Entity Model Test and Analysis of Local Scour of Three Different Structures of Artificial Reefs

Abstract

In this study, we aim to optimize the design of artificial reefs and improve their applicability and durability in the marine environment. Three types of reefs were selected to be placed in Indonesian waters as the target, and we analyzed the local scour characteristics and the influence of structural parameters of the artificial reefs at four different flow velocities through flume model tests and numerical simulations. The results showed that the local scour was insignificant when the flow velocity was less than 0.8 m/s and became severe when it reached 0.8 m/s. The structure of the reefs affects the degree of scouring, and a multi-column support structure will form a complex flow field, which can be optimized by combining with the design of sand content; the high and low values of the flow field, the bed shear, and the vortex field in the numerical simulation correspond to the areas of the local scour in the test. In conclusion, this study provides an essential basis for the design, deployment, and later management and maintenance of artificial reefs, which can help to improve their stability, better fulfill their ecological function, and promote the sustainable development of marine fisheries.

1. Introduction

With the increasing depletion of marine resources and the degradation of marine ecosystems, artificial reefs have received widespread attention as an effective means of marine ecological restoration. In recent years, Nature-based Solutions (NBS) have become a global research hotspot due to their potential to combat climate change, conserve biodiversity, and enhance ecosystem services [1]. Artificial reefs, as one of the typical NBS, can not only restore degraded habitats and enhance fishery resources by mimicking natural reef structures but also slow down coastal erosion by weakening wave energy [2]. Compared with traditional hard engineering measures (e.g., concrete revetments), NBS emphasize the self-regulation and sustainability of ecosystems, and the design and deployment of artificial reefs fit this core concept. Artificial reefs can provide habitats and breeding sites for fish, increase fish resources, and promote the growth of algae and other marine organisms, thus improving the marine ecological environment [3,4,5,6]. However, when artificial reefs are placed on the seabed, they will be affected by waves, tides, currents, and other factors, and then scouring, sinking, and burying will occur, which significantly reduces the stability of the artificial reefs, resulting in the loss of reefs for the sheltering marine organisms, loss of attraction, and loss of primary ecological regulation function [7,8,9]. As early as 2000, some researchers found that scour action is the leading cause of excessive reef subsidence or even burial [10]. Local scour refers to the phenomenon through which sediment is washed away from a specific area around a structure under the action of water flow, forming a scour pit [11]. For artificial reefs, local scour may lead to uneven settlement or even an overturning of the reef, thus affecting its stability and ecological function [12]. Therefore, the study of local scour characteristics of artificial reefs of different structures under the action of water currents is of great significance for optimizing the design of reefs and improving their applicability and durability in the marine environment.

In marine ecology and engineering research, the problem of localized scouring of artificial reefs has become one of the core concerns affecting their functioning and stability. The research contents are mainly flow velocity, reef structure, and substrate type [13,14,15,16]. For example, Li [17] studied two different structures of reefs and found that the bottom scour depth of the reef with smaller openings was relatively larger under the action of water flow. Yun and Kim et al. [18] investigated the local scouring characteristics of artificial reefs in an environment with different sand content and water depth. Through their systematic study, it is clear that the substrate characteristics, water velocity, and the structure of the reef itself are the key factors determining the degree of scour. Ali et al. [19] investigated the scouring of sediment around the bridge piers with three different cross-sectional shapes, namely cylindrical, square, and rhombic. They revealed that the scouring phenomenon is more important than other artificial reefs and the scouring depth is more important than other artificial reefs. The results revealed that the scour phenomenon is closely related to the structure’s shape and the water flow rate. Although the research object is not directly related to artificial reefs, it provides an analogy and reference for the research of artificial reefs. Ding et al. [12] studied the local scouring problem of artificial reefs under chalky substrate conditions in different flow conditions and analyzed it with the help of numerical simulation of the prototype flow field. Yang et al. [20]. focused on the local scouring problem around square artificial reefs and analyzed the local scouring mechanism and characteristics around the artificial reefs in a steady water flow with the help of a numerical model. Gong et al. [21]. comprehensively investigated the influence of artificial reefs on the scouring of monopile foundation and analyzed in detail the stability of the reefs and their protective effect on the pile foundation under the effect of different wave current conditions, reef types, arrangement, spacing between the reefs, and the pile foundations. Mohapatra et al. [22] established a multi-body hydrodynamic model and analyzed the responses of a 20-module articulated floating structure under wave, current, and wind actions through experimental validation. The study found that current velocity significantly influenced structural displacements (increasing with flow velocity), while wind loads had no significant effect. Additionally, it revealed the regulatory mechanisms of mooring stiffness, number of modules, and wave incidence angle on wave transmission efficiency and structural stability, providing a theoretical basis for the optimized design of floating offshore structures. In the past, the structure of the reef was relatively simple, and now, the structural design of the reef needs to be determined according to the conditions of the sea area and the function of the reef. In order to achieve better ecological benefits, the reef’s structural design needs to be as complex as possible in order to avoid a single simple shape, and the reefs tend to be large-scale and complex [23].

