Prediction method for critical condition of sudden response of riverbed incision of compound river channel after base level fall

Abstract

The application discloses a method for predicting critical conditions of sudden response of a riverbed of a compound river channel to incision after base level fall, which comprises the following steps: firstly, determining the flow of the main channel of the compound river channel, the critical flow of the main channel of the compound river channel to sudden response of the riverbed of the compound river channel to incision after base level fall, the critical water depth of the main channel, the critical width of the main channel, the critical depth of the main channel, then taking the difference between the critical depth of the main channel and the depth of the main channel as the critical base level fall height, and taking the critical flow of the main channel and the critical base level fall height as the critical conditions of sudden response of the riverbed of the compound river channel to incision. The application combines the stable slope calculation formula of the alluvial river and the calculation model of the flow distribution of the beach and channel of the compound river channel, and proposes the critical conditions of sudden response of the riverbed of the compound river channel to incision after base level fall, so that the critical base level fall height of the compound river channel under different frequency floods can be predicted, the blank of the technical field is filled, and the application has important significance for guaranteeing the stability of the river regime of the compound river channel in actual engineering.

Description

Technical Field

[0001] This invention belongs to the field of hydraulics and river dynamics, and relates to the prediction technology of critical conditions for abrupt changes in the response of a complex riverbed. Specifically, it relates to a method for predicting the critical conditions for abrupt changes in the response of a complex riverbed after a drop in erosion base level. Background Technology

[0002] In recent years, with global climate change, frequent natural disasters, and increased human activities, abrupt changes in river erosion base levels have become increasingly common. The erosion base level of a river is generally lower than the bottom elevation of structures within the river channel. Under the influence of natural factors (tectonic activity, climate, environmental factors, etc.) or human factors (dam removal, river sand mining, water resource development and utilization, etc.), the river erosion base level relatively decreases, the hydraulic gradient increases, leading to headward erosion of the riverbed, causing localized erosion that continues upstream.

[0003] Previous studies on the riverbed evolution mechanism after the erosion base level drops have suggested that coarsening causes the final stable slope of the river channel to be greater than before the erosion base level drops, with the incision depth gradually decreasing upstream. Headward scouring only affects a limited river section, a phenomenon known as river channel adaptation. However, some floodplain rivers exhibit a completely different pattern after the erosion base level drops. Their final stable slope is less than before the erosion base level drops, meaning the upstream riverbed incision depth is even greater than the height of the erosion base level drop. Headward scouring continues upstream, a phenomenon known as a sudden incision response, which can cause significant damage to the river. For example, in the Shuangsheng section of the Shiting River (a tributary of the Tuo River) in Sichuan Province, the erosion base level dropped by 5 meters during the 2013 flood season, with the incision depth exceeding 10 meters just 2.5 kilometers upstream of the erosion base level. In 2017, the erosion base level in this section continued to drop by 14 meters, with the maximum upstream incision depth exceeding 20 meters. The severe scouring and downcutting caused by the erosion of the base surface has damaged the foundations of two bridges in this section of the river; at the same time, the People's Canal culvert (drinking water pipeline) in the river channel has been severely damaged, threatening the drinking water safety of millions of people in Mianyang City, Sichuan Province.

[0004] Most natural alluvial rivers are complex channels with a main channel and floodplains. During the dry season, the water mainly flows in the main channel, while during floods, the water overflows the floodplains on both sides of the main channel, forming floodplains. Furthermore, complex cross-sections are frequently used in river regulation projects to achieve better flood control. If a complex river channel experiences a sudden change in response after the erosion base level drops, severe scouring and downcutting of the riverbed will drastically alter or even completely reshape the river's morphology. This not only seriously threatens the safety and stability of structures such as bridges and riverbank protection structures, but also leads to a drop in river and groundwater levels, affecting navigation, port operations, and water intake efficiency. Simultaneously, it significantly impacts river habitats, reducing habitat heterogeneity, decreasing ecosystem biodiversity, and ultimately causing a loss of ecological stability.

[0005] Therefore, a correct understanding of the evolution mechanism of complex riverbeds after erosion base level decline is not only of significant theoretical value, but also has broad practical application value in water conservancy, transportation, and ecology. However, currently, there are no methods, either domestically or internationally, to accurately predict the critical conditions for the abrupt downcut response of complex riverbeds after erosion base level decline. Summary of the Invention

[0006] In view of the current technical status quo, which makes it difficult to effectively predict the critical conditions of the abrupt downcut response of complex riverbeds after erosion base level drops, this invention aims to provide a prediction method to effectively predict the critical conditions of the abrupt downcut response of complex riverbeds after erosion base level drops.

