3D numerical simulation of turbulence and sediment transport within a tidal inlet

• Tidal inlet channel bed has been significantly eroded since inlet stabilization.
• 3D numerical model is developed to simulate the scour hole evolution within the channel.
• Flow constriction and turbulence generation appear to be the main mechanisms of scour hole development.

Turbulence and sediment transport models are incorporated into a three-dimensional hydrodynamics model to investigate the mechanisms of morphologic evolution of scour holes within the Indian River Inlet, Delaware, USA. The inlet bed had eroded slightly since stabilizing the channel walls in late 1930s through 1976. The mean rate of bed erosion roughly doubled as a response to anthropogenic activities within the inlet such as the removal of ~ 80 piles remaining from an old bridge. Severe erosion near the in-water bridge supports and cofferdams for the replacement bridge necessitated channel bed stabilization that along with the remained debris from the removal of old bridge piles enhanced the growth of two deep scour holes on the bayside and oceanside of the bridge cofferdams. Scour hole and channel bed evolution has decreased drastically since 1994. The present investigation suggests that, initially, flow concentration through the cofferdams of the replacement bridge was the main reason for scour hole development. Bed shear stress over the forward-facing slope of the scour hole entrains sediment from the bed and extends the scour hole along the inlet and in the vertical direction. Flow separation within the developed scour holes after channel bed stabilization enhances turbulence and appears to be the dominant mechanism for further scour hole development.