Refinement Task 19 Minne Lusa Basin Search for Hybrid Alternatives

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1 Refinement Task 19 Minne Lusa Basin Search for Hybrid Alternatives TO: Roger Coffey, PMT Document Control Center, CH2M Hill COPY: Jim Theiler, City of Omaha Scott Aurit, PMT Tom Heinemann, PMT FROM: Jim Schlaman, Black & Veatch DATE: February 10, 2009 FILE CODE: Refinement Task 19 Purpose The purpose of this refinement task is to search for ways to enhance the benefits provided by the proposed Program alternative facilities identified in the Substantively Complete Long Term Control Plan (SCLTCP), while maintaining or increasing the overall costeffectiveness of the controls, and by reducing Combined Sewer Overflow (CSO) rates and/or volumes. TM Review and Hybrid Identification The Program Management Team (PMT) Refinement Task Team reviewed the Minne Lusa Technology Screening TM dated January 18, 2007, the Minne Lusa Alternatives Evaluation TM dated October 9, 2007, and the Minne Lusa Sewer Backups & Street Flooding Recommended Approach TM dated May 15, 2008 to determine if any additional control technologies, not already incorporated in the Alternatives Evaluation TM or as part of the Program Alternative facilities, should be evaluated as part of this task. Consideration was also given to technologies that were previously eliminated by the Minne Lusa (ML) Basin team, but that might be applicable to the Program Alternative facilities. The PMT selected Adams Park in the Minne Lusa Basin as the hybrid analysis for the Black & Veatch (B&V) to evaluate. Specifically, B&V was tasked to evaluate the existing wetland and lagoon at the park. With sewer separation proposed upstream from Adams Park (see Figure 1), the wetland and lagoon could be reconfigured to detain storm flows and eliminate or reduce them from entering the combined sewer system. The hybrid analysis will consider how the system currently functions and how flows could be conveyed in the future to benefit the LTCP CSO program and better control downstream flows. Note that the analysis presented herein is a higher level review used to determine whether or not there is reason to further pursue an Adams Park hybrid alternative. Hybrid Alternatives Evaluation The Adams Park analysis presented herein is a higher level review used to determine whether or not a hybrid solution at the park should be further investigated and refined. Basic information and findings to help make this decision have been included herein. Based FILE CODE: ML PAGE 1

2 on discussions with City and PMT staff, specific questions and points of interest for this evaluation were: HY Is the hydraulic capacity of Adams Park being fully utilized? 2. Could the Adams Park wetland and lagoon accept more stormwater inflow? 3. Can a hybrid solution at Adams Park benefit the CSO program (i.e., reduce cost, reduce CSOs, etc.)? 4. Can Adams Park be modified to help downsize the proposed conveyance pipe connecting the proposed sewer separation area to the proposed stormwater tunnel? 5. Can an Adams Park hybrid solution help downsize the proposed stormwater tunnel? 6. Can a hybrid solution increase the water supply to the wetland and lagoon and enhance recreation and fishing opportunities? 7. Can a hybrid solution help to eliminate ongoing street flooding, near the intersection of 36th and Corby and 36 th and Maple streets, directly upstream from the park? Adams Park The Adams Park wetland and lagoon, as shown in Figure 2, was constructed in 1998, to receive separated stormwater from upstream sewer separation project RNC 5192 and RNC 5409C. Under existing conditions, stormwater overflows from the wetland and lagoon pass back to the combined sewer system (CSS) downstream from the park. The lagoon has historically had problems maintaining its designed normal pool elevation (elevation 1084 ft). Initially, to help address this problem, the City supplemented the lagoon using City potable water. However, this practice has since been minimized due to expense and because water pressure in the surrounding residential area was reduced to unacceptably low levels. In 2001, the City drained the lagoon and installed a bentonite layer to help reduce leakage back to the combined sewer and infiltration into the surrounding soil. In addition, a shallow 170 feet deep well was installed so water supplements could be made to the lagoon using a source other than using City potable water. As recently as 2007, City staff noted that the Adams Park lagoon was well below (~ 4 feet) its normal pool water surface elevation and surrounded by dense vegetation (see Figure 3 and Figure 4). The lagoon was intended to serve as a recreation and fishing destination. However, with the dense vegetation surrounding the lagoon and low water levels, this intent has yet to be fully realized. Adams Park receives separated stormwater from sewer separation project RNC 5192 and RNC 5409C. A 72-inch diameter storm sewer delivers stormwater inflows from RNC 5192 and RNC5409C to an inlet structure (i.e., riser) located in the southeastern portion of the wetland (see Figure 2 for the location of inlet structure, see Figure 5 for a picture of inlet structure). The riser inlet structure has an outbound 36-inch diameter pipe connecting it to the existing combined sewer as shown by Figure 6. This connection allows the first flush from a wet weather runoff event to pass under the Adams Park wetland and lagoon and back into the combined sewer. In addition, the 36-inch diameter pipe allows grit to pass downstream and prevent sediment build-up inside the inlet/riser structure. Once the 36- inch diameter pipe reaches its hydraulic capacity, water rises in the inlet structure until reaching the inlet structure side orifices. When the hydraulic gradeline in the inlet structure FILE CODE: ML PAGE 2

