Optimization design method for sandbag bank protection structure of erosive beach

Abstract

The invention belongs to the technical field of hydraulic engineering and ocean engineering, and particularly relates to an optimal design method for a sandbag bank protection structure of an erosive beach, which comprises the following steps: collecting basic data, selecting and determining materials, optimally designing the size of a single sandbag, and determining a structure combination installation form. In order to better apply the sandbag protection structure to protection of the corrosive sandy beach, the stability design formula of the beach sandbag structure is optimized through theoretical derivation, the design process of the beach sandbag bank protection structure is perfected, and the applicability of the protection structure is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of water conservancy engineering and ocean engineering, and in particular relates to an optimization design method for a sandbag bank protection structure of an erosive beach. Background Art

[0002] Under the dual influence of global climate change and human activities, coastal erosion has become a major challenge threatening coastal ecosystems and human safety worldwide. According to statistics, over 70% of the world's sandy coasts are experiencing varying degrees of erosion, with the rate of coastal retreat accelerating year by year. This not only leads to the loss of coastal land resources but also causes immeasurable damage to coastal infrastructure, tourism resources, and the ecological environment.

[0003] As a major maritime nation, my country's long coastline is also plagued by coastal erosion, especially on sandy coasts.

[0004] Currently, various coastal protection methods have numerous drawbacks. While "hard engineering" protection can protect the coastline, it fails to address the root causes of erosion and can even produce secondary impacts. "Soft engineering" beach maintenance, however, suffers from design and construction issues, leading to sand loss and poor restoration results. Existing protection methods often conflict with coastal erosion mechanisms, exacerbating localized beach erosion.

[0005] In this context, a scientific and effective solution for protecting erosive beaches is urgently needed. This is not only an inevitable requirement for protecting my country's coastal ecological environment and sustainable economic development, but also an important measure to meet the challenges of global climate change. Summary of the invention

[0006] In order to overcome the problems of the prior art, the present invention aims to provide an optimized design method for sandbag revetment protection structures for erosive beaches through innovative design concepts and technical means to address the problem of coastal erosion.

[0007] The object of the present invention is achieved like this:

[0008] This application provides a method for optimizing the design of sandbag revetment structures for erosive beaches, comprising the following steps:

[0009] Step 1, basic data collection:

[0010] Comprehensively collect multi-faceted data on the target beach and nearby waters, including sediment evolution characteristics, the median particle size of the beach, and wave elements and water level changes in the nearby waters over many years.

[0011] Step 2, material selection and determination:

[0012] Considering the acid resistance, abrasion resistance, ultraviolet resistance and microbial resistance of geosynthetics comprehensively, polypropylene material is determined as the geotextile to prepare a single sandbag;

[0013] The filling material in the sandbag is selected as the sediment with a median particle size of d 50 = 0.25 - 0.5 mm in the in-situ beach.

[0014] Step 3, optimization design of the single sandbag size:

[0015] The following design formula is used to determine the optimal sandbag size suitable for the target beach:

[0016]

[0017] In the formula, H s is the effective incident wave height; μ is the friction coefficient between sandbags; C w is the stability coefficient; ρ s is the density of sand, ρ w is the density of seawater; ξ0 is the breaking similarity parameter, L0 is the deep-water wave length, L0 = gT 2 / 2π, T is the spectral peak wave period, g is the acceleration of gravity; α is the facing angle of the sandbag structure; k is the number of sandbag layers affecting a single sandbag; A T is the bottom area of the sandbag;

[0018] Through the above formula, the influence of various factors including waves, sediment, and structure on the design of the single sandbag size is comprehensively considered, ensuring that the designed sandbag has good stability in a complex marine environment.

