IRP Approach to Water Supply Alternatives for Duck River Watershed:
Presentation to XII TN Water Resources Symposium
William W. WadeEnergy and Water Economics
Columbia, TNApril 4, 2002
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Overview
1 Duck River Reliability Planning
2 Demand Forecasts
3 Effects on Duck River Flows
4 Supply Planning
5 Hydrology Implications
6 Alternative Evaluation
7 Reliability Analysis8 Decision Issues
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Duck River Reliability Planning (1)
Two questions drive TVA Duck River Needs Analysis:
1.How much added water does the region need?
2.When will it be needed?
Don’t address the more fundamental questions . . .
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Duck River Reliability Planning (2)
Five Better Questions for Duck River Needs Analysis:
1. What is existing reliability & what does region need?
2. What can the region afford?
3. When needed and what is the least cost way of meeting future needs?
4. How do alternatives relate to other needs in the watershed?
5. Do alternatives add benefits other than supply reliability?– Can these benefits absorb some costs?
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Integrated Resource Planning• Supply reliability measures a water system’s
expected success in avoiding detrimental economic, social, and environmental effects from water shortages.– Effects, not shortage per se, are the targets.
• Supply reliability planning entails a water management strategy that provides the watershed with a reliable and affordable water supply for the next 50 years. – Reliability is an economic outcome of the process.
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Reliability TargetsReliability planning begins with Objectives
• DRA reliability targets:
1. Maintain Normandy above 864 elevation. [or ?]2. Meet 120/155 cfs at Shelbyville + 10 cfs demand. 3. Avoid breaching 130 cfs flow benchmark at
Columbia more than 1 day every five years. [or ?]
4. Meet consumptive demands in the watershed.
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Reliability Targets (2)
• DRA must set its own reliability targets before evaluating resource options.
• Cost effects of less than 100% supply reliability of any target need to be compared to cost of achieving target.
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Water Demand ForecastDemand forecast initiates water needs
assessment.– Average daily demand - beginning point for
water needs analytics.
USGS demand forecast updated based on revised population. – Table 1 shows revised water demands
increased by population growth ratios.– Based on input from USGS (Susan Hutson)
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Revised Forecasts - avg mgd annually
Ratio 2020 2050 2050
2020 Low High Low High
DRUC na na 8.7 11.0 11.0
Total Bedford Cty 1.150 1.2 1.31 8.3 11.0 12.3Total Marshall Cty 1.370 1.5 1.85 6.4 9.1 11.1Total Maury/Williamson Cties 1.110 1.1 1.25 20.3 26.6 30.0
Sources: DRUC, Hutson, August 27 Memo to TVA; TVA, Water Supply Needs Analysis, 1998Others, EWE, Sept 26, 2001
Ratio-2050
Table 1: Public Water Demands from the Duck River Watershed
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Peak Water Demand ForecastWater supplies must meet peak needs.
Chart 1 shows that peaks occur July - Sept.
Withdrawal demands in the spring are about 65% of peak demands.
Planning to meet demands amounts to planning for growth to meet peak.– Key is peak month.
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Chart 1: Cumulative Monthly Supply Withdrawals: 1999 - 2001
7.000
9.000
11.000
13.000
15.000
17.000
19.000
21.000
23.000
25.000AugJulyJuneM
ayAprM
arFebJanDecNovOctSeptAugJulyJuneM
ayAprM
arFebJanDecNovOctSeptAug
mgd
CPWS mgd L'burg mgd S'ville mgd
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Peak Water Demand Forecast (2)
Table 2 shows requirements to meet peak monthly requirements.
– Except DRUC: USGS predicts peak day demand.
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Revised Forecasts - peak mgd monthly
Peak Month 2020 2050 2050
Factor Low High
DRUC 1.5 13.1* 16.5* 16 -19*
Total Bedford Cty 1.150 9.6 12.6 14.2Total Marshall Cty 1.150 7.3 10.4 12.8Total Maury/Williamson Cties 1.350 27.3 36.0 40.5*Each of these estimates is linked to a 1.5 peak day benchmark. The high DRUC number is supported by 1.8 peak day.
Sources: DRUC, Hutson, August 27 Memo to TVA; TVA, Water Supply Needs Analysis, 1998Others, EWE, Sept 26, 2001
Table 2: Public Water Maximum Demands from the Duck River Watershed
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Effects on Duck River Flows
Can the Duck supply future water demands?
Table 3 begins to answer the question.
