Springfield is in south-western Massachusetts, New England, and is the area’s major urban center and largest community. The Connecticut River, one of the state’s key waterways, forms its western border. To the north of the city is the Chicopee River, a tributary to the Connecticut, and another small tributary, the Mill River, runs through the city. In all, 25 permitted CSOs discharge to the three rivers.
Under a third of the city’s total 21,000-acre area is served by combined sewers, which are mostly in its downtown and older parts. The SWSC, set up in 1996, is responsible for wastewater collection and treatment services to eight communities in the Greater Springfield area, including the city. The SWSC owns, operates and maintains the regional wastewater treatment facility, all the CSOs, the collection system, and pumping stations. The City of Springfield owns and operates the stormwater drainage system.
Like many cities with combined sewer systems, discharges from CSOs have polluted receiving waters, a problem which resulted in interceptor sewers being built, while excess flows during wet weather still discharged via the CSOs. Based on earlier control planning efforts, the Clinton Street CSO was believed to be one of the largest discharging to the Connecticut River.
The SWSC and City of Springfield have undertaken a number of CSO control and mitigation activities, including a regional CSO study. Because of the high costs of the recommendations, only the CSO regulator and in-system modifications were implemented. In line with national CSO control policy, a report on nine minimum control measures for the CSOs was developed and submitted to the regulators. The long-term control plan, drawn up in 2000, is the overall blueprint for CSO control. It sets out a range of projects in a number of phases that have to be implemented to meet an Administrative Order schedule negotiated with the Massachusetts Department of Environmental Protection and the US EPA.
The first major project achieved high levels of CSO control for the Mill River tributary. Before the second project, to control CSOs on the Chicopee River, the SWSC decided to update baseline conditions in the project area and reassess the Chicopee River area components of the overall plan. This was considered essential because of changes to the study area since the plan was completed, and to ensure that the SWSC was putting forward an optimized CSO control concept. As a result of this work, a series of projects that aligned better with the SWSC’s goal of upgrading its collection system infrastructure was defined.
The Clinton Street project
The Commission’s Clinton Street screening and disinfection facility project, the first one called for in the long-term control plan, is an example of this approach. This project consisted of separating the combined sewers to outfalls in Rowland Street and Springfield Street. Following the separation work, discharges from the CSO at Springfield Street would cease and discharges at Rowland Street would be eliminated for storms up to and including the two-year, 24-hour storm. Construction was due to begin in 2009.
The SWSC anticipated that field inspections and other data collected as part of the update could potentially change baseline CSO activations, volumes and flow rates, and identify potential infrastructure upgrade requirements.
These infrastructure improvements could in turn further change the design volume and flow rate at the Clinton Street outfall, which made it appropriate to assess the cost-effectiveness of proposed upgrades and other alternatives to the current plan for the screening and disinfection facility.
The update of baseline conditions covered two areas: field investigations, and updating and recalibrating of the InfoWorks CS collection system model, with field investigation data used to support the model update. Extensive field investigations included creating a database of collection system drawings, diving to inspect the interceptor’s condition, undertaking a topographic and planimetric survey of the whole interceptor and inspecting all of the CSO regulators and establishing their key dimensions. Other data such as invert elevations and pipe dimensions and connections into the structures was noted and key non-regulator manholes inspected to clarify connectivity for the modeling and to confirm possible cross-connections.
The pumping capacity at the interceptor’s two sanitary pumping stations was also tested, and pump curves developed and analyzed for use in the model. Flow metering, to monitor the system, and model calibration was undertaken in 2005. River stage data was also obtained and a 100-year data set created and incorporated into the model input file for the chosen ‘typical’ rainfall year, which had been set at 1976 in the previous regional CSO study.
The investigations found that the capacity of the York Street pumping station was around 30% less than had been thought, and that a substantial separate stormwater area, previously believed to be a tributary upstream of the Springfield Street regulator, was in fact a tributary to the outfall downstream of the regulator.
A connection between the CRI overflow and another one at Liberty Street was also found, and significant build-ups of sediment and grease were discovered.
The original collection system model was in a different format but as part of the Clinton Street screening and disinfection facility project it was converted to InfoWorks CS to facilitate evaluation of the system alternatives.
The InfoWorks model
The InfoWorks model was updated with all of the collected field information and the level of detail was expanded to include pipes of 24in diameter and above. Once updated and expanded, the model was recalibrated using the 32 flow meters deployed for the project.
After this, the model was used to define the baseline conditions for designing the Clinton Street facility. One early achievement was discovering that removing the restrictions found in the divers’ survey made little difference to CSO discharge performance, so a ‘half-removal’ program was undertaken.
The annual CSO discharge frequency and volume predictions between the original and updated InfoWorks models were also compared, and the predictions showed an 11% reduction or 53 million gallons using InfoWorks, with larger differences at individual outfalls.