[0019] Step 4, determination of the structural combination installation form:

[0020] The layered optimization design is adopted, and the cross-section from the inside to the outside is successively a fine sand filling layer, 1 layer of inner sandbag, and 1 layer of outer sandbag;

[0021] For adjacent sandbags, the stacking method is adopted, and a connection structure including geogrid is set as needed to enhance the overall anti-sliding and anti-scouring capabilities of the structure;

[0022] Finally, a sandbag revetment protection structure for an erosive beach is obtained by design.

[0023] Furthermore, in step 1, the wave elements include wave height, wave period, and wave direction; the water level change conditions include high tide level, low tide level, and tidal range.

[0024] Furthermore, in step 4, the material of the outermost sandbag in the layered optimization design is polypropylene woven geotextile, and its mass per unit area ≥ 180 g / m2 , the breaking strength is ≥35 KN / m in the radial direction and ≥25 KN / m in the weft direction, and the permeability coefficient is 10 -4 ~10 -3 cm / s.

[0025] Furthermore, the method further includes:

[0026] Step 5, determination of the filling rate of the geotextile bag:

[0027] Calculate the filling rate according to the theoretical maximum volume:

[0028]

[0029] In the formula, a and b are the dimensions of the empty sandbag, and V s is the maximum volume of the sandbag after filling with sand;

[0030] Among them, it should be ensured that the filling rate of a single sandbag > 75%.

[0031] Step 6, calculation of the design wave height:

[0032] Use equipment including ocean observation buoys and wave monitoring stations to obtain long-term measured wave data in a certain sea area;

[0033] Adopt the ANL method and POT method to select samples, substitute the sample data into the probability distribution function; perform fitting through the least squares principle, calculate the relevant parameters of each distribution function; compare and analyze according to indicators such as root mean square error and correlation coefficient to determine the best fitting distribution function; use the best fitting distribution function to calculate the design wave height of the target sea area, providing key parameters for the design of the sandbag revetment.

[0034] Step 7, optimization of the design elements of the sandbag structure:

[0035] According to the relevant formula for the wave run-up on the sandbag structure, combined with the actual slope and roughness conditions of the target beach, calculate the wave run-up value; compare the wave run-up calculation results considering the sandbag thickness and not considering the sandbag thickness to determine the reasonable top elevation of the structure.

[0036] The advantages and beneficial effects of the present invention are:

[0037] 1. By comprehensively collecting basic data and deeply analyzing the coastal environmental characteristics, the present invention provides a solid data support for subsequent designs, ensures that the design scheme meets the actual needs, and improves the applicability of the protection structure;

[0038] 2. In the optimized design method of the sandbag revetment protection structure of the present invention, the innovative design formula for the size of a single sandbag comprehensively considers multiple factors, significantly improves the scientificity and rationality of the sandbag design, enhances the stability of the sandbag in a complex marine environment, and reduces the risk of structural instability;

[0039] 3. The unique structural combination and installation method of the sandbag revetment protection structure optimization method of the present invention, through layered design and reasonable material selection, effectively prevents the loss of fine sand, enhances the integrity and anti-scour ability of the structure, and improves the protective effect and durability of the sandbag revetment;

[0040] 4. The sandbag revetment protection structure optimization design method described in the present invention uses an optimized stability control equation to more accurately evaluate structural stability, providing a more reliable theoretical basis for the design, construction, and maintenance of sandbag revetments, thereby helping to improve the safety and reliability of coastal protection projects.

[0041] 5. The precise design wave height calculation method of the present invention combines multiple sampling and analysis methods to determine the optimal probability distribution function, thereby obtaining a more accurate design wave height. This provides key design parameters for the design of sandbag revetment structures and ensures the safety of the structure under waves of different return periods.

[0042] 6. The optimization design method for sandbag revetment protection structure of the present invention optimizes the design elements of the sandbag structure, such as the reasonable determination of the structure top elevation and the selection of appropriate geotextile materials, which further improves the protective performance and durability of the sandbag revetment and reduces long-term maintenance costs.

