Experimental Study on the Straw Flexible Space Assembly Flood Control System

Abstract

By using straw to build an assembled flood wall, this study provided new ideas for useful applications of straw and for the development of assembly flood control facilities. A straw flexible space assembly flood control system is designed on the basis of Revit, and a hydraulic test of a scaled-down model of the system is conducted using a water-retaining test machine to prove the stability and impact resistance of the system. Finally, a numerical analysis model of the straw flexible space flood control system is established using ABAQUS software to perform finite element analysis of the system. A comparison of the test results and the numerical analysis results shows that they can be well fitted. Thus, this study lays a theoretical foundation for a large-scale engineering application of the proposed system.

1. Introduction

In recent years, hazy weather has led to significant challenges in our living environment. In north and northeast China, winter haze generally lasts for more than five days, and most areas experience severe haze caused by the burning of straw, which not only threatens people’s health and safety but also results in major losses to the national economy [1,2]. In this regard, the comprehensive use of straw, e.g., in new building materials, and straw building performance has emerged as an important research topic. Many other researchers have studied the comprehensive use of straw as an energy source through combustion for power generation as well as in bio-fermentation [3,4,5,6,7].

Straw also plays an important role in the field of construction and building materials. Zhang et al. [8] added wheat straw fibers to cementitious sands. Su et al. [9] studied the improvement in the strength and ductility of soil cubes by mixing them with wheat straw and coarse sand. Zhao et al. [10] proposed straw lightweight partitioned composite slats obtained by gluing together straw panels of different thicknesses and densities. Wang [11] proposed the concept of a straw–steel composite wall panel assembly building structural system. Soroushian et al. [12] studied the incorporation of straw into cement to obtain cement straw panels. Zhang et al. [13] studied the axial compression mechanical properties of cold-formed double-sided straw panels with thin-walled C-shaped steel composite walls.

In the context of global warming, the increasing frequency of typhoons and heavy rainfall is a serious concern worldwide, and flooding has become the most serious natural disaster in many countries [14]. Hence, flood control equipment has emerged as the most effective measure to tackle flooding [15]. Flood walls are widely used in Europe and the Americas. In recent years, the well-known German ‘Flood Artifice’ [16] has attracted widespread attention as a pioneering mobile flood wall. In 1997, the eastern part of the Czech Republic was flooded [17], so Prague built an aluminum alloy mobile flood control system, which is very lightweight and easy to store and transport. In 2013, weeks of continuous rainfall caused the Danube River to flood, so the Austrian town effectively prevented flooding by temporarily erecting a riverside flood break before the water level rose [18].

Many researchers have conducted numerous studies on flood walls, including the side-turning flapper type [19], the lift type [20], water-filled rubber bags, and water-filled woven bags [21], and mechanical tests and finite element simulations have been carried out. Ni et al. [22] carried out water retention tests on a lightweight mobile floodwall using a water retention testing machine to analyse the force forms and damage patterns of the lightweight mobile floodwall. Wu et al. [23] used ANSYS numerical simulation to construct a reasonable floodwall model and carried out field floodwall tests based on the simulation. Chen et al. [24]. conducted an experimental study on the mechanical properties and leakage characteristics of mobile flood control systems. Li et al. [25] studied the application of movable steel gate plates in the Harbin Daoli embankment section of the Songhua River. Pan et al. [26] studied the application of a new type of assembled flood wall for urban flood control in Wuhan. Song [27] studied a type of urban underground space entrance and exit based on mobile flood protection walls.

In summary, detailed studies on the comprehensive utilization of straw have been conducted worldwide, and major breakthroughs have been achieved in the fields of biology, construction, and energy. However, the straw research base is extensive, and the comprehensive use of straw requires continuous development. Various types of assembled flood walls have been developed, and the literature [22,23,24] analyses the mechanical properties and leakage characteristics of mobile floodwalls by means of tests and finite element simulations; however, existing mobile flood walls are rigid systems with poor flood protection performance. In this study, a straw flexible space assembly flood protection system is proposed, where straw is an important material of the water barrier developed to facilitate the reuse of the flood control device. Furthermore, straw flexible space theory is applied to not only enable the proposed system to mitigate the impact of floods and complement mobile flood walls but also ensure the comprehensive use of straw.

