Review on different shapes of spurs and their effects on channel morphology Open Access

Rivers and oceans are the backbones of the economy of the world. They are essential routes for transporting heavy cargo, benefiting from their navigational advantages and allowing unrestricted movement (Niedzielski et al. 2021). It has been observed that due to the meandering and braided nature of rivers, erosion is prevalent in many regions around the globe, thus, posing a challenge to both environment and human activities. It is evident that the migration of soil particles in the flowing rivers scours and erodes the riverbank. The eroded materials disturb the nature of flow downstream by depositing all the sediments (Rashedy et al. 2018). The action of erosion can be formed in different ways, such as through flow characteristics of the channel or due to the frictional effect between flow and bank/bed or obstruction of flow due to construction of dams, weirs. The erosive action causes failures of riverbanks and changes the course of flow, which leads to floods and damages human habitats, agricultural lands and infrastructures (Hansom & McGlashan 2000).

Scouring is another natural process that occurs when the erosive force of flowing water removes sediment and other materials from the bed and banks of a channel (Omar & Kumar 2021). This process is typically caused by high water velocities or changes in the flow direction, which can cause the water to pick up and transport sediment, rocks, and other materials. As the sediment is carried downstream, it can cause further erosion of the channel bed and banks, leading to deeper and wider channels over time. Scouring can have both positive and negative impacts on ecosystems and human infrastructure, depending on specific circumstances and magnitude. Scour is typically divided into three main types: general scour, contraction scour, and local scour. General scour refers to the erosion of sediment that happens throughout the bed and banks of a channel without any structures such as abutments, piers, or spur dikes (Shekhar et al. 2021). When scouring is caused by a reduction in channel width resulting from training structures such as dikes and J-hook vanes, it is referred to as constriction scour. The presence of these structures causes a substantial alteration in flow patterns and bed arrangements in a particular section of the channel, leading to excessive erosion around the structure. The scouring caused by such structures is limited to the area immediately surrounding the structure and is, therefore, called local scour (Zhang & Nakagawa 2008).

The protection of land by controlling the bank erosion caused by meandering rivers can be achieved through river training, which has been an ancient practice since the middle ages (Mosselman 2020). The process of maintaining proper cross-section and alignment by stabilizing the channel is termed river training (Bronstert et al. 2007). The prime objective of river training is to enhance the navigational feature of the river by maintaining a suitable channel depth. The river training also satisfies the objectives of increasing sediment transport efficiency, reducing erosion and scouring on the beds and banks, hence providing safety against river floods. Natural and human activities affect the equilibrium of the channel by disturbing the sediment transport capacity. Thus, in order to overcome this disturbance, the construction of river training structures is necessary. These training structures are classified as (i) bed fixation and bottom vanes and (ii) longitudinal and transverse structures (Blazejewski et al. 1995). The present study aims to evaluate spur dikes as a management measure for bank erosion and channel stability. Through a detailed analysis of existing research, it addresses the effectiveness of different shapes of spur dikes in minimizing erosion challenges, emphasizes optimal design factors, discusses the positive impact on channel morphology, and stresses the importance of considering environmental implications. The ultimate goal is to contribute to a better understanding of the long-term performance, durability, and effectiveness of spur dikes in erosion management. In addition, this paper provides a comprehensive summary of the latest findings regarding the influence of various shapes of spurs on hydrodynamics and channel morphology.

As erosion is a natural phenomenon and cannot be eliminated, therefore, to enhance riverbank protection, appropriate mitigative measures should be taken into consideration. Hydraulic structures, such as spurs, are considered to overcome these challenges, as discussed in the following.

Earthen embankments like levees or dikes are generally installed parallel to the channel and are incorporated to secure the regions behind it from high floods (Heerten & Werth 2012). Embankments have been used extensively for a long time. The functions are to avert inundation when the flow spills over its natural section and protect floodplains, habitations and property damages.

Revetments are artificially constructed longitudinal hydraulic structures which are usually made of materials such as stone pitching or cement concrete blocks. Revetments serve as a control structure on channel width, stabilizing the banks and preventing them from eroding (Rong et al. 2018).

Geo-synthetic materials are manufactured to be durable, flexible planar sheets. These materials serve various functionalities such as permeability, water tightness, strength with a wide range of applications such as improving soil quality, lining, drainage, reinforcement, and protection. For flood management and erosion control, various products such as geotextile bags, geo-tubes, geo-membrane, geo-grid, and geo-mattress are commonly used (Theisen 1992).

Porcupines are transverse permeable hydraulic structures comprising six RCC members joined or pinned with metal nuts and bolts. These structures create a slack flow that induces sedimentation in and around the region and protects the riverbank by dampening the flow velocity (Aamir & Sharma 2015). Due to its longer structural life and ease of construction, reinforced cement concrete is primarily used for RCC porcupines. However, traditionally, timber was one of the popular materials for such porous hydraulic structures.

Spurs or groynes are river training structures commonly adapted as an anti-erosion measure to protect the bank of an alluvial river against erosion and maintain the water surface profile by deflecting or repelling the flow direction (Dingorkar et al. 2017). Spurs are constructed across the river nearly perpendicular to the bank line. These structures have been commonly used for flood control, land reclamation and improving navigable depth. The prime objective of a spur is to train the channel along a preferred course to minimize the flow concentration at the vulnerable bank locations and significantly reduce the magnitude of thrust developed at the bank due to high-velocity current.

Spurs or groynes are considered prime hydraulic transverse structures to enhance navigational features and mitigate riverbank erosion. The structures constructed with various inclinations concerning the bankline to deflect water flow from the erosion-prone zone are spurs or groynes (Yossef & de Vriend 2011). Spurs are built using materials such as concrete, rock, timber, earth, or geosynthetics (Barkdoll et al. 2007; Dehghani et al. 2013). They are constructed across the channel to a certain extent from the riverbank based on the design specifications. Spurs also serve the purpose of controlling the flow depth, which enhances navigation and flood control. Installing such a structure, the local morphology changes as the water depth profile varies, generating efficient sediment movement.

Hydraulic structures such as spur dikes are one of the most common structures used to train a river. These structures divert the flow of water and reduce the erosion caused near the banks. Still, installing such structures generates eddies in the flow near the upstream and downstream sections (Keh 1984). These structures constrict the channel width and increase the velocity, which causes turbulent flow. This turbulence scours the riverbed, and the effect of erosion is indicated naturally (Rashedy et al. 2018). It is evident that the construction of spurs and groynes reduces erosion on the riverbanks, but the limitations are (i) transfer the impact to another location (Keh 1984), (ii) increases the velocity at the nose of the spur resulting in changing the flow characteristics (Ghodsian & Vaghefi 2009) and (iii) formation of eddies which causes turbulence (Prasad et al. 2016). The effect of all these limitations leads to the scouring of the riverbed. Thus, it is very crucial to understand various types of spur and their effect on the channel.

