A method and system for automatically identifying a crack flow based on flow field data

Abstract

The application discloses a kind of based on flow field data's crack stream automatic identification method and system, this method establishes grid in target place and obtains the flow rate and flow direction of each grid point, extracts connected object and calculates the area and minimum circumscribed frame of each connected object;Reserve the connected object of area greater than first threshold and the included angle between connected object long axis and target place off-shore direction is less than second threshold, determine as possible crack stream unit;Track the possible crack stream unit of multiple time, until all possible crack stream unit has been marked serial number;Reserve the label crack stream of duration greater than third threshold;The minimum circumscribed frame of each serial number crack stream is superimposed on flow field diagram to draw, by artificial check, obtain crack stream identification result.The application can identify two types of crack stream, such as groove crack stream and transient crack stream, and can be suitable for flow field data of different sources, for seaside crack stream early warning and protection.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of rip current identification, in particular to a rip current automatic identification method and system based on flow field data. BACKGROUND

[0002] Rip current is one of the important marine disasters in coastal tourist areas, which has the characteristics of narrow and rapid flow, short time and easy change, and can drag the unsuspecting beach tourists into the deep water area, resulting in drowning accidents. Rip current is usually easy to occur on the beach terrain with sand bar grooves, so it can be monitored and warned by placing sea current meters at single points according to the seabed topography. It is found that a type of transient rip current may also occur on the beach lacking of seabed topography, which is different from the fixed position groove rip current, and has the characteristics of position migration and short duration when occurring, so the monitoring of it needs to obtain the full field flow field information of the beach.

[0003] However, the identification of rip current from flow field data is still mainly through artificial visual interpretation at present, and there is a lack of automatic identification method to systematically extract rip current characteristics and other information from flow field data. Artificial analysis is mainly based on the existing description of rip current and the judgment of some observed rip current flow velocity, and the rip current usually occurs in the wave breaking zone and can extend outside the breaking wave zone, has a large flow velocity, usually 0.5-2m / s, a narrow width, presents an offshore flow direction opposite to its two sides, and often forms a circulation system with converging alongshore flow. However, the above description is more suitable for the fixed position groove rip current, and does not comprehensively cover the characteristics of the transient rip current which is not fixed in position and has a short duration. In addition, since the formation of transient rip current is often accompanied by the generation of breaking wave vortex, some scholars use the method of calculating vorticity and vortex detection to indirectly infer the occurrence of rip current, but this method is not a direct judgment of rip current, and will misjudge some vortexes without offshore flow characteristics as rip current, and also cannot be applied to the identification of groove rip current. It can be seen that there is a great difference in space position and duration between groove rip current and transient rip current, and there is currently a lack of a unified method to systematically automatically identify them. SUMMARY

[0004] The purpose of the present application is to provide a method and system for systematically extracting rip current characteristics from flow field data, and automatically identifying and tracking groove rip current and transient rip current.

[0005] Technical scheme: The rip current automatic identification method based on flow field data provided by the present application comprises the following steps:

[0006] (1) Establish a grid along the offshore direction and the alongshore direction at the target site, and obtain the flow velocity and flow direction of each grid point at each time;

[0007] (2) search possible flow separation units at each time, for grid points with flow velocity and flow direction within the threshold range, extract connected objects and calculate the area and minimum bounding box of each connected object; retain connected objects with area greater than a first threshold and the angle between the long axis of the connected object and the off-shore direction of the target site less than a second threshold, and determine as possible flow separation units;

[0008] (3) track possible flow separation units at multiple times, for each possible flow separation unit at each time, the remaining possible flow separation units are candidate units thereof, select the one candidate unit closest to the off-shore direction of the possible flow separation unit, and mark both as the same serial number of flow separation; repeat step (3) until all possible flow separation units have been marked with serial numbers;

[0009] (4) superimpose the minimum bounding box of each serial number of flow separation on the flow field diagram to draw, and obtain the flow separation identification result through manual verification.