[0043] 7. The sandbag revetment protection structure optimization design method described in the present invention improves the protective effectiveness of the sandbag revetment, effectively prevents the retreat of the erosive coastline, and has a limited, non-permanent impact on natural coastal processes. It is a new sandy coast protection solution with low cost, simple construction, and good protection effect. It has high cost-effectiveness, focuses on the sustainable development of protective measures and nature, and provides a better choice for beach restoration and coastal protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The present invention will be further described below with reference to the accompanying drawings and examples.

[0045] Figure 1 、 Figure 2 The seasonal average elevations of 11 sections and all sections of Chudao South Beach from 2017 to 2019 are shown;

[0046] Figure 3 Shows the changing trend of particle size parameters of sand dunes, beach shoulders and scour areas (the particle size parameters are the average of all previous data);

[0047] Figure 4 The distribution diagram of the design wave height return period calculated based on the Pearson type III curve;

[0048] Figure 5 The following are the top views of the three design schemes for the sandbag revetment test and the cross-sectional views of sections 1 to 3;

[0049] Figure 6 Show the protective efficiency of the sandbag structure under long-term field conditions; among them, (a) and (b) are the observation results of the structure under severe cold conditions; (c) and (d) are the observation results of the sandbag structure in the S2 and S3 sections under the field environment for more than two years.

[0050] Figure 7 Are the sectional view and cross-sectional view of the second test;

[0051] Figure 8 Show the change comparison of the test sections S4 - S8 before and after the storm wave; among them, (a - b) the S4 section failed during the typhoon "In-Fa" in 2021; (c - d) the grass seed bags and vegetation in the S6 section were damaged by the storm wave impact, and (e - f) the crushed stones and seagrass piled up in front of the S8 section.

[0052] Figure 9 Show the influence of the sandbag filling rate on the design size and structural stability; among them, (a) the influence of the sandbag filling rate on the design size; (b) the critical values of the structural stability of sandbags with the same size but different filling rates.

[0053] Figure 10 Show the change trend of the best - fitting standard judgment index calculated by the probability distribution function under different sample numbers; among them, (a) root mean square error, (b) correlation coefficient, (c) scale parameter, (d) shape parameter.

[0054] Figure 11 Show the debris line in the S2 section and the erosion line (yellow dotted line) in the S3 section after the typhoon to measure the wave run - up level. Specific implementation manners

[0055] The preferred implementation manners of the present invention will be described in detail below in conjunction with the embodiments. It should be understood that the following embodiments are given only for the purpose of illustration and are not used to limit the scope of the present invention. Those skilled in the art can make various modifications and substitutions to the present invention without departing from the purpose and spirit of the present invention. If not specifically specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0056] Embodiment:

[0057] Based on this, this embodiment conducts a field test study on the protection of the sandbag revetment on the erosive beach, and provides an optimized design method for the sandbag revetment protection structure on the erosive beach, including the following steps:

[0058] Step 1, basic data collection:

[0059] Comprehensively collect various data on the target beach and the nearby sea area, including sediment evolution characteristic data, the median grain size of the beach, and the wave elements and water level changes in the nearby sea area over the years. The wave elements include wave height, wave period, and wave direction; the water level changes include high tide level, low tide level, and tidal range.

[0060] Specifically, in this embodiment, by analyzing the measured data of 11 fixed beach profiles of the south beach of Chudao from September 2017 to November 2019, sediment evolution characteristic data are obtained, and the seasonal changes of the beach profile morphology, profile slope, and sediment grain size characteristics are analyzed. The erosion and recovery processes of the beach on the interannual scale and under the action of typhoon waves are clarified. For example, by statistically analyzing the average elevation of the profiles in different seasons, it is found that the beach profile changes between the normal wave type profile and the storm type profile with the seasonal changes of the hydrological wave elements during the year. It silts up to the normal wave type profile in winter and erodes to the storm type profile in summer, as Figure 1 、 Figure 2 shows the seasonal average elevations of 11 profiles and all profiles of the south beach of Chudao from 2017 to 2019.