3 reaches elevation feet, stormwater flows through the side orifices and into the Adams Park wetland. If the side orifices are not sufficient to pass the incoming stormwater runoff, water continues to backup in the inlet structure until water passes through the two 2-feet wide by 4-feet long grates located at the top of the inlet structure (elevation ft) The Adams Park wetland is connected to the Adams Park lagoon by a low flow 8-inch diameter orifice (invert elevation at ft) and high flow 35 foot wide weir (weir crest at elevation ft). Figure 7 and Figure 8 are presented to show how the wetland is hydraulically connected to the lagoon. In addition, the wetland has an emergency overflow weir (9.3 foot wide, weir crest elevation at ft) connected to the combined sewer. The Adams Park lagoon receives all stormwater inflows from the Adams Park wetland. The lagoon is connected to the combined sewer by a low flow 8-inch diameter orifice (invert elevation at 1084 ft) and high flow 7.3 foot long weir (weir crest at elevation ft). Figure 9 presents a schematic representation of the existing wetland, lagoon, and sewer system in Adams Park. For reference, Table 1 and Table 2 present the surface area to elevation relationship for the wetland and lagoon, respectively, used for the modeling simulations discussed later within this document. Table 1. Adams Park Wetland Surface Area to Elevation Relationship Elevation (NAVD 88 feet) Surface Area (acres) Volume (acre-ft) a a. Normal Pool Elevation Table 2. Adams Park Lagoon Surface Area to Elevation Relationship Elevation (NAVD 88 feet) Surface Area (acres) Volume (acre-ft) a a. Normal Pool Elevation One drainage related issue near Adams Park is the ongoing street flooding which occurs along Corby and Maple streets during wet weather events (streets are located directly adjacent (south) of the park). As mentioned earlier, a hybrid solution at Adams Park that helps to address this issue would be beneficial for the City. Upon review of the RNC 5409C as well as the Adams Park configuration, the difference in grade between the nearest Maple Street stormwater inlet and the wetland inlet side orifice invert elevation is very small. The inlet structure on Maple Street is located at elevation ft (see Figure 10) while the FILE CODE: ML PAGE 3