Indonesia has rich fishery resources, but for a long time, overfishing, climate change, and other unfavorable factors have led to a sharp decline in the amount of some fishery resources, and some traditional fishing grounds have even fallen into the plight of resource depletion [24]. Against this background, it is particularly crucial to actively conduct research in the field of marine ranching and to scientifically deploy artificial reefs. These measures can significantly improve the status quo of fishery resources and vigorously promote the sustainable development and utilization of fishery resources in Indonesia, thereby enhancing the stable development of the country’s fishery economy, the healthy balance of the ecological environment, and laying a solid foundation for the long-term prosperity of its marine industry.

Based on the characteristics of the ecological environment and biological resources in Indonesian offshore waters, this study systematically combined physical modeling experiments and numerical simulations to investigate the local scour characteristics of three new artificial reef structures under different flow velocities. The workflow of this study is as follows: First, flume experiments were conducted to measure the scour depths, morphological changes, and sedimentation patterns around the reefs at four flow velocities (0.2–0.8 m/s). Second, theoretical calculations of sediment critical velocities were performed to validate the experimental results. Third, computational fluid dynamics (CFD) simulations using the RNG k-ε turbulence model were performed to analyze the velocity field, bed shear stress, and vorticity distribution in order to provide mechanistic insights into scour formation. Finally, a comparative analysis of reef structures was conducted to determine the optimal design for reducing scour and improving stability. This integrated approach ensures a comprehensive understanding of hydrodynamic interactions and scour mechanisms for guiding practical reef deployment strategies in the Gulf of Indonesia.

2. Materials and Methods

2.1. Test Conditions and Model Parameters

The test was conducted at the Jiangsu RuDong test base of the Chinese Academy of Fisheries Research. The test tank (Figure 1) consists of a two-way current pump, frequency converter, current pipeline, equalization box, and computer control interface card.

Size of the test section: 4.5 m × 1 m × 1.5 m, 0.4 m thick sediment at the bottom, the reef was placed on the surface of the sediment layer centered at about 2 m from the front of the sand channel, and the water depth of the test model was 0.7 m. See Figure 2.

The test is conducted in accordance with the “Wave Model Test Procedure [25]”. Reef A has three parts: the top, middle, and bottom. Its top plate is designed as a stony coral fixation device. The connection between the top and bottom plates is achieved by using the grooves on them and assembling them with the two side plates. The side plates are designed with holes of different shapes, and the bottom plate is designed with three grooves, which can be used for installing the “I” type concrete bottom column to enhance the stability of the reef further. The bottom plate is designed with three grooves to enhance the stability of the reef further. The bottom plate is designed with three grooves for installing “I” shaped concrete bottom columns to further improve the stability of the reef; Reef B is also composed of three layers, the upper plate and the middle plate are embedded and connected by four columns, the top plate is equipped with a frame structure for fixing coral culture, and the middle plate is fitted with a number of structures on the top for coral limb transplantation, and the lower layer is surrounded by three side panels· as well as a The lower layer is surrounded by three side plates and a hole, and the bottom is a solid structure can effectively prevent the reef from sinking; Reef C includes a base and a top frame, the top frame is located above the base, with support columns between them, the top frame is a triangular beveled structure, the bottom plate is in the shape of a hexagonal star, the three equilateral triangles and the top triangles are directly designed with hooks and ropes, and the bottom is provided with a hemispherical fish-collecting device, and the bottom plate is a solid structure. Grouper (Epinephelus) is the main economic product and key species of coral reef ecology in Indonesia, but it is threatened by overfishing, and the complex structure of artificial reefs can provide hiding places and breeding sites for grouper, which is favorable for their survival and growth [26,27]. The three types of reefs are mainly designed according to the living habits of major fish species and the seabed ecosystem of the sea area where Indonesia is proposed to be placed in order to achieve the purpose of aquaculture and restoration of the marine environment.

According to the design water level, current conditions, test equipment conditions, and other factors, the model geometric scale is taken as 1:15. Three types of reef models were produced (Figure 3) with the following physical scales: length scale: Lr = 15; flow rate scale: Ur = Lr^1/2; weight scale: Wr = Lr³. The geometric parameters of the prototypes and models are shown in

Table 1. Geometric parameters of the reef.

	Prototype		Model
	Dimensions/m (L × W × H)	Weight/t	Dimensions/m (L × W × H)	Weight/t
A	3 × 2.7 × 3	8.44	0.2 × 0.18 × 0.2	2.5 × 10−3
B	3 × 3 × 3	7.09	0.2 × 0.2 × 0.2	2.1 × 10−3
C	3 × 2.7 × 3	3.44	0.2 × 0.18 × 0.2	1.02 × 10−3

. Since the artificial reefs are subjected to the direct force of the current after placement, the gravity similarity and drag similarity criteria are the first similarity criteria that must be met in the physical modeling tests. According to the ecological survey in Indonesia, the current velocity in the proposed sea area is relatively low, generally between 0.05 and 0.15 m/s, with some exceeding 0.3 m/s but not more than 0.4 m/s. Considering the extreme weather conditions, the maximum current velocity selected for this experiment is 0.8 m/s, and the prototype current velocities are selected as 0.2, 0.4, 0.6, and 0.8 m/s. According to Froude’s similarity law, the standard modeling method was used to design the water flow velocity as 5.2, 10.3, 15.5, and 20.7 cm/s.