[0007] This invention addresses complex waterways, which consist of a main channel (i.e., the main river channel) and two adjacent floodplains. During the dry season, water flows primarily within the main channel; during the flood season, water overflows the main channel and floods the wider floodplains on both sides, with the main channel and floodplains sharing the flow. The depth of the main channel refers to the height from the bottom of the main channel bed to the bottom of the floodplains on both sides.

[0008] The present invention provides a method for predicting the critical conditions of abrupt changes in the response of a complex channel bed downcut after erosion base level decline, comprising the following steps:

[0009] S1 determines the main channel flow rate Q of the complex river channel. mco ;

[0010] S2 determines the critical main channel discharge Q for the abrupt response of the compound channel bed after the erosion base level drops. mcc ;

[0011] S3 determines the critical main channel depth H of the complex channel abrupt response after the erosion base level drops. c Critical main channel width b c ;

[0012] S4 is based on the critical main channel water depth H. c Critical main channel width b c The critical main trench depth h is determined according to the following formula. c :

[0013]

[0014] In the formula, Q fpc For the critical beach flow, S fp f represents the slope of the beach. fp The beach resistance coefficient is calculated using the same method as f. mc B represents the total width of the complex river channel; n fp The roughness coefficient of the beach area;

[0015] S5 uses the critical main trench depth h cThe difference between the main channel depth h0 and the critical incipient bed-load transport velocity V bc ;

[0016] The critical main channel flow rate Q mcc and the critical incipient bed-load transport velocity V bc are the critical conditions for the sudden response of the riverbed incision of the compound channel.

[0017] In step S1, before the incipient bed-load transport, the compound channel is considered to be in an equilibrium state. Q mco The bankfull flow rate Q

[0018]

[0019] In the formula, b is the width of the main channel, H0 is the water depth of the main channel, f mc = 8gn mc 2 / R mc 1 / 3 is the resistance coefficient of the main channel, n mc is the roughness coefficient of the main channel, R mc is the hydraulic radius of the main channel, which is approximately the water depth in a natural river, S mc is the slope of the main channel, and g is the local gravitational acceleration.

[0020] In the above step S2, before and after the incipient bed-load transport, the stable slope of the main channel can be expressed as: (Nie Ruhua et al., Study on the Riverbed Evolution of Strong Earthquake Damaged Mountain Front River, Engineering Science and Technology, 2018, 50(03): 105-111), S mco and S mc1 are the stable slopes of the main channel before and after the incipient bed-load transport, Q mco and Q mc1 are the flow rates of the main channel before and after the incipient bed-load transport, d0 and d1 are the median particle sizes of the riverbed surface before and after the incipient bed-load transport, and the median particle size d 50 is selected as 0.8-1.0 for mountain and mountain front river sections and 0.5-0.8 for midstream river sections. As can be seen from the above formula, S mc1 increases with Q mc1It increases and decreases, and increases with increasing d1. In complex channels, after the erosion base level drops, the mainstream channeling and bed roughening effects cause Q to... mc1 Since d1 increases simultaneously, there exists a critical state where the increase in the main channel flow rate just offsets the effect of the coarsening effect on the stability slope of the main channel, i.e., S. mc1 =S mco Under this critical condition, parallel downcutting occurs in the riverbed, meaning that the coarsened surface layer cannot effectively protect the riverbed, and the median grain size of the riverbed surface layer reaches its limit. Chin et al. (Chin, CO, Melville, BW, & Raudkivi, AJ (1994). Streambed armoring. Journal of Hydraulic Engineering, 120(8), 899-918.) proposed that the median grain size limit of the coarsened surface layer of the riverbed is d. max / 1.8, d max Let d1 = d max Substituting / 1.8 into the above equation, we obtain the critical main channel flow rate Q for the complex channel abrupt change response. mcc Calculation formula:

[0021]

[0022] When the calculated Q mcc >Q0, where Q0 is the total flow in the river channel, means that even if all the water flow were channeled into the channel, it would not reach the critical main channel flow required for a sudden change in the complex river channel response, and therefore a sudden change in response would not occur. When Q mcc If ≤Q0, after the erosion base surface drops, the main channel flow rate may reach or exceed the critical main channel flow rate during the mainstream return process. Further calculation of the erosion base surface boundary drop height is required.

