A Method for Extracting the Flood Inundation Area and Water Depth of Small and Medium-sized Reservoir Dam Breaks in Hilly Areas Based on DEM

Abstract

The present invention provides a method for extracting the inundation range and water depth of dam-break floods in small and medium-sized reservoirs in hilly areas based on DEM, belonging to the technical field of dam-break flood inundation analysis. The steps are as follows: First, calculate the dam-break discharge at the dam site of the reservoir; second, extract the cross-sections of the downstream river channels of the small and medium-sized reservoirs, and deduce the peak discharge of the dam-break flood and its peak discharge at each control section along the downstream river channel; then, deduce the highest water level of each river cross-section based on the peak discharge of the section, and obtain the flood inundation range from the inundation water level; finally, based on the grid-type DEM terrain data, batch calculate the inundation water depth of each terrain grid unit within the inundation range, so as to realize the calculation and analysis of the dam-break flood inundation and water depth in the downstream area of the reservoir. The present invention can solve the problem that the traditional simple inundation model cannot calculate the inundation water depth. At the same time, the present invention calculates the inundation range and water depth based on the DEM terrain data, which is simpler than the traditional hydrodynamic model calculation and the model results are more stable.

Description

Technical Field

[0001] The present invention belongs to the technical field of dam-break flood inundation analysis, and relates to a method for extracting the inundation range and water depth of dam-break floods in medium and small-sized reservoirs in hilly areas based on DEM. Background Art

[0002] With the intensification of global climate change and the frequent occurrence of extreme weather events, the frequency and impact range of flood disasters caused thereby have also increased significantly. At present, flood disasters in small and medium-sized rivers in China have shown a trend of multiple, frequent, and severe occurrences, seriously threatening the safety of people's lives and property and social and economic development. At the same time, the terrain in hilly areas of China is complex, and a large number of medium and small-sized reservoirs have been built to meet the production and living needs such as irrigation, power generation, and water supply. According to statistics, there are about 98,000 reservoirs in China at present, more than 95% of which are medium and small-sized reservoirs, widely distributed in hilly areas, showing the characteristics of "numerous and widespread". These reservoirs have played an important role in flood control and disaster reduction, water resource utilization, etc., but there are also some potential safety hazards. Most of the medium and small-sized reservoirs in hilly areas were built in the last century. Limited by the technical conditions at that time, some reservoirs have potential safety hazards. Once a dam break occurs, it will cause a devastating blow to the downstream area. In addition, factors such as improper reservoir operation and imperfect flood discharge facilities may also increase the flood disaster risk in the downstream area of the reservoir. Therefore, quickly and accurately carrying out dam-break flood inundation analysis is of extremely important significance for flood warning, risk assessment, etc.

[0003] Traditional methods for calculating dam-break flood inundation analysis mainly rely on hydrodynamic models or simple inundation models. By generalizing the modeling and solving the Saint-Venant equations, information on the flood inundation range and water depth within the region is obtained. However, the hydrodynamic model modeling depends on high-precision basic geographic information data, and the modeling process is cumbersome, with problems such as easy divergence of model solutions and long calculation time, especially the solution of high-speed water flow under dam breaks in reservoirs is more unstable (Chinese Invention Patent 202311154591.3). Although the traditional simple inundation model does not require complex modeling and is relatively simple compared to the hydrodynamic model, it can only roughly calculate the inundation range along the river course and cannot deduce the spatial distribution of inundation water depth (Chinese Invention Patent 202210895949.7). Summary of the Invention

[0004] In view of the problems existing in the prior art, aiming at improving the calculation efficiency and stability of dam-break flood analysis, the present invention proposes a method for extracting the inundation range and water depth of dam-break floods in medium and small-sized reservoirs in hilly areas based on DEM. Here, DEM (Digital Elevation Model) refers to a digital elevation model. The present invention can solve the problem that the traditional simple inundation model cannot deduce the spatial distribution of inundation water depth and realize the rapid calculation and analysis of dam-break flood inundation and water depth in the downstream area of the reservoir.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A method for extracting the inundation range and water depth of dam-break floods in small and medium-sized reservoirs in hilly areas based on DEM. The present invention calculates the dam-break flow at the dam site of the reservoir by integrating traditional empirical formulas and terrain grid analysis methods. Secondly, the cross-sections of the river downstream of the small and medium-sized reservoir are extracted to calculate the peak flow of the dam-break flood and its peak flow at each control section along the downstream river course. Then, the highest water level of each river cross-section is calculated based on the cross-section peak flow, and the inundation range of the flood is obtained from the inundation water level. Finally, based on the grid-type DEM terrain data, the inundation water depth of each terrain grid unit within the inundation range is calculated in batches, so as to realize the rapid calculation and analysis of the inundation and water depth of the dam-break flood in the downstream area of the reservoir. The specific steps are as follows:

