Development of innovative groynes to establish fish passability of regulated rivers based on the example of the Wien River, Austria. Part I: Impact of groyne parameters on water depth and velocity

Rivers in Europe have been heavily modified over the last 200 years, with a significant impact on their ecology and environment. This also applies to rivers like the Wien River, Austria, which are designed as overwide concrete channels for the benefit of flood protection. To achieve a good ecological potential in such heavily modified water bodies, one key element is fish passability. This requires an increase in the water level at low flow conditions and a reduction of the flow velocity. The aim of this study is to assess whether groynes are suitable for this application. A design study was conducted to examine the effect of individual groyne parameters on water depths and velocities. Physical experiments were carried out in a laboratory flume at a scale of 1:8. In addition, a 2D numerical model was used. It was found that the groynes had to be submerged and the alignment had to be repelling to achieve both requirements. The configuration of the groyne height, distance and degree of obstruction parameters were crucial. The groyne angle and shape had a minor effect and can be used for fine‐tuning. The best groyne design created a passable section for fish. Thereby, and through sedimentation, the best design contributed to an ecological improvement. However, it did not create habitats and did not constitute a fullfledged restoration. In general, submerged groynes can fulfil the hydraulic requirements for fish passage in heavily modified water bodies with a fixed bed.

already been heavily altered over the last 200 years (Kollmann et al., 2019).The degradation of the hydromorphology leads to disturbances of the ecological continuum of water bodies (Kollmann et al., 2019) and habitats (Malmqvist & Rundle, 2002), and thus to a decrease in species diversity, density and biomass (Alp et al., 2011;Jungwirth et al., 2003).Other consequences include the deepening of the riverbed and the associated groundwater decline, which in turn has a major impact on adjacent moisture-dependent floodplain landscapes (Kollmann et al., 2019).The destruction of these floodplain landscapes and river degradation subsequently lead to an increased flood risk downstream (Kollmann et al., 2019).For the benefit of flood control, many rivers are designed as overwide channels and are therefore classified as heavily modified water bodies.This modification leads to an interruption of fish passage due to a water depth that is too low and flow velocities that are too high at low water levels.
These hydraulic parameters can be altered by implementing groynes, which are hydraulic engineering structures that dam parts of the cross-section.Groynes have been used for centuries.Starting in the mid-19th century, groynes were built in Europe's large rivers to improve navigation (DWA, 2023).These groynes are usually submerged only at higher flows (Sindelar et al., 2022).The focus of these early implementations was on engineering issues.The shift toward more natural river engineering is causing scientists to rethink groyne design and initiating investigations into specific issues.Recent research, for example, addresses the use of groynes in steep watercourses with a slope of 2%-7% for bank protection, bed load transport control, and ecological and structural enhancement (Bitterlich et al., 2018).Henning and Hentschel (2013) developed novel groyne designs for the ecological enhancement of groynes on the Elbe River.
They found that large flood events are largely responsible for sediment deposition in the bank areas of the groyne fields.However, they state that notched groynes increase the diversity of the current and the heterogeneity of the groyne field.Angled groynes, on the other hand, are more likely to slow down siltation.A detailed study of the effects of groynes on one bank on erosion, sedimentation and flow patterns at different lengths and orientations was conducted by Choufu et al. (2019).For example, they were able to determine the greatest sedimentation heights for groynes that are arranged from small to large lengths.They also found that groynes that are aligned from large to small lengths can reduce the maximum scour depth by up to 55%.Groynes that are arranged from small to large lengths can reduce the scour depth by up to 72%.Sukhodolov (2014) conducted field studies of three-dimensional structures of turbulent flow around non-submerged groynes on both bank sides.Sukhodolov described the formation of vortices based on the ratio of the length of the groyne to the distance between the groynes.A ratio <0.5 triggers two adjacent eddies in the direction of flow.Between 0.5 and 2.0, one large eddy predominates, while a ratio >2.0 creates two adjacent eddies transverse to the direction of flow.The field studies of Nakano and Nakamura (2006) showed that groynes can decrease the velocity of the river near the bank and increase the number of taxa and the total density of macroinvertebrates.Glas et al. (2018) carried out a study to determine the sensitivity of the flow and riverbed in the fairway to the groyne design using a 3D numeric model.They analysed groynes on one side of the bank and found the