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Appendix H Impacts of the Atlantic Yards Project on Infrastructure Summary Report


Forest City Ratner Company Brooklyn, New York

IMPACT OF THE ATLANTIC YARDS PROJECT ON LOCAL SEWER INFRASTRUCTURE: SUMMARY REPORT

Prepared by: HydroQual Environmental Engineers and Scientists, P.C.

NOTE: This report has been updated from its original July 7, 2006 version based on a reduced scope undertaken by FCRC within the Atlantic Yards project site.

FOCR.001 November 8, 2006


CONTENTS SECTION

PAGE

EXECUTIVE SUMMARY...............................................................................................................ES-1 1

INTRODUCTION ...................................................................................................................... 1-1

2

REPORT ELEMENTS............................................................................................................... 2-1

3

PROJECT SETTING.................................................................................................................. 3-1

4

POTENTIAL IMPACT ON COLLECTION SYSTEM:..................................................... 4-1 4.1 MANAGEMENT STRATEGIES: ............................................................................ 4-3

5

INDIVIDUAL REPORT ELEMENTS .................................................................................. 5-1 5.1 F&K – WATER DEMAND AND RE-USE ESTIMATES .................................. 5-1 5.2 VB – SEWER DESIGN, HYDRAULIC ANALYSIS, AND DETENTION .. STORAGE...................................................................................................................... 5-1 5.3 JNE – STORMWATER MANAGEMENT PLAN ................................................ 5-2 5.4 HYDROQUAL-COLLECTION SYSTEM SIMULATION ................................ 5-4

APPENDIX 1 : Sanitary Flow Estimates – Prepared by Flack + Kurtz (F&K), dated October 16, 2006. APPENDIX 2: Hydraulic Analysis and Sewer Design – Prepared by Vollmuth and Brush Planning and Engineering (VB), dated October 25, 2006 (revised) APPENDIX 3: Stormwater Management – Prepared by Judith Nitsch Engineering, Inc. (JNE), dated November 8, 2006. APPENDIX 4: Incremental CSO Volumes and Frequencies at Outfalls Discharging to Lower Gowanus Canal and the East River FIGURES Figure 1. Red Hook Drainage Area with CSO Discharge Points in Vicinity 3-1 Figure 2. Detail of Red Hook Drainage Area 3-3 Figure 3. Red Hook Drainage Area and Gowanus Canal Tributary Area 3-4 Figure 4. Atlantic Yards Site 4-1 Figure 5. Flow in a Combined Sewer 4-2 Figure 6. Storage Tank Schematic 5-4 Figure 7. Existing GPS/ Bond-Lorraine Sewer Configuration 5-6 Figure 8. Sewer Configuration after GPS Capacity Upgrade/Force Main Rehabilitation 5-7 Figure 9. Red Hook Modeling Network 5-8 Figure 10. Stormwater Facilities Schematic 5-12


ES-1

EXECUTIVE SUMMARY Forest City Ratner Company (FCR) plans to develop a site in Brooklyn, NY, the Atlantic Yards, in the vicinity of Flatbush and Atlantic Avenues. The project is in the drainage area tributary to the Red Hook water pollution control plant (WPCP), portions of which are also tributary to the Gowanus Canal. Water quality in the canal has been degraded in the past by overflows from combined sewers (CSO) during rainfall, and the Gowanus Pumping Station (GPS) at the head-end of the canal is the source of the largest of these discharges. In addition, the flows captured by the GPS are currently being re-routed through Bond-Lorraine sewer due to the force main being rehabilitated by the New York City Department of Environmental Protection (NYC DEP). The Bond-Lorraine sewer has capacity limitation to carry the entire GPS flows during wet weather to the interceptor sewer, due to heavy sedimentation and constrictions along the sewer. This re-routing has caused additional CSOs into the lower section of the Gowanus Canal well downstream from the GPS outfall. The CSO discharges to the canal, however, are significantly diluted by the flow pumped to the Gowanus Canal through the flushing tunnel that contains high dissolved oxygen levels associated with the East River water. FCR recognizes that development within the drainage area can have an impact on the local waters and is committed to producing a final project plan that will have no significant adverse impact on water quality in the Gowanus Canal or East River. To meet this water quality goal, FCR has pursued a design that includes a variety of stormwater management strategies, including provisions that are far and beyond those required by the existing regulations, as well as an aggressive approach to lowering water demand. The task is made difficult by the fact that the collection system tributary to the GPS is particularly sensitive to rainfall events. In addition, the footprint of the proposed project is very small relative to the total GPS drainage area, while the projected population in the project area is large compared to what is there now. Thus, the incremental increase of stormwater flow from the site is relatively small, while the corresponding increase in sanitary flow is greater. Stormwater flows must be retained and detained to meet the water quality goal. Stormwater regulations of NYC DEP require that new developments constructed in some areas of the City that will be connected to a city sewer must provide on-site detention storage if stormwater runoff will exceed the amount allowed for by the approved Drainage Plan for the site. This storage volume serves to attenuate peak-flows from the site into the public drainage network, and ensures that the existing network will not be overloaded during larger rainfall intensities designed for in the network for the approved Drainage Plan. The Hydraulic Analysis and Sewer Design report (Appendix 2), calculates the required volume as approximately 334,000 gallons for the full build-out project, including Site 5. The current project plan calls for storage volumes in excess of 900,000 gallons, far more than the DEP-required detention volume. The project also calls for storage of the rainwater at the site for rainfall intensities much smaller than those in the approved


