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CR 98-149 Consider Feasibility Report, Storm Drainage Problem Area 5September 11, 1998 Overview. 0 P K Consider Feasibility Report, Storm Drainage Problem Area 5 Proposed Action. Staff recommends adoption of the following motion: Move that Council accept the feasibility report on correction of flooding problems in Area 5 — alley between 8 and 9 Ayes North and 2" and 3 Streets North and approve proposed continued action. Last November City Council adopted by resolution an action plan to address last summer's storm and sanitary sewer problems. The plan required the city to confirm that the proposed 6th Avenue North storm sewer improvement project would also improve drainage in Area 5. The city's consultant confirmed that the 6 Ave project would not improve drainage in Area 5. Staff reported this information to Council on June 16th and Council authorized preparation of a feasibility study to identify potential improvements. The feasibility report recommendations were presented to Area 5 residents at a Sept 3 public meeting. Staff and Mike Foertsch, RCM Associates, will present the report to Council and answer questions. Primary Issues to Consider. • Report recommendations and proposed continued action. Storm sewer project funding Supporting Information. Feasibility Report Storm Sewer Fund — Cash Flow Steven J. Stadler, Public Works Director Council Report 98 -149 Council Report 98 -149 Page 2 Analysis of Issues Report recommendations and proposed continued action. Report recommendations: The report concludes that the most effective solution to this local drainage problem area is to better facilitate overland flow through this alley. It recommends that this be done by lowering the south end of the alley to reduce the depth of water backup prior to overflow to the south onto 2 " Street North. The attached report provides more detail on the analysis and alternatives. Proposed continued action: Staff recommends that a decision to proceed with this proposed improvement project wait until the storm water management plan update is completed. The plan update will consider the entire central Hopkins storm sewer system, identify shortcomings and recommend potential improvements. We must be certain that the design of improvements to Area 5 consider and incorporate the needs of the entire central Hopkins system. Staff would propose to complete a project design survey based on the feasibility report recommendations. In January, 1999 staff will report the results of the storm water management plan update and recommend a course of action for Area 5 improvements. Storm sewer project funding Storm sewer project funding is very tight until the year 2010 when the 1989 bonding debt service ($240,000/yr) is retired. In 1998 there was a $600,000 interfund loan from the sanitary sewer fund. Additional funding is needed in the storm sewer fund to cover the cost of this potential Area 5 project and other previously identified projects. Staff proposes that the 1998 loan of $600,000 become an equity transfer instead of a loan and an additional $200,000 be transferred from the sanitary sewer fund in 1999. There is sufficient cash balance in the sanitary sewer fund to absorb this $800,000 equity transfer. However, this proposed additional storm sewer funding decision should also be deferred until a better forecast of future needs are determined through the storm water management plan update. Area 5 - Alley Between 8 th & 9 th Avenue North Feasibility Report Storm Water Analysis Hopkins, MN RCM File No. 10441.05 10901 red circle drive • box 130 • minnetonka, mn 55343 • (612)935 -6901 • fax (612)935 -8814 September 1998 equal opportunity employer 10901 red circle dr. W ost office box 130 innetonka, mn 55343 -0130 (612) 935 -6901 fax (612) 935 -8814 www.rcm-assoc.com September 11, 1998 Mr. Steven J. Stadler, P.E., Director of Public Works City of Hopkins 1010 First Street South Hopkins, MN 55343 Re: Area 5 - Storm Water Flooding Analysis RCM File No. 10441.05 Dear Mr. Stadler: ICIll Enclosed is the feasibility report addressing the storm water flooding impacts to the alley between 8 and 9 Avenues North and between 2nd and 3r Street North within the City of Hopkins. The Hopkins City Council identified this study area by Council Resolution 97 -104 rieke for storm water flooding analysis. carroll muller ir sociates, inc. The improvements proposed herein are feasible from an engineering standpoint, in accordance ggineers with