Executive Summary Consent Decree

Transcription

1 OVERFLOW ABATEMENT PROGRAM Executive Summary Consent Decree The sewer system in Nashville dates back to the late 1800s and originally consisted of a combined sewer system, later transitioning to separate sanitary and storm sewers. A combined sewer system consists of a single set of pipes that conveys both sanitary sewage and storm water. Combined sewer systems were common in cities that developed in the 19th century to address public health problems caused by lack of proper sanitation. Since there were no facilities for treating wastewater in that era, sewage and storm water were both discharged directly to the rivers and streams. The treatment of wastewater began in the 20th century when interceptors were constructed to convey sanitary sewage along with storm water to treatment plants to improve water quality. Wastewater treatment plants have limitations, however, to the volume of flow that can be effectively treated during rainfall events. Intense rainfall often leads to flows of stormwater in the combined sewer system that exceeds treatment plant capacity. These high flows of primarily stormwater are discharged without treatment and referred to as combined sewer overflows or CSOs. Separate sanitary sewers are intended to convey only sanitary sewage, but the piping systems deteriorate over time allowing rainwater to leak into the lines. During extreme rainfall events, the volume of rainwater entering these older sewers can overwhelm the capacity of the system, leading to sanitary sewer overflows (SSOs). Metro Water Services began an aggressive program in 1990 to reduce the number of CSOs and SSOs, making tremendous progress toward improving water quality in the Cumberland River and its major tributaries. However, despite these improvements, a significant number of overflows still remained, prompting state and federal regulatory agencies to approach Metro in 2007 about the need for additional work within the sewer system. In March 2009, a consent decree between the United States of America, the State of Tennessee, and the Metropolitan Government of Nashville and Davidson County was approved and entered with the United States District Court for the Middle District of Tennessee. The consent decree was filed on behalf of the United States Environmental Protection Agency (EPA) and the Tennessee Department of Environment and Conservation (TDEC), and it requires Metro to use its best efforts to achieve the following goals: 1. Full compliance with National Pollutant Discharge Elimination System (NPDES) permits, the Clean Water Act, the Tennessee Water Quality Control Act, and their regulations. 2. Elimination of sanitary sewer overflows (SSOs). 3. Compliance with EPA s combined sewer overflow (CSO) Control Policy. ES 1

2 The schedule for the Consent Decree was amended in August 2010 to provide additional time due to the impacts of the May 2010 flood. The amended schedule includes the following requirements: 2 ½ years to develop a Corrective Action Plan/Engineering Report (CAP/ER) to address conditions causing overflows in the separate sanitary sewer system with the goal of eliminating SSOs 2 ½ years to develop an updated Long Term Control Plan (LTCP) to mitigate overflows from the combined sewer system and reduce water quality impacts to the Cumberland River 11 years after approval of each of these documents to complete the recommended improvements The Consent Decree also includes provisions for civil penalties for past violations and stipulated penalties for violations that may occur in the future. In lieu of the full civil penalty, Metro agreed to conduct and has completed Supplemental Environmental Projects to improve public health and the environment. At a total estimated cost of billion, the Clean Water Nashville program represents a major investment in overflow abatement for the community and provides the following benefits: Renews and improves aging infrastructure Enhances the environment for Nashville Improves water quality in the Cumberland River and its tributary watersheds in Metro Davidson County Provides major engineering and construction projects to boost the local economy Executive summaries for the Corrective Action Plan/Engineering Report and the Long Term Control Plan follow, along with Figure ES 2, which displays the recommended projects included in both reports. Corrective Action Plan/Engineering Report This report presents the approach to characterize Metro s sanitary sewer system and to evaluate potential alternatives for improvements to address hydraulic limitations. The major steps of this evaluation are described below: Extensive rainfall and flow monitoring data was collected through the study area to characterize base wastewater flows, groundwater infiltration, and rainfall derived infiltration and inflow (RDII). The primary focus of the analysis used data collected from February through April 2008; however, data collected from 2004 through 2010 was utilized to supplement this analysis. Data collection is described in Section 2 of this report, and the analysis of this data is included in Section 3. Hydraulic models of the sanitary sewer system were developed using Mike Urban software and EPA s Stormwater Management Model (SWMM) version 5.0. The models include most gravity sewers 10 inches or greater in diameter; additional smaller diameter sewers were also included for connectivity and to model areas of concern. For the purposes of this analysis, the collection systems serving each wastewater treatment plant service area were modeled ES 2

