The primary constraint controlling risk mitigation decisions at Oroville Dam has been flooding of the Hyatt Power Plant; if the Power Plant floods the Dam is lost. The Power Plant is at risk because the failed spillway is cutting the hillside resulting in a debris dam and water back flow to the power house. Its for these reasons that the Emergency spillway was utilized.
Without that bit of insight the decision to use the Emergency spillway seems incredible stupid, as it obviously has a greater risk of head cutting erosion into the Dam than the damaged spillway. But at the time, it would seem the California Department of Water Resources thought the risk of hillside giving way at the primary spillway and flooding the PowerHouse was the primary risk.
Unfortunately now that the Emergency Spillway is at risk of catastrophic failure, there currently are no safe ways of regulating lake levels. That situation may change as the debris dam is pared down and the Power House regains the ability to spill water through the turbines.
If lake levels can be maintained solely using the turbine exhaust the Dam failure risk is greatly reduced. But as long as water discharges from the primary spill way remain outside of their designed path, loss of control of the Dam remain high; the only safety buffer being low lake levels.
Lake Oroville Spillway February 27, 2017
Only Backup Generators Remain