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Revised Forecasts - peak cfs
1996 2020 2050 2050
Conversion Actuals Low Highcfs/mgd
DRUC 10.5 20.2 25.5 29.4
Total Bedford Cty Supply Intake 1.547 10.8 14.8 19.6 21.9Wastewater Discharge -4.2 -5.7 -7.5 -8.4Total Marshall Cty Supply Intake 1.547 5.3 11.3 16.1 19.7Wastewater Discharge -1.9 -4.0 -5.7 -7.0Total Maury/Williamson Cties Intake 1.547 22 42.3 55.6 62.7
Net Flow Reductions at Columbia Flow Gauge Mile 132.832.0 58.8 78.1 88.9Incremental withdrawal 26.8 46.1 56.9
Source: TVA, Water Supply Needs Analysis, 1998. EWE, Sept 17, 2001
Table 3: Duck River Maximum Water Withdrawals and Returns
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Effects on Duck River (2)Providing high quality drinking water under conditions of
drought and peak demand is difficult and expensive.
Chart 2 shows that DRUC’s target operating range between 865 - 875 elevation.
• Making drinkable water from Normandy is difficult . . .
Modeling shows that 20 mgd peak demand would bring the reservoir down to 858 feet in drought.
Treatment costs for DRUC to achieve drinking water quality for peak summer months and meet S’ ville flow targets are part of the needed data.
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Chart 2: DUCK RIVER UTILITY COMMISSION
860.0
865.0
870.0
875.0
880.0
J95F MAMJ J A SOND J
96F MAMJ J ASONDJ
97FMAM J J ASONDJ
98FMAMJ J A SOND J
99F MAMJ J A SONDJ
00FMAM J J ASONDJ
01FMAM J J
MONTH
EL
EV
AT
ION
(F
EE
T)
NORMAL LEVEL ACTUAL LEVEL
Normandy Reservoir Actual vs Normal Levels
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Planning Criteria
Assume that DRUC can meet Coffee County demands at reasonable costs while TVA meets Shelbyville flow target.
Can flows in the river meet future demands and TDEC requirement that flows exceed 130 cfs at river mile 133?
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Supply Planning Results
USGS flows at Columbia since 1904 can be used for statistical analysis.
But, flows before the river was regulated with opening of Normandy, January 1976, don’t directly apply.
Flows at Columbia since closure are used to compare supplies to requirements.
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Supply Planning Results (2)
Water supply shortfalls are measured in terms of:– Frequency– Duration– Magnitude
Hydrologic data post closure are “run” past future demands to determine frequency, duration and magnitude of breach to 130 cfs minimum flow.
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Supply Planning Results (3)Two scenarios are assumed:
– USGS demand forecast -- Revised by EWE.– Extraordinary industrial growth upstream of
Columbia withdrawing 12.5 cfs. (Four 2 MGD plants, one in each county.)
2050 demands are considered as Low or High.
Table 4 shows the equivalent benchmark minimum flows that will meet demands and minimum flow criteria.
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Table 4: Minimum Flow Conversions
Scenario 130 cfsEquivalent
2020 158 cfs2020 (HigherIndustrial Growth))
170 cfs
2050, low withdraws178 cfs
2050, high withdraws187 cfs
2050, high withdraws(Higher IndustrialGrowth)
199 cfs
These equivalent flows meet future demands and 130 cfs benchmark.
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Table 5: Critical Low-Flow Occurrences below 130 cfs and Probabilityat Columbia, based on 7,549 days, of data)
Variable 2020 2020
(HigherIndustrialGrowth)
2050, lowwithdraws
2050, highwithdraws
2050, highwithdraws
(HigherIndustrialGrowth)
1 Low Flow Occurrencesin 7,549 days 179 386 513 696 923
2 Low Flow Occurrencesper year (days) 8.7 18.7 24.8 33.6 44.7
3 Probability of anyshortfall in a year
.68 .68 .79 .84 .89
4 Average Duration LowFlow (days)
4.6 6.8 6.6 7.4 8.65 Average Daily
Shortfall (cfs) 15.0 16.0 19.1 22.0 27.46 Needed Expected
average annual addedsupply(2*5*3*.6464(million gallons) 1 mgd = 0.6464 cfs)
57 131 241 402 704
7 Acre Feet 175 402 740 1234 2,161
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Supply Planning Results (4)
Results show that a shortage could occur 68 % of the years, rising to 89 % of the by 2050.
Average shortfalls would rise from 15 - 22 cfs over time and scenario; 27 cfs with higher industrial growth.
Annual Shortfall days would rise from 9 to 34 within a year; 45 days with higher industrial growth.
Added supply of 1,234 AF would eliminate expected shortfall.
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Supply Planning Results (5)
A storage solution must target the maximum likely shortage event.– Table 6 shows this to range from 228 to
1,087 million gallons. [700 - 3,337 AF.]– Chart 5 interpolates the estimates to show
the growth in this requirement. – Chance of one year in 21 of 700 AF
shortfall rising to 2,450 AF by 2050.