ES-2 Drainage Plan so that CSOs to the Gowanus Canal can be minimized, if not totally prevented. The storage in excess of design requirements, water reuse in buildings, and additional capture through green roofs in the arena block are being provided in order to ensure that the final project will have no significant adverse impact on water quality in the Gowanus Canal or East River. The sewer design report also contains drawings and discussion of necessary changes in the site drainage network that will be required as part of the general site development process. The Atlantic Yards plan employs a variety of technologies and stormwater management strategies to meet the FCR goal of not affecting water quality in the Gowanus Canal or East River as compared to the project no-build condition in the planning years 2010 and 2016. These include: •

On-site detention and retention tanks for stormwater with multi-level discharge points to optimize storage

Detention of stormwater for rainfall intensities much lower than those planned for in the Drainage Plan for which local sewers are designed to convey without surcharging

Re-use of captured stormwater

On-site water features and site modifications to retain stormwater

Use of low-flow features to reduce water demand, and therefore, reduce wastewater flow, and

Use of green roof on the arena to intercept portions of runoff from reaching the sewer system during rain events.

The project team evaluated the proposed stormwater management plans by integrating them into a computer model of the Red Hook drainage area and simulating an entire year with a standard rainfall record. For the two planning years (2010 and 2016), the volume of CSOs at GPS, upper and lower portions of the Gowanus Canal, and the East River were tabulated for each of the planning conditions. Also included was the annual frequency of CSO events at GPS. In 2010, the stormwater controls will achieve reductions in CSO volumes compared to the no-build condition at the GPS. Due to re-routing of GPS flow through the Bond-Lorraine Sewer, the CSO discharge in the lower portion of Gowanus Canal is slightly increased. The aggressive stormwater controls at the Atlantic Yards site results in CSO reductions in excess of this increase in Lower Gowanus Canal discharge, ultimately resulting in a reduction of CSOs discharged into the Canal when compared with the project no-build condition. The discharges from outfalls along the Bond-Lorraine sewer and the GPS outfall are significantly diluted by flow from the East River pumped through the flushing tunnel with typical dissolved oxygen levels of 4-10 mg/L. Therefore, these discharges will have no significant adverse impact on the canal water quality. This revised baseline report accounts for the following changes in building scope or the model assumptions, in comparison to those used in the original HydroQual report dated July 7, 2006.


