###### Abstract  
Flow velocity in silt carrying flow is one key parameter to many river engineering problems. A visual measurement technique of velocity profile distribution in silt carrying flow is provided using a portable ultrasound imaging system and an improved iterative multi-grid deformation algorithm. A convex array probe in the system is used to obtain a series of ultrasonic images at different times. Window offset and an iterative computing scheme for reducing interrogation window size in the algorithm improve the accuracy and efficiency of flow velocity measurement in regions with velocity gradients. Results show that the measured profile velocities can be more acceptable after being compared with time-averaged stream-wise velocities of profiles at ten positions in the same silt carrying flow and subsequently verified by comparing the point-by-point standard value. The measured velocity is more in agreement with the theoretical value, with the minimum root mean square error in the ultrasound beam sweep effect calculated by using optimal interrogation size parameters. The system is a feasible alternative to the single-point measurement technique in silt carrying flow. The iterative multi-grid deformation algorithm can analyze velocity profile distribution with gradients simultaneously, which can help the real-time measurement of multiple spatial velocity distribution and turbulence. velocity profile; silt carrying flow; multi-grid deformation; ultrasound imaging; particle image velocimetry 2021  
The optical particle image velocimetry (PIV) technique is widely used in image measurement [10; 11; 12]. It is a reliable method and allows us to obtain quantitative information, including water flow spatial structures [13; 14; 15]. However, the well-known limitation of the optical measuring principle is that optical methods can only be feasible for transparent fluids through which optical light can pass. These limit their further applications in some opaque objects, such as suspensions and silt carrying flow. Recent PIV applications where these limitations are partially overcome include the use of granular PIV (g-PIV) exploit flash or LED lamps to obtain reliable near-wall velocity measurements of granular flows and liquid-granular mixtures [16; 17; 18]. Still, the setup may be too complex for a portable device during the application. To overcome these disadvantages and meet the requirement of the field velocity measurement of silt carrying flow, a visual and real-time measurement method of velocity profile distribution by using an ultrasound PIV system and iterative multi-grid deformation technique was developed [19], referred to as an improved ultrasound imaging device and a novel algorithm. The operating protocol for the ultrasound PIV system is composed of a medical ultrasound machine with a PIV algorithm and an interface program. This approach is appropriate for real-time measurement of velocity profile distribution in silt carrying flow condition by combining the ultrasound imaging technique with the digital PIV method [20]. The main advantage of the ultrasound PIV system compared with some Doppler-based techniques is that all the velocity components, both perpendicular and parallel to the ultrasound beam, can also be measured. Therefore, it can improve its accuracy, simplicity, and accessibility without some Doppler-based limitations [21]. For spatial velocity measurements in silt carrying flow or through optical image methods, the ultrasound PIV method is a promising technique to obtain two-dimensional flow fields and velocity profile distribution [22]; hence, this method can conduct a quantitative performance assessment through actual ultrasonic images obtained in a seeded flow. The method can improve the accuracy of the particle image displacement at a sub-pixel level. On the one hand, velocity profile distribution fields within the laminar flow field measured using the ultrasound PIV method are in excellent agreement with analytical profiles [23; 24; 25].