1. Balancing Supply and Demand
Ashokan
Reservoirs
Kensico
Balancing Supply and Demand
Hillview
Croton Reservoir Spillway

2. Water Supply and Demand Fluctuations
Seasonal supply fluctuations
Buffered using _________ reservoirs
Demand
Seasonal demand fluctuations
Daily demand fluctuations
Effect of flow fluctuations on system design
Size of balancing reservoirs
pipe sizes
watershed
distribution

3. Average Total Monthly Flow into Pepacton Reservoir (0.540 km3 storage)
Reservoir full (hopefully)
140
120
100 80
Million m3/month
average 60 40 20
Deficit provided by storage
May
July
January
March
September
November
____ million m3/month * __ month = ___ million m3 38 5
190
Better design is based on drought conditions!

4. 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

5. Reservoir Mass Balance Equations+ = +
Storage
Initial storage
Cumulative Inflow
Cumulative Outflow
O = Cumulative (________ + _________ + ___________ )
Demand
River flow
Evaporation
True at any time! or
Ii=
Di=

6. Density of Water 1000 kg/m3 Density (mass/unit volume) r
950
960
970
980
990
1000 50
100
Temperature (C)
Density (kg/m3)
Density (mass/unit volume) r
density of water:
1000 kg/m3
997
998
999
1000 10 20
Temperature (C)
Density (kg/m3)

7. 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 ______________
Based on regulations
drought status

8. Reservoir Rules in Equation Form
When Si = Smax
When is reservoir full? ___________________
Reservoir is overflowing
Overflow goes into river
No additional river flow
Smax=
Reservoir Capacity

9. Cannonsville Reservoir Storage (Demand of 1.04 x 106 m3/day)
How could we increase safe yield?
Increase reservoir volume

10. Storage vs. Safe Yield for Cannonsville Reservoir
What is the asymptote?
Average stream flow
0.5 1
1.5
250
500
750
1000
(million m3/day)
safe yield
367
storage volume (million m3)

11. 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 (
solution
                                 
Reservoir Levels

12. Empty NYC Reservoirs? What happens as Reservoir Levels drop?
__________________________________
Nutrient levels increase (less dilution)
Algae grows (taste and odors)
Water quality deteriorates
Shorter residence time (less time for pathogens to die)
Drought watch

13. Seasonal, Daily, and Hourly Fluctuations
Substantial increase in water demand during summer due to_______________________
Peak flows ___________________________________ ________________________
Low flows ______________________
watering lawns, swimming pools
Early morning as people get ready to go to work/school
Commercial Breaks (not any more)
Between midnight and 5 am

14. Estimates of Daily and Hourly Fluctuations*
As the time period of analysis decreases in length the maximum rate of water demand during that time period __________
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 )
the maximum hourly rate is 2.5 (range of )
for NYC the maximum instantaneous rate was _____
increases
1.75
*Henry and Heinke p 386

15. 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 every ________
two hours

16. Balancing Reservoirs
OK Fred, I’ll go give it another 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?

17. 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 ________ in analysis
Other unusual demands…
demand
Fire fighting needs
Main breaks
Maintenance of supply tunnels

18. 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

19. Catskill/Delaware Watersheds
Schoharie
Cannonsville
Pepacton
Ashokan
Neversink
Roundout

20. NYC Watersheds

21. Croton System

22. Ashokan Reservoir

23. Schoharie Reservoir

24. Neversink Reservoir

25. West Branch Reservoir

26. City Tunnels

27. Jerome Park Reservoir

28. Gaging Stations

29. 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?

30. Cannonsville Reservoir Storage (Demand of 0.5 x 106 m3/day)
Stream flow gage station map

31. Cannonsville Reservoir Storage (Demand of 0.75 x 106 m3/day)

32. Cannonsville Reservoir Storage (Demand of 1 x 106 m3/day)