20xxpereviewhydrologyandhydraulics(文件)
【正文】 pipe flow in a structure, Bernoulli’s equation is used: Frictional losses are multiples of the velocity head (V2/2g) and are additive. Head loss under pipe flow ? Entrance loss (Ke) ? Bend loss (Kb) ? Pipe friction loss (Kc) ? Each coefficient is documented in references Considering the Bernoulli equation for a spillway, the pressure at entrance and exit is atmospheric, the elevation difference is the water surface elevation difference between upstream and downstream, and the remaining term is the velocity head plus losses Consider the following Pipe flow Spillway considerations A given spillway may have several discharge relationships (weir, orifice, pipe) depending upon the head (stage). The stage discharge curve then bees a bination curve, with the type of relationship allowing the highest flow at a given head in control. Consider a drop inlet control structure: StageDischarge Curve Example An 18” CMP with an 18” vertical riser is used as the principal spillway for a pond. The pipe is 50’ long with one 90176。 77. The areaweighted CN is then (80*100+85*100+75*50+77*50)/300 = , which is more appropriately 80. Peak Discharge The CN method also provides for Peak Discharge estimation, using graphs or tables. Required information includes average watershed slope, watershed flow path length, CN, and rainfall depth. The graphical method from the EFM is: Peak Runoff Discharge Peak Discharge Example Same residential watershed that produced ” of runoff from a ” rainfall. Flow length is 2500’, slope is 2%. CN is 80, so S is ”. Ia = *S = ”. Ia/P = Tc = 2500^*(1000/809)^^ = Peak Runoff Discharge Example solution From graph, with Ia/P of and Tc of , unit peak discharge is cfs/ac/in or qp = *300* = 360 cfs Rational Method The Rational Equation is: Qp = CiA where: C is a coefficient i is rainfall intensity of duration tc A is area in acres C is approximately , A is 300ac, i is 2” in 30min, so 4iph, peak rate is then *300*4 = 480 cfs Questions on runoff? Hydraulics of Structures Flow through structures is important given that such structures control the rate of flow. Sizing of such structures is then important to allow flow to pass while protecting downstream areas from the effects of too high a flow rate. Structures may also be used for measurement of water flow. Each type of structure will produce different types of flow depending upon size and flow rate passing through it. Weirs ? Sharpcrested ? Broadcrested Weir Equation (from EFHCh03 Hydraulics
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