[0010] Further, in step (1), the flow field data of the target site is obtained and the time-averaged flow velocity field is calculated, the grid is established along the off-shore direction and the on-shore direction, the off-shore flow velocity u and the on-shore flow velocity v of each grid point are obtained by using linear interpolation method, and the flow velocity and the flow direction

[0011] Further, in step (2), the extraction of connected objects and the calculation of the area and the minimum bounding box of each connected object include:

[0012] If the flow velocity s and the flow direction θ of the grid point satisfy s≥U and θ<α, the grid point generates a mask with a mask value of 1, otherwise a mask with a mask value of 0; U is a flow separation velocity threshold, and α is a flow separation direction threshold;

[0013] The connected objects in the mask are extracted using 8-connected neighborhood connectivity, the area of each connected object is calculated, and the minimum bounding box of the connected object is calculated.

[0014] Further, in step (2), before retaining the connected objects with area greater than the first threshold and the angle between the long axis of the connected object and the off-shore direction of the target site less than the second threshold, it further includes:

[0015] determine whether the area a of the connected object satisfies a≥B 2 , if not, the connected object is deleted; B is a geometric threshold;

[0016] calculate the minimum distance d between the minimum bounding boxes of two connected objects, and merge the two connected objects that satisfy d<B, and update the area and the minimum bounding box of the connected object;

[0017] Delete the connected object which is close to the coastline and whose side close to the coastline is nearly parallel to the coastline, update the area and the minimum bounding box of the connected object.

[0018] Further, the step of judging whether the connected object contains the connected object whose side is parallel to the coastline includes:

[0019] (5.1) For the connected object S, divide its minimum bounding box into two along the coastline direction, and obtain two connected objects S1 and S2, S1 is the connected object close to the coastline;

[0020] (5.2) Calculate the angle between the long axis of S1 and S2 and the offshore direction respectively, if the angle is greater than α, take the length of the long axis as the characteristic width along the coastline direction, otherwise take the length of the short axis, and obtain the characteristic width w1 and w2 of S1 and S2; α is the threshold of the flow direction of the rip current;

[0021] (5.3) Calculate the median of the width of S2 along the coastline direction w3;

[0022] (5.4) If the determination condition max(w1, w2) > 2B and w1 > 1.5 * w3 is satisfied, move the side close to the coastline of the minimum bounding box of S to the offshore direction by one grid unit, obtain the new minimum bounding box, and repeat steps (5.1)-(5.3); until the determination condition is not satisfied, or w1 or w2 is 0, obtain the updated minimum bounding box;

[0023] (5.5) Extract the updated connected object S' using the updated minimum bounding box.

[0024] Further, in step (2), the connected object whose area is greater than the first threshold and the angle between the long axis of the connected object and the offshore direction of the target site is less than the second threshold includes:

[0025] Delete the connected object whose area a does not satisfy a ≥ B 2 , B is the geometric threshold;

[0026] Delete the connected object whose angle γ between the long axis of the minimum bounding box of the connected object and the offshore direction does not satisfy γ < 1.33α, α is the threshold of the flow direction of the rip current.

[0027] Further, in step (3), before selecting the candidate unit closest to the possible rip current unit in the offshore direction and marking them as the same serial number of the rip current, it further includes:

[0028] For each possible flow separation unit at each time, if the minimum circumscribed frame of the candidate flow separation unit and the overlapped area of the moved flow separation unit are less than 60%, the candidate unit is deleted; beta is the average flow direction on the off-shore side boundary of the possible flow separation unit, and B is a geometric threshold value.

[0029] Further, in step (3), the closest candidate unit in the off-shore direction of the possible flow separation unit is selected, and the two are marked as the same serial number of flow separation.

[0030] For each possible flow separation unit at each time, if the displacement of the candidate unit in the off-shore direction is not positive, the candidate unit is deleted.

[0031] In the remaining candidate units, the distance between the center point of the minimum circumscribed frame of the possible flow separation unit and the center point of the minimum circumscribed frame of the candidate unit is calculated, the candidate unit with the shortest distance is selected, and the possible flow separation unit and the candidate unit are marked as the same serial number of flow separation.