The literature review consists of two main parts. The first part deals with studies of spurs having rectangular/straight/I-shape in plan view. Most studies have been focused on straight spurs, and they provide a general comprehension of the fundamental morphological changes that may exist without considering additional complexities due to different shapes. The second part reviews previous studies of flows around different shapes of spurs (L, T, and hockey) installed in channels under various conditions. Also considered in the review are similar structures, such as stepped gabions, skimming walls, flow around a pile cap which may provide further insight into how morphological changes are affected.

A T-head spur has a shorter section branching off perpendicular to the shoreline, an L-head spur has one section extending parallel to the shoreline, and a hockey-shaped spur has a curved or rounded shape resembling a hockey stick blade. The terms ‘hockey-shaped’ and ‘inverted hockey-shaped’ are not widely recognized but rather informal descriptions. It is important to note that the design and shape of spurs vary based on local morphological conditions and engineering considerations. In this segment, the studied literature is associated with the influence of various shapes of spurs on scour and flow characteristics of the channel and its turbulent behaviour, as listed in Table 1.

Straight spurs redirect the flow away from the riverbank and into the centre portion of the channel, which can help reduce erosion and stabilize the riverbanks. Different studies throughout many years have been presented for straight spurs. The rectangular shape of a spur dike is one of the more economical constructions compared to other shapes. Due to its easier installation in the field and the lab, it has drawn substantial attention from researchers. Therefore, to analyze the flow characteristics surrounding the spur dike, researchers primarily use spur dikes with a rectangular shape. Giri et al. (2003) investigated the flow behaviour surrounding a solitary straight spur and a cluster of straight spurs situated in a meandering hydraulic channel featuring a slight curvature and a depth-to-width ratio nearly equivalent to 0.1. It has been observed that the flow's evolution is not significant due to the small bend angle and magnitude. However, this is not a matter of great concern as the state of fully developed flow is not crucial for design purposes (Odgaard & Bergs 1988). The vertical velocity profiles at a distance from the dike and the back-flow region adhere to a logarithmic distribution curve. In contrast, certain velocity profile values near the channel bed conform to Prandtl's logarithmic law (Giri et al. 2003). It is also observed that the placement of the spurs has a prominent impact on the flow behaviour in the meandering channel.

The cross-sectional area, blocked by all the different types of spurs, was kept uniform in all the cases. Based on the spurs' various types and submergence conditions, the eddy circulation zones were observed in the area lying between the two spurs. Factors like permeability and gradient of the spur head can control the turbulent behaviour in the vicinity of the spurs. These mentioned factors also impact flow resistance and sediment transport behaviour. Improvements in the velocity gradient in the horizontal axis are possible in the interaction region between mainstream flow and flow inside the spur field. These improvements can be made by altering the shape of the spur head along with permeability considerations.

The complexity of the flow would not be understood if only statistical flow properties were considered. So, to get a clear insight into shear stress, scour mechanism, and instant directions of streamlines, large-scale dynamic motions should be considered (Uijttewaal 2005). It is observed that the installation of large-scale impermeable straight spurs impacts the flow parameters (Ahmed et al. 2010). The pros and cons of incorporating spurs in flood mitigation and erosion prevention were also studied. The study was conducted by performing a series of experiments using impermeable straight spurs. The arrangement of the spurs is shown in Table 2. The velocity profiles in the longitudinal direction on both horizontal and vertical planes were determined at various points upstream and downstream of the spurs. Installation of spurs with greater length may incur floodplain erosion in the upper zone and downstream of the spurs as eddies and wake zone is developed (Ahmed et al. 2010).

Velocity increases for the mid and downstream sections, whereas it reduces at the upstream region of the channel. Negative velocities were developed for single spur with relative lengths 0.5, 0.75, and 1.0. The increase in velocity is up to 1.6 times the original velocity in the channel. The optimum distance between two symmetrical straight impermeable spurs fixed on the same side ranges from three to four times the spur length (Ahmed et al. 2010). That being the case, installing impermeable hydraulic structures such as spurs in a river floodplain with a large relative length increases the possibility of failure during substantial flood events. If inadequate protection works against the scouring process, embankment failures can also occur, and the river can divert easily. To avoid these consequences, the length of the spur should be less than half the floodplain width. Yazdi et al. (2010) conducted a study using a three-dimensional numerical hydraulic model to analyze the flow variations around a spur dike. They replicated the three-dimensional flow using a turbulence model and the Navier-Stokes equations for numerical modelling. The study examined the behaviour and characteristics of the model, with a focus on the generation of flux around the spur, and assessed the influence of discharge, angle of orientation, and length of the spur dike on the flow trend and shear stress of the channel bed. The results showed that the dimensions of the spur dikes had a significant impact on the distribution of the recirculation zone, with the vertical spur dike exhibiting the highest level of shear stress. The impact of bandal-like structures on the flow and river morphology was analyzed to improve the conception of these hydraulic structures. The study was first initiated by Teraguchi et al. (2011) by performing a series of experimental studies to observe flow patterns and changes in bed topography under controlled conditions. These experimental results were then validated through numerical simulations. The effects of bandal-like structures on attrition and deposition were also investigated. The experiments were carried out on a straight flume of specific dimensions, using a particular sediment diameter and flow rate. A comparative study was conducted between conventional impermeable spur structures and bandal-like structures. The results showed similarities in the formation of vertical vortex at the upstream section of both structures, but bandal-like structures produced higher flow velocity near the water surface, diverting it to the main channel. This resulted in reduced scouring near the structure and enhanced sedimentation near the channel sides. Furthermore, the bandal-like structures increased the depth of the main channel, improving its navigational aspects.

Two different flow rates are considered for the experiments: 2,600 m³/s as high flow and 650 m³/s as low flow. The sediment size is taken as 0.16 mm for the whole experimental domain. The study was conducted experimentally and computationally to have a clearer outlook on the optimum spur configuration. The study was performed to analyze the effect of installed spurs with various configurations on the efficiency of their performance for both high- and low-flow conditions. For the case of m2 spurs, the erosion was to be higher in the channel for both flow conditions. At high flow conditions, the thalweg is comparatively closer to the m2 spur, although, at low flow conditions, the location of the thalweg differs due to the development of shoals near the spurs. The sediments' deposition was relatively lower for the m2 and m3 spurs compared to the m1 spur. Moreover, erosion was not prominent near the bank in spur cases. Scour depth was also found to be negligible near the spurs for both m2 and m3 spurs. The m2 spurs demonstrate better results regarding increased channel depth and reduced bank erosion at both flow conditions. The authors suggest using this type of spur as it improves the navigable characteristics of the channel, considering minimal bank erosion.

