CN103388320A - Washing judging method for river bottom uncovering of high-sandiness flood - Google Patents

Abstract

The invention relates to a hydrologic forecasting method, in particular to a washing judging method for river bottom uncovering of high-sandiness flood, and belongs to the technical field of sediment dynamics. The IPC international patent classification node is E02B1/00. The method comprises the steps of firstly performing bending resistance tests on plaster blocks at a river bottom uncovering river reach; constructing a breaking uncovering model of the river bottom uncovering washing plaster blocks, analyzing various loads acting on the plaster blocks, determining the worst load combination, and establishing a plaster block critical breaking balance equation; and calculating various acting forces on the plaster blocks and resolving the equation to determine the breaking uncovering critical flow speed of the plaster blocks. The method corrects the parameter values of the formula (14) adopted by the traditional forecasting method, is more accurate in calculation, provides decision basis for flood prevention and also provides possibility for collecting the data of the river uncovering washing in real time, so a more accurate forecasting method is provided for judging whether a certain flood leads to the river bottom uncovering phenomenon or not.

Description

A kind of high sand-containing flood " is taken off river bed " and is washed away method of discrimination

Technical field

The present invention relates to a kind of hydrologic forecasting method, be specifically related to a kind of high moisture flood and produce the method for discrimination that " taking off river bed " washes away, belong to the sediment dynamics technical field, the IPC international Patent classificating number is E02B1/00.

Background technology

Phenomenon that high sand-containing flood " is taken off river bed " is one view on the Yellow River, generally occurs in Little Valleys In The Middle Reaches of The Yellow Xiao Bei master stream section and Downstream of The Weihe River section.When high sand-containing flood passes through this section, early stage is through the clay piece (channel aggradation thing) that density is large, intensity is higher of certain hour alluvial, started and surface by current, several square metres even tens square metres of areas, some clods erect vertical with water (flow) direction as wall, then " thump " pour (clod that has was just flutterred in water after upright two or three minutes) in water once into, by flood, swallowed up very soon.On river surface, a large amount of clods rise one after another, downstream stream seethe and under, bloom in full river, surging, the underwater sound is deafening." taking off river bed " and washing away is a kind of phenomenon that hyper-concentration flow and the borderline phase mutual effect of special riverbed produce." take off river bed " and wash away and often cause the strong incision in riverbed, sometimes a flood peak can rush riverbed dark several meters, causes riverbed and water level to decline to a great extent, and souring often causes migrating of river course major trough simultaneously, the littoral engineering in river course is done great damage, to the Yellow River flood control, bring immense pressure.

Last century the seventies, " taking off river bed " phenomenon has just caused showing great attention to of domestic and international water conservancy working person, and this problem has been carried out research." take off river bed " at present and wash away the research method of problem and divide three classes: the first kind is to utilize the field data at prototype hydrometric station, research and analyse the husky condition of water that " taking off river bed " phenomenon occurs for Xiao Bei master stream, the Yellow River and section, the Weihe River, but because the husky condition of high silt-laden river water is larger along Cheng Bianhua, therefore with " take off river bed " field data at upstream and downstream hydrometric station, section is inaccurate as the calculated data of the husky condition of " taking off river bed " flush water, and because the condition that " taking off river bed " phenomenon forms is more special, the randomness of its generation is larger, be difficult to capture in practice stronger " taking off river bed " field data of followability, also therefore caused the statistics length limited, therefore the husky condition of water of " taking off river bed " phenomenon generation of the method proposition does not have convincingness.The Equations of The Second Kind method is to utilize the husky condition of water in conceptual approach clay piece when starting of sediment transport capacity, but sediment carrying capacity reflection is transport conditions in silt downstream transport process, dynamic mechanical relation in the time of can not reflecting that the clay piece is instantaneous and start, and the clay piece is taken off the starting of lifting with single sand grain also very large difference, therefore, can reflect " taking off river bed " problem with the concept of sediment carrying capacity, be worth discussion.The 3rd class research method is experimental study, the method utilizes flume test simulation " taking off river bed " to wash away phenomenon, and built two kinds of clay pieces and take off and lift mechanical model: a kind of is that clay piece bottom is by the current erosion, when clod occurred upwards to rotate along the continuous section with riverbed (fixed end place), clay piece front end was in cantilever position; Another kind is not consider the water eroding and flooding situation, the adhesion strength of only considering clay block boundary and riverbed builds mechanics analysis model, but these two kinds of models are all to take off at clay piece full wafer the calculating of under the situation of lifting, the husky condition of water being carried out, and all do not consider the impact of the structural strength of clay piece (rupture strength) own.