ES-3 ƒ Buildings 3 and 6 have been reduced in scope, resulting in a reduction in dry weather flow from 0.254 million gallons per day (mgd) to 0.15 mgd ƒ Building program for Site 5 has been revised - increasing the residential units and eliminating office space, thereby reducing the size of the building, resulting in a reduction of dry weather flow from 0.119 mgd to 0.075 mgd ƒ The previous analyses used two (2) events with a full capacity of 20,000 people for the arena, however, the model assumption has been revised to reflect two events per day (one with 10,000 people and another with a full capacity of 18,000 people). Even the revised assumption is conservative considering the planned 225 events per year in the arena, and the limited number of full capacity events. ƒ The sewer system model incorporated further amendments to the drainage plan resulting from the NYC DEP’s comments to the September 1, 2006 Amended Drainage Plan (ADP) by Vollmuth and Brush. The ADP utilized here is dated October 13, 2006. In summary, these changes to the building scope and model inputs resulted in changes to the overflow volumes to the Gowanus Canal and East River. The results indicate that in the planning year 2010, the net volume of CSOs to the Gowanus Canal would decrease by 0.2 million gallons (MG) with the project as compared to the Project No-Build scenario. This reduction in CSO volume to the Gowanus Canal is an improvement from the results in the previous report which showed a slight increase in the overall CSO volumes to the canal. In the planning year 2016, the results show a net reduction in overflows to the Gowanus Canal of 2.3 MG with the project as compared to the no-build scenario which is again an improvement from the 1.7 MG reduction documented in the previous report. The already insignificant overflow volumes to the East River were reduced further in 2010 from 0.5 to 0.4 MG with the project and stayed constant at 0.7 MG in 2016. The frequency of overflows has increased by one event in both 2010 and 2016 over No Build condition. This is because one of the events that had less than 5,000 gallons (the minimum threshold used in this study consistent with the NYC DEP-accepted overflow definition) in the No Build scenario experienced a CSO volume slightly over 5,000 gallons in the Project scenario. Water quality analyses performed with the Project scenario exhibited immeasurable and insignificant differences in dissolved oxygen (DO) and pathogens at the worst-case locations (both temporally and spatially) between the No Project and Project scenarios. In conclusion, the Atlantic Yards project would result in a net decrease in CSO volumes to the Gowanus Canal and a minimal increase in CSOs to the East River that will have no significant adverse impact on the water quality of these two water bodies.


1-1 SECTION 1

1

INTRODUCTION

FCR is developing a final design plan for the Atlantic Yards project that will result in no significant adverse impact on the water quality of Gowanus Canal or East River. FCR took up this commitment independent of any regulatory requirements because it is aware of the importance to the community of this issue. The Gowanus Pumping Station, at the head end of the canal, is the most important source of CSO discharges that have significantly degraded water quality in the canal. The project site lies within the drainage area that contributes both sanitary flow and runoff to the pumping station. In addition, the force-main constructed by NYC DEP to convey flows from GPS to the Red Hook Main Interceptor is being rehabilitated. On an interim basis, the NYC DEP is currently diverting flows through the Bond-Lorraine sewer along the western end of Gowanus Canal. Considering the stress on hydraulic capacity of this sewer (with a potential for flooding) to convey the additional flows to the Red Hook Interceptor, the NYC DEP had created relief points along Bond-Lorraine sewer that allow overflows during rain events. Due to the sewer’s capacity limitation, the flows pumped by GPS can discharge into the Gowanus Canal through these relief points. It is important to note that the flushing tunnel that brings in flows from East River with high dissolved oxygen levels provides dilution to discharges from these relief points and other outfalls in the Gowanus Canal including the GPS outfall. The purpose of this summary stormwater report is to provide a single document that describes the relationship between the individual reports produced by several consultants working on design elements that would have the potential to affect the wastewater collection system. Each of these consultants has different responsibilities, and complementary technical approaches to work with a specific set of tools and assumptions that ultimately allow quantification of incremental CSO impacts on the Gowanus Canal or East River. The individual reports are attached to this document as appendices.


2-1 SECTION 2

2

REPORT ELEMENTS

FCR retained several consultants to address different aspects of the design related to wastewater and stormwater: •

Flack and Kurtz (F&K) performed plumbing design work for the project buildings. This entailed developing water demand estimates for each residential unit and use in the project, so that all elements of the water supply and wastewater drain system could be properly sized and configured. F&K is also carrying out design work for systems within the project boundary and within the buildings. To reduce water demand and wastewater flow from the site, many low-flow technologies and water recycling approaches are being proposed. F&K also sized the arena-block detention/retention tanks.

