I have an architectural/engineering question about a structure which is working only partially in my world. In my fortress I decided to build a waterfall room for my dwarves, which required building an aqueduct since the location of the proposed room was too far away from the river. Here is a rough schematic of the system:
Top level; | represents the wall grate
OOOOOOO
O#+#+#O
O+++++OOOOOOOOOOOOOOOOOO
O#+#+#| <- ÷÷_
O+++++OOOOOOOOOOOOOOOOOO
O#+#+#O _÷÷ ->
OOOOOOOOOOOOOOOOOOOOOOOO
Ground level; waterfall room is inside fortress
+++++++O W~~
+#+#+#+O *W~~
+++++++O OOO|W~~
+#+#+#+O O ÷÷~~~
+++++++OOOOOO OOOO~~~
+#+#+#+O_÷÷ O ~~~
+++++++OOOOOO
Basement level, drainage:
+++++++O
+++++++O
+++++++O
+++++++O
+++++++OOO
+++++++++O
+++++++OOO
These are crude schematics; the structure is bigger. The main water wheel is also connected via a couple of gears that can be disengaged by lever, as a safety shutoff. Also not shown is the drive mechanism for the pumps on the left, which run off a water wheel in the incoming channel (top). As you can see, the plan is to have the river (flowing south) drive a water wheel which pumps river water up two levels via connected pumps, into the main aqueduct channel, into the top room and through the grates. The top room should then rain down into the indoor waterfall room (ground level), and as water is coming into the inflow channel it's driving a second wheel that drives the pair of outflow pumps, so the drainage room can be emptied and send its water back to the river through the aqueduct's outflow channel.
My problem is not that this structure does not pump water; it does. I have successfully pumped water into the top room and verified that it does power the second wheel and the outflow pumps, which in turn send water to the river. The problems I did encounter, however, were twofold:
1) Water hitting the grates in the interior waterfall room (ground floor) also tended to spill out of the room a little, a problem that was magnified when:
2) The drainage room filled to capacity relatively quickly, despite being pumped out.
I have no explanation for #2. In Dwarf Fortress physics, the second water wheel should provide just as much power as the first, so the outflow pumps should be pulling just as much water out as the inflow pumps. In practice this is not happening. Water is flowing out, but apparently not at a rate that matches the inflow.
What I'm wondering is, what's the fatal flaw in this design? Is it that the drainage room can't fill up to 7/7 at the pump's source quickly enough so the pump is maybe pulling only 4/7, 5/7 at a time? If so, would a second nearby pump, emptying into a shared trough, alleviate the problem? E.g.,
+++++++O W~~
+#+#+#+O *W~~
+++++++OOOOOO OOO|W~~
+#+#+#+O_÷÷ O O ÷÷~~~
+++++++OO|O O OOOO~~~
+#+#+#+O_÷÷ O ~~~
+++++++OOOOOO
+++++++O
+++++++O
+++++++OOO
+++++++++O
+++++++OOO
+++++++++O
+++++++OOO
Even if two pumps could fill up the small reservoir fast enough though, the second-level outflow pump would probably have the same issue. Should I perhaps have designed my outflow system for double capacity to ensure overfilling was impossible?