the water weight increases incrementally until the pins 52 are forced out of the detents 66 this will allow for staged opening of the gate assembly 18 and will also work to prevent fluttering of the gate assembly as the water flow ebbs and increases it will be appreciated that the size and depth of the detents 66 must be controlled so as to not unduly hinder the movement of the trip plate in either the downward or upward direction as the water weight continues to increase eventually the biasing and the detents are overcome and the trip plate 20 will rotate entirely downward as shown in shadow in fig2 at this point the trip plate 20 ceases to exercise any limiting function on the gate assembly 18 which in turn is allowed to rotate entirely open by appropriate sizing and placement of the brackets 50 the side pins 44 and the other components the gate assembly 18 can be allowed to rotate through a full 90 degrees such that it comes to rest against the ceiling of the drain basis in which case the storm drain opening 12 is complete unobstructed maximum flow of water into the basis is allowed and even trip plate 20 is pulled up substantially away from the water flow once the water flow recedes the upward biasing on the trip plate 20 will again be greater than the water weight acting on the trip plate and it will again rotate upwardly simultaneously forcing the gate assembly 18 downward and into its closed position flush against the basin opening 12 referring back to fig1 it will been seen that the trip plate brackets 22 are attached to the large side brackets 36 and 38.