CN115630507A - Prediction method for critical condition of compound river bed undercut mutation response after erosion basal plane is reduced - Google Patents

Prediction method for critical condition of compound river bed undercut mutation response after erosion basal plane is reduced Download PDF

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CN115630507A
CN115630507A CN202211319284.1A CN202211319284A CN115630507A CN 115630507 A CN115630507 A CN 115630507A CN 202211319284 A CN202211319284 A CN 202211319284A CN 115630507 A CN115630507 A CN 115630507A
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聂锐华
刘超
王小凡
王路
刘兴年
黄尔
马旭东
马铭悦
杨克君
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Abstract

The invention discloses a method for predicting the response critical condition of a compound riverway bed undercut mutation after an erosion basal plane is lowered. The invention combines a alluvial river stable slope calculation formula and a compound river channel beach groove flow distribution calculation model, provides a critical condition of compound river channel riverbed undercutting abrupt response after the erosion basal plane is lowered, can predict the lowering height of the compound river channel critical erosion basal plane under different frequency floods, fills the blank of the technology in the field, and has important significance for guaranteeing the stability of the river situation of the compound river channel in the actual engineering.

Description

Prediction method for critical condition of compound river bed undercut mutation response after erosion basal plane is reduced
Technical Field
The invention belongs to the field of hydraulics and river dynamics, relates to a prediction technology of a sudden change response critical condition of a riverbed of a compound riverway, and particularly relates to a prediction method of a sudden change response critical condition of a downward cutting riverbed of the compound riverway after an erosion basal plane is reduced.
Background
In recent years, with global climate change, frequent natural disasters and increased human activities, the river erosion basal plane is in sudden change more and more frequently. The erosion floor of a river is generally below the elevation of the bottom end of the building in the river. Under the action of natural factors (construction effect, climate factors, environmental factors and the like) or artificial factors (dam dismantling, river channel sand mining, water resource development and utilization and the like), the erosion basal plane of the river relatively descends, the hydraulic gradient increases, the source-tracing erosion of the river bed is caused, and the local river bed is eroded and continuously erodes upstream.
The study of the scholars on the evolution mechanism of the riverbed after the erosion basal plane is reduced considers that the coarsening effect enables the final stable gradient of the riverway to be larger than that before the erosion basal plane is reduced, the undercut depth is gradually reduced towards the upstream, and the traceable scouring can only influence a limited river reach, namely riverway adaptive adjustment. However, some flood beach channels have quite different laws after the erosion base level is lowered, the final stable gradient of the channel is smaller than that before the erosion base level is lowered, namely the undercutting depth of the upstream riverbed is even larger than the lowering height of the erosion base level, and the upstream scouring continuously develops upstream, which is called the undercutting abrupt response of the channel, and the river is greatly damaged. For example, the erosion base surface of the double prime section of Shichengjiang river (Tuojiang tributary) in Sichuan province is reduced by 5 meters in the 2013 flood season, and the undercutting depth of a river channel which is only 2.5 kilometers upstream from the erosion base surface reaches more than 10 meters. In 2017, the elevation of the erosion base surface of the river reach is continuously reduced by 14 meters, and the maximum undercut depth of the upstream river reaches more than 20 meters. The foundation of two bridges in the river reach is damaged due to severe scouring undercut caused by the descending erosion basal plane; meanwhile, the culvert (drinking water delivery pipe) penetrated by the civil canal in the river channel is seriously damaged, and the drinking water safety of millions of people in the city of Mianyang province in Sichuan is threatened.
Most of natural alluvial rivers are compound riverways with a main groove and beaches, water flows mainly flow in the main groove in a dry season, and when flood occurs, the water flows over the beaches on two sides of the main riverway to form flood beach water flows. In addition, in river regulation projects, people also often adopt compound sections to achieve a better flood control effect. If the compound river channel has abrupt response after the erosion basal plane is reduced, the river bed is seriously washed and cut down, and the river morphology is changed violently and even completely remodeled. The method not only seriously threatens the safety and stability of buildings such as river-related bridges and the like and river bank protection structures, but also causes the river and the underground water level to be reduced, thereby influencing the shipping, port operation and water intake efficiency. Meanwhile, the method also can significantly affect the river habitat, reduce the heterogeneity of the living habitat and reduce the biodiversity of the ecological system, thereby causing the loss of ecological stability.
