>>“I don’t understand all I know about this.” (good quote)
Freeper abb had a good sketch drawing & pipe info at post 2094.
For the pipe to collect water there needs to be inlets, thus the inference of a type of perforated pipe (archives do not state this detail). The gravel enveloping the pipe allows a flow into the pipe from the “foundation”.
The pipes feed into a drain pipe section that has an outlet lower than the collection pipe(s) i.e. down hill. Thus each section & drain runs via gravity alone. Any increase in hydraulic fluid pressure, from main spillway flows, will pressurize any penetrating leakage of water under the main spillway slabs. This pressurization will cause both an increase in volume and an increase in pressure flow in the drain outlets. Thus the observation of “jetting” in the sidewall drains from these factors.
If there is a section that is not working, the issue becomes “what is the cause”. The lack of a drain flow typically indicates an unwanted problem (where is the water going?) - especially when water weeps upward on the main spillway (see 2013 pic in post 2,092).
Theoretically the water could migrate further down to the next drain sets (if the “foundation” provides for a continuous linear flow). In this case, you should see a greater flow in the next drain set below. However, if you look at the 2017 pic in post 2,092 you do not see an increase in the next set of drains. Thus, the water is penetrating and flowing in undesirable circumstances.
Again, as Scott Cahill, a renowned dam expert reminds, a well designed & maintained spillway should have very little drainage (no “jetting”) when the spillway is operating.
Well, that makes more sense, they are basically just gravity drains
angled in a downhill direction to account for the vertical rise of the discharge...
Another arm-chair way to explain sub-drains is to discuss what they normally prevent. Sub-drains normally prevent the build up of hydrostatic water pressure.
On your basement wall, the wall has some water-proofing that is probably water-resistive only. You backfill the foundation wall with clean drainage rock, top that backfill with impervious clay and hope that water pressure can’t build up against your wall. To assure that, you go to the bottom of the rock and add a perforated drain pipe to daylight. That way pressure can’t build up against the less than perfectly water resistant basement wall.
These sub-drains aren’t a lot different. You normally don’t want water in them at all. If you are getting a lot of water that means you are getting water through the spillway slab (that bad because it will only get worse) or you have water getting out of the spillway or sub-grade and getting under the spillway slab to damage it.
In the case where you have three consecutive sub-drains running a lot and water and then below you have a drain that is releasing none — what do you have? You have that hydrostatic pressure ready to bust the slab from below or at least blow out the joint sealants and once released, washing away sub-grade gravel.