Sediment Mobility in Overtopped Noncohesive Earth Embankments.

Mahmoud Al-Riffai, Ioan Nistor

Wednesday 1 july 2015

9:30 - 9:45h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 8A. Sediment - Transport

An experimental setup using 2-D overtopping tests on noncohesive earth embankment models was conducted in the Hydraulic Laboratory at the University of Ottawa. The novelty of this experimental setup is attributed to the instrumentation by which the transient pore-water-pressures are captured and subsequently recorded: miniature (2 mm in diameter) tensiometer assemblies. The erosion rate and subsequent slope retreat of the sand bed was captured by digital photo cameras and later digitized into bed profiles. Water surface profiles were similarly digitized from the captured images. Apart from investigating the influence of initial soil-water state on overtopping erosion, an experimental-numerical hybrid approach was developed to estimate an empirical mobility relationship taking into account the boundary seepage mechanism. Using the recorded pore-water-pressures, a transient flownet, comprising the exit hydraulic gradient, seepage angle and interstitial discharge velocity, was established throughout the embankment model body at 2-second intervals. Hydraulic parameters such as flow depth, bed slope angle and depth-averaged overtopping velocity were also extracted at 2-second intervals using the free water surface and bed profiles and the measured overtopping discharge. Using the one-dimensional Saint-Venant equations, an analytical expression for the bed shear stress was derived to take into account the effects of unsteady flow, boundary seepage and steep slopes. The Shields parameter was also revisited to account for the effects of unsteady and supercritical flow on a downstream steep slope in the presence of boundary seepage. This novel transient flownet approach will lead to the development of new sediment mobility relationships for breach flows, instead of the sediment transport-capacity formulations which are based on steady, subcritical and normal flow conditions.