following.The groyne height and length have the greatest impact.The influence of these parameters is strongest for a flow that lies exactly between the flow-around and flow-over conditions.Varying the groyne distance does not lead to any change in the water level gradient.Investigations with a changed angle and without a lowered groyne root result in only minor changes to the navigation channel.The effects are limited to localised scour formation.The sensitivity of the groyne parameters to the hydromorphological balance is also confirmed.Sindelar and Mende (2009) used micro-groynes (in German, 'Lenkbuhnen') as an ecological stabilisation measure.Micro-groynes are structures that are already completely submerged at low flow and therefore have a different effect than conventional groynes (Sindelar & Mende, 2009).
They describe the effect of micro-groynes with strong submergence.
The groynes induce secondary currents similar to that in a river bend.
Repelling micro-groynes direct slow-flowing fluid near the bottom into the groyne field, and the fast surface water is channelled into the centre of the river.The resulting slow velocities in the groyne field promote sedimentation in the groyne field and deepening in the centre of the river.For attracting micro-groynes, the flow patterns and sediment rearrangement are exactly the opposite.In addition, microgroynes do not cause a reduction in the water level gradient with large discharges.Mende (2014) developed design approaches for micro-groynes for bank protection on one bank.He found that the induced secondary flow of repelling groynes leads to a halving of the bank strain.Mende also reports that this secondary flow is hardly dependent on the relative height.Groynes with a repelling angle of 30 lead to stronger secondary currents at the same relative height compared to those with an angle of 60 .However, the reduction in near-bank velocities is somewhat smaller at 30 .The water depths are raised by a maximum of 6%.Müller et al. (2020) analysed the changes in the velocity, bed erosion and sedimentation caused by microgroynes at one bank.They found a decrease in the velocity on the protected bank (13.3%) and an increase on the other bank (2.1%).
During the morphological tests, a scour developed upstream of the groyne root and a second scour developed downstream of the groyne head.Downstream, a long and small deposit was found along the groyne root, demonstrating the bank-stabilising properties of this construction method.Morphological and purely hydraulic experiments yield comparable results in terms of velocity reduction on the protected bank.The study of Möws and Koll (2019) deals with submerged groynes on one bank side and their backwater effect and flow resistance.They observed that the height of the backwater remains constant with increasing groyne spacing and groyne field length.With a constant groyne field length, they observed a maximum of the backwater at a certain groyne spacing.It decreases again at greater distances.The maximum backwater is recorded at the groyne spacing at which the inclined groynes no longer overlap.However, a basic system understanding of groynes remains incomplete (Möws & Koll, 2019;Weitbrecht et al., 2008).There are hardly any available publications that deal with the rise in the water level and reduction in velocity caused by groynes on both bank sides.The scale of a water level rise of up to 90% also represents an unexplored order of magnitude.
This paper attempts to close this knowledge gap about the effect of groynes on both bank sides.The aim of this systematic study was the development of groynes to reestablish fish passability in heavily modified water bodies that suffer from low water depths and high velocities.Therefore, both physical and numerical experiments were conducted.Different groyne designs were tested to identify the effect of groyne parameters on the water depth, velocity and flow field.This study also aimed to determine the best groyne design for this application using the Wien River, Austria, as an example.This paper, 'Part I', focuses on the hydraulic aspects of the research.'Part II' will then focus on the fish passability.The functionality of the best groyne design was determined in a 1:1 ethohydraulic model with fish (Lasinger et al., in press).Commission, 2000), heavily modified water bodies must meet the environmental objective of good ecological potential.This is defined by, among other things, a self-sustaining fish population and sufficient biomass (Eberstaller et al., 2015).A restoration measure at km 12 has already been implemented, but complete fish passability does not yet exist.Keckeis (2002) proved that a previous restoration of the Wien River led to an increase in fish numbers and also stated that further improvement of the population age structure could be achieved by an ecological connection of the Wien River to the Danube Canal.This is also relevant because the Wien River, with its 124 tributary streams, is an important connection, as a biocorridor, to the Danube (Keckeis, 2002).However, a near-natural design along the entire stretch of the Wien River in this urban area is not possible due to flood protection and the existing vulnerable subway infrastructure (Figure 1).Therefore, in most areas only instream measures are possible.

