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Piece by piece Clues to the Erosion Channels under the Green Wet Area - 2011 to 2015 side-by-side imaged processed zooms

Like the game of "Clue": DWR has been trying to understand the mystery of the Green Wet Area and the "Erosion Channels" formed directly below the Wet Area. Using a "fingerprint" type of analysis of exact matching small erosion channel locations from 2011 and 2015, it reveals clues (to the question of: what it "is" and what "it's not"). DSOD inspectors have noted that when the Wet Area is saturated there is no waterflow observed in the downslope erosion channels. So how have these erosion channels formed?

These special forensic side-by-side images are high resolution zoom of exact matching channel locations. The images on the left are 2011 and the right are 2015. The light colored whitish to light green dots are the Zone 3 fill of the largest rock near the dam slope surface. The Zone 3 fill Shell is noted as sands, gravels, cobbles, and boulders to 24" max (including minus of 25% in gravels). The dark areas of the image are the finer materials. The light dots and brighter areas are the larger to largest materials. Marker dots in the images reveal that the larger material is not moving, rather it more of it is being exposed over time. The zoom with non-processing (for larger rock contrasting) shows that the channels are slowly widening and slowly exposing more of the larger to largest materials. These images reveal the depth increase by the increase in exposed larger rock. Clearly there is a process that is occurring. Yet DSOD inspectors could not see water flowing in the channels when the Green Wet area was saturated.

These erosion channels are unique to the lower section of the Green Wet Area. Heavy rains, if the cause, would form channels elsewhere including above the green wet area, yet there are none (dense channels). If the "water source" to the green area were proposed to somehow transition to high flow and then somehow transition back to wet and damp with no downhill flow… theoretically this may cause erosion channels. But the strong on/off part doesn't exactly follow a typical "water source" mechanism. What one piece is: Clearly there is a removal process of the lighter materials leaving the larger rock to result in "channels". (The clue to the next part of this mystery is hidden in other particular surface section on the dam). Due to the long term side-by-side forensic imaging & the DSOD "active" moment examination, "what it's not" is it is not a on/off of sudden water flow from the Green Wet Area.

Occam’s razor indicates that these erosion channels (and by inference, the “green spot”) are not formed by reservoir water because the channels are not flowing when the reservoir head is the highest. The simplest explanation would be a rainwater fed “seepage channel”, just as the dam builders had noted during construction. If this is a deep channel with a small catchment basin, analogous to the sink hole that almost opened up under the emergency spillway, then it might normally generate low flow seepage to keep the green area wet in the winter and spring. But during intense rain, it could gather enough water to generate the visible erosion.

It would be useful to include an assessment of current 2017 photos in this green spot erosion assessment. Did the erosion channel rock pattern change this year? If not, that wound’t necessarily be conclusive.

This year had the greatest aggregate precipitation, but not necessarily the most intense. The most intense precipitation in the Sierras is during short duration thunderstorms. It is plausible the (hypothesized) catchment basin only fills during intense thunder showers, which then flows down and into the dam with enough head to erode the surface channels as the flow perks out. This could explain why the (hypothesized hilltop) catchment basin is small enough to be unnoticed, and why no one (apparently) has seen these dam erosion channels being formed, because the rain was so intense that they were seeking shelter.

All speculation? No doubt. That’s largely what this forum is all about.

A Q&D diagnostic that could be done would be to place various size and colors of stones in the erosion channels that would move or get washed away at different flow rates. That way it would be relatively easy to do a remote assessment of if and when the erosion channels have been reactivated. My speculative hypothesis is this it would be after thundershowers.

In any case, if there are existing piping channels through the dam that can sustain enough flow to erode its surface, and those channels ever merge with the reservoir water, then this could quickly develop into an uncontrollable situation. We all need to put and keep the pressure on DWR to resolve this issue before it resolves itself.