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…)