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4a. West Central Lotus Lake Improvement Project-Feasibility Report Update CITY OF CHANHASSEN 7700 Market Boulevard PO Box 147 Chanhassen, MN 55317 Administration Phone: 952.227.1100 Fax: 952.227.1110 Building Inspections Phone: 952.227.1180 Fax: 952.227.1190 Engineering Phone: 952.227.1160 Fax: 952.227.1170 Finance Phone: 952.227.1140 Fax: 952.227.1110 Park & Recreation Phone: 952.227.1120 Fax: 952.227.1110 Recreation Center 2310 Coulter Boulevard Phone: 952.227.1400 Fax: 952.227.1404 Planning & Natural Resources Phone: 952.227.1130 Fax: 952.227.1110 Public Works 1591 Park Road Phone: 952.227.1300 Fax: 952.227.1310 Senior Center Phone: 952.227.1125 Fax: 952.227.1110 Web Site www.ci.chanhassen.mn.us 4~ MEMORANDUM 0 ~' TO: Paul Oehme, Director of Public Works/City Engineer \j Terry Jeffery, Water Resources Coordinator -,; i/t!' November 10,2008 ( FROM: DATE: SUBJECT: West-Central Lotus Lake Improvement Project No. 08-02: a. Update Council on Feasibility Report This is an informational presentation intended to apprise Council of the findings and recommendations from the feasibility study performed by WSB & Associates, Inc. Consulting engineers from WSB & Associates will be present to discuss their procedure, findings and recommendations. Staff will also be present to address additional questions which may arise. BACKGROUND The Meadow Green Park pond failed in 2005 during largeback-to-back rain events. This resulted in flooding within the area, erosion of the outlet channel with subsequent discharge of sediments into Lotus Lake and the exposure of a sanitary sewer line. The pond was repaired but the deficiencies were not addressed. Additional capacity issues are known to exist within the storm sewer conveyance system resulting in erosion and sediment deposition on private property and surcharging of manholes. This feasibility study is intended to address volume, water quality and rate control issues within the overall Meadow Green Park Watershed (aka West-Central Lotus Lake). The consultant contract was awarded to WSB & Associates at the February 25, 2008 City Council meeting. WSB began their study of this area in the spring of 2008 after Council approved the bid award in February of2008 and has prepared a feasibility report. The study returned a list of 17 potential components to address the overall scope of the known issues and the estimated costs associated with each option. It is important to note that since approval of the feasibility report, Lotus Lake was listed as an "impaired" water for nutrients. This development lends further merit to this project. FINDINGS The findings are best viewed as three separate phases of a larger project. Phase 1 would be performed first -likely in January through May of2009. This phase could potentially include Items A-4 through B-5 (see Table 1 on Page 4 of this staff report). However, staff and the consultant are not convinced of the need for Chanhassen is a Community for life - Providing for Today and Planning for Tomorrow West-Central Lotus Lake Imp. Project November 10, 2008 Page 2 Item A-5 (Installation of a redundant pipe under Kerber Boulevard) as the updated survey data and revised model indicate that adequate capacity exists if the downstream beml is lowered (Item A-6). It is known that increased storage capacity will be needed within the watershed. Originally, the intent was to get this additional storage within the Meadow Green Park Pond located easterly in the project area. However, after evaluation, it has been determined that this is a jurisdictional wetland and any impacts would need to be mitigated for. This need for mitigation would add additional costs to the project and the excavation costs would be greater for this area because of the need for dewatering and the general muck soil conditions. Based upon these findings, upstream areas were evaluated. In particular, the area southeast of the ball fields within Meadow Green Park. This would be Phase 2 of the project. What is indicated in the feasibility report as Items A-I through A-3 (see Table 2 on Page 5 of this staff report) is one possible scenario for increasing storage capacity, providing rate control and offering additional water quality treatment within the watershed. This may not necessarily be the only option or preferred