Minne Lusa Stormwater Tunnel and Pershing/Storz Detention Basins Hydraulic Evaluation

Transcription

1 TECHNICAL MEMORANDUM OMAHA CSO CONTROL PROGRAM Minne Lusa Stormwater Tunnel and Pershing/Storz Detention Basins Hydraulic Evaluation TO: Jim Theiler, City of Omaha Eitan Tsabari, City of Omaha Scott Aurit, Omaha CSO PMT Tom Heinemann, Omaha CSO PMT COPY: Matt Schultze, Black & Veatch Bill Glissman, HGM FROM: Kris Hahn, Black & Veatch Jim Schlaman, Black & Veatch DATE: February 6, 2009 Overview and Objectives The objectives of the task described herein are to perform a hydrologic and hydraulic evaluation of the 7,900 foot long stormwater tunnel proposed in the Minne Lusa (ML) Basin for the combined sewer overflow (CSO) Long Term Control Plan (LTCP). The proposed stormwater tunnel is planned to convey stormwater flows by gravity through a shallow soft ground tunnel to either the Storz West detention basin or the Pershing detention basin. The proposed stormwater tunnel will help to reduce the size of the retention treatment basin (RTB) at CSO 105, relieve sewer backups (SB) and street flooding (SF) from the existing and proposed separation areas shown in Figure 1 and reduce the CSO frequency, volume, and peak flows at the Minne Lusa basin outfalls. Both the Storz West and Pershing detention basins are located northeast of the intersection of Florence Boulevard and Storz Expressway. The Storz West detention basin detains stormwater inflows from the Sorensen Parkway storm sewer. The overflows from the Storz West detention basin pass through a pipe system which routes the flows to the Storz East detention basin. The Storz East detention basin is connected to the Storz East stormwater pump station which conveys flow through a 54-inch diameter force main to the Missouri River. The Pershing detention basin detains stormwater flows from sewer separation project RNC The outfall from the Pershing basin is located on the east side of the basin and directs flows north into the Minne Lusa (ML) CSO 105 outfall channel. The CSO 105 outfall channel is connected to the Missouri River. Unlike the Storz East basin, the conveyance system from the Pershing basin flows by gravity to the Missouri River (via the CSO 105 outfall channel). SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 1

2 The hydrologic and hydraulic evaluation presented herein for the proposed stormwater tunnel considers both the Storz and Pershing detention basin systems to determine the most effective means of conveying stormwater from the proposed tunnel to the Missouri River. For reference, Figure 2 shows the existing combined sewer outfalls and alignment of the CSO 105 outfall channel and Missouri River. Figure 3 shows the existing Storz and Pershing detention basins as well as the connectivity of the downstream stormwater conveyance system to the Missouri River. Figure 3 also shows the location of the downstream end of the proposed stormwater tunnel in relation to the detention basins. There are a number of existing and proposed stormwater inflows to the Pershing and Storz West detention basin systems that must be accounted for when determining the overall hydraulic capacity of the detention basin system and its ability to handle additional inflows from the proposed stormwater tunnel. Currently, the Storz West basin receives stormwater inflows from the Sorensen storm sewer. The Sorensen storm sewer collects stormwater runoff from a portion of the Highway 75 corridor in addition to areas along Sorensen Parkway (see Figure 4). Note that the Sorensen storm sewer has two overflow structures that connect the Sorensen storm sewer to the CSS. One is located near the intersection of Fontenelle Boulevard and Sorensen Parkway and the other located near the intersection of 44 th and Redman Ave. These connections are shown in schematic form on Figure 6. The purpose of the structures is to relieve the Sorensen Storm Sewer by allowing stormwater to pass from the storm sewer to the combined sewer system (CSS). The structures were intended to prevent combined sewer flows from passing back into the stormwater system from the CSS. For the Pershing/Storz hydraulic analysis, outflows from the Sorensen storm sewer to the CSS were not considered. By assuming that the overflows don t exist along the Sorensen storm sewer, a conservative assumption was made regarding the volume of stormwater runoff that is conveyed to the Storz West detention basin. The Pershing detention basin receives stormwater inflows from the area separated by sewer separation project RNC The project area is shown by Figure 5. Furthermore, sewer separation project OPW (i.e., 24 th and Ogden/Himebaugh, currently under design) is also planned to deliver separated stormwater to the Pershing basin. That project is shown by Figure 5 and is labeled SA Lastly, a future connection between the Miller Park detention basin and the Pershing detention basin is planned (see the Minne Lusa Combined Sewer Backup and Major Street Flooding Recommended Approach Technical Memorandum dated May 15, 2008 for more information) to redirect stormwater overflows that currently pass from the Miller Park detention basin back into the ML combined sewer system (CSS). That project is shown by Figure 5 and is labeled SA In addition to existing and proposed future inflows to the Storz and Pershing detention basins, downstream controls must also be accounted for in determining the detention basin system s hydraulic capacity. For example, outflow from the Storz West detention basin passes through approximately 5,000 linear feet of reinforced concrete pipe ranging in sizes between parallel twin 48-inch RCPs to a 84-inch RCP before reaching the Storz East detention basin (see Figure 3 for pipe locations and pipe sizes). This pipe reach inevitably has capacity limitations and may surcharge and flood during large rain events SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 2

3 thus limiting the peak outflow rate from the Storz West detention basin. Furthermore, in addition to the inflows from the Storz West detention basin, the Storz East detention basin collects stormwater runoff from local areas east of Florence Boulevard and north of Storz Expressway. Since the Storz East basin is dewatered by the Storz East stormwater pump station, the local inflows, in addition to the overflows from the Storz West detention basin must all be pumped to the Missouri River through the Storz East stormwater pump station and force main. In other words, the capacity of the pump station controls the rate at which the Storz West detention basin can be dewatered during and following rain events. The Pershing basin outflow is affected by the outfall pipe size (see Figure 3 for pipe locations and pipe sizes) and backwater in the CSO 105 outfall channel, which is directly affected by the water surface elevation in the Missouri River. In all, the backwater condition in the Missouri River, the Storz East pump station capacity, each basin s outfall pipe capacities, the local inflows, and each detention basin s storage volume affect the hydraulic capacity of the overall Pershing and Storz detention basin system. Therefore, all of these items were included in the proposed stormwater tunnel hydraulic evaluation discussed herein. A dynamic XP-SWMM computer model was developed to size the proposed stormwater tunnel and evaluate the hydraulic capacity of the downstream Pershing and Storz detention basin system. The model was used to determine sizing of the proposed stormwater tunnel and potential downstream improvements needed for the Pershing and Storz detention basin system. If the existing Pershing and Storz system is found to have inadequate capacity to handle additional inflows from the proposed stormwater tunnel, concepts will be investigated to increase the hydraulic capacity of the detention basin system to mitigate potential flooding and adverse impacts to the downstream conveyance system. Background As part of the October 2007 Substantively Complete Long Term Control Plan (SCLTCP), a 16.5 foot diameter stormwater tunnel was proposed for the ML Basin. The purpose of the proposed stormwater tunnel is to reduce the size of the proposed retention treatment basin (RTB) at CSO 105, reduce sewer backups (SB) and street flooding (SF) in the basin by conveying stormwater from areas previously, and proposed, to be sewer separated. Furthermore, by collecting separated stormwater from the ML combined sewer system and redirecting it to a dedicated stormwater outfall, the peak flow, volume, and frequency of combined sewer overflows (CSOs) from the ML basin will also be reduced. For the SCLTCP, the stormwater tunnel was sized based on a static flow rate of 3.9 cubic feet per second per acre (cfs/acre) of tributary area. This flow rate represented the approximate 10-year storm water runoff rate (i.e., design flow). As part of the recommendations in the SCLTC and Minne Lusa Implementation Plan TM dated September 27, 2007, the stormwater tunnel was proposed to daylight in the Storz West basin by retrofitting the existing Sorensen storm sewer outfall located in the southwest corner of the Storz West Basin to accommodate the stormwater tunnel. Note that as part of the refinement efforts discussed herein and within the Stormwater Tunneling Methods SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 3

