Stormwater Management Retrofit Costs: Why Is It So Hard To Predict?

Design, construction, and maintenance costs for stormwater control measures (SCMs) are a major concern as new regulations are under consideration which contain provisions for more rigorous stormwater management in the United States. Regulatory agencies often use past SCM cost information to predict the costs of implementing new permit programs, especially as it relates to urban retrofit considerations. Accurate predictions of SCM costs during the planning phase is challenging because of the variable unknown conditions that are encountered between the planning and construction phases of a project. Planners and engineers often use past examples of similar SCMs as a basis for predicting costs of proposed projects. Even using local, actual, construction costs information can lead to distorted cost estimates as unit costs for a project can vary from year to year depending on many market factors. This paper will explore this issue and suggest recommendations on how to address it.