[0023] The purpose of step S3 above is to determine the critical main channel water depth H of the complex channel abrupt response after the erosion base level drops. c Critical main channel width b c ;

[0024] The formula for calculating the stable channel width proposed by Ikeda et al. (Ikeda, S., Parker, G., & Kimura, Y. (1988). Stable width and depth of straight gravel rivers with heterogeneous bed materials. Water resources research, 24(5), 713-722) is as follows under critical conditions:

[0025]

[0026] where U is the average flow velocity in the main channel cross section; k s is the equivalent roughness, according to the assumption of Pitlick et al. (Pitlick, J., Marr, J., & Pizzuto, J. (2013). Width adjustment in experimental gravel-bed channels in response to overbank flows. Journal of Geophysical Research: Earth Surface, 118(2), 553-570), k s = 3d 50 . By combining equation (3) with the calculation model of the flow distribution between the main channel and the floodplain under the critical condition, the critical main channel depth b c can be eliminated, and the critical main channel water depth H c can be obtained:

[0027]

[0028] The Q mcc calculated by S2 is substituted into equation (4) to inversely calculate the critical main channel water depth H c . Then, H c is substituted back into equation (3) to calculate the critical main channel width b c .

[0029] The critical main channel depth h c under the response of the riverbed mutation of the compound river channel is calculated according to the following formula in step S4:

[0030]

[0031] where Q fpc is the critical floodplain flow, which is calculated by the calculation model of the flow distribution between the main channel and the floodplain proposed by Liu et al.: S fp is the floodplain slope, f fp is the floodplain resistance coefficient, and the calculation method is the same as f mc , f fp = 8gn fp 2 / R fp 1 / 3 is the main channel resistance coefficient, n fp is the roughness coefficient of the main channel, R fp is the hydraulic radius of the main channel; B is the total width of the compound river channel; and n fp is the roughness coefficient of the floodplain.

[0032] The critical erosion base surface drop height Δh bc.

[0033] In compound channel, due to the small water depth of the beach, it is usually assumed that the hydrodynamic force is mainly concentrated in the main channel, and the beach slope S fp is constant. The critical main channel depth h c is the difference between the main channel depth h bc :

[0034] Δh bc = h c -h0 (6).

[0035] In summary, when the riverbed composition (median particle size d0, maximum particle size d max ), the main channel shape (water depth H0, width b, depth h0, slope S mc ) and the erosion base surface drop height Δh b are known, the critical erosion base surface drop Δh bc can be calculated, and then it is judged whether the compound channel has a sudden response.

[0036] The application further proposes two other boundary conditions and prediction steps for the sudden response of the riverbed incision of the compound channel, specifically comprising:

[0037] S6 obtains the critical beach channel flow ratio according to the following formula:

[0038]

[0039] In the formula, Q fpc is the critical beach flow, Q c is the total flow of the compound channel when the flow is completely returned to the channel, that is, Q mcc = Q0 corresponding to the total flow of the compound channel;

[0040] S7 obtains the ratio of the critical erosion base surface drop height and the critical beach water depth according to the following formula:

[0041]

[0042] In the formula, H fpc is the beach water depth under the critical state, H fpc = H c -h c ; H fp0 is the beach water depth, which can be obtained by measurement;

[0043] The critical beach flow ratio and the ratio of the critical erosion base surface drop height and the critical beach water depth are used as the critical conditions for the sudden response of the riverbed incision of the compound channel.

[0044] The application further provides a method for predicting the sudden response of the riverbed incision of a compound river channel after the base level of the erosion surface is lowered, wherein the total flow Q0 of the compound river channel and the lowering height of the base level of the erosion surface are measured b According to the prediction method, the critical main channel flow Q mcc and the critical lowering height of the base level of the erosion surface are obtained bc If Q mcc ≤ Q0 and Δh b ≥ Δh bc are simultaneously satisfied, the stable gradient of the main channel is reduced after the base level of the erosion surface is lowered, the incision height of the upstream bed surface is greater than the lowering height of the base level of the erosion surface, and the compound river channel has a sudden response; otherwise, the compound river channel does not have a sudden response.

