Timothy Ruggaber firstname.lastname@example.org 574.855.1012 12441 Beckley St. #6 Granger, IN 46530
Tuesday, January 29, 2013 MEMORANDUM To: Dawn Zimmer, Hoboken City Council Re: Sewer Isolation/Pump Concept for Hoboken Flooding problem In February of 2008, EmNet, LLC was contacted by members of the Hoboken, NJ community to determine if EmNetâ€™s real-time monitoring and control technology (called CSOnet) could help alleviate the endemic flooding in several portions of Hoboken. There was concern over the high cost of the proposed four pump station solution and about the fact that untreated wastewater is being pumped directly into the Hudson River. EmNet was asked to determine if Real-Time Control technology could reduce the cost of the flooding solution and minimize the discharge of untreated wastewater into the environment. Over the last several months, EmNet has met with representatives the North Hudson Sewerage Authority (NHSA), CH2M Hill, and the City of Hoboken in order to fully understand the Hoboken sewer system, its problems, and the current strategies for fixing the flooding problem. This memo contains the results of EmNetâ€™s studies and evaluations. The Problem
Figure 1. A flooded street in Hoboken, NJ at the Housing Authority (left) and Sky Club (right) Several portions of Hoboken, NJ, particularly in the southwest area of the city, are prone to flooding when rain events occur during high tide (see figure 1). The Hoboken sewer system is a combined sewer system, which means that the same system of pipes carries both municipal wastewater and stormwater runoff. To prevent the system from flooding during wet weather, relief points called outfalls are built into the sewer, which are supposed to allow excess sewer flows to overflow into the Hudson River rather than
having it flood streets and basements. Unfortunately, large sections of Hoboken are either below the normal high tide elevation or the normal storm high tide elevation (see figure 2), and many of the associated outfalls are also either below normal high tide levels or the normal storm high tide levels. This means that these outfalls are not able to release this excess sewer flows into the river when storm events occur during high tide. Instead, all of this water stays in the sewer system, and the sewage levels throughout the sewer system keep rising until either the sewage level is higher than the tide level or street flooding begins.
Figure 2. The flood prone areas of Hoboken, NJ. The red areas are below the normal high tide elevation, the beige areas are below the normal storm high tide elevation, and the blue areas are in the 100-year storm floodplain. It is important to state how this flooding occurs. Sewer systems only have a finite amount of water that they can hold, and almost all of this storage volume is found in the pipes. Once the pipes are full, the water level in the sewer/manholes increases very rapidly as the small volume of the manholes is filled. The reason that the water level increases is simply because the water has nowhere else to go. However, once the water level reaches street level in one manhole, this volume restriction is gone. Water is then able to fill all of the bowls and depressions on the land surface, which may have a very large volume. In such a system, the higher elevation areas will not flood until all of the lower elevation areas are filled with water. Hence, even though a majority of Hoboken is
below the normal storm high tide, flooding may only be contained to only those lowest lying areas.
Figure 3. Hoboken’s drainage areas. The Hoboken sewer system is set up to be a grid, with larger east-west main pipes each collecting wastewater from a portion of town (called a drainage area, see figure 3). All of the drainage areas south of 5th St. send their water to the 5th St. Lift Station. This water is then pumped to the 11th St. Lift Station, which also collects water from other drainage areas and pumps it to the WWTP. Throughout the sewer system, there are several smaller north-south pipes that connect each of the drainage areas. This means that flows from one drainage area can enter a different drainage area through any of these pipes. The grid approach to sewer systems means that the water throughout the sewer system will all have the same hydraulic head (“head” refers essentially to the elevation of the water surface level in a pipe). The Existing Pump Solution Engineers from CH2M Hill have been working with the NHSA to develop a solution that addresses this flooding issue using traditional and available technology. In this solution, CH2M examined the entire City of Hoboken and developed an integrated plan that would prevent any portion of Hoboken that is below the normal storm high tide elevation from flooding during a large storm event at high tide. To do this, the plan calls for the consolidation of Hoboken’s eight outfalls into five outfalls and for pump stations to be installed at four of those outfalls. The purpose of the pump stations is to create
â€œlow tide conditionsâ€? within the sewer system. When a storm occurs during high tide, these pump stations collect all of the excess wastewater that would have naturally overflowed if the storm had occurred during low tide and pump it into the Hudson River. The locations of these pump stations are shown in figure 4.