4 Adams Park wetland inlet structure side orifice invert elevation is located at elevation ft (see Figure 6). Therefore, there is virtually no hydraulic elevation difference between these two structures (approximately 0.3 ft). As governed by basic hydraulics, if water is flowing into the Adams Park wetland, water is within 0.3 feet from pooling on Maple Street. If street flooding is to be minimized with a hybrid solution, this specific situation should be addressed. Hybrid Alternative Hydraulic Analysis HY Adams Park A XP-SWMM hydraulic model was developed to evaluate a hybrid solution at Adams Park. The hydraulic model was set up to simulate the response in the wetland and lagoon to a 10- year, 24-hour runoff event. A rainfall event with 4.6 inches of rain falling over a 24-hour period distributed with a Soil Conservation Service (SCS) Type 2 storm hyetograph was used to simulate the 10-year event. Note that this storm event is identical to the one used to size the proposed stormwater tunnel presented in the Minne Lusa Stormwater Tunnel and Pershing/Storz Detention Basin Hydraulic Evaluation Technical Memorandum (TM). Although the 10-year event is typically not the focus of a combined sewer long term control plan (LTCP), because sewer separation is recommended as part of the SCLTCP, per City requirements, any new storm sewer pipes constructed to achieve sewer separation must be designed with a 10-year capacity. A hybrid solution at Adams Park could reduce the size of the proposed Adams Park conveyance pipe and proposed stormwater tunnel (see Figure 1) which would be designed with a 10-year capacity. Therefore, reductions to the proposed storm sewers and stormwater tunnel with an Adams Park hybrid solution could help reduce the cost of the LTCP Program facilities as well as reduce the peak and volume of CSO discharged to the Missouri River from the ML basin. Three hydraulic conditions were simulated for the Adams Park hybrid solution. They were: 1) Existing Conditions: A simulation which represents the existing hydrologic and hydraulic conditions in the Adams Park wetland and lagoon in response to a 10-year rain event. Figure 9 presents a schematic of the existing condition hydraulic connectivity at Adams Park. 2) Hybrid Condition 1: A simulation which represents one proposed hydrologic and hydraulic condition in the Adams Park wetland and lagoon in response to a 10-year event. This condition assumes that the proposed sewer separation area, shown on Figure 1, is separated and the stormwater from the area bypasses the Adams Park wetland and lagoon and passes downstream to the proposed stormwater tunnel through the proposed storm sewer conveyance pipe (also shown on Figure 1). In addition, this condition assumes that overflows from the Adams Park wetland and lagoon pass to the proposed storm sewer conveyance pipe rather than the combined sewer. Modifications to the existing combined sewer are proposed to achieve this result. Figure 11 presents a schematic of the Condition 1 hydraulic connectivity at Adams Park. 3) Hybrid Condition 2: A simulation which represents one proposed hydrologic and hydraulic condition in the Adams Park wetland and lagoon in response to a 10-year event. This condition assumes that the proposed sewer separation area, shown on FILE CODE: ML PAGE 4

5 Figure 1, is separated and the stormwater from the area is directed into the Adams Park wetland. In addition, this condition assumes that overflows from the Adams Park wetland and lagoon pass to the proposed storm sewer conveyance pipe (also shown by Figure 1) rather than the combined sewer. Modifications to the existing combined sewer are proposed to achieve this result. Figure 12 presents a schematic of the Condition 2 hydraulic connectivity at Adams Park. The hydraulic model was simulated for each condition to compare the peak flow and volume of runoff through the sewer system. Table 3 presents a summary of the 10-year storm simulation results. Table 3. Hydraulic Results of Adams Park Hybrid Simulations Peak Flow Into Downstream Combined Sewer Peak Flow Into Downstream Proposed Stormwater Conveyance Pipe Volume Passing To Combined Sewer (downstream from Adams Park) Volume Passing To Proposed Stormwater Conveyance Pipe (downstream from Adams Park) Peak Water Surface Elevation in Wetland Peak Water Surface Elevation Lagoon Condition (cfs) (cfs) (cubic ft) (cubic ft) (ft) (ft) Existing Condition 368 a Not Applicable 4,492,000 Not Applicable Hybrid Condition a 576 2,071,000 2,340, Hybrid Condition a 340 2,063,000 2,462, a. Peak flow in the existing combined sewer system is restricted by undersized upstream and downstream pipe capacities as well as the pipe connectivity of each proposed condition. Note: The starting water surface elevation in the wetland and pond is assumed to be equal to the normal pool elevation (wetland = ft, lagoon = ft) Upon review of Table 3, several conclusions can be reached. First, the existing condition simulation shows that the runoff from the 10-year storm fits within the confines of the existing wetland and lagoon. For existing conditions, the 10-year storm raised the wetland water surface elevation from a starting elevation of ft to ft, or approximately 2.2 ft. Note that City information shows that the high water surface elevation of the wetland to be approximately Therefore, 1.3 ft of freeboard remains in the wetland during the peak of the 10-year event. The simulation also showed that the 10-year storm raised the lagoon water surface elevation from a starting elevation to 1084 ft to or approximately 0.8 ft. Note that City information shows that the high water surface elevation of the lagoon to be approximately 1090 ft leaving a freeboard in the lagoon of 5.2 FILE CODE: ML PAGE 5