[0007] Step 1: Calculate the dam-break flow at the dam site of the reservoir.

[0008] Step 1.1: Determine the average height of the dam body after dam break according to the sedimentation storage capacity, total storage capacity, and water depth in front of the dam of the reservoir. The calculation formula is as follows:

[0009] (1);

[0010] In the formula: The average height of the dam body after dam break; is the water depth in front of the dam before dam break, with the unit of m. Since it is usually difficult to obtain the water depth in front of the dam before dam break of the sediment dam, the present invention simplifies the treatment and takes the dam height as the water depth in front of the dam before dam break; V 淤 is the sedimentation storage capacity; V 设 is the total storage capacity, and the units are all m 3 .

[0011] Step 1.2: On the basis of Step 1.1, use the Shawclich formula to calculate the dam-break flow at the dam site of the reservoir. Calculate the dam-break flow at the dam site of the reservoir through the water depth in front of the dam, the average height of the dam body after dam break, and the average width of the dam site section. The calculation formula is as follows:

[0012] (2);

[0013] In the formula: is the dam-break flow at the dam site, with the unit of m 3 / s; B is the average width of the dam site section, with the unit of m.

[0014] Step 2: Extract the cross-sections of the river downstream of the small and medium-sized reservoir.

[0015] Step 2.1, batch extract the downstream river cross-sections (hereinafter referred to as river cross-sections) based on the digital elevation model (DEM) terrain data, and obtain the vector data of the river cross-sections and the river cross-section data (in txt format). Set the parameters as the river cross-section interval, the extraction width of the river cross-section, and the horizontal interval of the extraction points of the river cross-section. Among them, the river cross-section interval is set according to the along-river trend of the river; the extraction width of the river cross-section is extracted as 1.5 times the length of the dam crest, and the river cross-sections are densely arranged when the river turns, there are weir and dam structures, and it flows through villages and towns; the horizontal interval of the extraction points of the river cross-section is selected according to the accuracy of the DEM terrain data. The finally extracted river cross-section data at each point on the river cross-section contains X and Z attributes, where X represents the horizontal distance of each point on the river cross-section from the leftmost point of the river cross-section, and Z represents the terrain elevation of each point on the river cross-section.

[0016] Step 2.2, based on Step 2.1, according to the Z (terrain elevation) value obtained in Step 2.1 and the river cross-section interval, statistically analyze the thalweg point elevation of each river cross-section along the river and the distance of the river cross-section from the dam site section. The thalweg point elevation of each river cross-section is the minimum Z value in the river cross-section data of this river section, and the distance of the river cross-section from the dam site section is the sum of all adjacent river cross-section intervals before this river cross-section.

[0017] Step 3, calculate the dam-break discharge at each river cross-section.

[0018] According to the total reservoir capacity, the silted reservoir capacity, the distance of each river cross-section from the dam site section, and the maximum flow velocity of each river cross-section, calculate the dam-break discharge at each river cross-section. The calculation formula is as follows:

[0019] (3);

[0020] In the formula: is the total reservoir capacity, is the silted reservoir capacity, with the unit of m 3 ; L is the distance from the dam site section to the residential area, with the unit of m; is the maximum flow velocity of the river cross-section during the flood period, with the unit of m / s; K is an empirical coefficient; is the dam-break discharge at the dam site, with the unit of m 3 / s; is the dam-break discharge at the river cross-section.

[0021] Step 4, calculate the inundation level at each river cross-section.

