Pro-Series waterfall spillways have a plastic grating just below the water level that not only keeps the walls from bowing when buried in the ground, but also allows you to stack 3" - 4" of rock to hide the spillway.
The straight, inward and outward pieces are interchangeable and can be used in any combination.
Dual bulkheads (included) allow incoming water from either or both ends. A plug is included for whichever end is not used.
Waterfall spillways are designed to sit inside the liner eliminating any concerns of leaking. The liner simply goes under the spillway and up the back side. The bulkhead fitting secures liner to tub where the hose enters the spillway.