3 separately, resulting in three models representing the Dry Creek, Whites Creek, and Central systems respectively. Model development is described in Section 4 of the report. Using the collected rainfall and flow monitoring data, the hydraulic models were first calibrated to dry weather flow conditions. The second step of model calibration involved comparing model predicted RDII flows to those measured in the system as part of the flow monitoring program. Following completion of the calibration step, the models were compared to a second set of data for verification purposes. The model calibration steps result in models that meet the calibration goals and are acceptable for use in evaluating capacity under various scenarios to resolve capacity limitations. The calibration process is described in Section 4.3 of the report. To evaluate the need for improvements to the system, design conditions were applied to the models. This includes dormant season dry weather flows as well as the 2 year, 24 hour dormant season design storm. This storm event includes a peak intensity of 0.97 inches per hour and a 24 hour volume of 3.15 inches. Details of the development of this design storm are provided in Section 3.4 of the report. For SSOs listed in the CD and other model predicted, field verified overflows, the models were used to evaluate potential improvements to alleviate SSOs under design conditions. Improvements considered included pump station and pipeline improvements to increase conveyance capacity, storage to equalize peak flows, and sewer rehabilitation to reduce the amount of RDII entering the system. The alternatives analysis for the Dry Creek, Whites Creek and Central service areas are described in Sections 6, 7, and 8 of the report, respectively. Of the potential improvements capable of alleviating SSOs under design storm conditions, the project team evaluated each improvement considering costs, constructability, operations, and other factors to select the proposed improvement. This discussion is also included in Sections 6, 7, and 8 for the Dry Creek, Whites Creek, and Central service areas, respectively. Proposed projects were prioritized considering the frequency and severity (volume) of overflow being addressed, proximity of the SSO to public areas and 303d listed streams for E. coli, and project complexity, such as the need to acquire property, project sequencing, or estimated construction durations. As a result of this process, Metro has identified multiple projects that will be completed as this CAP/ER is implemented. These projects are listed in Table ES 1. This table is also presented in Section 9 of the report and includes projects previously identified by Metro that will contribute to the reduction of SSOs within the system. Table ES 1 also provides the preliminary, planning level schedule for the proposed projects. As shown in the schedule, Metro intends to complete all CAP/ER projects within the eleven year timeframe provided. The project start shown represents the start of project activities, including planning, land or right ofway acquisition, etc. The project end indicates the target completion date for the referenced project. All dates for proposed projects assume approval of the CAP/ER no later than January 1, If approval is received after that time, the schedule will be shifted forward to account for that delay. ES 3

4 Executive Summary Executive Summary Table ES 1 CAP/ER Implementation Plan and Schedule Project Start Project End Project Name Service Area (Fiscal Year) (Fiscal Year) Dry Creek WWTP Optimization Dry Creek Smith Springs Equalization Storage Central Barker Road / Omohundro Equalization Storage Phase I Central West Park Equalization Storage Phase I Whites Creek Mill Creek 36 Trunk Sewer System Rehabilitation Central Whites Creek WWTP Disinfection & Optimization Whites Creek Whites Creek Pump Station Improvements Whites Creek Dodson Chapel Equalization Storage Central Holiday Travel Park Gravity Conversion Central Lakewood Rehabilitation Dry Creek th Avenue Rehabilitation Central Cowan / Riverside Drive Rehabilitation Central Dodson Chapel Pipe Improvements Central Mill Creek / Opryland Equalization Storage Phase II Central Rockwood Conveyance Improvement Central Shelby Park Rehabilitation Central Smith Springs Rehabilitation Central Neely's Bend Rehabilitation Dry Creek Davidson Branch Equalization Storage Whites Creek Highway 100 / Tyne Boulevard Pipe Improvements Whites Creek Joelton Rehabilitation Whites Creek West Park Equalization Storage Phase II Whites Creek Dry Creek Pipe Improvements Dry Creek Gibson Creek Rehabilitation Dry Creek Vandiver Pump Station Upgrades Dry Creek Davidson Rehabilitation Whites Creek Brick Church Pike Equalization Storage Whites Creek Brick Church Pike Pipe Improvements Whites Creek Langford Farms Rehabilitation Central Mill Creek Trunk Improvements Central Berwick Trail Pipe Improvements Dry Creek Berwick Trail Pump Station Upgrades Dry Creek Gibson Creek Equalization Storage Dry Creek Hidden Acres Pump Station Upgrades Dry Creek Loves Branch Pump Station Upgrades Dry Creek Neely's Bend Pump Station Upgrades Dry Creek Hurricane Creek Pipe Improvements Central Mill Creek / Opryland Equalization Storage Phase III Central Notes: (1) Fiscal year is defined as the period starting on July 1 and ending on June 30 of the year shown. (2) Phasing is only shown for multiple storage tank locations. ES 4