Figure 4. Proposed sites of the four pump stations. In order to develop and test this proposed solution, the engineers from CH2M developed a computer model of the Hoboken sewer system using a program called XP SWMM. A schematic showing the setup of the computer model is shown in figure 5. The purpose of any model is to provide a general understanding of the key aspects of the sewer system and is not meant to capture every detail of a sewer system. Simplifications of the system have to be made in the model in order to prevent the model from becoming unwieldy and expensive to develop. Due to these simplifications, even the best models have errors rating of up to 20%. In the Hoboken sewer model, each of the drainage areas is represented by its associated main east-west sewer line. There are no cross connections between the drainage areas, which means that each drainage area behaves autonomously during the simulations. The modeler then inputs varying amounts of flow into these sewer lines that correlate to flows from a variety of storm. He or she then altered the model to include the four proposed pump stations and ran the storm simulations again in order to determine how effective the pumps were at reducing the flooding. This process was repeated a number of times with several different proposed solutions in order to determine the most effective and efficient one.
Figure 5. A schematic of the computer model of Hobokenâ€™s sewer system. It should be noted that the presence of grit and refuse inside of the sewer system can greatly affect how water flows through the pipes (see figure 6). If these factors are not included in the computer model, then the model is even less accurate in simulating conditions in the actual sewer system.
Figure 6. Inside of a surcharged manhole at 4th St. and Jackson St., taken on July 15, 2008
The overall cost of this solution is estimated to be $37 million, and the last pump station is not scheduled to be completed until June, 2012. A breakdown of the cost and timeline of each pump station is shown in table 1. Table 1. Description of four pump stations and timeline for installation. Wet Weather Pump Station 11th Street PS (CSO H5) 4th Street PS (CSO H3/4) 15th Street PS (CSO H6/7) Observer Highway PS (CSO H1)
Total Capacity (in MGD)
Funding Available From NJDEP
Completion of Construction
Estimated Construction Costs
Unfortunately, the pump station that will reduce the flooding in southwestern Hoboken the most is scheduled to be the last one to be installed. The installation of the northern pumps will provide some relief to the southwestern portion of Hoboken, but the problem will not be solved until the Observer Highway station is completed. The reason for this lies in how water moves within the Hoboken sewer system. There are two ways that wastewater from the southern part of Hoboken reaches the northern part of Hoboken. The primary way for the water to move north is through a large pipe running across each drainage area that collects the water and takes it to an existing lift station. All of the drainage areas south of 5th St. send their water to the 5th St. Lift Station. This water is then pumped to the 11th St. Lift Station, which also collects water from other drainage areas and pumps it to the WWTP. Due to the limited capacity of the 5th St. Lift Station, only a finite amount of water from the lower areas of Hoboken can reach the northern areas of Hoboken this way, creating a significant bottleneck. The secondary way is for the water to travel through the smaller pipes that interconnect the drainage areas and form a grid. However, the water has to snake its way though these smaller pipes over a fairly large distance, limiting the ability for the water to travel north. What causes this movement to occur is that the northern pumps created lower head conditions in the areas around the pumps, and water from the south moves north in order to even out the head. The head difference between these two points is likely to be rather small, meaning that the movement of water will be relatively slow. When the southernmost pump station is built, water will be able to flow through the larger, primary pipes to the pump, where it will then be pumped into the river. In this
scenario, there are no significant bottlenecks, and all of the excess water that would normally cause a flood is pumped out of the sewer system. If only the northernmost pump is installed, the scenario changes completely. In order for water from the south to be pumped out, it must first reach the pump. This is much more difficult due to the bottleneck at the 5th St lift station and the relatively difficult path through the interlocking grid sewers. Only a percentage of the water from the south is able to reach the pump, and the rest is left behind. This scenario is exacerbated by the fact that the capacities of the northern pumps are much smaller than that of the southernmost one. So, each northern pump will lessen the flooding in southwestern Hoboken by some degree, but the southern pump is needed to completely solve it. EmNet’s Proposed Solution Rather than examining the Hoboken sewer system as a whole and developing an integrated plan to address all potential flooding areas, EmNet instead sought to address the areas with the worst flooding first and then develop a custom design for the rest of the city. To do this, EmNet’s solution focuses on the H1 drainage area (see figure 7), which contains more than 80% of the Hoboken area that is below normal high tide elevation and 50% of the Hoboken area that is below normal storm high tide. This is the portion of the city with the lowest elevation, as low as 2.0 feet above mean sea level at Marshall and 1st St. This is also the area that floods with the greatest frequency and severity. Even during a 3 month storm, it is estimated that portions of the H1 service area will have up to 1.5’ of flooding. We estimate that solving the flooding in this drainage area will address 80% of Hoboken’s flooding problem.