[0032] Further, the duration of each serial number of flow separation is calculated, and the flow separation with a duration less than a third threshold value is deleted.

[0033] The flow separation automatic recognition system based on flow field data provided by the application comprises:

[0034] A data acquisition unit is configured to establish a grid in the off-shore direction and the on-shore direction at a target site, and acquire the flow velocity and flow direction of each grid point at each time.

[0035] A possible flow separation unit searching unit is configured to search for possible flow separation units at each time, extract connected objects and calculate the area and minimum circumscribed frame of each connected object for grid points with flow velocity and flow direction within a threshold range, retain connected objects with an area greater than a first threshold value and an angle between the long axis of the connected object and the off-shore direction of the target site less than a second threshold value, and determine the connected objects as possible flow separation units.

[0036] A possible flow separation unit tracking unit is configured to track possible flow separation units at multiple times, and for each possible flow separation unit at each time, the remaining possible flow separation units are candidate units thereof, the closest candidate unit in the off-shore direction of the possible flow separation unit is selected, and the two are marked as the same serial number of flow separation.

[0037] A flow separation recognition and verification unit is configured to superimpose the minimum circumscribed frame of each serial number of flow separation on a flow field diagram, and obtain a flow separation recognition result through artificial verification.

[0038] Beneficial Effects: Compared with existing technologies, the advantages of this invention are as follows: This invention addresses the limitations of current methods that require manual interpretation for crack flow identification, and the lack of a systematic method for identifying and judging the highly variable spatiotemporal characteristics of beach crack flows. This invention analyzes the velocity and direction of each grid point in the flow field data to identify potential crack flow units and track their direction, marking the crack flows. It can identify two types of crack flows: channel crack flows and transient crack flows. It is applicable to flow field data from different sources and can be widely used in high-crisis-occurrence sea areas such as recreational tourist beaches, providing technical support for coastal crack flow early warning and protection, and improving beach safety in coastal tourist areas. Attached Figure Description

[0039] Figure 1 This is a flowchart of the automatic crack flow identification method according to an embodiment of the present invention;

[0040] Figure 2 This is a schematic diagram of connected objects in an embodiment of the present invention;

[0041] Figure 3 This is a time-averaged flow field diagram and a schematic diagram of the minimum bounding box of the split flow unit in Example 1 of this invention.

[0042] Figure 4 This is a time-averaged flow field diagram and a schematic diagram of the minimum bounding box of the split flow unit in Example 2 of this invention.

[0043] Figure 5 This is a time-averaged flow field diagram and a schematic diagram of the minimum bounding box of the split flow unit in Example 3 of this embodiment of the invention;

[0044] Figure 6 This is a time-averaged flow field diagram and a schematic diagram of the minimum bounding box of the split flow unit in Example 4 of this embodiment of the invention. Detailed Implementation

[0045] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0046] like Figure 1 As shown, the automatic crack flow identification method based on flow field data includes the following steps.

[0047] Step 1: Collect flow field data and perform preprocessing.

[0048] Step 1.1: After averaging the collected flow field data over time, output the hourly average flow velocity field per minute.

[0049] Step 1.2: A two-dimensional square grid is established along the offshore x direction and the coastal y direction. Linear interpolation is used to obtain the offshore velocity u and the coastal velocity v at each grid point.

[0050] Step 1.3: Calculate the flow velocity at each grid point. and flow direction

[0051] Step 2, search possible crack flow units on the flow field at each time.

[0052] Step 2.1, set crack flow velocity threshold U, crack flow direction threshold a, geometric threshold B.

[0053] Step 2.2, determine whether the flow velocity size and direction of each grid point meet s> U and θ< a, and generate a mask (Mask) according to the determination result, if the above conditions are met, the mask value is 1, otherwise 0.

[0054] Step 2.3, use 8-connected neighborhood connectivity to extract connected objects in the mask, calculate the area a of each connected object and the position of the minimum circumscribed frame of the containing area.

[0055] Step 2.4, determine whether the connected object area meets a> B 2 , if not, delete the object from the object pool.