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Table 6. Maximum Shortfalls - 130 cfs
Scenario Max Hydrologic Shortage Event1
(130 cfs Benchmark)
Days Mil Gal2020, low 25 2282020, high 27 4322050, low withdraws 36 5792050, high withdraws 37 7942050, high withdraws(Higher IndustrialGrowth)
38 1,087
1Each of the Shortage Events occurs only once in the 20.7 year hydrology data set.
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Chart 5: Duck River Maximum Water Supply Needs to meet 130 cfs target
500
1,000
1,500
2,000
2,500
3,000
Acre
Fe
et
Supply Shortfall
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Hydrology Implications
Protecting the Columbia benchmark, 130 cfs, will require new water supply.
• Protecting the 21-year drought of record requires 700 - 1,250 AF to meet 2020 low to high demands.
• Protecting the 21-year drought of record requires 2,450 AF to meet high demand for a 37-day 2050 drought.
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Hydrology Implications (2)• Final conclusions about new water supply
require hydrology study including historic droughts before Normandy closure; i. e., with synthetic data.
• Synthetic data would simulate the Duck River flows post closure to show how the managed river would operate with Normandy.
• Provide a longer statistical basis for conclusions.
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Alternatives Evaluation• Challenge is to find the most economic
mix of resources to achieve an acceptable supply reliability objective.
• An affordable alternative with acceptable risk is preferable to elimination of any chance of shortage at much higher costs.– Depends on cost of effects of shortage.
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Alternatives Evaluation (2)• Policy determinants of the best mix are
DRA’s four reliability targets:– Reliability of Normandy water levels.– Reliability of flow rate at Shelbyville.– Reliability of flow rates past Columbia.– Reliability of watershed demand coverage.
• What risk of shortfall is acceptable?• 50 years demand forecast is a long time with
considerable uncertainty.• Planning for the high forecast may be too risk
averse, or it may be prudent.
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Alternatives Evaluation (3)• Empirical determinants of the best mix are
hydrologic, economic and regulatory driven.– What added supply do you need to achieve
acceptable reliability?– What can you pay for?– What can you permit?
• TVA 1998 and 2000 reports were not intended to and did not answer these questions.
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Alternatives Evaluation (4)
TVA Alternatives: (All costs approximate)
• Raise Normandy Dam $ 8 million• Downstream Intake $11 million• Tims Ford Pipeline $13 million• Fountain Creek Reservoir $50 million
Added Alternatives:• Shift Tullahoma Demand to Tims Ford $?• Conservation $ 1 million
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Alternatives Evaluation (5)Water Supply Created by Alternatives:
2050 Conservation 2,160 AF in Normandy
Move Tullahoma Demand 4,000 AF in Normandy
Tims Ford Pipeline 22 cfs when needed
Raise Normandy 17,000 AF in Normandy
Raise Normandy+Shift Tullahoma 21,000 AF
Fountain Creek Reservoir 44,000 AF in Fountain Creek
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Reliability Analysis • Table 7 compares the needed supplies
and added water.
– Worst case requirements are shown in top of Table 7.
– New supplies in storage are shown on bottom.
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Worst Case Requirements*2050 Low2050 High2050 High, AddedSupply Additions ConservationShift Tullahoma DemandRaise NormandyShift and RaiseFountain Creek* One event in 21 years.
Table 7: Worst Case Requirements v. New Storage Supplies
4,000
Acre Feet
177524403337
17,00021,00044,000
2,160
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Reliability Analysis (2)
Hydrologic modeling reveals likelihood, duration of shortages, average and worst case amounts of needed new water. – About 15 cfs, 7 days duration, 2 year in 3: 2020.– About 22 cfs, 8 days duration, 4 years in 5: 2050.
– Max shortage event: – About 16 cfs, 27 days duration, 1 year in 21: 2020– About 22 cfs, 37 days duration, 1 year in 21: 2050
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Summary of Decision Issues
• Four reliability targets govern the selection of a resource mix.– Maintain Normandy above 864 elevation. [or ?]– Meet 120/155 cfs at Shelbyville. [treatment
determined]– Avoid breaching 130 cfs benchmark at Columbia
more than 1 day every five years. [or ?]– Meet all consumptive demands in the watershed
[except during droughts of some quantifiable measure.]
• DRA must select planning targets.
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Summary of Decision Issues (2)
All alternatives provide adequate to more than adequate supply reliability.
Challenge: find the most economic way of adding increments of supply to meet reliability targets.
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Summary of Decision Issues (3)Cost of new water supply created by alternative
only can be amortized over new water required to meet targets. – Other purposes than water supply could absorb
some costs not born by DRA. • Recreation or Water Quality.
Costs of effects of not meeting DRA reliability targets are needed to set quantitative targets.– How much to spend depends on cost of
missing target.