Figure 7. Location of the H1 drainage system and its associated elevations.
The main concept of this proposed solution is to keep water from the higher elevations of Hoboken out of the flood-prone region and then to pump any remaining excess water out of the drainage area and into the Hudson River. We hypothesize that water from the northern service areas of Hoboken flows into the H1 drainage area when there is flooding because the flooding provides “storage capacity” for the excess water in the north. The stormwater that falls onto the H1 drainage area in a given storm may be enough to cause the initial flooding, but we hypothesis that once this flooding begins, the head in the flooded area increases much more slowly than that in the non-flooded areas. This is due to the fact that the flooded areas now have a very large storage volume with a large surface area, and the non-flooded areas still have very small storage volume with a very small surface area. This head difference would cause flows from the non-flooding areas to enter the flooding areas, making the problem worse. If this is the case, then the first step in reducing the total flooding volume is to keep water from the northern drainage areas from entering the H1 drainage area. This requires disconnecting between four and six of the smaller north-south lines that connect the drainage areas (shown with an “X” in figure 8). After this is done, the only way for the water from the drainage area to reach the wastewater treatment plant would be to drain into the main east-west pipe, which then curves north until it reaches the 5th St. Lift Station. This 5th St. Lift Station also collects flows from the H2, H3, and H4 drainage areas and has a capacity of 10 million gallons per day (MGD). In order to prevent this lift station from being overwhelmed with flows from the H1 drainage area, a moveable weir is installed on this north-south line (shown as a yellow “W”). A moveable weir is essentially a wall mounted in a sewer line with an automatically adjustable height. The moveable weir would be raised during a high-tide storm, thereby preventing flows from the H1 drainage area from reaching the lift station and enabling the flows from the other service areas to utilize the lift station’s full capacity. This will help to prevent the other service areas from flooding, as well. A pump station is then built in the southeast corner of Hoboken (on either Observer Highway or Newark St. – CH2M already has a design for a 150 MGD pump station at either of these sites). This pump station (shown as a blue “P”) would then pump all of the remaining excess water into the Hudson River. Once low tide comes or the storm passes, the moveable weir would be lowered, the pump station would be turned off, and the system would drain as normal. If capacity exists in the 5th St. Lift Station during a high tide storm event, then the moveable weir would be lowered, and some of the excess water from the H1 service area would be sent to the WWTP, thereby minimizing the total amount of sewage overflow.