[0056] Step 2.5, calculate the minimum distance between the minimum circumscribed frames of the connected objects in the object pool, if the minimum distance between the two frames is less than B, the two connected objects are considered to be the same connected object, and the position of the corresponding minimum circumscribed frame is updated, and the object pool is updated.

[0057] Step 2.6, for all connected objects in the object pool, delete the connected objects near the coastline that are nearly parallel to the coastline, as shown in Figure 2 , the specific method is as follows:

[0058] 1) for an object S, divide its minimum circumscribed frame in half along the y-axis direction, and obtain two connected objects S1 and S2 using the method in step 2.3, and designate S1 as the connected object near the coastline;

[0059] 2) calculate the included angle γ between the long axis of S1 and S2 and the x-axis respectively, if the included angle γ> threshold a, take the long axis length of the connected object as its characteristic width along the coast, otherwise take the short axis length, and obtain the characteristic widths w1 and w2 for the two connected objects S1 and S2 respectively;

[0060] 3) for the connected object S2, calculate the median width w3 along the coast;

[0061] 4) make the following judgment: max(w1, w2)> 2B and w1> 1.5*w3, if true, move the minimum circumscribed frame of S near the coastline to the offshore direction by one grid point unit, and repeat steps 1) to 4) after obtaining the new minimum circumscribed frame.

[0062] 5) If the condition in step 4) is not satisfied, or w1 or w2 is 0, stop updating, and extract the connected object in the minimum bounding box obtained in the last round using the method in step 2.3.

[0063] Step 2.7, determine whether the area of the connected object after updating in step 2.6 meets a≥B 2 , if not, delete the connected object from the object pool.

[0064] Step 2.8, determine whether the angle between the long axis of the connected object in step 2.7 and the x-axis direction is less than 1.33a, if not, delete the object from the object pool.

[0065] Step 2.9, regard each individual connected object in the object pool in step 2.8 as a possible rip cell, record its time, minimum bounding box position, and average flow direction β on the offshore side boundary of the connected object (the average flow direction of all grid points on the offshore side boundary of the connected object).

[0066] Step 3, based on the results of step 2, track the movement of possible rip cells at multiple times.

[0067] Step 3.1, set a rip time threshold T.

[0068] Step 3.2, for each possible rip cell at each time obtained in step 2, use the following steps in time order:

[0069] 1) For all possible rip cells at each time, determine whether each has been labeled one by one, if not, mark it as a new serial number;

[0070] 2) For all possible rip cells described in 1), perform the following steps one by one:

[0071] 2.1) For a possible rip cell R_0, list all k possible rip cells that have not been labeled from t_0+1 to t_0+T as candidate cells R_n (n = 1…k);

[0072] 2.2) Move the minimum bounding line of R_0 cell in the y-axis direction by B sinβ / sin45°, and extend it in the x-axis direction by B cosβ / cos 45°, if the area where the minimum bounding line of the candidate cell coincides with it is not greater than 60%, delete the candidate cell from the candidate pool;

[0073] 2.3) Calculate the displacement of R_0 to each candidate cell, if the displacement in the x-axis direction is not positive, delete the candidate cell from the candidate pool;

[0074] 2.4) If the number of the remaining candidate units is not 0, the candidate unit with the shortest distance between the center points of the minimum bounding boxes and the center point of the minimum bounding box of R_0 is determined as the same split unit as R_0 at t_0+1, and the two possible split units are marked as the same serial number of split, and the remaining candidate units are deleted from the candidate pool;

[0075] 2.5) The deleted candidate units in steps 2.2) to 2.4) are kept in the unmarked state and enter the next round of tracking.

[0076] Step 3.3, the above tracking process is repeated until the end of the data.

[0077] Step 3.4, check whether the duration of the marked split in step 3.3 is less than the time threshold T, if yes, delete it from the split pool.

[0078] Step 3.5, record the starting time, maximum flow rate, length, width, duration and the minimum bounding box position of the possible split unit at each time of the marked split in step 3.4.

[0079] Step 4, the splits determined in steps 2 to 3 are verified by manual interpretation.

