Method for extracting dam break flood inundation range and water depth of small and medium-sized reservoirs in hilly region based on DEM (Digital Elevation Model)

Abstract

The invention provides a DEM-based method for extracting dam break flood inundation range and water depth of small and medium-sized reservoirs in hilly areas, and belongs to the technical field of dam break flood inundation analysis. The method comprises the following steps: firstly, calculating dam break flow at a reservoir dam site; secondly, extracting a cross section of a downstream river channel of the small and medium-sized reservoir, and deducing dam break flood peak flow and peak flow evolved to each control section along the downstream river channel; then, the highest water level of each river channel cross section is calculated according to the peak flow of the section, and the flood inundation range is obtained through the inundation water level; and finally, on the basis of the grid type DEM topographic data, the submerging water depth of each topographic grid unit in the submerging range is calculated in batches, so that reservoir downstream region dam break flood submerging and water depth calculation analysis is realized. The problem that a traditional simple submerging model cannot calculate submerging water depth can be solved, meanwhile, submerging range and water depth calculation is carried out based on DEM topographic data, and compared with a traditional hydrodynamic model, calculation is simpler and more convenient, and a model result is more stable.

Description

Technical Field

[0001] The 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 of small and medium-sized reservoirs in hilly areas based on DEM. Background Art

[0002] As global climate change intensifies and extreme weather events occur frequently, the frequency and impact range of flood disasters caused by them have also significantly expanded. At present, flood disasters in small and medium-sized rivers in my country have become frequent, recurring, and have seriously threatened the safety of people's lives and property and social and economic development. At the same time, the terrain in my country's hilly areas is complex, and a large number of small and medium-sized reservoirs have been built to meet the production and living needs of irrigation, power generation, water supply, etc. According to statistics, there are about 98,000 reservoirs in my country, of which more than 95% are small and medium-sized reservoirs, which are widely distributed in hilly areas, showing the characteristics of "many points and wide areas". These reservoirs have played an important role in flood prevention and disaster reduction, water resource utilization, etc., but there are also some safety hazards. Most of the small and medium-sized reservoirs in hilly areas were built in the last century. Due to the limitations of the technical conditions at that time, some reservoirs have safety hazards. Once a dam breaks, it will cause a devastating blow to the downstream areas. In addition, factors such as improper reservoir scheduling and imperfect flood discharge facilities may also aggravate the risk of flood disasters in the downstream areas of reservoirs. Therefore, it is extremely important to quickly and accurately carry out dam break flood inundation analysis for flood early warning and risk assessment.

[0003] Traditional dam-break flood inundation analysis and calculation methods mainly rely on hydrodynamic models or simple inundation models. Through generalized modeling and solving the Saint-Venant equations, the flood inundation range and water depth information in the region are obtained. However, hydrodynamic modeling relies on high-precision basic geographic information data, and the modeling process is cumbersome. There are problems such as easy divergence of model solutions and long calculation time. In particular, the solution of high-speed water flow under reservoir dam breach is more unstable (Chinese invention patent 202311154591.3). Although the traditional simple inundation model does not require complex modeling and is simpler than the hydrodynamic model, it can only roughly calculate the inundation range along the river channel and cannot deduce the spatial distribution of the inundation depth (Chinese invention patent 202210895949.7). Summary of the invention

[0004] In view of the problems existing in the prior art, the present invention aims to improve the computational efficiency and stability of dam-break flood analysis and proposes 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. Here, DEM (Digital Elevation Model) refers to digital elevation model. The present invention can solve the problem that the traditional simple inundation model cannot deduce the spatial distribution of inundation depth and realize the rapid calculation and analysis of inundation and water depth of dam-break floods in the downstream area of ​​the reservoir.

[0005] In order 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 integrates traditional empirical formulas and terrain grid analysis methods to first calculate the dam-break flow at the reservoir dam site; secondly, extract the river cross section downstream of the small and medium-sized reservoir to deduce the dam-break flood peak flow and its evolution to the peak flow of each control section along the downstream river; then deduce the highest water level of each river cross section based on the cross-section peak flow, and obtain the flood inundation range from the inundation water level; finally, based on the grid-type DEM terrain data, batch calculate the inundation depth of each terrain grid unit within the inundation range, thereby realizing the rapid calculation and analysis of the inundation and water depth of dam-break floods in the downstream area of ​​the reservoir. Specifically, the following steps are included:

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

[0008] 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:

[0009] (1);

[0010] 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 .