Vollmuth and Brush Planning Environmental Engineering (VB) performed design and analysis work for the drainage network within and around the project site. Their design work is required as part of the NYC DEP permitting process to connect the proposed development to the existing city sewer network, and it includes modifications to meet current NYC DEP sewer design standards for future development. In addition, VB calculated the detention volume requirements for the project site as required by the NYC DEP in order to gain a permit for connection to the city sewers. Detention volumes hold back stormwater runoff for a period of time so that peak flows to the surrounding sewer system will be attenuated compared to flows from a direct connection. The detention volume required is calculated based on a 10-year storm using a DEP prescribed methodology. (A 10-year storm is expected to occur only once every ten years, and is far greater than in rain volume than a typical rainfall event that would be expected every year, or month.) Provision of this detention volume may help reduce CSO activity in the tributary area, but that is not its primary purpose.

Judith Nitsch Engineering (JNE) provided engineering and architectural services regarding site design and stormwater management strategies for the project site. Their work encompassed the design of retention and detention facilities (including use of existing storage tanks), landscape design with stormwater management features, and engineering analysis of the site’s response to storm events. In addition to detention/retention of stormwater, the use of green roofs on the arena as an additional stormwater management feature was evaluated by JNE using computational methods that are applicable at the sitelevel. JNE’s ongoing design work with FCRC examines a number of on-site reuse strategies,


2-2 but for the purposes of HydroQual’s analysis in this report, only the seasonal reuse for cooling tower makeup was taken into account. •

HydroQual Environmental Engineers and Scientists, P.C. (HQ) created computer simulations on a Red Hook drainage area basis to confirm the efficacy of the proposed design strategies in meeting the goal of maintaining water quality in the Gowanus Canal and East River. Site-level calculations performed by JNE were emulated by HQ in the drainage area-wide model to represent the stormwater management controls appropriately. The engineering work of the three firms mentioned above is integral to the work effort of HQ, as their reported data is used as input to the computer simulations of the local collection system. HQ performed simulations of the sewer system under a variety of development scenarios for the planning years specified in the EIS.


3-1 SECTION 3

3

PROJECT SETTING

Figure 1 shows the location of the Atlantic Yards project within the area tributary to the Red Hook WPCP. This plant collects flow from about 3,000 acres and discharges the clarified and disinfected effluent to the East River. Similar to many other drainage areas in New York City, the drainage area bounded by the blue line in Figure 1 is served by combined sewers, i.e., a single set of pipes carries both the sanitary flow from buildings and the storm flow that is routed into the pipes from catch basins in the street. Because they fulfill this dual purpose, many of the sewers in the collection system are quite large. During periods of dry weather, all sanitary flow is routed to the WPCP, and there are no overflows from discharge points, shown as red dots in Figure 1, in the area. The drainage areas outside of the blue Red Hook outline (e.g., properties along the waterfront) contribute dry weather flows and insignificant amounts of storm water to adjoining combined sewers. These flows are conveyed along with the combined sewage generated within the areas bounded by blue outline to be eventually taken to the WPCP or to a regulator.

Figure 1. Red Hook Drainage Area with CSO Discharge Points in Vicinity


3-2

The Red Hook WPCP has a New York State Pollutant Discharge Elimination System (SPDES) permit issued by the New York State Department of Environmental Conservation, which governs the allowable effluent flow volume and pollutant loads. The plant is permitted to treat a 12month rolling average dry weather flow of 60 mgd, and provides a secondary level of treatment (85% removal of solids and biological oxygen demand organics), and discharges the clarified and disinfected effluent to the East River. The flows to the WPCP can exceed 60 mgd during wet weather, and therefore, the WPCP is designed to provide treatment for a peak flow of up to two times the design dry weather flow or 120 mgd. When it rains over the Red Hook drainage area, the volume of water in the collection system can increase beyond the 120 mgd design wet weather capacity of the WPCP. To prevent the plant from being overwhelmed and disrupted, this excess flow must be discharged from the system into the local waters. These discharges comprise sanitary waste greatly diluted with stormwater, and in most locations, their impact on the local water quality is transitory. In the Gowanus Canal, however, because of its shape and circulatory pattern, CSO discharges from GPS can cause prolonged impairments in the form of depressed levels of dissolved oxygen (harmful to aquatic species) and elevated levels of coliform bacteria (present in sanitary wastewater.) This is particularly a concern at the head of the canal and less so nearer the outlet where tidal action mixing can better disperse CSO discharges. In New York City, the combined sewer system can be very sensitive to rainfall, i.e., it takes only a small amount of rain to trigger a CSO event in some portions of the system. This is partly due to the highly urbanized nature of the land surface, evident in Figure 2, in which most pervious areas (areas that will absorb water) have been eliminated. Currently, in the project site with the exception of the below-grade open rail yard, most of the runoff is from the impervious cover. Figure 3 shows the area that is tributary to the Gowanus Canal, i.e., the area that will contribute to CSOs discharged into the canal during wet weather. The canal tributary area is partially within the Red Hook drainage area, and the project site falls within Red Hook drainage area that is tributary to the canal. During dry weather, all sanitary flow drains to GPS, from which it is pumped under pressure into sewers that convey flows to the WPCP. The GPS is limited in the amount of flow it can convey. A pumping station’s conveyance capacity is defined as the design capacity of the mechanical pumps to lift wastewater from the influent well and force it under pressure through a pipe (referred to as a force-main) to another location. Inflow that exceeds the pumping station capacity must be bypassed in some way, i.e., discharged outside of the station by gravity flow, to avoid flooding the station and surrounding area. During wet weather, when the GPS cannot cope with the increased flow, the excess is discharged directly into the head end of the canal.


