Onondaga County, NY, is one of the first Regional Authorities in the U.S. to include the use of greeninfrastructures (GIs) in a consent judgment for combined sewer overflow (CSO) abatement. The Countyinitiated the Save the Rain program to implement GIs to prevent stormwater runoff from entering thecombined sewer system. The CSO abatement program also includes “gray” infrastructures, such as offlinestorage and regional treatment facilities, for CSO mitigation. One of the planning challenges is to figure outhow to strategically place the GIs among the sewersheds while maximizing its effectiveness for CSO capturein conjunction with planned gray infrastructures. A customized model was developed to link the SWMM5model’s LID module with genetic algorithms through the SimLink program. This presentation will discuss theautomated optimization procedures (Figure 1) with the customized model and analyze the modeling results for optimum placement of GIs and determination of GI technology types (Figure 2), the number of GIs needed,and their sizes. Pollution in Onondaga Lake was the subject of a Clean Water Act lawsuit which resulted in a consent judgment that requires Onondaga County, NY to mitigate the impact of lake water quality by the CSOs to the Lake and its tributaries. Prior to 2009, the County implemented many CSO control projects that consist of sewer separation, improve/upgrade of its wet weather treatment facilities, and conveyance systems. In November 2009, with the support of the stakeholders, which include U.S. EPA and NYSDEC, the federal courts approved a new plan to include innovative green infrastructures combined with gray infrastructures. The approval makes Onondaga County, NY one of the first Regional Authorities in the U.S. to include the use of GIs in a consent judgment for CSO abatement. The Save the Rain Program, planned and designed in 2010, includes a comprehensive GI program to reduce stormwater inflow into the sewer collection system. The program calls for an annual capture rate of 95% with the combination of green and gray infrastructure projects by December 2018. In 2011, the County Executive initiated the Project 50 campaign to build 50 distinct GI projects within the calendar year. With the most combined sewersheds within the City of Syracuse, it is a complex program that requires extraordinary coordination and commitment among various stakeholders. One of the planning challenges is to figure out how to strategically place the GIs among the sewersheds while maximizing its effectiveness for CSO capture in conjunction with planned gray infrastructures. For example, an offline storage facility in the Clinton Street (CSO080) sewershed would provide more than 6.0 million gallons of storage volume, and GI projects constructed within the tributary area of the storage facility may not be as effective as the same project located outside its tributary area for reducing CSO volumes. Using an automated optimization procedure, this study will evaluate the optimal placement of GI projects among the combined sewersheds to achieve the highest cost-effectiveness for the County’s CSO abatement program. A ten (10) day period of precipitation data extracted from the existing typical year rainfall data was used for optimization runs in order to closely match the typical year condition without compromise computation time.