Hi WildHighlander57, You ask a great question - this reveals a Sac Bee reporter's "editing liberty" on the BOC report (..where would that water be coming from?).
The Sac Bee reporter edited wording from the BOC report which changes the context of what the inspectors actually observed. Without looking at the actual Board of Consultants report, the Sac Bee reporter's writing could let a reader think that "gushing" water was flowing all of the time - even when the gates were closed and it wasn't raining. Wrong. If you read the actual BOC report wording, the "gushing water" from the herringbone structure spillway underdrains is when the spillway is in operation. (i.e. NOT "gushing" when the spillway is shut down.). The BOC correctly concludes the origin of this "gushing" water as from cracks in the slabs and from seams in the slabs that are "collected" by the drains.
Another very important point the BOC identifies is that water is coming back "up" through cracks and spalls in the slab when a light flow is present along the sandbagged rerouted side of the spillway. This means that there is a "channel" that the water can flow under the drains (i.e. not collected by the drains) and resurface further down the spillway. They also note, very importantly, that "piping" is part of the process. This is the same as water "channeling" through paths under the spillway & is a "wash erosion" issue - thus the statement of "..seems likely that piping of foundation material beneath the chute slab may be responsible for the voids that have been found and repaired in the past." Bingo!! The spillway was emplaced upon erodable material - see post link below.
Now the most interesting part is the BOC statement: "The drains appear to flow for some appreciable time after the gates are closed and not precipitation is occurring".
For this condition to occur there has to be either a "pooling" of water in an elevation just above the spillway undersurface, or, a pressurized form of a rebound spring of subsurface water from below. I suspect it is the former. In this case, water could be penetrating out of the sidewalls into the backfill areas & chute hillside. It then would re-flow back towards the lowest point where the sub drains are located. ANY water that escapes outside of the chute area has foundation consequences over time. "Fines" of small material could migrate from the backfill areas and chute hillside, forming voids, pockets of water, and potential alternate "piping" routes outside of the chute. Not surprising that the BOC stated "The BOC believes this situation should be investigated".