Spur dikes have been extensively used in river engineering to protect riverbank erosion, build proper navigation channels, and maintain overall diversity (Zhang et al. 2012). Building spur dikes around rivers is challenging due to sediment transport and bed deformation. A lot of research has been done on this topic, including laboratory experiments, field surveys, and numerical simulations. The channel bed around a spur is characterized by local scour and deposition. The maximum scour depth is a crucial factor in determining the safety of spur dikes and their environmental impacts. Despite extensive research, spur dikes still fail due to excessive scour. Different types of sediment beds have been tested to address this issue. The mean grain size and geometric standard deviation of the bed sediment are important practical parameters. To prevent spur dike failure and ensure maximum safety, sediment should be sorted longitudinally, laterally, and vertically, and the bed should be coarsened from upstream to downstream. The bottom sediment should be coarser at the bottom of the scour hole than at the top (Zhang et al. 2012). In the consequent year, Zhang et al. (2013) experimented on porous and non-porous spur dikes. A soft and porous pad is placed near the corner of the channel tank and the flume to interdict the flaccid congregation and reduce the wavy flow surface. The experiments are marked based on the type of residue bed and the type of spur dikes. In the outline procedure of spur dikes, there are a lot of frameworks to contemplate. For example, the distance, the height, the bearings, the top view and the porousness of spur dikes.

Porous spur dikes also bring down the action of the flaccid process between them and influence the flaccid track, but the impact is much more compact and different from those of the porous spur dikes. Besides, relatively the wake flaccid area in both porous and non-porous spur dikes and residue sublimation is commemorated in the downward drift of any spur dikes. Shamloo & Pirzadeh (2014) checked the correctness of the numerical results compared to the 3D simulation of six different angle spurs with the experimental data obtained by Yeo et al. (2005). The comparisons between these two results provide a very close value and establish the accuracy of the numerical results. The Froude number and critical velocity ratio (Uave/Ucr = 0.65) also influence the simulation accuracy (Pourshahbaz et al. 2022). The result value obtained from this set-up also showed that the turbulent flow patterns are indeed affected by the installed spur. It showed that the separation length downstream of a spur is 12 times the impermeable spur with 0.3 m length (Shamloo & Pirzadeh 2014).

As discussed earlier, the flow characteristics differ in the case of permeable straight spurs; Zhou et al. (2014) examines the flow pattern, erosion pattern and location of a permeable spur dike. In terms of fresh requirements regarding engineering architecture, it surveys the potential impacts and benefits of spur dikes which are permeable in nature in river training for mountainous terrain. Moreover, the study discusses the implementation as well as the adaptability prospects. Permeable spur dike had a slightly more common effect when the permeability rate was greater than 20%, and it had a probability of generating a slow flow region instead of the circumfluence region (solid spur dike) behind the deck. At that point, the depth of the local scour decreases with the rate of perviousness, and the placement field is controlled by adjusting the rate of perviousness. A permeable spur dike generally prevents further disruption by choosing the right level of water intake and can create the right flow of aquatic life at the upstream section of the dam. Permeable spur does not cause overgrowth to avoid the impact on the natural river basin. The depth of the spur area is relatively small, and it is structured with reinforcement helpful in protection. The permeable spur dike has proven extensive performance, satisfying many legal requirements to select the appropriate pervious standard. Moradinejad et al. (2019) observed that junctions in the channel and lateral intakes generally induce turbulence in the flow. This study explores sediment control methods for a few skimming walls and a spur. Three conditions were created: (i) without structure, (ii) including the skimming wall (iii) inclusive of both the skimming wall and spur. The study aims to observe the variation of discharge and sedimentation near the intake. The velocity distribution is also observed near the intake. Non-dimensional parameters like G_r and Q_r are established before the experimentation process. A 2.5 m long lateral channel having a width of 60 cm is installed in a 15 m long hydraulic flume of width 1.5 m. Bed material having median size D₅₀ along with all different properties are observed. The 25 cm high skimming wall has two parts of 75 cm (inclined at 10°) and 112 cm parallel to the channel and is installed at the entry of the lateral channel. The single spur dike installed is of length 0.25 of the breadth of the main channel (37.5 cm). It has been observed that there is a minor variation in the bed level in the first condition, without a skimming wall. In contrast, the bed morphology changes drastically after installing the skimming wall. The bed cross-section at X = 10 m and X = 10.7 m is divided into six sections (0, 0.3, 0.6, 0.9, 1.2, and 1.5 m). The results clearly illustrate that at 0.35 m from the side bank, the highest erosion is observed with the skimming wall and at 1 m for both the skimming wall and spur dike. But at 10.7 m, the bed profile has the least change. In the graph between G_r = Q_sl/Q_sm (ratio of discharge of sediment entering the intake channel to the discharge of sediment entering the main channel); VS Q_r = Q_L/Q_m (ratio of discharge of intake channel to main channel discharge intake ratio); it is observed that a higher amount of sediment enters the intake when no skimming wall is present compared to the condition with a skimming wall. Change in the position of maximum velocity is also observed. It can be concluded that combining a skimming wall and a spur dike reduces 15% of sediment entering the intake (Moradinejad et al. 2019). Pandey et al. (2019) considered an established equation to determine the maximum equilibrium scour depth (d_sa) for the channel bed with a mixture of sand and gravel. All the tests were completed under various parameters such as flow conditions, sediment properties, velocities, flow depth, length of the spur and mean diameter of bed particles, and their impact on maximum scour depth is discussed. A set of 32 experiments were conducted using a rectangular flume with four straight-head or I-head spur dikes with lengths (l) 0.06, 0.09, 0.115, and 0.140 m. The maximum depth of scouring is quantified at the point where the spur intersects the upstream nose and the upstream wall connection, at various time intervals. The d_sa is always observed to be at the nose of the spur upstream and is more compared to the spur wall junction. Increasing dimensionless parameters such as V_ac, F_dd, F_rsm and F_dl increases the maximum scour depth. Where V_ac represents the ratio of time average velocity and critical velocity of armour particle, F_dd represents the ratio of flow depth and median diameter of armour or gravel particle, F_rsm denotes Froude number of sediment mixture, and F_dl represents the ratio of flow depth and transverse length of spur dike. Additionally, the maximum scour depth decreases as the ratio of the median diameter of armour or gravel particle and the transverse length of the spur dike increases. The sediment proportion affects the scour process, and the scouring becomes more pronounced as the densimetric Froude number increases. and with the decrease of the non-uniform sediment mixture. There was found to be a good agreement between experimental and calculated values. It was also stated that the properties of the streambed mixture primarily influenced the scouring in such mixed bed conditions. A sensitivity analysis depicted that the maximum dimensionless scour depth parameter depends significantly on the densimetric Froude's number of the sediment mixture. Maximum scour depth also depends on sensible parameters like M_l, F_dl, and F_dd. M_l represents the ratio of the median diameter of armour or gravel particles and the transverse length of the spur dike.

