View
218
Download
0
Category
Tags:
Preview:
Citation preview
Advancing the management of water resources
Columbia, MD Raleigh, NC
Andrew D. Dehoff, P.E and
Daniel P. Sheer, Ph.D., P.E.
October 22, 2004
Position Analysis and Forecast-Based Water Supply Operations
Reducing Risk and Saving Money by Operating Smarter
River
Con
o win
go C
ree k
Susquehanna
W. Con
ewag
o C
reek
Creek
Peque
a
Y O R K C O U N T Y
L A N C A S T E R C O U N T Y
H A R F O R D C O U N T Y C E C I L C O U N T Y
PENNSYLVANIA
MARYLAND
York
LancasterMarietta
Columbia
LakeWilliams
LakeRedman
OctoraroLake
ChesapeakeBay
Riv
er
Conest
ogaC
odor
us Cree
k
E. Branch
S . Branch
Muddy Creek
Creek
Deer
Cre
ek
O ctor
aro
Marietta Gage
Conowingo Gage
City of LancasterPipeline
Chester WaterAuthority Pipelines
Muddy Run PumpedStorage Facility
Safe HarborDam
Holtwood Dam
Conowingo Dam
City of BaltimorePipeline
Peach BottomAtomic Power Station
York WaterCo. Pipeline
Conowingo Pool and surrounding area
Conflicting Objectives
FERC minimum pool elevation FERC minimum flow requirements FERC recreation requirements Municipal withdrawals Habitat/fishery needs Salinity intrusion
ConowingoDam
108.5 ft. Normal Pool
104.5 ft. Normal Minimum Level due to hydroelectric operations
102.5 ft. Critical Level for Peach Bottom
100.5 ft. Minimum Conowingo Pool
97.0 ft. Peach Bottom shuts down
Limited Range of Operations
No reservoir system is fail safe
There is always some risk of running out of water.
Water supply planning and operations can reduce, but not eliminate that risk.
Position Analysis and forecast-based operations can allow you to manage risk explicitly and effectively
Reducing Risk
Capacity expansion reduces the risk of water shortage
Smart operating policies can also reduce the risk of water shortage
What are you willing to do or pay to achieve an acceptable level of reliability?
Traditional Approach
Safe Yield:
The amount of water that can be safely withdrawn from a reservoir (system) during some specified drought.
Implied Reliability:
Safe yield of record
50-year safe yield
20-year safe yield
Etc.
Traditional Approach, cont’d.
Sup
ply/
Dem
and
Time
Safe Yield
Supply
Demand
Traditional Approach, cont’d.
Sup
ply/
Dem
and
Time
Safe Yield
Supply
Demand
Safe Yield Isn’t a measure of reliability
Safe yield in all but the simplest systems is more than the sum of the safe yields of individual facilities
How the system is operated is very important Safe Yield isn’t as Safe as It Sounds
A system operated at historical safe yield will run out of water in any worse drought
A system operated at 20 year safe yield will run out of water an average of 1 in 21 years
Operations are Crucial to Supply
Increasing water available during droughts increases reliability
Conjoint operations of all facilities can substantially increase the water available during droughts
The objective of such operations is to minimize water loss through spill or seepage
Conjoint Use Simple Example
Reservoir A - minimum spring inflow = 25% of storage
Reservoir B - minimum spring inflow = 125% of storage
Rule - empty reservoir B before drawing Reservoir A down more than 25%
If the drought is 2 years long, the system will benefit from a complete refill of Reservoir B in the spring of the second year.
Examples of Conjoint Operations
Upper and Lower Delaware Basin reservoirs
Conowingo Pool and Baltimore City reservoirs
Kansas River Basin
Potomac River Basin
G52G53
G54
G56
G57
G58 G59
G60
G61G62
Baltimore
Susq-17
Conod'Ck-1
LetterKenny
Hogestown
445
Susq-18
450Harrisburg
455
H'burgDem
460CampHill
YellowBrCkSusq-19
475 YorkHaven
480 TMI
485 BrunnerIsPP
490
Pinchot
492
Manchester
ConewagoCk
Susq-20
495 Marietta
496
Mar'Local
500
EDonegalPP
505
Marburg
CodorusCk-1
510SpringGrove
515Glatfelter 520IndianRock
525Redman
ECod'Ck-1
530Williams
535YorkPS
SBrCod'Ck
536 York
538
YorkWTP
York'Ret
540
YorkDem 545Lancaster
ConestogaR
548Lanc'Ret
Lanc'Dem
550
555
SafeHarbor
PumpStorage
556
SHLocal
560Holtwood
561
HoltLocal
565
MuddyRun
570Conowingo
BigInch-1
571
ConoLocal575
PeachBPP
580
DeltaPP OctoraroCk
585
OctoraroLk590
Chester
595SusqMouth
596
MouthLocal
600
Rocks
DeerCk-1605
OtherDem
Darlington
610
615
Aberdeen
620
Perryvil le
625
HavreDeGrace
630
CecilCo
700
Harford
BigInch-2
705
Harf'Dem
PrettyBoy
GunPow.R-1
735
MontebelloPipe
740LochRav en
GunPow.R-2
742
MontebelloFP
745
Montebello-2
750
HillenVernonPS
760
EBaltimore
765
Liberty
PatapscoR
AshburtonFS
770
Joint
998 CheBay-1
999
CheBay-2
Measuring Reliability
In actual operations, emergency measures will prevent a system from running out of water Short-term conservation Alternative higher cost or lower quality
sources Emergency measures will be undertaken in many
droughts, even if they are less severe than the drought used to calculate safe yield
Expected frequency, duration, and severity of drought emergencies are good measures
0
5
10
15
20
25
run1 run2Run
Phase 4
Phase 3
Phase 2
Phase 1
Water Shortage PhasesPercent of Years in Each Phase
Frequency, Severity and Duration
Evaluated with a simulation model like OASIS Model includes facilities, demands, facility
operating policies and drought management policies
Model runs for a long (~50 year or longer) record, usually of historical flows
Model outputs include when emergency measures are imposed
Model also produces output that can be used for environmental and economic evaluations
Operations to Increase Reliability
Low cost conservation measures implemented early can avert high cost draconian measures later.