[0080] Step 4.1, draw the minute mean flow field graph for the selected flow field data for debugging.

[0081] Step 4.2, superimpose the minimum bounding box line of the split unit appearing at each time within the starting time of the split determined by steps 2 to 3 on the flow field graph.

[0082] Step 4.3, manually check whether there is an obvious split that has not been identified, and whether the flow field within the minimum bounding box of the identified and marked split meets the definition of the split.

[0083] Step 4.4, adjust the geometric threshold B according to the comparison result of step 4.3 until the identification result reasonably meets the definition of the split.

[0084] Further, if the method is used for the first time at a certain place, part of the flow field data needs to be selected to determine the split by steps 2 and 3, and then compared with the manual interpretation to check whether the automatically identified split meets the definition, and the threshold parameters in steps 2 and 3 are adjusted, and after the suitable parameters for this place are determined, the remaining flow field data is determined by the split with the adjusted threshold parameters.

[0085] The method described in the application is verified by specific experiments.

[0086] (1) Obtain the flow field data, in this embodiment, the nearshore flow field data simulated by FUNWAVE-TVD model based on Bousinnesq wave model is adopted. The model setting method and general parameters are adopted as described in the mode manual, thus not repeated here. The simulation area is a rectangular area with 500m in the alongshore direction and 512m in the cross-shore direction, and a wave maker based on TMA wave spectrum is adopted, and the wave maker is located at a position 400m away from the shore. The simulation examples in this embodiment include the gully rip current under the sandbar terrain and the transient rip current on the flat and uniform beach, as shown in Table 1. The direct output is the average flow velocity per minute in the mode operation.

[0087] Table 1 Wave making conditions and terrain settings of simulation nearshore flow field data examples

[0088]

[0089] A two-dimensional square grid with a horizontal resolution of 1 meter is established along the offshore direction x and the alongshore direction, and the flow velocity and flow direction at each grid point are calculated.

[0090] (2) For example 1 and example 2 of the uniform terrain in Table 1, the rip current flow velocity threshold U = 0.5 m / s, the direction threshold α = 45°, and the geometric threshold B = 10 m are set; for example 3 and example 4 of the gully terrain in Table 1, the rip current flow velocity threshold U = 0.5 m / s, the direction threshold α = 60°, and the geometric threshold B = 15 m are set. Identify all possible rip current units, record and save their time, minimum bounding box position and average flow direction β on the offshore side boundary.

[0091] (3) For example 1-4 in Table 1, the rip current time threshold T = 2 minutes is set, and the possible rip current units are tracked and numbered, and the starting time, maximum flow velocity, length, width, duration and other characteristic information of each time of the possible rip current unit minimum bounding box position are recorded.

[0092] (4) Draw the time-averaged flow field diagram of each output time of the simulation flow field in Table 1, and superimpose the minimum bounding box of the possible rip current unit in the rip current pool on the time-averaged flow field diagram. The time-averaged flow field diagram and the minimum bounding box of the possible rip current unit of example 1 to example 4 are shown in Figures 3 to 6

[0093] Check if there is a rip current that is not identified or the flow field in the minimum bounding box does not meet the definition of the rip current. Figures 3 to 6 In Table 2, the rip current unit marked by the minimum bounding box is consistent with the artificial interpretation, wherein, Figure 2 、 3 , 5 is a transient rip current, and it can be seen that the rip current position migrates in the alongshore direction and the offshore direction at different times, and the rip current gradually falls off to form a vortex moving in the offshore direction, which is consistent with its generation and evolution mechanism,​Figure 4 For the split flow formed under the sandbar trench terrain, it is worth noting that for multiple split flows appearing on the flow field at the same time but their positions basically do not change at different times. It can be seen that the method can identify two types of split flows.

[0094] The split flow automatic identification system based on flow field data provided by the application comprises:

[0095] The data acquisition unit is configured to establish a grid in the offshore direction and the alongshore direction at the target site, and acquire the flow velocity and the flow direction of each grid point at each time point.