[0045] In order to facilitate the measurement in engineering, the application further provides another method for predicting the sudden response of the riverbed incision of a compound river channel after the base level of the erosion surface is lowered, wherein the total flow Q0 of the compound river channel and the lowering height of the base level of the erosion surface are measured b The ratio of the flow of the beach to the flow of the channel is calculated and the ratio of the lowering height of the base level of the erosion surface to the critical water depth of the beach is calculated Wherein, Q fp0 is the flow of the beach, and Q fp0 = Q0-Q mc0 ;

[0046] According to the prediction method, the critical ratio of the flow of the beach to the flow of the channel is obtained and the ratio of the lowering height of the base level of the erosion surface to the critical water depth of the beach is obtained If Q fp0 / Q0 ≥ Q fpc / Q c and Δh b / H fp0 ≥ Δj bc / H fpc are simultaneously satisfied, the compound river channel has a sudden response; otherwise, the compound river channel does not have a sudden response.

[0047] Compared with the prior art, the application has the following beneficial effects:

[0048] 1. The application combines the stable gradient calculation formula of the alluvial river and the flow distribution calculation model of the beach and the channel of the compound river channel, and proposes the critical condition of the sudden response of the riverbed incision of the compound river channel after the base level of the erosion surface is lowered, fills the gap in the field, and is of great significance for ensuring the stability of the river regime of the compound river channel in actual engineering.

[0049] 2、The application discloses the regulation of adaptive adjustment and mutation response of a compound river channel after the lowering of an erosion base surface, and can predict the critical lowering height of the erosion base surface of the compound river channel under different frequency floods.

[0050] 3、The application only needs to determine several common morphological indexes in the compound river channel, such as riverbed composition, water depth, width, depth and slope of the main channel, so as to realize the prediction of the critical condition of the sudden response of the compound river channel after the lowering of the erosion base surface, and the method is easy to operate in actual conditions and has wide universality in the field. BRIEF DESCRIPTION OF DRAWINGS

[0051] Figure 1 It is a schematic diagram of a test tank for the lowering of the erosion base surface of a compound river channel.

[0052] Figure 2 It is a schematic diagram of a terminal baffle of a test section for the lowering of the erosion base surface of a compound river channel.

[0053] Figure 3 It is a comparison of the adaptive adjustment and mutation response of the riverbed of a compound river channel after the lowering of the erosion base surface in the water tank test, comparative example 1 (2022) Shitingjiang prototype observation and comparative example 2 (2015) test and the critical condition obtained by the prediction method of the application. DETAILED DESCRIPTION

[0054] The technical solutions of the embodiments of the application will be described in detail below with reference to the drawings. Obviously, the described embodiments are only a part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative work belong to the application.

[0055] EMBODIMENT

[0056] This embodiment details the prediction method of the critical condition of the sudden response of the riverbed of a compound river channel after the lowering of the erosion base surface by water tank simulation test, Shitingjiang prototype observation data and previous water tank test data.

[0057] 1. Test purpose

[0058] The test result of the water tank is used to verify whether the prediction method of the critical condition of the sudden response of the riverbed of a compound river channel after the lowering of the erosion base surface is accurate and effective.

[0059] 2. Test equipment

[0060] The main equipment is shown in Table 1.

[0061] Table 1: Instrument and equipment for the lowering test of the erosion base surface of a compound river channel

[0062]

[0063]

[0064] 3. Test method

[0065] The test was conducted in a 20.7m long, 1.5m wide and 1.0m high straight flume. The test section was 12.5m long, including a 2.5m transition section and a 10.0m effective test section. The first 1.0m of the transition section was 16-25mm gravel, which did not move or coarsen under the maximum test flow conditions, and the last 1.5m was test sand. The effective test section was entirely paved with test sand. The test sand was natural sand with a size range of 0-6mm, a median particle size d 50 = 2.0mm, a maximum particle size d max = 6.0mm, and a standard deviation σ g = 2.90, and the grading curve approximately followed a normal distribution. The right side wall of the flume was made of organic glass, which was used to observe the incision process. The test flume was a partially self-circulating flume, and the flow was circulated in the flume by a centrifugal pump with a flow rate of 30L / s. The flume inlet flow was measured by a rectangular thin weir and controlled by adjusting the drainage valve in the front pool. A still pool and a brick-built stilling wall were provided at the flume inlet to dissipate energy and smooth the flow. The test was conducted with clear water, and the sediment transported downstream was discharged into the sedimentation tank downstream of the flume and did not circulate again.