4 TM, the proposed stormwater tunnel is no longer planned to be coupled with the existing Sorensen storm sewer outfall (i.e., no retrofit to the existing Sorensen storm sewer outfall is planned). For clarity, this assumption is also stated within the Recommendations and Cost Comparison Section of this TM. To provide additional background on the proposed stormwater tunnel, a portion of the Minne Lusa Implementation Plan TM dated September 27, 2007 as been reproduced below: This project will provide a large stormwater conveyance tunnel from the Storz/Pershing detention basins upstream to a strategic location within Minne Lusa where flows from three large upland subbasins (Sorenson, Fontenelle and Adams Park) converge in the same general vicinity. The preliminary tunnel diameter has been calculated at 16.5 feet, which will provide stormwater capacity into the upper reaches of the basin. This will allow peak flow relief to the large combined sewers in the lower portions of the Minne Lusa basin. The tunnel was sized to accommodate future public separation areas in these subbasins. Reconstruction of the outfall structure in the Storz detention facility and a new open ditch connecting to the Pershing detention facility will be required. The tunnel alignment extends from the Storz detention basin and rugby complex along the right-of-way of the Storz Expressway/30 th Street/Sorenson Parkway interchange westward to the Metro Community College (MCC) campus then southward to near the intersection of Paxton and John Creighton Boulevards (JCB). In this relatively small area of Sorenson Parkway to the Paxton/JCB intersection, the flows of the Sorenson, Fontenelle and Adams Park subbasins converge. These are the three largest stormwater flow-producing watersheds in the Minne Lusa basin. The primary access shaft for tunneling purposes, and where the greatest flows will be dropped into the tunnel, is located near the Paxton/JCB intersection. Flows from the Sorenson subbasin would be introduced via a drop shaft at the southern end of the MCC campus. The outlet at the Storz detention basin will include an open ditch to convey flow through to the newlyconstructed Pershing detention basin and continuing to the Minne Lusa outfall channel. The current stormwater tunnel concept is a rock tunnel assumed to be 170 feet from grade to the tunnel invert. The tunnel would operate as an inverted siphon with an open outlet at the Storz detention basin. A dewatering pump station will be required to dewater the tunnel after storm events. In addition, the existing outfall structure for the separate 108-inch Sorenson/North Freeway storm sewer will be reconstructed to accommodate the new tunnel outlet. The viability and cost-effectiveness of using a shallow soft ground tunnel in lieu of a deep rock tunnel should be evaluated during the refinement stage. As noted in the text from the Minne Lusa Implementation Plan TM dated September 27, 2007, Black & Veatch was preliminarily tasked to refine the viability and costeffectiveness of the proposed stormwater tunnel. However, further discussions held after the completion of the Minne Lusa Implementation Plan TM, determined the exact refinements to be considered. The specific refinements included: updating the size of the stormwater tunnel using a dynamic hydraulic model simulation of the 10-year storm (rather than the static 3.9 cfs per acre used for the SCLTCP), dynamic modeling of inbasin storage (i.e., Fontenelle Park Pond, Lake James, Adams Park Wetland and Adams Park Lagoon), refinement of the upstream sewer separation areas and modification of SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 4

5 the proposed tunnel configuration from a deep hard rock inverted siphon tunnel to a gravity soft ground tunnel. A gravity tunnel was viewed to be a more preferable alternative to a deep rock tunnel due to the following reasons: 1) Deep Tunnel Pump Station could be eliminated. 2) Cleaning and maintenance of a gravity tunnel would be easier 3) Grit could be transported by a gravity tunnel versus settling out in a deep tunnel. A deep tunnel would most likely require a clamshell or some other technology to periodically remove grit and debris. 4) A shallow gravity tunnel could be more easily accessed allowing future stormwater connections to be more easily incorporated. The areas proposed to be separated and deliver stormwater to the proposed stormwater tunnel were refined and presented in the Minne Lusa Combined Sewer Backup and Major Street Flooding Recommended Approach Technical Memorandum dated May 15, These areas have been reproduced herein and are shown in Figure 5 as: SA-105-1, SA-105-2a, SA-105-2b, SA-105-3, SA-105-4, SA-105-5, SA and SA The total tributary area proposed to deliver stormwater to the proposed stormwater tunnel from the areas previously and to-be separated is approximately 1,350 acres in size, or about 21% of the entire ML basin area (see Figure 7 for both existing and to-be separated areas that will deliver stormwater to the proposed tunnel). This assumption was confirmed at the June 16, 2008 Task Kickoff Meeting summarized in Attachment 2. It should be noted that there are more separation areas shown by Figure 5 than are relevant to this TM (Figure 5 was extracted from the Minne Lusa Combined Sewer Backup and Major Street Flooding Recommended Approach Technical Memorandum dated May 15, 2008). To be clear, the areas shown in yellow (Category 4 and 5 areas) by Figure 5 are planned to be separated via the CSO program as these projects will reduce the volume, frequency, and peak flow of CSOs. The areas shown in green and blue (Category 2 and 3 respectively) by Figure 5 are separation areas that will help to alleviate sewer backups and street flooding but will not benefit the CSO program. Therefore the Category 2 and 3 projects will not be implemented by the CSO program. In addition to the sizing and evaluation of the proposed stormwater tunnel, Black & Veatch was also tasked to evaluate the capacity of the Storz and Pershing detention basin system to handle the increased inflows as a result of the proposed stormwater tunnel. Although the Storz West and Pershing detention basins are not currently hydraulically connected, additional inflow from the proposed stormwater tunnel may dictate the need to make modifications to the Pershing and Storz detention basin system. Since the Pershing and Storz West detention basins are directly adjacent to each other, the overall system hydraulic capacity was evaluated to help determine appropriate system modifications to add hydraulic capacity if needed. SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 5