The prototype sand (both with Indonesia proposed to put the sea substrate sand) contains excellent particles that may cause flocculation settlement, resulting in the model sand being challenging to select in order to ensure the similarity of the acceleration of sediment particles and the consistency of the force of sediment particles under the action of the current, this study refers to the relevant literature [28,29] used with the same prototype sand as the natural sand as the model sand, whose particle size ranges from 0.2 to 0.5 mm, and whose particle size gradation curve is shown in Figure 4. The grain size grading curve is shown in Figure 4.

2.2. Test Methods and Data Acquisition

Flushing Test Prior to Initial Flow Rate Measurement:

Before placing the reef model in the test tank, a flushing test was conducted to calibrate the flow conditions. The procedure included the following steps:

(1)

Based on the target flow velocity (e.g., 0.2–0.8 m/s), the required current value was calculated.

(2)

The flow generation system was adjusted iteratively, and repeated sampling was performed to correct deviations.

(3)

The calibrated flow data were transmitted to the computer-controlled interface to ensure precise flow rate settings.

The following steps are performed after the flow measurement process is completed:

(1)

Soak the test sand in the water tank and then spread it out;

(2)

Put the test water into the test model water level; the water level is 0.7 m;

(3)

When the water is still, slowly place the reef model in the water tank;

(4)

After the reef is completely immersed, start the flow generation system to flush it;

(5)

Each group of flow rate scouring 20 h, scouring time continuous action cumulative every 5 h with a stylus instrument for data collection and scouring photos.

Repeat the above steps for the next flow rate.

The flow velocity measurement was carried out by LGY-II Intelligent Velocity Meter (Figure 5), with a measuring range of 1–300 cm/s and an accuracy of ±1 cm/s. The topography was measured by a stylus meter on the topography formed by scouring, such as scouring pits, with an accuracy of ±0.5 mm, and the photographs were taken with a Sony DSC-RX1RM2 camera. Combined with the results of topographic measurements, CFD-post (2022 R1) and Tecplot software were used to make velocity contour maps, flow field maps, bed shear stress maps, and vortex maps to analyze the relationship between the factors, and combined with the physical modeling test, to analyze the mechanism of the formation of local scour topography.

2.3. Theoretical Calculation of Starting Flow Rate

The particle size of the medium sand substrate selected for this test is 0.2–0.5 mm. In 2019, Cai and Zhang [30] corrected the sediment starting flow rate formula of Li [31] and compared it with the measured data and the formulas of each family. The results found that its corrected formula is quite in line with reality, and it has achieved a better validation effect. Therefore, according to the amended sediment starting flow rate formula:

V_c = [0.0035 (NR^1/6/g^1/2nRe_vd)² + 1.5]{[(γ_s − γ)/γ]gD}^1/2(R/D)^1/6

where N is a constant and is generally taken to be 11.6; R is the hydraulic radius; g is the acceleration of gravity; n is the roughness; Re_vd = V_cD/v is the Reynolds coefficient; D is the sediment particle size; v is the coefficient of viscous coefficient of water movement; γ_s, γ are the sediment particles and the water of the bulk weight, the unit of this formula is used in kg∙m∙s.

2.4. Numerical Simulation Methods

Numerical models were developed using Computational Fluid Dynamics (CFD) methods to investigate the velocity, flow, and eddy fields around the reef under different inflow conditions. Since Froude’s similarity law was used in the design of the experimental parameters, the corresponding dimensions of the prototype reefs are shown in Table 1, and in order to satisfy the similarity in the stress and stability of the reefs, the dimensions of the prototype reefs were adopted in the numerical model (Figure 6). The range of the computational area was set to be 3 times the length of the reef in front, 7 times the length of the reef in the back, 5 times the width of the reef, and 3 times the maximum height of the reef, i.e., 11L × 5W × 4H_MAX as shown in Figure 7, the mesh division of the computational area was adopted as the hexahedral mesh, and the tetrahedral mesh was used around the fish reef. The hexahedral mesh is used for most of the computational area, while the tetrahedral mesh is used around the reef, and the minimum size of the mesh is 0.1 m. In the numerical computation, the continuity equation and N-S equation under constant, incompressible flow are used in the control equations, and the RNG k-ε model is chosen as the turbulence model, and the turbulent kinetic energy k and turbulent dissipation rate ε are calculated with the density of the fluid of 1024 kg/m³, the characteristic length of 3 m, and the dynamic viscosity of 0.001 003 kg/ms. The three reef structures in this study are more complex and were numerically simulated using a computing platform equipped with an i9-13900K processor. Ansys Fluent Software (2022 R1) and Tecplot software (2022 R1) are used in the post-processing of velocity contour maps, flow field maps, bed shear stress maps, and vortex maps for analyzing the relationship between the factors and are combined with the physical model test to analyze the mechanism of the formation of local scour topography. The velocity contour map of the reef is selected from the vertical cross-section, the flow field distribution map is selected from the center cross-section of the reef, the bed shear stress is selected from the sediment bed surface, the vortex field is selected from the vertical cross-section, and the blank small boxes in each contour map represent the reef model.

