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Home My WebLink About2C, Townhouse Villages Drainage Issues EN HILLS Request for Council Action 2C i Prepared By: Terry Maurer,Public Works Work Session Date: April 16, 2012 Director Townhouse Villages Drainage Issues Budgeted Amount: Actual Amount: Funding Source: N/A N/A N/A Discussion Topic: The potential of implementing one of the scenarios discussed in the Bolton and Menk report and how it might be financed. �I Supporting Documents: 1. Memorandum from Terry Maurer, Public Works Director. 2. Drainage Report from Bolton and Menk. ,.It ,-ARZEN HILLS MEMORANDUM DATE: April 16, 2012 TO: Honorable Mayor and City Councilmembers Patrick Klaers, City Administrator FROM: Terry Maurer, Public Works Director SUBJECT: Townhouse Villages Drainage Issues Backl4round After the July 16, 2011 rainstorm the City Council hired Bolton and Menk, Inc., to analyze the existing drainage system in the Townhouse Villages and Townhouse Villages North areas to determine the capacity of the system and if there are improvements that could be undertaken to increase the capacity and provide more protection to the area in the event of a large quick rainfall event. A copy of the final Bolton and Menk report is attached. The drainage report's findings were discussed with the City Council at the October 10, 2011 Work Session and the final report was accepted by the City Council on November 14, 2011. After Council acceptance of the final report, I presented the Bolton and Menk, Inc., findings to the Townhouse Villages Board in late 2011. After discussing possible improvements to the drainage system amongst the Board, Jane Larson, Association Board President, would like to discuss with the Council the potential of implementing one of the scenarios discussed in the Bolton and Menk report and how it might be financed. Jane Larson and possibly other Board members will be in attendance at the Work Session. City Council Meeting PAAdmin\Council\Agendas&Packet Information\2012\04-16.12 Wk Sess\Agenda Page l of I DRAINAGE REPORT for Townhouse Villages North and Townhouse Villages at Arden Hills October 2011 City of Arden Hills, MN BMI PROJECT NO. T16.103924 CERTIFICATION SHEET I hereby certify that this report was prepared by me or under my direct supervision, and that I am a duly Licensed Professional Engineer under the laws of the State of Minnesota. La L. Le' y Date: October 31, 2011 Lic. No. 20846 Introduction The Townhouse Villages North and Townhouse Villages at Arden Hills experienced a significant flood event in July of 2011. This incident caused street, yard and home flooding, along with damage to some vehicles. Bolton & Menk, Inc. was retained to investigate the storm sewer drainage system to determine if feasible solutions could be made to the system that would provide relief from future heavy rainfall events. Background The watershed area that drains into the storm sewer system analyzed consists of approximately 46 acres (see Figure 1). Land use in the watershed consists of single family residential dwellings (townhomes),open space and some natural woodland. The storm sewer system contains reinforced concrete pipe(RCP)ranging in size between 12"and 27" in diameter. The drainage outlet for the main storm sewer system empties into in a pond located near the southwest corner of the property. On July 18, 2011 the Minnesota Climatology Working Group website reports that the general area near the Townhouse Villages received 5.45 inches of rainfall. Localized flooding was reported at two road low points and adjacent townhouses. These two areas are identified as the "northern low point" and the"southern low point" on Figure 1. To complicate matters, the 2011 Pavement Management Program was being constructed in this area when the storm event occurred. As a result, the Contractor had inlet protection installed in the street catch basins to catch sediment from discharging into the downstream water bodies, but also restrict the flow of water into the storm sewer castings. Methodology Using Autodesk®Storm and Sanitary Analysis(SSA),the drainage area was modeled to evaluate