[0066] A baffle was provided at the end of the test section to simulate changes in the erosion base surface. The baffle included three parts: a 17cm high fixed baffle across the flume to ensure that the bed sand did not erode to the flume floor; an 8cm high beach fixed baffle to provide a constant erosion base surface for the beach; and four 1.5cm high removable movable baffles to simulate the successive lowering of the main channel erosion base surface.

[0067] The study adopted an asymmetric test to simulate the response of a semi-complex river channel. The main channel was pre-set on the organic glass side, with an initial width of 0.5m and a depth of 2cm. In a river channel with a similar width-depth ratio to the actual river, the vertical lateral boundary has little effect on the flow velocity and shear stress distribution in the center of the river channel. In addition, in the test, no significant erosion or deposition occurred in the main channel along the organic glass side, indicating that the organic glass had no significant effect on the flow in the main channel.

[0068] The test process is arranged as follows. Before starting each group test, the mixed sand is evenly laid in the flume, the initial slope of the sand laying is 5 ‰, the sand laying thickness of the beach at the end of the test section is 25 cm, and the sand laying thickness of the main channel is 23 cm. After opening the water, a small flow is maintained for a period of time first to discharge the air in the bed, and then slowly increased to the design flow of 7.5 L / s to shape the initial river bed until the river bed reaches the equilibrium state. After the bed surface of the flume is dry, the bed surface topography and sand gradation are measured. Then, the water is opened to the design flow again, a movable baffle is removed to make the erosion base surface of the river channel main channel drop by 1.5 cm, the flow is kept unchanged until the flume reaches a new equilibrium, and the topography and gradation are measured again. Continue to remove the movable baffle for 3 times to make the erosion base surface of the main channel drop by 3.0 cm, 4.5 cm and 6.0 cm, and measure after each drop respectively. Then, the design flow is changed to 10.0 L / s and 12.5 L / s, and the above experimental steps are repeated.

[0069] The test overview is shown in Table 2.

[0070] Table 2 Summary of parameters of erosion base surface drop test of compound river channel

[0071]

[0072]

[0073] 4. Previous prototype observation and test data

[0074] In order to further verify the accuracy of the critical condition prediction method for the response of the compound river channel to the sudden change of the erosion base surface after the drop provided by the present application, the prototype observation data of Shiting River (Wang Xiaofan, Sediment supply and river bed evolution characteristics of mountainous river under sudden change of erosion base surface, Doctoral dissertation of Sichuan University, 2022) are used to verify the comparative example 1, and the large-scale flume test data (Ma Xudong, Experimental study on the evolution characteristics of the beach channel of the mountainous river after the 5.12 Wenchuan earthquake, Postdoctoral research report of Sichuan University, 2015) are used to verify the comparative example 2. The research methods of the two scholars are introduced below.

[0075] (1) Comparative Example 1

[0076] The study section is a 2.5-kilometer stretch of the Shiting River near Shuangsheng Town, Deyang City. There are four water-related structures: from upstream, the People's Canal through the Shiting River culvert (built in 1956), the Chengdu-Lanzhou Railway Bridge (construction began in 2011), the Chengdu-Mianyang Expressway Bridge (built in 2011), and the Sichuan Provincial Highway 105 Shiting River Bridge (damaged during the 2017 flood season and rebuilt in 2018). Prior to the 2008 Wenchuan earthquake, the study section was relatively stable, with a relatively straight, compound channel. The average channel width B is 350 meters, the main channel width b is 50 meters, the slope of both the main channel and the floodplain is 6.25‰, and the main channel depth j0 is 1.7 meters. Q0 represents the 2-year flood level of the Shiting River at 600 meters. 3 / s, and the measured maximum peak flow rate of 2710m³ 3 / s, corresponding to main channel water depths H0 of 3.17m and 6.15m respectively. The roughness coefficient n of the main channel and the beach is taken as 0.06, and the parameter j is taken as 0.8. The main channel is mainly composed of sand and gravel with a well-developed roughened surface layer, with a particle size between 50 and 200 mm and an average particle size of about 100 mm; the beach is a pebble riverbed with a median particle size of about 300 mm and a maximum particle size of 600-800 mm.