3. Results

3.1. Model Test Results

3.1.1. Reef A

From the viewpoint of the scouring process under various working conditions of Reef A, when the flow velocity is 0.2 m/s, there are almost no evident scouring traces around the reef, and there is only slight subsidence after the reef is taken out in Figure 8a; when the flow velocity is 0.4 m/s, the sediment around the reef is mainly scouring craters at two corners of the headwater in Figure 8b; when the flow velocity is 0.6 m/s, the degree of scouring is aggravated in the scouring situation at 0.4 m/s, and the scope of scouring crater becomes larger and deeper, and there are apparent scouring traces on the backwaters, and there are evident scouring traces on the backwater in Figure 8c; When the flow velocity is 0.8 m/s, the terrain around the reef changes rapidly. Two arc-shaped scouring pits quickly appear in front of the reef. As time passes, the scope and depth of the scouring pits increase continuously and eventually merge with the scouring pits at the two side corners. However, the depth of the merged pit does not reach that of the side scouring pits. Horseshoe-shaped scouring pits emerge on the sides of the reef. As time goes by, the sediment accumulation gradually moves backward and becomes higher, and the scouring pits become deeper. Two accumulations are formed on the leeward side of the reef and close to it. With the increase in time, more minor accumulations and scouring marks appear at a distance of 50 mm from the reef, and the height of the accumulations gradually increases over time. The overall length of the scouring marks at the tail becomes larger in Figure 8d.

3.1.2. Reef B

From the scouring phenomenon of reef B, when the flow velocity is 0.2 m/s, there is no change in the sediment around the reef in Figure 9a. When the flow velocity is 0.4 m/s, small scouring pits appear on the front side of the reef, and scouring piles appear on the back side in Figure 9b; when the flow velocity is 0.6 m/s, evident scouring traces appear on the two corners of the reef’s current-facing surface at the early stage of scouring, and with the increase in the scouring time, the scope of the scouring pits and piles is enlarged, and the height is gradually increased, and the degree of scouring increases with the increase in the time in Figure 9c; when the flow velocity is 0.8 m/s, there are apparent scouring marks at the corners of the reef’s current-facing surface at the early stage of the scour in Figure 9d. At the initial stage, there are apparent scour marks at the corners of the reef facing the current, and its depth increases slowly with time. After scouring for 4 h, scour marks of blade-like accumulation appear on both sides of the reef. After 10 h, the number of scour pits on both sides of the face increases and the depth becomes larger, the accumulation of the height of the backward shift increases, the degree of scour increases, and the scope of scouring topography around the reef is gradually expanding, and after 20 h, scouring continues to expand outward on the two feet. After 20 h, the scouring at the two feet expanded outward, and the scouring at the two sides was asymmetric.

3.1.3. Reef C

From the scouring phenomenon of the reef, it can be seen that when the flow velocity is 0.2 m/s, there is basically no change in the sediment around the reef in Figure 10a; when the flow velocity is 0.4 m/s, the scouring phenomenon is not too obvious, and scouring pits appear at the top of the first triangle on both sides of the face in Figure 10b; when the flow velocity is 0.6 m/s, scouring pits appear at the top of the first triangle on both sides of the reef, and there is a pile-up between the two triangles, and the extent of scouring pits and pile up expand gradually in height and the degree of scouring increases with time in Figure 10c. When the flow velocity is 0.8 m/s, there are apparent scouring traces at the top of the triangle on both sides of the reef at the beginning of the flow, and its depth increases rapidly with the increase in time; after 5 h of scouring the scouring pits at the two triangles can be seen at the headward surface, and the piles between the two triangles at the two sides appear to be stacked, with long tail traces appearing behind the piles and scouring pits. After 10 h, the number of scouring pits on both sides increases, the depth becomes more extensive, the degree of scouring increases, and the scope of scouring topography on both sides of the reef gradually expands. After 20 h, the scouring on both sides continues to expand backward, and the scouring on both sides is basically symmetrical in Figure 10d.

3.1.4. Calculation of Starting Flow Rate

The calculated starting flow rate of the sand in the test was 0.22 m/s. When the test flow rate was 0.2 m/s, it was less than the starting flow rate of sand, but due to the bottom plate’s structure and thickness, only a tiny amount of sand was washed up by the clear water.