[0022] Step 4.1, take multiple elevations (water levels) for each river cross-section, from the lowest water level to the highest water level that can be reached on both banks, and extract the corresponding cross-sectional area, hydraulic radius, and wetted perimeter at different elevations (water levels) according to the shape of the river cross-section.

[0023] Step 4.2: Calculate the flow rates at different water levels for each river channel cross-section using the Chezy formula. Extract the water level-discharge relationship curve for the river channel cross-section from the two parameters of water level and flow rate, where the abscissa is the water level and the ordinate is the flow rate. The flow rate calculation formula for the river channel cross-section is as follows:

[0024] (4);

[0025] (5);

[0026] In the formula: is the dam-break flow rate at the river channel cross-section; is the cross-sectional area of the water passing through the settlement section, with the unit of m 2 ; C is the Chezy coefficient; R is the hydraulic radius of the settlement section, with the unit of m; i is the hydraulic gradient, which is obtained by the ratio of the difference in elevation between the lowest point of the river channel at the dam site and the lowest point of the river channel cross-section to the distance between the river channel cross-section and the dam site cross-section; n is the roughness coefficient, which is selected according to the characteristics of the gully.

[0027] Step 4.3: Calculate the inundation water level of the dam-break flow rate at the river channel cross-section.

[0028] According to the water level-discharge relationship curve of the river channel cross-section extracted in Step 4.2, find the inundation water level corresponding to the dam-break flow rate at the river channel cross-section on the curve to obtain the inundation water level of the dam-break flood.

[0029] Step 5: Extract the inundation range of the dam-break flood.

[0030] Step 5.1: Based on the DEM terrain data on the basis of Step 4.3, extract the contour lines corresponding to the inundation water levels of each river channel cross-section, and then combine with the vector data of the river channel cross-section extracted in Step 2.1 to carry out elevation intersection analysis to obtain the intersection points of the inundation water levels of each river channel cross-section and the terrain elevations on the left and right banks. This intersection point is used as the boundary point of the inundation range;

[0031] Step 5.2: Connect the boundary points of the inundation range on the left and right banks of adjacent river channel cross-sections along the river trend to obtain the inundation range of the downstream area of the reservoir under the dam-break flood (hereinafter referred to as the inundation range).

[0032] Step 6: Deduce the inundation depth of the dam-break flood.

[0033] Step 6.1: Based on the inundation range obtained in Step 5.2, rasterize the inundation range according to the DEM terrain data;

[0034] Step 6.2: According to the river course trend, divide the grid cells within the flood inundation range obtained in Step 6.1 into several groups longitudinally or horizontally. When the river course trend is horizontal, the grid cells are grouped longitudinally, and the grid cells in the same vertical column form a group. When the river course trend is vertical, the grid cells are grouped horizontally, and the grid cells in the same horizontal row form a group. The inundation water level values of the grid cells at both ends of each group of grid cells are equal to their terrain elevations, and the inundation water level value of the middle grid cells is obtained by linear interpolation from the elevations of the grid cells at both ends.

[0035] Step 6.3: On the basis of Step 6.2, subtract the terrain elevation of the DEM terrain data from the inundation water level value of each grid cell to obtain the inundation depth of the dam-break flood, and extract the spatial distribution of the inundation depth within the inundation range.

[0036] The beneficial effects of the present invention are as follows:

[0037] Based on the traditional simple inundation range model, the present invention enables the model to be applied to the calculation of dam-break floods through Step 1 and Step 3 in combination with traditional empirical formulas. Moreover, through the grid-based processing and interpolation of the inundation range described in Step 6, the spatial distribution of the inundation depth is obtained, solving the problem that the traditional simple inundation model cannot calculate the inundation depth. At the same time, the present invention calculates the inundation range and water depth based on DEM terrain data, which is simpler than the traditional hydrodynamic model calculation and the model results are more stable. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is a flow chart of the method for extracting the inundation range and water depth of the dam-break flood of medium and small-sized reservoirs in mountainous areas according to the present invention;

[0039] Figure 2 is a distribution map of the river network water system (including terrain) of the research case;

[0040] Figure 3 is a cross-sectional distribution map of the river downstream of the Ergehao Dam;

[0041] Figure 4 is an example diagram of the cross-section of the extracted river;

[0042] Figure 5 is a schematic diagram of the water level-flow relationship calculation for the river cross-section;

[0043] Figure 6 is a map of the inundation range of the dam-break flood of the Ergehao Dam;

[0044] Figure 7 is a schematic diagram of the method for calculating the inundation depth of the dam-break flood;

[0045] Figure 8 is a spatial distribution map of the inundation depth of the dam-break flood of the Ergehao Dam. DETAILED DESCRIPTION OF THE INVENTION

[0046] The present invention will be further described below in conjunction with specific examples.