Therefore, the correct understanding of the evolution mechanism of the riverbed of the compound riverway after the erosion basal plane is reduced not only has important theoretical value, but also has wide practical application value in the aspects of water conservancy, traffic, ecology and the like. However, at present, no method capable of accurately predicting the critical response condition of the undercutting mutation of the riverbed of the compound riverway after the erosion basal plane is reduced exists at home and abroad.
Disclosure of Invention
Aiming at the technical current situation that the critical response condition of the compound river bed undercut mutation after the erosion base surface is reduced is difficult to effectively predict at present, the invention aims to provide a prediction method to realize effective prediction of the critical response condition of the compound river bed undercut mutation after the erosion base surface is reduced.
The invention aims at a compound river channel, which consists of a main trough (namely a main river channel) and two lateral beaches. In the dry period, water flows mostly in the main groove, in the flood period, the water flows over the main groove to overflow the wider beaches at the two sides, and the main groove and the beaches flow over together. The depth of the main groove refers to the height from the bottom of the main riverway riverbed to the bottoms of the beaches at two sides.
The invention provides a method for predicting the critical condition of compound river bed undercut mutation response after erosion basal plane decline, which comprises the following steps:
s1 determining flow Q of main channel of compound river channel mco ；
S2, determining critical main tank flow Q of sudden change response of riverbed of the compound riverway after erosion basal plane is reduced mcc ；
S3, determining critical main trough water depth H of sudden change response of compound river channel after erosion basal plane is reduced c Critical main slot width b c ；
S4, according to the critical main tank water depth H c Critical main slot width b c Determining the critical main groove depth h according to the following formula c ：
Figure BDA0003909649560000021
In the formula, Q fpc The flow rate of the critical tidal flat is the critical flow rate,
Figure BDA0003909649560000022
S fp slope of beach land, f fp The calculation method is the same as f for the beach resistance coefficient mc (ii) a B is the total width of the compound river channel; n is a radical of an alkyl radical fp The roughness coefficient of the beach land;
s5 is criticalDepth h of main groove c Depth h from main groove 0 The difference is used as the critical erosion basal plane drop height Deltah bc ；
The critical main tank flow Q mcc And critical erosion basal plane descent height Δ h bc Namely the critical response condition of the down-cut mutation of the riverbed of the compound riverway.
In step S1, before the erosion basal plane is lowered, the compound river is considered to be in an equilibrium state. Q mco Liu et al (Liu, C., shan, Y., liu, X.),&the calculation model of beach trough flow distribution proposed by Yang, K.,2016, method for associating discharge in networking channels in Proceedings of the institute of Civil Engineers-Water Management,169 (1): 17-29, 2016) calculates:
Figure BDA0003909649560000023
wherein b is the width of the main groove H 0 Is the main tank depth, f mc ＝8gn mc 2 /R mc 1/3 Is the coefficient of resistance of the main channel, n mc Is the roughness coefficient of the main groove, R mc Is the hydraulic radius of the main channel, approximately the water depth in natural rivers, S mc Is the main tank slope and g is the local gravitational acceleration.