[0061] Table 1 Seasonal sediment transport rate of the beach

[0062]

[0063] Note: "+" represents mainly alongshore sediment transport, "-" represents mainly offshore (onshore) sediment transport, and "±" represents the simultaneous existence of offshore (onshore) sediment transport and alongshore sediment transport.

[0064] By analyzing the change amount of the area of adjacent beach profiles, the main sediment transport forms of the beach are determined, and the beach sediment transport rate is estimated. As shown in Table 1 above, the seasonal sediment transport rate of the beach is shown. The median grain size of the beach is determined, and its variation law in the dune, beach shoulder, and scouring area is mastered by combining multi-year data ( Figure 3 The variation trends of parameters such as the average grain size of sediments in the dune, beach shoulder, and scouring area of the profile are statistically analyzed). Collect the wave elements in the nearby sea area over the years, including wave height, wave period, and wave direction, as well as the water level changes, such as high tide level, low tide level, and tidal range. Among them, wave data are obtained by using a buoy for observation. The buoy is 2.74 km in a straight line distance from the test project and has collected wave and meteorological data such as wind speed, wind direction, air pressure, air temperature, wave height, wave period, and wave direction since June 2010; water level data are obtained through a long-term monitoring station, providing a basis for subsequent design.

[0065] Step 2, Selection and determination of materials:

[0066] According to the beach sandbag structure design process, using data such as wave dynamics and sediment particle size of Chudao Beach over the years, long-term on-site tests were carried out on the severely eroded coastal section on the north side of Chudao. The main design contents cover the stability of the structure and the durability of geotextiles under hydrodynamic conditions, with key attention paid to design elements such as the top elevation of the structure, the size of a single sandbag, and the type of sandbag material. Considering the acid resistance, abrasion resistance, ultraviolet resistance, and microbial resistance of geotextiles, polypropylene material was selected as the geotextile to prepare a single sandbag.

[0067] The outer layer of the sandbag uses woven-high-strength geotextile (polypropylene) material (geotextile-Type Ⅰ), with a unit area mass ≥ 280 g / m 2 , a radial breaking strength ≥ 70 KN / m, a weft breaking strength ≥ 56 KN / m, and a permeability coefficient of 10 -4 -10 -3 cm / s; the inner layer (filter layer) of the outer layer of the sandbag selects polyester needle-punched geotextile (geotextile-Type Ⅱ), with a unit area mass ≥ 230 g / m 2 , a breaking strength ≥ 12 KN / m; the material of the inner layer of the sandbag uses polypropylene woven geotextile (geotextile-Type Ⅲ), with a unit area mass ≥ 135 g / m 2 , a warp breaking strength ≥ 30 KN / m, a weft breaking strength ≥ 22 KN / m, and a permeability coefficient of 10 -2 -10 -3 cm / s. All types of materials are subjected to anti-aging treatment (Table 2 shows the material characteristics of geotextile sandbags). The filling material in the sandbag selects sediment with an in-situ median particle size of d 50 = 0.25 - 0.5 mm on the beach to ensure compatibility with the local sediment characteristics.

[0068] Table 2 Material characteristics of geotextile sandbags

[0069]

[0070] Step 3, optimization design of the size of a single sandbag:

[0071] Use the following design formula to determine the optimal sandbag size suitable for the south beach of Chudao:

[0072]

[0073] In the formula, H s is the incident effective wave height; μ is the friction coefficient between sandbags; C w is the stability coefficient; ρ s is the density of sand, ρ w is the density of seawater; ξ0 is the breaking similarity parameter, L0 is the deep-water wave length, L0 = gT 2 / 2π, where T is the spectral peak wave period, g is the acceleration due to gravity; α is the facing angle of the sandbag structure; k is the number of sandbag layers affecting a single sandbag; A T is the bottom area of the sandbag.