[0096] The possible split flow unit searching unit is configured to search for possible split flow units at each time point, extract connected objects and calculate the area and the minimum circumscribed frame of each connected object for the grid points with flow velocity and flow direction within the threshold range, and retain the connected objects with an area greater than a first threshold value and an included angle between the long axis of the connected object and the offshore direction of the target site less than a second threshold value as possible split flow units.

[0097] The possible split flow unit tracking unit is configured to track possible split flow units at multiple time points, select a candidate unit for each possible split flow unit at each time point from the remaining possible split flow units, mark the possible split flow unit and the nearest candidate unit in the offshore direction as the same serial number of split flow, and repeat the execution of the possible split flow unit tracking unit until all possible split flow units are marked with serial numbers.

[0098] The split flow identification and verification unit is configured to superimpose and draw the minimum circumscribed frame of each serial number of split flow on the flow field diagram, and obtain a split flow identification result through artificial verification.

[0099] The computer readable storage medium can include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, flash memory or any other medium that can be used to store program codes in the form of instructions or data structures and can be accessed by a computer.

[0100] The processor is configured to execute the computer program stored in the memory to implement each step in the method related to the above embodiments.

Claims

1. A method for automatic identification of a split flow based on flow field data, characterized in that, The method comprises the following steps: (1) establishing a grid in the off-shore direction and the along-shore direction at the target site, and obtaining the flow velocity and flow direction of each grid point at each time; (2) searching for possible flow separation units at each time, extracting connected objects for the grid points with flow velocity and flow direction within the threshold range, and calculating the area and minimum circumscribed frame of each connected object; retaining the connected objects with an area greater than a first threshold and an angle between the long axis of the connected object and the off-shore direction of the target site less than a second threshold, and determining the connected objects as possible flow separation units; (3) tracking the possible flow separation units at multiple times, for each possible flow separation unit at each time, the remaining possible flow separation units are candidate units thereof, selecting the candidate unit closest to the possible flow separation unit in the off-shore direction, and marking the two as the same serial number of flow separation; repeating step (3) until all possible flow separation units have been marked with serial numbers; (4) superimposing the minimum circumscribed frame of each serial number of flow separation on the flow field diagram to draw the minimum circumscribed frame, and obtaining the flow separation identification result through manual verification; In step (1), the flow field data of the target site is acquired and the time-averaged flow velocity field is calculated, the grid is established in the offshore direction and in the onshore direction, the linear interpolation method is used to obtain the offshore flow velocity u and the onshore flow velocity v of each grid point, and the flow velocity of each grid point is calculated and flow direction In step (2), the extraction of connected objects and the calculation of the area and minimum circumscribed frame of each connected object comprise: If the flow velocity s and flow direction θ of the grid point satisfy s≥U and θ<α, the grid point generates a mask with a mask value of 1, otherwise a mask with a mask value of 0 is generated; U is a flow separation velocity threshold, and α is a flow separation direction threshold; The connected objects in the mask are extracted using 8-connected neighborhood connectivity, the area of each connected object is calculated, and the minimum circumscribed frame of the connected object is calculated.

2. The method for automatic identification of split flow based on flow field data according to claim 1, characterized in that, In step (2), before retaining the connected objects with an area greater than a first threshold and an angle between the long axis of the connected object and the off-shore direction of the target site less than a second threshold, it further comprises: determining whether the area a of the connected object satisfies a ≥ B 2 , and if not, deleting the connected object; B is a geometric threshold value; Calculating the minimum distance d between the minimum circumscribed frames of two connected objects, merging the two connected objects that satisfy d<B, and updating the area and minimum circumscribed frame of the connected object; Deleting the connected object in the connected object that is close to the shore line and is nearly parallel to the shore line, and updating the area and minimum circumscribed frame of the connected object.