In step S2, before and after the erosion basal plane is lowered, the main tank stabilizing gradient can be expressed as:
Figure BDA0003909649560000031
(Niezuelua, et al, study on evolution of river bed in pre-mountain river with damaged strong earthquake, engineering science and technology, 2018, 50 (03): 105-111), S mco And S mc1 The main trough stabilizing gradient, Q, before and after the decline of the erosion base mco And Q mc1 Flow of main channel before and after the decline of erosion base, d 0 And d 1 Selecting median particle diameter d of surface layer before and after the erosion base surface is decreased 50 J is an empirical parameter and is determined by the position of the river reach, 0.8 to 1.0 percent of the river reach of the mountain area and the foot and 0.5 to 0.8 percent of the river reach of the midstream. From the above formula, S mc1 With Q mc1 Increase and decrease with d 1 And increases with an increase. In the compound river channel, after the erosion basal plane is reduced, the main flow returning groove and the bed surface coarsening function lead Q mc1 And d 1 And simultaneously, the flow rate of the main groove is increased, so that a critical state exists, and the influence of the coarsening effect on the stable gradient of the main groove is just counteracted by the increase of the flow rate of the main groove, namely S mc1 ＝S mco . Under the critical condition, the river channel is cut downwards in parallel, which means that the coarsened surface layer cannot effectively protect the riverbed, and the median diameter of the riverbed surface layer reaches the limit. Chi et al (chi, c.o., melville, b.w.,&raudkivi, A.J. (1994), stream armoring, journal of Hydraulic Engineering,120 (8), 899-918) have proposed that the median particle size limit of a roughened surface layer of a riverbed is d max /1.8，d max Is the maximum particle size of the particles in the bed sand. Will d 1 ＝d max The 1.8 is brought into the above formula and has critical main tank flow Q of compound river channel abrupt change response mcc Calculating the formula:
Figure BDA0003909649560000032
when calculated Q mcc >Q 0 ，Q 0 The total flow in the river channel is that the water flow can not reach the critical main channel flow of the compound river channel abrupt change response even if all the water flows return to the channel, and the abrupt change response can not occur. When Q is mcc ≤Q 0 After the erosion basal plane is decreased, the main trough flow in the main flow trough returning process may reach or exceed the critical main trough flow, and the critical decreasing height of the erosion basal plane is further calculated.
The step S3 aims to determine the critical main trough water depth H of the sudden change response of the compound river channel after the erosion basal plane is reduced c Critical main slot width b c ；
Ikeda et al (Ikeda, S., parker, G., kimura, Y. (1988) Stable widths and depth of straight gradient with terrestrial materials Water resources research,24 (5), 713-722) set forth the following equations at critical conditions:
Figure BDA0003909649560000033
in the formula, U is the average flow velocity of the section of the main tank; k is a radical of formula s For equivalent roughness, the grain size is determined according to Pitlick et al (Pitlick, j., mar, j.,&pizzuto, J. (2013). Width adjustment in experimental gradient-bed channels in response to overflash flows. Journal of geographic Research: earth Surface,118 (2), 553-570), assumption of s ＝3d 50 . Combining the formula (3) with a beach tank flow distribution calculation model under the critical condition, and eliminating the critical main tank depth b c Obtaining the critical main trough water depth H c Calculating the formula:
Figure BDA0003909649560000041
q calculated from S2 mcc The depth H of the critical main tank can be calculated reversely by the drive-in type (4) c Then, H is introduced c The critical main slot width b can be obtained by the formula (3) c 。
Step S4, calculating the critical main groove depth h of the sudden change response of the riverbed of the compound riverway according to the following formula c ；
Figure BDA0003909649560000042
In the formula, Q fpc For critical beach traffic, the beach trough traffic distribution calculation model proposed by Liu et al calculates:
Figure BDA0003909649560000043
S fp is the slope of the beach land, f fp For the drag coefficient of the beach land, the calculation method is the same as f mc ，f fp ＝8gn fp 2 /R fp 1/3 Is the coefficient of resistance of the main channel, n fp Is the roughness coefficient of the main groove, R fp Is the hydraulic radius of the main trough; b is the total width of the compound river channel; n is fp The roughness coefficient of the beach.
The step S5 determines the compound river channel after the erosion basal plane is reducedBed abrupt response critical erosion basal plane descent height delta h bc 。
In a compound river channel, due to the fact that the depth of water in the beach land is small, generally, the hydrodynamic force is mainly concentrated on the main trough, and the slope S of the beach land fp And is not changed. Critical main groove depth h c Depth h from main groove 0 The difference is the critical erosion basal plane drop height delta h of the compound riverway riverbed undercutting abrupt response bc ：
Δh bc ＝h c -h 0 (6)。
In summary, when the riverbed composition (median diameter d) in the compound riverway is 0 Maximum particle diameter d max ) Main trough shape (water depth H) 0 Width b, depth h 0 Slope S mc ) And erosion basal descent height Δ h b The critical erosion base level drop Δ h can be calculated under known conditions bc And then judging whether the compound river channel has mutation response or not.
