Computer programs dealing with hydrology and hydraulics must support the many sections that are used. While there are many references for shapes, the primary source used by this program is Table 2-1 of Open Channel Hydraulics, Ven Te Chow, 1959, McGraw-Hill.
The key relationship for hydraulic computations is the Hydraulic Radius, R
Where Area is the cross sectional area of water and WP is the wetted perimeter. For each section, knowing the area and wetted perimeter allows the program to compute hydraulic radius.