Key differences in system configuration and understanding explain the differences. These are:
- Restriction and sediment levels in InfoWorks model not available in the original;
- The reduced pumping station capacity that was discovered;
- The effect of the downstream stormwater tributary to the Clinton Street outfall;
- More extensive flow metering for the InfoWorks calibration;
- More detailed modeling of CSO regulators in InfoWorks.
Non-model related volumes aside, the difference in total volumes between the two models was just 4% but it was seen as clearly appropriate to increase the level of detail, accuracy and confidence in the InfoWorks model to support the design of the Clinton Street facility and the assessment of alternatives to it.
Modeling of alternatives
On completing the model calibration and update of baseline conditions, a review of hydraulic profiles and flow conditions in the interceptor and CSO tributary trunk sewers was undertaken. Based on the profiles and system storage, as well as input from the SWSC, a list of alternatives was identified for further evaluation.
Model runs of the free discharge option at the York Street pumping station indicated that unlimited capacity here would significantly reduce the peak hydraulic grade line at the pumping station, a decrease that tapered to nothing about 5000ft upstream. This option reduced CSO volumes from the baseline 423MGAL to 294MGAL, but to handle the additional CSO flows generated, high-cost dedicated wet weather facilities would be required. An analysis of the effectiveness, costs and flexibility of this option caused it to be eliminated.
Separation of five target areas for disconnecting separate stormwater pipes from the combined sewer system was also modeled, which showed the baseline CSO volume could be reduced to 353MGAL. Because of the relatively modest reduction, this option was eliminated.
Two sub-alternatives were also modeled, the first involving a relief pipe to convey peak flows from the largest storm in a typical year from the dry weather flow connections and overflows from the three CSOs just downstream of CSO 010 to a free discharge in the York Street pumping station areas. The second sub-alternative was similar except that the relief pipe would be sized to store all flows from the dry weather flow connections and overflows from the three CSOs in the largest storm in the typical year. The latter would not involve providing additional facilities at the pumping station, though a pump-out station to dewater the storage pipe would be needed.
The modeling showed that both alternatives were relatively complex and needed a number of new flow control structures that would cost more than the screening and disinfection facility.
Because of the stormwater separation already found, it was decided to re-examine sewer separation as an alternative, as this information had not been available for the original long-term plan that had found it not to be cost-effective.
The InfoWorks model was used to assess the feasibility of closing the outfalls from the CSO 007 and 049 regulators following sewer separation. This was achieved by comparing peak hydraulic grade lines and flooding for the alternative against the baseline conditions.
The modeling showed that flooding was still predicted at the CSO 049 regulator in the five-year, 24-hour storm. In the 10-year, 24-hour storm flooding upstream of the CS 007 regulator was eliminated by the hydraulic grade line at the regulator was predicted to increase by around 0.8ft compared to baseline conditions, which would take it to within 5ft of the surface, thus increasing the risk of basement flooding near the regulator.
The InfoWorks model was then used to optimize performance of this alternative. By leaving the CSO 007 outfall open and raising the regulator weir, and increasing the size of the CSO 049 dry weather flow connection to the interceptor, peak hydraulic grade lines were found to be lower than under baseline conditions. No CSO discharges were predicted at CSO 007 up to and including the two-year, 24-hour storm, and an extremely small discharge volume was predicted in response to the five-year, 24-hour storm.
In this way, a very high level of CSO control could be achieved at this outfall, even though it would have to remain open for system relief in extreme storms. Additional InfoWorks simulations showed that the outfall could be closed in future if the next downstream area tributary to the interceptor was separated.
Comparison with the original plan
In terms of all the factors – reductions in surcharges and flooding, CSO activation and volume, E. coli loading and receiving water impacts – the alternative was found to perform as well as or better than the Clinton Street facility.
This option will also provide a better net reduction in annual bacteria load to the Connecticut River, compared to the Clinton Street screening and disinfection facility, and will allow the SWSC to have more flexibility to implement greater CSO controls along the river while staying in line with the long-term control plan.
In terms of construction costs, both the facility and the sewer separation alternative costs were found to be nearly the same: $26,100 for the facility and $26,200 for the separation project. The facility also had additional operation and maintenance costs, whereas the alternative did not.
It was also found that the alternative could be designed and constructed in time to meet the dates established for the Clinton Street facility schedule. And as the project would be divided into two construction contracts, water quality benefits would be available ahead of the anticipated 2011 completion date.
The SWSC met with representatives of the US EPA and DEP throughout the development of the project so both agencies accepted the basic data supporting the potential change. Following the evaluations, SWSC sought and obtained approval from the regulators to substitute the sewer separation project for the previous screening and disinfection project.