2. Design of the Flood Control System

2.1. Composition of the Straw Flexible Space Assembly Flood Control System

The straw flexible space assembly flood control system is divided into water baffles, columns, and inclined rod supports located behind the columns. The straw water baffles are divided into oncoming and backing boards, which are connected using connecting rods, and the straw is filled between the oncoming and the backing board. The columns are divided into side columns and central columns. The columns are used to fix the water baffles. To ensure that the straw flexible space assembly type flood control system has sufficient anti-overturning performance, inclined rods are used to provide support behind the columns. The straw flexible space assembly type flood control system designed using BIM-Revit software is shown in Figure 1.

2.2. Material Selections for Straw Flexible Space Flood Control System

Water baffles are one of the main component materials of the flood control system, and aluminum alloy is used in the water baffle panels. Aluminum alloy has good corrosion resistance and does not rust easily even after prolonged contact with water; thus, it increases the service life of the water baffles. As one of the main force members, columns must have high resistance to overturning, good stability, and other characteristics. Moreover, they must have good welding performance. Hence, Q235 steel is selected as the column material so that the columns are easier to process. In addition, it can ensure that the flood control system can withstand larger waves and maintain the stability of the overall structure. Straw has a strong deformation capacity as well as good water absorption and expansion characteristics. Hence, straw is used as a filling material for flexible spaces. The mechanical property parameters of the component materials of the straw flexible space assembly flood control system are summarized in

Table 1. Mechanical property parameters of each component material.

	Type	Density (KN/m3)	Modulus of Elasticity (Pa)	Poisson’s Ratio
Water baffles	Aluminum alloy 6061	27	7 × 1010	0.34
Column	Q235B	78.9	20.6 × 1010	0.3
Inclined rod support	Q235B	78.9	20.6 × 1010	0.3
Connecting rod	EN1.4301	78.9	20.6 × 1010	0.3
Bolt plate	Q235B	78.9	20.6 × 1010	0.3
Straw	-	4	1.9 × 109	0.3

.

2.3. ABAQUS Finite Element Model for the Flood Control System

As shown in Figure 2, the ABAQUS numerical model components mainly consist of columns, water baffles, and straws. The columns of the oncoming and backing boards of the flood control system are set up. The columns of the oncoming boards only serve to fix the water barrier within a certain range, and do not have a force-bearing role. The columns of the backing boards serve to hold the backing boards in place, and the forces applied to the backing boards are transferred to the columns, so the columns of the backing boards must be reflected in the numerical model. The oncoming and backing boards are five each. The flexible space flood control system focuses on the forces on the water baffles, so the water baffles must be represented in the numerical model. The innovation of the straw flexible space flood control system lies in the addition of a flexible space in the middle of the water baffle. The flexible space is characterised by its ability to be deformed by forces and to assume the role of enhancing the stability of the flood control system, so the flexible space must be reflected in the numerical model.

There are four types of contact in the numerical model of the flood control system: the two-sided contact between the headwater panel and the flexible space, two-sided contact between the backwater panel and the flexible space, self contact in the flexible space, two-sided contact between the backing board and the column, and tie restraint between the backing boards. The mechanical properties of the contact are mainly defined in the contact properties. Flood control systems generally do not study heat transfer and define the tangential and normal properties of the mechanical properties. The tangential properties are mainly defined according to the coefficient of friction between the materials, which is around 0.2 between aluminium alloy and wood. The boundary conditions of the flexible space flood control system are set without rotation and only fixed as well as directional displacements exist. The main cell type for grid assignment in the gridding of flexible space flood control systems is the C3D8R cell type. The numerical model of the flood control system uses a structured sub-grid, which will give the maximum value of the grid density of the solid model. The grid division density also follows the guideline that the greater the stiffness, the greater the density of the grid division should be, so the grid seed interval of the flexible space is 1.5 mm, the grid seed interval of the water barrier is 1 mm, and the grid seed interval of the column is 0.5 mm.