6 Proposed Stormwater Tunnel and Pershing and Storz Hydraulic Connectivity One of the challenges in determining the hydraulic capacity of the Pershing and Storz Detention basin system is accounting for the number and connectivity of inflows, outflows, and downstream conditions/constraints that affect the overall hydraulic capacity of the system. These relationships were touched on briefly in the Overview and Objectives Section but should be detailed further. Figure 6 presents the existing hydraulic connectivity of the Pershing and Storz Detention basin system in schematic form. In addition, the figure shows the location of the proposed inflows from the proposed stormwater tunnel and Miller Park detention basin. Note that these two inflows do not currently exist but may in the future. Furthermore, the connection of the proposed stormwater tunnel and Miller Park overflow may also need to be adjusted from what is shown in this figure so that the detention basin system has enough hydraulic capacity to accommodate the increased inflows. If a modification to the proposed location of the stormwater tunnel outfall, as shown in Figure 6 is required, the recommendation will be clearly noted later within this document. Figure 6 presents the initial hydraulic model setup (i.e., starting point, Alternative 1) used to evaluate the capacity of the detention basin system. Table 1 provides a written description of each detention basin and pump station presented in Figure 6 along with the sources of inflows, outflows, and potential backwater constraints at each structure. SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 6

7 Table 1. Pershing and Storz Detention Basin Connectivity Structure/Detention Basin Direct Inflows Direct Outflows Possible Downstream Hydraulic Controls and Structures Comments Storz West Detention Basin Existing Sorensen Parkway Storm Sewer Proposed Stormwater Tunnel (as proposed in Alternative 1) Two 48 inch diameter RCPs, increasing to 66-, 72-, and 84-inch pipes to Storz East Basin (gravity flow, See Figure 3) Storz East Basin 66-, 72-, and 84-inch diameter outflow pipes The Storz West Detention basin delivers flow, via a gravity pipe system, to the Storz East detention basin. The water surface elevation in the Storz East Basin and connecting pipe capacity can cause backwater and may affect the outflow rates from the Storz West Basin. (see Figure 3 and Figure 6 and for schematic representations) Storz East Detention Basin Existing Storz West Outflows Existing Local Drainage Storz East Pump Station (3) cfs pumps (221.5 cfs total) Controlled by Storz East Pump Station The Storz East detention basin collects local runoff (in addition to the inflows from Storz West overflows) and is dewatered by the Storz East Stormwater Pump Station. (see Figure 3 and Figure 6 and for schematic representations) Storz East Stormwater Pump Station Existing Storz East Detention Basin 54-inch diameter force main to the Missouri River Carter Lake North Stormwater Pump Station The Storz East stormwater pump station delivers flow from the Storz East basin to the Missouri River. The outgoing 54-inch diameter force main joins with the 24-inch diameter force main from the Carter Lake North stormwater pump station before reaching the Missouri River. High river levels might affect pump station capacity which may need to be verified during detailed design. 18-inch diameter force main to Carter Lake. This is not normally operated. Missouri River (see Figure 3 and Figure 6 and for schematic representations) Carter Lake North Stormwater Pump Station Existing Carter Lake 24-inch Diameter Force Main to 54-inch diameter Storz East Force Main. There are two pumps, each with a 10 cfs capacity Missouri River (20 cfs total) Storz East Pump Station 54-inch diameter force main The Carter Lake North stormwater pump station helps control the water surface elevation in Carter Lake by pumping water from the lake, via a 24 inch diameter force main, to the Missouri River. The 24-inch diameter force main joins the 54 inch diameter force main from the Storz East pump station before reaching the Missouri River. The Carter Lake North Stormwater pump station is assumed to not be operating during the 10-year, 24-hour event discussed herein. This assumption was made in coordination with City and PMT staff. (see Figure 3 and Figure 6 and for schematic representations) SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 7

8 Structure/Detention Basin Direct Inflows Direct Outflows Possible Downstream Hydraulic Controls and Structures Comments Pershing Basin Existing inflow from Sewer Separation Project RNC Proposed Stormwater Tunnel (as proposed in Alternative 2.) Sewer Separation Project OPW is under design and is proposed to deliver separated stormwater to the Pershing basin through the RNC 5946 outfall Sewer Separation Project OPW is in design. Modifications to the original plan, detailed in the Minne Lusa Combined Sewer Backup and Major Street Flooding Recommended Approach Technical Memorandum dated May 15, 2008, proposes a new connection between Miller Park and the Pershing Basin (instead of a connection from Miller Park back to the ML CSS.) This connection is shown in Figure 5 as SA foot wide by 4 foot tall Reinforced Concrete Box to a pipe system (See Figure 3 for pipe sizes) The water surface elevation (i.e., Hydraulic Gradeline) in CSO 105 Channel from 10-year runoff event. This is affected by the peak flow in the channel and the Missouri River backwater elevation. Inflows from Sewer Separation Project OPW into the outflow pipe from the Pershing Detention basin affect the overflow rate from the Pershing Basin. The Pershing Basin collects stormwater runoff from sewer separation project RNC In addition, sewer separation project OPW is under design and is planned to deliver additional stormwater to the Pershing basin through the outlet pipe already constructed during sewer separation project RNC As part of the SB and SF efforts, additional inflows are proposed from the Miller Park Detention basin to the Pershing Basin. The Pershing Basin is dewatered via a 8 foot wide by 4 foot tall concrete box outlet structure. The overflows pass to the north and are delivered to the CSO 105 relief channel. Before reaching the CSO 105 channel, the overflows from the Pershing basin mix with separated stormwater collected from sewer separation project OPW Therefore, the runoff from OPW may reduce the overflow capacity from the Pershing basin. (see Figure 3 and Figure 6 and for schematic representations) SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 8

9 Hydrologic and Hydraulic Assumptions and Evaluation The Omaha Regional Stormwater Design Manual dated April 2006 was used for guidance for the hydrologic and hydraulic evaluation of the proposed stormwater tunnel and detention basin system. Since the hydraulic evaluation of the proposed stormwater tunnel is based upon a 24-hour, 10-year flood event (rather than a water quality wet weather event, like the InfoWorks Combined Sewer (IWCS) model), B&V referred to the Omaha Regional Stormwater Design Manual as a guide for developing the hydrologic parameters. The inflow hydrographs from the stormwater separation areas (as shown by Figure 7) into the proposed tunnel were determined using the US Army Corps of Engineers HEC- HMS software application. Note that the areas shown by Figure 7 were derived by aggregating the hydrologic subareas contained in the Infoworks Combined Sewer (IWCS) model. To fix sewer backups and street flooding in the southwest portions of the Minne Lusa Basin (see the Minne Lusa Combined Sewer Backup and Major Street Flooding Recommended Approach Technical Memorandum dated May 15, 2008 for more detailed information) it was determined that selected sewer system laterals and associate lateral watersheds should be separated. Therefore, appropriate hydrologic subareas were selected from the IWCS model to determine the sewer lateral watershed extent to be included within each sewer separation study area. Note that the IWCS model was reviewed to help with the selection of the hydrologic variables used in the Stormwater Tunnel Analysis. However, since the IWCS model has been calibrated for use in smaller wet weather events, it was determined that this model would not be used for the 10-year simulation. Although not performed for this task, comparisons between the IWCS model and the stormwater tunnel hydraulic model results could be made in the future at the discretion of the City and PMT. The Soil Conservation Service (SCS) methodology was used within the HEC-HMS hydrologic model with composite curve numbers and an SCS Type 2 storm hyetograph with a 10-year recurrence interval, distributing a total rainfall of 4.6 inches over a 24- hour period for the Omaha area. Times of concentration for each of the subareas were determined using GIS, aerial photographs, elevation contours, and assumptions for the size and location of the future stormwater collection system that will be required to collect stormwater and convey it to the tunnel. Table 2-8 of Omaha s Stormwater Manual (provided in Attachment 4) along with aerial photos were reviewed to select appropriate the Soil Conservation Service (SCS) curve numbers and land use types to calculate composite curve numbers for the proposed stormwater tunnel drainage area. Composite curve numbers were determined by weighting the percent of land use (land use was determined by reviewing City s aerial photography) within each subarea, using a residential curve number of 75, park/open space curve number of 69, and commercial curve number of 92. Table 2 shows the inputs and outputs from the HEC-HMS model for the Stormwater Tunnel tributary area. Figure 7 shows a schematic of the HEC-HMS subareas (with labels) which represent the Stormwater Tunnel tributary area. Note that the hydraulic SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 9

