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Water & Sewer Authority of Cabarrus County <br />Water and Wastewater System Master Plan <br /> <br />Safe Yield Update and Regional Drought Operations <br /> <br />· The reservoir behaves similarly to Lakes Fisher and Concord during drought <br /> events, andrthe 50-yr safe yield is defined by the 1986-1987 drought event. <br /> <br />· Duration of the critical drought is related to storage volume. <br />· Stream flow, precipitation, and evaporation are not considered. <br /> <br /> Table B-1 <br /> Comparison of Reservoir Parameters <br /> Black Run Res. Lake Concord Lake Fisher <br />Drainage Area 6.7 sq. mi. 4.7 sq. mi. 18.7 sq.mi <br />Volume 177.4 regal 179.2 mgal 749.6 regal <br />Duration of 50-yr Assume 365 days 365 days 396 days <br />Drought, days <br />Zero Inflow Yield 0.49 mgd 0.49 mgd 1.89 mgd <br />(=Volume/Duration) <br />Mass-Balance Yield -- 1.2 mgd 5.15 mgd <br /> <br />Based on these assumptions, a "zero inflow" yield value w?s calculated by dividing the <br />usable reservoir volume by the duration of the 50-year drought event, assuming that no <br />inflow occurs. The duration of the event is defined from the time that the reservoir was <br />full until it reaches its minimum elevation and then refills. At Lake Fisher the event <br />lasted 396 days, and at Lake Concord, that event lasted 365 days. <br /> <br />The "zero inflow" value can be compared to the safe yield computed using the mass <br />balance model. The mass'balance models consider the effect of precipitation, <br />evaporation and stream flow. It is apparent that not considering these effects results in <br />underestimating the reservoir's yield. Following the methodology described above to <br />compute the zero inflow yield at Lake Fisher resulted in a value of 1.89 mgd, while the <br />use of a detailed mass-balance model yielded 5.15 mgd. For Lake Concord a "zero <br />inflow" yield of 0.49 mgd was calculated, while a mass balance model yields 1.2 mgd. <br /> <br />Taking these relationships into consideration it is assumed that the Black Run Reservoir, <br />similar to Lake Concord based on drainage are and storage volume, will respond to <br />drought in a similar mann,er. Considering the 1.2 mgd safe yield computed using the <br />mass balance model and subtracting the 0.2 cfs (0.13 mgd) minimum release <br />requirement, it is reasonable to assume a safe yield of 1.0 mgd for the Black Run <br />Reservoir, not considering additional flows from Dutch Buffalo Creek. <br /> <br />PN 096873.0800 35 E'3, <br />January 23, 2004 <br /> ~, ~,,, BLACK & VEATCH <br /> · Intenlltlonll Complny <br /> <br /> <br />