When the test flow rate is 0.2 m/s, which is smaller than the starting flow rate of sediment, for the clear water scouring, the three kinds of reefs are only a tiny amount of sediment washed up, but due to the structure of the reef as well as the thickness of the bottom plate on the blocking effect of the water flow, sediment in the deceleration area around the reef settlement siltation. When the test flow rates of 0.4, 0.6, and 0.8 m/s all exceeded the starting flow rate of sediment, for the dynamic bed scouring, at this time, the sediment is mainly in the form of negative mass movement, the bed formed a uniform distribution of sand scales, and move backward from the reef to the current side, is the main form of negative mass movement [12,32] (Ding et al., 2019; Wang et al., 2020).

In the two sides of Reef A and the openings on the backflow side, especially at the corners of the two sides of the flow-facing side, the water velocity is large, the bottom sand is transformed into suspended sand, and a scouring pit is formed. There is a flow velocity low-value zone at the two corners of the backflow side, which makes the nudging mass move to this place to pile up, and the suspended mass settles here, and thus, the pile-up occurs at the two corners. However, due to the blockage of water flow by the I-shaped bottom column at the bottom of the reef and the open hole structure in the middle of the reef, there is a low-value zone in the flow velocity at the back of the reef. The buildup of the reef moves backward. It separates from the reef, and at the same time, it is limited by the flow velocity high-value zones at the two sides, and it continues to move backward to the tailing vortex flow velocity low-value zone at about 0.5 times the width of the reef. The bottom of Reef B is a square structure with multiple openings, and the flow velocity inside the reef is complex, with high-value zones of flow velocity located on both sides of the reef and scour pits and piles also on both sides. The two sides of the reef are also high-velocity areas, and scour pits are formed at the top of the triangles on both sides of the headwater side. Due to the bottom’s solid structure and the bottom plate’s thickness, the flow velocity between the triangles on both sides is low, making the nudging mass move to this area to form a pile, and the middle is a hollow structure. In contrast, the flow velocity on the dorsal side is low, and there is no apparent scouring trace.

3.2. Numerical Simulation

3.2.1. Analysis of Numerical Simulation Results for Reef A

Velocity Field

When the water velocity is 0.2 m/s, the flow velocity is lower than the starting flow velocity of the sediment, and there is no apparent change around the reef in Figure 11a. When the water velocity is 0.4 m/s, the two sides of the reef are the velocity low-value zone. The sediment is easy to settle and silt up. The position behind the backflow surface of the reef is the velocity low-value area in Figure 12. The position of the hole is the velocity relatively high-value area, and the distribution characteristics of the flow field correspond to the scouring and siltation topography of the backflow surface of the reef in the scouring test. The vertical flow field distribution at the hole on the backflow side of the reef shows that the thickness of the bottom plate causes the flow field to become weaker in the area near the bottom plate on the backflow side of the reef. There is almost no siltation on the backflow side of the reef because of the suspended sediments in the water body, and only the flow velocity at the corners on the two sides of the reef facing the flow changes in Figure 11b. When the water velocity is 0.6 m/s, there is a high-speed water flow inside the reef spreading outward through the position of the side face hole, the range of the velocity low-value area decreases, and the velocity high-value area in front of the side of the reef aggravates the degree of scouring of the terrain. The flow velocity at the location of the backflow face hole of the reef increases, the range of the velocity high-value area increases, and the range of the low-value area decreases, increasing the degree of scouring and a decrease in the siltation phenomenon in Figure 11c. When the current speed is 0.8 m/s, the current speed in front of the reef is twice as fast as the bottom sand’s starting speed, the bottom sand is started a lot, and the topographic scouring is severe. The flow velocity on the back side of the reef is higher than the previous three flow rates, the sediment accumulation is the least, and the topographic scouring is the most serious in Figure 11d.

Bed Shear Force

Bed shear refers to the shear drag force of the bottom water flow on the sediment bed surface, which is one of the important factors affecting the change in sediment bed surface scouring. The sediment starts when the bed shear force is greater than the critical starting shear force of bed sediment. For the fine particles of sediment, the sediment can be suspended after starting the sediment bed surface scouring; when the bed shear force is less than the critical stop-suspension shear force of sediment, the fine particles of sediment in the water column settle to the surface of the sediment bed, forming siltation.

The distribution results of bed shear under four flow conditions show that the bed shear is relatively tiny in a small area in front of the reef, relatively large on both sides of the reef, and the bed shear is the smallest in the wake of the reef on the dorsal side of the reef. The maximum value appeared at the two corners in front of the two sides of the reef, and the area of high shear value extended along the flow direction. At the same time, the area of high shear value was also the most serious area of scouring in the scouring test (Figure 13).

Vorticity Field

The larger values of vorticity were mainly concentrated around the reef columnar structures, and the vorticity values increased with the increase in flow velocity (Figure 14).