3. Proposal and Validation of the Straw Flexible Space Theory

3.1. Straw Flexible Space Theory

The flood control system is not rigid. It has a flexible connection between the oncoming and backing panels. The oncoming and backing boards are connected by connecting rods and are designed with space for movement. When the water board is subjected to wave load impact, the straw-filled water baffles will be compressed. The elastic–plastic deformation of the straw plays a buffering role; it does not directly affect the water board and the column. After the impact force disappears, the deformed straw rapidly returns to its original state. The water board provides support to the initial position, so that the impact of the wave and the straw after the water absorption and expansion of the water board generates a space. This is called the straw flexible space, which is formed by the combined effect of the wave impact and the supporting force of the straw on the headboard.

Therefore, the straw flexible space theory is proposed, according to which, in the process of the flood control system, the static water in the straw flexible space becomes dynamic water owing to the gap between the water board and the two sides of the column. Thus, part of the wave impact kinetic energy is converted into water kinetic energy, and the straw deformation buffer in the flexible space drains off part of the impact kinetic energy. Thus, the straw flexible space enables the backing board and column to withstand the impact force. It reduces the impact force on the backsplash and columns, thus improving the stability and impact resistance of the flood control system.

3.2. Force Analysis of a Straw Flexible Baffle under the Uniform Water Flow

When flooding occurs, the flow force of the flood water on the water baffles is calculated as follows [28]:

$$F_w=C_w\frac{\rho }{2}{v}^{2}A$$

(1)

where F_W is the water flow force, A is the area of the baffle board, V is the design flow velocity, C_W is the water flow resistance coefficient, and ρ is the density of water.

The hydrostatic pressure on the water baffle is given by

S₀ = P_cA

(2)

where P_c is the pressure at the center of the water baffles shape, P_c = ρgh_c, g is gravitational acceleration, and hc is the depth below the liquid surface at the center of the water baffles shape.

The support force of the straw on the baffle under hydrostatic conditions is given by

$$F=\sigma A_1$$

(3)

where σ is the compressive strength of the straw material and A₁ is the cross-sectional area of the straw between the water-bearing and backing plates.

The support force of the straw on the baffle at a constant flow velocity together with the water flow force and the water pressure form a pair of flat constant forces, whose relationship is expressed as

$$F=F_w+s_0$$

(4)

When the oncoming board is subjected to wave impact loads due to the deformation of the straw, the support force of the straw on the oncoming board increases and it will exceed the hydrostatic pressure of the floodwater on the oncoming board after the wave, thus pushing the oncoming board to its original position. This cycle is repeated. Between the oncoming board and the backing board, an impact load is formed and it can be deformed, and after unloading, it can be restored to the flexible space. Thus, the straw flexible space theory satisfies the formula derived.

3.3. Straw Flexible Space Theory Validation Test

A straw flexible water stop was prepared and the straw device was made as dense as possible. The straw device was placed in the water stop tester as shown in Figure 3, without squeezing the straw device excessively. The distance between the water baffles and the backing board was measured as the initial condition for the straw space test. Once the assembly was complete, the tester system was filled with water to 750 mm and left to stand for a while. A certain amount of time elapsed before the straw absorbed water and expanded, and the distance to which the water-bearing board moved was measured at regular intervals.

The test was carried out on an hourly basis, recording the water absorption after 1, 3, and 6 h, and analyzing the water absorption and swelling of the straw by the size of the flexible space at different times. As shown in Figure 4, water was added to the test machine reservoir to 750 mm and test data was obtained after 1, 3, and 6 h. The swelling of the straw in the test space was analyzed. The test results show that the straw swelled after a certain water absorption time. The width of the straw space in the initial state was 165 mm; after 1 h it was 174 mm; after 3 h it was 183 mm; and after 6 h it was 186 mm. The distance that the swelling pushed the water baffles showed that it pushed 9 mm at the beginning of 1 h, 9 mm from 1–3 h, and only 3 mm from 3–6 h. The swelling started with a rapid expansion, and for the later expansion rate, it slowed down. After 6 h, the straw expansion was 11%, and at this time, the straw water absorption and expansion of the reaction force was seen, as straw water absorption and expansion of the water baffle has a supporting role, and straw as a flood control system filler can enhance the stability of the flood control system. The theoretical assumption of a flexible spatial flood control system with straw is satisfied.

