the river does not change considerably as the flow moves through the groyne field, the biological and gravimetrical composition of the suspended material changes substantially within the groyne field. These changes are due to preferential deposition of coarse mineral particles and the replacement of those minerals with finer organic material. Copyright © 2002 John Wiley & Sons, Ltd.

Permanent and semi-permanent groyne structures and method for shoreline and land mass reclamation

Abstract: Porous groynes (30) for land reclamation which includes a plurality of spaced stanchions (31) and at least one screen grid (32) means having a plurality of openings (39) therein through which fluid and fluid conveyed solids may pass and wherein the screen grid means is formed of a self supporting high density polyethylene, polypropylene, polymers, co-polymers, polymer mixtures or laminates.

The effect of groynes on rivers: Literature review

Abstract: Groynes are structures constructed at an angle to the flow in order to deflect the flowing water away from critical zones. They are made of stone, gravel, rock, earth, or piles, beginning at the riverbank with a root and ending at the regulation line with a head. They serve to maintain a desirable channel for the purpose of flood control, improved navigation and erosion control. In the River Rhine, which is considered the backbone of North-western European waterways network, the primary objective of groynes is to provide a fairway of sufficient depth and width. For example the River Waal, the most important branch of the Rhine River in the Netherlands is regulated by around 500 groyne. Within the framework of the research project "Space for the Rhine Branches" several measures have been devised to achieve a decrease of the water levels at peak discharges, one of those measures, is lowering of the existing groynes. The rationale behind this proposal is that; due to large-scale erosion of the low-water bed through the past decades, the groynes are now higher than necessary for keeping the main channel at depth. Lowering the groynes along certain reaches of the river would result in a reduction of the effective roughness during high water conditions thus, increasing the river's flood conveyance capacity. If the groynes are lowered, however, the balance of hydrodynamic forces acting on the groyne-fields will change, and there will be a large-scale morphological impact. To identify this impact, a thorough understanding to the effect o f groynes on the morphology of the river is necessary. The sediment exchange between the groyne-fields and the main channel needs to be more comprehensible. The purpose of this report is to acquire the background knowledge required to study the effect of groynes on a river. The characteristics of the existing groyne-fields along the Waal River are presented. The hydrodynamic and morphological impact of groynes on a river is described. Moreover, because navigation plays an important role in the interaction between the groyne fields and the main channel, the navigation induced water motion and its effect on the flow in groyne-fields is described. Finally, a review of some prediction attempts to the interaction between the groyne-fields and the main channel is presented.

Morphological modelling in the vicinity of groynes: An extended application in Delft3D-RAM including tidal impact

Abstract: The shoreline develops on several time scales. At some places along the coast the amount of sand in a cross-section decreases continuously and the shoreline shifts landward. This phenomenon is called structural erosion and is often caused by an increasing longshore sediment transport along the coast. When it is necessary to protect the coast, e.g. because the sea defense function of the coast is threatened, groynes can be applied to reduce the longshore transport. The erosion is then prevented locally. Nevertheless past the protected area leeside erosion occurs due to the full presence of the original sediment transport. In order to make a proper design for a shore protection with groynes, insight in the effectiveness as well as the side-effects is essential. In a previous study a model application was set up for the simplified case of a coast subjected to a uniform and stationary wave field in absence of the tidal impact. However, the tide does influence the morphological development, e.g. due to the displacing beaker zone, the drying and flooding of the beach and the presence of tidal currents. In this study the application is extended with the tide and an improved representation of the groyne within the computational modules is proposed. The applied computer model is Delft3D-RAM, a part of the Delft3D software-package, developed by WL Delft Hydraulics. The model is a process-based model that computes the 2dimensional current, wave and sediment transport field and translates these into bed level changes. The computational time stays limited due to the "Rapid Assessment of Morphology" (RAM) procedure. In the RAM-procedure, it is assumed that the local current and wave field do not vary much as long as the bed level changes are small. Within a certain period, defined as the morphological time step, the current and wave field are not recomputed, and the transports are simply adapted for the changed bathymetry. The RAM-procedure also includes a method to account for the development of the dry beach resulting from the morphological changes in the wet area. This approach, the "uniform beach profile method", redistributes bed level changes close to the waterline in such a way that the upper part of the beach profile shifts forward or backward while retaining its shape as initially predescribed. The groyne is implemented in the numerical grid using the "thin dam" option in the flow module and the "sheet" option in the wave module. The combination of two thin dams perpendicular to the shore and one connecting the two tips not only blocks the currents and sediment transports completely, but also efficiently separates the morphological development at both sides of the groyne. A set of three sheets is required to block the waves and prevent interpolation effects from the numerical wave grid to the flow grid. The proposed modelling of the groyne leads to satisfactory results. The model set up of the original application without tide is used as a starting point for the set up of an extended application including the tidal effects. Both the flow grid and the wave grid are refined around the groyne to obtain a more detailed representation of the local flow and wave field. In the extended application, the tide is implemented as a single sinusoid with a period of 12 hours. The propagation of the simulated tide corresponds more or less with the tide along the Dutch coast. The amplitude is taken higher than found in practice, namely 1.5 m. The application calculates the flow and waves iteratively to account for the interaction between the tide and the waves. The transports are then calculated throughout the tide and averaged over the tidal period. The bed level changes are computed from the averaged sediment transport field. Due to the computations over the full tidal period and an iterative calculation of currents and waves the application including tidal impact is computationally intensive.

