WEF Collection Systems Conference 2017

Transcription

1 Davenport, Iowa 14th District Diversion Study Karol Giokas, P.E., RJN Group Tom Leabhart, P.E., Davenport, IA ABSTRACT The 14 th District sanitary sewer basin in Davenport, Iowa has high levels of inflow and infiltration (I/I), resulting in sanitary sewer overflows and basement backups. When the West Side Tunnel came online in 2013 much of the flow that previously was tributary to the south Duck Creek interceptor was diverted to the tunnel. This resulted in the south Duck Creek interceptor having additional capacity. The purpose of this study was to evaluate ways to reduce the occurrence of overflows and basement backups in the 14 th District area by utilizing that potential excess capacity in the south Duck Creek interceptor. Different diversion options were evaluated along with different levels of flow reduction to come up with a recommended option for the city to reduce basement backups and overflows. KEYWORDS Basement backups, sanitary sewer overflows, flow metering, flow monitoring, flow diversion, hydraulic modeling. INTRODUCTION The City of Davenport, Iowa (City) has had problems with sanitary sewer overflows (SSOs) and basement backups in the sewershed known as the 14 th District. The area served by the 14 th District is one of the City s older areas, situated on a bluff northeast of downtown Davenport, with homes dating from the 1870s. The basement backups in the area have resulted in several lawsuits being filed against the City. The City set a goal to eliminate SSOs and basement backups in wet weather conditions up to a 5-year storm recurrence interval. RJN Group, Inc. (RJN) had been working with the City performing flow monitoring and sanitary sewer evaluation services (SSES) in multiple location in the City including the 14 th District. RJN was also selected to evaluate options for diverting flow from the 14 th District to the south Duck Creek interceptor. FLOW MONITORING Over a period of 4 years 25 flow meters and multiple rain gauges would be installed in and around the 14 th District basin to understand the system response to rainfall. The original metered area is shown as the small area outlined in yellow as 14 th District Study area in Figure 1. This metering in 2011 indicated that a restriction was present downstream (out of the study area) and was restricting the flow in the 14 th District interceptor. In 2012, the metering was expanded to include the downstream sewers that connected to the south Duck Creek interceptor. Unfortunately, there was insufficient rain during the metering period in 2012, and the flow meters were reinstalled in Copyright 2017 by the Water Environment Federation 899

2 FIGURE DIVERSION ANALYSIS STUDY AREA Copyright 2017 by the Water Environment Federation 900

3 The West Side Tunnel (WST), installed to allow for continued development in the northwest portion of the City, was completed in The WST was also designated to take the flow from the western portions of the north and south Duck Creek interceptors. This resulted in available capacity in the two Duck Creek interceptors, also shown in Figure 1. The three periods of flow metering provided a good baseline for flows in the 14 th District interceptor. However, no metering had been performed on the north and south Duck Creek interceptors. Therefore, in 2014 RJN installed meters along both Duck Creek interceptors, as well as the 14 th District interceptor. This data would provide the information needed to calibrate a model of all three interceptors. The meters used were ADS Triton meters on telemetry, with dual depth sensors and a Doppler velocity probe. The meters were installed, calibrated, and maintained to obtain high-quality data to use in a hydraulic model. DPS-03 Meter Installation In this case, where the City is considering committing millions of dollars for construction it is important to get good rain data, both dry days and multiple storms of varying sizes to be confident in the model calibration. For this reason the metering period was designated as 90 days, with an option to extend if that would become necessary. In this case that was not required, as Figure 2 shows the 1-hour rain events that occurred during the study. Copyright 2017 by the Water Environment Federation 901

4 FIGURE 2 1-HR RAIN EVENTS MANHOLE INVESTIGATION AND SURVEY For the accurate modeling of the sewers in Davenport, GPS sub centimeter rim elevations were collected for manholes along the model route. Many of these manholes were located along Duck Creek which is heavily wooded in some areas, resulting in some challenges to the survey crew. Collecting Manhole Inspection Data at MH DN-40 Invert depths were collected on the structures to be incorporated into the model during inspections of the manholes. Because of the critical nature of the model and the importance of getting the pipe diameters correct, descent inspections were performed on structures where pipe diameter transitions occurred. Manhole inspections also documented structural and I/I defects present in the manholes. Recommendations for remediation of those defects was provided to the City. Map changes, resulting from the field investigations were also communicated back to City staff so that the GIS asset inventory could be updated to more closely match the field configuration. Copyright 2017 by the Water Environment Federation 902

