Drainage basin unit extraction method for debris flow susceptibility evaluation and early warning

Abstract

The invention discloses a drainage basin unit extraction method for debris flow susceptibility evaluation and early warning. The drainage basin unit extraction method comprises the following steps: (1) filling pits; (2) flow direction analysis; (3) calculating the number of catchment pixels; (4) extracting a river network; (5) river network grading; (6) determining the grade of the river network where the debris flow disaster point is located and the converged river network; (7) determining the highest river network grade of the debris flow basin unit; (8) determining the outlet position of the debris flow basin unit; and (9) generating a drainage basin unit. The drainage basin unit extracted by the method not only retains the drainage basin with a smaller area, but also retains the integrity of the drainage basin with a larger area, and does not contain a water collecting area formed by a main river and slopes on two banks.

Description

Technical Field

[0001] The invention relates to the field of debris flow basin unit extraction, and in particular to a basin unit extraction method for debris flow susceptibility assessment and early warning. Background Art

[0002] Debris flow susceptibility assessment and debris flow early warning are important non-engineering measures for debris flow disaster prevention and mitigation. Debris flow susceptibility assessment can show the probability of debris flow in the national territory. The evaluation results can provide a scientific basis for national territory space planning, major engineering line selection, etc., and can also provide early warning targets for debris flow early warning. Debris flow early warning can use meteorological data to provide meteorological warnings for medium and high debris flow susceptibility basins, thereby serving the local pre-disaster transfer of personnel and property and reducing disaster losses. Debris flow is a geomorphological phenomenon that occurs in a certain basin. Therefore, debris flow susceptibility assessment and regional early warning need to take appropriate basin units as research objects. Therefore, the extraction of basin units is the basis for debris flow susceptibility assessment and regional early warning.

[0003] To this end, the present invention proposes a watershed unit extraction method that takes into account the characteristics of the river network level where debris flow occurs. This method can provide a more suitable research object for debris flow susceptibility assessment and regional early warning. Summary of the invention

[0004] To solve the above problems, the present invention aims to provide a method for extracting watershed units for debris flow susceptibility assessment and early warning. The watershed units extracted by this method retain both the integrity of watersheds with smaller areas and those with larger areas, and do not include the catchment area formed by the main river and the slopes on both sides.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for extracting watershed units for debris flow susceptibility assessment and early warning, characterized in that it comprises the following steps:

[0007] (1) Depression filling: Based on DEM data, the depressions in the terrain are removed or filled to obtain a depression filling set file named demf;

[0008] (2) Flow direction analysis: The flow direction is determined by the hydrology module in ArcGIS, and a flow direction set file is obtained, named dir;

[0009] (3) Calculate the number of water collection pixels: The total number of upstream pixels that flow into each pixel is calculated through the hydrology module in ArcGIS, that is, the number of pixels contained in the water collection area corresponding to the pixel, and the water collection pixel set file is obtained, named acc;

[0010] (4) River network extraction: Set a threshold for the number of water collection pixels. Pixels with a water collection pixel number greater than the threshold are defined as river network pixels. The river network pixel set is obtained through raster calculation processing in ArcGIS, and the resulting river network set file is named net;

[0011] (5) River network classification: Based on the river network set file, the river network is classified using the STRAHLER method. Specifically, the river network directly originating from the river source is defined as a first-level river network. The river network formed by the intersection of two or more river networks of the same level is defined as one level higher than the original one. The river network formed by the intersection of two or more river networks of different levels is defined as the same level as the higher level of the original river network. The hydrology module in ArcGIS is used for classification processing to obtain a river network classification file named net_order.

[0012] (6) Determine the river network where the debris flow disaster point is located and the level of the river network into which it flows: Overlay the geographical location of the historical debris flow disaster data with the river network classification data to determine the river network level of each disaster point and the higher-level river network level into which the disaster point will flow. In this step, the geographical location and the river network classification data are overlaid, that is, the historical debris flow disaster point vector data and the river network classification raster data are overlaid on the DEM.

[0013] (7) Determination of the highest river network level of debris flow basin unit: Analyze the distribution characteristics of the river network level of the disaster point and the river network level of the river it merges into. Take the river network level of the debris flow disaster point ≤ N and the river network level of the river it merges into> N as the standard. Count the number of debris flow disaster points that meet the above conditions when N takes different values. The N corresponding to the largest number is the highest river network level extracted from the debris flow basin unit.

