Groynes are popular hydraulic structures often used to control the erosion of banks by altering flow and sediment transport. In this paper, the effects of altering groyne orientation and spatial setup (from large to small and vice versa) on flow patterns, bed erosion, and sedimentation are numerically investigated. Studied groynes were parallel to each other, non-submerged, and impermeable. Numerical simulations were conducted in FLOW-3D. A nested mesh configuration combined with Van-Rijn formula on sediment transport yielded more accurate results when comparing numerical results to experiments. Groynes arranged from large to small at an angle of 45° decreased the scour depth by up to 55%, and an arrangement from small to large at an angle of 135° reduced the scour depth by up to 72%. Additionally, it was observed that simulations with an orientation closer to 90 degrees needed more equilibrium time when compared to other simulations.

Investigation of Flow, Erosion, and Sedimentation Pattern around Varied Groynes under Different Hydraulic and Geometric Conditions: A Numerical Study

This PhD thesis investigates the effects of changing discharge regimes on the morphological development of meandering rivers. The reduction of discharge causes bed morphology and planform changes in alluvial rivers with erodible banks, and restoring the discharge may alter the existing morphology, leading to a new (so far unpredictable) river course. The present thesis explores the capability of the open-source software Delft3D to simulate the adaption of both planform dynamics and bed topography to changed discharge regimes. A specific reach in the Dhaleshwari River (Bangladesh) is taken as a case study as it provides relevant data for model validation. 
Both two-dimensional (2D) and three-dimensional (3D) modelling approaches were applied. The 3D model, which was calibrated against measured 3D flow data, was used for predicting bed level changes over a one-year period. The simulated morphological changes showed a certain degree of resemblance with the available field data, but the required computational time prevented further analyses. Therefore, a 2D model, in which the parameterization of the 3D flow effect was validated against curved flume data, was used to simulate the morphological development for different discharge scenarios over a 10-year period.
The results of the simulations revealed that the 2D model could predict scour depth, bank erosion, and riffle-pool sequences under both constant and varying discharge scenarios. However, the prediction of channel bankfull width showed some deficiencies. The simulations with varying discharge demonstrated a more realistic prediction of the meander planform than the simulations with constant discharge. The conclusion from this research is that a 2D modelling approach, in combination with a time-varying discharge, can be used to simulate the natural dynamics of meandering rivers, both in terms of the development of bed topography and channel planform. The results further revealed that a discharge magnitude of about 90% of the bankfull discharge represents the dominant discharge, and that the meander wavelength increases with the discharge magnitude. The results of the simulation for time-varying discharge records revealed that larger floods favor enlargement of meander wavelength and smaller floods favor shortening of meander wavelength during the first 50% of the simulation period. However, after the first 5-year period, the meander wavelength becomes nearly unresponsive to altered high- and low- floods.

Morphological Development of Meandering Rivers Due to Changing Discharge Regimes

Design guidelines were developed for a number of in-stream structures; however, the knowledge about their morphological and hydraulic function is still incomplete. A variant is submerged groynes, which aim to be applicable for bank protection especially in areas with restricted flood water levels due to their shallow height. Laboratory experiments were conducted to investigate the backwater effect and the flow resistance of submerged groyne fields with varying and constant field length and groyne distance. The effect of the shape of a groyne model was investigated using two types of groynes. The validity of different flow types, from “isolated roughness” to “quasi smooth”, was analyzed in relation to the roughness density of the groyne fields. The results show a higher backwater effect for simplified groynes made of multiplex plates, compared to groynes made of gravel. The relative increase of the upstream water level was lower at high initial water levels, for short length of the groyne field, and for larger distance between the single groynes. The highest roughness of the groyne fields was found at roughness densities, which indicated wake interference flow. Considering a mobile bed, the flow resistance was reduced significantly.

https://www.mdpi.com/2073-4441/11/6/1253/pdf

Roughness Effect of Submerged Groyne Fields with Varying Length, Groyne Distance, and Groyne Types

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Morphologische Versuche zur Ableitung einer hydraulischen und geometrischen Definition von Lenkbuhnen

The transport of large wood (LW) has been a focus of research in the last decades because of its importance for river ecosystems, fluvial dynamics and flood protection. Riverine processes connected with the presence of LW have largely been based on laboratory experiments carried out in straight or weakly bended channels. The dynamics of LW in sharp river bends has received less attention, so there exist many open questions regarding how LW is transported through such channel parts. For this reason, this paper presents results of unique laboratory experiments designed to study the trajectories of single wooden dowels in sharp bends. Dowel trajectories were determined for four dowel sizes, two initial dowel orientations with respect to the main channel axis and three inlet positions. Information about the 3D velocity field in the bend were obtained from a previous study that used the same channel with identical hydraulic boundary conditions. Furthermore, a force‐balance analysis was developed for interpreting the dowel trajectory and drag coefficient of LW in sharp river bends. The experimental results show that the dowel length is the main parameter controlling dowel trajectory in a sharp bend, and indicate the role of inertial effects that drive the dowel drift along the bend. The obtained drag coefficients ranged between 0.35 and 1, which is in agreement with previous values for submerged, or semi‐submerged, fixed pieces of wood in straight river reaches.

Characterization of trajectories and drag coefficients of large wood in sharp river bends from flume experiments

The need for river bank protection in an ecologically appropriate manner has led to the investigation of shallow groynes as a nature-oriented countermeasure against bank erosion. Shallow groynes are in-stream structures which have a horizontal crest and are submerged even during low flow conditions in order to minimize the effect on the flood level. Rather than enhancing the resisting forces like traditional bank protection measures, e.g. riprap, shallow groynes are used to redirect the attacking flow away from the river bank. In this way, the bank itself becomes available as habitat and the water-land interface is rehabilitated or preserved. Moreover, the shallow groynes increase the heterogeneity of the flow field, bed topography, and bed material, which is one of the key elements requested by the EU Water Framework Directive to improve the ecological condition of rivers. However, guidelines are necessary for a successful and sustainable design of shallow groynes.
The objective of the study was to develop design guidelines for shallow groynes in river bends by experimentally investigating the design parameters, namely groyne inclination, length, width, location and spacing in a river bend. The experiments were carried out in a curved flume in the laboratory of Leichtweis-Institut fur Wasserbau (LWI). The three-dimensional flow field was measured with an Acoustic Doppler Velocimeter. The hydraulic boundary conditions were kept constant throughout the experiments. A reference run was carried out without a groyne providing the basis for quantification of the hydraulic effects due to the shallow groynes. The optimum groyne parameters were defined based on the highest reduction of stream-wise velocity close to the outer bank. The manipulation of the flow field throughout the bend was investigated with a single groyne as well as with a group of groynes varying the number of groynes as well as the spacing. Further experiments with a fixed bed and a mobile outer bank confirmed the effectiveness of shallow groynes for bank protection. 
The results of the different sets of experiments were combined to develop a design method for bank protection with shallow groynes throughout a river bend. The method incorporates geometric parameters of the bend as well as the hydraulic boundary conditions.

Zaid, Bahaeldeen, A.

Papers

Development of Design Guidelines for Shallow Groynes