[0074] Through the above formula, the influences of various factors including waves, sediment, and structure on the design of a single sandbag size are comprehensively considered, ensuring that the designed sandbags have good stability in a complex marine environment.

[0075] Specifically in this embodiment, based on the water level data of the nearby sea area of the test area for many years, the designed high water level at the research site is estimated to be 1.42 m. Using the ERA5 reanalysis data provided by the European Centre for Medium-Range Weather Forecasts, the annual maximum wave height of the ERA-5 reanalysis wave data in the Chudao Sea area for 35 years is statistically analyzed, and the effective wave height value with a return period of 25 years (finite water depth) at the calculation point is obtained as 4.4 m, and the effective wave height with a return period of 50 years is 4.6 m ( Figure 4 ).

[0076] Considering that the bottom elevation of the test section is greater than the high water level and the test cost, the size of the test sandbag is adjusted to be 1.4 m in length and 0.85 m in width (empty sandbag), and the size of the sandbag after filling with sand is 1.3×0.8×0.2 m.

[0077] Step 4, determination of the structural combination installation form:

[0078] The specific sandbag revetment test plan is as follows:

[0079] The total length of the test section of the eroded shoreline in the north of Chudao is 74 m, which is located in the middle section of the northern shoreline of Chudao. As Figure 5As shown in the figure, the first test consists of three test sections. Each test section is designed to have the same slope, height, and trench excavation depth, but the sandbag types in each test section are different (Table 3 gives the geotechnical material models of the sandbags used in each shore section for each test): Test section I (S1), with a length of 20 m, from the inside to the outside, it is filled with fine sand, multiple layers of inner sandbags, and 1 layer of outer sandbags; Test section II (S2), with a length of 24 m, from the inside to the outside, it is filled with fine sand, a geotextile filter layer, and 1 layer of outer sandbags, and a geogrid is used to wrap the outer sandbags into a whole to improve the overall stability of the sandbag protection structure; Test section III (S3), with a length of 30 m, from the inside to the outside, it is filled with fine sand, a geotextile filter layer, 1 layer of inner sandbags and 1 layer of outer sandbags. The geotextile filter layer is to prevent the loss of fine sand inside. The slope angle of all test sections is designed to be 50°, the top elevation of the revetment structure is designed to be equal to the sum of the extreme high water level and the wave run-up, which is 3.7 (m, MSL). The top of the shore is paved with 0.1 m thick undisturbed soil, with a total of 3.8 m. The three trenches are excavated from the beach surface to the bedrock layer about 1 m deep. The widths of the trenches are 2 m, 2.5 m, and 4 m respectively. The backfill of the trenches is all filled with sandbags, and the gaps between the sandbags are filled with fine sand and tamped. The outer sandbags (￥25) of the beach sandbag protection structure have better tensile strength and durability compared with the inner layer (￥5). The filling material in the sandbag is selected as sediment with a particle size of d 50 = 0.25 - 0.5 mm.

[0080] Table 3 First test section plan

[0081]

[0082] Observation on the protection efficiency of the sandbag revetment in the first test:

[0083] The geotextile material itself has limitations, which is an important issue in the on-site application of the beach sandbag protection structure. Compared with most other geotextile application environments (such as water supply and drainage and inland river use), the GSS is long-term exposed to strong ultraviolet rays, cold environment, and periodic action of waves, which means that the damage of the geotextile sandbag is difficult to avoid, and this is the key factor restricting the service life of the geotextile sandbag structure. The types of damage observed on-site can be divided into two categories: accidental damage (from sharp gravel / shells, floating wood and other sharp substances) and material aging caused by strong ultraviolet rays. They are easily pierced and worn by vandals and debris without effort, and the expected service life after long-term exposure to ultraviolet radiation is not clear, and it is difficult to construct an accurate aging curve. Figure 6 In (a) and (b), they are the observation results of the test section in the cold environment, and there is no obvious change in the structure; After two years, the S2 test section is relatively stable as a whole because of the geogrid wrapping, but the geotextile has aged and lost strength ( Figure 6In (c)); The outer layer of sandbag geosynthetics in the S3 section is completely damaged, and the damaged materials on the top layer form a wavy erosion line, but the inner layer of sandbags still plays a protective role. Figure 6 In (d)), there are many steel fishing nets washed by the sea on the beach surface.