[0014] (8) Determination of the debris flow basin unit exit position: Based on the flow direction set file dir and the river network classification file net_order, if the river network level of a certain pixel is ≤ N and the river network level of the next pixel it flows into is > N, then the pixel is marked as the debris flow basin unit exit, and the resulting pixel data set is marked as the debris flow basin unit exit data set;

[0015] (9) Generate watershed units: Based on the debris flow watershed unit outlet dataset and flow direction set, the watershed unit division results are obtained through the hydrological analysis module in ArcGIS.

[0016] The inventors found that the commonly extracted watershed units have two problems: (1) The watershed units are fragmented. For a large watershed, a debris flow watershed will be divided into multiple watershed units, destroying the integrity of the watershed unit; (2) The catchment area between the outlets of two watershed units is divided into a watershed unit. This catchment area is actually the area composed of the main river and the slopes on both sides, not the watershed unit where the debris flow occurs. Based on this, the inventors provide a method of superimposing the river network classification raster data and the geographical location points of debris flow disasters, i.e., vector data, and determining the outlet position of the watershed unit based on the number of debris flow watershed unit disaster points, and dividing the watershed units, so that the extracted watershed units retain both the smaller watersheds and the larger watersheds. The integrity, and finally, the excellent result of not including the catchment area composed of the main river and the slopes on both sides.

[0017] A debris flow early warning method includes the watershed unit extraction method as described above, firstly extracting the debris flow watershed unit.

[0018] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0019] Compared with the traditional method, the watershed unit extracted by the present invention retains both the smaller watershed and the integrity of the larger watershed, and does not include the watershed area formed by the main river and the slopes on both banks. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0021] Figure 1 This is the result of DEM data after depression filling processing;

[0022] Figure 2 This is the flow direction analysis result diagram;

[0023] Figure 3 This is the result map of the number of water collection pixels;

[0024] Figure 4 This is the result map of river network extraction;

[0025] Figure 5 This is the river network classification result map;

[0026] Figure 6 It is the location map of the watershed unit outlet;

[0027] Figure 7 A watershed unit map extracted for the present invention;

[0028] Figure 8 The watershed unit map extracted for the existing technology;

[0029] Fig. 9 This is a schematic diagram of river network classification results;

[0030] Fig.10 A schematic diagram of river network classification superimposed on the debris flow disaster point. DETAILED DESCRIPTION

[0031] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0032] Example 1

[0033] Based on the 50m resolution DEM data of Ya'an City, Sichuan Province, the following analysis was carried out (due to the large area of ​​the entire study area, the following example image only shows a part of the study area):

[0034] (1) Filling the depression. This is done using the hydrology module in ArcGIS. The resulting file is named demf, as follows: Figure 1 shown.

[0035] (2) Flow direction analysis. This is done using the hydrology module in ArcGIS, and the resulting file is named dir, such as Figure 2 shown.

[0036] (3) Calculate the number of water collection pixels. This is done using the hydrology module in ArcGIS, and the resulting file is named acc, such as Figure 3 shown.

[0037] (4) River network extraction. The threshold of the number of water collection pixels is set to 180 (i.e. 0.45 km 2 There are many existing technical methods to determine the threshold. This example uses the water system map of the study area to obtain it. The pixels with the number of water collection pixels greater than the threshold are defined as river network pixels. This is done through the raster calculator in ArcGIS, and the resulting file is named net, such as Figure 4 shown.

[0038] (5) River network classification. The river network classification is performed using the STRAHLER method and the hydrology module in ArcGIS. The resulting file is named net_order. The classification method is as follows: Fig. 9 As shown in the figure, the river network directly originating from the river source is defined as a first-level river network. The river network formed by the intersection of two or more river networks of the same level is defined as one level higher than the original one. The river network formed by the intersection of two or more river networks of different levels is defined as the same level as the higher level of the original river network. The final classification result is as follows Figure 5 shown.

[0039] (6) Determine the river network level where the debris flow disaster point is located and the river network level it will flow into. Overlay the geographical location (point vector) of the 243 debris flow disaster data in history with the river network classification data (raster data) to determine the river network level where each disaster point is located and the higher-level river network level that the disaster point will flow into. Fig.10 The medium disaster points A and B are located in the 1st-level river network and the 3rd-level river network respectively, and the river networks where they are located flow into the 3rd-level river network and the 4th-level river network respectively.

