If we allow flood water to flow down naturally it will follow free surface flow equation Q= cLH^1.5. Q is volumetric flow rate, cL empirical constant and H height of water column above riverbed. Or you want to use Manning equation.... This is actually the natural way water can transport itself given even a slight pressure gradient for it to move. It builds up itself layer by layer to increase H so that Q increases. Allow it undisturbed, the higher the flood flow the higher it builds up H. This nature of water flow that makes the flood looks the way we usually see.

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If you observe free surface flow equation you can see that the water velocity higher on the top and zero at the bottom. If this equation governs out going discharge of a a bottleneck cross section the flood plain just upstream will be flooded. The water velocity is almost zero and hydraulic gradient between the bottle neck section and upstream end of the "pond" has been created is very small. It may take a long time before the this pond to be emptied. The nature of this discharge formula in fact is used for flood control for a very big reservoir (surprisingly your Bhumibol does not have this kind of flood control gate-as far as I can see-This is one of the reasons why the flood is soooo big) to slow down water release to downstream. But you don't want this equation dominates at downstream. What shall you do? Let we make up some numbers for one of the bottle necks.

Q= 4500m3/s. H =5m. Then cL 866. That makes Q=866H^1.5. Assuming the flood plain water surface area 5km2. Then we have about 25million cubic meter of water that need to be drained out based on that equation. How we can make a 25million m3 above faster? Just put t pump and suck the water out via the bottle neck. So the discharge equation for the pond becomes

Q = 866H^1.5 + pump discharge. Then the pond water level is going down faster. Or just force the water out using boat propeller. The equation becomes:

Q= 866H^1.5 + propeller discharge.

Either one the discharge will be higher. As long as the flood plain of this kind that propellers can provide meaningful impact. Otherwise .....

I am loving this semi-useless thread but I am also very very stupid, i am not sure if there is a conection HOWEVER, in your mathmatics and phyics i just have one question. well probably 2.

you, regardless of the amount, state that the exit for the water is a bottle neck. you assume that exit capacity to be Q

you now partially block that exit with a static mounted propeller.

therefore add resistance to the natural flow lets call that reduction amount Y

so regardless of the efficiency of the new propeller unit, your equasion cant be right, because you have not deducted the loss Y.

so wouldnt it be.. Q-y+ propeller discharge ( which i still think is a small amount)??

at the end of the day I admit to being a complete idiot so have probable misread all of this and need to go back to page one

Nice remark. I will right back soon.

1. Yes if you put the propeller static there then you increase resistance. But if we put energy to propeller it rotates. What the impeller of the propeller does is it provides water with momentum to move across its impeller. Thus the water travels faster. There is NO different with transfer pump concept. We provide water with kinetic energy then it moves faster.

Many people here have conceptually wrong view by believing that only the water near the propeller can be moved. The water movement vector naturally (forget about any propeller) parabolic in nature if view from the top of a straight straight river. For the river of equal depth and symmetry around its center the maximum velocity is achieved exactly at the center of the river. Near both of the river the banks that is a phenomenon called "a boundary layer effect". Water velocity is theoretically zero or close to zero.

If you look from site maximum water velocity occurs at around 0.7 X Depth (Can remember exactly but I promised I'm not far off) . Why I'm telling you this? There is theory that so called a shear stress & boundary of water. In our case that theory explains why the water just near the bank tries to maintain its zero velocity due to friction with river bank. The next layer of water tries to do so but there is not enough friction force to hold its position. Thus it travels slightly higher than zero. At the center, boundary layer effect has the least. Therefore the water travels at the maximum velocity for the given pressure gradient. Similar thing happens looking from side of the flow. Since boundary layer friction between air and water is smaller than of any solid wall (riverbed) therefore its maximum velocity does not lie symmetry with the dept.

Similar thing happens when you try to drag the water at any section to move. The water nearby will be dragged too but it will not move at the same velocity as the section you drag. It may form a very "thin" shaped parabola. I'm not really sure whether I'm right about it shape. But I can assure you I'm conceptually right This does not in any way violate conservation of energy since the some of kinetic energy being supplied to water at any point shall be equal to sum of kinetic energy for all tiny masses of water + losses.

Google parabolic water flow (image)