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“The panel report also said that while touring the spillway, consultants spotted “extraordinarily large” amounts of water gushing out of drains designed to move water out from beneath the intact portion of the chute. The water was flowing even though the spillway’s gates were closed and it wasn’t raining,..”

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".

Subsurface Seam at Blowout Failure location. Photograph provides insight into how "un-captured" waterflow in the sub-par drain design could "wash" a void layer between the bedrock and the concrete slab pour

Regarding the amount of water pouring out of the side drains, I did some comparison between what’s coming out now at 40,000 and what appeared to be flowing out of the drains prior to the shutdown and partial sealing/caulking of the spillway. It’s not much different between today’s 40,000 and the 50,000 it was at just prior to shutting it down. Maybe a small difference, but not much.

It was enormous when they had it at 100,000 a few weeks ago. This tells me what you and others have said - that water is pushing under through cracks and the seams between pieces of concrete. There could also be some issues close to the spill gates at the top, possibly in an area that they were unable to address due to the small amount of leakage flow constantly present.

2014/2015 Dam Inspection Reports: "Long Standing..Wet Area" - midslope dam backside seepage (being ignored?)

Just an FYI - A persistent, long existing, seepage wet area on the midslope area backside of the main earthen dam is noted in the 2014 & 2015 Oroville Dam Inspection Reports. A photograph of this location was included in the 2015 report where the inspector placed a red arrow in the pic to denote the green vegetation from the wet area. The safety Inspector recommended that this should be investigated**. Recommendations included taking measures, if needed, "to ensure the long term stability of the slope". The picture description stated that this was a "long standing…wet area". A prior June 2014 Inspection report also noted this "wet area" and that "Staff should continue to monitor the mid-slope seepage at the left side of the downstream slope". It continues: "A means of monitoring the extent of the seepage area should be implemented".

What would seem odd is that the same recommendations are made in 2014 AND 2015, but with no apparent change in action. You would expect that the 2015 report would include the status of the implementation of "monitoring" as recommended in the 2014 report. So why wasn't any action taken? Why is the 2015 report stating "this should be investigated"?

Then 2016 rolls along with the drought and the left side of the dam "seepage" area has dried up. Yet no discussion of the recommendations from 2014 and 2015. The 2016 report just states "The long standing seepage area at mid-slope on the left side of the dam has dried up due to the prolonged drought." i.e. infers that this issue has been sidelined as it went away due to the drought (and low water levels eh?). Let's hope that this issue doesn't recur in the midst of the Spillway crisis now that the water levels in the dam are above "mid-slope".

**note: Oroville dam uses a thick shell barrier of an inclined compacted clay core as the water barrier of the main dam. Any presence of water seepage or "wet areas" on the downstream face of the dam could infer a leakage or crack within this water barrier.

Oroville Dam Inspection Report June 2014 - mid-slope seepage & monitoring the "extent"

Oroville Dam Inspection Report 2015 - Wet Area Mid-slope on Dam (Picture 4)

Oroville Dam Inspection Report 2016 - seepage area dried out from prolonged drought

CA Division of Safety of Dams Inspection Report 2014 - Recommendation to implement a method to "monitor the mid-slope seepage" on the backside mid-slope of Oroville's earthen dam

CA Division of Safety of Dams Inspection Report 2015 - Stronger Recommendation to "investigate" wet area "to ensure the long term stability of the slope"

CA Division of Safety of Dams Inspection Report 2015 - Inspector picture with red arrow pointing to wet area that "should be investigated"…"to ensure the long term stability of the slope"

CA Division of Safety of Dams Inspection Report 2016 - "*never mind*"… the wet spot has dried up due to the prolonged drought…!?!