6 ft. This result may help to explain why the lagoon has historically had trouble maintaining its normal pool elevation. Even in a 10-year storm, the lagoon is only raised about 0.8 feet. If the lagoon is somewhat leaky and receives minimal inflows, it could lead to a chronic water shortage. The modeling results may help to explain why this condition has been noted at the lagoon site. The Condition 1 simulation results show that inflows to the wetland and lagoon are decreased due to the proposed Condition 1 modifications. The proposed Condition 1 modifications included the following changes (see Figure 11 for a schematic representation of the modifications): 1) Convert the existing combined sewer underneath the wetland and lagoon into a storm sewer by bypassing the upstream combined sewer flows around Adams Park. 2) Direct overflows from the wetland and lagoon to the proposed stormwater conveyance pipe instead of the combined sewer. 3) Bypass stormwater flows from the proposed upstream separation areas around the wetland and lagoon and direct them into the proposed stormwater conveyance sewer. The Condition 1 simulation results show that if the lagoon and wetland are reconfigured as proposed, inflows to the wetland and lagoon would be reduced from existing conditions (actually eliminated for the 10-year event). This occurs because under existing conditions, the hydraulic gradeline in the combined sewer is high which encourages stormwater inflows from RNC 5192 and RNC 5409C to pass into the wetland rather than pass through the low flow 36-inch diameter pipe connected to the combined sewer. However, by changing the configuration of the Adams Park as noted above (and shown by Figure 11), the backwater elevation is reduced downstream from Adams Park which allows more storm flows to pass from RNC 5192 and RNC 5409C through the 36-inch diameter low flow pipe. Without additional modification, Condition 1 may reduce the water supply to the wetland and lagoon and exacerbate the ongoing water supply. Condition 1 does, however, differ significantly from the existing conditions as approximately half of the runoff volume from the upstream separation areas is kept out of the downstream combined sewer system. Additionally, this Condition allows overflows from the wetland and lagoon to pass into the proposed storm sewer versus back to the existing combined sewer. This modification reduces the volume passing to the combined sewer by an additional 540,000 cubic-ft during the 10-year storm. The Condition 2 simulation results show that inflows to the wetland and lagoon are increased due to the proposed Condition 2 modifications. The proposed modifications for Condition 2 included the following changes (see Figure 12 for a schematic representation of the modifications): 1) Convert the existing combined sewer underneath the wetland and lagoon into a storm sewer by bypassing the upstream combined sewer flows around Adams Park. 2) Direct overflows from the wetland and lagoon to the proposed stormwater conveyance pipe instead of the combined sewer. 3) Direct stormwater from the proposed upstream separation areas to the wetland. FILE CODE: ML PAGE 6

