Environmental Valuation & Cost-Benefit News

The Center for Neighborhood Technologies (CNT) Green Values Stormwater Calculator (greenvalues.cnt.org), released in 2007, is designed to arrive at a first approximation of the hydrologic and financial conditions for a site that is defined by the user. In order to calculate the conditions, a variety of assumptions are made. The calculations are based on procedures contained in the report “Urban Hydrology for Small Watersheds,” Technical Release 55, which are commonly used in the Chicago region. (The TR-55 Report)

Based upon the input assumptions several scenarios are characterized on the “Hydrologic” screen, and also shown on the “Financial,” “Financial Detail,” and “Scenario Detail” screens. The cost of a scenario is estimated by calculating the costs of components of the infrastructure and adding them together. Both construction and annual maintenance costs are shown. The annual benefits of a scenario can be found in the same way. An attempt has been made to include financial estimates for each type of benefit, but
many of them have not been quantified.

For simplicity’s sake, only the costs of the stormwater infrastructure components that
change between the green infrastructure and conventional infrastructure scenarios are
utilized. For example, the cost of the rain garden as compared to a turf area of the same size was modeled, but the cost of the remaining turf that makes up the yard was not, because the cost would be the same for both scenarios.

The “Financial” screen includes additional information on how costs or are determined
under the link “See how costs are calculated.” A table in that section gives low, middle and high estimates of construction costs, annual maintenance costs, and component lifespan for each component. In each case, CNT has used the “mid” cost Sources of figures are linked from the estimates, allowing users to investigate and assess what costs are most
appropriate. In many cases there is a direct link to the original source of the costs.

Green Value Stormwater Calculator http://greenvalues.cnt.org/calculator
from the Center for Neighborhood Technology www.cnt.org