[0047] The present invention proposes a method for extracting the inundation range and water depth of dam-break floods for small and medium-sized reservoirs in hilly areas based on DEM. Taking the Ergehao Dam in the hilly area of Jungar County, Ordos City, Inner Mongolia as an example, the inundation range and inundation water depth during its dam break are calculated. The Ergehao Dam is located in the Husitai River Basin. The dam height of this dam is 9m, the dam crest length is 360m, and the total reservoir capacity is 388,700 m 3 , and the sedimentation reservoir capacity is 135,000 m 3 . The downstream river is the Haolai Gully. See the river network water system in Figure 2 .

[0048] Step 1: Calculate the dam-break flow at the dam site of the reservoir.

[0049] Step 1.1: Determine the average height of the dam body after dam break according to the sedimentation reservoir capacity, total reservoir capacity, and water depth in front of the dam. The water depth in front of the dam is simplified to take the dam crest elevation as 9m. The average height of the dam body after dam break:

[0050]

[0051] In the formula: The average height of the dam body after dam break; is the water depth in front of the dam before dam break, with the unit of m. Since it is usually difficult to obtain the water depth in front of the dam before the sediment dam breaks, the present invention simplifies the treatment and takes the dam height as the water depth in front of the dam before dam break; V 淤 is the sedimentation reservoir capacity, and V 设 is the total reservoir capacity, and the units are all m 3 .

[0052] Step 1.2: On the basis of Step 1.1, use the Shawclich formula to calculate the dam-break flow at the dam site of the reservoir. Calculate the dam-break flow at the dam site of the reservoir through the water depth in front of the dam, the average height of the dam body after dam break, and the average width of the dam site section. The dam-break flow at the dam site is:

[0053]

[0054] In the formula: is the dam-break flow at the dam site, with the unit of m 3 / s; B is the average width of the dam site section, with the unit of m.

[0055] Step 2: Extract the river cross-section downstream of the small and medium-sized reservoir.

[0056] Step 2.1, batch extract the downstream river channel cross-sections (hereinafter referred to as river channel cross-sections) based on the digital elevation model (DEM) terrain data to obtain the vector data of the river channel cross-sections and the river channel cross-section data (in txt format). Set the parameters as the river channel cross-section interval, the extraction width of the river channel cross-section, and the horizontal interval of the extraction points of the river channel cross-section. Among them, the river channel cross-section interval is set to 200 m according to the river's along-course trend; the extraction width of the river channel cross-section is extracted based on 1.5 times the length of the dam crest. The length of the dam crest is 360 m, and the cross-section width is taken as 360×1.5 = 540 m. Since the river channel of Haolaigou, downstream of the Ergehao Dam, is relatively straight and the cross-section extraction interval is small, there is no need to add additional cross-sections. Finally, a total of 69 cross-sections are extracted; the horizontal interval of the extraction points of the river channel cross-section is selected according to the DEM terrain data accuracy. The DEM accuracy is 12.5 m×12.5 m, and the horizontal interval of the extraction points of the river channel cross-section is taken as 5 m. The finally extracted river channel cross-section data are the points on the river channel cross-section that contain the X and Z attributes, where X represents the horizontal distance of each point on the river channel cross-section from the leftmost point of the river channel cross-section, and Z represents the terrain elevation of each point on the river channel cross-section.

[0057] Step 2.2, based on Step 2.1, statistically analyze the thalweg point elevation of each river channel cross-section along the river and the distance of the river channel cross-section from the dam site section according to the Z (terrain elevation) value obtained in Step 2.1 and the river channel cross-section interval. The thalweg point elevation of each river channel cross-section is the minimum Z value in the river channel cross-section data, and the distance of the river channel cross-section from the dam site section is the sum of all adjacent river channel cross-section intervals before this river channel cross-section. The extraction results of the cross-section data, thalweg point elevation, and the distance along the river from the dam site are shown in Table 1;

[0058] Table 1 is the cross-section data table

[0059]

[0060] Step 3, calculate the dam-break discharge at each river channel cross-section.

