By Ted Blahnick, PE


BMP: best management practice

C/I: cost/square foot of impervious surface managed/square foot of BMP

GI: green infrastructure

ISA: impervious surface area

LID: low-impact design

To better comply with the Clean comply with Water Act, EPA is preparing to expand its reach beyond separate and combined-sewer communities to bring the nation under a new, uniform quality standard for old as well as new development. It's the next step toward replacing our patchwork of widely varying — city by city, state by state, region by region — control requirements with one standard that applies to all watersheds.

In addition to focusing on flood control — i.e., designing for individual events — regulators will expect National Pollutant Discharge Elimination System (NPDES) permittees to (NPDES) permittees also control annual runoff volume to mimic predeveloped watershed conditions for typical-year rainfall. The agency's strategies for managing nonpoint pollution emphasize “green infrastructure” that reduces impervious surfaces, increases infiltration, and/or reuses runoff.

The new requirements are expected to be released by November 2012, and they won't affect only stormwater operations. If you haven't already, start reaching out to colleagues in other departments to review and update standards for streets, subdivisions, and other land use.

Control measure metrics

How to calculate the potential cost and impact of best management practices.

The figures on this page represent standalone stormwater projects. But many operations are incorporating runoff-control measures into planned capital improvements like street or curb replacement, a new park, or a roof replacement.

In such cases, the “premium” for a green retrofit may be little or nothing. Which, along with being environmentally friendly, is one of the beauties of green infrastructure.

In addition, many communities are trying to minimize future rate increases by deploying both traditional gray and newer “green” stormwater control measures. In general, gray infrastructure collects and diverts excess runoff, while green infrastructure keeps it from entering the collection system. Gray measures like pipes and basins aren't typically integrated into the surrounding infrastructure, while green measures often become part of the street, building, or green space in which they lie. Consequently, green infrastructure delivers benefits beyond controlling runoff that are difficult to quantify for comparison to conventional alternatives.

Another budgeting challenge is that low-impact designs like green roofs, pervious pavements, and rain gardens lack the decades of engineering and construction experience associated with curbs, inlets, and pipes. Nor has EPA promulgated standards for such designs, which is why you may have received a request from the agency in August for performance data on your operation's low-impact designs.

Two years ago, the Indianapolis Department of Public Works retained our firm (Williams Creek Consulting Inc.) to create a Green Infrastructure Master Plan to manage runoff in the combined sewer watershed portion of the city. Since then, we've identified, designed, and/or implemented pilot projects to study the cost and effectiveness of green measures.

Figure 1 (top) shows the anticipated reduction in rainfall volumes for varying amounts of green measures in varying soil types. A 6% to 8% BMP:ISA ratio manages 20% to 40% of impervious surfaces with minimal disruption to existing or planned infrastructure.

To prioritize project implementation, we reviewed aerial photographs to identify open spaces adjacent to impervious surfaces and the department's GIS to avoid existing utilities. Control measures applied at the 6% to 8% BMP:ISA ratio to those projects remove 80% of the annual runoff volume — 1 to 2 billion gallons annually — from the collection system, saving $1 million to $2 million annually at the wastewater treatment plant based on current rates.

Figure 2 (bottom) was developed for the climate conditions modeled in Figure 1 and soils with an infiltration rate of 0.13 inches/hour. Results indicate that cost efficiencies are greatest where control measures are phased into application, with each phase intentionally undersized in case later phases aren't needed to achieve the desired management goal.


There are various databases (see Web extra for links) to help estimate installation costs for individual control measures, but they don't tie costs to the measures' common denominator: impervious surface. Follow these three steps to develop metrics that apply to your operation's unique situation.

Determine management goal. Are you going to control annual runoff volume infiltration or single events? The goal defines how much runoff a measure is expected to infiltrate or otherwise remove from the collection system.

Choosing annual infiltration requires defining a typical year rainfall data set and modeling using a continuous simulation; results are expressed as percent volume removal or gallons per year.

Events are expressed as a “water quality event.” They range between 0.75 and 1.25 inches of runoff, but vary based on local and state climate and watershed conditions.

Calculate amount of surface area managed. As you can see in Table 1, this figure varies widely depending on the control measure, site constraints, and climate and soil type.

Green roofs usually don't receive runoff from other areas, so they manage 1 square foot of impervious surface per square foot of green roof.

Premanufactured systems such as mechanical separators, tree planters, carbon filters, catch basin inserts, and any combination thereof have a high per-unit cost/square foot relative to bio-infiltration, but may do better when their potential for high flow-through capacity is taken into account.

Green alleys and other pervious pavements that don't receive impervious surface runoff from other areas have a 1:1 BMP:ISA ratio. But when they receive runoff from driveways or downspouts, or when pervious strips are placed in parking spaces or curbs, the ratio can dramatically increase.

Residential rain gardens, green space basins, green streets and parking lots, and other bio-infiltration areas manage a wide range of impervious surfaces and consequently vary widely in price. Rain gardens may be limited by the roof area draining to them and not the area available for infiltration, while a green space may be located to create a regional basin capable of managing dozens of acres.

Site constraints also affect a measure's effectiveness.

Existing conditions, such as the amount of paved surfaces and number of downspouts, dictate where to place the measure for maximum impact within existing development.

Constructability. Sometimes you just have to oversize. Even if a smaller space would accommodate the same anticipated flow, a rain garden installed in a former parking space must assume the dimensions of the parking space.

Property type. Rights of way, parking lots, public green spaces, and residential lots affect control measure type, size, and location.

Rights of way contain competing utilities. A parking lot may drain to inlets easily intercepted by a green measure, or it may make collection more difficult because it sheets flow into the right of way. A green space may or may not be located in an area receiving runoff from an adjacent impervious surface. Residential lots may or may not be conducive to downspout disconnection and onsite rain gardens.

Finally, arid climates with rapidly draining soils need a fraction of the infiltration area required by a humid climate subject to intense storms with poorly drained soils.

Determine local per-unit construction costs.Table 2 shows this figure also varies according to variables such as design standards. Storm events and rainfall patterns differ from community to community; and the formulas for parameters like depth of biosoil, inlet and curbing requirements, and lining requirements differ from technical manual to technical manual.

Local labor and material costs.

Project type. Costs increase when the design extends beyond controlling stormwater; park benches, tree grates, monitoring devices, and other premiums are valuable but skew costs reported in databases.

Service level. Larger runoff volumes require more capacity per square foot.

Dividing the unit cost values in Table 2 by the unit area managed costs in Table 1 yields the budget manager's Holy Grail: cost per square foot of impervious surface managed per square foot of best management practice, otherwise known as C/I. Return on investment depends on what your operation wants to accomplish.

Let's look at two examples. Indianapolis is retrofitting an urban residential neighborhood and a commercial street. At $1.14 and $1.67, respectively, costs per square foot of impervious surface managed are right where they should be for green streets (see Table 3).

The residential project was completed in September, and the commercial project is scheduled for completion later this year. Lessons learned include:

  • Get the buy-in of the purchasing and streets departments by letting them review and comment on the proposed design.
  • Community development corporations have access to grants and loans that public works may not, so make friends with yours; you'll be pleasantly surprised by how helpful they can be.
  • Resist the urge to include inlets inside rain gardens; they cause cost creep, require maintenance, and can be installed later if necessary. Instead, use existing gray infrastructure to the maximum extent practicable.
  • — Blahnik ( is a principal with Williams Creek Consulting Inc. in Indianapolis.

    Web Extra

    We search so you don't have to. For links to the sources referred to in Table 2, click here.