[0077] Researchers used Real-Time Kinematic (RTK) technology to measure the elevation of 10 cross-sections along the studied river section before the flood season in 2009 and after the flood season in 2013, with cross-sections spaced approximately 250 meters apart. Aerial photography of the river section was then conducted using a DJI Phantom 3 drone (with a 12.4-megapixel camera) and a Mavic 2 drone (with a 20-megapixel camera) after the flood seasons in 2016 and 2020, respectively. The aerial photographs were then processed using the image stitching software Agisoft Metashape. Figure Three 3D reconstruction was performed to obtain a digital elevation model (DEM) and digital orthophoto (DOM) of the studied river section. Accuracy was verified using RTK measurement data, with the DEM elevation error within 1m. The main equipment used for prototype observation is shown in Table 3.

[0078] The post-earthquake study shows that the river morphology of the study reach has changed dramatically. The trend of the longitudinal profile from 2009 to 2020 can be divided into three stages: (1) from 2009 to 2013, the erosion base level decreased by 5m, and the upper building was undercut by 15m, 7m and 8m respectively. The average slope of the study reach decreased from 6.25‰ to 3.57‰. (2) From 2013 to 2016, the erosion base level remained stable, and the thalweg of the river changed little. (3) From 2016 to 2020, due to the destruction of GCS downstream of the Shitingjiang Bridge on the 105 line, the erosion base level decreased by 14m in 2017, and the upper building was undercut by 25m, 20m and 22m respectively, and the average slope of the river reach was about 3.97‰. Under the coupling action of flood and sudden drop of erosion base level, the response of the study reach to the undercutting is obviously different from the results of previous single river studies. After the erosion base level drops, the slope of the study reach does not increase but decreases, and the undercutting gradually intensifies in the source development process, resulting in a larger undercutting depth in the upper river than the drop height of the erosion base level, which is consistent with the sudden response mode in the invention.

[0079] Table 3 Instrument and equipment for erosion base level drop test of compound river channel

[0080]

[0081] (2) Comparative Example 2

[0082] The test is based on the measured terrain of the study reach in 2013, simulating a river reach of about 3.3km long from the upstream 800m of the Shitingjiang Bridge on the 105 line of Sichuan Provincial Highway to the Shitengjiang Bridge on the 105 line of Sichuan Provincial Highway. To ensure the consistency of the model test and the prototype, the normal model of the prototype river is used to design the test tank and test scheme. The geometric scale λ l = λ h = 80, the test tank is about 45m long and 6m wide. The width b of the main tank, the depth h0 of the main tank, the slope of the main tank and the beach, the roughness coefficient n of the main tank and the beach, and the parameter j are consistent with those of Comparative Example 1. The test sand is obtained by pusher motion similarity, which is 8-160mm natural sand, the median particle size d 50 = 45.8mm, the maximum particle size d max = 160.0mm, and the standard deviation σ g = 2.02. The test simulates the riverbed evolution trend of the study reach under the action of different frequency floods when the erosion base level height drops, and the undercutting depth of the key water-crossing building. The riverbed evolution law of the study reach under the action of different frequency floods and different erosion base level heights is explored.

[0083] The test overview is shown in Table 4.

[0084] Table 4 Summary of test conditions of Comparative Example 2

[0085]

[0086] 5. Theoretical prediction results

[0087] In the embodiment of the present application, ρ s is the density of the sediment (=2650 kg / m 3 ), ρ is the density of water (=1000 kg / m 3 ), and g is the local gravity acceleration (=9.8 m / s 2 ).

[0088] The water tank test data of the present application, the prototype observation data of Shiting River in Comparative Example 1, and the water tank test data of Comparative Example 2 are brought into steps S1-S5 of the critical condition prediction method for the sudden response of the riverbed incision of a compound river channel after the lowering of the erosion base surface, to normalize the critical beach slot flow rate ratio ((Q fp0 / Q0) / (Q fpc / Q c )) and the critical erosion base surface lowering height ((Δh b / H fp0 ) / (Δh bc / H fpc )) under the critical beach slot flow rate ratio as dimensionless parameters, and the calculation results are shown in Table 1. Figure 3 It can be seen that the calculated critical condition for the sudden response of the riverbed incision of a compound river channel has good prediction effects on both the model test and the natural river, which indicates that the critical condition prediction method for the sudden response of the riverbed incision of a compound river channel after the lowering of the erosion base surface can accurately predict the critical erosion base surface lowering height under different frequency floods.

