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Article: Is ‘Green Spot’ a Sign of More Trouble for Oroville Dam?

A good article that describes both sides of the story on the Green Spot (DWR's internal puzzlement and references to Green Spot history).

This article reveals DWR's position of the Green Spot being observed before the reservoir was filled. Quote: "The consultants' report noted that the final construction report for the dam, which went into service in 1968, indicated that the green spot area was observed even before Lake Oroville began rising behind the structure."

This is key. Exactly what did the final construction report say on the dam concerning the Green Spot? The article uses the word "indicated". Yet there are historical photographs that show this area to be "clean" just before its filling of the lake. Springs in metavolcanic rock are common. However the best evidence is photographs (as shown a few posts above) of the prior green spot. Otherwise, DWR should release the full wording reference of this "final construction report" & "indicated" sections of the report document to the public.

= = Article clips (emphasis mine): "From Day 1 of the Oroville spillway crisis in February, the California Department of Water Resources has never wavered in its declarations that, despite the disintegration of the massive concrete flood control outlet and a near-disaster caused by uncontrolled emergency reservoir flows down a rapidly eroding hillside, the stability of the massive dam itself was not and has never been threatened."

"Despite those oft-repeated assurances, public questions about the dam’s integrity have persisted — in internet forums, in community meetings and, most recently, in a report released last week under the auspices of UC Berkeley’s Center for Catastrophic Risk Management."

"That’s in part a reflection of public distrust of DWR after the spillway incident and in part a recognition that anything that seriously compromises the 770-foot-tall dam could endanger tens of thousands of lives, cripple a key element of California’s water-supply network and put the state’s entire economy at risk."

"Specifically, the questions have focused on an extensive area of moisture on the left side of the dam’s downstream face that’s known, even to the Department of Water Resources, as “the green spot.”

"The spot, characterized by what state inspectors have termed “lush” vegetation during wet seasons that turns into dense thickets of dry weeds by late summer, is clearly visible on satellite images and measures about 700 feet long by 130 feet wide. That’s roughly the size of two football fields.

"Last week’s study, led by internationally known civil engineer and risk management analyst Robert Bea, included several subreports asking whether the moisture at the green spot is a sign that water is leaking through the dam and weakening its inner structure."

"Publicly, DWR officials have tended to dismiss those concerns. In response to questions at community meetings in Oroville and Yuba City in May, for instance, the agency said the green area is due to rainfall, that it first appeared while the dam was under construction, and that it poses no risk to the dam."

"But outside public view, documents KQED obtained under the California Public Records Act show the Department of Water Resources has puzzled for years over the source of the seepage feeding the “green spot” and has been slow to act on a 2014 recommendation from independent experts to investigate the issue."

. . . "The consultants’ report noted that the final construction report for the dam, which went into service in 1968, indicated that the green spot area was observed even before Lake Oroville began rising behind the structure."

“The green spot is believed to be associated with pre-existing natural springs in the downstream left abutment area of the dam foundation,” the Part 12D report says. The concentration of moisture, the document speculates, could be due to the composition of the rock and earth used to build the dam’s downstream embankment. The fill, which may contain excessive volumes of very fine, dense material, “may prevent free drainage of flows from those underlying springs.”

"The four consultants said that although there was no evidence of movement or instability in the green spot area, they recommended the Department of Water Resources investigate to see whether the persistent moisture could pose a risk to the dam, especially in the event an earthquake occurred during a period of particularly wet conditions."

= = end clips More at link:

Is ‘Green Spot’ a Sign of More Trouble for Oroville Dam?

https://ww2.kqed.org/news/2017/07/28/oroville-dam-green-spot-department-of-water-resources/

How Good - or Bad - is using a Toe Drain to detect Dam Anomalies? (When you don't have any working piezometers in the dam)

This will be a series of posts in discussing this question as DWR has zero working (non-useful) piezometers** in Oroville Dam. All of the array of these very important instrument sensors have been allowed to expire in usefulness without replacements. Thus, this issue directly relates to the Green Spot discussion as a decision was eventually made to use the Dam's Toe drain seepage as an "anomaly detector" to any internal issues with the dam. FERC has repeatedly asked DWR that they be able to measure the internal water levels (Phreatic level) and conditions (Phreatic condition) internal to the dam (translation, please install piezometers).

