Lecture 2. Flow through culverts

Transcription

1 Lectre Flow throgh clverts

2 Clverts Clverts are the condits, which convey storm water throgh highway and railways embankments. A clvert consists of three parts: the intake (also called inlet or fan), the barrel (or throat) and the diffser (also called otlet or expansion fan). Commonly sed shapes are circlar, rectanglar, elliptical, shapes inclding arch and pipe arch clverts are sed. Plan Fig.5. A Typical Box clvert (Akan, 006)

3 The bottom of the barrel is called the invert while the barrel roof is called the soffit or obvert. The training walls of the inlet and otlet are called wing walls. Clverts are mostly designed to operate as a open channel with the critical flow occrring in the barrel in order to maximize the discharge per nit width ths minimizing the cross section of the barrel. In clverts, the characteristics of the flow are very complicated as the flow is controlled by many variables, inclding the inlet geometry, slope, size, roghness, approach and tail water conditions etc. Inlet configration plays a vital role in the performance of clverts. Inlet configration can be either projected barrel, cast in place concrete headwall and wing wall, precast end section etc. Clvert can flow either fll or partially fll depending pon pstream and downstream conditions. Flow can either be sb critical or spercritical and flow conditions can change with time in a clvert. Flow in a clvert is either inlet (pstream) controlled or otlet (downstream) controlled. Inlet control occrs when the clvert barrel capacity is higher than inlet will accept it. Inlet control clverts In the inlet control, the flow primarily depends on initial conditions sch as area, shape and configration of the inlet. Flow nder sch condition is spercritical and does not depend on tail water level. The hydralic behavior of inlet is similar to that of a weir if the inlet is not sbmerged and if sbmerged the behavior is similar to an orifice.

4 Fig 5.3 Types of inlet controlled flow in a clvert (after Normann et al. 1985) According to FWA (Normann et al., 1985) the inlet will be considered n sbmerged if Q A g 0.6 Where Q is discharge, A is the cross-sectional area of the clvert. is interior height of the clvert and g is gravitational acceleration.

5 Two forms of eqation are commonly sed for nsbmerged inlets (Akan, 006; Page 14-17). The form I eqation for n sbmerged inlet is yc Vc Q K I g A g 0. 5 M I k s S Where is headwater depth above the pstream invert of the clvert, Vc is the velocity at the critical depth, inlets, S is the clvert barrel slope, and The form II for n sbmerged inlet is yc is the critical depth, ks is 0.70 for metered inlets and is - for non metered K I, M I are empirical constants. Q A g K II 0. 5 M II K II, M II are empirical constants. The inlet will be considered sbmerged if Q A g 0.70 The flow eqation for sbmerged inlets is Q c A g Y k s S S is the slope, c and Y are empirical constants and for non metered inlets Otlet controlled clverts k s is 0.70 for metered inlets and is - In the otlet control, the clvert can either flow fll or partially fll condition. When the clvert is partially fll, the flow will be sbcritical. The flow capacity depends pon the

6 clvert area, shape, length, bottom slope, head losses in the clvert, and the headwater and tail water levels. Fig 5.4 Types of otlet controlled flow in a clvert (after Normann et al. 1985) The energy eqation for a clvert flowing fll is written as SL 1 k e gn L 4 / 3 k R n Q ga

7 Where is tail water depth measred from the downstream invert of the clvert; S is the clvert slope; L is the clvert length, g is the gravitational acceleration, n is Manning's roghness factor; R is the hydralic radis; A is the cross sectional area, 3 k n 1.0m 1/ / s and k e is the entrance loss coefficient. Flow is most likely to be governed by otlet control if the clvert slope is mild, for mild slopes, fll flow will occr if otherwise flow will be partly fll. The flow will be mostly inlet control if the clvert slope is steep. Example 5.:- A discharge of.5 cmec flows throgh a RCC rectanglar box clvert having =1.0m, b=1.0m, L=50m, n=0.013, and S=0.00. Otlet of the clvert is sbmerged with the tail water head of 1.6m. etermine the headwater depth. Take ke = For the box clvert the area and hydralic radis is given by A B 11 1m B R 0. 5m B SL 1 k e gn L 4 / 3 k R n Q ga m 4 /

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