10 routing of the hydrographs determined in HEC-HMS was performed by the XP-SWMM hydraulic model (discussed later within this document). Table 2 presents the input variables and the model output from the HEC-HMS hydrologic model. A more detailed table presenting model results is included within Attachment 4. Table 2 only presents the model results from the subareas upstream from the proposed stormwater tunnel while Attachment 4 presents all of the hydrologic results. Note that the peak flows simulated by the HEC-HMS model were verified to within 10 percent of hand calculated peak flows using the SCS TR-55 methodology. Subarea (see Figure 7) Table 2. Stormwater Tunnel Subarea HEC-HMS Input and Output Area (acres) Composite Curve Number Time of Concentration (min) Lag Time (min) 10yr Peak Flow (cfs) a b c d e RNC a. Subareas 5a through 5e include the area encompassed by sewer separation projects RNCL 5970B, RNC 5097, and RNC 5790A. Hydrographs were also generated using the HEC-HMS model for the Sorensen stormwater collection area draining into the Storz West basin (see Figure 4), the local drainage into the Storz East basin, the area draining into the Miller Park basin (OPW 51205), and CSO 105 outfall from the ML trunk sewer. Hydrographs developed for the separation areas RNC 5946, OPW 51497, and OPW were calculated in Excel using the TR-55 method and directly input to the XP-SWMM hydraulic model. The hydrologic results for all of these areas is presented in Attachment 4. SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 10

11 The XP-SWMM model was used for dynamic modeling of the proposed stormwater tunnel system, using the hydrographs from HEC-HMS as input. Ground elevations were reviewed to set the invert elevation of the downstream tunnel outfall and to define the maximum surcharge elevation at the upstream end of the tunnel. The model was configured with the proposed conveyance sewers by sizing them to handle the incoming flow (without flooding) and extending them up into the stormwater separation areas. The conveyance sewers collected and routed the runoff hydrographs generated in HEC- HMS to the stormwater tunnel. It was assumed that the tunnel system would convey the flows from the 10-year storm below ground (i.e., no flooding for the 10-year storm). In addition, the tunnel was allowed to pressurize so that the diameter could be reduced. In addition, the XP-SWMM model was configured to account for the flow attenuation effects to the proposed stormwater tunnel from in-basin wet weather storage facilities. The stage-surface area curves were obtained from GIS and from the City for the following eight wet weather detention facilities and input to the XP-SWMM hydraulic model: 1) Storz West detention basin (dry bottom facility) 2) Storz East detention basin (dry bottom facility) 3) Pershing detention basin (dry bottom facility) 4) Miller Park pond (wet bottom facility, permanent pool is set at elevation 1,015 ft) 5) Fontenelle Pond (wet bottom facility, permanent pool is set at elevation 1,115 ft) 6) Lake James (dry bottom facility) 7) Adams Park Wetland (wet bottom facility, permanent pool is set at elevation 1,086.5 ft) 8) Adams Park Lagoon (wet bottom facility, permanent pool is set at elevation 1,084 ft) See Figure 8 through Figure 15, respectively, for the existing condition stage-surface areas curves of each detention basin listed above. The stage-surface area curves were input to the XP-SWMM model to simulate the flow attenuation through each storage basin. Note that the Miller Park Pond improvements that are currently under construction by project OPW were included in the pond s stage-storage curve (i.e., the storage improvements were considered to be completed for the routing analysis discussed herein). The XP-SWMM simulations also assume that each facility is filled to its normal pool elevation (if applicable). This is a typical initial condition modeling assumption made to simulate the flow attenuation from wet weather control facilities during flood events. In addition to modeling the storage facilities, the existing stormwater pipe systems downstream from the Storz, Pershing, Lake James, and Fontenelle Park detention basins were input to the XP-SWMM model to account for the effects of downstream conditions on the hydraulics of the basins. Available as-built plan and profile sheets along with information obtained from the IWCS model (where applicable) were used to populate the sizes and inverts of the existing stormwater conveyance system. Note that the SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 11

12 hydrologic and hydraulic components presented in Figure 3 and Figure 6 and discussed in Attachment 4 (See Table 4.1), with the exception of the 44 th and Redman and Sorensen Parkway Stormwater Overflows (see Figure 6) are accounted for in the XP-SWMM hydraulic model. Available information from the City shows that the pump station at the Storz East Detention basin has three pumps with a total capacity of approximately 221 cfs. Based upon discussions with City and Program Management Team (PMT) staff, it was assumed for the purposes of the evaluation discussed herein that the pump station will be able to continuously pump 221 cfs through the 54-inch force main during the 2-year Missouri River backwater condition. It was also assumed that the 24-inch diameter connection from Carter Lake to the 54-inch force main will be closed during the 10-year storm event, so that the only flow in the 54-inch force main is from the Storz East pump station (i.e., the Carter Lake North Stormwater pump station will not take conveyance capacity away from the Storz East Stormwater Pump Station). This assumption was reviewed and agreed to by City and PMT staff. Also based upon discussions with City staff, it was assumed for purposes of this study that the Missouri River will have a 2-year backwater flood stage when the 10-year storm falls on the Minne Lusa basin. A Weibull distribution was used to determine the Missouri River 2-year backwater flood stage elevation (see Figure 16 for flood stage frequency plot). Using historical stage data obtained from USGS Gage and a slope profile of the Missouri River bed from available FEMA FIS mapping, it is estimated that the 2-year backwater elevation of the Missouri River is approximately 978 feet where the Minne Lusa 105 outfall channel empties into the Missouri River (see Figure 3). It is also estimated that the 2-year backwater elevation of the Missouri River is approximately 977 feet where the Storz East pump station discharges to the Missouri River (see Figure 3). Note that changes in the backwater assumption could significantly impact the findings of this study. In other words, as this project moves forward, it is recommended that the City review the backwater assumption for flood control facilities located within major floodplains. For example, simulations can be performed to determine the performance of the system when the Missouri River experiences a 10- or 100-year flood event. A uniform design criterion could be adopted to ensure that the standard meets the Community s flood protection level of service goals. Results and Discussion The following two alternatives were evaluated for the proposed stormwater tunnel 10- year storm hydraulic evaluation: Alternative 1. Convey stormwater from the stormwater tunnel to the Storz West basin and discharge it to the Missouri River via the Storz East basin stormwater pump station (shown in concept by Figure 6). Alternative 2. Convey stormwater from the proposed tunnel, via a new open channel, by bypassing the Storz West basin and directing the tunnel outflows SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 12

