Measuring the Cost-Effectiveness of Stormwater Management Plans Using

Life Cycle Costs & Performance Metrics

Alex Foraste, P.E. (ORISE Fellow)1, Robert Goo1, Joel Thrash, CPESC2, Lisa Hair, P.E.1

1U.S. EPA, Office of Water, Nonpoint Source Control Branch2Cardno JFNew

Low Impact Development SymposiumPhiladelphia, PASeptember 2011 

• Overview & discussion of problem with conventional development

• Explored ways that StormwaterCosts are reported

• Describe 4 STEP process used to measure cost-effectiveness

• Identified several built LID projects to serve as case studies to measure cost-effectiveness.

• Compared each LID project to an alternatively designed conventional development plan for an ‘apples to apples’ comparison.

• Compiled results from 8 case studies analyzed and report trends

• Compared $/lb/yr and $/cf/yr results to other published values to ‘ground-truth’

• Assessed Environmental and Social Benefits of LID (Triple Bottom Line)

Two dominant surface features in today’s conventional development:

1.) Surface Parking lots2.) Turf grass

Turf vs. Trees (short vs. tall vegetation)

Prairie & Turf Root Depths (Agrecol 2011)

Measuring Cost-EffectivenessLinking Cost to Performance

STEP 1 - Measure Performance (cf/year, lbs/year)

STEP 2 - Estimate Construction Capital Cost ($)

STEP 3 – Estimate Life Cycle Costs ($/year)

STEP 4 – Calculate Cost-Effectiveness ($/cf/year)

Measure Stormwater Performance (STEP 1)

Model/measure the pollutant of interest and its rate of removal or reduction from site contributing runoff

Cubic Feet of runoff volume reduced per year (cf/year), Pounds of Phosphorus (TP) removed per year (lbs/year) Pounds of Nitrogen (TN) removed per year (lbs/year) Pounds of Sediment (TSS) removed per year (lbs/year)

Must be in terms of unit removed/reduced per time (rate)

Capital Costs (Step 2) - Many ways to represent cost

BMP Unit Costs ‘in space’ ($/cf, $/sf) Capital Cost per storage volume (cf or gallon) of installed BMP (e.g. cistern) Capital Cost per unit area of installed BMP (e.g. permeable pavers) MMDS (2009), Weiss (2005, 2007), Olson et. al. (2010), UNHSC 2011)

BMP Unit Costs ‘in the ground’ ($/cf, $/sf) Accounts for costs needed to tie BMP into site E.g. grading and outlet protection required to tie bioretention cell into slope, conveyance

system required to direct runoff to BMPs, additional clearing, grubbing, and land acquisition costs for detention pond, retaining walls for ponds on tight, sloped site, etc.

Stormwater Costs per acre ($/acre, $/impervious acre) Cost per developed acre, Olsson (2007) Cost per impervious acre, CSN (2010 a,b), CDM (2010) Costs irrespective of SWM standard; can embed details making it difficult to generalize

Stormwater and Site development Project Capital Costs ($) EPA (2007), ECONorthwest (2007), UNHSC (2011)

Life Cycle Costs & Savings (STEP 3) - Shifting the decision basis from Capital to Life Cycle Costs

Maintenance Costs WERF (2009) Lampe et al. (2005), rain gardens, swales, green roofs, permeable

pavement, large cisterns, In-curb planter vaults, extended detention basins, retention ponds

Olson, Roesner, Urbonas, & MacKenzie (2010) BMPRealcost EPA (2005) & CWP (1998) as % of construction cost Weiss et al. (2005, 2007) as % of construction cost Erickson et al. (2010) survey of 38 cities in MN & WI

Estimate Cost Additions or Savings Think about the USGBC & accepted green building & energy efficiency industry Same applies to site – need to consider long term operations, maintenance, high

performance, efficiency, resource savings, and design life. Savings may be realized through reduced corrective actions, decreased purchases of

municipally supplied water and commercial fertilizer, utility fee reductions, and extended design life.