3-3

Figure 2. Detail of Red Hook Drainage Area

The GPS has a current capacity to divert up to 20.2 mgd through the force-main to the Red Hook Main Interceptor. Since this force-main is not functional, the GPS flow is routed to the sewer beneath Bond and Lorraine Streets, up to a maximum flow of 28.5 mgd. The Bond-Lorraine sewer now receives flow from its own contributing area, and the GPS flows. The sewer also has capacity limitations due to sedimentation and constrictions along the sewer. Three relief points have been established by NYC DEP along the Bond-Lorraine sewer to prevent potential flooding due to the increased flow into this sewer. The GPS is currently being rehabilitated by NYC DEP. The planned capital project, scheduled for completion by 2012, includes pump upgrades and construction of a new force-main to convey flows to the Red Hook interceptor. The rehabilitated operating capacity of pump station will be 30 MGD. As part of water quality improvement initiative in Gowanus Canal, the NYC DEP has been operating a flushing tunnel that brings flows in the range of 100-150 million gallons per day from the Buttermilk Channel (East River) to the canal to substantially improve dilution and flush the sediment and carbonaceous matter from the head-end, and also offset any impacts from poor


3-4 circulation in the canal. All outfalls that discharge into Gowanus Canal receive the flushing tunnel’s water quality benefits in terms of the significant dilution of solids and oxygen demanding substances. For the purpose of the analyses performed here, it has been assumed that the station is rehabilitated and operating at full capacity for the full project scenario in 2016, which will be the case when NYC DEP capital programs that are in place for the station are completed in 2012. However, for Phase 1 of the Atlantic Yards project to be completed in Year 2010, the pumping station has been assumed to operate at the current capacity of 28.5 MGD and to discharge into the BondLorraine sewer for conveyance to the Red Hook WPCP. The sedimentation and constriction along the Bond-Lorraine sewer are conservatively assumed to exist under the planning years 2010 and 2016, although the NYC DEP had periodically cleaned this sewer in the past.

Figure 3. Red Hook Drainage Area and Gowanus Canal Tributary Area


4-1 SECTION 4

4

POTENTIAL EFFECT ON COLLECTION SYSTEM

The detailed photo-map in Figure 4 shows that the Atlantic Yards site is partially developed and includes a functioning rail yard. The effect of the proposed development on the local sewer system is driven by the changes the proposed land uses will bring to stormwater flows from the affected blocks and by the increase in sanitary wastewater generated. The current sanitary wastewater flow is small, as the area has a small permanent population. The magnitude and temporal patterns of the area’s wastewater flow were confirmed by on-site flow monitoring carried out over a period of weeks.

Figure 4. Atlantic Yards Site

To understand the potential for changes in CSO behavior at the GPS, it is important to be clear on the relationship between three quantities: average dry-weather flow; wet-weather capacity; and capacity at the pumping station. These terms are defined as follows:


4-2 •

Average dry-weather flow is the total daily flow (usually as million gallons per day, mgd) leaving a given area. Dry-weather flow includes only sanitary wastewater, i.e., contributions from residential, commercial, and industrial users, and all of it is treated at the downstream WPCP. The actual flow rate of sanitary wastewater from a given area will vary greatly over the course of a day (known as the diurnal pattern), which is why the daily average is an important figure for capacity planning purposes.