[0089] Those skilled in the art will realize that the embodiments described herein are for the purpose of aiding the reader in understanding the principles of the present application and should be construed as not limiting the scope of the present application to such specific embodiments and examples. Those skilled in the art can make various other specific modifications and combinations according to the technical disclosures of the present application without departing from the spirit of the present application, and these modifications and combinations are still within the scope of the present application.

Claims

1. A method for predicting the critical condition of response of incised channel bed incision sudden change after base level fall, characterized in that, The method comprises the following steps: S1 determines the main channel flow Q of the compound river channel mco ; S2 determines the critical main channel flow Q of the sudden response of the compound river channel bed after the base level of erosion falls mcc ; S3 determines the critical main channel water depth H of the sudden response of the compound river channel after the base level of erosion falls c , the critical main channel width b c ; S4 The critical main groove water depth H is determined according to the following formula: c S5 The critical main groove width b is determined according to the following formula: c S6 The critical main groove depth h is determined according to the following formula: c h = 0.5b where Q fpc is the critical floodplain flow, S fp is the floodplain slope, f fp is the floodplain friction coefficient; B is the total width of the compound channel; n fp is the roughness coefficient of the floodplain; S5 with critical main groove depth h c Difference from main groove depth h0 as critical erosion face base surface lowering height Δh bc ; The critical main channel flow rate Q mcc And the critical erosion surface base surface drop height Δh bc As a complex river channel bed incision mutation response critical condition.

2. The method according to claim 1, wherein the method is characterized by, In step S1, Q mco The result is calculated according to the following formula: where b is the main channel width, H0 is the main channel water depth, f mc = 8gn mc 2 / R mc 1 / 3 is the main channel resistance coefficient, n mc is the main channel roughness coefficient, R mc is the main channel hydraulic radius, S mc is the main channel slope, and g is the local gravitational acceleration.

3. The method according to claim 1 or 2, characterized in that, In step S2, the critical main channel flow rate Q of the compound river channel sudden response mcc The calculation formula is as follows: where j is an empirical parameter; d0is the median grain size in the riverbed surface layer before the base level falls; d max is the maximum grain size of the bed load.

4. The method according to claim 1, wherein the method is characterized by, In step S3, The critical main channel width b of the complex river channel bed sudden change response is calculated according to the following formula c : where U is the average flow velocity in the main channel cross section; k s is the equivalent roughness; The critical main channel water depth H of the complex river channel bed sudden change response is calculated according to the following formula c :

5. The method according to claim 1, wherein the method is characterized by: In step S5, the critical incision base surface drop height Δh of the compound river channel riverbed mutation response bc : Δh bc = h c - h0 (6).

6. The method according to claim 1, wherein the method is characterized by: Further comprising: S6 obtains the critical ratio of the beach groove flow according to the following formula: where Q fpc is the critical floodplain flow, Q c is the total flow at which the sudden response occurs when the flow in the compound channel is fully channeled; and j is an empirical parameter. S7 obtains the ratio of the critical erosion base surface drop height and the critical beach water depth according to the following formula: where H fpc is the water depth on the beach at the critical condition, H fpc = H c - h c ; H fp0 is the water depth on the beach; The critical bankfull discharge ratio and the critical ratio of incised base level lowering to bankfull depth as a critical condition for abrupt response of the river bed in braided channels 7. A method for predicting the abrupt change in the downcut response of a complex riverbed after erosion base level decline, characterized in that, The total flow Q0 and the erosion surface base surface drop height Δh of the compound river channel are measured b The critical main channel flow Q and the critical erosion surface base surface drop height Δh are obtained according to the prediction method in any one of claims 1 to 5 mcc bc If Q0≥Q mcc and Δh b ≥Δh bc are simultaneously satisfied, the compound river channel has a catastrophic response; otherwise, the compound river channel does not have a catastrophic response.​ 8. A method for predicting the abrupt change in the downcut response of a complex riverbed after erosion base level decline, characterized in that, The total flow Q0 and the height of the falling erosion base Δh are measured b The flow ratio of the bar and groove is calculated The ratio of the falling erosion base and the critical water depth of the bar is calculated Wherein, Q fp0 = Q0-Q mc0 ; The critical bankfull discharge ratio obtained by the prediction method of claim 6 and the ratio of the critical erosion base level drop height to the critical floodplain water depth If Q fp0 / Q0≥Q fpc / Q c , and Δh b / H fp0 ≥Δh bc / H fpc , then the compound channel has a sudden response; otherwise, the compound channel does not have a sudden response.

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