36.14.502    HYDROLOGIC STANDARD FOR EMERGENCY AND PRINCIPAL SPILLWAYS

(1) Spillway conveyance for high-hazard dams will be based on estimated loss of life downstream from the dam caused by spillway failure.

(2) The minimum inflow design flood for an estimated loss of life of 0.5 or less shall be the 500-year recurrence interval flood.

(3) The minimum inflow design flood recurrence interval for estimated loss of life greater than 0.5 and less than or equal to 5 shall be determined by multiplying the estimated loss of life by 1,000.

(4) The minimum inflow design flood for estimated loss of life greater than 5 and less than 1,000 shall be determined from the design precipitation derived using the following equations:

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P_s = P_5,000 (10^rd) where

r = -0.304 + .435 log₁₀ (lol)

d = log₁₀ (PMP) - log₁₀ (P_5,000)

lol = estimated loss of life

PMP = probable maximum precipitation

P_5,000 = 5,000-year recurrence interval precipitation

P_s = design precipitation to meet spillway standard

(5) The minimum inflow design flood for estimated loss of life greater than or equal to 1,000 shall be the probable maximum flood.

(6) The reservoir and spillway must safely store and pass the runoff resulting from the minimum inflow design flood.

(7) As a minimum, the reservoir will be assumed at normal operation pool before the minimum inflow design flood begins.

(8) The spillway capacity may be smaller if an analysis shows that there is no additional loss of life expected from the dam failure flood than that observed for the minimum inflow design flood.

History: 85-15-110, MCA; IMP, 85-15-210, MCA; NEW, 1988 MAR p. 2489, Eff. 11/24/88; AMD, 1995 MAR p. 541, Eff. 4/14/95; AMD, 1999 MAR p. 2573, Eff. 11/5/99.