[0040] (7) Determination of the highest river network level of debris flow basin units. The distribution characteristics of the river network level of the disaster point and the river network level of the river it flows into are analyzed. The river network level of the debris flow disaster point is ≤ N and the river network level of the river it flows into is > N. The number of debris flow disaster points that meet the above conditions when N takes different values ​​is counted, as shown in Table 1. When the number is the largest (202), the corresponding N is 3, so the highest river network level extracted from the debris flow basin unit is level 3.

[0041] Table 1 The number of debris flow basins corresponding to different values ​​of the highest river network level in the debris flow basin

[0042] N 1 2 3 4 5 6 7 8 Number of debris flows 81 186 202 184 81 32 11 0

[0043] (8) Determination of the location of the debris flow basin unit outlet. Based on the flow direction (file dir) and river network classification (file net_order), if the river network level of a pixel is ≤3 and the river network level of the next pixel it flows into is >3, then the pixel is marked as the basin outlet. The results are as follows: Figure 6 shown.

[0044] (9) Generate watershed units. The watershed unit division results are obtained based on the watershed outlet and flow direction. This step is completed through the hydrological analysis module in ArcGIS. The results are as follows: Figure 7 (color picture). Compared with the watershed units extracted by traditional methods ( Figure 8 Compared with the color map), the watershed unit extracted by the present invention retains both the smaller watershed and the integrity of the larger watershed, and does not include the catchment area composed of the main river and the slopes on both sides ( Figure 7 The traditional method obviously includes the catchment area composed of the main river and the slopes on both sides, and does not retain the integrity of the larger basin. Among the 24 debris flow basin units in the display area, the present invention accurately extracts 19 basin units, and the traditional method accurately extracts 8 basin units. The present invention is significantly better than the traditional method.

[0045] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A watershed unit extraction method for debris flow susceptibility assessment and early warning, characterized in that: The following steps are involved: (1) Depression filling: Based on DEM data, the depressions in the terrain are removed or filled to obtain a depression filling set file; (2) Flow direction analysis: Based on the depression filling set file, the water flow direction is determined through the hydrology module in ArcGIS to obtain the flow direction set file; (3) Calculate the number of water collection pixels: Based on the flow direction set file, the total number of upstream pixels that flow into each pixel is calculated through the hydrology module in ArcGIS, that is, the number of pixels contained in the water collection area corresponding to the pixel, and the water collection pixel set file is obtained; (4) River network extraction: Set a threshold for the number of water collection pixels. Pixels with a water collection pixel number greater than the threshold are defined as river network pixels. The river network pixel set is obtained through raster calculation processing in ArcGIS to obtain a river network set file. (5) River network classification: Based on the river network set file, the river network is classified using the STRAHLER method. Specifically, the river network directly originating from the river source is defined as a level 1 river network. The river network formed by the intersection of two or more river networks of the same level is defined as a level higher than the original one. The river network formed by the intersection of two or more river networks of different levels is defined as a level equal to the higher level of the original river network. The hydrology module in ArcGIS is used for classification processing to obtain a river network classification file. (6) Determine the river network level where the debris flow disaster site is located and the river network level it will flow into: Overlay the geographical location of historical debris flow disaster data with river network classification data to determine the river network level where each disaster site is located and the higher-level river network level that the disaster site will flow into; (7) Determination of the highest river network level of debris flow basin unit: Analyze the distribution characteristics of the river network level of the disaster point and the river network level of the river it merges into. Take the river network level of the debris flow disaster point ≤ N and the river network level of the river it merges into> N as the standard. Count the number of debris flow disaster points that meet the above conditions when N takes different values. The N corresponding to the largest number is the highest river network level extracted from the debris flow basin unit. (8) Determination of the location of the debris flow basin unit outlet: If the river network level of a certain pixel is ≤ N and the river network level of the next pixel it flows into is > N, then the pixel is marked as the debris flow basin unit outlet. Based on the flow direction set file and the river network classification file, the resulting pixel data set is marked as the debris flow basin unit outlet data set; (9) Generate watershed units: Based on the debris flow watershed unit outlet dataset and flow direction set, the watershed unit division results are obtained through the hydrological analysis module in ArcGIS.

2. A debris flow early warning method, characterized in that: The method comprises the watershed unit extraction method as claimed in claim 1, wherein the debris flow watershed unit is firstly extracted.

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