The unregulated flows have zero firm yield but provide a significant increase in system firm yield if supplemented by reservoir releases during low flow periods. Reservoir storage capacity is sized to include a reserve for 50 to 100 years of sedimentation in addition to the usable design capacity. In most water resources planning reports and other documents, yield is viewed in terms of usable storage capacity, excluding sediment reserve. Prior to being filled with sediment, the sediment reserve can be used for other purposes. The BRA uses sediment reserve capacity for temporary water supply commitments.
Sediment reserves represent a significant amount of storage capacity available for many years.
In the present study, firm yield and other analyses were performed for alternative conditions of sedimentation. In the case study, reallocation between conservation purposes focused on using hydropower inactive storage capacity for water supply. System firm yields for the BRA reservoir system can be significantly increased in this manner. Adverse impacts on hydroelectric power generation can be minimized by releasing from the inactive pool only after the storage capacity of the other reservoirs in the system has been depleted. Thus, encroachments into the inactive pool occur very infrequently, but the contingency water supply storage significantly increases system firm yield.