[0084] The second test was modified based on the previous test. Except for the outermost layer of sandbags in the first test, the characteristics of the original structure were retained. In the second test, the structure was reinforced with geogrids, so the size of the sandbags was reduced to 0.85 m in length and 0.55 m in width (empty sandbags, the size of the filled sandbags is: 0.7×0.4×0.2 m). Polypropylene woven geotextile was selected as the main material (Table 4). A total of 5 comparison test sections were designed, and each test section was stabilized with "reinforced Mac mat" to prevent damage to the sandbags caused by the reciprocating erosion of gravel. Test section Ⅳ (S4): 12.5 m in length, the slope of the sandbag structure is 45°. Since the inner sandbags were not damaged before, they were retained, and only the outer layer design needed to be upgraded, mainly by installing geogrids, stacking from the outside, and the outer layer of sandbags used geotextile - type Ⅳ; Test section Ⅴ (S5): about 9.20 m in length, the slope of the sandbag structure is 60°; Test section Ⅵ (S6): 24 m in length, the slope of the revetment is 60°. This test section is divided into two parts, and needle-punched geotextile grass-seed bags are placed at positions with an elevation exceeding 2.7 m (MSL); Test section Ⅶ (S7): 5 m in length, the slope of the revetment is 60°. The placement form of the sandbags in this test section is that the short side of the sandbag is perpendicular to the shoreline, and the rest of the test sections are parallel to the shoreline; Test section Ⅷ (S8): 15 m in length, only 2 layers of sandbags are laid.

[0085] Figure 7 They are the cross-section and sectional view of the second test. The second test increased the comparison of the slope of the revetment structure, the size of the sandbags, the stacking form of the sandbags, and the feasibility of the ecological grass-seed geotextile bags.

[0086] Table 4 Second test section plan

[0087]

[0088] Observation on the protection efficiency of the sandbag revetment in the second test:

[0089] During 2022, three typhoons had a significant impact on the waters of Shandong Peninsula, among which Typhoon Fireworks No. 2106 had the strongest impact. Typhoon Fireworks moved slowly and stayed on land for the longest time since 1949. The direct economic losses totaled about 800 million yuan, accounting for 33% of the annual storm surge disaster losses (Ministry of Natural Resources 2019 and 2021 "China Marine Disaster Bulletin"). In the second test, the S4 section was in the same position, 12.5m long, and the sandbag structure had a slope of 45°. Since the original internal sandbags were not damaged, they were retained. Only the outer layer design needed to be upgraded. Every 4 layers of sandbags were reinforced with reinforced Mac mats, and the outer sandbags used geotextile-IV type. On July 30, 2021, under the action of typhoon waves generated by Typhoon Fireworks, the overall protection function was lost ( Figure 8 (a), (b)). The first test results showed that the use of geogrids to wrap sandbags can significantly improve the stability of sandbag revetments. Therefore, the second test section reduced the size of the sandbags and used a denser pore size micro-pad for testing. Figure 8 The comparison of the changes of test sections S5 to S8 before and after the action of storm waves is given in the figure. The S5 section is about 9.20m long, the sandbag structure has a slope of 60°, and multiple layers of sandbags are laid horizontally. It is located in the curved bank section; the S6 section is 24m long, the revetment slope is 60°, and the test section is divided into two parts. The needle-punched geotextile grass seed bags are placed at the position above 2.75 (m, MSL), and the rest of the positions are geotextile-IV sandbags and covered with Mac mats; the S7 section is 5m long, the revetment slope is 60°, and the sandbags in this test section are placed in the form of the short side of the sandbags perpendicular to the shoreline, while the rest of the test sections are parallel to the shoreline; the S8 section is 15m long, and only two layers of sandbags are laid vertically, and covered with Mac mats ( Figure 8 (a), (c) and (e)). Under the repeated impact of waves from multiple storms and Typhoon "Fireworks", the outermost layer of the MICRO pad was damaged. Under the influence of storm surge and wave height, overtopping occurred in the test section S6, and two sandbags were removed. However, the overall structure is still stable. The grass seed bags and vegetation in the S6 section were damaged by the impact of storm waves ( Figure 8 (d) The accumulation of debris, including gravel and seaweed, in front of sections S7 and S8 has damaged the beach and seriously damaged the landscape. It is worth noting that the Type IV geotextile used has not yet shown any signs of aging.