7 The Condition 2 simulation results show that if the lagoon and wetland are reconfigured as proposed, inflows to the wetland and lagoon would be increased from existing conditions. For Condition 2, the 10-year storm raised the wetland water surface elevation from a starting elevation of ft to ft or approximately 3.6 ft. Note that City information shows that the high water surface elevation of the wetland to be approximately 1090 ft so all of the available freeboard would be used (i.e., wetland would be completely filled). For Condition 2, the 10-year storm raised the lagoon water surface elevation from a starting elevation of 1084 ft to or approximately 5.4 ft. Note that City information shows that the high water surface elevation of the lagoon to be approximately 1090 ft leaving a freeboard in the lagoon of 0.6 ft. Changes to the Adams Park configuration, as proposed by Condition 2, would help to alleviate the water supply problems at Adams Park. Condition 2 is similar to Condition 1 in that half of the runoff volume from the upstream separation areas is kept out of the downstream combined sewer system. However, Condition 2 is better than Condition 1 at reducing the peak flow through the proposed stormwater conveyance pipe since all of the water from the upstream sewer separation project is directed into the Adams Park wetland rather than bypassing the Park altogether. Similar to Condition 1, Condition 2 allows overflows from the wetland and lagoon to pass into a proposed storm sewer versus back to the existing combined sewer as they do under existing conditions. This modification is necessary to keep all of the overflows from Adams Park separated from the combined sewer. Hybrid Alternative Summary and Recommendations HY Adams Park Basic observations from Adams Park modeling and hybrid investigation are summarized by the following points: 1) The Adams Park lagoon has a history of low water levels and poor natural water supply. 2) The Adams Park wetland and lagoon have additional capacity to accept stormwater inflows including the 10-year storm. 3) The grade differential between the Adams Park wetland inlet structure ( ft) and the RNC 5490C stormwater inlet along Maple Street ( ft) is minimal (see Figure 6 and Figure 10). Ponding water in Maple Street is almost certain to exist if the wetland inlet structure is operating. 4) Condition 1 reduces inflows to the wetland and lagoon. 5) Condition 1 helps alleviate upstream street flooding along RNC 5490C because it lowers the hydraulic gradeline in the sewer system at Adams Park. 6) Condition 1 helps reduce the stormwater inflows (peak and volume) passing into the combined sewer system. 7) Condition 1 helps reduce the stormwater inflows (peak and volume) passing into the proposed conveyance pipe and stormwater tunnel. These proposed structures may be able to be downsized. FILE CODE: ML PAGE 7

8 8) Condition 2 increases inflows to the wetland and lagoon. 9) Condition 2 will not help and may exacerbate upstream street flooding because the water surface elevation in the wetland is increased from existing conditions. Since the Maple Street stormwater inlet (elevation ft) is tied hydraulically to the wetland, increasing the wetland water surface elevation may cause additional street flooding even though the wetland and lagoon are designed to accommodate a high water level of 1090 ft. 10) Condition 2 helps reduce the stormwater inflows (peak and volume) passing into the combined sewer system. 11) Condition 2 is better than Condition 1 in reducing the peak inflow and overall runoff volume directed to the proposed stormwater conveyance pipe leading from Adams Park to the proposed stormwater tunnel. The proposed conveyance pipe and stormwater tunnel may be able to be downsized. 12) Condition 2 requires a new inlet structure be constructed at the wetland. The existing inlet structure is not big enough, both physically, or hydraulically, to accept more inflows (i.e., without a new inlet structure, additional upstream flooding along Maple Street would occur) Based upon the findings presented herein, recommendations for pursuing a hybrid solution at Adams Park are: 1) An option, similar to Condition 2, would further benefit the LTCP CSO program. The proposed downstream stormwater conveyance pipe and tunnel might be reduced in size by maximizing the use of the available storage at Adams Park for peak flow attenuation. 2) Weir elevations and cross connections in Adams Park could be optimized (optional) to help alleviate the upstream street flooding along Maple street and balance the water supply between the wetland and lagoon. Changes could include: a. Lowering the elevation of the wetland inlet structure weir to reduce backwater effects along Maple Street. b. Lowering the elevation of the weir connecting Adams Park wetland and lagoon to better distribute flow between the wetland and lagoon during peak events and to reduce the backwater elevation. c. Lowering the elevation of the weir connecting the Adams Park wetland and lagoon to the outbound sewer system (proposed to be a stormwater system) to reduce the overall system backwater elevation. 3) The existing footprint of the wetland and lagoon appears to be sufficient in accepting the 10-year inflow for Condition 2 (some small modifications may be needed). However, the Adams Park wetland and lagoon footprint (i.e., storage volume) could be increased to achieve additional City objectives. Is not anticipated, however, that changes to the storage alone will eliminate the upstream street flooding. Modifying the existing weir elevations may be the only way to effectively reduce ongoing street flooding problems. FILE CODE: ML PAGE 8