[0061] According to the total reservoir capacity, the silted reservoir capacity, the distance of each river channel cross-section from the dam site section, and the maximum flow velocity of each river channel cross-section, calculate the dam-break discharge at each river channel cross-section. The calculation formula is as follows:

[0062]

[0063] Among them, the dam-break discharge at the dam site = 4103.48 m 3 / s; the total reservoir capacity = 388,700 m 3 ; the silted reservoir capacity = 135,000 m 3 ; the maximum flow velocity of the river channel cross-section during the flood period = 3.5 m / s, K = 1.25. The calculated results of the flow rate for each cross-section are shown in Table 2.

[0064] Table 2 is the table of calculated flow rates for cross-sections

[0065]

[0066] Step 4, calculate the inundation levels of each river channel cross-section.

[0067] Step 4.1, take multiple elevations (water levels) for each river channel cross-section, starting from the lowest water level up to the highest water level that can be reached on both banks. According to the shape of the river channel cross-section, extract the corresponding cross-sectional area, hydraulic radius, and wetted perimeter at different elevations (water levels).

[0068] Step 4.2, use the Chezy formula to calculate the flow rates passing through each river channel cross-section at different water levels. From the two parameters of water level and flow rate, extract the water level - flow rate relationship curve of the river channel cross-section, where the abscissa is the water level and the ordinate is the flow rate. The water level - flow rate relationship curve is shown in Figure 5 ; The flow rate calculation formula for the river channel cross-section is as follows:

[0069]

[0070]

[0071] In the formula: is the dam-break flow rate at the river channel cross-section; is the cross-sectional area of the residential area cross-section, with the unit of m 2 ; C is the Chezy coefficient; R is the hydraulic radius of the residential area cross-section, with the unit of m; i is the hydraulic gradient, obtained by the ratio of the difference in elevation between the lowest point of the river channel at the dam site and the lowest point of the river channel cross-section to the distance between the river channel cross-section and the dam site cross-section; n is the roughness coefficient, selected according to the characteristics of the gully.

[0072] Step 4.3, calculate the inundation level of the dam-break flow rate at the river channel cross-section.

[0073] According to the water level - flow rate relationship curve of the river channel cross-section extracted in Step 4.2, find the inundation level corresponding to the dam-break flow rate at the river channel cross-section on the curve to obtain the inundation level of the dam-break flood corresponding to the dam-break flood. The inundation levels of each cross-section are shown in Table 3.

[0074] Table 3 is the calculation result table of inundation levels for different cross-sections

[0075]

[0076] Step 5, extract the inundation range of the dam-break flood.

[0077] Step 5.1: Based on the DEM terrain data obtained in Step 4.3, extract the contour lines corresponding to the inundation levels of each river cross-section, and then, in combination with the vector data of the river cross-sections extracted in Step 2.1, conduct an elevation intersection analysis to obtain the intersection points between the inundation levels of each river cross-section and the terrain elevations on the left and right banks. These intersection points serve as the boundary points of the inundation area.

[0078] Step 5.2: As Figure 6 shown, connect the boundary points of the inundation areas on the left and right banks of adjacent river cross-sections along the river trend to obtain the inundation area of the dam-break flood of the Ergehao Dam.

[0079] Step 6: Deduce the inundation depth of the dam-break flood.

[0080] Step 6.1: Based on the inundation area obtained in Step 5.2, rasterize the inundation area according to the DEM terrain data.

[0081] Step 6.2: According to the river trend, divide the raster cells within the inundation area obtained in Step 6.1 into several groups longitudinally or horizontally. Since the trend of the Haolai Gully is horizontal, the raster cells are grouped longitudinally. The raster cells in the same vertical column form a group. The inundation level values of the raster cells at both ends of each group are equal to their terrain elevations, and the inundation level values of the intermediate raster cells are obtained by linear interpolation from the elevations of the raster cells at both ends. The calculation principle is as Figure 7 shown.

