River Flow Inversion Algorithm Research Based On UHF Radar

River discharge,as the most important hydrological data,has always been one of the most concerned and also difficult hydrological information for hydrologists.It is a quantity that cannot be measured directly and always calculated from velocities measured by current velocity instrument.Up to now,the velocity-area method is the most widely used discharge calculation method at home and abroad.The velocity-area method requires selecting a cross-section on the river,then measuring single point or multi-point velocity,and finally calculating the river discharge by the product of the mean cross-sectional velocity and the cross-sectional area.Therefore,two important tasks must be accomplished in river discharge monitoring,the first is cross-section velocity measurement,and the second is velocity-discharge inversion.With the development of fluid mechanics for more than a century,the flow measuring instruments and flow inversion algorithms have made great progress.These developments have been fully applied to river discharge measurement and the efficient use of water resources.Today's flow monitoring has formed a relatively complete system,but there are still many problems that limit the convenience and accuracy of measurement.These problems are mainly reflected in two aspects:The first is the problem of current measuring instrument,which mainly includes the following two points:1)Traditional flow measurement instruments have some obvious defects: 1)they are in contact with the water;2)some of them are difficult to keep stable when the water velocity is large;3)some of them are required to be installed below the water surface,and there are clear requirements for their installation and site selection;4)they are easy to be affected by the rise and fall of water level.2)The non-contact instruments bring a lot of convenience for velocity measurement.However,these non-contact methods also have some drawbacks: PIV has a limited measurement range and is mainly applicable to smaller rivers.The temporal and spatial resolution of microwave remote sensing method is rough,which is difficult to meet the demand of real-time and continuous flow measurement.Hand-held radar has high requirements for installed position,and it is easy to be affected by the change of the flow condition.The second is the problem of the velocity-discharge inversion algorithm,which mainly include the following three points:1)The application index-velocity method is mature in contact flow measurement.However,due to the influence of the contact measurement instruments,it is often difficult to obtain the optimal index velocity,which brings extra error to the calculation of discharge.2)The semi-experience and semi-theory formulas such as the power law and the log law are mainly applicable to wide channels,which can calculate vertical velocity distribution only when the water-surface velocity is known.However,when applied to narrow channels,it is often necessary to know the underwater velocity or related parameters.3)The methods derived based on the principle of fluid mechanics has a sufficient theoretical basis,but the form is too complex and unsuitable for engineering application.At present,most of the researches focus on the channels with regular shape(such as rectangular channel,semicircular channel),which are difficult to be applied to the varied shape of natural rivers.On the basis of fully understanding the existing methods,this thesis conducts relevant research on the following aspects:1)Firstly,UHF radar is used as the river velocity measuring instrument to automatically obtain river surface flow field in real time.Then the cross-section water-surface velocities are extracted from the flow field.In the case of single station,this thesis presents several methods to extract cross-section water-surface velocities from radial flow field.These methods have been applied well in practice.Together with the results of two-station vector synthesis,the results of all methods are compared with actual values.The overall error is small,which is mainly within ±10%.At the same time,the sources of the error is analyzed,and it is concluded that the error is mainly caused by the difference of the two flow measurement methods,the comparison error between them is not the real error from radar measurement.The effective extraction of the cross-section water-surface velocities provides a solid foundation for the following discharge algorithm research.2)Considering that the index velocity method has been well applied in the contact flow measurement method,the index velocity method based on the cross-section water-surface velocity is proposed.In this thesis,the maximum velocity on the surface of the cross-section is used as the index velocity to analyze several widely used index velocity method models,which shows that the moel considering both velocity and velocity has better performance than others.Taking the velocity at each distance,the maximum velocity and the average velocity in the middle area as the index velocity respectively,which results in that the maximum velocity is the optimal index velocity.At the same time,the feasibility and reliability of the index velocity method based on the cross-section water-surface velocity are verified.The estimated discharge error of the model is very small and mainly within ±5%.This research makes the index velocity method be applied well in UHF radar discharge measurement,and it has more advantages than the application in the contact measurement instrument.3)Based on the Reynolds-Averaged Navier-Stokes(RANS)equations and surface velocities detected by UHF radar as the boundary condition,a cross-section velocity distribution model for irregular natural channel is derived.Compared with the existing methods,this model is applicable to a wider range.At the same time,it is more suitable for engineering applications.Because the influence of river sidewall is considered,this model is not only applied well in wide channels,but also has greater advantages in narrow channels when compared with other methods.The error is mainly within ±10%.The calculated discharge error is mainly within ±5%.This model solves the problem that the index velocity method cannot be directly applied to a new site and the existing velocity-discharge inversion algorithm is limited in the application of non-contact instruments in narrow and deep channels.