There’s a drainage ditch at the toe of the Addicks downstream face, which is currently flowing a lot more than I’d like to see, but I’ll hope that’s backwash, curling around from the gate release. Not seepage.
When they cut the old embankment, they already knew they’d want to compact soils evenly around the new gate structure one layer at a time, and there, the new outlet channel they’ll have to construct would be an obstruction, if they’d piled all the spoil on one side or the other, so they divided it.
They used the eastern half to construct the ramp down to outlet level, and just piled the western half down in the drainage ditch, blocking it, not realizing Harvey was coming, and on up the embankment itself.
I’m not up on slump nechanics, but I could see your description in action, you have loose spoil, over a 60 year old compacted, grassy, monitored slope, exactly the conditions you specify for soil mobility, with Harvey itself providing the lubricant.
Your other reference, is, I believe, a much closer shot of an angle similar to that seen at 1:23. I’m not 100% clear of the mechanics of that myself. Water is clearly spilling over the right (looking downstream) spillway training wall, back into the spillway, but I’m hard pressed to account for the dynamics of that.
Best shot...south moving outflow at velocity, dammed up by the roadway bridge between spillway and I-10, further in the distance, rebounds north, up both left and right side, outside the main channel. On the left, it’s turned east by the embankment itself and left spillway training wall. On the right, it can’t flow west, because of the ramp mentioned above. It piles up, against the toe of the embankment, and with nowhere else to go, flows back over the right training wall into the spillway. It always feels counter-intuitive to me when liquids exhibit laminar flow uphill, for extended periods, but it does happen, and I think it’s happening here.
Open to other ideas though.
What do you make of the two guys near the top of the downstream face, 0:36 to 0:58?
After further review (and a nights sleep) ...
Indeed the it looks like the material west of the cofferdam at Addicks is a temporary spoils pile as you say.
Also, what looks like water draining into the spillway outlet may indeed be the water released from the reservoir. What we are observing is a rough looking “hydraulic jump” where water exiting at a high velocity results in a lower level than the receiving water. That explains the water from the sides draining into the spillway channel. I’m unclear though from just this video as to whether at the end of the video we are looking at the Addicks or Barker video.
As for the two guys, they appear to be employees, maybe just doing an observation.