Wet-weather capacity is the ability of a pipe to convey wastewater that includes both stormwater runoff and sanitary wastewater. In NYC, most drainage areas are served by combined sewers, so all facilities must be sized to handle flows that are much greater than the average dry-weather flow, or even the peak dry-weather flow. Wet-weather flows, which last beyond that actual cessation of rainfall, are much greater than peak dry-weather flows.

Pumping station capacity is simply the rated capacity of the mechanical pumps to lift wastewater from the influent well and force it under pressure through a pipe (force-main) to another location. Inflows that exceed the pumping station capacity must be bypassed in some way, i.e., discharged outside of the station by gravity flow, or the station and surrounding area will be flooded.

These quantities are shown schematically in Figure 5, which illustrates a typical trunk sewer line. In this schematic, a combined sewer is flowing nearly full during a storm event. (The slope of the pipe is greatly exaggerated.) The sanitary wastewater occupies only a small part of the total sewer capacity, and the small space at the crown of the sewer indicates the remaining capacity in the conduit. (The schematic diagram presents the stormwater and sanitary waste as discrete, while, in fact, they are thoroughly mixed.) Additional Sewer Capacity Stormwater Flow

Pump Station Capacity

Sanitary Wastewater

Figure 5. Flow in a Combined Sewer

The pumping station, in this diagram, has a capacity equivalent to the total flow reaching it at the moment represented in the figure. In that moment, there will be no CSO event at the pumping station since the mechanical pumps will successfully route all inflows to the interceptor sewer, and


4-3 on to the WPCP. When there is additional flow resulting from continued rain events, the pipe has extra capacity to convey to the pumping station, however, the pumps cannot convey the entire flow to the interceptor sewer. The flows in excess of the pumping capacity would likely result in a CSO event. The Atlantic Yards development will increase the sanitary flow to GPS because the many residential units, the arena, and the retail and office space will increase the water demand by full and part-time residents. The development also has the potential to increase stormwater flows by increasing the percentage of the area that directly contributes runoff to the sewer system. This is an effect of the covering over of the existing rail yard with a foundation slab on which numerous structures will be placed. Because the GPS has little excess capacity, even after planned capital upgrades, very small changes in the collection system and its tributary area can easily cause changes in the CSO behavior at the canal. This is the context in which the project team had to work in order to ensure that developing the Atlantic Yards does not cause any significant adverse impact on the water quality in the Gowanus Canal or East River. Although the project area is small compared to the entire Red Hook drainage area, and though the projected increases in sanitary wastewater flow is also a small percentage of the total flow in the area, the bottleneck created by the GPS means that any change caused by the project could potentially have a significant impact on CSO behavior. This impact will be attenuated somewhat because the site is located in the upstream portion of the drainage area, i.e., flows from the Atlantic Yards take a considerable time to reach the GPS, and therefore any increases in the peak-flows are damped. Nevertheless, the sensitivity of the system to any changes is marked. 4.1

MANAGEMENT STRATEGIES

There are three strategies for managing stormwater that the project team has adopted to meet its water quality goals for the Gowanus Canal and East River. They share the purpose of reducing the burden placed on the local sewer system, and also decreasing the CSO activity at GPS, by the project either by effecting an absolute reduction of input or by spreading out the load to reduce the temporal peak. •

Detention is the withholding of stormwater from the local sewer system for a given period of time. This can be accomplished with simple passive technologies, e.g., special roof drains that cause rainfall to pond on a flat roof to a certain depth before draining down roof leaders. More intensive approaches will supply engineered storage tanks that fill with stormwater during design storms (storms of a given intensity and recurrence period) and then let the water slowly drain to the sewer. In all cases, the water is held back only temporarily, but the reduction in peak-flow can be substantial. Detention of stormwater is required by the NYC DEP for specified new developments, and this requirement has been


2006 Atlantic Yards Sewage Impacts on Gowanus - HydroQual  

A summary report detailing the impacts of the Atlantic Yards Project on Infrastructure. This is a revised baseline report accounting fo the...

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