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Lake propertiesKey
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Lake surface area83acres 3,615,480 sq ftInput
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First foot volume (approximate) 3,615,480 cu ft27,045,670gallonsEstimate
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Output
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Required flow - assuming no inflow/rain
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Desired number of days to lower first foot15days
Unit conversions
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Required flow to draw down first foot in time241,032cu ft / day 167 cu ft / min 2.79 cu ft / sec43560sq ft / acre
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7.48052gallon / cu ft
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Lake volume increase due to inflow/rain
This is the toughest thing to estimate! Note: all inflow is ultimately due to rain.
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Watershed (50% runoff): Inflow to lake per inch of rain14.84inches / inch rain4,470,345cu ft / inch rain
From sheet: Watershed
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Watershed (saturated): Inflow to lake per inch of rain28.67inches / inch rain8,639,400cu ft / inch rain
From sheet: Watershed
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Watershed (76% runoff): Inflow to lake per inch of rain22.03inches / inch rain6,638,254cu ft / inch rain
Matches 2018 conditions. From sheet: Watershed
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Copy one of the numbers in orange above to the blue cell below, or enter a different number to try
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Volume increase per inch of rain22.03inches / inch rain 6,638,254 cu ft / inch rain49,657,589gallons / inch rain
76% runoff, matches 2018 conditions
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RainfallHistorical20172018
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Average rain per day Sep - Dec0.150.110.20
From sheet: Drawdown rainfall
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Copy one of the numbers above to the blue cell below, or enter a different number to try
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Average rain per day over proposed drawdown period0.20inches rain / day2018 actual
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Average volume increase per day due to rain (includes stream inflow)4.41inches / day 1,327,651 cu ft / day 9,931,518 gallons / day
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Required outflow
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Average volume increase per day due to inflow and rain (above)1,327,651cu ft / day9,931,518gallons / day
Estimated based on the calculation above
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Desired number of days to lower first foot15days
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Total volume that must flow to lower first foot23,530,241cu ft 176,018,437 gallons
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Required flow to draw down first foot in time1,568,683cu ft / day 1,089 cu ft / min 18.16 cu ft / sec
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 8,149 gallons / min 136 gallons / sec
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Pilot siphon (6" diameter pipe)
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Time for (marker in) water to traverse pilot siphon5.5seconds
Scot's measurements using video frames, reported 5/28/19. Range of other mesaurements: 4.9 - 5.4 seconds
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Pilot siphon pipe length57.67feet
Scot's measurements, reported 5/28/19
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Linear flow though pilot siphon629.1ft / min10.49ft / second
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Flow through piot siphon123.5cf ft / min2.06cf ft / second
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15.4gallons / second
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Planned siphons
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Each planned siphon pipe diameter12inches
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Each planned siphon pipe cross-section area113.10sq in0.79sq ft
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Expected linear flow per pipe relative to pilot1.00
Estimated same as the pilot
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Expected linear flow per pipe629.1ft / min
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Expected flow per pipe494cu ft / min / pipe8.24cu ft / second / pipe
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 62 gallons / sec / pipe
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Number of pipes required3pipes 2.20 pipes
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Important cases 2.2 12" pipes 8.8 6" pipes
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Important cases, rounded312" pipes96" pipes
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Summary: Siphons required based on rainfallHistorical avg: 0.15inches rain /day2018: 0.20inches rain /day
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12" pipes1.712" pipes2.212" pipes
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6" pipes76" pipes8.86" pipes
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Even more: 0.25inches rain /day
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2.712" pipes
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10.76" pipes
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Estimated flow per 12" pipe494cu ft / min / pipe
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Number of 12" pipes112" pipe212" pipes312" pipes
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Expected outflow494cu ft / min988cu ft / min1482cu ft / min
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Ballpark drawdown time estimate for 3x12" siphons, assuming 76% runoff when it rains, on average (24% absorbed by the ground in the watershed)
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This section presents a rough estimate of drawdown time based on rainfall. It makes the assumption that, as the lake level drops, two opposing factors will cancel each other out:
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The flow through the siphons, which will slow down as the level gets lower, tending to increase the drawdown time.
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The surface area of the lake, which will reduce as the level gets lower thereby reducing the volume yet to remove, tending to decrease the drawdown time.
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This assumption is optimistic. One reason is that the reduction in surface area makes very little difference to the increase in lake volume from rain, most of which comes from the watershed.
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Since volume added by rain is very significant compared to initial lake volume, this really matters. Unfortunately we don't have the data to get anywhere without this assumption.
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Drawdown times are therefore very likely to be longer than the estimates below.
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First foot volume of lake (approximate) 3,615,480 cu ft
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5-foot volume (assuming per-foot volume remains the same, see above)
18,077,400cu ft
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Volume increase per inch of rain (from B14 based on runoff percentage) 6,638,254 cu ft / inch rain
Can change B14 to try different cases
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Estimated flow per 12" siphon (from measurements of 2019 pilot)494cu ft / min / pipe
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711,360cu ft / day / pipe
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Estimated flow for all 3 siphons (assumed unchanging, see above)2,134,080cu ft / day / pipe
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Number of Days Required based on RainfallThe rainfall numbers in the blue cells below can be changed to analyze different rainfall scenarios Overall average below
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Average rain per day over proposed drawdown period, inches0.20.220.240.260.280.13
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Average volume increase/day due to rain (incl. stream inflow), cu ft / day
1,327,6511,460,4161,593,1811,725,9461,858,711844,718
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Average drawdown/day: difference between outflow and inflow, cu ft / day
806,429673,664540,899408,134275,3691,289,362
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Number of days required - very likely an under-estimate (see above)
232734456615
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Looks unrealistically low!
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30-year averages:
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Nov3.90inches rain/month0.13inches rain/day
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Dec4.53inches rain/month0.15inches rain/day
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Jan3.73inches rain/month0.12inches rain/day
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Feb3.11inches rain/month0.10inches rain/day
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Overall average0.13inches rain/day
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