Positive and negative surges in open channels

The phenomena of positive and negative surges in an open channel describe waves caused by a sudden change in the discharge. In pipes, there is the similar phenomenon with water hammers. The sudden change of the discharge may occur for example, when opening and closing a gate or switching off turbines. The positive surge wave is formed steeply (propagation velocity of the wave increases with increasing water depth), while the negative surge wave is rather flat. As a first approximation, positive and negative surge heights are equal in size and can be calculated using the continuity equation. In the case of a sudden opening (left illustration) we refer to a discharge surge and fill surge, and in the case of closure (right illustration) we refer to backwater surge and downstream negative surge.

Positive surge wave