Measuring Cost-Effectiveness(STEP 4) - Linking cost to performance

Determine Discount Rate (r) & timeframe (t)

Calculate Net Present Value (NPV)

Calculate Annuity Rate (At,r)

Calculate Equivalent Annual Cost (EAC)

Calculate Cost Effectiveness: Divide EAC by pollutant load removed per year (Pr)

t

nn

rR

NPV1 1

 

r

rA

t

rt

1

11

,

rtANPVEAC

,

rPEACivenessCostEffect

($)

($/yr)

($/lb/yr)

Case Study ApplicationUNION RURAL ELECTRIC

Source: Cardno JFNew

Conventional Site Plan LID Site Plan

Location: Marysville, Ohio Pre‐development

Storm Event DischargeGreenfield Condition

No Controls LID DesignConventional 

Design

Peak Flow (cfs) 0.04 2.61 0.26 0.43Volume (Ac‐ft) 0.016 0.13 0.01 0.10Peak Flow (cfs)  2.55 11.62 1.71 2.58Volume (Ac‐ft)  0.27 0.57 0.08 0.53Peak Flow (cfs) 5.60 18.93 2.94 5.68Volume (Ac‐ft) 0.57 0.95 0.14 0.89

100 year storm = 5"

10 year storm (3.8")

2 year storm (2.6")

Post‐development

WQv (1")

STEP 1 - Measuring Stormwater Performance

Hydrologic Criteria:

Q2post<=Q2preQ10post<=Q10pre

Assess Q100post overflow/freeboard

Hydrologic Performance – 1 year storm

STEP 2 – Estimate Capital CostsStormwater & Drainage Conveyance

UnitsConventional 

designLID       

designQualitative Difference

Cost Difference 

metric Cost Cost # units $

L.F.& width $116,214 $120,000Conv. Asphal t: subgrade  

compaction @ $3.60/SF, Perv. Conc @ $3.75/SF

$3,786

L.F.  $35,462 $19,3101,084 L.F. less  curb to 

encourage  sheet flow to bioswale

‐$16,152

L.F. $71,699 $34,250

840 L.F. less  s torm dra in pipe  @ $27/LF for 12", $38/LF for 18" and $45/LF for 24"; $3.01/LF for 

gravel  bed

‐$37,448

# grates, drop inlets

$28,600 $6,600 4 less  grates , 6 less  drop inlets  @ $2,200 each ‐$22,000

# ponds & size, risers

$58,082 $90,179

El iminated bas in of 33,760 CF; added 3 ra in gardens  & bio‐

swale  @ $8.17/SF avg. Includes  overs ight, l andscaping, 

materia l s , labor.

$32,097

SF $4,818 $11,651 turf vs . bioretention plants $6,833

$ $310,056 $270,340 ‐12.8% ‐$39,717

 course (1.5") and Asphalt concrete base (6") both excluded from URE conventional paving costs to more closely match standard asphalt detail.

Landscaping3

Site Work Category

Roadway & Access1

Curb & Gutter

Stormwater Management (SWM ponds, bio cells & swales w/out Plants)2

Stormwater Structures (grates & inlets)

Stormwater Drainage Pipes2

Total Stormwater & Roadway Cost 

1 Conventional pavement costs from City of Marysville US 36 roadway expansion project public records.  Unit costs include: $0.11/SF for subgrade compaction, $1.85/CF for aggregate base (6"), $5.19/CF for Asphalt Concrete suface course (1.5").  Original Asphalt concrete Intermediate

3  Landscpaing costs only reflect areas where rain garden and bioretention plants were used in the place of Turf.  Sod costs (@$0.5/sf). 

2 Underdrain costs reflected in Stormwater Management category for LID site. City of Marysville public records for Crazy Burrito commercial project used for cost basis to estimate Detention Basin, stormwater drainage, and structure SWM costs for URE.

STEP 3 – Estimate Life Cycle Costs & Savings

Traditional asphalt still requires maintenance:

Sealing cracks, coat sealing, resurfacing, filling of potholes, periodic replacement (mill and overlay).

Street sweeping; Litter & minor debris removal; Inspection, & reporting; Periodic removal, washing of aggregate and/or replace and reinstall.

