Cost-Benefit Analysis of Urban Stormwater Retrofits and Stream Daylighting Using Low Impact Development Technologies

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ABSTRACT
This project incorporates Low Impact Development (LID) concepts into the City of Caldwell (Idaho) Indian Creek daylighting and downtown redevelopment project, which is a prime candidate for a “showcase” project producing benefits in urban runoff within the lower Boise River watershed. The primary goal of this project was to incorporate LID concepts into the Indian Creek daylighting project. Indian Creek is an excellent focus stream because it is a high priority sub-watershed to the lower Boise River, both of which are 303(d) listed for sediments, bacteria, and phosphorus. The project recognized that the existing collection systems could be upgraded to mitigate the discharge of untreated stormwater into a highly-visible, restored stream. The LIFE™ model was used to determine a cost-benefit ratio. LIFE™ is a physically-based, continuous simulation tool that represents the stormwater and runoff processes that occur within bioretention facilities, vegetated swales, green roofs, infiltration devices, and other LID controls. On a site-specific basis, LIFE™ predicted increased improvements due to the LID stormwater design as compared to tradition upstream technologies. From a cost-benefit perspective, traditional stormwater controls have a cost of $8,500, with a removal efficiency of 5% (the resulting cost-benefit ratio is $1,700 per % of load removal). In contrast, the LID technologies cost more ($20,648), but the removal efficiency is much higher (32%) for a resulting cost-benefit ratio of $645 per % of load removal. On a cost-benefit basis, the improvements in LID certainly suggest a better return on investment.

THE PROBLEM
This project incorporates Low Impact Development (LID) concepts into the City of Caldwell (Idaho) Indian Creek daylighting and downtown redevelopment project, which is a prime candidate for a “showcase” project producing benefits in urban runoff within the lower Boise River watershed. Indian Creek meanders through farmland, residential, and industrial areas before it enters downtown Caldwell, Idaho, where it joins the lower Boise River. As Caldwell grew, it buried Indian Creek beneath asphalt and concrete. The daylighting project provides important habitat improvement along Indian Creek and is one component of a larger effort aimed at creating an attractive core area for the revitalization of downtown Caldwell. This work builds on existing efforts in the Indian Creek watershed, including a 2002 redevelopment design charette and an existing “Urban Ecology Design Manual for the Lower Boise River”.

GOALS AND OBJECTIVES
The primary goal of this project was to incorporate LID concepts into the Indian Creek daylighting project. Indian Creek is an excellent focus stream because it is a high priority subwatershed to the lower Boise River, both of which are 303(d) listed for sediments, bacteria, and phosphorus. The project recognized that the existing collection systems could be upgraded to mitigate the discharge of untreated stormwater into a highly-visible, restored stream.

A secondary goal of the project was to develop an LID handbook that can be distributed to developers looking to build in the redeveloping downtown area.

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