Culvert Design：涵洞设计.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * CE154 Design Considerations Flared ends improve efficiency Use culverts as wide as stream width Use same gradient as stream channel Use same alignment as stream channel Single large culvert is better for debris passage than several small ones Fall 2009 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * CE154 Key Approaches Critical flow does not occur on mild slopes, except under certain special, temporary condition [such as inlet control (3)] Critical flow always occurs at the inlet of a steep slope, except when the inlet is deeply submerged [H/D > 1.2-1.5] On mild slopes, most likely it’s outlet control Fall 2009 * CE154 Approaches For unsubmerged inlet control, - for culvert on steep slope, use critical flow condition to determine the discharge- for culvert on mild slope, use weir equation to compute flow For submerged inlet control, use orifice flow equation to compute discharge For outlet control, perform energy balance between inlet and outlet Fall 2009 * CE154 Critical Flow Condition yc = (q2/g)1/3 Fr = vc/(gyc)1/2 = 1 vc = (gyc)1/2 Ec = yc + vc2/2g = 3/2 yc q = unit discharge = Q/width (for non-circular conduit; for circular pipe use table to find critical condition)Fr = Froude numberE = specific energyy = depthc = subscript denotes critical flow condition Fall 2009 * Fall 2009 CE154 * Weir Flow Weir flow equation  B = culvert widthCw = weir discharge coefficient, aninitial estimate may be 3.0note that this eq. is similar to equations for ogee crest weir, broadcrested weir, sharp crest weir Fall 2009 CE154 * Orifice Flow Inlet control with submerged inlet,  Cd = orifice discharge coefficient, an initial estimate ? 0.60 b = culvert height HW-b/2 = average head over the culvert CE154 Outlet control hydraulics Energy balance between inlet and outlet Fall 2009 * CE154 Outlet c