This paper presents an extension of the method that can be applicable to arbitrary geometry of the channel cross section by replacing the hydraulic depth with the hydraulic radius and adjusting formulations associated with wave celerity coefficient and kinematic wave equation in the original Fread method.

K. Lee, M. Muste, “Refinement of the Fread Method for Improved Tracking of Stream Discharges during Unsteady Flows,” American Society of Civil Engineers, DOI: 10.1061/(ASCE)HY.1943-7900.0001280.