Gate hinge

I'd like to get your expert opinion on a problem I'm having coming up with a design for a flush fit gate. My house is below grade in San Diego and for the past few years we have one or two rains that runover the curb and flood my house. Last year I built a flower box / retainer wall to keep the water moving down hill to the sea but I need to finish it with two gates that will keep the water from coming thru the side walk enterances. Both enterances are about 40" wide. It doesn't have to be as tight at tupper ware but the general idea is to stop what amounts to a 6-8"wall of water from moving onto the property. Sand bags have done the trick up till now but my wife dowsn't like them for the 2-3 months of the rainy season. So I'd like to come up wth a hinge design that will close the gate fairly flush and tight and if possible be removeable for the 9 months of the year they are not called for. The flower box is 4x8x16 concrete block with a stucco finish 24" high. Any input is welcome and thanks for your help...Gus