3.2.2. Numerical Simulation Analysis of Reef B

Velocity Field

The scour location of Reef B is mainly located on both sides of the reef, and no more obvious sediment siltation and scouring occurred. When the water current speed is 0.2 m/s, the flow velocity is lower than the starting flow velocity of sediment, and there is no apparent change around the reef in Figure 15a. When the water current speed is 0.4 m/s, there is a flow velocity low-value area on the headward side of the reef. The square shape structure at the bottom of the reef blocks part of the water flow, and after the hole is opened, the internal water flow is disordered in Figure 16. The flow velocity on the backward side decreases, and meanwhile, there is almost no sinking of the reef, the scour is mainly located in the corners at the sides of the headward side, and there is a slight scouring trace in Figure 15b. When the current speed is 0.6 m/s, the sediment is washed away at the two feet of the reef on the flow surface, which corresponds to the high-velocity area at the two feet of the flow surface in the velocity contour plot in Figure 15c. Compared with the velocity contour plot at 0.4 m/s, the range of the relatively high-value area of the velocity on the backflow side of the reef increases, and the range of the low-value area of the velocity decreases at 0.6 m/s. However, since the flow velocity on the backflow side is smaller than the starting velocity of the sediment, there is no apparent scouring trace on the backflow side, and the range of the high-value area on the two sides becomes larger. When the water velocity is 0.8 m/s, the velocity increases at the two feet of the reef face, and the range of the high-value zone also increases, resulting in the deepening of the scouring degree and increasing the scouring range and the flow velocity at the backflow face increases relatively, but its range is small in Figure 15d.

Bed Shear Force

Reef B shows an increase in bed shear with increasing flow velocity. The areas of high values of bed shear at the two feet of the flow-facing side of the reef and the side of the reef correspond to the areas of severe sediment scour (Figure 17). The high bed shear area has a high scouring capacity of the flow on the sediment bed surface, resulting in severe sediment scour, especially at 0.8 m/s flow velocity on the medium sand substrate, where the scour pits at the feet of the headward side are the most obvious.

Vorticity Field

Similar to the support structure at the bottom of Reef A, the high vortex region of Reef B is also distributed around the square-shaped structure at the bottom of the reef, which is mainly caused by the complex flow regime around the square-shaped structures, especially the complex flow regime inside the reef (Figure 18).

3.2.3. Numerical Simulation Analysis of Reef C

Velocity Field

Reef C is a polygonal structure, and the distribution of high- and low-velocity areas on the velocity contour distribution map has apparent differences. The scouring topography of Reef C is mainly located at the top of the triangle. When the water velocity is 0.2 m/s, the flow velocity does not reach the sediment starting flow velocity, and the scouring is relatively insignificant in Figure 19a; when the water velocity is 0.4 m/s, the area with noticeable flow velocity changes appears at the top of the triangle, and inside the reef, but its flow velocity is small, and there is almost no subsidence of the reef in Figure 19b; when the water velocity is 0.6 m/s, the flow velocity at the triangle of the reef increases, and obvious scouring occurs, but the scope of scouring is not enormous, and it can be seen in the flow field diagrams in Figure 19c. The flow field can be seen in the flow field diagram. The sloping structure on the top of the reef makes the flow field change, and the flow velocity on the backflow side is obviously smaller than that on the inflow side (Figure 20). When the current speed is 0.8 m/s, the sediment scouring is evident at the top of the triangle of the reef face, which corresponds to the velocity high-value area at the top of the two triangles of the face in the velocity contour map. The range of the velocity high-value area increases, and the position of the hemisphere in the middle of the reef face is the high-value area of the velocity. There are traces of scouring on the backflow surface of the reef. Compared with the velocity contour plot at 0.4 m/s, the range of the velocity high-value area increases, and the range of the velocity low-value area decreases at 0.8 m/s, so the scouring behind the back current surface is more apparent, and the degree of scouring is deepened in Figure 19d.

Bed Shear Force

Bed shear stress in Reef C increases with increasing flow velocity. The high value of bed shear at the top of the two triangles on the headward side of the reef and the side of the reef corresponds to the grave area of sediment scouring. At the same time, the scouring in the middle and backward side of the reef is also more serious in the area of the high value of bed shear (Figure 21). The high bed shear area has a strong scouring ability on the sediment bed surface, the two triangular fronts of the headward side are hollowed out at the maximum flow rate of 0.8 m/s, and the reef is not tilted.

Vorticity Field

Reef C four flow velocities, with the increase in flow velocity, the vortex around the reef increased significantly, mainly distributed around the complex structure of the reef, such as the open hole, the internal hemispherical structure, etc., the maximum flow velocity of 0.8 m/s vortex high-value area is distributed around the columnar structure, the sediment is hollowed out at the triangular front end of the reef facing the flow surface (Figure 22).