4. Testing of Flood Control System

4.1. Hydrostatic Test for Flood Control Systems

The strain gauges were attached to the preset positions and connected using wires. The strain gauges were attached in the middle of the first board, in the middle of the third board, and in the middle of the fifth board, respectively, with the three strain gauges at the same height on the left, in the middle, and on the right. The strain gauges were labeled as L1, L2, and L3 on the left side, with heights of 75, 375, and 675 mm, respectively; M1, M2, and M3 in the middle; and R1, R2, and R3 on the right side. The strain gauges were arranged, as shown in Figure 5. Finally, the test machine system was filled with water at a height of 750 mm and a hydrostatic test of the flood control system was carried out, as shown in Figure 6. The hydrostatic test did not require consideration of the location of the centrifugal pump of the pipeline; hence, the strains at the three water levels of 75, 375 and 675 mm under hydrostatic pressure could be observed and the strains of the baffles on the strain gauge were recorded. A hydrostatic test of the flood control system without straw was also carried out for comparison of the results of the test with and without straw.

4.2. Hydrostatic Test Results

The strain gauge data were exported and the data were processed to obtain the strain–strain gauge height dotted line graph. As can be seen from Figure 7a, as the water level became deeper, the effect of hydrostatic pressure on the flood control system became more significant. At the strain gauge height of 75 mm, the strain values of R1 and L1 were greater than M1, and at the strain gauge heights of 375 and 675 mm, the strain values of M2 and M3 were greater than R2 and L2, and the strain data measured on the left and right baffles were the same. As the structure of the baffle tester and the reduced scale model were symmetrical, the strain gauges were also arranged in symmetrical positions and the strain values measured on the left and right baffles were the same. As can be seen from Figure 7b, the value with straw was greater than that without straw, indicating that the straw absorbed water and expanded to support the water baffles, causing the strain value to increase. Thus, the straw as a filler can enhance the stability of the flood control system.

Figure 8 shows the results of the ABAQUS numerical analysis of the hydrostatic test of the flood control system. The pressure equation (P = ρgh) was used to calculate the pressure at different water level heights. The numerical model was established under hydrostatic conditions, the parameters were input into the numerical model, and the strain values of the water baffles were analyzed. As can be seen from the figure, the strain values of the flood control system under the hydrostatic test were symmetrically distributed to the left and right. The maximum strain values of the whole flood control system appeared symmetrically in the lower left and right corners, which are the position of the column foot, indicating that the hydrostatic pressure causes the column to deform and the column is the main force-bearing structure of the flood control system. Figure 9 shows the water level–strain diagram obtained from the comparison between the hydrostatic test results and the numerical analysis results. The data obtained from the hydrostatic test and the numerical simulation were the same in terms of law and magnitude. The results of the hydrostatic tests showed that the flood control system can maintain stability under normal conditions, and a good fit was observed between the test results and the numerical analysis results, with an error within 5%.

4.3. Dynamic Water Test for Flood Control Systems

Water was injected into the reservoir zone of the water baffle test machine to carry out a dynamic water test of the system. The strain on the water baffles was recorded under different operating conditions. Depending on the number of centrifugal pumps in the pipeline and the different water level heights (300, 450, 600, 750 mm), the number of test groups was set to 12, with each group of dynamic water tests divided into three positions: left, center, and right. The actual test process required 36 different sets of dynamic water test data to be recorded. The straw flexible water baffle strain was analyzed at the water level. The strain patterns of the water baffles with and without straw were compared, and the flexible space in moving water was analyzed. The water retention test motorized water device could convert the static water state of the water retention test machine storage area into a dynamic water cycle by means of an electric gate box that controlled three centrifugal pumps. The dynamic water environment was divided into three working conditions (1 gear, 2 gear, 3 gear): in the 1 gear condition, only the middle pump was started; in the 2 gear condition, only the two side pumps were started; and in the 3 gear condition, all three pumps were started at the same time. As the gear increased, the impact generated by the centrifugal pump increased, causing the water flow to gradually increase. The dynamic water test for the flood control system is shown in Figure 10.

