Abstract: The Qiantang estuary is a strong alluvial estuary.The sedimentation/erosion extends usually reaches 5-10 under the action of the flood or tidal flow.So the riverbed erosion extend is the key technical problem for the tunnel project across the Qiantang estuary.In the paper,three prediction methods on the maximum erosion depth of the river bed nearby this tunnel are set up based on the analysis of the hydraulic geometry of river channel,the sediment mathematical model and the movable bed physical model.These models are calibrated using hydrographic field data as well as those on the historical morphological changes in the Qiantang estuary.After the calibration these models are used to investigate the maximum scour depth caused by an exceptional flood in the cross-section of a tunnel across the Qiantang estuary.Finally,the maximum erosion depth and erosion curve of the riverbed cross-section nearby the tunnel under the 100-year flood is predicted by the three predicting methods,and the predicted result agrees very well with the geologic drilling data.It is shown that the predicted result is basically reasonable and the scientific basis is offered for the study of this tunnel.