| Project area
The project area is 4.4 km long and is located between river kilometres 3.3 and 7.9.The fixed profile is about 20 m wide and about 10 m high, and it has a bed width of 3.5 m.The banks are sloped at around 1:7.5 and are laterally bounded by vertical walls.In the project area, the Wien River has low water levels and high velocities at low and mean flow conditions.Under the current conditions, it is not possible for fish to migrate.Within the scope of the systematic study, groynes should be developed for the project area to establish fish passability.This was the first step in connecting the Wien River with the Danube Canal and the Danube in the future.
A representative cross-section and bed slope of the project area were determined from data of a survey of Municipal Department 41 in 2016 and from our own measurements in 2021 (Lasinger & Leutgöb, 2022).The parameters affecting hydraulic conditions were selected conservatively.For example, different bed slopes were measured in the project area.However, the experiments were only carried out with a slope of 4.6 ‰.The selected characteristic bed slope of 4.6 ‰ represents the steeper slope and therefore leads to lower water depths in the cross-section.This ensured that the requirements for raising the water level and reducing velocity were met throughout the project area.The selected bed slope of 4.6 ‰ and the characteristic cross-section are shown in Figure 3b,c.Two design flow rates were determined from the Austrian Hydrographic Service data at the 'Kennedybrücke' gauging station (km 7.9).
Table 1 summarises the most important hydrological parameters at 'Kennedybrücke'.Flow rates were calculated using data from 1976 to 2018.The flow rate that exceeded 330 days per year was called Q 330 and was calculated using the long-term median daily flow rates.Mean annual flow rates were used to determine the mean flow rate (MQ).

| Hydraulic requirements for fish and requirements for operation
The Wien River is classified in the fish bioregion 'epipotamal small' (BML, 2022).The key fish species in the region are the chub (Saqualius cephalus/Leuciscus cephalus), gudgeon (Gobio gobio) and stone loach Based on an extensive literature study of the hydrodynamic requirements for the key fish species at different ages, we have set the following target values for water depths and velocities.The water level should be raised from at least 0.11-0.20 m at Q 330 , and the flow velocity should be below 1 m s À1 (Lasinger et al., in press).The detailed investigations and results will be published in Part 2. For maintenance work on bridges or the nearby subway, the Wien River still has to be navigated by cars and mobile cranes despite the groynes.Therefore, the groynes must be 20 m apart so that cars can drive around the groynes or the groynes must have a maximum height of 0.2 m so that cars can drive over the groynes.To ensure that the groynes can be driven over at a height of 0.2 m, they must also be sloped upstream and downstream.

| Groyne design parameters
The investigated designs differed in various groyne parameters.A total of eight groyne parameters were varied and combined in different ways.All considered parameters are listed in Table 2 with their abbreviations and units, as well as the range used.The height H, the length L and the width W describe the dimensions of the groyne (Figure 2b).The angle α was measured between the groyne and the upstream bank line (Figure 2a).Since the groynes were installed alter- [Color figure can be viewed at wileyonlinelibrary.com] defined as the ratio of the projected groyne length at the riverbed to the bed width (Figure 2a).The groynes also differed in shape.There were designs in which the groyne head was sloped or the entire groyne was modelled with a slope.In addition, the groynes were aligned to be repelling or attracting.