4.4. Dynamic Water Test Results

Figure 11 shows the results of the dynamic water test of the straw flood control system under 12 operating conditions. The overall strain pattern is that the greater the water flow force, the greater is the strain on the water baffles. The strain across the center is significantly higher than the strain on the left and right sides. The strains on the left and right sides largely overlap. There are some significant local differences, with the greatest difference between the middle strain and the left- and right-side strains at three levels of the flow force, and the smallest difference between the middle strain and the left- and right-side strains at one level of the flow force. The line graph shows that as the water flow force continues to increase, the gap between the strain in the middle and the strain on the left and right sides increases. As can be seen from Figure 12, the change in the baffle strain under the dynamic water test is the same for the flood control system with and without straw. The overall pattern of change in the flood control system in the dynamic water test shows that the test strain with the straw baffle is less than that without the straw baffle, indicating that the straw flexible space cushions the impact of the water force in the water baffle test so that the baffle strain is reduced to a certain extent.

Figure 13 shows the strain distribution diagrams of the three levels of water flow forces under the action of different water levels (300, 450, 600, and 750 mm, respectively). Under the action of the water level forces of 300, 450, and 600 mm, the height of the highlighted part of the strain distribution diagram shows a slight difference from the height of the water level. Owing to the interaction between the water baffles, the height of the highlighted part of the stress distribution diagram is slightly higher than the height of the water level, and the strain value in the middle is higher than the strain values on both sides of the baffle. When the water level reaches 750 mm, the stress distribution in the strain distribution diagram changes somewhat. The stress distribution gradually stratifies from the inside to the outside, and the strain in the middle of the water baffles is greater than that around the water baffles. At different water levels, the lower left and right corners of the water baffles, where the foot of the column is located, have the highest strain values. This indicates that the column is the main stress-bearing structure in the flood control system.

Figure 14 compares the results of the dynamic water test and the numerical analysis of the water baffles under different operating conditions. The strain values from the dynamic water test and the numerical simulations are the same in terms of the variation pattern at each location in the flood control system. The strain values for each condition of the dynamic water test are slightly less than the strain values from the numerical analysis. As the water flow increases, the line graphs of the dynamic water test and the numerical simulation show an increasingly better fit, and the errors decrease. The error in the data for all conditions is between 5% and 8%.

4.5. Discussion of Tests Results and Finite Element Modelling Results

The error between the above test results and the finite element simulation results is small, indicating the correctness of the test and the validity of the finite element modeling. The above results of tests and finite element modelling show that straw has a supporting effect on the water barrier when it absorbs water and expands, and that straw as a filler in the system can enhance the stability of the flood control system. The straw flexible space can transform the dynamic energy of the wave impact into the dynamic energy of the static water in the flexible space into dynamic water and the potential energy of the straw compression, thus dissipating part of the impact force of the flood and playing a buffering role, improving the stability and impact resistance of the flood protection system. The straw flexible space flood control system has a good flood control performance and applicability.

5. Conclusions

A straw flexible space assembled flood control system was designed on the basis of Revit, and the straw flexible space theory was proposed. Further, a straw flexible space theory verification test was conducted on the scaled-down model of the flood control system using a water-retaining test machine. The test results were compared with the ABAQUS finite element analysis results through static and dynamic water tests. Thus, the feasibility of the straw flexible space assembled flood control system was verified. The main conclusions of this study can be summarized as follows:

• The flood control system is mainly composed of a straw flexible water barrier, column, tilt rod support, and base. According to the elasticity of the straw material proposed by the straw flexible space theory, the straw flexible space can transform the wave impact kinetic energy from static water to dynamic water in the flexible space as well as into potential energy of straw compression and thus drain away part of the impact of the flood. Therefore, it plays a buffering role, improves the stability of the flood control system, and provides impact resistance;
• The straw has a supporting effect on the baffle after water absorption and expansion, which makes the strain value of the baffle with straw under hydrostatic conditions greater than that without straw. Thus, straw as a filler in the flood control system can not only enhance the stability of the system but also satisfy the theoretical assumption of a flexible spatial flood control system with straw;
• The straw strain value of the water barrier with straw was less than that without straw under different working conditions of dynamic water. The straw flexible space plays a buffering role to offset the impact force, and the system is stable and well-sealed under different working conditions, which proves the feasibility of the straw flexible space assembly flood control system;
• A comparison of the test results with the finite element analysis results showed that the overall change pattern is the same. The test results were slightly smaller than the numerical analysis results. Nevertheless, both were in good agreement, and the comparison error was between 5% and 8%, which verifies the accuracy of the numerical analysis method.