" take off river bed " at present and wash away the research of problem, the differences such as emphasis due to research means, method, object and research, therefore the criterion of " taking off river bed ", discriminant criterion, mechanism etc. are still lacked unified understanding, and because the condition that " taking off river bed " phenomenon forms is more special, the randomness of its generation is larger, be difficult to capture in practice stronger " taking off river bed " field data of followability, also caused the research of " taking off river bed " phenomenon at present to be restricted.

can channel aggradation thing block be started, can produce in other words " taking off river bed " phenomenon, depend primarily on the synthesis result of various conditions, comprising channel aggradation in early stage form and adjustment situation, deposit density and relative roughness, deposit block formational situation and lift height, deposit Block Boundary condition, the mechanical strength of alluvial piece itself, peak flood flow and sand content size, peb process duration length, the channel geometry parameter, the layer vertical pressure fluctuation meter is strong and weak, pressure fluctuation meter is at the PHASE DISTRIBUTION of deposit block upper and lower surface and Phase Stacking probability etc.

Wash away the present achievement in research of problem from " taking off river bed ", deriving " taking off river bed " while washing away critical criterion, consider the Research Literature that above-mentioned " taking off river bed " wash away influence factor and not yet find.The present invention utilizes " taking off river bed " to wash away clay piece mechanical strength experimental data, and consideration " taking off river bed " is washed away the combined influence factor, the critical discrimination formula that derive " taking off river bed " washes away.

Summary of the invention

The present invention washes away under combined influence factor condition in consideration " taking off river bed ", and structure clay piece fractures to take off and lifts mechanics analysis model, and utilizes clay piece rupture strength experimental data, calculates the critical flow condition that the clay piece fractures and causes " taking off river bed " phenomenon to occur.The technical scheme that the present invention takes is:

A kind of high sand-containing flood " is taken off river bed " and is washed away method of discrimination, comprises the following steps:

(1), " take off river bed " and wash away an easily section clay piece rupture strength test:

The test soil sample washes away for taking from " taking off river bed " the clay piece of easily sending out section, at first the test soil sample is reinvented, concrete grammar is: will wash away and easily send out the clay piece that the former horseshoe bend in section fetches and pulverize from " taking off river bed ", put into container and add the water stirring, make silt flocculation deposition, by air-dry formation clay piece sample, and utilize the different moisture content of controlling clay piece sample of air-dry time; Then,, on the same group time clay piece sample not, utilize Intensity of Breaking Tester to measure rupture strength;

Test method is as follows:

Clay piece sample is disposed across on two support cylinder of Intensity of Breaking Tester, the test specimen major axis is perpendicular to the Intensity of Breaking Tester support cylinder, with the speed of the 50N/s ± 10N/s loading cylinder by Intensity of Breaking Tester, vertical load is added on the horizontal plane of prism equably, until clay piece sample fractures; Utilize formula (1) to calculate the rupture strength M of clay piece sample,

$M=\frac{1.5F_{f}L}{B^3}---\left(1\right)$

In formula, F _fVertically be added in the load at its top planes middle part when-test specimen fractures, unit is N;

Distance between L-support cylinder, unit is mm;

The length of side of B-prism square sectional, unit is mm;

Use the average of one group three clay piece sample rupture strengths as result of the test, utilize experimental result to set up the relation equation of clay piece rupture strength and moisture content, calculate " taking off river bed " by this relation equation and wash away rupture strength M under the clay piece saturated aqueous rate of easily sending out section _s

(2), build " taking off river bed " while washing away the clay piece fracture to take off and lift stressed Mathematical Modeling:

F _DFor the positive thrust of current: $F_D=-\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(2\right)$

In formula, V _bFor clay piece bottom water flow flow velocity, C _DFor resistance coefficient, γ _mFor muddy water unit weight, b is clay piece width, and c is clay piece thickness.