option. Phase 3 involves the stabilization of the outlet channel to Lotus Lake. This would occur only after the causes of the erosion within the channel are addressed through the actions of the first two phases. This phase would include Items C-l through C-3 of the feasibility report as shown in Table 3 on Page 5 of this staff report. g:\eng\public\_2008 projects\08-02 west-central lotus lake improvements\staffreporta lllO08.doc . ".' s I- U W ., o II: Q. W l- II Z ~ W ...J ! CI en w Z ::J > - I- 0 en o II: ~ ...J .. I ! Q. .J _ ~ W II: ! .J ... W ~ Z I- en w en en U >- 0 I- U1 Q. en II: w w :; ~ ~ ! II: o I- U1 ~ ~ 1inch~ss04001M West-Central Lotus Lake Imp. Project November 10,2008 Page 3 Phase 1 would address many of the issues surrounding the culvert crossing under Kerber Boulevard as well as the channel which flows to the Meadow Green Park Wetland and the wetland itself. The storm sewer manhole east of Kerber Boulevard surcharges. This is a condition where water pressure builds up at some point within the system due to inadequate capacity. This has resulted in water coming up through the manhole and, in at least one instance, removing the casting from the structure. Additionally, the backyards adjacent to the stream have experienced erosion and washing out of the backyards. Options A-4 through A-7 are intended to address these issues and provide rate control of runoff into the Meadow Green Park Wetland. The reduction of the height of the berm east of Kerber Boulevard should address the backflow conditions under Kerber Boulevard. It is because of this finding that Item A-5 is likely not necessary. In order to accomplish the same goal which the berm was intended to fulfill, a series of rock veins (ditch checks) will be placed in the channel. This will slow the water through the channel and provide an opportunity for sediment to settle out of suspension providing downstream water quality benefit. Finally, Phase 1 will involve corrections within the pond which failed during the 2005 rain event. The current inadequate outlets will be replaced and the berm will be reconstructed so that it is less likely to fail during large rain events. Phase 2 is intended to look for additional water treatment opportunities upstream as well as additional storage and opportunities to decrease the rate at which runoff enters the system. The current feasibility report looks at expanding the existing wetland area immediately west of Kerber Boulevard. While Phase 1 is relatively static, there is flexibility in how Phase 2 may be accomplished. Staff will continue to evaluate this area for other opportunities which may be more cost effective. This may involve construction of a pond south of the wetland, the installation of alternative stormwater management techniques along the southern park boundary, increasing the bounce within the wetland or some combination of these techniques. The goal will be to achieve the same net benefit but with less excavation - the largest expense associated with Phase 2. Phase 3 involves the stabilization of the channel which outlets to Lotus Lake. The presence of erosion throughout the channel speaks to the importance of this phase. However, it would not be prudent to mitigate the channel erosion without addressing the upstream flow conditions which have led to the erosive conditions. Other activities appurtenant to the overall project include inflow and infiltration issues. Currently, sanitary sewer flows under or immediately adjacent to the Meadow Green Park Wetland. Further, as alluded to earlier, the forcemain that runs within the outlet channel has been exposed in the past. This area is not scheduled for 1&1 inspection in the immediate future but given the scope of work, it may be beneficial to investigate the sanitary sewer at this time. g:\eng\public"-2008 projects\08-02 west-central lotus lake improvements\staffreporta 111008.doc West-Central Lotus Lake Imp. Project November 10, 2008 Page 4 SUMMARY OF COSTS The following tables indicate a breakdown of the opinion of cost for each option and each phase. The overall project is anticipated to occurin three phases with each phase occurring at one- to two-year intervals depending upon the funding available. Given the recent report by Ehlers, it may be necessary to push the time frame out even further into the future unless outside funding sources can be procured. It is important to note that cost associated with each option is estimated as though it is a standalone item. It is reasonable to assume that when bid as a whole project, costs will be reduced 10% to 15%. 