The invention further provides another two boundary conditions and prediction steps of the compound riverway bed undercut mutation response, which specifically comprise the following steps:
s6, obtaining the flow ratio of the critical beach tank according to the following formula:
Figure BDA0003909649560000044
in the formula, Q fpc Is the critical beach flow, Q c The total flow rate, namely Q, of the composite river channel with the sudden change response just happened when the water flow is completely returned to the channel mcc ＝Q 0 The total flow of the compound riverway is corresponding;
s7, obtaining the ratio of the critical erosion base surface descending height to the critical beach water depth according to the following formula:
Figure BDA0003909649560000051
in the formula, H fpc Is the beach water depth H in the critical state fpc ＝H c -h c ；H fp0 The depth of water in the beach can be measuredObtaining the quantity;
at the critical tidal flat flow rate ratio
Figure BDA0003909649560000052
And the ratio of the critical erosion basal plane descent height to the critical beach water depth
Figure BDA0003909649560000053
The method is used as a critical condition for response of the compound riverbed undercutting mutation.
The invention further provides a prediction method for the down-cut mutation response of the riverbed of the compound riverway after the erosion basal plane is lowered, and the total flow Q of the compound riverway is measured 0 And the basal plane descent height Deltah of the erosion surface b Obtaining the critical main tank flow Q according to the prediction method mcc And critical erosion surface basal plane descent height delta h bc If Q is satisfied simultaneously mcc ≤Q 0 And Δ h b ≥Δh bc After the erosion basal plane descends, the stable gradient of the main groove is reduced compared with the stable gradient, the undercut height of the upstream bed surface is greater than the descending height of the erosion basal plane, and the compound river channel has abrupt response; otherwise, the compound riverway does not have mutation response.
In order to facilitate the measurement in engineering, the invention further provides another prediction method for the switching-off and sudden change response of the riverbed of the compound riverway after the erosion basal plane is reduced, and the total flow Q of the compound riverway is measured 0 And the basal plane descent height Deltah of the erosion surface b And calculating to obtain the flow ratio of the beach tank
Figure BDA0003909649560000054
And the ratio of the erosion basal plane descending height to the critical beach water depth
Figure BDA0003909649560000055
Wherein Q is fp0 For tidal land flow, Q fp0 ＝Q 0 -Q mc0 ；
Obtaining the critical tidal flat flow ratio according to the prediction method
Figure BDA0003909649560000056
And critical erosion baseRatio of surface descent height to critical beach water depth
Figure BDA0003909649560000057
If Q is satisfied simultaneously fp0 /Q 0 ≥Q fpc /Q c And Δ h b /H fp0 ≥Δj bc /H fpc If so, carrying out mutation response on the compound riverway; otherwise, the compound riverway does not have mutation response.
Compared with the prior art, the invention has the following beneficial effects:
1. the invention combines a alluvial river stability slope calculation formula and a compound river channel beach groove flow distribution calculation model, provides a critical condition of compound river channel riverbed undercutting mutation response after the erosion basal plane is reduced, fills the blank of the technology in the field, and has important significance for ensuring the compound river channel river stability in the actual engineering.
2. The method disclosed by the invention starts from the limit states of the increase of the main groove flow and the coarsening of the bed surface of the compound riverway, reveals the adaptability adjustment and the abrupt change response rule of the compound riverway after the erosion basal plane is reduced, and can predict the critical erosion basal plane reduction height of the compound riverway under floods with different frequencies.
3. The invention only needs to determine a few common morphological indexes in the compound river channel: the river bed composition, the depth, the width, the depth, the gradient and the like of the main trough can predict the critical response conditions of the compound river channel undercut mutation after the erosion base surface is reduced, the operation is easy in practical situations, and the method has wide universality in the field.
Drawings
Figure 1 is a schematic view of a compound river erosion floor descent test flume.
Fig. 2 is a schematic diagram of a tail end baffle of a compound riverway erosion basal plane descent test segment.
Fig. 3 is a comparison of the adaptation and mutation response of the riverbed of the compound riverway after the erosion basal plane is reduced in the water tank test of the invention, the observation of the stone pavilion river prototype of the comparative example 1 (2022) and the test of the comparative example 2 (2015) with the critical conditions obtained by the prediction method of the invention.
Detailed Description
The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings, and it is to be understood that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, belong to the present invention.
Examples
In this embodiment, the method for predicting the critical condition of the compound river channel riverbed undercut sudden change response after the erosion basal plane is decreased is explained in detail through a water tank simulation test, observation data of a shiting river prototype and test data of a predecessor water tank.
