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C R E A T I N G A N D D E L I V E R I N G B E T T E R S O L U T I O N S. In-Ground Disposal of Captured Stormwater: Is it Worth It?. by Scott Schillereff, Ph.D., P.Geo., EBA, Kelowna, BC Darryl Arsenault, M.Sc., R.P.Bio., EBA, Kelowna, BC - PowerPoint PPT PresentationTRANSCRIPT
C R E A T I N G A N D D E L I V E R I N G B E T T E R S O L U T I O N S
E B A E N G I N E E R I N GC O N S U L T A N T S L T D .
by Scott Schillereff, Ph.D., P.Geo., EBA, Kelowna, BC
Darryl Arsenault, M.Sc., R.P.Bio., EBA, Kelowna, BCLeon Gous, P.Eng., MBA, GM, Community Services, City of Vernon, BC
In-Ground Disposal of Captured Stormwater:
Is it Worth It?
BC Stormwater Planning Guidebook
• Released May 2002 - endorsed by BC Gov., stakeholder groups and BC municipalities
• Objective: “To offer a common sense, effective and affordable approach to integrated stormwater management”
• Best management practice
Source: Stormwater Planning - A Guidebook for British Columbia, 2002
Source: Stormwater Planning - A Guidebook for British Columbia, 2002
If you capture it - the up side:
If you capture it - the up side:
If you let it go - the downside:
If you let it go - the downside:
Tools and Techniques:
• Drywells• Infiltration Trenches• Soakage Trenches• Green roofs• Permeable pavements/driveways• Other infiltration structures
Tools and Techniques:
Source: Stormwater Management Manual, Portland, Oregon, 2002
Tools and Techniques:
Source: Stormwater Planning - A Guidebook for British Columbia, 2002
Tools and Techniques:
Source: Stormwater Management Manual, Portland, Oregon, 2002
Tools and Techniques:
Source: Stormwater Management Manual, Portland, Oregon, 2002
So, capture seems like a good idea,but...
What are the global costs of capture/reuse (best practice) vs. release (standard practice)?
Income Statement Approach
• An Income statement is a measure of financial performance over a period of time
• Expressed as:
Revenues (asset flow, reduced liabilities)
- Expenses (costs, increased liabilities)
= Net (gain or loss over the period)
• Compare Income statements for in-ground disposal with standard conveyance for an urban sub-basin over a municipal life cycle
Framing the question - parameters and constraints
• Control area = urban subbasin - Neighborhood in Okanagan, 100 ha, 400 lots, lot area 60 ha, roads etc. 40 ha
• Annual precip. ~ 400 mm; 75% capture = 300 mm
• Existing storm sewerage (constructed for severe design storm)
• Time period 20 years; periodic maintenance (replacement)
• Global cost analysis (regardless who pays)
Stream1 km1 km
1 km1 km
400 Lots
In-ground disposal “Revenues”:
• reduced sewer maintenance costs• reduced stream erosion, maintain base flow• maintain/enhance recreational fishing value• enhanced residential/parkland values• recharge to aquifer, avoid lowering water table• reduced irrigation water costs• reduced need for irrigation infrastructure• Development Cost Charges (DCC) benefits
In-ground disposal “Expenses”:
• capital costs for in-ground structures• periodic O&M costs• design costs• increased potential for slope failures (slides)• municipal review/approval costs (time)
Standard conveyance disposal “Revenues”:
• reduced design/approval costs• quicker, cheaper installation (tie in to storm sewer)
Standard conveyance disposal “Expenses”:
• stream degradation and restoration cost• stream channelization capital costs• increased O&M costs• loss of recreational fishing value, income• degraded property values (less appreciation)• irrigation infrastructure costs• irrigation water costs• degraded stream water quality (1st flush events)
Income Statement - In-ground Capture (Lots Only)
Revenues - In-ground Disposal Annual 20-yearReduced sewer maintenance costs $5,745 $114,900Healthy stream, property value up ($12,500/yr for 40 lots) $500,000 $10,000,000Enhanced recreational fishing revenue $2,500 $50,000Aquifer recharge, avoid lowered water table (well rehab) $80,000Reduced Irrigation water costs ($75/yr/lot) $30,000 $600,000Reduced irrigation infrastructure/ O&M ($42/yr/lot) $16,800 $336,000DCC benefits $0 $0Total In-ground revenues $11,180,900
Expenses - In-ground DisposalCapital costs for in-ground structures ($5,250 per lot) $2,100,000O&M costs ($250/yr/lot) $100,000 $2,000,000Design costs (one time) $2,500 $2,500Increased potential for slope failure (one resid. failure & repairs) $50,000Municipal/regulatory approval costs (one time) $3,500Total (In-ground expenses): $4,156,000
Net (Revenues less Expenses for In-ground): NET GAIN $7,024,900
Income Statement - Standard Conveyance (Lots only)
Revenues - Conveyance for Lots Annual 20-yearReduced design costs (std; one time; 1/2 of in-ground) $1,250Reduced slope failure (avoidance of residential failure cost) $50,000Total (conveyance for lots): $51,250
Expenses - Conveyance for LotsStream restoration costs (1 km rehab at $500K/km) $500,000Stream channelization capital costs (assume 200 m) $60,000Stream channel O&M costs $10,000 $200,000Loss of fishing income $2,500 $50,000Loss of property appreciation (do not realize 4%) $400,000 $8,000,000Increase irrigation infrastructure costs $0 $0Increased irrigation water costs $30,000 $600,000Stream water quality degradation (assume one $10K fine) $10,000Total (conveyance for lots): $9,420,000
Net (Revenues less Expenses for Conveyance): NET LOSS -$9,368,750
Income Statement Summary over 20 yr
In-ground disposal: Net Gain ~$7.0 M
Std Conveyance: Net Loss ~$9.4 M
Differential: >$16 M over 20 years
Conclusions:
Global economic benefit of >$800K per year for this test scale.
Coarse economic analysis shows strong cost benefit for in-ground disposal
Different benefit values expected for other scenarios (e.g., rural, dense urban, retrofit)
A strong driver is who pays (another tale…)