Definition - What does Water Prism mean?
A water prism is a section with similar ends and parallel sides. Sewer pipes are circular in cross section and cylindrical in shape. A cylinder is a prism whose cross section is a circle. Technically, a cylinder isn't a prism but can be imagined as a prism with infinite number of sides tending to a circle. Similar to a prism, a cylindrical pipe has the same cross-section in all places and its area is derived by multiplying the area of a base by its height.
Trenchlesspedia explains Water Prism
Sewer pipes never flow at full capacity; they flow less than half full or more than half full depending on the pipe design. The prism formed by the water depending on the type of flow is used to determine flow characteristics of the fluid flowing through the pipe.
Due to deposits formed by solids present in the water, the water may not always flow in the shape of the pipe, reducing its efficiency with which water is able to pass through a particular cross-section. Trenchless rehabilitation methods such as cured-in-place pipe (CIPP), pipe bursting and sliplining can be used to rehabilitate pipes that have been damaged by scouring with solids present in the flow.