What would be the implications of an unanticipated drastic rise in the water surface elevation in the inlet channel upstream of an inverted siphon with all the rest design parameters (e.g. discharge, downstream water elevation etc) remaining the same as considered in the original design? What kinds of malfunctions would affect the siphon's operation in that case?
Well,
Inlet and outlet should be so layout to meet the requirements for hydraulic conditions, reliable operation, sufficient resistance against sliding and scouring. Inlet and outlet construction availability, allowable siphon velocities determines the size of the siphon. Thus, it is necessary to assume internal dimensions for the siphon and compute all head losses such as entrance, friction, bend, and exit. The computed losses should approximate the difference in energy grade elevation between the upstream and downstream ends of the siphon.
Regards
Thank you Bayan Hussien for your answer.
Let us assume that an inverted siphon has been designed with an incorrect upstream water level elevation as an input design parameter. With this in mind, what problems may arise if the upstream water level elevation under operational conditions is much higher than anticipated? I'm looking for an understanding of the implications in terms of hydraulics.
Armin Hajighasem Kashani
Sometime they name inverted siphons as "Culvert on Sag". Siphon is working like a culvert in principle, but at some part works under pressure. Back to your question:
- If the head of water on the inlet increases, inlet usually becomes the bottle neck of the flow and it works in the "inlet control" situation.
- The above scenario almost never happens! as siphons are sensitive to head water (and they work properly in a certain head), usually there is a structure upstream to divert overflow and maintain the head in the range.
- Just a not related to your question but just consider this: Siphons are rarely used these day. They are almost anywhere in cities are illegal to use due to hazard to life and codes do not permit using of a siphon. In the rural area preference is to raise the road and have culvert under or lower the road and have a overhead pipe. Maintenance of a siphon is expensive compared to a channel or culvert. And out of cities there is not much cost to raise a road.
Hope it helps,
Kaveh
PS: See this book
https://www.usbr.gov/tsc/techreferences/mands/mands-pdfs/SmallCanals.pdf
Thank you Kaveh Zamani
Based on your comments and my understanding, I can make the following inference and would really appreciate your correcting me if I'm still on the wrong track.
For design purposes, the inverted siphon is basically supposed to function similar to type 6 flow, i.e. a full flow with a free outlet, with the control section at the outlet, according to the classification presented in Open-Channel Hydraulics (Richard H. French, 1985).
However, if the available head (under operation conditions) exceeds the design head by far for any reason (just for your information, this is due to a considerable rise in the hydraulic resistance in the upstream channel which is resulted from ice floes accumulation on the flow surface, in the case of my study), the flow regime would switch to the type 5 flow, i.e. a rapid flow would form at the inlet with the inlet control.
If the above is correct, the following questions arise.
- Does the discharge decrease when the flow regime switches from type 6 to type 5?
- Does the possibility of air entrainment (and the consequent blowback phenomenon) increase? In more general terms, what are the adverse consequences if this change of flow regime happens?
- How the transition between these two flow regimes is determined?
I really appreciate it if you could help me out.
Armin Hajighasem Kashani
1- I do not use Culvert specific equations for design of an inverted siphons, there are many manuals for inverted siphons (e.g. this and many more http://eng2.lacity.org/techdocs/sewer-ma/f200.pdf ) and your other option is CFD modeling of the siphon.
2- So your question is "if inlet control capacity is always less than barrel control or outlet control?" the answer is no. There are exceptions, so it needs a case by case study (by the way there are funnel shape, concrete culverts and siphons!)
3- No air entertainment, only derbies trap in the inlet. And closing due to derbies.
4- If you model with CFD in automatically captures that, if not and you use equations for inlet control and barrel control, solve for both and find the common point, that is the transient point from one regime to an other regime (and that transition is a RVF, 3D and turbulent, so the only way to study that is CFD)
Kaveh Zamani
I'm more than grateful for your invaluable comments. However, I'm still unclear. Maybe it's best to briefly describe my study first, then I hope I can clarify my point better.
I have investigated the effects of the ice floes accumulation on the channel's surface upstream of an inverted siphon (see figure 1). As you can imagine, due to the dramatic increase in the hydraulic resistance under the action of the surface ice (which acts as an extremely rough boundary added to the flow surface), a considerable backwater occurs in the inlet channel. My objective is to evaluate the ice-related backwater.
Let’s get back to my inquiry now. I want to briefly discuss what would happen if the ice-related backwater is not considered in the design. In other words, what problems may affect the hydraulic performance of the inverted siphon or even other upstream structures working in conjunction with the siphon? In my previous comments, I was trying to describe these consequences in terms of the similar flow regimes in culverts. I speculate that when the upstream water levels rise above the design value, the flow regime switches from type 6 to type 5 (see figure 2). So if this is true, what would be the consequences affecting the inverted siphon performance?
You answer can be find via 3D CFD modeling. (BTW, I think this never happens and it is an only "made up" case in academia. If there is ice jam, water surface goes up and reach the freeboard of the system and spill out.)
It is could be causes the flooding in the inlet structures of the inverted siphon as well as increasing the inlet velocity of the hydraulic structures.
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