4. Discussions

4.1. Effects of Water Flow on Scouring

Under the specific condition that the bottom bed is medium sandy substrate set in this experiment, the degree of local scour suffered by the artificial reefs shows an undeniable trend: it increases with the gradual increase in the incoming current velocity. This is consistent with the results obtained by Wang et al. [32], that is, the distribution of bed shear appears in the peak area on both sides of the reef surface, which is the most serious area of local scour and is also the first area to start scouring. Comparing the magnitude of bed shear under the four flow conditions, it can be concluded that the bed shear is also larger at higher flow velocities. When the flow velocity is larger, the flow velocity on the vertical line increases faster, resulting in larger shear deformation and increased shear force. Bed shear not only reflects the size of the sediment bed surface resistance to the water flow but also reflects the strength of the water flow on the sediment bed surface scouring ability. The larger the shear force, the stronger the water flow on the sediment bed surface scouring ability. Therefore, under the action of 0.8 m/s current, the scouring degree around the reef is the most serious, and the development of the scouring pit is faster, and there are apparent scouring traces on the backflow surface of the reef. When the incoming velocity is at a slow value of 0.2 m/s because the velocity has not yet reached the starting speed of the model sand, at this time, the disturbance of the sediment is minimal, the whole bed appears to be extremely flat, and the local scour phenomenon around the reef is also microscopic, almost negligible, in this case, the main presentation of the floating sand siltation state. The situation changed significantly when the incoming velocity was increased to 0.4 m/s and 0.6 m/s. As the velocity was enough to start the sediment, sand waves began to appear on the bed, and the scouring pattern around the reef became more visible compared to the previous 0.2 m/s, but the overall scouring degree was still in the weaker category, and the more serious hollowing phenomenon had not yet appeared. At the maximum flow rate of 0.8 m/s, the scouring around the reef reached the most significant level, but fortunately, the reef remained stable even under such strong scouring.

This result shows that the three types of reefs selected for this experiment have good stability. The results of this experiment are consistent with the conclusions of Ding et al. [12] and Wang et al. [32] on the local scour of reefs at different flow velocities. Taking the three types of reefs involved in this experiment as typical examples for in-depth analysis, we can make a more scientific and reasonable choice of reef casting area based on the results obtained from the experiment. If we choose the medium sandy substrate with a relatively slow current as the reefing area, we can effectively reduce the local scour condition around the reef. Therefore, the first step in selecting artificial reef sites should be to use rigorous and scientific methods. Through on-site sampling, one can accurately obtain the actual nature of the sediment in the area to be selected; with the help of theoretical analysis, it can be based on the relevant scientific principles of the characteristics of the sediment and the possible situation for in-depth analysis and prediction; the use of physical modeling tests can be in the simulated environment to visualize and study the region’s sediment starting speed and hydrological conditions, and other aspects of the important factors. The comprehensive use of these methods can determine the most suitable site for the construction of artificial reefs to obtain a more ideal and excellent artificial reef construction effect, to ensure the adequate performance of artificial reefs in the improvement of the marine ecological environment, the enhancement of fishery resources and other aspects of the function of artificial reefs, and to promote the smooth implementation of the strategy of the sustainable development of the marine fisheries industry.

4.2. Effects of Reef Structure on Scouring

Vortex is the main driving factor for local scour of structures. Due to the obstruction by the structure, the water moves downward at a faster speed on the surface of the column structure, forming a secondary flow, which in turn forms a vortex. The sediment will actuate when the gravity or inter-particle bonding of sediment particles on the bed surface cannot resist the force exerted on them by the vortex. From the vortex plots, it can be seen that although the vortex values are higher inside the reef and at the openings on the current-facing side, no sediment scouring is caused in this region because the bottom of the reef model is a solid structure, and severe scouring or even hollowing out may be formed at these locations if the bottom of the reef is of an open-hole design. The local scour patterns of the artificial reefs are somewhat related to the results of previous studies on square columns and underwater production facility foundations, with similarities and significant differences [33,34,35,36,37]. The similarities are that the more serious scour areas are concentrated in the sharp corners and sides of the flow-facing surface of the structure, while the differences are that the local scour patterns are more complicated and diverse due to the unique design of the openings and the multi-supporting column structure of the reef.

The flow velocity at the open hole of Reef A is larger, the vortex inside the reef is simple, and the flow velocity in the wake area is small, especially at the back of the reef, 1~2 times the width of the reef, the flow velocity difference is larger with the surrounding flow velocity, and the sediment is easy to settle and silt, and this analysis is consistent with the phenomenon of the sand pile at the back of the reef obtained from the model test; the flow velocity at the upper part of Reef B does not have a big difference with the surrounding current velocity, the vortex inside the bottom box structure is more complicated than that in the reef A, the flow velocity at the back of the dorsal surface of the reef is lower than the surrounding current velocity. However, the formation of the sand pile is more complex than that at the back of the reef. It is lower than the surrounding flow velocity, but the formation of the tail vortex is complex. There is no more obvious flow velocity low-value area consistent with the results obtained from the model test. The back of the reef did not appear in the sand pile; Reef C, the top of the triangular sloping structure, the formation of the back of the flow velocity low-value area, the middle of the structure of the multi-supporting column, the back of the reef for the flow velocity low-value area, but the difference between the flow velocity and the flow velocity around the reef is not as large as that of Reef A, the flow velocity at the back of the reef 1 to 2 times the width of the reef The flow velocity at 1~2 times the width of the reef after the reef is relatively small, and the sediment is slightly settled and silted. This analysis is consistent with the phenomenon obtained from the model test, which corresponds to the velocity high-value area at the top of the two triangles on the flow surface in the velocity contour map, and the range of the velocity high-value area increases. The position of the hemisphere in the middle of the reef on the flow surface is the high-value area of the velocity, the flow velocity on the dorsal surface is larger than that of Reef B, and there are traces of scouring on the dorsal flow surface of the reef. The flow field effect and the scouring situation behind the reef can be effectively used with the spacing of the reef placement, and it has been shown that the optimal spacing of the reef combination between the reefs is 1~1.5 times the size of the reef [38]. However, the flow field effect of the reef is mainly considered without taking into account the scouring situation of the reef. The obstruction of the flow by the openings on the flow-facing side of the reef and the bottom support structure increases the flow velocity on both sides of the flow-facing side and at the openings on the back-facing side, forming a complex flow pattern with multiple eddies inside the reef, especially in Reef B and Reef C, which enhances the degree of scouring in some areas. Therefore, when optimizing the structural design of the artificial reef structure while ensuring a better flow field effect, consideration can be given to reducing the scouring intensity in local areas and increasing the stability of the reef by improving the design of the reef’s current-facing surface, multi-supporting structure, and solid bottom plate.