[0011] 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:

[0012] (2);

[0013] 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.

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

[0015] 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 (txt format). Set the parameters as river cross section interval, river cross section extraction width, and river cross section extraction point horizontal interval, where the river cross section interval is set according to the river trend; the river cross section extraction width is extracted according to 1.5 times the dam crest length, and the river cross sections are densely arranged when the river turns, there are weir structures, and when it flows through villages and towns; the horizontal interval of river cross section extraction points is selected according to the accuracy of DEM terrain data. The final extracted river cross section data is a point 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 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, statistically analyze the elevation of the deep point of each river cross section along the river and the distance between the river cross section and the dam site section according to the Z (topography elevation) value and river cross section interval obtained in step 2.1. The elevation of the deep point of each river cross section is the smallest 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.

[0017] Step 3: deduce the dam break flow at each river cross section.

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

[0019] (3);

[0020] 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.

[0021] Step 4: Calculate the flooding water level of 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) based on the shape of the river cross section.

[0023] Step 4.2, use Xie Cai's 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 horizontal axis is the water level and the vertical axis is the flow. The formula for calculating the flow rate of the river cross section is as follows:

[0024] (4);

[0025] (5);

[0026] 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 the Xie Cai coefficient; R is the hydraulic radius of the residential section, in meters; i is the hydraulic gradient, which is obtained by the ratio of the difference between the elevation of the lowest point of the river channel at the dam site and the elevation of the lowest point of the river channel cross section to the distance between the river channel cross section and the dam site section; n is the roughness, which is selected according to the characteristics of the channel.

[0027] Step 4.3, calculate the flooding water level of the river cross section due to dam break flow.

[0028] 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.

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

[0030] Step 5.1: Based on step 4.3, 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 vector data extracted in step 2.1 to carry out 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, and the intersection point is used as the boundary point of the flooded range;

[0031] 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 reservoir downstream area under the dam break flood (hereinafter referred to as the flood inundation range).

[0032] Step 6, calculate the water depth of the dam breach flood.

[0033] 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;

[0034] Step 6.2, according to the river channel trend, the grid cells within the flood inundation range obtained in step 6.1 are divided into several groups along the longitudinal or transverse direction. When the river channel trend is transverse, the grid cells are grouped longitudinally, and the grid cells in the same vertical column are grouped together. When the river channel trend is longitudinal, the grid cells are grouped transversely, and the grid cells in the same horizontal 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.

[0035] 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.

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

[0037] Based on the traditional simple inundation range model, the present invention combines the traditional empirical formula through steps 1 and 3 to enable the model to be applied to dam break flood calculations, and obtains the spatial distribution of inundation depth through the inundation range rasterization and interpolation processing described in step 6, thereby 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 to calculate and has more stable model results than traditional hydrodynamic models. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0039] Figure 2 The distribution map of the river network for the case study (including topography);

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

[0041] Figure 4 Example of extracting cross sections for a river;

[0042] Figure 5 Calculate the water level and flow relationship diagram for the river cross section;

[0043] Figure 6 Map of the flood inundation area caused by the Ergehao Dam breach;

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

[0045] Figure 8 This is the spatial distribution map of the water depth caused by the Ergehao Dam breach flood. DETAILED DESCRIPTION

[0046] The present invention is further described below with reference to specific examples.

[0047] This paper proposes a method for extracting the flood inundation range and water depth of small and medium-sized reservoirs in hilly areas based on DEM. Taking Ergehao Dam in the hilly area of ​​Junggar County, Ordos City, Inner Mongolia as an example, the inundation range and water depth of the dam are calculated. Ergehao Dam is located in the Hustai River Basin. The dam is 9 meters high and 360 meters long at the top. The total storage capacity is 388,700 m 3 , siltation storage capacity 135,000 m 3 The downstream river is Haolaigou, and the river network system is Figure 2 .

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

[0049] Step 1.1, according to the reservoir siltation storage capacity, total storage capacity, and water depth in front of the dam, determine the average height of the dam body after the dam breach. 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 the dam breach is:

[0050]

[0051] 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 is the silted reservoir capacity, V 设 is the total storage capacity, in m 3 .

[0052] 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 section. The dam-break flow at the dam site is:

[0053]

[0054] 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.