For F on plane of rupture _DReaction force: $F_D^{\′}=-\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(3\right)$

τ is the current drag power of clay piece upper surface: $\mathrm{\τ}=\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{lbg}---\left(4\right)$

G _sClay piece weight is: G _s=γ _sLbcg(5)

In formula, γ _sFor the unit weight of clay piece, l is the erosion jib-length, and g is acceleration of gravity, and other symbol is the same.

P ₁For the suffered water pressure of clay piece: P ₁=γ _mLbhg(6)

In formula, h is the clay piece surface depth of water, and other symbol is the same.

P ₂Buoyancy for the clay piece: P ₂=γ _mLb (h+c) g(7)

Maximum flow of water pulsation uplift force: $F_{d\max }={\mathrm{K\γ}}_m{\mathrm{JV}}_b^2\mathrm{lb}---\left(8\right)$

In formula, K is linear coefficient, and units/kg ms, span: 3～4.2, J are water-surface gradient.

(3), consider to set up the torque equilibrium equation of each power to the y axle under least favorable Load Combination condition:

$F_{d\max }l+P_2\frac{l}{2}+\mathrm{\τc}-G_s\frac{l}{2}-P_1\frac{l}{2}=M_s\mathrm{bc}---\left(9\right)$

(4), equation solution, draw critical flow velocity V _b:

At first, to the formula (2) in step (two) and (three)-(9) simultaneous solution, obtain formula (10):

$V_b^2=\frac{2M_{s}c+l^2\mathrm{cg}({\mathrm{\γ}}_s-{\mathrm{\γ}}_m)}{2\mathrm{KJ}{l}^2{\mathrm{\γ}}_m+\mathrm{lcg}{\mathrm{\γ}}_m}---\left(10\right)$

The clay piece is adopted empirical formula by the erosion jib-length of scour within the Δ t time period:

$l=C_1\mathrm{\Δt}({\mathrm{\τ}}_s-{\mathrm{\τ}}_c)e^{-0.013{\mathrm{\τ}}_c/{\mathrm{\γ}}_c}---\left(11\right)$

In formula, C ₁For bed scour coefficient, τ _sFor the suffered drag force of sand grain around the clay piece, τ _cCritical incipient motion shearing stress when the channel aggradation thing washes away, γ _cUnit weight for the channel aggradation thing.

${\mathrm{\τ}}_s=0.16D^2{\mathrm{\ρ}}_m{V}_b^2---\left(13\right)$

$\frac{{\mathrm{\τ}}_c}{({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)d_i}=A(1+\mathrm{\ξ}\frac{d_m}{d_i})---\left(14\right)$

In formula, ρ _mFor the density of high sand-containing flood, D is the average grain diameter of movable deposit around the clay piece, and A is the combined influence coefficient, and its value is 0.044, and coefficient ξ value is 1.35, d _mFor the average grain diameter of erosion deposit, d _iFor washed out sand grain particle diameter,

Simultaneous formula (10), (11), (13) and (14) solve, and can obtain V _b, this V _bProduce exactly the critical flow velocity that " taking off river bed " washes away;

(5), judge whether certain flood produces " taking off river bed " and wash away phenomenon.

When " taking off river bed " washes away easy upstream, section generation high sand-containing flood, according to the observation data of this upstream, section streamflow measuring station to flood flow rate of water flow and sand content, actual riverbed flow velocity, sand content when calculating this flood and arriving this section and wash away duration, then utilize the method for step (four) to calculate the critical flow velocity V that generation " is taken off river bed " and washed away _b, when this actual riverbed flow velocity greater than this critical flow velocity V _bThe time, " taking off river bed " can occur wash away phenomenon, otherwise, " taking off river bed " can not occur wash away phenomenon.

Method of discrimination of the present invention, whether the high moisture flood that can forecast with unerring accuracy can produce " taking off river bed " is washed away phenomenon, for flood control provide decision-making foundation, simultaneously, also for " take off river bed " real-time data acquisition that washes away provides may.

