There is a concrete box girder bridge in a large stream. On top of the box girder bridge is 1 meter of earth and stones. At a large flow rate, the water runs over the bridge despite the siphon effect. What happens if you remove the earth and stones above the box bridge and place a 2nd box bridge on the 1st? Does the siphon effect still exist with accelerated inflow at duct 1? and is the amount of water that flows through when the stream is completely filled larger with 2 tubes? There are people who claim that the amount of water that flows through should be about the same. But is this true?
Answer
The flow rate is the volume (quantity in m3) water passing per unit time (second) through a given cross-section of area (in m2) of the stream. Assuming that no water flows in from the side or that water flows laterally out of the stream (so the water level remains lower than the bank), then the water can do nothing but continue to follow the stream. In other words, the flow rate will not change. This is called the fundamental conservation law of continuity in fluid mechanics (of incompressible fluids), or conservation of volume. So the amount (per unit of time) will not change.
What you call the “siphon effect” is actually a change in the state of flow due to the local narrowing of the stream in the chute. This speeds up the current as the water descends. Area multiplied by velocity is the flow rate, which therefore does not change.
If the flow rate increases, it is more than likely that the lower chamber will fill completely first and the water in front will rise. Here one will no longer have free surface currents. Then it will only happen in the upper chamber (if it is low enough in relation to the bank). However, the obstruction of the reduced flow area of ​​the shaft openings will cause water to accumulate in front of the bridge more quickly than one can expect flooding of the banks sooner.
Answered by
prof. dr. ir. Erik Tooman
Fluid Mechanics Sedimentology Hydraulic Engineering
Old Market 13 3000 Leuven
https://www.kuleuven.be/
.