Field studies of tidal flows in largely pristine mangrove swamps suggest that the momentum equation simplifies to a balance between the water surface slope and the drag force. The controlling parameters is the vegetation length scale LE, which is a function of the projected area of mangrove vegetation and the volume of the vegetation. The value of LE varies greatly with mangrove species and water depth. It is found that the drag coefficient is related to the Reynolds number Re defined using LE. The drag coefficient decreases with increasing values of Re from a maximum value of 10 at low value of Re (&lt
104), and converges towards 0.4 for Re &gt
5 × 104.