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

[0056] Step 2.1, based on the digital elevation DEM terrain data, batch extract the downstream river cross section (hereinafter referred to as the river cross section) to obtain the river cross section vector data and river cross section data (txt format). Set the parameters as the river cross section interval, river cross section extraction width, and river cross section extraction point horizontal interval. Among them, the river cross section interval is set to 200m according to the river trend along the river; the river cross section extraction width is extracted according to 1.5 times the dam crest length, the dam crest length is 360m, and the cross section width is 360×1.5=540m. Since the river channel of Haolaigou downstream of Ergehao Dam is relatively straight and the cross section extraction interval is small, there is no need to add additional cross sections. In the end, a total of 69 cross sections were extracted; the horizontal interval of the river cross section extraction point was selected according to the accuracy of the DEM terrain data, the DEM accuracy is 12.5m×12.5m, and the horizontal interval of the river cross section extraction point is taken as 5m. The finally 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.

[0057] Step 2.2, based on step 2.1, the elevation of the deep point of each river cross section along the river and the distance between the river cross section and the dam site section are statistically analyzed according to the Z (terrain elevation) value obtained in step 2.1 and the river cross section interval. The elevation of the deep point of each river cross section is the smallest 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. The extraction results of cross section data, deep point elevation and distance along the dam site are shown in Table 1;

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

[0059]

[0060] Step 3: deduce the dam break flow at each river cross section.

[0061] According to 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, the dam break flow at each river cross section is calculated. The calculation formula is as follows:

[0062]

[0063] Among them, the dam break flow at the dam site =4103.48m 3 / s; total storage capacity =388,700 m 3 ; Siltated reservoir capacity =135,000 m 3 ; Maximum flow velocity of the river section during flood period =3.5m / s, K=1.25. The flow calculation results of each section are shown in Table 2.

[0064] Table 2 is the cross-section flow calculation table

[0065]

[0066] Step 4: Calculate the flooding water level of each river cross section.

[0067] 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) based on the shape of the river cross section.

[0068] Step 4.2, use Xie Cai's 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 horizontal axis is the water level and the vertical axis is the flow. Figure 5 ; The formula for calculating the flow rate of the river cross section is as follows:

[0069]

[0070]

[0071] 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 the Xie Cai coefficient; R is the hydraulic radius of the residential section, in meters; i is the hydraulic gradient, which is obtained by the ratio of the difference between the elevation of the lowest point of the river channel at the dam site and the elevation of the lowest point of the river channel cross section to the distance between the river channel cross section and the dam site section; n is the roughness, which is selected according to the characteristics of the channel.

[0072] Step 4.3, calculate the flooding water level of the river cross section due to dam break flow.

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

[0074] Table 3 is the calculation results of flooding water level in different sections

[0075]

[0076] Step 5: Extract the inundation range of the dam break flood.

[0077] Step 5.1: Based on step 4.3, 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 vector data extracted in step 2.1 to carry out 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, and the intersection point is used as the boundary point of the flooded range;

[0078] Step 5.2, such as Figure 6 As shown, by connecting the boundary points of the left and right banks of adjacent river cross sections along the river direction, the inundation range of the Ergehao Dam breach flood is obtained.

[0079] Step 6, calculate the water depth of the dam breach flood.

[0080] 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;

[0081] Step 6.2, according to the river channel trend, divide the grid cells in the flood inundation range obtained in step 6.1 into several groups along the longitudinal or transverse direction. Since the Haolai ditch trend is transverse, the grid cells are grouped longitudinally, and the grid cells in the same column are grouped. 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. The calculation principle is as follows: Figure 7 shown.

[0082] Step 6.3: Based on step 6.2, the flooding water level of each grid cell is subtracted from the terrain elevation of the DEM terrain data to obtain the flooding depth of the dam break flood, and the spatial distribution of the flooding depth within the flooding range is extracted. Figure 8 ,from Figure 8 It can be seen that the flood inundation range caused by the Ergehao Dam breach is within the river channel, with little impact on nearby villages. The flood depth near the dam site is relatively deep, reaching a maximum of 6.17m.

[0083] The above-described embodiments merely express the implementation methods of the present invention, but they should not be understood as limiting the scope of the present invention. It should be pointed out that for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to 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 inundation water level corresponding to the dam break flow at the river cross-section on the curve, and obtain the dam break flood inundation water level corresponding to the dam break flood.

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