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Ashokan
Reservoirs
Kensico
Balancing Supply and Demand
Croton
Reservoir
Spillway
Hillview
Monroe L. Weber-Shirk
School of Civil and
Environmental Engineering
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
Supply
Seasonal supply fluctuations
Buffered using _________
watershed reservoirs
Demand
Seasonal demand fluctuations
Daily demand fluctuations
Buffered using distribution
_________ reservoirs
Effect of flow fluctuations on system design
Size of balancing reservoirs
pipe sizes
Average Total Monthly Flow into
Pepacton Reservoir (0.540 km3 storage)
Million m3/month
Reservoir full (hopefully)
140
120
100
80
60
40
20
0
average
Deficit provided
by storage
35 million m3/month * __
____
___ million m3
5 month = 175
Better design is based on drought conditions!
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?
_________________________________
The average stream flow into the reservoir.
What are the critical events in history that
determine how big the reservoir has to be?
__________
Droughts
Reservoir Mass Balance
Equations
+
=
+
S0 + Ii = Oi + Si
O = Cumulative (________
Demand + _________
River flow + ___________
Evaporation )
S0 Ii Di R i Si
i
Ii=
å Q Dt
i
1
True at any time!
or Ii 1 Qi t
a f
Di= QNYC t t0
1000 kg/m3
1000
990
980
970
960
950
0
Density (kg/m3)
Density (mass/unit volume) r
density of water:
Density (kg/m3)
Density of Water
50
100
Temperature (C)
1000
999
998
997
0
10
Temperature (C)
20
Downstream River Flow?
Simplest operating rule
Waste from reservoir when reservoir is full
Don’t waste from reservoir if reservoir isn’t full
More complex rules could easily be incorporated
into a spreadsheet model
Minimum discharge into stream as a function of
reservoir storage volume or ______________
drought status
Based on regulations
Reservoir Rules in Equation
Form
When Si = Smax
When is reservoir full? ___________________
S0 Ii Di R i Si
Si S0 Ii Di R i
6/1/90
6/1/86
6/1/82
6/1/78
6/1/74
6/1/70
6/1/66
6/1/62
Smax= Reservoir Capacity
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
6/1/58
ELSE R i R i-1
6/1/54
THEN R i S0 Ii - Di - Smax
Reservoir is overflowing
Overflow goes into river
No additional river flow
6/1/50
S0 Ii - Di - R i-1 Smax
percent of full
IF
How could we increase safe yield?
6/1/90
6/1/86
6/1/82
6/1/78
6/1/74
6/1/70
6/1/66
6/1/62
6/1/58
6/1/54
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
6/1/50
percent of full
Cannonsville Reservoir Storage
(Demand of 1.04 x 106 m3/day)
Increase reservoir volume
Storage vs. Safe Yield for
Cannonsville Reservoir
safe yield
(million m3/day)
What is the asymptote? Average stream flow
1.5
1
0.5
0
0
250
367
500
750
storage volume (million m3)
1000
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? solution
Current Reservoir levels
(http://www.ci.nyc.ny.us/nyclink/html/dep/html/current.html)
Reservoir Levels
Seasonal, Daily, and Hourly
Fluctuations
Substantial increase in water demand during
watering lawns, swimming pools
summer due to_______________________
Peak flows
___________________________________
Early
morning as people get ready to go to work/school
________________________
Commercial
Breaks (not any more)
Low flows
______________________
Between
midnight and 5 am
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 __________
increases
If the average annual flow rate is 1.0 then
the 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)
1.75
for NYC the maximum instantaneous rate was _____
*Henry and Heinke p 386
Methods to Even Out
Fluctuations
Seasonal fluctuations
Source (watershed) reservoirs
Kensico and West Branch Reservoirs
Daily fluctuations
Hillview and Jerome Park Reservoirs (directly
connected to distribution tunnels)
Hillview has 3.4 million m3 useable storage
Flows from Kensico to Hillview are adjusted
two hours
every ________
Balancing
Reservoirs
OK Fred, I’ll go give it a turn.
Did you say you have more water
than you need?
Hey Bob, I need some more
water. Could you open the
valve another turn?
Where are the largest
tunnels in the NYC
water supply and
distribution system?
How Can You Estimate Required
Balancing-Reservoir Capacity?
Variable supply
Variable demand
Analyze historic record to search for worst
case conditions
Use same Mass Balance analysis
Include variable ________
demand in analysis
Other unusual demands… Fire 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
Catskill/Delaware Watersheds
Schoharie
Ashokan
Neversink
NYC
Watersheds
Ashokan Reservoir
riovreseR eirahohcS
Neversink Reservoir
Croton System
West Branch Reservoir
City Tunnels
Ashokan Reservoir
Schoharie Reservoir
Neversink Reservoir
West Branch Reservoir
NY 301 crosses West Branch
Reservoir
Kensico Reservoir
City Tunnels
Jerome 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/s
How long could NYC go without any
inflow into the reservoirs?
1000 mfO s
a
day
L
2 km M
380
days
P
km
N Q61m 86400 s
3
3
3
6/1/90
6/1/86
6/1/82
6/1/78
6/1/74
6/1/70
6/1/66
6/1/62
6/1/58
6/1/54
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
6/1/50
percent of full
Cannonsville Reservoir Storage
(Demand of 0.5 x 106 m3/day)
Stream flow gage station map
6/1/90
6/1/86
6/1/82
6/1/78
6/1/74
6/1/70
6/1/66
6/1/62
6/1/58
6/1/54
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
6/1/50
percent of full
Cannonsville Reservoir Storage
(Demand of 0.75 x 106 m3/day)
6/1/90
6/1/86
6/1/82
6/1/78
6/1/74
6/1/70
6/1/66
6/1/62
6/1/58
6/1/54
6/1/50
percent of full
6/1/90
6/1/86
6/1/82
6/1/78
6/1/74
6/1/70
6/1/66
6/1/62
6/1/58
6/1/54
6/1/50
percent of full
Cannonsville Reservoir Storage
(Demand of 1 x 106 m3/day)
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0