In this study, Formulae were developed for rehabilitated weirs (i.e., combined weirs and orifices, weirs with 1 or 2 pipes in their bodies). Several sites of the existing rehabilitated weirs in El-Fayoum were visited. During the visits, measurements were undertaken (i.e., discharge, velocity, and water levels) before and after the rehabilitation of weirs. The measurements were analyzed and used as input data for model SOBEK. The model was calibrated and verified against field data. Formulae were developed from the data measurements. Moreover, two Formulae were developed, based on the model results for weirs with 1 or 2 pipes. These Formulae were validated against field measurements. The developed weir Formulae related the parameters, defining the weir with the pipe dimensions, and the discharge. These Formulae would assist engineers in designing the modification required to develop the existing weirs in order to increase the discharge.