Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering Reservoirs Balancing Supply and Demand Ashokan Kensico Hillview Croton Reservoir Spillway.
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Monroe L. Weber-Shirk
School of Civil and Environmental Engineering
ReservoirsReservoirs
Balancing Supply and Demand
AshokanKensico
HillviewCroton
Reservoir Spillway
How Big must the Reservoirs be?
What is the objective that you are trying to meet?
What information do you need in order to solve this problem?
What algorithm could you use to solve the problem?
Water Supply and Demand Fluctuations
SupplySeasonal supply fluctuationsBuffered using _________ reservoirs
DemandSeasonal demand fluctuations Daily demand fluctuationsBuffered using _________ reservoirs
Effect of flow fluctuations on system designSize of balancing reservoirspipe sizes
watershed
distribution
Average Total Monthly Flow into Pepacton Reservoir (0.540 km3 storage)
020406080
100120140
Janu
ary
Marc
hM
ay July
Septem
ber
Novem
ber
Mil
lion
m3 /
mon
th
Reservoir full (hopefully)
average
Deficit provided by storage
Better design is based on drought conditions!____ million m3/month * __ month = ___ million m335 5 175
What is the safe yield from the Cannonsville Reservoir?
What is the maximum rate that we can withdraw water from the Cannonsville Reservoir without emptying the reservoir?_________________________________
What are the critical events in history that determine how big the reservoir has to be?__________
The average stream flow into the reservoir.
Droughts
Reservoir Mass Balance Equations
Di=Ii=
+ +=
0 i i iS I O S+ = +
iiii0 SRDIS
O = Cumulative (________ + _________ + ___________ )O = Cumulative (________ + _________ + ___________ )
Initial st
orage
Cumulative I
nflow
Cumulative O
utflow
Storage
Demand River flow Evaporation
True at any time!
1
i
iQ tDå I Q ti i 1 or Q t tNYC 0a f
Density of Water
950960970980990
1000
0 50 100Temperature (C)
Den
sity
(kg
/m3 )
997
998
999
1000
0 10 20
Temperature (C)
Den
sity
(kg
/m3 )
Density (mass/unit volume) density of water:
1000 kg/m3
drought status
Downstream River Flow?
Simplest operating ruleWaste from reservoir when reservoir is fullDon’t waste from reservoir if reservoir isn’t full
More complex rules could easily be incorporated into a spreadsheet modelMinimum discharge into stream as a function of
reservoir storage volume or ______________Based on regulations
Reservoir Rules in Equation Form
Smax=
When is reservoir full? ___________________
iiii0 SRDIS
1-ii
maxii0i
max1-iii0
RR ELSE
S-D-ISR THEN
SR-D-IS IF
iii0i RDISS
Reservoir Capacity
When Si = Smax
Reservoir is overflowingOverflow goes into riverNo additional river flow
0.010.020.030.040.050.060.070.080.090.0
100.0
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ll
0.010.020.030.040.050.060.070.080.090.0
100.0
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llCannonsville Reservoir Storage (Demand of 1.04 x 106 m3/day)
How could we increase safe yield?Increase reservoir volume
Storage vs. Safe Yield for Cannonsville Reservoir
0
0.5
1
1.5
0 250 500 750 1000
storage volume (million m3)
safe
yie
ld(m
illi
on m
3 /da
y)
367
Average stream flowWhat is the asymptote?
NYC Reservoirs
NYC supply reservoirs have a storage capacity of 550 billion gallons (2 km3)
How long could NYC go without any inflow into the reservoirs?
Current Reservoir levels (http://www.ci.nyc.ny.us/nyclink/html/dep/html/current.html)
Reservoir Levels
solution
Seasonal, Daily, and Hourly Fluctuations
Early morning as people get ready to go to work/school
Commercial Breaks (not any more)
Between midnight and 5 am
Substantial increase in water demand during summer due to_______________________
Peak flows___________________________________________________________
Low flows______________________
watering lawns, swimming pools
Estimates of Daily and Hourly Fluctuations*
As the time interval of analysis decreases in length the maximum rate of water demand during that time interval __________
If the average annual flow rate is 1.0 thenthe maximum season rate is 1.25 (summer)the maximum daily rate is 1.5 (range of 1.2-2.0)the maximum hourly rate is 2.5 (range of 1.5-3.5)
for NYC the maximum instantaneous rate was _____*Henry and Heinke p 386
increases
1.75
Methods to Even Out Fluctuations
Seasonal fluctuationsSource (watershed) reservoirsKensico and West Branch Reservoirs
Daily fluctuationsHillview and Jerome Park Reservoirs (directly
connected to distribution tunnels)Hillview has 3.4 million m3 useable storage
Flows from Kensico to Hillview are adjusted every ________two hours
Balancing Reservoirs
Hey Bob, I need some more water. Could you open the
valve another turn?
OK Fred, I’ll go give it a turn. Did you say you have more
water than you need?
Where are the largest tunnels in the NYC water supply and distribution system?
How Can You Estimate Required Balancing-Reservoir Capacity?
Variable supplyVariable demandAnalyze historic record to search for worst
case conditionsUse same Mass Balance analysisInclude variable ________ in analysisOther unusual demands…
demandFire fighting needs
Main breaks Maintenance of supply tunnels
Summary
An understanding of the variability in supply and demand are essential for the sizing of reservoirs and pipes in a water supply system
Supply Reservoirs must be sized to store water during drought periods
Balancing Reservoirs must be sized for daily or hourly fluctuations
Distribution pipes must be sized to handle peak flows
NYC Watersheds
Ashokan ReservoirAshokan Reservoir
Schoharie ReservoirSchoharie Reservoir
Neversink ReservoirNeversink Reservoir
Empty NYC Reservoirs
NYC supply reservoirs have a storage capacity of 550 billion gallons (2 km3)
Average demand is 61 m3/sHow long could NYC go without any
inflow into the reservoirs?
21000
61 864003803
3
3kmm
kmsm
days
daysa fLNM
OQP
Cannonsville Reservoir Storage (Demand of 0.5 x 106 m3/day)
0.010.020.030.040.050.060.070.080.090.0
100.0
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Stream flow gage station map
0.010.020.030.040.050.060.070.080.090.0
100.0
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llCannonsville Reservoir Storage (Demand of 0.75 x 106 m3/day)
0.010.020.030.040.050.060.070.080.090.0
100.06/
1/50
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ll
Cannonsville Reservoir Storage (Demand of 1 x 106 m3/day)
0.010.020.030.040.050.060.070.080.090.0
100.0
6/1/
50
6/1/
54
6/1/
58
6/1/
62
6/1/
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ent o
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