1. Here is an Original "Artist Conception" of the Oroville dam. These renditions are done in consultation with engineers and was used to promote the project to the public. Notice the "wing shaped" Main Spillway design. Also notice that the spillway was irregular going down the mountain as if it were cutting into natural rock (could have been a "liberty" of the artist).
2. There are indications that initial construction & grading of the "saddle" spillway area had the option of implementing a "wing shaped" Main Spillway (denoted in red shading in this Google Earth view). Also, test drill locations tend to support the thinking of a "wing shape" option.
3. Special 1:78 and 1:42 scale working models were built and carefully tested in a lab environment. The scale models show it became obvious that ideal "laminar flow" was the design of choice. This meant that the additional "wing" portion would have been too turbulent at a focus region in the spillway chute.
4. If the initial grading was performed to facilitate a "winged" section, but not ultimately used, then the Emergency Spillway Wier would need to account for the grade differential - thus the higher section near the Main Spillway Gate Structure area. These effects resulted in a focused wash basin topology that ultimately initiated the extreme gash erosion from the overspill flow.
5. Engineers were wise to have concerns about this focused wash basin. They "armored" the side area berm of the access road (see where concrete boom truck is covering a layer of placed rock in the image). They did this work before the storm overtopped the spillway. Also notice that they did some moderate grading of the hillside & removed vegetation - in an attempt to maintain a laminar flow (light emphasis on "attempt" & "laminar flow").