9 4) A hybrid solution to benefit the CSO program, like Condition 2, is possible at Adams Park but many variables must be balanced to achieve the proper result. In summary, the Condition 2 hybrid alternative is recommended to be used at Adams Park to help the CSO program. At this time, it is recommended that a new outfall be constructed to allow stormwater inflows from the proposed separation area to enter the wetland and lagoon. In addition, a CSO bypass should be constructed to reroute the remaining combined sewer flows around the Park area. This will allow the separated stormwater that enters the wetland and lagoon to remain permanently separated downstream. At this time, making recommendations to modify the existing wetland and lagoon outlet works and weir elevations would be premature and require further refinement. Costs associated with the recommended Hybrid Condition 2 are not included in this Hybrid Alternative TM due to the following: 1. The Task 21 Sewer Separation refinement incorporates many of the costs for the pipes and manholes needed to modify the configuration of Adams Park. Therefore, the remaining incremental cost associated with Hybrid Condition 2 are those needed to construct a new wetland stormwater outfall and reroute the existing combined sewer with a new concrete structure (i.e., junction box). Those two structures and associated costs are not included in the Task 21 Sewer Separation work. 2. The Task 2 Green Solutions refinement investigated green solutions options at Adams Park. Since there appears to be a reasonable green (i.e. Hybrid Condition 2) option to reconfigure the wetland and lagoon to help the CSO program, the incremental costs for the Adams Park Hybrid Condition 2 are included in the Minne Lusa Basin Green Solutions TM rather than within this Hybrid document. The costs are not included in this document to avoid the potential of double counting costs associated with Hybrid Condition 2 facilities. FILE CODE: ML PAGE 9

10 Acronym/Term City CFS CSO CSS FT LCTP ML NAVD PMT SCLTCP TM Definition City of Omaha Cubic Feet per Second Combined Sewer Overflow Combined Sewer System Feet Long Term Control Plan Minne Lusa North American Vertical Datum Program Management Team Substantively Complete Long Term Control Plan Technical Memorandum FILE CODE: ML PAGE 10

11 Figures Figure 1. Overview of Existing and Proposed Stormwater Tunnel Separation Areas Figure 2. Adams Park Figure 3. Adams Park Lagoon in 2007 (looking south) Figure 4. Adams Park Lagoon in 2007 (looking east) Figure 5. Adams Park Wetland Stormwater Inlet Structure Figure 6. Adams Park Inlet Structure Configuration (From RNC 5409C) Figure 7. Adams Park Wetland and Lagoon Hydraulic Connection (looking northwest) Figure 8. Adams Park Wetland Low Flow Orifice and Weir (looking northwest) Figure 9. Adams Park Existing Hydraulic Connectivity Figure 10. RNC 5409C Configuration Figure 11. Condition 1: Adams Park Proposed Hydraulic Connectivity Figure 12. Condition 2: Adams Park Proposed Hydraulic Connectivity FILE CODE: ML PAGE 11

12 Proposed Stormwater Conveyance Pipe Adams Park Proposed Separation Area Figure 1. Overview of Existing and Proposed Stormwater Tunnel Separation Areas FILE CODE: ML PAGE 12

13 Inlet Structure Figure 2. Adams Park FILE CODE: ML PAGE 13

14 Figure 3. Adams Park Lagoon in 2007 (looking south) Figure 4. Adams Park Lagoon in 2007 (looking east) FILE CODE: ML PAGE 14

15 Figure 5. Adams Park Wetland Stormwater Inlet Structure FILE CODE: ML PAGE 15

16 Inlet Structure Figure 6. Adams Park Inlet Structure Configuration (From RNC 5409C) FILE CODE: ML PAGE 16

17 Figure 7. Adams Park Wetland and Lagoon Hydraulic Connection (looking northwest) Figure 8. Adams Park Wetland Low Flow Orifice and Weir (looking northwest) FILE CODE: ML PAGE 17

