Gravity currents play an important role in many aspects of natural processes and engineering practice. This paper presents a systematic lock-exchange experimental study to investigate the influence of linearly stratified environments and ramp slope on the hydrodynamics of gravity currents. A high-speed camcorder and particle image velocimetry (PIV) are applied to analyze the macro- and microstructures of gravity currents. It is observed that the front velocity of the gravity currents increases first then decreases for both stratified and unstratified environments. However, the density contrast that drives the currents decreases more quickly in stratified environments as the currents descend the ramp, causing more intense deceleration of the front velocity, which indicates the damping of ambient stratification on gravity currents. Based on the thermal theory, a new set of analytical formulas is then developed to determine the front location and velocity at the deceleration stage by considering the ...

Fluvial bed load transport is often considered to assume a capacity regime exclusively determined by local flow conditions, but its applicability in naturally occurring unsteady flows remains to be theoretically justified. In addition, mathematical river models are often decoupled, being based on simplified conservation equations and ignoring the feedback impacts of bed deformation to a certain extent. So far whether the decoupling could have considerable impacts on the fluvial processes with bed load transport remains poorly understood. This paper presents a theoretical investigation of both issues. The multiple time scales of fluvial processes with bed load sediment are evaluated to examine the applicability of bed load transport capacity and decoupled models. Numerical case studies involving active bed load transport by highly unsteady flows complement the analysis of the time scales. It is found that bed load transport can sufficiently rapidly adapt to capacity in line with local flow because sediment...