Water saved = #days * demand_reduction/day Costs are more than monetary costs Costs of reducing 50% may be orders of
magnitude larger than the cost of reducing 10%
Operating Rules
Rules will also include provisions that minimize the negative effects of implementing emergency measures
Rules will include operations to increase water available
Rules should be adequate to handle any historical drought with a reasonable margin of safety.
Rules will NOT be traditional safe yield rules
Operating Rules Can Impact
Demands
Environmental requirements
Balancing of supplies
Operating Rules Can Consider
Present state of the resource Storage Groundwater levels
Inflow forecasts Time of year Demand forecasts Other factors
We Can’t Forecast Weather, But
Streamflow depends on both weather and the “dryness” or “wetness” of the drainage basin
The snow, soil and aquifers in the basin are reservoirs and we can get some useful information about how much water they will or can contribute to a stream or to wells over the next several months
That information can be used to help make better operating decisions
NOT Particularly Useful Forecasts
Expected value of flow over the next x months -on average, there will always be enough water
Flows are likely to be higher than normal or lower than normal - OK, worry a little less or a little more
Worst case scenario - wonderful if you’re paranoid, otherwise useless unless you know its probability (most are arbitrary)
Useful Forecasts
Flow time series and their probabilities
Flow totals for various periods into the future and their probabilities
timet0
Historical Time Series
Str
eam
flow
Conditional Time Series Forecasts
All start from today’s flows Can be produced using statistical methods or
rainfall runoff models that represent surficial and deep aquifer interactions (and snow, if appropriate
Usually produce “equally likely” traces based on the assumption that any year’s historical weather is equally likely to repeat
timet0
Time series conditioned upon today’s basin conditions
Str
eam
flow
Forecasts to Useful Information
Run the simulation model for each of the equally likely forecasts
If there are 50 equally likely traces, and the reservoir falls below a given level in only 5 of the simulations, then the probability of falling below that level is 10% given the rules used in the simulation
Probabilities of other consequences, including environmental consequences and utility revenue can also be evaluated
0 10 20 30 40 50 60 70 80 90 100
Probability of Non-exceedance
100
105
110
115
120
125
130
Stag
e in
feet
Reservoir Stage930911, Tue Aug 17 2004 17:15
2 week 4 week 8 week 12 week 16 week
0 10 20 30 40 50 60 70 80 90 100
Probability of Non-exceedance
100
105
110
115
120
125
130
Stag
e in
feet
Reservoir Stage930731, Tue Aug 17 2004 17:18
2 week 4 week 8 week 12 week 16 week
0 10 20 30 40 50 60 70 80 90 100
Probability of Non-exceedance
100
105
110
115
120
125
130
Stag
e in
feet
Reservoir Stage930814, Tue Aug 17 2004 17:17
2 week 4 week 8 week 12 week 16 week
0 10 20 30 40 50 60 70 80 90 100
Probability of Non-exceedance
100
105
110
115
120
125
130
Stag
e in
feet
Reservoir Stage930828, Tue Aug 17 2004 17:16
2 week 4 week 8 week 12 week 16 week
0 10 20 30 40 50 60 70 80 90 100
Probability of Non-exceedance
100
105
110
115
120
125
130
Stag
e in
feet
Reservoir Stage930911, Tue Aug 17 2004 17:15
2 week 4 week 8 week 12 week 16 week
How to respond
If the probabilities of untoward events are too high, change the rule and test again. If it works and you’re happy with the rule, do it.
Find a rule that works for the long-term so you don’t have to change rules “on the fly”
Creating a Rule That Uses Forecasts
Many forms are possible The Rocky Mount rule
If the probability of the reservoir falling below 25% in the next 8 weeks is > 20% institute water use restrictions and reduce instream flow
A possible (and untested) Conowingo Rule If the probability of Marietta flow falling
below 2500 cfs in the next summer is > 20% begin Baltimore pumping
Effective Rules
Can NOT miss any droughts Must provide enough advance notice for
remedial action to be effective Minimize the number of false alarms
Developed by trial and error through the use of simulation models.
Evaluating Rules
Scenario Number of
Days in Water Restriction
Number of Years with Water Restrictions
Volume of Water Not Delivered
(million gallons)
1 10 1 25
2 16 3 30
3 5 5 5
4 25 3 140
5 30 6 130
6 18 2 65
Checking that the Rule is Sufficient
PERFORMANCE SUMMARY(15 mgd Demand; 80 cfs Minimum Release)
DROUGHTS OFRECORD
REDUCTION TRIGGERED INLONG-TERM SIMULATION(20%, 2 months, Elev. 115)
REDUCTION BASED ON REAL-TIMEEVALUATION (weeks)
(3) 1932 Yes 4
(5) 1933-34 No 0
(2) 1968 Yes 5
(4) 1980-81 Yes 16
1986 Yes 2
1987 Yes 1
1988 Yes 0
(1) 1993 Yes 12
Rocky Mount Results
Change in operating rule replaced $70M pipeline Required emergency measures infrequent and
not severe NCDNR agreed to tie reductions in instream
flow requirements to imposition of demand restrictions
Environmental impacts of infrequent reductions in low flow (still well above “natural flows”) judged less than impacts of building and operating the pipeline
Advancing the management of water resources
Columbia, MD Raleigh, NC
Thank You
Recommended