The specific embodiment

Committed step of the present invention is that " taking off river bed " section clay piece is carried out the rupture strength test; Consider " take off river bed " wash away the combined influence factor and build the clay piece and fracture to take off and lift model mathematics, and model is solved.Its concrete implementation step is as follows:

(1) " take off river bed " and wash away an easily section clay piece rupture strength test.

The test soil sample is that river bed " is taken off " and wash away the clay piece of easily sending out section in the Yellow River.At first wash away according to " taking off river bed " the formation mechanism of easily sending out section clay piece flocculation deposition, reinvent wash away the easy soil sample of fetching section of sending out from " taking off river bed ", concrete grammar is: will pulverize from the clay piece that former horseshoe bend is fetched, put into container and add the water stirring, make silt flocculation deposition,, as sample, and utilize the different moisture content of controlling clay piece sample of air-dry time by air-dry formation clay piece., on the same group time clay piece sample not, utilize Intensity of Breaking Tester to measure rupture strength.

Test method is as follows:

Clay piece sample is disposed across on two support cylinder of Intensity of Breaking Tester, the test specimen major axis is perpendicular to the Intensity of Breaking Tester support cylinder, with the speed of the about 50N/s ± 10N/s loading cylinder by Intensity of Breaking Tester, vertical load is added on the horizontal plane of prism equably, until clay piece sample fractures.The design formulas of rupture strength M is suc as formula (1), and unit is newton/square millimeter (MPa).

$M=\frac{1.5F_{f}L}{B^3}---\left(1\right)$

In formula, F _fVertically be added in the load at its top planes middle part when-test specimen fractures, unit is N;

Distance between L-support cylinder, unit is mm;

The length of side of B-prism square sectional, unit is mm.

Use the average of one group three clay piece sample rupture strengths as result of the test.Utilize experimental result to set up the relation equation of clay piece rupture strength and moisture content, calculate " taking off river bed " by this relation equation and wash away rupture strength M under the clay piece saturated aqueous rate of easily sending out section _s

(2) build " taking off river bed " while washing away the clay piece fracture to take off and lift stressed Mathematical Modeling, calculate clay piece load.

F _DThe positive thrust of current: $F_D=-\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(2\right)$

In formula, V _bFor clay piece bottom water flow flow velocity, C _DFor resistance coefficient, value is 1, γ _mFor muddy water unit weight, b is clay piece width, and c is clay piece thickness.

For F _DReaction force: $F_D^{\′}=-\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(3\right)$

τ ' is the fracture vertical shearing of section part of clay piece, and it does not produce moment to the y axle, can ignore.

τ is the current drag power of clay piece upper surface: $\mathrm{\τ}=\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{lbg}---\left(4\right)$

τ " be the current drag power of clay piece soffit, it does not produce moment to the y axle, can ignore.

Clay piece weight G _s: G _s=γ _sLbcg(5)

In formula, γ _sFor the unit weight of clay piece, l is the scour jib-length, and g is acceleration of gravity, and other symbol is the same.

The suffered water pressure P of clay piece ₁: P ₁=γ _mLbhg(6)

In formula, h is the clay piece surface depth of water, and other symbol is the same.

The buoyancy P of clay piece ₂: P ₂=γ _mLb (h+c) g(7)

Maximum flow of water pulsation uplift force: $F_{d\max }={\mathrm{K\γ}}_m{\mathrm{JV}}_b^2\mathrm{lb}---\left(8\right)$

In formula, K is linear coefficient, and units/kg ms, span: 3～4.2, J are water-surface gradient.