the City's Comprehensive Storm Water Management Plan, cost effective, and necessary land t su r y t o provide an increased level of flooding protection for the roe owners adjacent surveyors P g p property rtY acent to the J study area. We are available to meet with the City Council, staff and interested parties to review any aspect of this report. Thank you for providing RCM the opportunity to be of service to the City of Hopkins. Respectfully submitted, RIE . ARROLL MULLER ASSOCIATES, IN Michael P. Foertsch, P.E./L. Civil/Municipal Department Manager MPF/ka Enclosure Date: Date: AREA 5: ALLEY BETWEEN 8TH & 9TH AVENUE NORTH CITY OF HOPKINS, MINNESOTA SEPTEMBER 1998 RCM FILE NO. 10441.05 Prepared by: Rieke Carroll Muller Associates, Inc. 10901 Red Circle Drive, Suite 200 Minnetonka, Minnesota 55343 Reviewed by: Steven J. Stadler, P.E., Public Works Director I hereby certify that this report was prepared by me or under my direct supervision and that I am a duly Registered Professional Engineer under th qg FEASIBILITY REPORT STORM WATER FLOODING ANALYSIS f the State of Minnes Michael P. Foertsch, P.E./L.S. Reg, No. 18139 TABLE OF CONTENTS Section Page Issues 1 Analysis 1 Recommendations 2 Estimated Construction Costs 5 Appendix Figure 1 - Local Drainage Area 7 Figure 2 - Existing Tributary Areas 8 Figure 3 - Construction Area 9 Figure 4 - Garage Floor Elevations 10 AREA -5: ALLEY BETWEEN 8TH AND 9TH AVENUES NORTH ISSUES: The alley between 8 and 9 Avenues North and 2nd and 3r Streets North, reportedly flooded on two occasions during July of 1997 and once during May 1998. Drainage from this area was improved slightly in 1992 with the 1st Street North storm sewer project. The intent of the 1992 improvements was to maximize the available pipe capacity within the existing storm sewer system. That project was not expected to solve the problem, but simply allow a slight improvement. The 6th Avenue North storm sewer improvements do not provide an increased level of protection for this area. ANALYSIS: This area (Figure 1) is drained by a 24 -inch storm sewer pipe which drains southerly to the existing storm sewer system in 2nd Street North. The low point in the alley is drained by two existing catch basin inlets. The capacity of the two inlet catch basin grates is approximately 6.5 cfs. The capacity of the existing 24 -inch storm sewer pipe is estimated at 13 cfs. The "localized" drainage area to this alley low point is approximately 3 acres. The estimated storm water flows from this 3 acre area are about 2.1 cfs and 3.8 cfs during the 10 year and 100 year frequency storms, respectively. The difficulty in draining this alley is that the middle of the alley is at a lower elevation than the south end, some of the stormwater flowing along the alley is trapped. This causes the flooding condition. Overland flow from approximately 25 acres (Figure 2) is tributary to this alley area low point. During the rainfall events which exceed the existing pipe capacities within the drainage district, the flow to this alley area in the 100 year storm event is estimated at approximately 35 cfs from this 25 acre area. In addition, there is testimony from residents who live in this area that storm water flow southerly along 8 Avenue North, splits and goes west on 3r Street North and a portion flows south along 8 Avenue North. The portion of the flow which flows westerly along 3r Street North collects in the low point at 9 Avenue and 3rd Street North. On May 15, 1998, residents in this area reported as much as 2- inches of rainfall within a 20 minute period. During that rainfall event, the water ponded to a depth of approximately 1.7 feet. This 1.7 foot depth was verified by water lines on the existing structures adjacent to the low point in the 1 alley. The low point elevation of the overland flow point from this location is at the intersection of 2nd Street North and the alley. This intersection elevation is approximately 1.8 feet above the low point in the alley. As a result, in rainfall events which exceed the inlet capacity of the existing storm sewer system, storm water ponds to a depth of approximately 1.8 feet before overflowing southerly onto r Street North. The overland flow on 2n Street North is easterly to 7 Avenue North and then southerly along 7 Avenue North. RECOMMENDATIONS: The existing storm sewer pipe system which services this area is currently at capacity. There are no feasible cost effective pipe construction alternatives which would alleviate the 100 year flood levels in this area of study. The following alternatives are identified for consideration: 1. Provide an overland overflow route at a lower elevation. 