[0090] It was finally decided to adopt a layered optimization design. The cross-section from the inside to the outside is composed of a fine sand filling layer, 1 layer of inner sandbags, and 1 layer of outer sandbags. For adjacent sandbags, a stacking method is adopted, and connecting structures including geogrids are set up as needed.

[0091] Step 5, determination of geobag filling rate:

[0092] Calculate the filling rate based on the theoretical maximum volume:

[0093]

[0094] Wherein, a and b are the dimensions of the empty sandbag, and V s is the maximum volume of the sandbag after filling with sand.

[0095] In actual construction, by controlling the sand filling height and measuring the weight of the sandbag, ensure that the filling rate of the sandbags used in test sections S1 - S3 is between 75% - 85%, and the filling rate of sections S4 - S8 is 90% - 95%, and ensure that the filling rate of a single sandbag > 75% to meet the stability requirements of the structure under storm conditions ( Figure 9 showing the influence of the sandbag filling rate on the design dimensions and structural stability).

[0096] Step 6, design wave height calculation:

[0097] Use the No. 07 comprehensive observation and research buoy system with a diameter of 3 meters deployed by the Institute of Oceanology, Chinese Academy of Sciences to obtain long - term measured wave data in the Chudao Sea area from 2010 to 2019. Select samples using the ANL method and the POT method, and substitute the sample data into the probability distribution function; perform fitting through the least - squares principle to calculate the relevant parameters of each distribution function.

[0098] Specifically, use the ANL method to compare and analyze the RMSE, correlation coefficient, scale parameter, and shape parameter of 8 probability distribution functions. The results show that the RMSE of FT - Ⅲ is the smallest and the R 2 is the largest, which is the best - fitting distribution function for the study area ( Figure 10 showing the change trend of the best - fitting standard judgment index calculated by the probability distribution function under different sample numbers).

[0099] Use the best - fitting distribution function to calculate the design wave height of the target sea area, providing key parameters for the design of the sandbag revetment.

[0100] Step 7, optimization of sandbag structure design elements:

[0101] According to the relevant formula for the run - up of waves on the sandbag structure, combined with the actual slope and roughness of the target beach, calculate the wave run - up value; in this embodiment, compare the wave run - up calculation results considering the sandbag thickness and not considering the sandbag thickness to determine a reasonable structural top elevation.

[0102] In the first test, the combined value of the designed high water level and the wave run-up value was used to calculate the top elevation. Overtopping did not occur in the S1 - S3 section, but the wave run-up value was still explored and optimized. In the second test, the wave rising level was measured according to the debris line of the S2 section and the scouring line of the S3 section after the typhoon. The elevation of the sandbag shore top was reduced to 2.75 m (the average value of the debris line and the scouring line elevations), and at the same time, test sections with different top elevations were set up to further optimize the design scheme. Figure 11 The wave rising level was measured according to the debris line of the S2 section and the scouring line of the S3 section after the typhoon).