1. Purpose of the experiment
And (4) testing whether the prediction method of the critical condition of the undercutting mutation response of the riverbed of the compound riverway is accurate and effective after the erosion basal plane is reduced by using a water tank test result.
2. Test apparatus
The main equipment is shown in table 1.
TABLE 1 Instrument and Equipment for compound river erosion basal plane descent test
Figure BDA0003909649560000061
Figure BDA0003909649560000071
3. Test method
The test was carried out in a straight circulating water bath having a length of 20.7m, a width of 1.5m and a height of 1.0 m. The test section of the water tank is 12.5m in length and comprises a transition section of 2.5m and an effective test section of 10.0 m. The gravel with the diameter of 16-25 mm is arranged 1.0m in front of the transition section, the gravel is not started and coarsened under the condition of the maximum test water flow, and the sand for the test is arranged 1.5m behind the transition section. And all the effective test sections are paved with test sand. The sand for test is 0-6 mm natural sand with median diameter d 50 =2.0mm, maximum particle diameter d max =6.0mm, standard deviation σ g =2.90, the grading curve approximately follows a normal distribution. Sink right sideThe wall is made of organic glass and is used for observing the undercutting process. The test water tank is a local self-circulation water tank, water flow circulates in the water tank through a centrifugal pump, and the pump flow is 30L/s. The inlet flow of the water tank is measured by a rectangular thin-wall weir and is controlled by adjusting a drainage valve in the forebay. A stilling pool and a stilling wall built by bricks are arranged at the inlet of the water tank and used for dissipating energy and smoothing water flow. The test is that the silt transported to the downstream enters a desilting basin at the downstream of the water tank and does not circulate.
The end of the test section was equipped with baffles to simulate the change in the erosion floor. The baffle includes three parts: a fixed baffle plate with the height of 17cm and crossing the water tank ensures that bed sand cannot erode to a bottom plate of the water tank; a beach land fixing baffle plate with the height of 8cm provides a invariable erosion base surface for the beach land; four removable flaps 1.5cm high were used to simulate the historical decline of the main trough erosion floor.
An asymmetric test is adopted to simulate the response of a half-edge compound river channel, a main groove is preset on one side of organic glass, the initial width of the main groove is 0.5m, and the depth of the main groove is 2cm. In the river channel with the width-depth ratio similar to that of an actual river, the vertical side boundary has little influence on the central flow velocity and the shear stress distribution of the river channel, and in addition, in the test, the main groove does not have obvious scouring or silting along one side of the organic glass, which shows that the organic glass has no obvious influence on the water flow of the main groove.
The experimental procedure was arranged as follows. Before each group of tests, the evenly mixed silt is paved into a water tank, the initial slope of the paved silt is 5 per mill, the beach sand paving thickness at the tail end of the test section is 25cm, and the main tank sand paving thickness is 23cm. After boiling water, keeping a period of small flow, exhausting air in the bed surface, slowly increasing the flow to 7.5L/s, and molding the initial river bed until the river bed reaches an equilibrium state. And measuring the topography and the sand gradation of the bed surface when the bed surface in the water tank is dry. Then, boiling water again to the designed flow, removing a movable baffle plate to enable the erosion basal plane of the main channel of the river channel to be reduced by 1.5cm, keeping the flow unchanged until the water channel reaches new balance, and measuring the terrain and the gradation again. The flapper was continuously removed 3 times, dropping the main trough erosion floor by 3.0cm, 4.5cm and 6.0cm, measured after each drop. Thereafter, the design flow rates were varied to 10.0L/s and 12.5L/s, and the experimental procedure described above was repeated.
The test profiles are shown in table 2.
TABLE 2 summary of Compound river erosion basal plane descent test parameters
Figure BDA0003909649560000072
Figure BDA0003909649560000081
4. Observation and test data of predecessor prototype
In order to further test the accuracy of the method for predicting the sudden change response critical condition of the compound river channel after the decline of the erosion basal plane, the method adopts stone pavilion river prototype observed data (Wangcaifan, silt supplement to the research on the evolution characteristics of the river bed of the river in the mountain area under the sudden change of the erosion basal plane, doctor's academic thesis of Sichuan university, 2022) and Dabichi water tank test data (Ma Xudong, 5.12 research on the evolution characteristics of the river beach tank in the mountain front area after the earthquake of Wenchuan, a research work report after the doctor of Sichuan university, 2015) for verification in comparative example 1 and comparative example 2. The study methods of two scholars are described in detail below.