2008 I 2009 IMPROVEMENT PROJECT DEBRIS CATCHER STRUCTURE UPSTREAM OF KERBER BOULEVARD LOWER EXISTING BERM EAST OF KERBER CONSTRUCT CHECK DAMS (3) EAST OF KERBER RECONSTRUCT POND OUTLET TO NORTH RAISE AND STABILIZE EXISTING MEADOW GREEN PARK WETLAND BERM; RECONSTRUCT EMERGENCY OVERFLOW REMOVE EXISTING WETLAND OUTLET RECONSTRUCT OUTLET CONTROL STRUCTURE EXTEND STORM SEWER OUTFALL TO LOW POINT TOTAL ESTIMATED COST Table 1. Phase 1 Opinion of Cost A-4 A-6 A-7 B-1 B-2 B-3 B-4 B-5 $29,093 $17,658 $97,335 $44,186 $46,467 $10,800 $129,128 $23,321 $397,987 The table below indicates construction in 2010 carrying over into 2011. This is based upon the consulting engineer's feasibility report which would indicate that this is the third phase proposed. This is one option which has been proposed. Staff would like to explore other options which may result in less excavation and less cost to see if it would be feasible from a cost-benefit perspective. Staff is of the opinion that this would be Phase 2 of the construction given that stabilization of the channel would best be left until after the source has been remedied. However, the years are representative of when this project might occur. g:\eng\public\_2008 projects\08-02 west-central lotus lake improvements\staff reporta III 008.doc West-Central Lotus Lake Imp. Project November 10, 2008 Page 5 2010 I 2011 IMPROVEMENT PROJECT RECONSTRUCT POND 10.11 OUTLET EXPAND POND 10.11 PROVIDE WATER QUALITY VOLUME IN POND 10.11 TOTAL ESTIMATED COST Table 2. Phase 2 Opinion of Cost $75,128 $533,318 $157,950 $766,395 A-1 A-2 A-3 Once upstream conditions are addressed, the symptom of the volume and discharge issue should be addressed. This is primarily the stabilization of the outlet channel to Lotus Lake. It was erosion within this channel from upstream discharge which resulted in the force main being exposed and the sediment deposition into Lotus Lake. 2012 I 2013 IMPROVEMENT PROJECT STABILIZE STREAM BANK & ARMOUR TOE OF STREAM EMBANKMENT; CONSTRUCT ROCK VANES/CHECK DAMS SUSTAINABLE LANDSCAPING NEAR LOTUS LAKE TOTAL ESTIMATED COST Table 3. Phase 3 Opinion of Cost C-1 & C-2 C-3 $496,800 $37,800 $507,600 CONCLUSIONS AND RECOMMENDATIONS It is staff s goal to finalize the details of Phase 1 and begin to prepare plans to submit for bid. Even if the City does not proceed to Phase 2, staff feels that the benefits of Phase 1 are immediate and warrant the expense. Previous attempts to gamer financial cooperation from partner agencies have not met with success. This appears to be a result of having priorities that were not coincidental with one another. The City will again request that the Riley-Purgatory-Bluff Creek Watershed District partner with the City on this project. It is reasonable, based upon past discussions, to conclude that this partnership may not occur. As such, the City must consider whether to proceed with this project alone and how to make the most efficient use of available Surface Water Management funds. g:\eng\public'-2008 projects\08-02 west-central lotus lake improvements\staffreporta II I008.doc West-Central Lotus Lake Imp. Project November 10,2008 Page 6 If the subsequent phases are postponed into the future, it is conceivable that the Total Maximum Daily Load (TMDL) study for Lotus Lake may be performed. If a TMDL study is performed, this project may qualify for Clean Water Legacy Act grant funds which may help defray future costs. Attachment g:\eng\public'-2008 projects\08-02 west-central lotus lake improvements\staff reporta III 008.doc :i. n :r CD .0 C OJ iil .... o o en ~ 11'1 .. ~ l ." ~ ~ N _ WEST CENTRAL LOTUS LAKE IMPROVEMENT PRO.JECT OVERALL AREA OF INTEREST ~.z v [Il " ."...", c::...:!!b $'- , ,,',: :' _,-""I '.,,~ .... -- S" 111~ (") ::T 2 c IlL rJ) g) f f f <D '" '" - :Po WEST CENTRAL LOTUS LAKE IMPROVEMENT PRO.JECT PHASE 1 AREA OF INTEREST ~ if..-Z :T1 =i" Iii; n ~ 2 <:: Ill. Ul g: ! ~ ~ a; '" " - ~ WEST CENTRAL LOTUS LAKE IMPROVEMENT PRO.JECT PHASE 2 AR EA OF INTEREST ~ ~.-z :T1 ::i" Iii; " ::::r ~ c m. lSl ~ ~ [ [ (i) '" '" - ~ WEST CENTRAL LOTUS LAKE IMPROVEMENT PROJECT PHASE 3 AREA OF INTEREST ~ if...Z :t1