13 Alternative 1 into the Pershing basin. From the Pershing detention basin, flows would pass to the Missouri River, via gravity flow (shown in concept by Figure 21). The inflows developed in HEC-HMS and TR-55 were input to the XP-SWMM hydraulic model. A summary of all of the hydrologic inputs to the XP-SWMM model is detailed by Table 4.1 in Attachment 4. The XP-SWMM model configuration for Alternative 1 is presented by Figure 17 and Figure 18. Figure 17 presents the overall XP-SWMM model connectivity for Alternative 1 while Figure 18 presents a detailed schematic of the upstream connectivity of the model. Note that there is no difference between Alternative 1 and Alternative 2 for the upstream connectivity so Figure 18 represents the upstream model connectivity for both Alternative 1 and Alternative 2. For Alternative 1, the dynamic XP-SWMM model simulated that the proposed stormwater tunnel must be 12.5 feet in diameter to convey flow for the 10-year storm event. During the peak of the 10-year event, the 12.5-foot diameter tunnel surcharges, but water levels do not rise above ground. Table 3 presents the conditions of the stormwater tunnel during the peak 10-year storm conditions. Figure 19 provides an illustration of the peak 10-year hydraulic gradeline profile in the proposed stormwater tunnel. Table 3. Stormwater Tunnel Hydraulic Peak Flow Conditions Upstream Tunnel Invert feet Upstream Tunnel Crown Upstream Maximum Water Surface Elevation Upstream Ground Elevation feet feet 1052 feet The 7,900-foot long tunnel was simulated with a slope of 0.2 percent, with the upstream invert at an elevation of feet (about 51 feet below ground surface). The depth of the tunnel below the existing City infrastructure (i.e., existing combined sewers) will help ensure that the existing infrastructure is not disturbed during the construction of the tunnel. The downstream invert of the tunnel is governed by the existing grade of the Storz West detention basin, at an elevation of approximately 985 feet. Based upon discussions with City staff as well as review of the geologic information along the tunnel alignment, a gravity soft ground tunnel having these hydraulic characteristics appears reasonable. Furthermore, it may be necessary for a drop shaft to be located near the intersection of Sorensen Parkway and 31 st street so that the tunnel can make the tight radius needed to transition it from northbound to eastbound. Furthermore, with the incorporation of the drop shaft, it would be possible that additional stormwater flows could be connected to the stormwater tunnel at a later date, an operational flexibility requested by City staff. Note that the stormwater tunnel may SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 13

14 need to be upsized if additional inflows are significant. For the purposes of the study described herein, additional storm water inflows, besides those already associated with the separation areas presented in Figure 7, were not considered. For additional discussion about the tunnel construction and tunneling method, a Stormwater Tunneling Methods Technical Memorandum will be completed and submitted at a later date. Upstream from the proposed tunnel, the future stormwater collection pipes will need to approach the size of the tunnel itself, reaching a diameter of approximately 12 feet before dropping into the soft ground stormwater tunnel near the intersection of 31 st and Sprague. Furthermore, the constructability of the large diameter stormwater collection pipe feeding inflows from the west to the stormwater tunnel (proposed along Paxton Boulevard) could be investigated a later date. In the Alternative 1 simulation, the hydraulic model simulated that the maximum water surface in the Storz West basin will exceed the top of the berm surrounding the basin (elevation 994 feet), with some flow spilling to the north, over the existing Storz West embankment, into Pershing basin, and some flow continuing to the east through the pipe system into the Storz East basin and then to the river via the Storz East stormwater pump station. The hydraulic model also simulated that the pipe system between the Storz West and Storz East basins is undersized, allowing water to surcharge and flood the system during the 10-year event. To eliminate flooding along this pipe segment, a new parallel pipe would be required between the basins, and the Storz East pump station would need to be upgraded to increase the pump capacity from 221 cfs to 400 cfs. Alternative 1 also included an evaluation of the proposed future connection of the Miller Park basin to the Pershing basin. Note that it was assumed that inflow from separation project OPW was routed through the expanded Miller Park Pond and to the Pershing Basin through the proposed connection. Even with this potential connection from Miller Park, and with water spilling out of Storz West basin into Pershing, the model indicated that there may be a possibility of directing more stormwater from the tunnel into the Pershing system and allowing it to drain to the river by gravity through the CSO 105 outfall as opposed to conveying to the river through the Storz East pump station (i.e., the model indicated additional capacity in the Pershing basin). Therefore, Alternative 2 was developed and investigated to maximize the use of the Pershing detention basin. Table 4 presents the hydraulic results from all the simulated wet weather storage facilities for Alternative 1. SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 14

15 Storage Facility Table 4. Alternative 1 10-yr Storage Facility Hydraulic Results Peak 10-yr Inflow (CFS) Peak 10-yr Outflow (CFS) Lake James Fontenelle Pond 323 Starting Water Surface Elevation (ft) Peak 10-yr Water Surface Elevation (ft) (bottom of basin assumed dry at beginning of storm) 1,169.2 (two inflow pipes) 72 1,115 1,117.1 Adams Park Wetland ,086.5 Adams Park Lagoon ,084 1,084 Miller Park ,015 1,018.9 Storz West 2 2,790 Storz East Pershing ~460 ~192 (downstream flooding) (upstream flooding) ,582 (182 cfs is from the Miller Park inflow and inflow from the RNCL 5946 and OPW separation areas. 1,400 cfs is from spillover out of Storz West) 187 (bottom of basin assumed dry at beginning of storm) (bottom of basin assumed dry at beginning of storm) (bottom of basin assumed dry at beginning of storm) ~994 (embankment overtopped) 971 Upstream Flooded ~991 (Facility Flooded) 1. By assuming the upstream area (Subarea 8 in Figure 7) is separated and a new stormwater connection links Adams Park to the proposed stormwater tunnel, backwater on the existing stormwater pipe system was reduced and eliminated inflows to Adams Park. The Hybrid Analysis (to be documented by the Minne Lusa Basin Search for Hybrid Alternatives TM) will investigate methods to better utilize the Adams Park facilities. 2. Alternative assumes that the Proposed Stormwater Tunnel Is Connected. SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 15