(1) Comparative example 1
The research river segment is a river segment with the length of about 2.5 kilometers of a stone pavilion river near the double prosperity town of Germany and yang, and 4 wading buildings are respectively arranged from the upstream to a civil canal stone pavilion river culvert (built in 1956), a blue railway bridge (built in 2011), a finished-cotton compound line highway bridge (built in 2011) and a Sichuan province 105-line stone pavilion river bridge (damaged in 2017 and rebuilt in 2018). The research river reach is in a stable state for a long time before 5.12 Wenchuan earthquake in 2008, the river channel is a relatively straight and compound river channel, the average width B of the river channel is 350m, the width B of the main groove is 50m, the slopes of the main groove and the beach land are both 6.25 per thousand, and the depth j of the main groove is j 0 Is 1.7m. Q 0 Respectively 600m flood in 2 years for Shiting Jiang 3 S, and measured maximum peak flow 2710m 3 S, corresponding main tank water depth H 0 Are respectively 3.17mAnd 6.15m. The coefficient n of the roughness of the main trough and the beach is 0.06, and the parameter j is 0.8. The main groove mainly takes sand and pebbles, and has a well-developed coarsened surface layer, the grain diameter is between 50 and 200mm, and the average grain diameter is about 100mm; the beach land is a pebble river bed, the median particle size of the bed surface is about 300mm, and the maximum particle size of the bed surface reaches 600-800 mm.
Researchers used Real-Time Kinematic (RTK) technology to measure 10 section elevations of the research river reach along the course before 2009 flood and after 2013 flood, and the section intervals are about 250m. And after 2016 and 2020 flood, aerial photography is carried out on the river segment under study by using a Xinntom 3 unmanned aerial vehicle (a camera has 1240 ten thousand effective pixels) and a Mavic 2 unmanned aerial vehicle (2000 ten thousand effective pixels) respectively. And performing multi-view three-dimensional reconstruction on the aerial photo through image splicing software Agisosoft Metashape, and acquiring a Digital Elevation Model (DEM) and a digital ortho-image (DOM) of the researched river reach. And performing precision checking by using RTK measurement data, wherein the DEM elevation error is within 1 m. Prototype observation main equipment is shown in table 3.
The trend of longitudinal section change after severe change in 2009-2020 of river channel morphology study after earthquake is generally divided into three stages: (1) In 2009-2013, the elevation of the erosion basal plane is lowered by 5m, and the upstream buildings are cut down by 15m, 7m and 8m respectively. The average slope of the river reach is reduced from 6.25 per thousand to 3.57 per thousand under study. (2) In 2013-2016, the erosion base surface is kept stable, and the river deep body line changes little. (3) In 2016-2020, due to the damage of the GCS at the downstream of the 105-line Shixingjiang bridge, the erosion base level is reduced by 14m in 2017, the up-stream buildings are cut by 25m, 20m and 22m in 2020, and the average gradient of the river reach is about 3.97 per thousand. Under the coupling action of flood and erosion basal plane sudden drop, the research result of the undercut response of the river reach is obviously different from the previous research result of a single river channel. After the erosion basal plane is decreased, the slope of the river channel at the river reach is researched not to increase or decrease, and the undercutting is gradually intensified in the tracing development process, so that the undercutting depth of the upstream river channel is larger than the decreasing height of the erosion basal plane, and the method is consistent with a sudden change response mode in the invention.