Since DWR hasn't followed FERC's requests, the only "trigger" detector to any internal dam problem, such as leakage increases or changes, is anticipated to be caught by the toe drain pipe at the bottom of the dam (downstream "Toe" of the Dam).

A Cross section side view of Oroville Dam is shown in Fig 1. Orange & Brown colored vertical sections are designed to work together to retain the reservoir water. A Clay/Clayey Core (orange) is the plastic material that is the most waterproof. But this highly compacted material requires stability support as it can bend or arc without a "sandwich" type of side compressive support. The Brown layers next to the core are called the Transition zones where they provide an immediate abutting mechanical stability/support with the ultimate help of the massive Zone 3 outer zones/layers. The transition (Zone 2) layers also perform an important function of (1) maintaining a soil particle/materials "compatibility" between the Core and the Zone 3 different compositions and (2) the Zone 2 layer can "heal" any imperfection cracks in the clay/clayey core by allowing silts/sands within the Zone 2 layer to migrate into the Core if a small crack (leak) develops. Migrating materials to seal these cracks are carried by the leakage water. Note: The upstream Zone 2 layer provides most/all of (2) as the water pressure & flow is upstream to downstream (left to right in image).

Even a near perfect Earthen dam will leak given a very large surface area. A process of high water pressure differential causes a slow migration of moisture between the tiny grains of the Core and Zone 2 layers - even though the tiny grains remain in place. This is considered normal seepage through the core.

To maintain the structural integrity of the downstream massive Zone 3 "sandwich" layer, it is important to keep the Zone 3 layer from becoming saturated, -or- keep it in a somewhat "dry" state. This is accomplished by making the Zone 3 layer such that water can migrate through it by incorporating "voids" into the materials. Thus the smallest material mixed within is sand (in combination with Boulders, cobbles, rocks, and gravels). Rainfall thus will migrate as shown in the light blue arrows in Fig 1.

The other key to keeping the Zone 3 layer from being saturated is in using a "Vertical Drain Chimney" of Boulders, cobbles, rocks, gravels (20 feet thick - all along the backside of Zone 2) that allows any normal seepage from the Core & Zone 2 layers to be drained into this chimney down to the secondary part of the drain - the horizontal drain zone (10 feet thick). See the dark blue arrows in Fig 1 for the ideal (non clogged) flows.

At the lower right hand side of Fig 1 is where all of this collected seepage (through the core and rainfall) would be trapped in a mini "barrier dam" and a 30 inch RCP pipe (Reinforced Concrete Pipe) would transmit this "seepage" flow to a meter station where flows are measured in gallons per minute (gpm).

Keep in mind that ANY seepage, core or rainfall, relies on the left and right canyon abutments (canyon walls) to help "channel" this flow towards the narrow horizontal drain channel at the bottom (near the "v" bottom in the canyon). This "horizontal" drain channel is not much wider than the original Feather River channel width (at bottom). See post 4,117, Fig 1.

To "detect" any internal problems with leakage in the dam by using the "Toe Drain" seepage flows requires an understanding of all of the possible internal dam issues that could affect this flow - such that may introduce errors in interpretation.

Over the years, The Division of Safety of Dams (DSOD), DWR, & O&M, should have records of the Toe Drain seepage flows in known reservoir levels and rainfall conditions. If a good history of measurements were conducted consistently over the years, a "profile" of expected performance could be compared against.

NOTE: DWR is using both the Toe Drain seepage and the Right and Left Gallery drain seepage channels to form a "sense of health" of the dam. For this discussion (Green Spot), only the Toe Drain is the point of focus (gallery drains are mainly "core" and Zone 2 canyon wall side abutment seepages from the reservoir and or "springs" at these tunnel locations).

(**Piezometer = sensor, detects water presence and water pore pressure levels).

Fig 1. Cross section side view of Oroville Dam. Orange & Brown colored vertical sections are designed to work together to retain the reservoir water.