16 Alternative 2 Alternative 2 differs from Alternative 1 as it proposes directing flows from the proposed stormwater tunnel directly to Pershing detention basin via a trapezoidal open channel. The trapezoidal channel is proposed to connect the stormwater tunnel outfall (proposed to daylight near the southwest corner of the Storz West detention basin) to the Pershing detention basin. The open channel would be aligned, from south to north, along the western portion of the Storz West detention basin. Furthermore, a berm to form the open channel is proposed within the existing Storz West detention basin. The proposed berm will form the eastern embankment of the open channel while the western embankment of the open channel will be the existing hillside. A notch will need to be cut through the existing embankment separating the Storz West and Pershing basins to allow the flow from the open channel to enter Pershing basin. Note that the two detention basins will still remain hydraulically independent by this proposal. By directing the proposed tunnel outflows to the Pershing basin, stormwater will be conveyed to the Missouri River via gravity rather than being pumped as would have been required by Alternative 1. Please see Figure 20 for a 3D rendering of this open channel and embankment. The XP-SWMM model configuration for Alternative 2 is presented by Figure 21 and Figure 18. Figure 21 presents the overall XP-SWMM model connectivity for Alternative 2 while Figure 18 presents a detailed schematic of the upstream connectivity of the model. Note that there is no difference between Alternative 1 and Alternative 2 for the upstream connectivity so Figure 18 represents the upstream model connectivity for both Alternative 1 and Alternative 2. Since the connectivity and drainage area upstream from the proposed stormwater tunnel are the same for Alternative 1 and Alternative 2, the size and configuration of the proposed stormwater tunnel is identical for Alternative 2 as it was for Alternative 1. A 12.5 ft diameter stormwater tunnel will be required for Alternative 2. The 7,900-foot long tunnel was simulated with a slope of 0.2 percent, with the upstream invert at an elevation of feet (about 51 feet below ground surface). The depth of the tunnel below the existing City infrastructure (i.e., existing combined sewers) will help ensure that the existing infrastructure is not disturbed during the construction of the tunnel. The downstream invert of the tunnel is governed by the existing grade of the Storz West detention basin, at an elevation of approximately 985 feet. Upon review of the initial hydraulic results, Alternative 2 requires that the Pershing basin embankment be raised from its current spillcrest of 988 feet to an elevation of 995 feet. If the Pershing embankment is not raised, flows would overtop the existing embankment and flood adjacent low lying areas. By raising the berm elevation, the top of the modified Pershing Basin embankment would be at approximately the same elevation of the Storz West basin embankment (i.e., elevation 995 feet). Furthermore, by raising the embankment, the model simulated that the Pershing Embankment still had approximately 3 feet of freeboard during the 10-year storm event. By raising the embankment, the new storage capacity of the Pershing basin would increase from approximately 42 acre-feet to approximately 177 acre-feet. SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 16

17 With both the stormwater tunnel and Miller Park detention basin inflows (assumes OPW and SA shown in Figure 5 are constructed) passing to the Pershing detention basin, the Pershing detention basin s peak 10-year water surface elevation was simulated to be feet, or about 3 feet below the spillcrest of the modified embankment crest of 995 feet. Furthermore, the water in the modified Pershing detention basin does not spill back to the south over the existing Storz West/Pershing embankment into the Storz West detention basin so no overtopping of the existing Storz West embankment exists for this condition. Although the stormwater tunnel was not hydraulically connected to the Storz West detention basin for the Alternative 2 scenario, the model simulated that the existing stormwater pipes connecting Storz West and Storz East basins have inadequate capacity and flood for the 10-year, 24-hour storm event (the existing pipe running parallel to Storz Expressway as shown on Figure 3). However, it should be noted that the model also indicated that under existing conditions, the Storz conveyance pipe system is currently inadequate to convey the flow from the 10-year 24-hour design storm. Since the Storz system is shown to have flooding for existing conditions, the Recommendation and Cost section for Alternative 2 does not include costs and upgrades for the Storz conveyance system since the existing conditions already showed a problem. Note that to completely eliminate flooding along the Storz conveyance pipes, a new pipe between Storz West and Storz East in addition to an upgrade to the Storz East pump station may be required. However, as stated previously, these costs have not been added to Alternative 2. For reference, a schematic of the XP-SWMM hydraulic model for Alternative 2 is shown by Figure 21. Also note that the modified stage versus surface area rating curves for the Storz West and Pershing Detention basins required for Alternative 2 are presented by Figure 22 and Figure 23 respectively. Since the Pershing system relies on gravity drainage to the river, the backwater condition in the CSO 105 outfall channel during the 10-year, 24-hour storm is important. Under existing conditions, the combined sewer peak flow from the ML Trunk Sewer to the CSO 105 outfall is limited by the pipe capacity of this sewer. The full pipe flow capacity (i.e., Manning s capacity) is estimated to be approximately 4,800 cfs for the 18-ft by 12-ft RCB ML 105 inbound pipe to the CSO 105 outfall. However, if the CSS pipe limitations are ignored, the actual peak flow from the drainage area of the CSO 105 outfall (minus the proposed stormwater separation areas shown by Figure 5) would be approximately 8,800 cfs. Future evaluation of the entire ML CSS for larger storm events (such as the 10-year event) could be performed in an attempt to determine where the excess flow escapes from the CSS and if any of this excess sewer flow affects the capacity of the stormwater and detention basin systems. If overland runoff from larger events appears to be significant and affects the volume and peak flow through these components, the detention basins and pipe sizes recommended herein may need to be increased and/or refined during detailed design. SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 17

18 Storage Facility Table 5. Alternative 2 10-yr Storage Facility Hydraulic Results Starting Water Peak 10-yr Peak 10-yr Surface Inflow Outflow Elevation (CFS) (CFS) Lake James Fontenelle Pond 323 (ft) Peak 10-yr Water Surface Elevation (ft) (bottom of basin assumed dry at beginning of storm) 1,169.2 (two inflow pipes) 72 1,115 1,117.1 Adams Park Wetland , ,086.5 Adams Park Lagoon ,084 1,084 Miller Park ,015 1,018.9 Stroz West 769 Storz East Pershing ~150 (downstream flooding) (upstream flooding) 221 2,216 (sum of Proposed Tunnel inflow, Miller Park inflow, inflow from RNCL 5946 and OPW 51497) 197 (bottom of basin assumed dry at beginning of storm) ~992 (downstream flooding) (bottom of basin assumed dry at beginning of storm) ~971 (bottom of basin assumed dry at beginning of storm) By assuming the upstream area (Subarea 8 in Figure 7) is separated and a new stormwater connection links Adams Park to the proposed stormwater tunnel, backwater on the existing stormwater pipe system was reduced and eliminated inflows to Adams Park. The Hybrid Analysis (to be documented by the Minne Lusa Basin Search for Hybrid Alternatives TM) will investigate methods to better utilize the Adams Park facilities. 2. Alternative 2 assumes that the Proposed Stormwater Tunnel Is Connected. SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 18