TABLE 3 Instrument and Equipment for compound river erosion basal plane descent test
Figure BDA0003909649560000091
(2) Comparative example 2
The test is based on actual measurement terrain in 2013 of the researched river reach, and the simulated area covers the river reach of the double-full section of the stone pavilion river from the civil canal to the upstream of the stone pavilion river culvert of 800m to 105 lines of the Sichuan province road, wherein the length of the bridge of the stone pavilion river is about 3.3 km. In order to ensure the consistency between the model test and the prototype, the test adopts the normal model of the prototype river to design the test water tank and the test scheme. Geometric scale lambda l ＝λ h =80, the test flume was about 45m long and 6m wide. Width b and depth h of main groove 0 The main trough and beach slope, main trough and beach roughness coefficient n, parameter j are all the same as in comparative example 1. The test sand is obtained by the similar movement of bed load, 8-160 mm natural sand is adopted, and the median grain diameter d 50 =45.8mm, maximum particle diameter d max =160.0mm, standard deviation σ g =2.02. The experiment simulates the research of the riverbed evolution trend of the river reach and the undercut depth of key wading buildings under the action of floods with different frequencies when the elevation of the erosion base plane suddenly drops, and the research of the riverbed evolution law of the river reach under the action of floods with different frequencies and the elevations of different erosion base planes is carried out.
The test profiles are shown in table 4.
Table 4 summary of test conditions of comparative example 2
Figure BDA0003909649560000101
5. Theoretical prediction results
In the embodiment of the invention, rho s Is the density of silt (= 2650 kg/m) 3 ) ρ is the density of water (= 1000 kg/m) 3 ) G is the local gravity acceleration (= 9.8 m/s) 2 )。
The water tank test data, the stone pavilion river prototype observation data in the comparative example 1 and the water tank test data in the comparative example 2 are introduced into steps S1 to S5 of the prediction method of the critical condition of the compound river channel riverbed undercut mutation response after the erosion basal plane is reduced, and the critical beach tank flow ratio ((Q) fp0 /Q 0 )/(Q fpc /Q c ) ) and critical beach flowCritical erosion base reduction height at volume ratio ((Δ h) b /H fp0 )/(Δh bc /H fpc ) Normalized as a dimensionless parameter and calculated as in figure 3. Therefore, the critical condition of the compound river channel bed undercut mutation response calculated by the method has a good prediction effect on both model tests and natural rivers, and the method for predicting the critical condition of the compound river channel bed undercut mutation response after the erosion base surface is reduced can accurately predict the reduction height of the critical erosion base surface of the compound river channel under the flood of different frequencies.
It will be appreciated by those of ordinary skill in the art that the embodiments described herein are intended to assist the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited embodiments and examples. Those skilled in the art can make various other specific changes and combinations based on the teachings of the present invention without departing from the spirit of the invention, and these changes and combinations are within the scope of the invention.

Claims (8)

1. A method for predicting the critical condition of compound river bed undercut mutation response after erosion basal plane decline is characterized by comprising the following steps:
s1 determining flow Q of main channel of compound river channel mco ；
S2, determining critical main groove flow Q of sudden change response of river bed of compound river channel after erosion base surface is reduced mcc ；
S3, determining critical main trough water depth H of sudden change response of compound river channel after erosion basal plane is reduced c Critical main slot width b c ；
S4, according to the critical main tank water depth H c Critical main slot width b c Determining the critical main groove depth h according to the following formula c ：
Figure FDA0003909649550000011
In the formula, Q fpc The flow rate of the critical tidal flat is the critical flow rate,
Figure FDA0003909649550000012
S fp is the slope of the beach land, f fp The coefficient of drag of the beach land; b is the total width of the compound river channel; n is fp The roughness coefficient of the beach land;
s5, critical main groove depth h c Depth h from main groove 0 The difference is used as the critical erosion surface basal plane descent height delta h bc ；
The critical main tank flow Q mcc And critical erosion surface basal plane descent height delta h bc The method is used as a critical condition for response of the compound riverbed undercutting mutation.
2. The method for predicting critical conditions for the undercutting abrupt response of the riverbed of the compound riverway after erosion basal plane decline according to claim 1, wherein in the step S1, Q mco Calculated according to the following formula:
Figure FDA0003909649550000013
wherein b is the width of the main groove, H 0 Is the main tank depth, f mc ＝8gn mc 2 /R mc 1/3 Is the main channel resistance coefficient, n mc Is the roughness coefficient of the main groove, R mc Is the hydraulic radius of the main channel, S mc Is the main tank slope and g is the local gravitational acceleration.