[0082] Step 6.3: On the basis of Step 6.2, subtract the terrain elevation of the DEM terrain data from the inundation level value of each raster cell to obtain the inundation depth of the dam-break flood, and extract the spatial distribution of the inundation depth within the inundation area. See the inundation depth distribution map of the example in Figure 8 , from Figure 8 which it can be seen that: The inundation area of the dam-break flood of the Ergehao Dam is within the river channel, with little impact on nearby villages. The inundation depth is relatively deep near the dam site, reaching a maximum of 6.17 m.

[0083] The above-described embodiments merely represent the implementation modes of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that for those skilled in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all fall within the protection scope of the present invention.

Claims

1. A method for extracting the flood inundation range and water depth of small and medium-sized reservoirs in hilly areas based on DEM, characterized in that: The method for extracting the inundation range and water depth of dam-break floods of small and medium-sized reservoirs in hilly areas first calculates the dam-break flow at the reservoir dam site; secondly, extracts the cross section of the river downstream of the small and medium-sized reservoir, and deduces the peak flow of the dam-break flood and its evolution to the peak flow of each control section along the downstream river; then, the highest water level of each river cross section is deduced according to the peak flow of the section, and the flood inundation range is obtained from the inundation water level; finally, based on the grid-type DEM terrain data, the inundation depth of each terrain grid unit within the inundation range is calculated in batches, thereby realizing the calculation and analysis of the inundation and water depth of the dam-break flood in the downstream area of ​​the reservoir; the specific steps are as follows: Step 1, calculating the dam-break flow at the reservoir dam site; Step 2, extracting the river cross section downstream of small and medium-sized reservoirs; Step 3, deriving the dam-break flow at each river cross section; The dam-break flow at each river cross section is calculated based on the total reservoir capacity, the silted reservoir capacity, the distance between each river cross section and the dam site section, and the maximum flow velocity of each river cross section; Step 4, calculating the flooding water level of each river cross section; Step 5: Extract the flood inundation range caused by the dam breach; specifically: Step 5.1: Based on step 4, based on the DEM terrain data, extract the contour lines corresponding to the flooded water level of each river cross section, and then combine the river cross section data extracted in step 2 to conduct elevation intersection analysis to obtain the intersection point of the flooded water level of each river cross section and the terrain elevation of the left and right banks. The intersection point is used as the boundary point of the flooded range; Step 5.2, connect the boundary points of the inundation ranges on the left and right banks of adjacent river cross sections along the river flow to obtain the flood inundation range of the downstream area of ​​the reservoir under the dam break flood, referred to as the flood inundation range; Step 6: Calculate the flood depth caused by the dam breach. Specifically: Step 6.1, based on the flood inundation range obtained in step 5.2, rasterize the flood inundation range according to the DEM terrain data to obtain the raster cells within the flood inundation range; Step 6.2, according to the river channel trend, the grid cells in the flood inundation range obtained in step 6.1 are divided into several groups along the longitudinal direction or the transverse direction. When the river channel trend is transverse, the grid cells are grouped longitudinally, and the grid cells in the same longitudinal column are grouped together. When the river channel trend is longitudinal, the grid cells are grouped transversely, and the grid cells in the same transverse column are grouped together. The flooded water level values ​​of the grid cells at both ends of each group of grid cells are equal to their terrain elevations, and the flooded water level values ​​of the middle grid cells are obtained by linear interpolation of the elevations of the grid cells at both ends. Step 6.3, based on step 6.2, the inundation water level value of each grid cell is subtracted from the terrain elevation of the DEM terrain data to obtain the inundation depth of the dam break flood, and the spatial distribution of the inundation depth within the inundation range is extracted.