[0103] Finally, it should be noted that the above is only used to illustrate the technical solution of the present invention and not to limit it. Although the present invention has been described in detail with reference to the preferred layout scheme, those of ordinary skill in the art should understand that the technical solution of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solution of the present invention.

Claims

1. An optimized design method for a sandbag revetment protection structure of an erosive beach, characterized in that, The method includes the following steps: Step 1, basic data collection: Comprehensively collect various data of the target beach and the nearby sea area, including sediment evolution characteristic data, median grain size of the beach, and wave elements and water level changes in the nearby sea area over the years; Step 2, selection and determination of materials: Comprehensively consider the acid resistance, wear resistance, ultraviolet resistance, and microbial resistance of geotechnical materials, and determine polypropylene materials as geotextiles to prepare single sandbags; The filling material in the sandbag is selected as sediment with an in-situ median grain size of d 50 = 0.25 - 0.5 mm in the beach; Step 3, optimized design of the size of a single sandbag: Use the following design formula to determine the optimal sandbag size suitable for the target beach: where H s is the incident significant wave height; μ is the friction coefficient between sandbags; C w is the stability coefficient; ρ s is the density of sand, ρ w is the density of seawater; ξ0 is the breaking similarity parameter, L0 is the wavelength of deep-water waves, L0 = gT 2 / 2π, T is the spectral peak wave period, g is the acceleration due to gravity; α is the facing angle of the sandbag structure; k is the number of sandbag layers affecting a single sandbag; A T is the bottom area of the sandbag; Through the above formula, comprehensively consider the influence of various factors including waves, sediment, and structure on the design of the size of a single sandbag, and ensure that the designed sandbag has good stability in a complex marine environment; Step 4, determination of the structural combination installation form: Adopt hierarchical optimization design, and the cross-section is successively a fine sand filling layer, one layer of inner sandbags, and one layer of outer sandbags from the inside to the outside; For adjacent sandbags, use the stacking method, and set connection structures including geogrid as needed to enhance the overall anti-sliding and anti-scouring capabilities of the structure; Finally, design a sandbag revetment protection structure for the erosive beach.

2. The method according to claim 1, characterized in that, In Step 1, the wave elements include wave height, wave period, and wave direction; the water level changes include high tide level, low tide level, and tidal range.

3. The method according to claim 1, characterized in that In step 4, the material of the outermost sandbag in the layered optimization design is polypropylene woven geotextile, and its unit area mass is ≥180g / m 2 , radial strength ≥35KN / m, latitudinal ≥25KN / m, permeability coefficient is 10 -4 ~10 -3 cm / s.

4. The method according to any one of claims 1 to 3, characterized in that, The method further includes: Step 5, determination of the filling rate of geotextile bags: Calculate the filling rate according to the theoretical maximum volume: Where a and b are the dimensions of the empty sandbag, and V s is the maximum volume of the sandbag after filling with sand; Among them, ensure that the filling rate of a single sandbag > 75%.

5. The method according to any one of claims 1 to 3, characterized in that, The method further includes: Step 6, calculation of the design wave height: Use equipment including ocean observation buoys and wave monitoring stations to obtain long-term measured wave data in a certain sea area; Adopt the ANL method and POT method to select samples, substitute the sample data into the probability distribution function; perform fitting through the least squares principle, calculate the relevant parameters of each distribution function; compare and analyze according to indicators such as root mean square error and correlation coefficient to determine the best fitting distribution function; use the best fitting distribution function to calculate the design wave height of the target sea area, providing key parameters for the design of the sandbag revetment.

6. The method according to any one of claims 1 to 3, characterized in that, The method further includes: Step 7, optimization of the design elements of the sandbag structure: According to the relevant formula for the run-up of waves on the sandbag structure, combined with the actual slope and roughness conditions of the target beach, calculate the wave run-up value; compare the wave run-up calculation results considering the sandbag thickness and not considering the sandbag thickness to determine a reasonable structural top elevation.

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