impacts associated with various storm events. A theoretical 100-year, 24-hour storm event was used to model the options that were looked at. The 100-year event is defined as that event likely to occur on the average once every 100-years. This is not to say that a 100-year rainfall cannot occur in consecutive years or even twice in the same year. In reality, the 100-year storm has a 1% chance of occurring in any given year. The 100-year,24-hour storm is defined as a 5.9-inch event for Ramsey County. Existing Condition Analysis In July of 2011,the two land-locked low points in the development flooded due to what appeared to be an approximately 100-year storm event. The model results show that the existing drainage system has the capacity to handle a 10-year storm event without surcharging. The results also showed that the existing drainage system has the capacity to handle a 100-year storm event with minimal,localized flooding at the low points, which would not cause flooding into adjacent homes. When the model was run with a 100-year, 24-hour storm event, assuming no restrictions on the inlets or pipes,the high water levels(HWL)for each low point was: Northern Low Point(on Arden View Drive) =973.35 Southern Low Point(rectangular grate east of Arden View Ct)=965.15 There is no emergency overflow to provide relief if the catch basin grates or storm sewers are plugged, or if water in the pipe surcharges and comes back out of the catch basins. When either one of these conditions occur there is potential that the low points will flood. Water will not escape overland from the north low point until it reaches an elevation of approximately 980. At this point, from reviewing the survey, if water reached an elevation of 976.7,it would enter the window sills of the adjacent townhomes. Water will not escape overland from the south low point until it reaches an elevation of approximately 968.5. At this point, the survey shows that if water reached an elevation of 966.6, it would enter the garages/doors of the adjacent townhomes adjacent to this low point. Options An on-site field visit and a review of existing topography maps and construction drawings were used to develop several drainage options. The storm sewer map for this project area shows that approximately 75% of the northern portion of the site flows into the southwest corner pond through a single run of storm sewer pipe. The improvement options considered were based upon the location of where the flooding occurred and what appeared to be potential solutions based upon a visual review of the existing drainage system. The following options were modeled and their associated costs calculated: Option 1 (Figure 2) This option involves building a berm around the southwest corner of Pond 2 and reconstructing the pond outlet. The existing overflow for Pond 2 is in the southwest corner where an existing pathway is located. Building a berm in this area and replacing the existing outlet with a new structure would allow additional storage(pond bounce)to occur,while at the same time limit the pond discharge to the downstream storm sewer system. Modeling Option 1 showed that this action would have an insignificant impact on either the north or south low points. The north low point HWL was 973.26(-0.09')and the south low point HWL was 965.08(-0.07')with this alternative. The estimated cost for Option 1 is shown in Table 1. Option 2(Figure 3) This option would provide an emergency overflow pipe for the low point area located to the southeast of the Arden View Court cul-de-sac. A pipe or a swale could be constructed that would extend from the driveways between units 4364 Arden View Court and 4324 Arden View Court. For the purposes of this analysis a pipe was considered,since this would likely be less disruptive to the existing landscape. To maximize pipe flow potential and limit head build up,a 24"arch pipe was assumed for cost estimating purposes. This was an option originally considered in the preliminary analysis. Once the survey data was obtained it was recognized that this may not be a viable alternative. Because the invert of an emergency overflow pipe at