An Innovative Storm Sewer Design in an Urban Environment

New York State Department of Conservation (NYSDEC) issued a CSO (Combined Sewer Overflows) Abatement Consent Order to New York City Department of Environmental Protection (NYCDEP) to reduce the CSO into Thurston Basin, a tributary of Jamaica Bay. The CSO could only be reduced by separating the flows from the existing Combined District. In a conventional design the existing combined sewers would be abandoned and a separate sanitary and storm sewer system would be designed. However, the abandonment of the existing combined sewers will cause severe disruptions of existing house connections as well as being quite expensive. NYCDEP developed a unique design that retained the existing combined sewers to carry 100 percent sanitary flow and 50 percent storm flow and a new High Level Storm (HLS) Sewer System that carried the remaining 50 percent storm flow from street runoffs. This innovative design will separate the flow from the Combined District and reduce the CSO at Thurston Basin. A 57-acre (23 ha) triangular area (Figure 1) on the east side of the Cross Island Parkway (the Parkway), however, presented a great challenge. The flows from the HLS sewer system from this low lying area, which gets flooded frequently, could not be conveyed to Thurston Basin via the proposed HLS trunk sewers. After considering several alternatives, it was decided to use an existing natural ponds and stream system, running parallel to the Parkway to Thurston Basin, as a trunk sewer for the transmission of HLS sewer discharges from this low lying area. NYCDEP has used this concept of using natural streams as trunk sewers in Staten Island known as Bluebelt Project. NYCDEP's Bluebelt Project has been highly acclaimed by environmentalists and regulatory agencies. The unique design of the HLS sewer system and the Bluebelt concept provided the solutions for an innovative storm sewer design in an urban environment.