Basics of Reservoir

Operations 2

Computer Aided Negotiations

Fall 2008

Megan Wiley Rivera

A few words about precision, accuracy and significant figures

• This had no meaning to me as an undergraduate (what’s the harm in using as many digits as my calculator gives me?), but has become very important to me

• Precision is how finely you can read the instrument (degree to which further measurements show the same result)

• Accuracy is how closely the instrument reflects reality (you need a point of comparison)

Reference value

accuracy

precision

0:00

A few words about precision, accuracy and significant figures

• Please do not create false precision in your calculations—carry through the number of significant digits in the number with the fewest

• Similarly, don’t degrade precision by too few digits in your conversion factors

• It is not uncommon for flow gages to be off by 15%

• Wind alone can cause a stage reading to be off by many inches

Reference value

accuracy

precision

Now that we know how to do the accounting…

• Let’s look at the impact of adding a reservoir to a river basin.

• Does the amount of water you’re working with change? (consider consumptive uses)

0:40

Impacts of reservoirs

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With Reservoir Without Reservoir

• What about the hydrograph? (flow in the river)

4:10

Some benefits of reservoirs

• The ability to store water allows you to “use” it when you need it.– Water supply (fire protection, drinking water, irrigation,

manufacturing)– Recreation (on the lake and on the river)– Fisheries– Ecosystem enhancement– Hydropower generation– Cooling water

• Similarly, the ability to store water allows you to hold it when you need to (flood protection)

5:20

A Word About Flood Control

• The amount of water flowing downstream of the reservoir can be reduced if there is void (empty space) when the rain starts

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inflow

void

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inflow Uncontrolled spill

7:00

A Word About Flood Control

• In addition, the reservoir itself results in attenuation of flood peaks

11:00

Some limitations of reservoirs

• They are only so big– Lose benefits when they are empty (with

serious consequences)– Reduce flood control when they are full

(intentional releases versus uncontrolled spill)

• They are subject to local hydrology

12:30

Subject to Local Hydrology

• They only refill as quickly as they refill

• Take Chattahoochee River as an example– Lake Lanier (Buford) has

about 1.1 million af of storage, but only 4% of the basin drainage area

– West Point and WF George have about 300 and 250 kaf of storage respectively

13:50

Lanier therefore has a much lower probability of refill

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Usable Storage in Major Reservoirs

Lanier West Point WF George Lanier Full West Point FullWF George Full

(Historical stages, http://water.sam.usace.army.mil/acfframe.htm)

14:30

To Illustrate Potential Impact on System Storage

• Two model runs were done• Both runs started on June 1 with the SAME amount of

water in storage– Run 1: Lanier started full, other two empty (blue line)– Run 2: Lanier started empty, other two full (red line)

• Model was run from June 1 to March 15 62 times (Historical hydrology from 1939-2000)

• In wet years, Lanier refills under both starting conditions• In worst case, Lanier has 340 kaf less in Run 2 (340 kaf

would meet the needs of NYC’s 8.2 million people for 100 days)

• Imagine what could happen in a multi-year drought

16:35

In addition to being less likely to refill, Lanier is the most upstream

• “Spill” from Lanier can be captured by downstream reservoirs, if they have void.

• “Spill” from the most downstream reservoir is “lost” (flows out of the system)

• Therefore, when there is concern about refill, holding water in most upstream reservoir gives you more options

• Should be balanced against other needs

18:30

Some problems caused by reservoirs

• Ecosystem degradation– Flooding to create reservoirs– Physical barrier of dam– Alterations to hydrograph

• Reduced floodplain inundation• Seasonal shifts• Changes in character of pulses, low-flows, etc

– Alterations to channel morphology

• Displacement of people to create reservoirs• Water wars (reservoirs have to be managed)

20:00

Reduced floodplain inundation

Causes of long-term water-level decline

Floodplain Impacts

Example of channel straightening

Impact on the Apalachicola River

Impact on Apalachicola River

Impact on the Apalachicola River

From Biological Opinion and Conference Report on the U.S. Army Corps of Engineers, Mobile District Interim Operating Plan for Jim Woodruff Dam and the Associated Releases to the Apalachicola River. US Fish and Wildlife Service, Panama City Field Office, Florida, 9/15/2006.

So let’s talk about how to manage them?

• You need to consider specific needs of your basin—performance measures (this is the first third of the class)

• For now, let’s talk about some common management practices

• Start with a brainstorm: think, pair, share– Look over benefits and think about how

reservoirs could be operated to address them– Start with water supply and flood control

Some benefits of reservoirs

• The ability to store water allows you to “use” it when you need it.– Water supply (fire protection, drinking water, irrigation,

manufacturing)– Recreation (on the lake and on the river)– Fisheries– Ecosystem enhancement– Hydropower generation– Cooling water

• Similarly, the ability to store water allows you to hold it when you need to (flood protection)

Some ideas

• Leave void for flood control, but this needs to be balanced against water supply reliability (drawdown when flooding is likely—seasonal, forecasts, drought conditions)

• Consider accounts for different users, allowing them to “call” on the water as they see fit

• Condition operations on probability of refill: desire to start your dry season full; otherwise, you’re likely to accumulate debt over multi-year drought

• Try to keep reservoirs high on weekends (recreational need, reduced power needs)

• Schedule hydropower based on prices rather than fixed levels of generation