Monroe L. Weber-Shirk

School of Civil and Environmental Engineering

Reservoirs

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

Janua

ryMarc

hMay Jul

y

Septem

ber

Novem

ber

Mill

ion

m3 /m

onth

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 In

flow

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 ELSES-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

6/1/

50

6/1/

54

6/1/

58

6/1/

62

6/1/

66

6/1/

70

6/1/

74

6/1/

78

6/1/

82

6/1/

86

6/1/

90

perc

ent o

f ful

l

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/

66

6/1/

70

6/1/

74

6/1/

78

6/1/

82

6/1/

86

6/1/

90

perc

ent o

f ful

lCannonsville 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

illio

n m

3 /day

)

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/schoolCommercial 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

Pepacton

Schoharie

Ashokan

NeversinkRoundout

Cannonsville

Catskill/Delaware Watersheds

NYC Watersheds

Ashokan ReservoirAshokan Reservoir

Schoharie ReservoirSchoharie Reservoir

Neversink ReservoirNeversink Reservoir

Croton System

West Branch ReservoirWest Branch Reservoir

City TunnelsCity Tunnels

Ashokan Reservoir

Schoharie Reservoir

Neversink Reservoir

West Branch Reservoir

NY 301 crosses West Branch Reservoir

Kensico Reservoir

City Tunnels

Jerome Park ReservoirJerome Park Reservoir

Jerome Park Reservoir

Gaging Stations

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

6/1/

50

6/1/

54

6/1/

58

6/1/

62

6/1/

66

6/1/

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ent o

f ful

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Stream flow gage station map

0.010.020.030.040.050.060.070.080.090.0

100.0

6/1/

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6/1/

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ent o

f ful

lCannonsville 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

6/1/

54

6/1/

58

6/1/

62

6/1/

66

6/1/

70

6/1/

74

6/1/

78

6/1/

82

6/1/

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6/1/

90

perc

ent o

f ful

l

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/

66

6/1/

70

6/1/

74

6/1/

78

6/1/

82

6/1/

86

6/1/

90

perc

ent o

f ful

l