Abstract: In this paper, bore propagation was theoretically analyzed at first. It was then pointed out that the well known formula of bore propagation speed couldn't be used if the water depth is very shallow in comparison with the height of the bore, even though the formula is derived from the conservation of mass and momentum. Therefore, a new formula was developed, based on the conservation of mass and energy. the calculated result agrees well with the measurement, so it can be concluded that the new formula can be used to compute bore propagation on a floodplain if the water depth is very shallow.