(3) consider to set up the torque equilibrium equation of each power to the y axle under least favorable Load Combination condition

$F_{d\max }l+P_2\frac{l}{2}+\mathrm{\τc}-G_s\frac{l}{2}-P_1\frac{l}{2}=M_s\mathrm{bc}---\left(9\right)$

(4) equation solution, draw flow velocity V _b, the critical flow velocity that " taking off river bed " washes away occurs exactly:

In formula (4), (5), (6), (7), (8) substitution formula (9), arrange:

$V_b^2=\frac{2M_{s}c+l^2\mathrm{cg}({\mathrm{\γ}}_s-{\mathrm{\γ}}_m)}{2\mathrm{KJ}{l}^2{\mathrm{\γ}}_m+\mathrm{lcg}{\mathrm{\γ}}_m}---\left(10\right)$

The clay piece is adopted empirical formula by the erosion jib-length of scour within the Δ t time period:

$l=C_1\mathrm{\Δt}({\mathrm{\τ}}_s-{\mathrm{\τ}}_c)e^{-0.013{\mathrm{\τ}}_c/{\mathrm{\γ}}_c}---\left(11\right)$

In formula, C ₁For bed scour coefficient, τ _sFor the suffered drag force of sand grain around the clay piece, τ _cCritical incipient motion shearing stress when the channel aggradation thing washes away, γ _cUnit weight for the channel aggradation thing.

${\mathrm{\τ}}_s=C_D^{\′}{\mathrm{\α}}_D{d}_i^2\frac{{\mathrm{\ρ}}_m{V}_b^2}{2}---\left(12\right)$

In formula,

For the drag force coefficient, value is 0.4, α _DFor the area coefficient of sand grain, value is

ρ _mFor the density of water, d _iFor washed out sand grain particle diameter, other symbol is the same.Replace d with the average grain diameter D of movable deposit around the clay piece _i, arrangement formula (12):

${\mathrm{\τ}}_s=0.16D^2{\mathrm{\ρ}}_m{V}_b^2---\left(13\right)$

Channel aggradation thing critical incipient motion shearing stress τ _c: $\frac{{\mathrm{\τ}}_c}{({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)d_i}=A(1+\mathrm{\ξ}\frac{d_m}{d_i})---\left(14\right)$

In formula, through test of many times, determine, the accurate value of combined influence coefficient A is 0.044, and the span of coefficient ξ is 1.25～1.65, and its accurate value is 1.35, d _mFor the average grain diameter of erosion deposit, d _iFor washed out sand grain particle diameter.

In formula (13), (14) substitution formula (11), can obtain the design formulas of scour jib-length l:

$l=C_1\mathrm{\Δt}[0.16{\mathrm{\ρ}}_m{V}_b^2{D}^2-A{d}_i({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)(1+\mathrm{\ξ}\frac{d_m}{d_i})]e^{-0.013A{d}_i({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)(1+\mathrm{\ξ}\frac{d_m}{d_i})/{\mathrm{\γ}}_c}---\left(15\right)$

Around consideration clay piece, but the particle diameter of erosion sand grain is 10 ^-5(m) magnitude left and right, the span of A is 10 ^-2Magnitude, and

Therefore, in formula (15) Replace d with the average grain diameter D of movable deposit around the clay piece _i, formula (15) can be expressed as:

$l=C_1\mathrm{\Δt}[0.16D^2{\mathrm{\ρ}}_m{V}_b^2+\mathrm{AD}({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)(1+\mathrm{\ξ})]---\left(16\right)$

Will

L is as variable, simultaneous formula (10), formula (16) solve: with formula (10) substitution formula (16) and arrange:

$l^3-\frac{C_1\mathrm{\ΔtAD}}{2}(2{\mathrm{\γ}}_c+2\mathrm{\ξ}{\mathrm{\γ}}_c-{\mathrm{\γ}}_m-\mathrm{\ξ}{\mathrm{\γ}}_m)l^2-\frac{C_1\mathrm{\ΔtADcg}}{2\mathrm{KJ}}({\mathrm{\γ}}_c+\mathrm{\ξ}{\mathrm{\γ}}_c-{\mathrm{\γ}}_m-\mathrm{\ξ}{\mathrm{\γ}}_m)l----\left(17\right)$

$C_1\mathrm{\Δt}{D}^2{\mathrm{\ρ}}_{m}c[0.32M_s+0.16g({\mathrm{\γ}}_s-{\mathrm{\γ}}_m)]=0$