2. The method for extracting the flood inundation range and water depth of small and medium-sized reservoirs in hilly areas based on DEM according to claim 1 is characterized in that: The step 1 is specifically as follows: Step 1.1: Determine the average height of the dam after the dam breach based on the reservoir siltation capacity, total storage capacity, and water depth in front of the dam. The calculation formula is as follows: （1）； Where: Average height of the dam after the dam breach; V is the water depth in front of the dam before the dam burst, in meters. Since the water depth in front of the dam before the dam burst is usually difficult to obtain, the present invention simplifies the process and takes the dam height as the water depth in front of the dam before the dam burst; 淤 V is the silted reservoir capacity; 设 is the total storage capacity, in m 3 ; Step 1.2: Based on step 1.1, the Shockley formula is used to calculate the dam-break flow at the reservoir dam site. The dam-break flow at the reservoir dam site is calculated by the water depth in front of the dam, the average height of the dam body after the dam-break, and the average width of the dam site section. The calculation formula is as follows: （2）； Where: is the dam-break flow at the dam site, in m 3 / s; B is the average width of the dam site section, unit: m.

3. The method for extracting the flood inundation range and water depth of small and medium-sized reservoirs in hilly areas based on DEM according to claim 2 is characterized in that: The step 2 is specifically as follows: Step 2.1, based on the digital elevation DEM terrain data, batch extract downstream river cross sections, hereinafter referred to as river cross sections, to obtain river cross section vector data and river cross section data; set parameters including river cross section interval, river cross section extraction width, and river cross section extraction point horizontal interval; the final extracted river cross section data are points on the river cross section containing X and Z attributes, where X represents the horizontal distance between each point on the river cross section and the leftmost point on the river cross section, and Z represents the terrain elevation of each point on the river cross section; Step 2.2, based on the Z value and the river cross-section interval obtained in step 2.1, statistically analyze the elevation of the deep-sea point of each river cross-section along the river and the distance between the river cross-section and the dam site section. The elevation of the deep-sea point of each river cross-section is the minimum Z value in the river cross-section data, and the distance between the river cross-section and the dam site section is the sum of the intervals of all adjacent river cross-sections before the river cross-section.

4. The method for extracting the flood inundation range and water depth of small and medium-sized reservoirs in hilly areas based on DEM according to claim 3 is characterized in that: During the parameter setting process of step 2.1, the river cross-section interval is set according to the trend of the river along the way; the river cross-section extraction width is extracted according to 1.5 times the dam crest length, and the river cross-section is densely arranged when the river turns, there are weir structures, and flows through villages and towns; the horizontal interval of the river cross-section extraction points is selected according to the accuracy of the DEM terrain data.

5. The method for extracting the flood inundation range and water depth of small and medium-sized reservoirs in hilly areas based on DEM according to claim 1 is characterized in that: The formula for calculating the dam break flow at each river cross section in step 3 is as follows: （3）； Where: is the total storage capacity, is the silted reservoir capacity, in m 3 ; L is the distance from the dam section to the residential area, in meters; is the maximum flow velocity of the river cross section during flood period, in m / s; K is the empirical coefficient; is the dam-break flow at the dam site, in m 3 / s; is the dam break flow at the river cross section.

6. The method for extracting the flood inundation range and water depth of small and medium-sized reservoirs in hilly areas based on DEM according to claim 3 is characterized in that: The step 4 is specifically as follows: Step 4.1, take multiple water levels in each river cross section, from the lowest water level to the highest water level that can be reached on both banks, and extract the corresponding cross-sectional area, hydraulic radius, and wetted perimeter at different water levels according to the shape of the river cross section; Step 4.2, use the Xie Cai formula to calculate the flow rate of each river cross section at different water levels, and extract the water level-flow relationship curve of the river cross section from the two parameters of water level and flow, where the abscissa is the water level and the ordinate is the flow; the flow calculation formula of the river cross section is as follows: （4）； （5）； Where: is the dam-break discharge at the river cross section; is the water-passing area of ​​the residential section, in m 2 ; C is Xie Cai coefficient; R is the hydraulic radius of the residential section, unit is m; i is the hydraulic gradient, which is obtained by the ratio of the difference between the lowest point elevation of the river channel at the dam site and the lowest point elevation of the river channel cross section to the distance of the river channel cross section from the dam site section; n is the roughness, which is selected according to the channel characteristics; Step 4.3, calculate the flooding water level of the dam-break flow in the river cross section; According to the river cross-section water level-flow relationship curve extracted in step 4.2, find the submerged water level corresponding to the dam break flow at the river cross-section on the curve, and obtain the submerged water level of the dam break flood corresponding to the dam break flood.

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