| Experimental setup and procedure
Physical experiments were carried out in the hydraulic laboratory flume of the University of Natural Resources and Life Science Vienna.
In this model, a total of 10 different designs were investigated (Supplementary material Table A1).A straight channel with the characteristic cross-section was modelled at a scale of 1:8.The experimental section was 1 m wide and 7 m long (Figure 2a,c).To create the required slope of 4.6 ‰ and the trapezoidal profile, three-ply sheets made of spruce wood were used.The groynes were made from XPSpanels.The water depth was measured by a mobile pointing gauge with an accuracy of 8 mm in nature.The velocity was measured by a vane wheel flow sensor ZS16 with an accuracy of 0.12 m s À1 in nature.The measuring range for this device is between 0.23 and 24.64 m s À1 in nature.The calibration was carried out with the two flow rates, Q 330 and MQ, without groynes.The necessary water levels were determined by a rating curve at the 'Kennedybrücke' gauging station.In the physical model, the water levels in relation to the required water level were slightly higher at Q 330 and slightly lower at MQ. Therefore, there was no tendency for the surface to be too rough or too smooth.The deviation was 0.01 m in nature.This value was smaller than the expected accuracy of the rating curve.Therefore, the model surface was not changed.The experimental section was long enough to adjust a uniform flow.At the beginning of each experiment, the flow was visualised with ink (Figure 3d).All measurements were made in a defined pattern, with 14 to 19 points per groyne field (Figure 4).In addition, qualitative information on sedimentation was obtained by adding sediment once for four designs.In each experiment, 51.2 kg (in nature) of sediment were added.Half of the added grains had sizes of 3.2-6.4mm and the other half had sizes of 5.6-9.6 mm in nature.H (m) 0.10, 0.15, 0.17, 0.20, 0.32, 0.5 simulated under steady flow conditions (Supplementary materials Table A1).The grid of the model had a resolution of 0.11 m.The groynes were modelled as a geometric structure.For calibration, Manning's roughness coefficient for Q 330 without groynes was varied to achieve the target water depth.The Manning value n = 0.015 approximated the water depth with a deviation of 0.002 m.The focus was on the model results for water depths and depth-averaged velocities.

| Hydrodynamic 2D simulation
In addition, four zones with a certain water depth and velocity were defined (cf. Figure 5c).The migration corridor (black) was defined as an area with a water depth of at least 0.2 m and a velocity between 0.2 and 1.0 m s À1 .The green area represented a resting area, where the velocity was below the positive rheotactic velocity (<0.2 m s À1 ).
The blue and white areas represent those with velocities that were too large (>1.0 m s À1 ) and water depths that were too low (<0.2 m), respectively.

| RESULTS AND DISCUSSION
First, the results of the two models are compared.Furthermore, due to the large number of modelled designs (Supplementary materials Table A1), the effect of each parameter on the water depth and flow velocity could be isolated.Therefore, the impact of each parameter is individually presented using example designs.Finally, the design that was best for this study region is shown.The results are given in natural dimensions.

| Comparison of the two models
The water depths and velocities of the physical and numerical models are compared below.By comparing the differences in the min- The water depths were, at both flow rates, sometimes one colourscale bracket higher, but the gradation was similar.The velocities showed a higher variation than the water depths.Since the measurement grid in the physical model was not as fine as that in the numerical one, the standard deviation in the differences of velocities was higher.The minimum and maximum water depths at Q 330 were 0.02 m and 0.01 m higher in the physical model, respectively.At MQ, the minimum and maximum water depths were 0.01 m s À1 lower and 0.01 m s À1 higher in the physical model, respectively.The maximum velocities were the same for Q 330 in both models and 0.01 m s À1 higher for MQ in the numerical model.This investigation proved that the two models agreed well.The illustration of the numerical model showed the flow pattern better due to the areal visualisation.Due to this and the fact that the lower water depth and higher velocities in the numerical model are on the safe side, the numerical model is shown here for illustrative purposes.

| Alignment
In the following, two designs are compared; they only differ in terms of their alignment.Design 4 was repelling and design 11 was The alignment of the groynes had different effects on the two hydraulic parameters.The distribution of the water depths was similar, except for the minimum water depth.For the repelling design, the minimum water depth was downstream of the groyne head.The minimum water depth was located downstream of the groyne root for the attracting design.The minimum and maximum water depths differed only slightly (0.01 m).The alignment had a considerable effect on the velocities, however.The maximum velocity was 0.5 m s À1 higher for the attracting groynes.However, if the mean velocity was considered, it was again very similar for both variants (0.02 m s À1 difference).The reason for this is that attracting groynes led to very high velocities in the channel centre, and downstream of the groynes, the velocity was close to 0.0 m s À1 .The repelling groynes, however, reduce the flow velocity in the flow corridor more effectively and still have slow areas behind the groynes.Overall, repelling groynes are more effective at reducing the flow velocity in the flow corridor and distribute the velocity better over the entire groyne field.Thus, repelling groynes are more effective for achieving our goal.

