Estimate the mean residence time of a sand grain on a floodplain. Assume that the sand grain was deposited in a scrollbar deposit as the river meandered within the floodplain. The floodplain is neither aggrading nor eroding. The floodplain width is 5 km, the channel width is 80 m, and repeat air photos indicate that it is migrating laterally at a mean rate of 10 cm/year.