The mathematical model is an idealization of a real system and it can be used to study the operation
scenarios of an existing or a proposed system. The 1-D Sobek model is a mathematical model which
is chosen to simulate the Fayoumi Standard weirs which have been modified by adding pipes to
convey much water to downstream direction. Measurements of discharge with these structures are
complicated due to existence of the pipes. Field measurements are carried out and simulated using
mathematical model to investigate the effect of installing pipes on a clear over fall weir. Bahr
Hassan Wasef is simulated with six cross-sections. A kilometer from the beginning of the channel
including weir is represented. The used data of cross-sections altitudes, discharges, water levels
upstream and downstream the weir, and the head over weir for Bahr Hassan Wasef were measured
at the field by a Staff from Ministry of Water Resources and Irrigation in Al-Fayoum and Assiut
Irrigation Directorates. The effect of downstream water depth is taken into consideration. The flow
results of piped weir are compared with those of weir without pipes. It is found that there is a large
difference between them. Equations and curves for computing the discharge of piped weirs are
developed using multiple regression analysis on the bases of dimensional analysis. This research
might help irrigation engineers to accurately estimate the discharge for such structures.