Chung et al. (2020) scientifically explored spurs' efficacy in protecting and rehabilitating the river system and their respective ecosystem in Korea. Several experimental conditions using different straight spur designs, such as their shapes, orientation angles and spacing between the spurs, were considered for a straight and a meandering channel. In both the channel conditions, a constant width, length and depth of water were considered. In the case of the meandering channel at a distance of 3.2 m, a 60° upward curve is incorporated, whereas at a distance of 7.2 m, a downward curve is incorporated to give a meandering nature to the channel. The bed characteristics with the same size of sand and gravel were considered for both channels. The dimensions of the spurs were kept the same for all the cases, and they are as follows: the width, length and height of the spur are taken as 48, 38, and 45 cm, respectively. The experiments were categorized into five cases, explained in Table 3.

The experiment categorized as case 5 reflects effective results in producing a diverse topography regarding well-distributed deposition volume and erosion throughout the channel, which is a positive sign of sustenance for aquatic life. The effectiveness of installed spurs was evident by significant topographic changes in contour lines and the formation of shallow puddles. Moreover, the increase in aquatic life supports the theory that the distributed erosion and depositions induced by the installation of spurs lead to an enriched ecosystem by providing an abode to aquatic life.

The impact of changes in the spacing between enhanced permeable spurs in series on bed topography in a 180° flume bend was experimentally explored by Shokrian Hajibehzad et al. (2020). The flume width used for the experiment was 0.6 m, and the radius of internal and external curvatures was 1.8 and 2.4 m, respectively. The sediment used for the channel bed has a mean diameter of 0.00088 mm. The flow depth of the channel was 0.12 m and was kept constant for all experiments. The modified spur constituted a permeable part with a length L_p = 0.09 m and a triangular shape impermeable part with a length L_t = 0.05 m. Therefore L_p/L_t was equal to 0.55. The effective length (L_e) or total length was kept at 23% of the channel width. Four various intervals between the hydraulic structures were investigated for three different flow rates with Froude's numbers 2.18, 2.37, and 2.55. These intervals are 4, 5, 6, and 7 times the effective length of the modified spur, i.e., 4L_e, 5L_e, 6L_e, and 7L_e. The experiments reveal that the maximum scour depth is developed around the initial spur, specifically near the tip of the triangular impermeable part. No significant variations around the initial spur regarding scour depth were observed due to changes in spacing intervals between the structures for Froude's numbers 2.18 and 2.37. However for Froude's number 2.55, a 15% increase in maximum scour depth is observed for spacing of 4L_e, 5L_e and 6L_e. For the highest Froude's number, the maximum scour width in the third and fifth spur was minimal. The maximum scour width is noted to increase for the first and last two spurs in the channel. Again, for Froude's number 2.55, the minimum scour length is observed from the second to fifth spur, mainly because of the closely spaced structures nearby. The maximum scour length diminishes when the spacing interval between the spurs increases, compared to the maximum scour depth and width. The spacing interval of 4L_e sedimentation was observed to be nearer to the banks on the outer part of the bend. The scouring in the centre of the channel increased when the spurs were installed in series. Also, scour depth in the centre of the channel is directly dependent on Froude's number and indirectly on the spacing interval. The reduction of spacing between the spur leads to closer thalweg nearby the inner side of the bend. According to the outcome of this study, the distance between the spurs should be less than four times the spur's effective length (Shokrian Hajibehzad et al. 2020). Yarahmadi et al. (2020) compared two straight spurs of rectangular and triangular profile shapes to analyze the scour and sedimentation process. The flow characteristics considering all the dimensions for the profile as mentioned above shapes are also studied. The experiments were conducted with various flow rates, i.e., 0.018, 0.020, 0.022, and 0.024 mm³/s having Froude's number 0.176, 0.196, 0.216, and 0.235 with a constant depth of 0.15 m. The spurs were set-up in groups of four for both rectangular and triangular profiles. The sediment's scouring-deposition trend occurs around the tip and around the spurs, and a substantial scour hole occurred at the tip of the foremost spur for both cases. The only difference is that an asymmetrical scour hole is developed in the case of spurs of the triangular profile. In contrast, an approximately symmetric scour hole can be seen for spurs of rectangular profile. However, the proportion of scour hole is higher in spurs with a rectangular profile. At Froude's number values, such as 0.176, 0.196, 0.216, and 0.235 for spurs with a triangular profile, the maximum depth of scouring, and the span between maximum deposition to the channel wall, are obtained as 64.2, 49.3, 42.2 and 21, and, 20, 46.7, 60, and 100% respectively, less as compared to rectangular spurs. For triangular spurs, the sedimentation pattern was observed to be in the longitudinal direction of the channel near the wall. In contrast, the sedimentation primarily occurs around the tip of the downstream spur for rectangular spurs. In both types of spurs, it is noted that the streamlines are rerouted towards the centre of the channel; as a result, the velocity at that portion increases. Velocity increases at a height of 0.03 and 0.14 m from the channel bottom. For triangular spurs, the increment percentages are 70.3 and 84.3%, respectively. Similarly, the increment percentages for rectangular spurs are 15.14 and 29.11%, respectively. At immediate downstream, a vortex of approximately 8 and 5.5 times the effective length of the spur is generated at a height of 0.03 m from the bottom for both types of spurs. The vortices generated for both types of spurs appear to be different. In the case of the rectangular spur, the alignment of the vortex is parallel to the channel wall, whereas, for the triangular spur, the vortex is at an angle with respect to the horizontal axis. Due to this, the downstream wall becomes vulnerable as the vortex strikes the same. Alauddin et al. (2017) found that perpendicular and upstream aligned impermeable spurs cause flow disturbances, lower stability, and strong vortices and scours. Adding a downstream component and angling the crest of the structures can reduce the vortices. Equations for determining scour depth should be checked with physical models before use. Stable flow conditions are necessary for maintaining stability. Indulekha et al. (2021) observed complex flow patterns around a series of spurs for a meandering river. Understanding the turbulence mechanism and stream profiles of such channels can simplify the complexity of the phenomena. This study analyzed the flow behaviour of a meandering erodible channel, where the flow was observed around the different orientations of spurs. The study was conducted using numerical simulations in ANSYS Fluent. The results were verified with an experimental study. Experiments were conducted by mimicking a meandering stretch of the Vamanapuram River in Kerala. The curved portion was scaled to fit the hydraulic flume, consisting of three bends with angles 67, 134, and 67°. The flume was then bedded with sand from the same river. Discharge and velocity for the experiment were maintained at 27 litres/s (0.027 m³/s) and 0.41 m/s, respectively, to achieve a subcritical and turbulent flow. A series of six spurs having different orientation angles were incorporated in the outer bank of the centre line of the meandering stretch, a dimensionless parameter S/L (spacing between spurs/length of the spur) = 2.5 was used. Fundamental fluid dynamics principles such as conservation of mass, momentum, and energy were applied for numerical modelling. A range of different flow parameters was simulated in five cross-sections in the second bend, and the velocity obtained in the simulation was then compared to the experimental results. The pressure was maximum in the 150° orientation of the spurs, and a significant pressure variation was observed in the third and fourth spurs. Maximum pressure variation was seen in the 90° orientation of the spurs, with significant variation in the second, fifth, and sixth spurs. Velocities were observed to be exceptionally low for 15° and 150° orientations, but vortices were developed due to dynamic velocity fluctuation. The orientation 30 and 45° velocity decreases while entering the spur field and increases while exiting the field. The maximum velocity was observed in the 90° orientation. In the 90° orientation, the turbulent kinetic energy (TKE) was maximum with the least variation. Most variations were concentrated near and around the third spur for all the spur orientation angles. In the simulation, it was noted that, for 15, 90, and 120° orientation, strong vortices were observed. In the streamlined simulation, a 15° orientation displayed the probability of erosion at the bottom of the spurs. The 45° orientation was found to be the most appropriate as the TKE was reduced as it passed through the spur field, and variation in velocity profile was the least.