STEP 3 – Estimate Life Cycle Costs & Savings

Maintenance: Irrigation maintenance , Mowing, Fertilizer application

Resource Inputs: Fertilizer, Herbicide, Water for Irrigation, Gas for mowing & trimming

Maintenance: Vegetation Management, periodic Mulch replacement, 1-2 years of irrigation

Resource Inputs: Initial water for irrigation

STEP 4 – Calculate Cost-Effectiveness

Conventional Design

LID Design DifferenceCost‐Effective 

Ratio

$/(unit) $/(unit) LID ‐ Conv Conv/LID$4.20 $2.69 ‐$1.52 1.6

$67,684 $53,631 ‐$14,053 1.3

$13,548 $3,341 ‐$10,206 4.1$2,230 $465 ‐$1,765 4.8

$0.58 $0.10 ‐$0.48 6.0

1 Fi rs t year conventiona l  storage  refers  to capaci ty of detention pond, not permanent pool . BMP, storm dra in pipe, and inlet costs  included.  Additional  pervious  pavement and landscaping costs  as  wel l  as  avoided costs  included in LID option and used as  cost bas is  for al l  metrics  in this  table. 

WQv volume Retained (cf/50 yrs)4

Stormwater & Drainage Costs per Treated Impervious Acre ($/Ac)2

Annual 

1st Y

ear

Ann

ual

Storage Unit Cost (cf)1

Total Phosphorus Removed (lbs/50 yrs)4

2 Includes  stormwater BMPs, storm dra in pipes  and inlets .  Pervious  concrete  included as  "treated impervious  acre".  If quanti fied for BMP only, conv. Is  $24,821/ac and LID i s  $36,174/ac for bioretention & additional  pervious  concrete.4 Equiva lent Annual  costs  represents  NPV for 50 year period, includes  al l  maintenance  costs , and i s  discounted at a  5.5% rate.

Total Nitrogen Removed (lbs/50 yrs)4

Pollutant 

Overall Results & Findings

Overall Results: Capital Cost Basis

Key Finding #1 – Site development capital costs of LID plans were found to be 19% less than conventional plans.

Key Finding #1 – Site development capital costs of LID plans were found to be 19% less than conventional plans.

Overall Results: Cost-Effectiveness

Key Finding - On average, the LID plans analyzed in this study were found to be four to five times more cost-effective at removing nutrient pollutant loads and retaining

stormwater runoff volumes than the conventional plans.

Key Finding - On average, the LID plans analyzed in this study were found to be four to five times more cost-effective at removing nutrient pollutant loads and retaining

stormwater runoff volumes than the conventional plans.

Key Finding - Storage Volume vs. Volume Retention Unit Costs - Installation costs are often reported in $/volume of storage. Volume retention over time compared to storage volume

may differ significantly from one BMP to the next.

Key Finding - Storage Volume vs. Volume Retention Unit Costs - Installation costs are often reported in $/volume of storage. Volume retention over time compared to storage volume

may differ significantly from one BMP to the next.

Compounding Effects

Key Finding -The primary factor underlying the improved cost-effectiveness of LID over conventional methods is its superior ability to retain stormwater and reduce runoff volumes

at a comparable or lower cost.

Key Finding -The primary factor underlying the improved cost-effectiveness of LID over conventional methods is its superior ability to retain stormwater and reduce runoff volumes

at a comparable or lower cost.

Key Finding -Stormwater plans that exhibit lower life cycle costs and higher performance metrics realize compounding benefits, when compared on a cost per performance metric

basis, such as cost-effectiveness.

Key Finding -Stormwater plans that exhibit lower life cycle costs and higher performance metrics realize compounding benefits, when compared on a cost per performance metric

basis, such as cost-effectiveness.

Comparison to published Results42 total projects, 9 studies

Triple Bottom Line

Image © John Foraste Photography

Alex Foraste, P.E.: foraste.alex@epa.gov; Robert Goo: goo.robert@epa.govJoel Thrash, CPESC: joel.thrash@cardno.com; Lisa Hair, P.E.: hair.lisa@epa.gov

Contact Information:

DISCLAIMER: Any opinions expressed in this publication are those of the Author and do not, necessarily, reflect the official positions and policies of the U.S. EPA. The data and results generated in the report that this presentation summarized, and hispresentation itself are in draft form only, and therefore subject to change. Neither has been officially reviewed by the EPA, and therefore does not represent an Agency publication. Reference herein to any specific commercial products, process, or service bytrade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the Author or the U.S. EPA. Some of the photographs, figures, tables, and other graphics that are used in this document are copyrighted material for which permission are marked [pending], and are still under copyright by the original authors and publishers. If you wish to use any of the copyrighted photographs, figures, tables, or other graphics in any other publication, you must contact the owner and request permission