| Height, distance, length and degree of obstruction
The effect of different combinations of the groyne height, distance, length and degree of obstruction on the water depth and velocity is presented below.Figure 5 compares designs 3 and 4. Design 3 had a height of 0.5 m, a length of 5.36 m, a degree of obstruction of 0.67, a distance of 20 m and an angle of 90 .Design 4 had a height of 0.2 m, a length of 3.95 m, a degree of obstruction of 0.45, a distance of 6 m and an angle of 45 .With increasing groyne height, a smaller distance and a greater degree of obstruction or length, the water depths became greater.By adjusting these parameters accordingly, any targeted water depth could be achieved.For example, with a large distance, only the degree of obstruction and the height had to be large enough to achieve a certain water depth (Figure 5a).With this combination, however, the required velocities could not always be achieved.
The reason for this is that a high degree of obstruction concentrated the flow in a narrow section.The same applied to groynes that were not submerged.In both cases, the cross-section available for flow was reduced and the velocity increased as a result.Figure 5c shows the migration corridor (black) where the target velocities and water depths meet.It was observed that the migration corridor was larger in design 3 due to the higher water depths upstream of the groyne compared to design 4.However, this corridor was interrupted by zones of too-high velocities (blue) in design 3 (Figure 5c).If the groyne spacing is too high, the resistance is too low, meaning that the flow velocities are not sufficiently slowed down.Therefore, the groynes should be submerged and the distance should be small in order to achieve low velocities (Figure 5b).

| Angle
By varying the inclination angle, the following was determined.Different groyne angles had a slight effect (±0.02 m) on the maximum and minimum water depths during submergence.There was no tendency for a larger or smaller angle to cause greater water depths.In the case of the investigated non-submerging groynes, the effect was somewhat larger (±0.04 m).The variation of the angle did not cause any change in the constricted flow cross-section.The area of impounded water above the groyne and the resistance at the groyne head did change, but these did not have a major impact on the hydraulic parameters.The magnitude of the velocities was also not greatly affected (±0.1 m s À1 for submerged groynes and ± 0.2 m s À1 for nonsubmerged groynes), but the flow pattern was.Due to the different angles, the water was directed in different directions for submerged and non-submerged groynes, thus causing a different flow pattern.In particular, the direction of the main current changed.Small changes were observed with a variation from 55 to 70 (Supplementary material Figure A1).Angles of 90 affected the pattern of the oscillating main stream more strongly (Supplementary material Figure A2).It can be said that the variation of the angle in the case of a fixed bed does not have a strong effect on the extreme values of water depths and flow velocities, but it can change their distribution.Once the groyne height, distance and degree of obstruction parameters have been determined and the groyne configuration has been largely set, the angle can only be used for fine-tuning.When the same parameters are used and only the angle is changed, a flatter angle results in material savings.

| Head slope
Two designs were compared that differ only in terms of the slope of the groyne head.Design 5 had a non-sloped head and design 6 had a 1:1 sloped head.The measured maximum and minimum water depths and velocities differed by only 0.01 m and 0.1 m s À1 , respectively.The reason for highly similar water depths is that the cross-section was only slightly changed and thus the same effect could be achieved.However, if the head was sloped at a ratio of up to 1:14, the velocities became greater with increasing slope.the head slope (1:3 for design 29 and 1:14 for design 26).The water depth was decreasing and the velocity was increasing due to the greater slope.The reason for this is that the strong slope meant that the cross-section was no longer sufficiently narrowed and the water could flow through almost unhindered, with a slight oscillation in the middle.In addition, the water was less dammed with increasing head slope, and the water depths thus became smaller.
From this, it can be concluded that the effect of the head slope on the hydraulic parameters depends on the strength of the slope, as well as on the configuration of the remaining groyne parameters.
However, if two designs with almost the same parameters but different lengths are compared, a longer groyne with a slope can be quite advantageous (Figure 6c,d).In contrast to a short groyne without a slope, it dams the water better due to its greater length but does not narrow the cross-section as much due to the slope, which means that the velocities are slower.

| Side slope
Designs were modelled with and without a slope upstream and/or downstream (2:3 and 1:1) (Figure 2c).The results were similar.The same minimum and maximum water depths and the same maximum velocities were measured regardless of the side slopes of the groynes.
At Q 330 , the slope had a slight influence on the distribution of both the water depths and velocities, but the extreme values remained the same (Supplementary material Figure A3).For this reason, the influence of the groyne side slope in a river with a fixed bed can be neglected.This ensures that all designs can be compared with each other.