19 Recommendations and Cost Comparison For either Alternative 1 or Alternative 2, a 12.5-foot diameter stormwater tunnel is needed to convey flow for the 10-year, 24-hour event from the separated subbasins shown in Figure 7. The 7,900-foot long tunnel was configured to have a slope of 0.2 percent with an upstream invert elevation of feet (about 51 feet below ground surface) and a downstream outlet invert elevation of approximately 985 feet (i.e., the floor of the existing Storz West detention basin). The updated cost of the proposed stormwater tunnel will be presented in the upcoming Minne Lusa Stormwater Tunneling Methods Technical Memorandum and has therefore been withheld from the discussion presented herein. Alternative 1 assumes that the proposed stormwater tunnel will daylight in the existing Storz West detention basin and the stormwater flows be conveyed to the Missouri River via an upgraded pipe system and Storz East stormwater pump station. A significant disadvantage of Alternative 1 is that it forces all flow from the proposed stormwater tunnel to pass through the Storz East pumping station, and be pumped, rather than flowing to the Missouri River via gravity flow (as is proposed by Alternative 2). Besides the construction of the stormwater tunnel, the following three additional facilities need to be constructed for Alternative 1 to convey flows for the 10-year storm event without significantly more flooding occurring along Storz Expressway (i.e., along the conveyance pipes connecting the Storz West and Storz East detention basin): 1. A new, 4,800 foot long, 84-inch diameter pipe (parallel to the existing pipe) from the Storz West detention basin to the Storz East detention basin. 2. Upgrade Storz East pump station from a 221 cfs station to a 400 cfs station. 3. Add a new 3,200 foot long, 54 inch force main from the Storz East basin to the Missouri River. Note that the proposed stormwater tunnel is no longer proposed to be coupled with the Sorensen stormwater outfall. Thus a retrofit of this facility is no longer required as was initially stated within the Minne Lusa Implementation Plan TM dated September 27, Also note that the project cost for the proposed stormwater tunnel is presented in the Stormwater Tunneling Methods TM and not herein. This TM presents the costs for the facilities downstream from the proposed storm tunnel for two distinct alternatives, while the Stormwater Tunneling Methods TM presents the costs for the stormwater tunnel and associated appurtenances. The project cost for the Alternative 1 detention basin improvements, excluding the stormwater tunnel cost (which is the same for each alternative), is presented by Table 6. See Attachment 3 for the detailed cost and construction phasing sheets for Alternative 1. SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 19

20 Table 6. Project Cost for Alternative 1 Detention Basin Improvements Project Construction Cost Estimate 1 Project Capital Cost Estimate Alternative 1 $24,832,000 $41,941, Proposed stormwater tunnel cost is not included since it is the same for both Alternative 1 and Alternative 2, will be detailed later in the Minne Lusa Stormwater Tunneling Methods Technical Memorandum Alternative 2 differs from Alternative 1 as it plans to convey flow from the proposed stormwater tunnel to the Pershing detention basin through a new trapezoidal channel constructed through the Storz West basin. The following facilities would need to be constructed for Alternative 2 to convey the flow from the proposed tunnel to the CSO 105 outfall channel and Missouri River for the 10-year, 24-hour storm event: 1) Construct a new 1,800 foot long trapezoidal open channel embankment to connect the proposed stormwater tunnel to the Pershing Detention basin (the western embankment will be formed using the existing hillside). The channel dimensions would have a bottom width of 20 feet, have a total depth of 11 feet, and have embankments with 3:1 side slopes. The open channel construction would require an estimated 33,000 cubic yards of fill at a construction cost of $30 per cubic yard installed. (See Figure 20 for a conceptual rendering of this open channel and embankment). 2) Raise the Pershing detention basin embankment from elevation 988 feet to elevation 995 feet using 3:1 side slope embankment. The embankment modification will require an estimated 35,000 cubic yards of fill to construct. The installed price is estimated to be $30 per cubic yard of fill. 3) Approximately 500 cubic yards of concrete (at $900 per cubic yard installed, construction cost) to build a concrete apron at the stormwater tunnel outfall to prevent scour and erosion. Note that the proposed stormwater tunnel is no longer proposed to be coupled with the Sorensen stormwater outfall. Thus a retrofit of this facility is no longer required as was initially stated within the Minne Lusa Implementation Plan TM dated September 27, Also note that the project cost for the proposed stormwater tunnel is presented in the Stormwater Tunneling Methods TM and not herein. This TM presents the costs for the facilities downstream from the proposed storm tunnel for two distinct alternatives, while the Stormwater Tunneling Methods TM presents the costs for the stormwater tunnel and associated appurtenances. The project cost for the Alternative 2 detention basin improvements, excluding the stormwater tunnel cost which is the same for each alternative, is presented by Table 7. See Attachment 3 for the detailed cost and construction phasing sheets. Since the Storz system is shown to have flooding for existing conditions, Alternative 2 does not include costs for upgrades to the Storz stormwater conveyance system since the existing conditions model already showed a problem along this pipe extent. SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 20

21 Table 7. Project Cost for Alternative 2 Detention Basin Improvements Project Construction Cost Estimate 1 Project Capital Cost Estimate Alternative 2 $2,423,000 $4,047, Stormwater tunnel cost is not included since it is the same for both Alternative 1 and Alternative 2. Note that a preliminary geotechnical engineering study may be required to refine the proposed open channel and Pershing embankments to ensure stability under wet weather conditions. Also, peak outflow velocities during the 10-year, 24-hour storm event from the tunnel reach velocities that could cause scour problems (greater than 15- feet per second). Therefore, a concrete apron is recommended to reduce this risk and is included in the cost of this alternative. Furthermore, other constructability issues such as railroad easements and locations of utilities may affect the final alignment of the proposed Pershing channel embankment which could cause the proposed volume of the Pershing detention basin to change. Also, although the model results for Alternative 2 show that the Pershing and Storz basins would not need to be hydraulically connected (i.e., neither basin overtopped under the 10-year, 24-hour event), an evaluation of the 100-year event (i.e., a basin wide stormwater masterplan) as part of a future study may indicate that these basins need to be connected or an emergency spillway is required to direct flow to the river during larger flood storm events. The configuration and cost of an emergency spillway and hydraulic connection between the Pershing and Storz west basin is not included herein since the 10-year, 24-hour event did not require one. Furthermore, it is recommended that an evaluation of the 10-year and/or 100-year storm event be performed for the entire ML basin to determine if overflow will escape the ML CSS and stormwater system. If flow does escape the CSS or stormwater system, causing overland flow and flooding, the recommendations presented herein could be refined for the Pershing and Storz system to better convey the overflow toward the river and minimize flooding. It could also be used for contingency planning purposes so the City has an idea of where the overflow will go following a very large storm event. Upon review of the costs and benefits of each alternative, it appears that Alternative 2 is the preferred alternative. It is less expensive than Alternative 1, allows flows to pass to the Missouri River via gravity flow (instead of pumping as would be required in Alternative 1), minimizes the construction of new conveyance system components (i.e., less complex than Alternative 1), and maximizes the City s use of land already dedicated to detention of wet weather flows. Furthermore, Alternative 2 will avoid worsening flooding along the undersized Storz system caused by connecting tunnel flow to Storz West basin as proposed by Alternative 1. Summary and Conclusion In summary, the hydraulic analysis of the proposed stormwater tunnel shows that a shallow depth 12.5-foot diameter tunnel with a 0.2% slope can pass the 10-year, 24-hour flow from the proposed separation areas presented by Figure 7. Furthermore, the SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 21