The design of L-head spurs is similar to T-head spurs, but instead of a single wing extending from the main body of the spur, L-head spurs have two wings, forming a right angle. The angle of the L-head spurs can vary depending on the specific hydraulic conditions of the river. L-head spurs, like T-head spurs, help to reduce flow velocity and guide water towards the channel's centre, which can assist in minimizing bank erosion and sedimentation. They also aid in producing eddies and reducing flow energy, allowing sediment to be deposited and forming new habitats for aquatic plants and animals.

Several studies have been carried out to investigate the impact of L-head type spurs on flow characteristics such as velocity, thalweg line, and separation zone in channels. Kang & Yeo (2011) investigated the effect of varied arm lengths on five types of L-head type spurs with a fixed length-to-width ratio of 0.15. The study aimed to predict flow changes and optimize spur design for practical application. The Large-Scale Particle Image Velocimetry was used to measure the flow characteristics for the channel. The study observed that the distance between the thalweg line and the spur was reduced compared to I or straight-head spurs in the case of L-type spurs. The velocity was amplified by a factor of 1.2–1.6 of the average velocity at the upstream zone. The highest velocity was developed downstream, and a recirculation zone was formed at a distance of about 3–7 times the spur length towards the downstream. Another study by Dehghani et al. (2013) aimed to compare the magnitude of scouring after installing a straight and an L-type spur and study the impact on scouring due to variations in the angle between both the walls of the L-type spur. The investigation considered five different flow discharges, and the ratio of mean velocity to critical velocity was taken as a constant value of 0.95. The L-type spur was researched for several angles between the spur's two walls to attain a favourable angle. According to the study, the scouring depth was less in the case of the L-type spur facing upstream than in the other two cases of straight and L-type spur facing upstream. An L-type spur facing upstream and downstream should have an angle of 60 and 110°, respectively. Overall, the studies shed light on the effects of L-head type spurs on flow characteristics and channel scouring. These findings can help improve spur structure design and mitigate the negative effects of water flow in rivers and streams.

L-head spurs can be especially beneficial when there is a sharp bend in the river or a high erosion risk. They can also be used with other river engineering techniques, such as gabion baskets and riprap, to stabilize the bank erosion. Overall, L-head spurs are vital in river management, providing an effective, long-lasting solution for flow control and riverbank erosion protection.

T-head spurs are hydraulic structures that control flow and reduce scouring in open-channel flows. They comprise a vertical stem or web that runs perpendicular to the channel bottom and two lateral wings angled at right angles to the stem. The T-head spurs' primary function is to create a small disruption in the flow field, resulting in secondary currents and shear stresses that improve sediment movement and encourage scouring.

Mansoori et al. (2012) evaluated the impacts of T-shaped and straight spur dikes and discovered that the presence of wings creates a larger stagnant zone and provides higher stability against erosion. They also discovered that the T-shape influences the distribution of high-velocity zones and the creation of point bars and bed topography, resulting in a superior navigation route. Mehraein et al. (2017) investigated scour hole characteristics and flow behaviour around T-head spurs in a 90° bend. They found that two eddy zones are developed near the wing of the T-head spurs and in proximity to the spur's downstream section. The submergence ratio, which is the ratio between the difference in flow depth and spur height from the initial bed level to spur height from the initial bed level, is an important parameter affecting the size of the scour hole. The study revealed that the scour hole size reduces with an increase in the submergence ratio, but it becomes greater when certain other parameters such as Froude's number at the spur, the ratio between the radius of the bend to the channel width, the ratio of the length of the T-spur to the channel width, and the ratio of the wing length to spur length increase. The applicability of the conclusions derived from the study is constrained to other variants of spur dikes and angles of bend. In a further study, Akbari et al. (2021) measured 3D flow velocity using Acoustic Doppler Velocimetry (ADV) on a 180° bend with T-head spurs of varying lengths. They found that installing T-head spurs generates turbulence and affects the flow pattern. They observed the development of a vertical vortex of clockwise nature at the cross-section of 60° in the channel bend. The location of the vortex's midpoint is nearly 0.05 times the channel width from the inner side of the channel wall. The study also noted that the centre of the vortex shifts towards the inner wall with an increase in spur length. The existence of spurs helps protect the outer wall against erosion. The study discovered that as spur length increased, the percentage increment of maximum velocities increased, and that mean secondary flow increased by 250% when placement of the spur was at the apex of the channel bend. The mean secondary flow strength is similarly seen to rise with the length of the spur. T-head spurs provide various advantages over straight spurs, according to research. In another recent study, Patel & Kumar (2023) conducted an experimental analysis to investigate the flow behaviour in the vicinity of T-shaped spur dikes, focusing on the effects of varying levels of downward seepage. To comprehensively examine the phenomenon, the researchers performed tests under two distinct discharge conditions. Within each discharge condition, they meticulously assessed three different seepage scenarios, namely, no seepage, 5% seepage, and 10% seepage. The experimental investigations were carried out until a state of equilibrium was achieved, which was precisely defined as when the scour depth exhibited minimal fluctuations of no more than 1 mm within a specific time interval. The phenomenon of downward seepage was noted to have a significant impact on both the elevation of the channel bed and the creation of scour depth. The maximum scour depth was observed at the periphery of the initial spur dike, which was oriented towards the flow. Furthermore, it was observed that the magnitude of the scour depth also increased in correlation with the rate of seepage. The study underscores the significance of incorporating non-uniform sediment sizes in future research endeavours, as this will afford a more comprehensive understanding of the intricate interactions that are involved in the process of downward seepage and the subsequent scouring behaviour.