Application of timber groynes in coastal engineering

Abstract: Timber groynes have been used for centuries to retain and control beaches. They are found throughout Europe but little information is available about the design or detailing of the structures. During recent years alternatives such as beach renourishment, detached breakwaters and rock groynes have been used increasingly, however timber groynes can still offer advantages in terms of sustainability, cost and appearance. These include the relative ease with which the level and profile of the groyne may be adapted (by adding or removing planks) or maintained. Their appearance and small footprint are particularly advantageous on amenity beaches. Beaches constantly change as they respond to natural processes including waves, currents and the wind. These processes can result in material being transported from the beach, which if not replaced by material from adjacent areas, will result in erosion. Groynes are constructed more or less perpendicular to the shoreline to restrict the movement of sediment along the shore. They usually work in two ways, by providing a physical barrier to the movement of the beach material, which enables the beach to re-orientate approximately perpendicular to the incident wave direction, and by diverting longshore currents (which may be caused by tides or waves) away from the beach. In the past, groyne design was often limited to the slightly improved replacement of existing structures. As most local authorities in the United Kingdom are responsible for their own coastal protection, groyne design differs largely along the coastline. The performance and durability of timber groynes is highly dependent on the design and detailing of the structure. By comparing several schemes, various advantages and disadvantages of groyne design have been identified. Keeping a structure and its connections as simple as possible will benefit the ease of construction and maintenance. Design details such as attaching softwood or recycled timber rubbing pieces to the piles at critical levels, thus protecting them from abrasion, can considerably increase the lifetime of a groyne. In order to assure the sustainable use of timber, certified timber should be specified. Additionally, reuse and recycling of timber can substantially reduce the need for new timber, and should be incorporated in the design and maintenance of structures. If designed properly, timber groynes can be an effective and sustainable solution to retain a beach where the main erosion problem is caused by longshore sediment transport.

Combining phase-resolving wave models with photogrammetric measurement techniques

Abstract: A class of models, termed Boussinesq wave models, has been developed in the past to provide time dependent (phase-resolving) wave information for shallow and intermediate water depths. Recent extensions include wave breaking, runup and expansion into deeper water. There is a need for appropriate field data (spatial and time dependent) to control, calibrate and validate the models. Spatial quasi-continuous measurement techniques using high resolution digital cameras are applicable for this purpose. The research area is a groyne field on a North Sea island in Germany. First results are presented.

Bank erosion and protection on the Brahmaputra (Jamuna) River

Abstract: Bank erosion is a major problem for river and floodplain management along the Brahrnaputra-Jamuna river system of Bangladesh. Considerable resources are spent protecting the river banks from erosion. Consequently an understanding of the erosion processes as well as the suitability of protection methods are important issues to be explored. In this paper bank erosion processes are examined considering the hydrogeological conditions with particular reference to the Jamuna river. The critical condition for bank erosion by mass failure can be defined by the critical bank height for cohesive soils and the critical slope angle for noncohesive soils. Basic analyses show that the critical condition for failure, and hence the critical bank height or critical slope angle, change considerably depending on the hydrogeological condition as affected by river and groundwater levels. A river bank that appears to be stable in the dry season may fail subsequently in the wet season without any significant change of bed levels in the river. This phenomena, combined with fluvial action, would explain the temporal variation of bank erosion rate on the Jamuna from pre-monsoon to postmonsoon period. To protect banks of the Jamuna river from widespread erosiou several types of structural measures have been used. These include groyne and revetment type constructions. As evident from the variable degree of performance of such structures in this river, a critical evaluation should be undertaken before implementing such measures. The performance of the structures is drastically impaired by the generally unpredictable development of the multiple charnels in the braid belt and resulting damage to the structures by the combined effects of fluvial action and geotechnical instability. Based on the field evidence, important issues related to the design of bank protection structures are discussed.