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

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

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!

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

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)

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

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

Cannonsville 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 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)

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) solution                                   Reservoir Levels

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

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

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 1.2-2.0) the maximum hourly rate is 2.5 (range of 1.5-3.5) for NYC the maximum instantaneous rate was _____ increases 1.75 *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 every ________ two hours

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?

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

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 Cannonsville Pepacton Ashokan Neversink Roundout

NYC Watersheds

Croton System

Ashokan Reservoir

Schoharie Reservoir

Neversink Reservoir

West Branch Reservoir

City Tunnels

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?

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

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

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