Based on the model test, theoretical analysis, and numerical analysis of three different structures of artificial reefs, it is known that the design and deployment of the reef should not only consider the optimization of the flow field effect but also study the impact of scouring and siltation phenomena on the stability of individual reefs and reef groups and put forward the optimization of the structure of the reef, the layout of the reef and the configuration mode of the reef group after a comprehensive evaluation of the flow field effect and the degree of sediment scouring. Therefore, future designs need to optimize the bottom opening rate by combining the substrate characteristics to reduce the scouring risk while safeguarding the flow field effect, in order to achieve synergy between ecological benefits and engineering stability [2].

5. Conclusions

In our study of the local scour of the three types of artificial reefs applicable to Indonesian waters, the following conclusions were drawn:

• The degree of local scour around the artificial reef is directly proportional to the flow velocity, and a scour’s scope and depth increase with a flow velocity increase. When the flow velocity is less than 0.8 m/s, the local scour around the reef is not apparent; when the flow velocity is 0.8 m/s, the scouring around the reef is severe. When selecting a site, analyze the relationship between the flow velocity in the chosen area and the starting speed of sediment in the area to avoid severe scouring and reduce the reef’s service life and ecological benefits.
• The degree of scouring of the reef has a significant relationship with its structure, and the multi-column support structure of the reef can form a complex flow field. Still, its flow-blocking effect is not too noticeable, and it is easy to cause suspended sediment to settle and accumulate. It should also be combined with sand content for design. The bottom of the three reefs in this test were solid structures, and no tilting occurred when they were scoured with a high flow rate of 0.8 m/s on a medium sand substrate. Therefore, in an open bottom design of the reefs, we can try to minimize the bottom opening while reducing the construction cost, and the bottom opening ratio can be designed according to the characteristics of the substrate to reduce the chance of the bottom being hollowed out.
• The numerical simulations of the flow fields of the three reefs show that the high and low values of the flow field, bed shear, and vorticity field correspond to the local scour strengths and weaknesses of the three reefs in the experiment. A numerical simulation can effectively illustrate the scouring conditions of the reefs, and more critically, it can significantly reduce the actual cost of reef tests. This opens up a new path for related research and practice to be conducted efficiently and economically.

Among the three reef types tested, Reef A (Prismatic Artificial Reef) demonstrated superior performance in balancing scour resistance and habitat complexity, making it the most suitable for long-term ecological restoration in medium-sand substrates. In contrast, Reef C (Multi-functional Tri-prismatic Reef), with specialized features such as fish-collecting devices, shows potential for targeted fishery enhancement but requires maintenance to prevent sediment blockage in hollow structures. For regions prioritizing ecological recovery (e.g., Indonesia’s degraded coral reefs), Reef A is recommended. Future designs could integrate Reef C’s modular fish-attracting components into Reef A’s framework to synergize ecological resilience and fishery productivity.

While this study provides valuable insights into the local scour characteristics of artificial reefs, several limitations should be acknowledged. First, the experiments were conducted under controlled laboratory conditions with simplified flow velocities, neglecting the combined effects of waves, tides, and variable sediment properties, which are common in natural marine environments. The use of a 1:15 geometric scale model may not fully capture the complexity of prototype hydrodynamics, particularly in terms of turbulence and long-term sediment transport processes. Additionally, the study focused on three specific reef designs, limiting the generalizability of the findings to other structural configurations. For future research, several directions are recommended. First, expanding the scope to include wave-current interactions and mixed sediment substrates would enhance the applicability of results to real-world scenarios. Second, parametric studies on additional structural parameters (e.g., porosity, column spacing, and base geometry) could further optimize reef stability. These advancements will contribute to the development of robust, eco-friendly artificial reef designs tailored to diverse marine environments.