[Ray76]

To see how the dam fit the landscape a current topographic map was superimposed with one pre-dating the dam. The older map showed a stream originating on the hill adjacent to the spillway, in the vicinity of "841" near the center of the map, the stream does not appear on the current map. I doesn't really matter, I just thought it was an interesting feature.

fig 1

 

Flowing water cuts a "v" shaped profile into hillsides. Some of these profiles are highlighted in blue on this map:

fig 2

 

Here is an overall view of the complex prior to the emergency. The profiles highlighted in figure 2 are visible in this photo.

fig 3

 

This photo shows the heavily eroded area downstream from the emergency spillway.

fig 4

 

Several channels were cut.

fig 5

 

The channels in figure 5 correlate with the flows illustrated in figure 2 (channels A and C), and with a road (channel B-blue).

fig 6

So...

1) the topography was known, stream cuts are obvious
2) the geology was known, the weathered "rotten" rock was known
3) the emergency spillway is expected to be used

Water followed known paths. The erosion was foreseeable. It was foreseeable yet not mitigated. Why not?

Another question is, why is the erosion most severe at the head of channel B?

 

[Ray76]

Water leaving the spillway at the blowout is following a predictable path.

In figure 2 water to the left of the spillway runs roughly parallel to the spillway, takes a brief jog toward the spillway, and then moves progressively further from the spillway.

In figure 2 water to the right of the spillway runs roughly parallel to the spillway, bows sharply away from the spillway, and then returns to a roughly parallel course down to the river.

That's pretty much what happened:

[EarthResearcher333]

Hi Ray76, the answers to your questions require starting way back at the original total flood control "concept" design. From this starting point, thereon followed a historical accounting of engineering testing, analysis, expert inputs & change process - forming long series of design decisions. These design decisions fall into "Engineering Judgement". Along the way, some interesting events occurred (very important "tradeoff decisions"). To get this understanding, a person has to walk back through all of the design archives & studies, and reconstruct the process, to see how some original design specifications were transitioned into "assumptions"** when non-solution space was encountered.

**Assumptions: Term to identify where unanticipated alternate solution options were chosen where the decision(s) had a sense of a political tradeoff. "Political" in the sense of a decision that did not have the "scrutiny" of the engineering processes of the prior decisions. Whether this was done because of schedule reasons, cost reasons, plain assumption, or "being overruled" is not stated (not stated fully in the archives), but is recognizable by experienced engineers familiar with large & complex projects were command decisions occur.

I will post more on some key decisions and their significant impact on the MS & ES final design.