Notwithstanding any other provision of this chapter, the board of retirement may, at the request of the board of supervisors, adopt a funding period of 30 years to amortize unfunded accrued actuarial obligations, as determined by their actuary or by an actuary employed by the board of investments, for benefits applicable to all membership categories for the purpose of determining employer contribution rates for counties and districts. The board of retirement shall approve a new amortization period based upon a request from the board of supervisors that demonstrates

a financial necessity. The board of retirement may deny a request when the request would subject the fund to an unsound financial risk.

A board of retirement may take an action pursuant to this section only once.