The US Bureau of Reclamation distinguishes three classes of spillway. Friends of the River et al., refer to the Oroville spillway as an auxiliary spillway, the design docs call it an emergency spillway.
3.1.1 Spillways
[] There are three classifications of spillways typically employed by Reclamation, which are based on frequency of use. They are explained in more detail in the following sections.
3.2.1.1 Service Spillway
A service spillway provides continuous, or frequent regulated, or staged releases5 (controlled) or unregulated (uncontrolled) releases from a reservoir without significant damage to the dam, dike, or appurtenant structures due to releases up to and including the maximum design discharge. Service spillways are typically very robust, erosion-resistant structures consisting of mostly cast-in-place reinforced concrete and riprap channel protection. Some examples of service spillways are illustrated in figure 3.2.1.1-1.
3.2.1.2 Auxiliary Spillway
An auxiliary spillway is infrequently used and may be a secondary spillway (augmenting a service spillway discharge capacity). During operation there could be some degree of structural damage or erosion to the auxiliary spillway due to releases up to and including the maximum design discharge. Auxiliary spillways may be less robust, erosion-resistant structures consisting of some cast-in-place reinforced concrete, riprap channel protection and/or unarmored excavated channels. Some examples of auxiliary spillways are illustrated in figure 3.2.1.2-1.
3.2.1.3 Emergency Spillway
An emergency spillway is designed to provide additional protection against overtopping of a dam and/or dike and is intended for use under unusual or extreme conditions such as misoperation or malfunction of the service spillway or outlet works during very large, remote floods (such as the PMF), or other emergency conditions. As with auxiliary spillways, some degree of structural damage and/or erosion may be expected due to releases up to and including the maximum design discharge. Emergency spillways are the least robust, erosion-resistant structures consisting of some cast-in-place reinforced concrete, riprap channel protection, and/or unarmored excavated channels. Some examples of service spillways are illustrated in figure 3.2.1.3-1.
see generally https://www.usbr.gov/tsc/techreferences/designstandards-datacollectionguides/designstandards.html#final
I think it would be wise for a dam this large to have an auxiliary spillway in addition to an emergency spillway.
By those definitions, it sounds like
what we have been calling the “Main Spillway” is an “Auxiliary Spillway” and
what we have been calling the “Emergency Spillway” is the correct term.
(1) Oroville's design is stuck with the 1,730 ft long 901ft elevation crest for the original 350,000 cfs "Emergency Spillway" requirement for PMF (now bumped up to 369,000 cfs by the BOC). DWR has played PR "name games" switching auxiliary spillway with the original design stated "emergency spillway". Oroville's design is 100% accurate to "emergency spillway" per definition of 3.2.1.3 above.
(2) It is physically impossible to have an "auxiliary spillway" function (as stated in 3.2.1.2) unless a linear section of the 1,730 ft is rebuilt to a lower elevation, thus preventing the 901 ft elevation "emergency spillway weir" from self-activating.
However, IF cash were flowing in heaps, and there was a luxury of time, a redundant "less sophisticated non-radial gate" chute lined w/ sidewalls parallel structure could theoretically serve as an "augmenting to a service spillway discharge capacity" (i.e. augmenting the main spillway). This would kick in at a lower elevation than 901ft with perhaps a simple release mechanism (other engineering technicalities arise due to the fixed main spillway head gate structure & Head requirement for flow, but ignoring these complications just for discussion sake).
(3) However, the sharp elevation drop to the Feather River, due to the tremendous height of this dam, creates a "solution killer" in that erosion must not dam the Feather river. Severe damming of the Feather river causes a backflow to the toe of the dam. This can flood the power plant, thus *failing* the turbine infrastructure & losing the ability to drain the reservoir below 813.6ft.
So - (1) without full hillside armor will cause (3), (2) will prevent (1) from fulfilling the spec requirement of rated 369,000 cfs, but (2) will still cause (3) without full hillside armor or a "walled chute". (2) will require big cash & time on top of nearly an impossible window. (2) cannot handle a PMF flow, thus (1) has to kick in and you have (3) again (unless there is full hillside armoring). Also, (2) cannot fail in concert with (1) as it must handle a PMF flow.
No matter how you look at it, the only solution with time & wise spending is to fully armor the hillside (& buttress the concrete weir to secure its anchoring). Only this solution will protect the main dam in a full "safe" redundancy.