5 Executive Summary Executive Summary Table ES 1 CAP/ER Implementation Plan and Schedule (continued) Project Start Project End Project Name Service Area (Fiscal Year) (Fiscal Year) GC14 Trunk Improvements Dry Creek Madison Heights / Rainbow Terrace Rehabilitation Dry Creek West Park Equalization Storage Phase III Whites Creek Cleece Ferry Rehabilitation Whites Creek Cowan Street Pipe Improvements Central Riverside Drive Pump Station Upgrades Central Henry Ford Drive Pipe Improvements Whites Creek River Drive Rehabilitation Whites Creek Cowan Street Pump Station Upgrades Central GL01 Rehabilitation Central Notes: (1) Fiscal year is defined as the period starting on July 1 and ending on June 30 of the year shown. (2) Phasing is only shown for multiple storage tank locations. Other projects may be separated into phases as the project design progresses. ES 5

6 Long Term Control Plan The Long Term Control Plan Update presents a characterization of the MWS combined sewer system and the recommended improvements to reduce the occurrence and impact of combined sewer overflows (CSOs) on the Cumberland River. MWS followed a rigorous engineering, quantitative, and scientific process for identifying and evaluating alternatives to control CSOs. Financial considerations and engineering analyses were also used to develop the recommended improvements, in conjunction with four key objectives that were established early in the planning process: Improve the water quality of the Cumberland River by reducing impacts from combined sewer overflows. Provide a level of CSO control that results in improvements in water quality that are consistent with the community s needs and use of the Cumberland River. Align investment in CSO controls to be commensurate with the contribution of CSOs to water quality standards violations relative to other sources. Consider the impact of the overall program cost on the ratepayers in the current economic climate. These goals and objectives were developed based on feedback provided by representatives from MWS, local government, and the community through a Public Engagement Campaign developed to solicit input from affected stakeholders. Prior Efforts to Control CSOs Attention and focus on CSOs over the past 23 years has resulted in significant improvements in the combined sewer system and has aided in improving the water quality of the Cumberland River. When the Overflow Abatement Program began in 1990, there were a total of 32 CSO discharge points in downtown Nashville. With an expenditure of over $265 million on improvements, significant reductions in CSO discharges have been attained through the Overflow Abatement Program. These improvements have eliminated 24 CSO discharge sites and greatly reduced the total volume of untreated discharges into the Cumberland River on an annual basis. Currently, only eight CSO locations remain as depicted in Figure ES 1. Characterization of CSO Discharges Hydraulic models of the combined sewer system were developed to determine the relationship between rainfall events and CSO discharges. The hydraulic models were calibrated using an extensive array of flow monitors and a network of 16 Metro rainfall gauges. The benefits of reductions in CSO discharges were assessed on an annual basis. To accomplish this task, precipitation records for the past 54 years were examined to select a year reflective of average conditions for rainfall volume, intensity, duration and number of events. The year 1995 was selected to represent the typical year for rainfall in Nashville, and rainfall data for that year were utilized for hydraulic modeling of the combined sewer system. Additionally, modeling was conducted to assess the water quality impact on the Cumberland River during rainfall events from CSO discharges along with the impact of other sources, such as tributary ES 6