One of their key advantages is the ability of T-head spurs to limit the formation of stagnant zones in the inner embayment of the spur, which can lead to sediment deposition and the construction of point bars. Furthermore, T-head spurs are more stable against erosion caused by the migration of high-stress zones away from the spur. Submergence ratio, Froude's number, radius of bend, wing length, spur length, and sediment size all affect the performance of T-head spurs. These parameters influences the flow behaviour near the spur and also affect the size and shape of the scour hole. In open-channel flows, T-head spurs are an excellent and versatile flow control and sediment management tool.

Spurs can vary in shape and size depending on the individual coastal or river circumstances and desired effects. The term ‘Hockey Head’ or ‘Hockey Stick Head’ refers to a spur with a curved or hooked end that resembles the shape of a hockey stick or the letter ‘J.’ This design is frequently used to improve the effectiveness of sediment capture and flow pattern modification. Much research work has been conducted to determine the usefulness of hockey head spurs or spurs in preventing bank erosion. These constructions are intended to stabilize the shoreline or riverbank by minimizing undercutting and reducing the lateral flow of silt.

Rashedy et al. (2018) confirmed that utilizing a hockey spur dike causes the scour hole morphology to shift downstream, resulting in smoother flow deflection. The maximum scour depth is located at a distance of roughly 10–15% of the spur dike length from the spur dike's upstream face. It can reduce the maximum depth of scour surrounding spur dikes by 75% compared to straight spur dikes. Compared to other spur dike layouts, the hockey spur dike is the most successful in preventing scour in the spur zone. Bajelvand et al. (2022) noticed that the downstream section of the hockey spur dike near the channel wall has no scouring, which is better employed to maintain the wall and improve the river's course than the L-head spurs dike. According to previous research studies, hockey-shaped spur dikes display a better performance in minimizing scour depth (Patel et al. 2022).

There is currently little study on the efficiency of hockey-shaped spurs for river erosion management. This suggests a gap in the existing body of information, implying a lack of extensive studies assessing the use and impact of hockey-shaped spurs in erosion management. While studies on various forms of groynes or spurs may exist, hockey-shaped spurs' specific configuration and efficiency in combating erosion have not been fully examined or documented. As a result, more research is needed to determine the effectiveness of this particular spur design in mitigating riverbank erosion.

A combination or succession of numerous spurs is commonly used to achieve good results in river engineering and erosion control. The river's morphology, flow characteristics, sediment transport patterns, and erosion concerns play a role in the precise design and placement of spurs. Engineers and scientists use various spur layouts depending on the unique requirements of a river system. The placement of these structures can be carefully considered to alter sediment distribution, stabilize riverbanks, and reduce erosion. Spurs are often designed according to the river's unique hydraulic and sediment transport characteristics to accomplish the desired results.

Mansoori et al. (2013) conducted an empirical investigation to examine the alterations in the topography of the bed surrounding two dissimilar headed types of spur dikes. This particular study sought to scrutinize the dissimilarities in scouring hole characteristics and the resemblance pattern between the two distinct head shapes of spur dikes. The equilibrium of bed variations in both types of spur dikes, namely a series of straight spur dikes and a series of T-head shaped spur dikes, was demonstrated. The presence of wings in T-shaped spur dikes exerted a notable influence on the alterations in the bed within the initial and subsequent embayments. However, the impact was less pronounced in the third embayment, resulting in a consistent arrangement along the interfaces of the embayment and main channel. It was discovered that when the opening length exceeded five, the variation in bed characteristics within the embayment was localized and limited. Safarzadeh et al. (2016) studied the flow structure and compared two different spurs. The influence of the wing wall and its length on the turbulent flow was investigated. A horseshoe vortex was generated near the base of the straight spur, while it was weaker for the T-head spur with a short wing length. The horseshoe vortex did not form near the base in T-head spurs with long wing walls. Complications in flow behaviour increased upstream of the T-head spur due to boundary separation caused by the wing walls. The vortex was dominant upstream for the T-head spur with a short wing wall, located at a greater distance from the spur and channel bed compared to the straight spur. A strong horizontal vortex was generated upstream for the T-head spur with a long wing wall. The horseshoe vortex disappeared faster for the T-head spur with a short wing wall compared to the straight spur. In the downstream section, the T-head spur with a long wing wall had a faster disappearance of the horseshoe vortex compared to the other spurs. The wing section of the T-headed spur was effective in preventing scouring.

Rashedy et al. (2018) established the maximum scour depth to examine the relationship between Froude number and flow discharge. The objective of the study is to investigate the influence of different shapes of spur dikes, considering variations in key flow properties such as Froude number and flow discharge. The maximum depth of scour and other important factors for various spur dike geometry were determined through a series of laboratory studies. This study contrasts various spur dike designs, such as straight, hockey, mole head, L-shape, and T-shape, taking into account variables like Froude number and flow discharges. The hockey shape outperforms the others, according to research on how clean water affects the maximum scour depth. However, the straight shape has a positive impact on the habitat by minimizing scour depth, despite the formation of a large hole. The scour depth or relative scour depth for all discharges increases as the Froude number increases for all spur layouts. Similarly, as the discharge increases, the scour depth also increases for all flow depths. Moreover, a universal equation was formulated to forecast the maximum scour depth by employing empirical correlations for any form of spur dikes, taking into account both the Froude number and shape factor. The effect of discharge and Froude number on the scour process was investigated around straight, hockey, mole, L, and T forms. When the discharge is increased, the relative scour depth increases. The hockey form has the lowest relative scour depth value, whereas the straight shape has the highest. In other words, as demonstrated by the greatest scour hole, the straight shape generates the most bed level fluctuations.