5 These included incorrect connectivity, new structures, non-existent structures, and pipe diameter changes. MODEL DEVELOPMENT The skeletal model, which had been developed previously to include all sewers 12 and larger for the 14 th District interceptor, was now expanded to include the north and south Duck Creek interceptors. The collected flow data was used to calibrate this larger model to match the system response to various wet-weather events. Not only was the model used to evaluate the effect of changes on the 14 th District, but also on the south and north Duck Creek interceptors. It is important that options for relief in the 14 th District do not simply move the problem to another area and, in this case, negatively impact the south Duck Creek interceptor. CAPACITY OF THE INTERCEPTORS To understand the effect of the extra flow from the diversion area on the existing Duck Creek Interceptor flow, it is first important to understand the current capacity of the interceptors. This can be assessed by computing the Sustainable Peaking Factor (SPF), or the ratio of full pipe flow to dry-weather flow for each segment along the sewer. Flow data collected during dry-weather, low-groundwater periods was analyzed to determine the average dry-weather flow (ADWF) for each basin in the study area. A summary of the ADWF for each basin is given in Table 1. The ADWF was then used to model the current condition of flow during dry weather conditions. The calculated capacity, based on pipe diameter, slope, and material, for each pipe segment was compared to the ADWF of that segment to get the segment SPF, and is shown in Figure 3. Sustainable Peaking Factors can be categorized as follows: Less than 4 The sewer capacity is lower than design standards for new pipe dry weather flow and capacity improvements are necessary. I/I reduction and storage are also likely components of a full solution. TABLE 1 ADWF SUMMARY ADWF Meter (mgd) DP DP DPN DPN DPN DPS DPS DPS DPS Between 4 and 8 Sewer has sufficient capacity for dry weather flow, but wet weather capacity is limited. Moderate inflow and infiltration (I/I) during major rain events may surcharge the system. Capacity improvements, storage, and I/I reduction options should be evaluated. Between 8 and 15 Sewer has sufficient capacity for dry weather flow and should have sufficient wet weather capacity up to a 5-year storm for wet weather flow with moderate I/I. However, systems with significant I/I may still surcharge the system in major rain events. Capacity improvements are generally not justified, but I/I reduction Copyright 2017 by the Water Environment Federation 903

6 improvements are typically utilized with storage included in lieu of addressing expensive private sector sources. Greater than 15 Sewer has sufficient capacity for dry weather flow and should have sufficient wet weather capacity through a 10-year storm event unless I/I is severe. Figure 4 shows the SPF of all the sewers modeled for this study. As can be seen, the south Duck Creek interceptor has greater than 15 SPF through the upper reaches of its span. This extra capacity was created when all of the tributary flow west of DS-53 near Wilkes Ave, was diverted to the West Side Tunnel in Also seen in Figure 4 is the limited capacity in the north Duck Creek interceptor, the 14 th District interceptor, and the stretch of sewer after the two Duck Creek interceptors merge together south of Duck Creek and east of the railroad tracks. A new sanitary system is typically designed to convey 3-4 times the assumed dry-weather flow. Therefore, a peaking factor less than 4 for any specific basin is generally not considered to be an I/I issue as the sewer should have capacity to handle the excess flow. A system hydraulic model is calibrated according to the meter data gathered and standard rain distributions are applied to the study area in order to pinpoint areas with surcharges and/or backups. In smaller wet weather events, such as a 1-Year storm, the north and south Duck Creek interceptors currently have large amounts of freeboard, or vertical distance between the ground elevation and the sanitary flow hydraulic grade line. When freeboard gets to zero, the model is predicting a sanitary sewer overflow. Freeboard is also an important tool used to indicate the risk of basement backups in the area. Because of the variability of basement depths relative to sewer depths the allowable freeboard for each house will be different, but it is clear that as freeboard decreases the risk of a basement backup increases. Copyright 2017 by the Water Environment Federation 904

7 FIGURE 3 CURRENT CONDITION SUSTAINABLE PEAKING FACTORS Copyright 2017 by the Water Environment Federation 905

8 For larger rain events, such as a 5-year or 10-year rain event, the hydraulic model predicted that the Duck Creek interceptors begin to take on significant amounts of I/I causing backups where the two interceptors come together. The volume of excess flow in the Duck Creek interceptors results in loss in freeboard, as can be seen in Figures 4-6. FIGURE 4 CURRENT CONDITION 1-YR STORM FREEBOARD FIGURE 5 CURRENT CONDITION 5-YR STORM FREEBOARD Copyright 2017 by the Water Environment Federation 906

9 FIGURE 6 CURRENT CONDITION 10-YR STORM OPTIONS EVALUATED The limitations in the 14 th District interceptor can be seen in the preceding Figures 4 through 6 at manhole F-59 just downstream of meter location DP F-59 is a site where the combination of high levels of excess flows and the manhole depth of only approximately 4 feet resulted in frequent overflows. The location that was assessed for possible diversion of flow north to the south Duck Creek interceptor was just upstream of this manhole at manhole F-61. The hydraulic model indicated that, while flow reduction was necessary, it alone would not be sufficient to meet the City s goal of reducing the occurrence of basement backups and the SSO at F-59. Four scenarios for flow storage and/or flow diversion utilizing the excess capacity in the south Duck Creek interceptor resulting from the recently installed the West Side Tunnel were evaluated by RJN using the enhanced system model. Minimum freeboard in the modeled system was the criterion used to determine how effectively the various options resolved the excess flow issues in the 14 th District. The four scenarios evaluated were: Scenario 1 - Diversion of all of the flow from the 14 th District interceptor at manhole F- 61 to the south Duck Creek interceptor Scenario 2 - Partial diversion of flow from the 14 th District interceptor at manhole F-61 to the south Duck Creek interceptor Scenario 1A - Diversion of all of the flow from the 14 th District interceptor at manhole F- 61 to the south Duck Creek interceptor and incorporating storage Scenario 2 - Partial diversion of flow from the 14 th District interceptor at manhole F-61 to the south Duck Creek interceptor and incorporating storage Copyright 2017 by the Water Environment Federation 907