22 hydraulic model simulated that the Pershing detention basin system has the hydraulic capacity to handle the increased inflows from the stormwater tunnel for the 10-year storm event if the embankments are raised (Alternative 2). During detailed design, additional analysis could be performed to refine the hydraulic evaluation of the proposed stormwater tunnel and Pershing/Storz Detention further. Suggested tasks are: 1) Complete a stormwater masterplan for the Minne Lusa basin to determine the overall basin response to a large flood event (i.e., the 10 or 100-year flood event). This would ensure that the final stormwater tunnel and the Pershing and Storz detention basin system accommodates any overland flow and in-basin flooding as effectively as possible. 2) Conduct additional field surveys at selected locations. Specifically, the existing stormwater pipes and sump areas could be surveyed to better detail the topography and overland flow/culvert connectivity in the flat areas near the Storz East detention basin. 3) Review the Storz East pump station information (i.e., obtain pump curves) and assumptions. 4) Review the Missouri River 2-year backwater assumption and determine if the assumption meets the City s flood protection level of service goals. A sensitivity analysis could be performed to determine the impacts of the Missouri River stage on the hydraulics in the Pershing and Storz system. 5) Conduct a geotechnical analysis of the proposed Pershing basin and open channel embankments to ensure that the embankments will maintain stability during long periods of saturated conditions. 6) Review the construction feasibility (i.e., perform a corridor study detailing the potential utility conflicts) of the proposed stormwater conveyance collector sewers to ensure that these large, near surface sewers can be constructed as proposed. SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 22

23 Acronym/Term City CFS CSO CSS EQ FT 3 HGL IWCS LTCP MG MGD ML NA PMT RTB SCLTCP SSS TM Definition City of Omaha Cubic Feet Per Second Combined Sewer Overflow Combined Sewer System Equalization Cubic Feet Hydraulic Gradeline Infoworks Combined Sewer Long Term Control Plan Million Gallons Million Gallons per Day Minne Lusa Not Applicable Program Management Team Retention Treatment Basin Substantively Complete Long Term Control Plan Sanitary Sewer System Technical Memorandum SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 23

24 Attachment 1 Figures Figure 1. Overview of Proposed Stormwater Tunnel and Separation Areas Figure 2. Minne Lusa Basin Outfalls Figure 3. Existing Conveyance System from Proposed Tunnel Outlet Figure 4. Sorensen Storm Sewer Drainage Area Figure 5. Proposed Separation Study Areas Figure 6. Pershing and Storz Detention Basin Connectivity Figure 7. Proposed Stormwater Separation Areas to Deliver Runoff to Proposed Stormwater Tunnel Figure 8. Storz West Detention Basin Stage-Surface Area Curve Figure 9. Storz East Detention Basin Stage-Surface Area Curve Figure 10. Pershing Detention Basin Stage-Surface Area Curve Figure 11. Miller Park Detention Basin Stage-Surface Area Curve Figure 12. Fontenelle Park Pond Stage-Surface Area Curve Figure 13. Lake James Stage-Surface Area Curve Figure 14. Adams Park Wetland Stage-Surface Area Curve Figure 15. Adams Park Lagoon Stage-Surface Area Curve Figure 16. Missouri River Water Surface Elevation Exceedance Probabilities (USGS Station ) Figure 17. Alternative 1 XP-SWMM Model Configuration Figure 18. Upstream XP-SWMM Model Connectivity Schematic Figure 19. Tunnel Hydraulic Profile During Peak of 10-Year Event Figure 20. 3D Rendering of Proposed Open Channel and Embankment for Alternative 2 Figure 21. Alternative 2 XP-SWMM Hydraulic Model Schematic Figure 22. Storz West Detention Basin Stage versus Surface-Area Curve for Alternative 2 Figure 23. Pershing Detention Basin Stage versus Surface-Area Curve for Alternative 2 SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 24

25 Figure 1. Overview of Proposed Stormwater Tunnel and Separation Areas SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program Page 25

26 Figure 2. Minne Lusa Basin Outfalls SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

27 Local Inflow 8 Existing Sorensen Storm Sewer Figure 3. Existing Conveyance System from Proposed Tunnel Outlet SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

28 Figure 4. Sorensen Storm Sewer Drainage Area SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

29 Figure 5. Proposed Separation Study Areas SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

30 Minne Lusa Outfall Channel Figure 6. Pershing and Storz Detention Basin Connectivity (Note: Location of Proposed Inflows Have Yet to Be Finalized) SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

31 Figure 7. Proposed Stormwater Separation Areas to Deliver Runoff to Proposed Stormwater Tunnel (HEC-HMS Model Subareas 1 through 8 are identified) SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

32 Storz West Existing Surface Area Volume Surface Area (acres) Volume (acre-ft) Elevation (feet) Figure 8. Storz West Detention Basin Stage-Surface Area Curve SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

33 Storz East Existing Surface Area Volume Surface Area (acres) Volume (acre-ft) elevation (feet) Figure 9. Storz East Detention Basin Stage-Surface Area Curve SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

34 Pershing Existing Surface Area Volume Surface Area (acres) Volume (acre-ft) elevation (feet) Figure 10. Pershing Detention Basin Stage-Surface Area Curve SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

35 Miller Park Existing Surface Area Normal Pool Elevation Volume Surface Area (Acres) Volume (Acre-ft) elevation (feet) Figure 11. Miller Park Detention Basin Stage-Surface Area Curve SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

36 Fontenelle Pond Existing Area Normal Pool Elevation Volume Surace Area (acres) Volume (Acre-Feet) Elevation (feet) Figure 12. Fontenelle Park Pond Stage-Surface Area Curve SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

37 Lake James Existing area volume Surface Area (acres) Volume (Acre-Feet) Elevation (feet) Figure 13. Lake James Stage-Surface Area Curve SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

38 Adams Park Wetland Existing Surface Area Normal Pool Elevation Volume Surface Area (acres) Volume (Acre-Feet) Elevation (feet) Figure 14. Adams Park Wetland Stage-Surface Area Curve SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

39 Adams Park Lagoon Existing Surface Area Normal Pool Elevation Volume Surface Area (acres) Volume (Acre-Feet) 0.0 Elevation (feet) Figure 15. Adams Park Lagoon Stage-Surface Area Curve SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

40 Return Frequency (Years) Missouri River Stage (NAVD 1929) % 10% 100% Exceedance Probability ( Gage Datum = feet above sea level NGVD29) Figure 16. Missouri River Water Surface Elevation Exceedance Probabilities (USGS Station ) SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

41 CSO_Inflow OPW_51306 MillerPark Subarea 6 Storz West localdrainage Subarea 3 Subarea 2 Pershing Storz East Subarea 1 Subarea 7 Subarea 4 Fontenelle Pond Adams Park Wetland and Lagoon B B St_James Subarea_8a Subarea_8b Figure 17. Alternative 1 XP-SWMM Model Configuration (Red Items Are XP-SWMM Pour Points, Blue Items Are Wet Weather Storage Facilities) SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

42 Figure 18. Upstream XP-SWMM Model Connectivity Schematic (Same for Alternative 1 and Alternative 2) SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

43 Ground Surface Peak 10-yr Hydraulic Gradeline (purple line) Hydraulic Gradeline at Time of Screen Capture Figure 19. Tunnel Hydraulic Profile During Peak of 10-Year Event SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program

44 Proposed Embankment Existing Storz West detention basin Proposed Stormwater Tunnel Outfall Figure 20. 3D Rendering of Proposed Open Channel and Embankment for Alternative 2 SWTunnel_Pershing_Storz_Hydraulic_Eval_V5.doc Black & Veatch File Code: ML Omaha CSO CONTROL Program