7 streams and urban runoff. Water quality modeling focused on two sensitive areas, Riverfront Park and Cleece Ferry. There are currently four CSO discharge sites upstream of Riverfront Park, and four additional CSO discharge sites upstream of Cleece Ferry Figure ES 1: Eight MWS CSO Locations in 2010 Summary of Recommended Improvements to CSO Outfalls Extensive hydraulic modeling was conducted to analyze the effect of various alternatives for each of the areas within the combined sewer system. These alternatives typically consist of reduction in flow by removing a portion of the storm water from the system, conveyance improvements to transport additional flow, or storage facilities to temporarily store excess flow during rainfall events. The hydraulic modeling estimates the number of annual overflows for each alternative utilizing the rainfall data for the typical precipitation year. The recommended improvements and the typical year overflow frequency for each of the eight CSO discharge sites are summarized in Table ES 2. ES 7

8 Table ES 2 Summary of Major Long Term Control Plan Projects CSO Site Overflow Frequency for Recommended Project Type Typical Year Boscobel 0 storage tank Benedict & Crutcher 2 storage tank Driftwood 0 additional volume for existing storage tank Broadway N/A eliminate by conveyance improvement Washington 11 conveyance improvement and system storage Kerrigan 9 conveyance improvement and storage tank Van Buren N/A eliminate by conveyance improvement Schrader 2 storage tank Benefits of Recommended Plan MWS has put forth an aggressive CSO control plan that is consistent with its long history of improving the water quality of the Cumberland River. The recommended improvements under the LTCP will provide the following benefits: Incorporate green infrastructure and low impact development techniques to reduce storm water loading to CSS Eliminate two CSO overflow sites Reduce typical year discharge frequency for four of the remaining CSO sites to two or less per year Reduce typical year discharge frequency at the two largest CSO sites from times/year to 9 11 times/year Reduce annual CSO volume for a typical year by 53% from current levels Achieve near zero hours of water quality standard exceedance from CSOs at Riverfront Park in a typical year Reduce water quality standard exceedance from CSOs at Cleece Ferry to approximately 29 hours (or less than 1% of the hours) in a typical year Select alternatives that limit neighborhood impact from construction and minimize property acquisition Address public concerns of affordability to ratepayers in difficult economic times Provide a level of water quality attainment in the Cumberland River consistent with the community s uses and needs The projects recommended under the LTCP, along with planned starting and ending dates for the 11 year implementation period, are listed in Table ES 3. ES 8

9 Project Name Table ES 3 Planned Schedule for Combined Sewer System Improvements Projected MWS FY* Start Projected MWS FY* End Affected CSO Broadway Improvements U/C 2012 Broadway Washington CSO Facility Improvements U/C 2012 Washington Van Buren Improvements U/C 2012 Van Buren Driftwood EQ Expansion Driftwood CWWTP Optimization + EQ Conversion (5mgal) Kerrigan Kerrigan Weir Dynamic Addition Kerrigan Apex Sewer Corrections Washington Parthenon Area Improvements Kerrigan Green Improvements (Annual) Various CWWTP EQ Addition Ph 1 (13 mgal) Kerrigan CWWTP Pumps/EQ Grit Equipment Kerrigan CWWTP EQ Expansion Ph Kerrigan Benedict & Crutcher PS/EQ Benedict & Crutcher Boscobel PS/FM/EQ Boscobel FAT Access Van Buren Schrader PS/EQ Schrader CWWTP EQ Expansion Ph Kerrigan Trash Trap Replacement Kerrigan CSS + FAT Rehab Various Notes: *FY is Metro fiscal year starting July 1 and ending June 30 of the year shown. U/C is under construction. ES 9

10 I 24 "! "! "!!.!.!.!.!. I 40 Legend CAP/ER Pump Station Project CAP/ER Storage Project LTCP Storage Project!. Other LTCP Project Conveyance Improvements Rehabilitation Area I 65 I 24 "! WWTP Modeled Conduits Metro Sewered Area Miles Figure ES-2: Overview of CAP/ER and LTCP Projects