9 wet weather freakonomics
TRANSCRIPT
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Wet-Weather Freakonomics: A Cost-Benefit Calculus for a Sanitary
Sewer Backup Reduction Program
Presented by
Vinnie Bergl, P.E.
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The Past
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Knee of the Curve ca. 1984
• Many I/I sources still
“low-hanging fruit”
• Treatment capacity
more feasible
• Relief capacity? Justtell us where to send it!
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Knee of the Curve Present Day
• I/I sources are higher
up in the trees
• Treatment capacity a
tougher sell
• Relief capacity?There’s nowhere to
send it!
+ i n f l a t i o n
+storage
???
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I/I Source Distribution ca. 1984
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I/I Source Distribution Present Day
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The Situation Today
• Rare storms increasingly less rare
•
Single performance criteria elusive
• Growing emphasis on basement backups
• Infinite possibilities, finite resources
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A Metric for 2014
• How can we compare the costs of:
– I/I reduction
– capacity improvements
– private backup protection
– and many combinations thereof
marginal cost per potential backupprevented
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Marginal Cost per Potential BackupPrevented
A. Establish level of protection (LOP) throughout systemB. Determine number of properties within each LOP
C. Annualize potential basement backups (PBBs)
D. Compare to existing conditions for reduction in PBBs
E. Price each alternativeF. Divide reduction in PBBs by cost for marginal cost
A × B = C
D1 = C0 – C1 ; D2 = C0 – C2 ; etc.
F = E ÷ D
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Marginal Cost per Potential Backup Prevented
A × B = C
D1 = C0 – C1 ; D2 = C0 – C2 ; etc.
F = D ÷ E
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Level of Protection Analysis
partially full (plenty of freeboard)
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Level of Protection Analysis
surcharged (danger)
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Level of Protection Analysis
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Level of Protection Analysis
1-year storm
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Level of Protection Analysis
2-year storm
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Level of Protection Analysis
5-year storm
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Level of Protection Analysis
10-year storm
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Level of Protection Analysis
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Level of Protection Analysis
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Level of Protection Analysis
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Level of Protection Analysis
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Level of Protection Analysis
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Level of Protection Analysis
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Level of Protection Analysis
(special methodology for the mayor’s block)
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Basin-Wide Level of Protection
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Basin-Wide Level of Protection
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Case Study 1: City of Elmhurst SouthwestWet-Weather Control Facility
• Model run for existing conditions
•
Less than 1-year LOP in many locations
• Level 1 flow reduction % from I/I source
data
• Model used to assess improvement
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I/I Source Quantification
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Level 1 Flow Reduction
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Existing Conditions
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After Level 1 Flow Reduction
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Development of Alternatives
• Level 1 flow reduction short of desired LOP
• Target LOP of 10 to 25 years
• Additional measures considered
–Option A: Level 2 flow reduction
–Option B: Relief and storage
–PLUS targeted overhead sewers
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Option A: Level 2 Flow Reduction
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Option A: Level 2 Flow Reduction
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Foundation Drain Removal
Excavation aroundfoundation
Overhead sewerrecommended
BMPs for overlanddischarge
Up to $15,000 per
home
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Option A: Level 2 Flow Reduction
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Option A: Level 2 Flow Reduction
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Option A: Level 2 Flow Reduction
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Option B: Relief and Storage
•
Targeted two of lowestLOP areas – upstream of Saylor and
Jackson Lift Station – vicinity of FM discharge
•
Concept included: – upsizing the Saylor and
Jackson lift station – upsizing the tributary sewer – wet-weather force main – wet-weather storage
• Both 10- and 25-year LOPalternatives modeled
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Option B: Relief and Storage
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Option B: Relief and Storage
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Analysis of Alternatives
*Each alternative included targeted overhead
sewers
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Analysis of Alternatives
*Each alternative included targeted overhead
sewers
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Option X: Overhead Sewers for All!
•
Some ideas sound great on paper…
• Major coordination challenge
• High variability in costs
• Slow, plodding process
• Still wouldn’t get rid of SSOs!
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Cost-Benefit Analysis
blue = marginal reduction in potential backups
green = marginal cost
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Epilogue
• Capacity improvements under construction – 24” influent sewer
– 18” wet-weather force main
–2.0 million gallon storage tank at WRF
• I/I reduction plan continues to evolve
• Incentives for private sector remain crucial
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Case Study 2: Village of Wilmette HarmsRoad Region Wet-Weather Storage Plan
• Continuation of 2012 Village-wide modeling
•
2012 model calibrated from flow monitoringperiod with little rain
• Estimated 3.2 MG storage for 10-year LOP
• Model recalibrated in 2013 with 6-month monitoring program
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Case Study 2: Village of Wilmette HarmsRoad Region Wet-Weather Storage Plan
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1. Larger sample to refine RTK parameters
2. Outfall conditions!
What changed?
36” Wilmette sewerMWRD
Harms
Road
Interceptor
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2013 Metering
April 18
~16 hrs. reverse flow
June 26
~7 hrs. reverse flow
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2013 Metering
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• Need for storage more evident
• How much is optimal?
Development of Alternatives
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Analysis of Alternatives
Backups Per Annum by Alternative
Storm
Event
Average #
of Events
Per Year
Existing
Condition
1.5
MG
3.0
MG
4.5
MG
5.5
MG
8.0
MG
11.0
MG
9.0
MG
(P)
2-Month 6 0 0 0 0 0 0 0 0
6-Month 2 116 0 0 0 0 0 0 0
1-Year 1 209 40 38 17 7 7 7 72-Year 0.5 473 441 97 48 48 29 29 29
5-Year 0.2 91 141 284 229 54 49 45 49
10-Year 0.1 4 5 3 43 118 92 92 87
25-Year 0.04 0 1 0 0 5 10 6 7
50-Year 0.02 0 0 0 0 0 4 2 6
100-Year 0.01 0 0 0 0 0 0 2 0Total 893 628 422 337 232 191 184 185
Reduction n/a 265 471 556 661 702 709 708
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Storage Volume Optimization
5.5 MG
(optimal)
3.2 MG
(previous)
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Storage Volume Optimization
ExistingCondition 3.0 MG 4.5 MG 5.5 MG
9.0 MG(P)
Existing 471 556 661 708
3.0 MG $14,642 85 190 2374.5 MG $17,791 $35,211 104 151
5.5 MG $16,643 $21,615 $10,528 47
9.0 MG (P) $25,427 $46,901 $53,476 $148,968
blue = marginal reduction in potential backups
green = marginal cost
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Epilogue• 5.5 MG storage facility under construction
• Village addressing I/I sources to further increase LOP
• Discussions with MWRD and neighboring communitiesfor regional solutions
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The Future
• Improvements to RDII hydrology and I/Isource quantification
• More complete SSO and backup data
•
Integrated storm / sanitary modeling
• Cost optimization engines
…the future, Vinnie?