2. Increase the inlet capacity to maximize available pipe capacity in the 24 -inch storm sewer pipe serving this local area. 3. Divert all overland storm water flow around the alley. 4. Flood proofing of the existing structures below the current overflow elevation. To determine the scope of the improvements necessary to divert all the overland flow from this area, a detail topographic survey of area between 2nd and 3 Street North and 8 & 9 Avenues North was performed. Alternative No. 1- To provide an overland overflow route at a lower elevation. The detailed survey information obtained, provided information which allows the determination of the overland flow elevation at the southerly end of the alley at approximately 0.2 to 0.3 feet below the existing garage floor elevations within this area. The extent of the improvement necessary to provide this level of protection includes the following (Figure 3): • Reconstruction of the alley from the low point catch basins southerly to 2 Street North. 3 • Reconstruction of 2n Street and a lower elevation from its intersection with 9 Avenue North to the intersection of 7 Avenue North. With the reconstruction of 2 Street North from 9 Avenue to 7 Avenue North, flow which currently drains easterly on 2n Avenue North and northerly at its intersection with 9 Avenue North is diverted easterly to the 7t Avenue North intersection of 2n Street North. This alleviates some of the flooding potential at the 3' Street North and 9 Avenue North intersection. The downside to this street regrading is that it would result in a slightly flatter street grades than what currently exists today. Today the street grades are approximately 0.4 %. With this proposed improvements, the street grades would be approximately 0.2 %. A typical minimum desirable grades along a curb and gutter street section is approximate 0.5 %. As a comparison, the existing street grades along 9 Avenue North, north of r Street North, is approximately 0.2 %. Therefore, there are areas within the City of Hopkins which have street grades which are less than typical 0.5% minimum. Alternative No. 2 - Increase the inlet capacity at the alley low point. The existing inlet capacity of the catch basins at the low point in the alley is currently less than the available pipe capacity within the 24 -inch pipe which services this area. The addition of the 2 catch basins would increase the inlet capacity to that of approximately the capacity of the existing 24 -inch storm sewer pipe. Alternative No. 3 - Divert all overland storm water flow away from the alley area. This alternative was reviewed and it was determined that due to the extremely flat street grades within this area, no feasible cost effective overland diversion route could be established to divert all overland flow away from this alley area. Alternative No. 4 - Flood proofing the existing structures below the current overflow elevation. Due to the fact that Alternatives 1 and 2 are a viable option to mitigate the flooding within this alley area, flood proofing of the existing structures below the current overflow elevation was not considered. Figure No. 4 shows that 7 structures (Elevation 923) are impacted given the current overflow situation within the alley area. After the proposed improvements identified in Alternatives 1 and 2, the ponding area is limited to the localized areas around the proposed catch basins as well as the reconstructed alley area. Informational Meeting with Residents. An informational meeting with the residents adjacent to this alley area was held on Thursday, September 3. The information in this report was presented to them along with a tentative time frame to complete the improvements. The residents were in agreement that Alternatives 1 and 2 provided a reasonable solution to the problem. They stated they were not aware of the water levels in the alley area ever rising to a level such that an overflow condition existed at 2nd Street North. The proposed improvements would provide an over flow at approximately 0.5' above the existing alley low point, currently at the catch basin locations. The information provided the residents indicated that the City is currently updating their Comprehensive Storm Water Management Plan. This plan update will be completed by the end of this year. From a local drainage district