Nayyer et al. (2019) determined how well a riverbank is protected from erosion. To look at ways to reduce scouring, figure out the attributes of various geometric shapes, and recognize the harmful effects of scouring to assess the intervention's effectiveness in reducing scouring and flow characteristics. Spur dikes are utilized to keep the primary mitigation channel open while simultaneously protecting riverbanks from erosion. Using spurs in a series could be a modern approach to reduce scouring without the need for additional constructions. An algorithmic fluid dynamic model was created to examine the properties of triple combinational series with various geometries. I-, L-, and T-head spur dike individually, and T-, L- and I-shaped spur in a combinational series were analyzed in the laboratory to verify the numerical model, and flow parameters around them were measured. The numerical data was utilized to establish a connection between spur dikes in a series. The goal was to simulate a three-dimensional flow around a sequence of three objects in a rigid condition involving spur dikes on the eroded bed. To achieve this, the researchers employed FLOW-3D software for their investigation. The most effective series of spur dikes in reducing flow characteristics, such as flow speed, shear stress, pressure, and turbulent energy, and thus reducing the scouring process, was found to be the (L T T) series of spur dikes.

Sumi et al. (2021) studied the method of installing spur dikes in the Yodo River. More than 50 bridges cross the Yodo River for cars and trains. By improving the navigation condition in the river, alternative transportation can be secured during natural disasters. These studies discuss the installation methods of two types of spur dikes, T-shaped and L-head. The methods of improving them are also discussed. Various materials can be used to build spur dikes. The proposed geometry is a C-shape with a height of 0.7 m from the water level. The shape and height of the spur dike affect sediment deposition.

Yu (2021) conducted a study on five types of spur dike structures, comparing their flow characteristics. The impact of the dike shape is more noticeable on the left side of the river, near the dam's starting point. The water level changes are smaller for the arc section dike. The trapezoid fan-shaped dam has the highest water level in the blocking area, while the circular straight-head dam has the lowest downstream water level. The straight-head dam has a greater transverse drop change compared to the hook head dam. The fan-shaped hook dam has the highest water level behind the dam. The contour curve of the circular arc straight-head dam is smoother than the trapezoidal section circular arc straight-head dam. The vertical velocity distribution varies for different sections of the dam. The trapezoidal section fan hook has the highest velocity, followed by the trapezoidal section circular arc straight head, circular arc straight head of circular arc section, and trapezoidal section circular arc straight head.

Using various configurations of spur dikes placed in sequence can effectively decrease the erosion caused by scouring. It is critical to remember that the efficiency of spurs or groynes in erosion control might vary based on the specific project and site conditions. Engineering studies, field observations, and numerical modelling are commonly used to evaluate the effectiveness and optimize the design of these structures to maximize their erosion control benefits.

Several investigations have been carried out to examine the scour phenomenon in the vicinity of a spur dike to forecast the maximum scour depth. Determining the depth of scour presents a considerable challenge due to its dependence on numerous conditions; nonetheless, numerous satisfactory approximations have been derived from prior investigations. Various equations have been suggested over time to estimate the depth of scouring adjacent to piers of bridges, abutments, and spur dikes. Some of the recently propsed formulas are shown in Table 4, and these were established on the basis of experiments conducted in laboratories and empirical observations from the field.

Table 4

Some recently proposed relationships for maximum scour depth around spur dikes

Authors .	Experimental conditions .	Equations used .	Remarks .
Pandey et al. (2016)	Spur type : Rectangular impermeable spur Median diameter (d50) of sediment mixture : 0.27 mm Spur length: 6 cm, 8.5 cm, 10 cm, 12 cm, and 20 cm Discharge : 0.0159, 0.0189, and 0.021 m3/s Critical velocity for sediment : 0.252 m/s		Equation for predicting the maximum scour depth near a spur dike: It was noted that the highest scour depth is observed in the vicinity of the upstream side, specifically near the tip of the spur dike, while erosive sediment is deposited on the downstream side
Pandey et al. (2019)	Spur type : Rectangular impermeable spur Conditions of two different sediment mixture : (i) ds = 0.27 mm and da = 2.7 mm, and (ii) ds = 0.27 mm and da = 3.1 mm Spur length: 6.0 cm, 9.0 cm, 11.5 cm, and 14.0 cm		Non-linear empirical equation to estimate the scour depth at the upstream nose of a rectangular spur dike: The maximum equilibrium scour depth is influenced by critical velocity ratio, the ratio of water depth to particle size, Froude number for the sediment mixture, the ratio of water depth to spur dike length, and the ratio of armour particle size to spur dike length. It has been observed that the scour depth increases as the non-uniformity of the sediment mixture decreases
Pandey et al. (2021)	Spur type : Rectangular impermeable spur Conditions of two different sediment mixture : (i) ds = 0.27 mm and da = 2.7 mm, and (ii) ds = 0.27 mm and da = 3.1 mm Where da = Median diameter of gravel particle and ds = Median diameter of sand Spur length: 6.0 cm, 9.0 cm, 11.5 cm, and 14.0 cm Scour depth (dst) was measured with a point gauge at different time intervals, i.e. t = 1, 3, 5, 10, 15, 30 and 60 min		Empirical relation to calculate the temporal scour depth Variation: The equation estimates of the temporal evolution of scour depth around the nose of a verticalwall spur dike under clear-water scour conditions
Özyaman et al. (2022)	Spur dikes orientation angle, α = 45°, 90° and 135° Spur dike lengths: 15 cm, 20 cm, and 25 cm. Median grain diameters for non-uniform and uniform sediments : 1.12 mmand 1.43 mm,	For Uniform Sediment: For Non-uniform Sediment:	The equations proposed in this investigation provide estimations for the scour depth in the vicinity of bridge piers, bridge abutments, and spur dikes for uniform as well as non-uniform bed material

Authors .	Experimental conditions .	Equations used .	Remarks .
Pandey et al. (2016)	Spur type : Rectangular impermeable spur Median diameter (d50) of sediment mixture : 0.27 mm Spur length: 6 cm, 8.5 cm, 10 cm, 12 cm, and 20 cm Discharge : 0.0159, 0.0189, and 0.021 m3/s Critical velocity for sediment : 0.252 m/s		Equation for predicting the maximum scour depth near a spur dike: It was noted that the highest scour depth is observed in the vicinity of the upstream side, specifically near the tip of the spur dike, while erosive sediment is deposited on the downstream side
Pandey et al. (2019)	Spur type : Rectangular impermeable spur Conditions of two different sediment mixture : (i) ds = 0.27 mm and da = 2.7 mm, and (ii) ds = 0.27 mm and da = 3.1 mm Spur length: 6.0 cm, 9.0 cm, 11.5 cm, and 14.0 cm		Non-linear empirical equation to estimate the scour depth at the upstream nose of a rectangular spur dike: The maximum equilibrium scour depth is influenced by critical velocity ratio, the ratio of water depth to particle size, Froude number for the sediment mixture, the ratio of water depth to spur dike length, and the ratio of armour particle size to spur dike length. It has been observed that the scour depth increases as the non-uniformity of the sediment mixture decreases
Pandey et al. (2021)	Spur type : Rectangular impermeable spur Conditions of two different sediment mixture : (i) ds = 0.27 mm and da = 2.7 mm, and (ii) ds = 0.27 mm and da = 3.1 mm Where da = Median diameter of gravel particle and ds = Median diameter of sand Spur length: 6.0 cm, 9.0 cm, 11.5 cm, and 14.0 cm Scour depth (dst) was measured with a point gauge at different time intervals, i.e. t = 1, 3, 5, 10, 15, 30 and 60 min		Empirical relation to calculate the temporal scour depth Variation: The equation estimates of the temporal evolution of scour depth around the nose of a verticalwall spur dike under clear-water scour conditions
Özyaman et al. (2022)	Spur dikes orientation angle, α = 45°, 90° and 135° Spur dike lengths: 15 cm, 20 cm, and 25 cm. Median grain diameters for non-uniform and uniform sediments : 1.12 mmand 1.43 mm,	For Uniform Sediment: For Non-uniform Sediment:	The equations proposed in this investigation provide estimations for the scour depth in the vicinity of bridge piers, bridge abutments, and spur dikes for uniform as well as non-uniform bed material