the east end of the driveway would be higher than the low garage floor elevations,this option would not serve a practical purpose. Modeling Option 2 showed that this action would have no impact on either the north or south low points. The estimated cost for Option 2 is shown in Table 1. Option 3(Figure 4) Option 3 would serve to isolate the low point catch basins on Arden View Court from the main line storm sewer. A 24"storm sewer bypass pipe would be constructed between units 4401 and 4402 of Arden View Court. Currently,the entire flow upstream of the low point catch basins in Arden View Court must make a 90-degree turn at the catch basin to continue downstream. Given the momentum and velocity of a 24"pipe flowing full,it is likely that the storm sewer pipe and grate in the driveway to the east of this low point is surcharged and backs up during heavy rainfall events, as occurred in July of 2011. It was thought that a proposed bypass pipe would help to relieve flow pressure from the low point in Arden View Court and likely reduce the flooding potential in this area. To minimize disruption to the existing landscape in this area,a new pipe was considered to be jacked. Modeling Option 3 showed insignificant improvement to either the north or south low points. The HWL at the north low point was 973.33(-0.02')and the HWL at the south low point was 964.98 (-0.17'). The estimated cost for this improvement is shown in Table 1. Option 4(Figure 5) This option involves constructing a new outlet from Pond 2 towards the west,which would outlet near Pond 5. A new outlet pipe from Pond 2 would reduce the outflow from the downstream storm sewer conveyance system. This would provide extra carrying capacity of the existing system,thus allowing for greater storm events to be handled before surcharging occurred. A directionally drilled pipe from Pond 2 towards Pond 5 would limit the overland disruption. Modeling Option 4 showed no significant improvements at either low point. The HWL at the north point was 973.29(-0.06')and the HWL at the south low point was 965.09(-0.06'). The estimated cost for this improvement is shown in Table 1. Option 5(Figure 6) This option focused on reducing the flooding in the southern low point and is similar to Option 2. The difference is that a new storm sewer outlet was simulated being constructed near the existing rectangular driveway grate,which is east of the south low point on Arden View Court. This pipe system was routed toward the southeast pond(Pond 3),where it would outlet. This scenario placed a new grate near the existing low point,which could start taking water immediately once runoff from the catchment occurred and divert some of this flow to the southeast. This scenario was modeled simulating the July,2011 storm,with the existing low point catch basins in this area essentially plugged. Constructing a new catch basin structure and a 24" outlet pipe resulted in a HWL of 966.4 at the southern low point. The existing low garage elevation is 966.63. This option would protect the low townhomes adjacent to the low point and prevent flooding from a 100-year storm event,or an event similar to that experienced in July, 2011. The estimated cost for Option 6 is shown in Table 1. Option 6(Figure 7) This option looked at what improvements to the existing storm sewer system would be necessary to keep the pipes downstream of the southern low point from surcharging during a 100-year event. Approximately 865-feet of storm sewer pipe would need to be removed and replaced from the southern low point to the last manhole,prior to outletting into Pond 4. Extensive removal of existing driveways would be required,along with removal of recent improvements that occurred in 2011 Pavement Management Program. The estimated cost for Option 6 is shown in Table 1. Estimated Costs of Options Proposed construction costs for each option are summarized in the table below. These cost estimates are based upon public construction cost information. Estimated construction costs presented in the following table include a 20 percent contingency factor. Table 1-Cost Summary. -Option 1 - Berm&New Outlet