Order $a^{,}=-\frac{C_1\mathrm{\ΔtAD}}{2}(2{\mathrm{\γ}}_c+2\mathrm{\ξ}{\mathrm{\γ}}_c-{\mathrm{\γ}}_m-\mathrm{\ξ}{\mathrm{\γ}}_m),b^{,}=-\frac{C_1\mathrm{\ΔtADcg}}{2\mathrm{KJ}}({\mathrm{\γ}}_c+\mathrm{\ξ}{\mathrm{\γ}}_c-{\mathrm{\γ}}_m-\mathrm{\ξ}{\mathrm{\γ}}_m),$

$c^{,}=-C_1\mathrm{\Δt}{D}^2{\mathrm{\ρ}}_{m}c[0.32M_s+0.16({\mathrm{\γ}}_s-{\mathrm{\γ}}_m)],l^{\′}=l-\frac{a^{\′}}{3},$ Formula (17) is converted into:

${\left(l^{\′}\right)}^3+p{l}^{\′}+q=0---\left(18\right)$

In formula, $p=(\frac{{\left(a^{,}\right)}^2}{3}+b^{,}),q=\frac{2{\left(a^{,}\right)}^3-9a^{,}{b}^{,}+27c^{,}}{27}$

Obtain according to the radical formula of simple cubic equation:

$l{=\{-\frac{q}{2}+{[{\left(\frac{q}{2}\right)}^2+{\left(\frac{p}{3}\right)}^3]}^{\frac{1}{2}}\}}^{\frac{1}{3}}={\{-\frac{q}{2}-{[{\left(\frac{q}{2}\right)}^2+{\left(\frac{p}{3}\right)}^3]}^{\frac{1}{2}}\}}^{\frac{1}{3}}+\frac{a^{,}}{3}---\left(19\right)$

In l substitution formula (10), can obtain $V_b^2=f(C_1,\mathrm{\Δt},{\mathrm{\γ}}_c,{\mathrm{\γ}}_s,{\mathrm{\γ}}_m,c,J,K,M_s,D).$

(5) judge whether certain flood produces " taking off river bed " and wash away phenomenon.

When " taking off river bed " washes away easy upstream, section generation high sand-containing flood, according to this upstream, section streamflow measuring station to flood flow rate of water flow and sand content observation data, according to this section hydrological observation data for many years, actual riverbed flow velocity, sand content in the time of can calculating at an easy rate this flood and arrive this section and wash away duration, then utilize step (four) method to calculate the critical flow velocity V that generation " is taken off river bed " and washed away _bWhen this actual riverbed flow velocity greater than this critical flow velocity V _bThe time, " taking off river bed " can occur wash away phenomenon, otherwise, " taking off river bed " can not occur wash away phenomenon.

Claims (1)

1. a high sand-containing flood " is taken off river bed " and is washed away discrimination technology, comprises the following steps:

(1) " take off river bed " and wash away an easily section clay piece rupture strength test:

The test soil sample washes away for taking from " taking off river bed " the clay piece of easily sending out section, at first the test soil sample is reinvented, concrete grammar is: will wash away and easily send out the clay piece that the former horseshoe bend in section fetches and pulverize from " taking off river bed ", put into container and add the water stirring, make silt flocculation deposition, by air-dry formation clay piece sample, and utilize the different moisture content of controlling clay piece sample of air-dry time; Then,, on the same group time clay piece sample not, utilize Intensity of Breaking Tester to measure rupture strength;

Test method is as follows:

Clay piece sample is disposed across on two support cylinder of Intensity of Breaking Tester, the test specimen major axis is perpendicular to the Intensity of Breaking Tester support cylinder, with the speed of the 50N/s ± 10N/s loading cylinder by Intensity of Breaking Tester, vertical load is added on the horizontal plane of prism equably, until clay piece sample fractures; Utilize formula (1) to calculate the rupture strength M of clay piece sample,

$M=\frac{1.5F_{f}L}{B^3}---\left(1\right)$

In formula, F _fVertically be added in the load at its top planes middle part when-test specimen fractures, unit is N;

Distance between L-support cylinder, unit is mm;

The length of side of B-prism square sectional, unit is mm;

Use the average of one group three clay piece sample rupture strengths as result of the test, utilize experimental result to set up the relation equation of clay piece rupture strength and moisture content, calculate " taking off river bed " by this relation equation and wash away rupture strength M under the clay piece saturated aqueous rate of easily sending out section _s

