Abstract: Scour and siltation are the forms of riverbed evolution in the vertical direction, which have some impacts on river flood control and drainage, safety of wading engineering, shipping and natural landscape. So it is important practical and theoretical significance to research the riverbed evolution. Taking the engineering of Ningbo Subway Line 2 undercrossing the Yaojiang River as an example, the method of 2-D mathematical model of tidal flow and sediment combined with the method of empirical formula calculation were used to research the greatest scour depth of riverbed near subway line under different flood and tide conditions. The research results show that the flood frequency become smaller, the maximum scour depth of riverbed near the engineering line become deeper, and riverbed scour develops quickly at first, and then slow down gradually.

Key words: river bed evolution, tidal current and sediment, mathematical model, maximum scour, rail transit