Friend's Formula

Friend's Formula

This equation appeared in early versions of Australian Rainfall and Runoff and is still presented in the 2013 Queensland Urban Drainage Manual (QUDM) and other guidelines. It has the form:


The graphical version of this equation is presented in QUDM as follows:


DRAINS can use this equation or the kinematic wave equation to calculate times of flow across plane surfaces, with the choice of equation being made in the hydrological model property sheets.  Both equations use the same factors - L, n and S, but the kinematic wave formula also considers rainfall intensities.


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