3. The method for automatic identification of split flow based on flow field data according to claim 2, characterized in that, Judging whether the connected object contains a connected object with a boundary parallel to the shore line, if it contains, deleting the connected object, and updating the area and minimum circumscribed frame of the connected object comprise: (5.1) for the connected object S, the minimum circumscribed frame is bisected along the along-shore direction to obtain two connected objects S1 and S2, and S1 is the connected object close to the shore line; (5.2) calculating the angle between the long axis of S1 and S2 and the off-shore direction, respectively, if the angle is greater than α, the length of the long axis of the connected object is taken as the characteristic width in the along-shore direction, otherwise the length of the short axis is taken, and the characteristic widths w1 and w2 of S1 and S2 are obtained; α is a flow separation direction threshold; (5.3) calculating the median w3 of the width of S2 in the along-shore direction; (5.4) if the determination condition max(w1, w2)>2B and w1>1.5*w3 is satisfied, the minimum circumscribed frame of S on the side close to the coast is moved in the off-shore direction by one grid point unit to obtain a new minimum circumscribed frame, and steps (5.1)-(5.3) are repeated; until the determination condition is not satisfied, or w1 or w2 is 0, the updated minimum circumscribed frame is obtained. (5.5) Extracting the updated connected object S' using the updated minimum bounding box.

4. The method for automatic identification of split flow based on flow field data according to claim 1, characterized in that, In step (2), the connected object whose reserved area is greater than the first threshold value and the included angle between the long axis of the connected object and the off-shore direction of the target site is less than the second threshold value is included. deleting connected objects whose area a does not satisfy a ≥ B 2 , B being a geometric threshold value; Delete the connected object whose included angle γ between the long axis of the minimum bounding box of the connected object and the off-shore direction does not satisfy γ < 1.33α, and α is the threshold value of the flow direction of the rip current.

5. The method for automatic identification of split flow based on flow field data according to claim 1, characterized in that, In step (3), before selecting the candidate unit closest to the possible rip current unit in the off-shore direction and marking the two as the same serial number of the rip current, it further includes: For each possible rip current unit at each time, move it in the on-shore direction by B sinβ / sin45° and extend it in the off-shore direction by B cosβ / cos45°, and if the overlapping area of the minimum bounding box of the candidate rip current unit and the moved possible rip current unit is less than 60%, delete the candidate unit; β is the average flow direction on the off-shore side boundary of the possible rip current unit, and B is the geometric threshold value.

6. The method for automatic identification of split flow based on flow field data according to claim 1, characterized in that, In step (3), selecting the candidate unit closest to the possible rip current unit in the off-shore direction and marking the two as the same serial number of the rip current includes: For each possible rip current unit at each time, if the displacement of the possible rip current unit to the candidate unit is not positive in the off-shore direction, delete the candidate unit; Among the remaining candidate units, calculate the distance between the center point of the minimum bounding box of the possible rip current unit and the center point of the minimum bounding box of the candidate unit, select the candidate unit with the shortest distance, and mark the possible rip current unit and the candidate unit as the same serial number of the rip current.

7. The method for automatic identification of split flow based on flow field data according to claim 6, characterized in that, Calculate the duration of each labeled rip current, and delete the rip current with a duration less than a third threshold value.

8. A flow field data based split flow automatic identification system based on the method of claim 1, characterized in that, It includes: A data acquisition unit for establishing a grid in the off-shore direction and the on-shore direction at the target site, acquiring the flow velocity and flow direction of each grid point at each time; A possible rip current unit searching unit for searching possible rip current units at each time, extracting connected objects for grid points with flow velocity and flow direction within a threshold range, and calculating the area and minimum bounding box of each connected object; connected objects with an area greater than a first threshold value and an included angle between the long axis of the connected object and the off-shore direction of the target site less than a second threshold value are determined as possible rip current units; A possible rip current unit tracking unit for tracking possible rip current units at multiple times, for each possible rip current unit at each time, the remaining possible rip current units are its candidate units, and the candidate unit closest to the possible rip current unit in the off-shore direction is selected, and the two are marked as the same serial number of the rip current; repeat the possible rip current unit tracking unit until all possible rip current units have been marked with serial numbers; A rip current identification verification unit for superimposing the minimum bounding box of each serial number of the rip current on the flow field diagram, and obtaining the identification result of the rip current through artificial verification.

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