I’m tired too; gotta check a groundwater monitoring network next to a river in the morning so shouldn’t be trying to figure this out, but agree with your last remarks. However, two comments:
At Addicks water from the water-filled cofferdam appears to have drained from there into the water coming from the release gates. At minute 1:04 at lower left you can see a small braided channel, now drying, as evidence of that. Also at 1:04, notice the brown scar at top left center with no green grass. At 1:46 there is telltale evidence of water draining from the area of the scar as shown by debris fan where it meets the cofferdam water. I wonder if there was slumpage similar to what you have with landslides in the US northwest. These occur when water infiltrates until a less permeable layer is encountered and the seepage water then acts a lubricant overcoming soil friction allowing slumping to occur. Though it appears no water is seeping from the other side, there was movement of soil from that slope and that has to be worrying to ACE.
Also notice the water rushing out at minute 2:18. It’s not identified except being at Addicks. From the lamppost shadows, I have to conclude its water coming out of the northeast emergency spillway (water that started spilling when it rose to a level of 108.00 feet). Certainly it’s a lot more than the small volume I would expect to see. Sort of reminiscent of water going over the emergency spillway at Oroville Dam.
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?