(2) build " taking off river bed " while washing away the clay piece fracture to take off and lift stressed Mathematical Modeling:

F _DFor the positive thrust of current: $F_D=\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(2\right)$

In formula, V _bFor clay piece bottom water flow flow velocity, C _DFor resistance coefficient, γ _mFor muddy water unit weight, b is clay piece width, and c is clay piece thickness,

For F on plane of rupture _DReaction force: $F_D^{\′}=-\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(3\right)$

τ is the current drag power of clay piece upper surface: $\mathrm{\τ}=\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{lbg}---\left(4\right)$

G _sClay piece weight is: G _s=γ _sLbcg(5)

In formula, γ _sFor the unit weight of clay piece, l is the erosion jib-length, and g is acceleration of gravity;

P ₁For the suffered water pressure of clay piece: P ₁=γ _mLbhg(6)

In formula, h is the clay piece surface depth of water;

P ₂Buoyancy for the clay piece: P ₂=γ _mLb (h+c) g(7)

Maximum flow of water pulsation uplift force: $F_{d\max }={\mathrm{K\γ}}_m{\mathrm{JV}}_b^2\mathrm{lb}---\left(8\right)$

In formula, K is linear coefficient, and units/kg ms, span: 3～4.2, J are water-surface gradient;

(3) consider to set up the torque equilibrium equation of each power to the y axle under least favorable Load Combination condition:

$F_{d\max }l+P_2\frac{l}{2}+\mathrm{\τc}-G_s\frac{l}{2}-P_1\frac{l}{2}=M_s\mathrm{bc}---\left(9\right)$

(4) equation solution, draw critical flow velocity V _b:

At first, to the formula (2) in step (two) and (three)-(9) simultaneous solution, obtain formula (10):

$V_b^2=\frac{2M_{s}c+l^2\mathrm{cg}({\mathrm{\γ}}_s-{\mathrm{\γ}}_m)}{2\mathrm{KJ}{l}^2{\mathrm{\γ}}_m+\mathrm{lcg}{\mathrm{\γ}}_m}---\left(10\right)$

The clay piece is adopted empirical formula by the erosion jib-length l of scour within the Δ t time period:

$l=C_1\mathrm{\Δt}({\mathrm{\τ}}_s-{\mathrm{\τ}}_c)e^{-0.013{\mathrm{\τ}}_c/{\mathrm{\γ}}_c}---\left(11\right)$

In formula, C ₁For bed scour coefficient, τ _sFor the suffered drag force of sand grain around the clay piece, τ _cCritical incipient motion shearing stress when the channel aggradation thing washes away, γ _cUnit weight for the channel aggradation thing;

${\mathrm{\τ}}_s=0.16D^2{\mathrm{\ρ}}_m{V}_b^2---\left(13\right)$

$\frac{{\mathrm{\τ}}_c}{({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)d_i}=A(1+\mathrm{\ξ}\frac{d_m}{d_i})---\left(14\right)$

In formula, ρ _mFor the density of high sand-containing flood, D is the average grain diameter of movable deposit around the clay piece, and A is the combined influence coefficient, and its value is 0.044, and coefficient ξ value is 1.35, d _mFor the average grain diameter of erosion deposit, d _iFor washed out sand grain particle diameter,

Simultaneous formula (10), (11), (13) and (14) solve, and can obtain V _b, this V _bProduce exactly the critical flow velocity that " taking off river bed " washes away;

(5) judge whether certain flood produces " taking off river bed " and wash away phenomenon:

When " taking off river bed " washes away easy upstream, section generation high sand-containing flood, according to the observation data of this upstream, section streamflow measuring station to flood flow rate of water flow and sand content, actual riverbed flow velocity, sand content when calculating this flood and arriving this section and wash away duration, then utilize the method for step (four) to calculate the critical flow velocity V that generation " is taken off river bed " and washed away _b, when this actual riverbed flow velocity greater than this critical flow velocity V _bThe time, " taking off river bed " can occur wash away phenomenon, otherwise, " taking off river bed " can not occur wash away phenomenon.