Google axial flow, and mixed flow pumps; these are what you want. These pumps are perfect for throwing water over a levee wall (the kind that Louisiana would never pay for and could have prevented Katrina flooding...but thats another story).
Axial flows typically between 5-40 ft of head, mixed flow between 30- 80' or thereabouts. As said above, they are typically vertical pumps; axials (sometimes called propeller pumps)are named such because axial forces are dominant in moving the fluid axially along the shaft. Their energy is used to create flow, not head. Specific speeds usually range from....maybe 10K-25K....
1500 gpm is really not a huge volume; some of these monsters can easily move 100,000 gpm. Go check out Goulds, Floway, Flowserve, Patterson, Sulzer (they now own the old Johnston pump line). A cheaper version was made by a company called Low-Lift. They were totally fabbed pumps, no castings, and had huge installation base. Don't know if they are still around.