| Sediment
The sediment experiment was done for four groyne designs.At

| Implementation design
To determine the best design for this study, all modelled designs were compared with each other.The following criteria were considered: the water depth, velocity, material input, construction, navigability, sedimentation, continuity of migration corridor, area of migration corridor and resting areas.Overall, design 10 was the best.This groyne design was submerged with a height of 0.2 m and had a length of about 4 m, an angle of 70 , a distance of 6 m and a degree of obstruction of 0.57.The shape of this design was complex, with a 1:8.25 sloped groyne head and a slope of 1:1 upstream and 1:1.5 downstream (Figure 7a).The measured water depths at Q 330 were between 0.20 and 0.28 m.The maximum measured velocity at Q 330 was 0.9 m s À1 .Therefore, the required water depth (≥0.2 m) and velocity (<1.0 m s À1 ) could be achieved and the migration corridor was continuous (Figure 7b-d

| CONCLUSIONS
The aim of this study was to determine the impact of each groyne parameter on the water depth and velocity.Furthermore, it considered whether groynes are suitable for achieving the hydraulic conditions for fish passability in hard-banked small rivers with overwide river profiles using the Wien River, Austria, as an example.Therefore, a hydraulic model of the Wien River was built in the hydraulic laboratory of the University of Natural Resources and Life Sciences Vienna at a scale of 1:8.Using this model, 10 different designs were modelled.In addition, 50 designs were modelled in a 2D numerical model.
The following eight different groyne parameters were varied: the height, length, width, angle, degree of obstruction, shape, distance and alignment.
Repelling groynes are more effective at reducing the flow velocity in the flow corridor and distribute the velocity better over the entire groyne field.To raise the water level from 0.11 to 0.20 m, many combinations of the groyne height, distance, degree of obstruction and length can be used.However, to achieve velocities of less than 1.0 m s À1 , the groynes have to be submerged with a height of 0.2 m and placed at a distance of about 6 m.Non-submerged groynes with a high degree of obstruction result in a narrow cross-section for the flow and high velocities.Large groyne distances lead to areas of high velocities between the groynes.Altering the angle has different but very minor effects on the water depths and velocities in the case of a fixed bed but changes the flow pattern.Therefore, the angle can be changed for fine-tuning.When the same parameters are used and only the angle is changed, a flatter angle results in resource savings.
Different head slopes can also help with the fine-tuning of the groyne design.The influence of the groyne side slopes on the water depths and velocities can be neglected for a fixed bed.In a qualitative sediment test, it was found that the groynes lead to sedimentation upstream and/or downstream of the groynes.For a more accurate and detailed assessment of sedimentation, either more extensive sediment experiments in a physical model must be carried out or a 3D numerical model with a sediment focus must be created.To summarise, groynes are suitable for achieving the hydraulic conditions for fish passability in hard-banked small rivers with overwide river profiles.A submerged design with a height of 0.2 m, a length of about 4 m, an angle of 70 , a distance of 6 m and a degree of obstruction of 0.57 proves to be the best design.The shape of this groyne design is complex, with a 1:8.25 sloped groyne head and a slope of 1:1 upstream and 1:1.5 downstream.These groynes are innovative, as they are even submerged at low flow conditions and have a special shape.They are longer than the classical groynes to achieve the desired meandering flow pattern, which elongates the thalweg and thus reduces the flow velocities.However, as the groynes are so long, the groyne head must be sloped over a large length.Due to their side slope, the groynes can be driven over by vehicles.This combination of length, height and shape is novel and innovative.Furthermore, the effect of the groynes is considerable.The implementation design leads to an increase in the water level of 82%-154% and a reduction in the flow velocity of at least 10% at low flow rates.The fish passability of this groyne design was then tested with fish in a 1:1 scale model (Lasinger et al., in press).
This systematic study uses the hydrological and geometrical parameters of the Wien River in Vienna, Austria.It flows 17 km through Vienna.Due to hydromorphological modification, the Wien River is classified as a heavily modified water body (BMLRT, 2022).According to the European Water Framework Directive (European