18 Figure 9. Adams Park Existing Hydraulic Connectivity FILE CODE: ML PAGE 18

19 Street Inlet Figure 10. RNC 5409C Configuration FILE CODE: ML PAGE 19

20 Figure 11. Condition 1: Adams Park Proposed Hydraulic Connectivity FILE CODE: ML PAGE 20

21 Figure 12. Condition 2: Adams Park Proposed Hydraulic Connectivity FILE CODE: ML PAGE 21

22 Attachment 1 Adams Park Wetland and Lagoon Design Information (Provided by City Staff)

23 7/19/06 Adams Park Wetland & Retention Pond (RNC 5409A; RNC 5409C; RNC 5192) 36th & Maple St. Detention area consists of a 0.75 surface acre wetland with islands (average depth 18" with 4 ft freeboard for stormwater detention) and which overflows to a 1.75 surface acre wet pond (with approx. 3 ft of freeboard for stormwater detention) Ponds can handle approx. 10-year storm event and are released over a period of 2-3 days so wetland vegetation can reduce the pollutants with biological activity & settling. Total stormwater detention = (0.75 acx 4 ft) + (1.75 ac x 3 ft) =8.25 acre-ft. I could not find any design calculations, but found project summary sheet with the following information: Existing 78" comb sewer. New 72" storm sewer comes in from south to Adams Park and has inflow/outflow structure to wetland pond. Inlet/outlet box has 36" outgoing pipe that taps back into existing combined 78". Stormwater greater than the 36" RCP storm sewer pipe capacity overflows thru structure out to wetland then to pond (thru wall openings & weir blockouts) and is detained. Pond also has high level overflow and feeds back into the existing 78" sewer system (that runs south to north through Adams Park) with a 36" RCP pipe & flow control gate. I BELIEVE this overflow connection is upstream of MH (but downstream of MH ), this is not clear on any plan. The wet pond also has lower level overflows (thru 2" holes & 8" weir wall blockouts) that allows water flow to transfer between wetlands & pond or vise-versa. After the larger wet pond (east pond) was constructed (1998), it would not hold water so had to be supplemented with MUD water. It was known that the east pond bottom soil had debris that was removed during construction. However, seepage paths seem to exist even though there was clean, compacted fill placed during construction. The pond was emptied end of June After drying, the top + 12inches of soil was removed from the top 4 ft of side slope banks and set aside (the area not normally under water). To the best of my knowledge, the bentonite liner material was compacted into the pond side slopes from approx. elev 1076 to 1084 and the pond floor from elev to 1076 (worked it in if area was dry and possibly just sprinkling on top if area was too wet to work). Work was done during July 2001 by City of Omaha crews (Gordon Andersen) at cost of about $11,000 (approx. 90 tons of material at +3lbs/SF). Bentonite was also to be concentrated around the bridge structure in both the wetland and pond areas. The 12inch cover soil was replaced on pond side slopes to protect the bentonite from drying out and cracking, and the pond was refilled. A concrete flume spillway was also constructed in the east pond (approx. $9,000) to help stop erosion from incoming stormwater flow at the bridge area. The file is a bit confusing regarding a well--i believe that a shallow well was drilled to 170 ft. to provide supplemental water without affecting the water pressure in neighborhood/area (when using MUD water). A second well was looked at but would require drilling down 900 ft (six figure cost), so this was not done (Parks Dept was handling the well aspect of the project). The pond likely needs supplemental water to compensate for evaporation or dry years (will use shallow well and MUD water), but severe leakage seems to have slowed or stopped (based on No complaints from Parks Dept). Wetland Inflow/Outflow structure: High water elev (structure top) Normal water level Bottom elev. varies In-coming pipe: 72" RCP at 0.98% at IE Out-going pipe: 36" RCP at 9.63% at IE

24 Wet Pond Inflow/Outflow information: Wet pond bottom elev ; normal pond level ; high pond level Also 2" weep holes in wall between wetland & pond at ; overflow weir elev ; weir 8" blockout holes IE Overflow at weir wall elev will allow wet pond drainage to 60" dia. opening leading to existing 78" combined sewer. Plans of wetland, pond, & control structures (see RNC 5409A sht 5, 13 & 14 and RNC 5409C sht. 3 & 9).

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