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It is important to note that scour at hydraulic structures, such as spur dikes, bridge piers, and abutments, is a significant process that modifies the sediment-flow equilibrium and has an impact on cross-sectional ecosystems, the vulnerability of the area to flooding and changes in local geomorphologic features. In order to forecast the environmental consequences of structures on a local scale, further research must examine the mechanisms and estimations of scour depth evolution at different forms of hydraulic structures.

Despite the fact that various research studies have been conducted to better understand the 3-D flow patterns around spur dikes, there are still several issues that must be addressed in future studies. Some of these challenges have been investigated through experiments to add to current knowledge, whereas others have been investigated through computer simulations and compared to earlier experimental findings. Nonetheless, there are certain gaps in our knowledge about spur dikes, including:

• The shape of spurs can significantly impact flow and bed morphology, playing a crucial role in designing effective river management strategies. While previous studies have focused on straight spurs, it is important to investigate other shapes like L-head and T-head spurs. L-head spurs may enhance riverbank stability, while T-head spurs could more effectively redirect flow and prevent erosion. More research is needed to understand the impacts of these various spur shapes on flow and bed morphology and their appropriate application in river management schemes.

• While prior research has concentrated on the impact of individual spurs on river morphology and flow, there is a research gap concerning the collective action of spurs in a series. Some research has examined several spurs' combined effects and spatial distribution. The placement and spacing of spurs within a series can cause flow convergence or divergence, resulting in changes to the river channel's shape, velocity, and sediment transport patterns. Furthermore, a sequence of spurs can have a cumulative influence on river ecology, influencing erosion and deposition patterns and aquatic ecosystems. As a result, further research is needed to understand the aggregate impacts and interconnections of spurs within a series.

• Investigating the effect of fluvial factors on hydro morphology near spur dikes is an important study topic that should be pursued further. Vegetation significantly impacts the form of the river channel and its response to spur dikes. However, the literature on this topic is scarce, highlighting the need for additional research. Vegetation in the river channel increases flow resistance, altering flow velocity, direction, sediment transport, and deposition. Understanding vegetation interactions with spur dikes and their environs is crucial for sustainable river management and ecological conservation.

• Installing spur dikes can affect the river channel's flow and sediment transport patterns, causing changes in the hydraulic forces acting on the riverbanks. These modifications may enhance the likelihood of bank erosion and instability. However, many researchers have not included riverbank susceptibility in their studies on spur dikes, and there is a lack of sufficient literature on this topic. Therefore, future research should also focus on investigating the susceptibility of riverbanks to erosion and instability in the presence of spur dikes.

• A research gap exists regarding the temporal changes in bed morphology around spur dikes. Existing studies on spur dikes have mainly focused on static conditions and have not examined how the bed morphology evolves over time in their presence. Investigating these temporal variations can enhance our understanding of sediment transport dynamics in river channels and the role of spur dikes in modifying sediment transport patterns.

• Sinuous streams are characterized by a meandering channel with alternating pools and riffles, which can create complex flow patterns and sediment transport dynamics. Sinuosity can influence how flows interact with spur dikes and potentially affect the scour depth and bed morphology around these structures. While numerous numerical simulations and experimental studies have been conducted to observe the bed morphology and scour depth around spur dikes in straight channels, relatively few studies have explored the same phenomenon in sinuous streams. This represents a significant research gap that needs to be addressed.

• Although lab experiments provide valuable insights, it is important to recognize their limitations, especially regarding their relevance to real-world scenarios. Many studies focus on uniform sand when examining the scour depth and turbulent characteristics near spur dike. Still, future research should include non-uniform sediment sizes to better match field conditions. To bridge this gap, future studies need to consider the presence of non-uniform sediment sizes.

Spur dikes have emerged as a well-studied approach among the numerous ways investigated. Several important conclusions and recommendations can be taken from a detailed assessment and analysis of the research on spur dikes. Research shows spur dikes are effective in minimizing erosion problems. Spur dikes provide safety to channel banks. They reduce erosive forces by redirecting water and preventing gradual weakening and erosion. When implementing spur dikes, it is critical to consider various design factors. Efficiency is influenced by head shape, spacing, alignment, size, and installation angle. L-head spurs improve riverbank stability, while T-head spurs counter bed scouring by minimizing horseshoe vortex development. Researchers advise considering these characteristics for optimal performance and long-term stability. Research consistently shows spur dikes benefit the channel's overall shape. They help create stable cross-sectional forms and encourage sediment deposition, making the channel more erosion-resistant. Spur dikes must be thoroughly evaluated for their impact on ecology, aquatic life, and sediment transport.

After conducting a comprehensive and meticulous analysis of all the research findings pertaining to this subject matter, it becomes exceedingly apparent and unquestionably evident that spur dikes possess an immense and noteworthy potential as a highly viable and effective solution for effectively managing the pervasive issue of erosion, while concurrently preserving the integrity and stability of channel banks. The study on the optimization of spur shapes is limited. So, to increase the spur's serviceability by effectively reducing erosion, a comparison of performances for different head shapes of spurs should be given considerable interest. The study of spurs installed in a series, with the same or a combination of different shapes, should also be given utmost importance. These structures have been proven to be remarkably successful in mitigating erosion, thus playing a pivotal role in fostering and perpetuating the much-needed stability that is essential for the long-term preservation of channel banks. However, there is still potential for development and more knowledge is required.

Further research and field studies are required to improve the efficiency of spur dikes. These studies can aid in the refinement of design principles, ensuring that structures are optimum for various situations and locations. By understanding how spur dikes perform over the long term, we can gain confidence in their durability and effectiveness.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.