perspective, the improvements identified offer a viable solution to the storm water problems they have been experiencing. The updated Storm Water Management Plan may or may not confirm these findings from an overall City storm water management perspective. If changes result, the residents will be informed prior to any final City Council action in this regard. Proposed Schedule Informational Meeting with Residents September 3, 1998 City Council Consideration September 15, 1998 Storm Water Management Plan Update Completed January 1, 1999 City Council Project Authorization March 1999 Project Construction May, June 1999 4 ESTIMATED CONSTRUCTION COSTS: A. Increase inlet capacity in alley to utilize available pipe capacity. Item Ouantity Unit Cost Amount Reconstruct catch basins by structure 2 $500 $1,000 Construct catch basins by structure 2 $1,000 $2,000 Total $3,000 B. Reconstruct alley from existing catch basins south to 2 Street at a lower elevation to provide overflow in alley at approximately 0.5' depth at catch basin. Item Quantity Unit Cost Amount Remove existing concrete pavement by square yard 623 $10 $6,230 Common excavation by cubic yard 210 $6 $1,260 Aggregate base 6" thick by ton 105 $15 $1,575 Concrete pavement by square yard 623 $25 $15,575 Bituminous and sod restoration by square yard 249 $10 $2.490 Total $27,130 C. Reconstruct 2nd Street from 9 Avenue to 7 Avenue at 0.2% grade, and install valley gutters, 2 at 8 Avenue and 1 at 7 Avenue. Item Ouantity Unit Cost Amount Remove bituminous pavement by square yard 3120 $5 $15,600 Remove concrete curb and gutter by linear feet 1560 $2 $3,120 Common excavation by cubic yard 1040 $6 $6,240 Aggregate base 6" thick by cubic yard 520 $15 $7,800 Bituminous base 3" thick by ton 515 $25 $12,875 Bituminous wear 1 -1/2" thick by ton 258 $28 $7,224 5 Item Quantity Unit Cost Amount 18" concrete valley gutter by square yard 270 $7 $1,890 Concrete curb and gutter by linear feet 1560 $10 $15,600 Sidewalk/pedestrian ramps by lump sum 1 $2,500 $2,500 Driveway restoration by lump sum 1 $4,500 $4,500 Sod by lump sum 1 $3,000 $3,000 Structure adjustments by lump sum 1 $5,000 $5,000 Total $85,349 Total Alternatives 1 and 2 $115,479 15% Contingency $17,300 Administrative and Engineering $34,500 Grand Total $167,279 APPENDIX EXHIPIT1.DWG •- FWURE 4, - -25 -98 PLOT 923.9 9,_ 15" X 9 15" 2nd —0._ 4_ ---- rieke carrot, mutter associates, inc. engineers architects • land surveyors 3rd STREET 924.8 FIGURE NORTH ( X 2 X 923.9 925.2 922.4 931.6 SHEET NO. , 924.6 t rn 9 -I GO PN. 1()441.05 3 ACRE LOCAL DRAINAGE AREA HOPKINS, MINNESOTA REVISIONS � DATE UGUST 1998 24" STREET N 2 4" ORTH 15" - - . . ±L _0_ - - - — _ - -923.3 SCALE: 1" = 100' 7 FILE NO. 10441.05 FIG -5.DWG 6 -10 -98 PLOT 1 -1 PN.10441.05 rieke carroll muller associates, inc. engineers architects land surveyors FIGURE 2 EXISTING TRIBUTARY DRAINAGE AREAS HOPKINS, MINNESOTA REVISIONS DATE AUGUST 1998 SHEET NO. C FILE NO. 10441.05 924.6 X 924.5 24' X 925.3 gla Jr 000/ r 01 r a: 92 5 / ifs. r :3 ! / i � x23.9 �� ! \ \ \ \ \\ \ 1 011 924.8 X 9 25.2 lo"I q 1 I, I 1 i ■ pa r '0°0 h & 1 11 YO 4 * 1 1 i - - -- WO _ _1 ._ _ _ _ - .! , "A",, 4* :,./ 7 ' A i Tr .( x a 9 22.4 r 6:. yam: ! CM rieke corral! muller associates, inc. engineers architects • land surveyors 3rd STREET X 924.8 NORTH 925.0 FIGURE 3 926.3 924.8 Xx � X 923.9 931.6 ! i s �3 926. w z 6 -- I -98 PLOT ;- DO PN.10441.)5 <<3 CONSTRUCTION AREA HOPKINS, MINNESOTA SCALE: 1" = 100' REVISIONS DATE A UGUST 1998 FILE NO. 10441.05 29. C EXHIBIT-- 4. DWG 9-2-98 PLOT -100 PN.1 04 4 1 .05 924.6 924.5 X 925.3 CI" c ri a e r k r e o i i 1 mulier associates, inc. engineers architects and surveyors 24" 3rd STREET \ 924,8 NORTH O 925.0 ,== INDICATES GARAGE FLOORS BELOW 923 923.8 FIGURE 4 92.4,8 X x X 923.9 (z3 925.2 SHEET NO. 926. 15"„X 925,1 — 15 . 2nd >k/ t 19. GARAGE FLOORS BELOW ESTIMATED 923.0 STORM WATER ELEVATION HOPKINS, MINNESOTA 0 SCALE: 1" = 100' REVISIONS DATE SEPT.1998 FILE NO. 10441.05 29.8 24" STREET 923.3 ---> 24" - 9 - NORTH 15. Working Capital Beginning Year Add From Operations Debt Service: '89 Bonds Interfund loan payback $600,000 Capital Outlay CIP Working Capital Ending Year Rate History 1989 $4.75 1991 $6.00 1992 $7.50 1993 $9.00 1998 $9.60 2000 $10.80 planned increase STORM SEWER FUND - CASH FLOW (000'S Omitted) 1999 2.0.Q 2001 2002 2QQ (717) (832) (865) (725) 410 500 500 500 500 (467) 239 60 7 347 (717) 39 243 246 238 238 60 60 60 60 0 0 0 0 312 227 62 50 (832) (865) (725) (573) Balance 12/31/2003