The City of Terre Haute and its consultants completed an evaluation of “green” infrastructure retrofit potential in the CSO Long Term Control Plan (LTCP). The study was completed to evaluate the potential for “green” infrastructure to reduce the size of “gray” infrastructure control measures (pipes, tanks, etc.) for CSO overflow reduction as part of the City’s LTCP.
The goal of the evaluation was to identify potential green infrastructure retrofits in the City’s combined sewer area, particularly basins 009 (Chestnut), 010 (Spruce) and 003 (Turner), estimate the costs of those retrofits and assess their benefits in terms of storm water volume capture.
Green Roofs- are one of the commonly cited technologies for controlling rooftop runoff. Where it is structurally feasible, large, relatively flat roofs can be retrofitted to accommodate growth media and vegetation to provide stormwater storage and runoff reduction benefits.
Cisterns are large storage tanks installed adjacent to buildings, either above or below ground. Rooftop runoff is directed into a cistern, where it is stored to be used for irrigation or other purposes, such as watering, car washing or even toilet flushing.
Rain Gardens are most commonly designed for residential use, although larger installations have been used in commercial or institutional settings. These are vegetated depressions that provide stormwater capture and temporary storage.
Bioretention islands and bioswales can be suitable retrofits in virtually any highly impervious area, depending on site conditions. These features are installed adjacent to buildings, parking lots, and/or roadways to collect and temporarily store runoff.
Permeable Pavement can consist of pavers and/or porous media that allow stormwater to infiltrate into the underlying soil and be collected by an underdrain system or storage/infiltration bed, rather than running off directly to combined or storm sewers.
Infiltration Bed (or storage beds) can be installed beneath existing pervious areas to improve their infiltration capacity and reduce stormwater runoff. A common application for infiltration beds on university campuses is to install them beneath athletic fields.
Tree box filters are a means of capturing, storing, and infiltrating runoff in places that are highly impervious and where space is extremely limited.
Bioretention Planters serve a similar function as tree box filters and are best suited for highly impervious areas with limited space for green infrastructure.
Rain Barrel is a small-scale version of the cistern. While cisterns are typically hundreds to thousands of gallons in capacity, rain barrels typically range from 50 to 250 gallons.
Based on the green infrastructure evaluation, it was determined that there are widespread opportunities for green infrastructure implementation in the system, particularly the CSO 009 drainage area. These opportunities are more prominent in part of the drainage area occupied by the Indiana State University (ISU) campus, as compared to other areas occupied mainly by single family residences. (Opportunities also existing in the CSO 010 basin, in which the Union Hospital campus lies, and the CSO 003 area along the Margaret Avenue corridor.) In the ISU campus area and larger commercial areas, the large impervious areas created by buildings, parking lots, and streets present a variety of green infrastructure retrofit opportunities. Controlling storm water runoff from these impervious areas can potentially have significant impact on reducing wet weather flows from the drainage area which outlet to combined sewer. In addition, large athletic fields, in combination with permeable soils, present a unique opportunity for construction of infiltration beds that can provide large stormwater storage volume without compromising the primary use of the fields.
While it is unlikely that 100% implementation of green infrastructure retrofits can be achieved in any or all of these areas, even the ISU campus area, the studies clearly showed that significant storm water storage potential exists for even partial implementation and the costs per gallon are lower than the cost of CSO storage tanks. This storage potential can be further enhanced by extending green infrastructure retrofits in other parts of the CSO 009 drainage area and other basins, including predominantly residential areas. It is theorized that the green infrastructure implementation in the 009 drainage area (and others) can provide equivalent storage to offset the need for millions of gallons in storage tank volume.
Phases III and IV of the LTCP include implementation of green infrastructure which will be monitored after implementation to measure the effectiveness of the measures. CSO storage included in Phase V of the LTCP could potentially be minimized or eliminated based upon the effectiveness of the green infrastructure coupled with other LTCP recommendations. Phase V will be re-evaluated following the post-construction monitoring and modeling completed following Phases III and IV.