(
Barbatula barbatula) (BMNT, 2019).The barbel (Barbus barbus) and F I G U R E 1 Example image of the cross-section from the Wien River with neighbouring infrastructure and buildings.[Color figure can be viewed at wileyonlinelibrary.com] the nase (Chondrostoma nasus) are accompanying fish species in the Wien River and key fish species in the Danube Canal (epipotamal middle) (BMNT, 2019) and were, therefore, also taken into account.
nately, the distance D expresses the distance between a left-banked groyne and a right-banked groyne.The degree of obstruction O was F I G U R E 3 Physical model built at a scale of 1:8: (a) top view, (b) section A-A longitudinal profile, (c) section B-B characteristic profile, and (d) section C model top view with ink.[Color figure can be viewed at wileyonlinelibrary.com]F I G U R E 2 Groyne parameters: (a) top view of the channel, (b) side and top view of groyne and (c) groyne cross-section (A-A).

ForFlow
preliminary investigations and parameter study, a numerical hydrodynamic model was created using the software Hydro_as-2D.A straight channel of 200 m in length with the characteristic crosssection was modelled at a scale of 1:1.Fifty different designs were F I G U R E 4 Physical and numerical results of design 10: (a) water depth (m) and (b) flow velocity (m s À1 ) at Q 330 and MQ; dot plot: physical model; areal view: numerical model.[Color figure can be viewed at wileyonlinelibrary.com]T A B L E 1 Flow parameters without groynes: Q = flow rate, h = water depth, v = flow velocity, Re = Reynolds number = v4R/ν, R = hydraulic radius = cross-sectional area/wetted perimeter, Fr = Froude number = v/√gh, S = bed slope and water level slope (Lasinger & Leutgöb, 2022).Groyne parameters (nature): L p = projected groyne length, D = groyne distance, α = angle, H = groyne height, h Q330 / H = relative submergence, S head = head slope, S side,us = side slope upstream and S side,ds = side slope downstream.
imum and maximum values of both models, the following averages were obtained.For Q 330 , both the minimum and maximum water depths were on average 0.02 m greater in the physical model.For MQ, the minimum water depths were the same on average.The maximum water depths were greater by 0.03 m in the physical model.The maximum velocities were the same on average at Q 330 and greater by 0.1 m s À1 in the numerical model at MQ.Comparing each measuring point of design 10 of the physical model with that of the numerical one, the water depths at Q 330 and MQ were, on average, equal and greater by 0.01 m, respectively, in the physical model.The standard deviation of the water depths was 0.01 m for both flow rates.The velocities were, on average, 0.05 m s À1 greater at Q 330 .At MQ, the mean deviation was 0.00 m s À1 .The standard deviations were 0.19 m s À1 at Q 330 and 0.12 m s À1 at MQ. Figure4shows the measurement points of the physical model of design 10 superimposed on the numerical model.

Q 330 ,
not all of the added sediment was transported.Compared to MQ, less sedimentation occurred.At MQ, almost all of the added sediment was eroded and sedimentation occurred upstream and/or downstream of the groyne.With the exception of one design, sedimentation upstream of the groyne was always greater than sedimentation downstream.Based on the observations, the sedimentation downstream resulted from particles going around the groyne head and then being flushed behind the groynes by the transversal flow.A difference between grain sizes was only observed for two designs; the smaller sediment particles were closer to the middle of the flume.
). Due to the low angle and height, the material input with a 3D degree of obstruction of 4.3% is minor.It is possible that flora will develop because of the observed sedimentation and possible natural succession.Although the construction of this design F I G U R E 6 Variation of head shape: (a) water depth (m) and (b) flow velocity (m s À1 ) for design 29 (H = 0.2 m, O = 0.57, D = 6 m, α = 55 , L = 4.58 m, S head = 1:3) and design 26 (H = 0.2 m, O = 0.57, D = 6 m, α = 55 , L = 4.58 m, S head = 1:14); (c) water depth (m) and (d) flow velocity (m s À1 ) for design 4 (H = 0.2 m, O = 0.45, D = 6 m, α = 45 , L = 3.95 m) and design 10 (H = 0.2 m, O = 0.57, D = 6 m, α = 70 , L = 3.99 m, S head = 1:8.3).[Color figure can be viewed at wileyonlinelibrary.com] is complex due to the different slopes, the advantages outweigh the disadvantages.