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<br />WEIR FUlW <br /> <br />Flow over the top of roadway is computed by subdividing the roa.dw~ <br /> <br />cross section intc;> sel!lllOnts and computing tbe :flow over each by the weir <br /> <br />formula. Corrections for submergence caused by flow over the weir and <br /> <br />; <br /> <br />throU8h the bridge are presently based on relations developed for ogee <br /> <br /> <br />overflow crests (reference 3) and are shown in Table 2. It is planned <br /> <br />to incorporate submergence relations determined recently by W. A. Thomas <br />for broad-crested weirs (reference 8). The basic weir :flow equation <br />used in the program is <br />Q .. CL i3/2 <br /> <br />where : <br /> <br />Q .. The discharge for weir now <br /> <br />L .. The effective length of. flow <br /> <br /> <br />H." The head over the roa~which is measured to the upstream <br /> <br />energy grade line <br /> <br />C .. Coefficient of discharge <br /> <br />The various values of C required to reconstruct profiles through bridges <br />are much lower than the theoretical coefficient (3.1 for English system, <br />and 1.72 for metric system) for critical depth in a rectangular channel <br />since the weir is broad-crested. A typical value for C is 2.5. <br /> <br />II <br />