The Role of Receiving Water Models in CSO Long-Term Control Plan Decision-Making and Water Quality Standard Revisions

Many communities have or are developing Long-Term Control Plans (LTCPs) to address their combined sewer overflows (CSOs). Often, however, the most cost-effective strategy for controlling CSOs will still result in occasional violations of current water quality standards. Many states have provided relief to CSO communities through mechanisms that temporarily modify the designated use of the water body or the numeric criteria. Indiana allows communities to apply for a Wet Weather Limited Use designation for up to four days after a CSO event. River models have provided Indiana communities with the flexibility and versatility needed to assess site-specific conditions required for the LTCP and subsequent application for limited use designation. This paper presents a brief overview of the issues communities face in their LTCP planning, a general discussion of how river models can assist communities in decision-making, and illustrates their use with several case studies from communities in Indiana.