from Pond 2 $17,400 -Option 2—Overflow Pipe from SE Low Point $20,300 Option 3—Diversion Pie $75,100 -Option 4—New Outlet Pipe from Pond 2 $59,000 -Option 5—New Outlet Pipe from SE Low Point $68,900 -Option 6—Increased Storm Sewer Pipe Size $100,800 Recommendations The most cost effective solution would be Option 5. This option would provide an alternative route for storm water to discharge from the southern isolated low point, without relying on the existing storm sewer pipe system to carry runoff away from this area of concern. The existing storm drainage system in this townhome development should be inspected on a regular basis to ensure that there is no blockage of the grates or in the storm sewer pipe that would cause water to backup during storm events. There is no emergency overflow adjacent to the two low points studied. Storm events greater than the 100-year design storm will continue to pose a flooding threat to adjacent homes, even if the proposed recommendation is constructed. Ll WATERSHED BOUNDARY a � ZJ o ORT LtIQ P NT 0 Q o � 0 o PO 2 OPTION #1 z � OPTION #4 o 11 T "17 Q Z :3 0 ❑ o0 �D OPTION #3 E ❑ ND 5 OPTION #6 ❑ E❑ 0 OPTION #2 o d o0 CD POND 4 F= OUT ERN LOW PCII E Ez OPTION #5 P jv 0 ❑ C1 � aQ � �Tt- n .C:3 tiJ COLLEEN FE BOLTON aL MENK. If-4C. CITY OF ARDEN HILLS, MINNESOTA Consulting Engineers & Surveyors TOWNHOUSE DRAINAGE ANALYSIS MMKATO. MN FAIRMONT.MN SLEEP'r EYE, NN PROJECT SITE DURNSM�E, MN MILLMNI, MN CHtiSKA� MN RAJdISEY. MN MAPLEw00D, MN BRAINERD. MN AMES, M FIGURE NO. 1 M\ARON\T1610302�\sad\S'd.Ma,.d.y 10/31/11 §:�am 01'—,M-1.Iz.2011..w,-R--d OCTOBER. 201 POND 2 OUTLET ONTROL STRU-TURE �JJ JJ JJ 0 30 SCALE FEET BOLTON 8c MENK. I"C. CITY OF ARDEN HILLS, MINNESOTA Consulting Engineers & Surveyors TOWNHOUSE DRAINAGE ANALYSIS MAN ISSUILLE, MNAF WIILLMAR,MN SLEEPY M M NN OiAWA. YNN OPTION 1: RAISED BERM AND NEW OUTLET RAMSEY. MN MgUVOOU. MN BIWNEM. MN AMES. IA FIGURE N0. 2 M:\A110N\T1610].31\MJ-2R C. ..m G.g 9/15/11 7o7.,. 0..-r... i...mll..RIgM ie....w SEPTEMBER. 2011 o,. JJ QI w W J Q W Z J � Q 2 o {0 SCALE BOLTON dk MENK. INC. CITY OF ARDEN HILLS, MINNESOTA Consulting Engineers & Surve"ra TOWNHOUSE DRAINAGE ANALYSIS M9UAt�lSOLLEN,MFN��MAR. MN SLEEPY MN OPTION 2: ARDEN VIEW COURT OVERFLOW RAMSEY, MN MAP EW000, MN aRMNERD, MN Awes,IA FlGURE N0. 3 M\MIOn\T161t!]BI�\MA-3k Co---q 9/15/11 rw.m Oe.R.,�&M.. i...W11.M Righu n....�.a SEPTEMBER. 2011 Oct 0 Q D 0 P55 CO LL JJ L rr BOLTON aL MENKq I"C. CITY OF ARDEN HILLS, MINNESOTA Consulting Engineers & Surveyors TOWNHOUSE DRAINAGE ANALYSIS MANKATO, NN FAIRMONT. MN SLEEPY EYE, MN OPTION 3: STORM SEWER BYPASS BURNSNU wO LLMNt.BRNNERO, MN AMES, M RAMSEY, MN AIAPL FIGURE N0. 4 H:\Mdl\T1610J01d\wd\/�Nh-1N Centun.dvq 10/1!/11 1:15 pm e9sN.n k M.nK 1.��1011,N Rghb Rw�,Md OCTOBER, 2G1 -ZZ POND 1 PO D2 DIRE' TIONAL \� O ..................................... ....................... so FEET E3C:>L--rC:)" &a KAMP4F<, ll"C- CITY OF ARDEN HILLS, MINNESOTA Consufflng Engineers & Surveyors TOWNHOUSE DRAINAGE ANALYSIS MANV AM NN FNr. MN SLEEPY EYE. R"svur' MN AF=mum. MN CKASICA, INN OPTION #4: NEW POND OUTLET PIPE RMSEY, MN WILEW000.MN BRANERD, MN AMM A C.--d.q 9/15/11 707— c—&mal,1RC!w114 m SEPTEMBER. 2011 nGURE NO. 5 rr LL \ JJ JJ LL qN, �L G POND 0 SCALE �ET BOLTON 8. MENK. iNC. CITY OF ARDEN HILLS, MINNESOTA Consulting Engineers & Surveyors TOWNHOUSE DRAINAGE ANALYSIS MANKATO, MN FAIRMONT, MN SLEEPY EYE, MN BVRNSMLLE MN WILLMAR. MN CHASKA, MN OPTION 5: NEW OUTLET PIPE FROM SE LOW POINT RAMSEY, MN MAPLEWOOD, MN BRAINERD, MN AMES, IA OCTOBER, 201 FIGURE N0. 6 R:\�RDH\T1610]9N\ead\ANh-2f1 Cm4w.n.d.g 10/28/11 11:13 om ORoRon k Nm4,lu.2011,NI Riyhl.R.....�d PO D 2 Ll/ Li 4 0 ° 0 0 o a D o d P SIZE 1 CREASED STORM E O O 0 000 e � OO o SCALE FEET BOLTON aL MENK, I IVc. CITY OF ARDEN HILLS, MINNESOTA Consulting Engineers & Surveyors TOWNHOUSE DRAINAGE ANALYSIS MANKATO, NN FAIRMONT, MN SLEEPY EYE, MN OPTION 6: INCREASED STORM SEWER PIPE SIZE BURNSVILLE MN WILLMAR, MN CHASKA, MN RAMSEY, MN MA�LEWDOD, MN BRAINERD, MN AMES, IA FIGURE NO. 7 u\�noNV+sioaosa\..a\ebe—xn co„c d, q to/m/ii 11 13— Cml—k w.,,k,i.=.2011,ui a�gn�.a...,..a OCTOBER, 2011