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A Novel Experimental Technique for the Planar Breach Outflow of an Overtopped Sand Embankment Using 2DV-PTV

Author(s): AMANJ RAHMAN, Ioan Nistor, Julio-Angel Infante

Linked Author(s): AMANJ RAHMAN, Ioan Nistor

Keywords: Particle Tracking Velocimetry; Infiltration; Supercritical Flow; Sediment Erosion; Embankment Breaching;

Abstract: The estimation of an overtopped embankment breach outflow in experimental modeling has been a major challenge for the past several decades because of several factors such as supercritical flow conditions, rapid change in bed and water surface elevation, and in-slope infiltration. This study presents a novel experimental approach for measuring the time history of the velocity profile during overtopping flow of a homogenous, non-cohesive sediment embankment using 2-D vertical particle tracking velocimetry (2DV-PTV) technique. In this work, the overtopping discharge is calculated as the product of the integrated velocity profile and the measured flow cross-section area at the crest of the embankment. 1mm median diameter Fluorescent Green Polyethylene Microspheres were used as seeding particles. These particles are specifically designed and manufactured with a density of 1 g/cc to allow neutral buoyancy in fresh water, enabling thus conducting PTV velocity measurement at different depths. Prior to mixing them with water, particles are hydrophobic and need to be coated with a surfactant such as Tween 80 Bio-compatible Surfactant. They also exhibit a strong fluorescent response and are color stable in water. To track them at different instants during the breaching process, a high output ultraviolet LED backlight was used to augment color intensity. Also, to achieve the ultimate possible spatial and temporal resolution, two high-speed Phantom VEO 410 cameras with recording capability of up to 500 fps were used in filming the event. Furthermore, the image processing and tracking particles velocity were achieved using the 2D-PIV DaVis 8.4.0 software. Coupled with PTV technic, nineteen micro-tensiometer-transducer probe assemblies (MTTPA), were assembled and calibrated in the Hydraulic Engineering Laboratories at the University of Ottawa, Canada. These were installed below the downstream face of the embankment slope to monitor the time-history of pore water pressure (PWP) during overtopping. The measured breach hydrograph was compared to most used methods such as the hydrologic routing method, and formulas using the circular crested weir, and the flume outlet v-notch weir. The novel method was shown an improvement of the traditional methods. The measured PWPs were used to further analyze the sub-surface flow and determine the exit hydraulic gradient at the soil-water boundary layer along the eroding downstream face. Image analysis tools were used to extract data required to calculate the erosion rate on a steep slope and under supercritical flow with considering infiltration


Year: 2019

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