Figure 8. Schematic of the isolated H1 drainage area. This proposed solution is focused on solving a majority of Hobokenâ€™s flooding problems as quickly and inexpensively as possible. We estimate that this proposal will cost roughly 1/3 of the four pump station solution and address 80% of Hobokenâ€™s flooding. Although we do not believe that this solution will make flooding in the rest of Hoboken worse than it is currently, it should be noted that this solution will not reduce flooding there. After this initial solution is implemented, its impact will be evaluated and another tailor-made solution will be proposed to resolve any remaining issues in northern Hoboken. However, since a bulk of the flooding issues would already be resolved, solving any northern flooding may require fewer pump stations and be less expensive than previously estimated. One important thing to note is that the use of any pump station to purposefully discharge sewage into the Hudson River will likely become regulated when the NJ Department of Environmental Protection and the US EPA begin to enforce CSO regulations in Hoboken, and these pump stations may become obsolete due this enforcement. Additional Sewer Monitoring Unfortunately, EmNetâ€™s proposed solution cannot be tested using the existing computer model of the Hoboken system because that model does not include the northsouth interconnecting lines, which are a key aspect of the Hoboken system and our solution. We believe that a further study of how water moves in these lines during storm events is necessary in order to determine the potential effectiveness of our solution and to prove or disprove our hypotheses. In addition to monitoring these lines, we also propose monitoring each of the outfalls and locations within the lowest lying areas within Hoboken. Such a system would be able to:
• • • • • • • • •
Determine the effectiveness of the pumps in reducing flooding, especially as the pumps are incrementally installed. Confirm/validate the selection of pump sizes for the pumps that will be installed at a later date Aid the City and NHSA in the detection of problem spots in the sewer areas before they become significant problems Explore alternative control strategies to reduce the flooding problem and to reduce the amount of CSO overflows (especially after the NJDEP becomes more strict in enforcing the EPA CSO mandates) Provide data for the further calibration of the City’s computer model of the sewer system. Characterize flows into and out of the flood prone areas, providing the data necessary to minimize flows into these areas during wet weather. Determine how much overflow occurs during each storm event and how effective different control measures are in reducing this amount over time. This will particularly important as the City and NJDEP transition into developing a CSO reduction plan. Coordinate the operation of the pumps with actual conditions in the flood-prone areas of Hoboken rather than basing control on conditions in the areas of the pumps (which tend not to flood). Reduce the amount of overflowed sewage and the operation and maintenance costs of the pumps by only having the pumps on when necessary to prevent flooding.
Figure 9. Approximate locations of proposed monitoring points. Key:Yellow circle = monitoring of service area connections, Red circle = monitoring of flood prone area, Blue circle = monitoring of outfalls (not all shown), Pink area = flood prone service area, Green line = sewer line that runs to the treatment plant, “D” = suggested depth sensor, “F” = suggested flow meter We propose installing real time monitoring points at the following locations (see figure 9): • The sewer pipes that connect this service area with the rest of the sewer system (4-6 locations, monitored with flow meters). This will help determine if flows from other service areas are entering the flood-prone areas during storm events. • Sections of the city that are most prone to flooding (2-3 locations, monitored with depth sensors). This will help determine the effectiveness of the pumps, provide an early
detection system for flooding, and enable the pumps to be coordinated with actual conditions in the flood prone areas. The CSO outfalls (5 locations, monitored with depth sensors). There are the areas where the overflows occur. These monitoring points will provide data on current overflow volumes, which will be necessary as the City and the NHSA transition into a CSO abatement strategy, and also provide data on the overflow reductions that occur as the strategy is implemented.
It has been brought to our attention that accurately monitoring flows in interconnected â€œgridâ€? sewer system can be very difficult due the lateral flow directions in the manhole and that flow values may be off by as much as 20%. We do not foresee this being problematic, since we are more concerned with the overall flow directions and depths than in highly accurate flow values. These parameters should be accurate, even in interconnected sewers. We believe that the addition of real-time monitoring to the flood-reduction strategy will increase the effectiveness and efficiency of the pump system, while potentially reducing its cost. The monitoring system will also provide necessary data to the City as it transitions from flood control to CSO volume reduction. The estimated cost of such a monitoring system is approximately $100,000 installed, which is a small percentage of the overall cost of the solutions proposed. In the best case scenario, data from this monitoring system shows that the number of pump stations can be reduced, thereby decreasing the cost of the project. In the worst case scenario, the data will justify to the people of Hoboken that all four pump stations are the necessary and prove that this is a worthwhile and necessary expense.
Published on Dec 22, 2008
Figure 1. A flooded street in Hoboken, NJ at the Housing Authority (left) and Sky Club (right) Figure 2. The flood prone areas of Hoboke...