10 Each scenario was also modeled with no flow reduction or 20% flow reduction in either the entire 14 th District study area or in just the area that was being diverted to the south Duck Creek interceptor. Concurrent with the flow monitoring and hydraulic modeling study, the City initiated a comprehensive sanitary sewer evaluation survey (SSES) with RJN. The work included public system inspections including manhole inspections and survey, closed-circuit televising (CCTV), and smoke testing. In addition, targeted private property inspections were also completed. Upon completion of the SSES inspections, a flow balance was completed to estimate inflow and infiltration (I/I) into the system and to provide the City with recommended flow reduction options. The results of the modeling indicated that the best solution for the excess flow issues was a combination of diversion of all flow from the 14 th District interceptor at F-61to the south Duck Creek interceptor, 20% flow reduction in the 14 th District, parallel relief piping in select localized areas, and inclusion of a storage tank. This is basically Scenario 1A with the additional local sewers. Figures 7 and 8 show the resulting modeled freeboard for Scenario 1A for a 1-year rain event and a 5-year rain event. The other scenarios modeled did not resolve the excess flow issues for up to a 5-year rain event. Also note that the model indicated that for all scenarios the north Duck Creek interceptor had very low levels of available freeboard, indicating high levels of surcharge in the sewer. This issue could not be resolved by any of the options being considered in the study. Copyright 2017 by the Water Environment Federation 908

11 FIGURE 7 SCENARIO 1A, 20% FLOW REDUCTION 1-YEAR RAIN Copyright 2017 by the Water Environment Federation 909

12 FIGURE 8 SCENARIO 1A, 20% FLOW REDUCTION 5 YEAR RAIN RECOMMENDATIONS The recommendations from the study included: Continuation of system rehabilitation and clear water source removal in the 14 th District with a goal of at least 20% reduction of wet weather flows through manhole and mainline rehabilitation and quick fix private sector defects; Complete diversion of flow from the 14 th District to the south Duck Creek Interceptor; Incorporation of parallel relief sewers in a local area; Design considerations to allow for a future 1.5 MGD storage facility; and Continuing flow monitoring to measure the change in system response during wet weather events resulting from improvements. It was recommended that the City implement these recommendations in a phased approach, using flow metering as system changes were completed to confirm the resulting flows in the system matched those predicted in the model. This implementation would involve starting with a modified Scenario 2, using a variable flow diversion structure at F-61, to start with partial flow diversion. The ultimate solution for the area will require the incorporation of the storage and parallel sewer shown in Scenario 1A. This will eventually result in full diversion of all flow from the 14 th District interceptor at F-61. Copyright 2017 by the Water Environment Federation 910

13 Flow reduction would need to continue by addressing public sector sources, and some easy private sources such as downspouts or cleanouts. With the goal of no basement backups or SSOs are experienced during a 5-year storm event, the level of flow reduction achieved by the City could impact the size of the excess flow storage tank that will be needed. This is the major reason for the phased approach recommended. Flow metering data collected and analyzed following the construction of the diversion structure and implementation of flow reduction would result in more accurate sizing of the storage structure and the parallel sewers needed for the resulting system. SUMMARY AND CONCLUSIONS In order to relieve surcharging and the resulting overflows and potential backups in the 14 th District study area, the City of Davenport needs to reduce flows in the areas tributary to the 14 th District interceptor. However, based on system models, flow reduction through I/I removal alone is not sufficient for system wide relief. RJN evaluated potential options for diversion of flow to the south Duck Creek interceptor. It is important that options for relief in the 14 th District do not simply move the problem to other areas and, in this case, negatively impact the area tributary to the Duck Creek interceptor. Available freeboard was the criterion used for determining how well a scenario addresses the needs in both the 14 th District and the south Duck Creek area. Only Scenario 1A, with flow reduction, will reduce the risk of overflows in the 14 th District at all locations without increasing the potential of overflow or basement backups in the south Duck Creek interceptor area. However, a partial Scenario 2 implementation could be utilized as an initial step to provide some immediate benefit without significantly impacting the Duck Creek interceptor. Implementing Option 2 initially with flow reduction can reduce the cost of implementing scenario 1A by reducing the size of the wet-weather storage needed or it can provide an extra measure of storm protection over the 5-year storm summarized here. The City completed the implementation of the variable diversion structure in There have not been significant rain events to date, nor are there flow meters installed, to provide any data indicating the impact on flow levels of the approach. Copyright 2017 by the Water Environment Federation 911