3. The method for predicting the critical condition of the compound river channel riverbed undercut sudden change response after the erosion basal plane is lowered according to claim 1 or 2, wherein in the step S2, the critical main tank flow Q of the compound river channel sudden change response is provided mcc The calculation formula is as follows:
Figure FDA0003909649550000014
in the formula, j is an empirical parameter; d 0 Front bed for erosion basal descentThe median diameter of the surface layer; d max Is the maximum particle size of the particles in the bed sand.
4. The method for predicting critical conditions for the undercutting abrupt response of the riverbed of the compound riverway after erosion basal plane decline according to claim 1, wherein in step S3,
calculating the critical main groove width b of the sudden change response of the riverbed of the compound riverway according to the following formula c ：
Figure FDA0003909649550000015
In the formula, U is the average flow velocity of the section of the main tank; k is a radical of s Is an equivalent roughness;
calculating the critical main channel water depth H of the sudden change response of the riverbed of the compound riverway according to the following formula c ：
Figure FDA0003909649550000021
5. The method for predicting the critical condition of the compound river bed undercutting abrupt change response after the erosion basal plane is lowered according to claim 1, wherein in the step S5, the critical erosion basal plane lowering height delta h of the compound river bed abrupt change response is bc ：
Δh bc ＝h c -h 0 (6)。
6. The method for predicting critical conditions of compound riverway riverbed undercut transition response after erosion basal plane decline according to claim 1, further comprising:
s6, obtaining a critical beach tank flow ratio according to the following formula:
Figure FDA0003909649550000022
in the formula, Q fpc Is the critical beach flow, Q c The total flow rate is the total flow rate of the sudden change response just happened when the water flow of the compound riverway completely returns to the tank; j is an empirical parameter;
s7, obtaining the ratio of the critical erosion basal plane descending height to the critical beach water depth according to the following formula:
Figure FDA0003909649550000023
in the formula, H fpc Is the beach water depth H in the critical state fpc ＝H c -h c ；H fp0 The depth of the beach water;
at critical beach trough flow ratio
Figure FDA0003909649550000024
And the ratio of the critical erosion basal plane descent height to the critical beach water depth
Figure FDA0003909649550000025
The method is used as a critical condition for response of the compound riverbed undercutting mutation.
7. A prediction method for compound river bed undercut mutation response after erosion basal plane decline is characterized in that the total flow Q of the compound river bed is obtained through measurement 0 And the basal plane descent height Deltah of the erosion surface b The critical main tank flow Q is obtained by the prediction method according to any one of claims 1 to 5 mcc And critical erosion surface basal plane descent height delta h bc If Q is satisfied simultaneously 0 ≥Q mcc And Δ h b ≥Δh bc If so, carrying out mutation response on the compound riverway; otherwise, the compound riverway does not have mutation response.
8. A prediction method for compound river bed undercut mutation response after erosion basal plane decline is characterized in that the total flow Q of the compound river is measured 0 And the basal plane descent height Deltah of the erosion surface b And calculating to obtain the flow ratio of the beach tank
Figure FDA0003909649550000026
And the ratio of the erosion basal plane descending height to the critical beach water depth
Figure FDA0003909649550000027
Wherein Q is fp0 ＝Q 0 -Q mc0 ；
Obtaining the critical beach tank flow ratio according to the prediction method of claim 6
Figure FDA0003909649550000028
And the ratio of the critical erosion basal plane descent height to the critical beach water depth
Figure FDA0003909649550000031
If Q is satisfied simultaneously fp0 /Q 0 ≥Q fpc /Q c And Δ h b /H fp0 ≥Δh bc /H fpc If so, carrying out mutation response on the compound riverway; otherwise, the compound riverway does not have mutation response.
CN202211319284.1A 2022-10-25 2022-10-26 Prediction method for critical condition of compound river bed undercut mutation response after erosion basal plane is reduced Pending CN115630507A (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN117091997A (en) * 2023-10-13 2023-11-21 四川省生态环境科学研究院 Method and device for directly measuring longitudinal dynamic dispersion coefficient of river colloid

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN117091997A (en) * 2023-10-13 2023-11-21 四川省生态环境科学研究院 Method and device for directly measuring longitudinal dynamic dispersion coefficient of river colloid
CN117091997B (en) * 2023-10-13 2023-12-22 四川省生态环境科学研究院 Method and device for directly measuring longitudinal dynamic dispersion coefficient of river colloid

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