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<br />class A low flow. Because the calculation is based on the presence of <br />piers, both the coefficient and a total width (BWP) must be read on the <br />SB card. If there are no piers, both variables can be left blank and <br />the program will use a standard step solution for low flows. The follow- <br />ing table gives values of XK for various pier shapes. <br /> <br />Pier Shape K <br /> <br />Semicircular nose and tail 0.90 <br />Twin-cylinder piers with connecting diaphragm 0.95 <br />Twin-cylinder piers without diaphragm 1.05 <br />900 triangular nose and tail 1.05 <br />Square nose and tail 1.25 <br /> <br />The Yarnell equation is a semi-empirical equation based on hydraulic <br />model data~ As such, it probably should not be applied in cases where <br />the flow obstruction is something other than a pier; for example, the <br />fill separating twin circular culverts. <br />loss Coefficient XKOR is used in the orifice flow equation, Q = <br />A "-!2gH/K. This fonn of the equation can be derived by applying the <br />energy equation from a point just downstream from the bridge (2) to <br />a point just upstream (1). <br /> <br />Yl + Zl + "1 <br /> <br />y2 <br />1 <br />2g <br /> <br />= Y2 + Z2 + "2 <br /> <br />y2 <br />2 <br />2g + Hl <br /> <br />(1) <br /> <br />where: <br /> <br />Y = depth of water <br /> <br />Z = invert elevation <br /> <br />30 <br />