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 (firstname.lastname@example.org) is a principal with Williams Creek Consulting Inc. in Indianapolis.
We search so you don't have to. For links to the sources referred to in Table 2, click here.