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Risk analyses for a water-supply system; Occoquan Reservoir, Fairfax and Prince William Counties, Virginia

Open-File Report 78-452

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Abstract

Two techniques for evaluation risks in the operation of a water-supply system are demonstrated. Both rely on reconstructed historical streamflow data to develop estimates of the probabilities of certain specific events occurring in the future. These techniques are applied to the Occoquan Reservoir in Fairfax and Prince William Counties, Va., which was experiencing an unprecedented low level of storage in the autumn of 1977. In the first technique, the general risk analysis model (GRAM), simulations of the reservoir 's contents are carried out under a set of assumptions about withdrawal rates and emergency procedures. The results of the GRAM simulation for the Occoquan Reservoir are in the form of estimates of the probabilities that in any year certain emergency procedures will have to be invoked. These estimates are given for a range of rates of withdrawals and for four different stages of emergency actions. The second technique is position analysis. In this procedure probability distributions of future storages are estimated under existing storage conditions and an assumed rate of withdrawal from the reservoir. The position analysis which was initialized at the October 1, 1977, conditions indicates that the probability of entering a Stage III emergency (the prohibition of all uses of water nonessential to life, health, and safety) in the autumn of 1977 or winter of 1978 was 10 percent at that time. (Woodard-USGS)

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Risk analyses for a water-supply system; Occoquan Reservoir, Fairfax and Prince William Counties, Virginia
Series title:
Open-File Report
Series number:
78-452
Edition:
-
Year Published:
1978
Language:
ENGLISH
Publisher:
U.S. Geological Survey,
Description:
v, 61 p. :ill., map ;27 cm.--