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HomeMy WebLinkAbout4 - Handout - Matters from the Board MEETING SUMMARY CH211VIIHILL South Boulder Creek Flood Mitigation Study Workshop #3 Notes ATTENDEES. Kurt Bauer/City of Boulder Mark Glidden/CH2M HILL Bob Harberg/City of Boulder Alan Turner/CH2M HILL Annie Noble/City of Boulder Aaron Cook/CH2M HILL Shea Thomas/UDFCD Alan Taylor/Alan Taylor Chuck Howe/Boulder Water Consulting Resources Ad. Board FROM: CH2M Hill DATE: May 11, 2010 PROJECT NUMBER: 40016$ Review Progress to Date Mark Glidden from CH2M Hill presented a brief overview of the progress to date. Work to date includes identification of the sources of the problems, development of a suite of potential solutions, development of alternative plans, determination of design discharge, alignment refinement, cost detennh-Cation, and benefit estimation. See the Workshop No 3 Review Packet for a complete summary of progress to date. Recommended Best Alternative Plans High Hazard/Critical Facilities Protection The High Hazard and Critical ]facilities Protection alternative includes flood proofing of all critical facilities within the West Valley and re-grading two properties to remove two structures from the high hazard zone. The two structures in the high hazard zone are located at the location where flow overtops US 36 into the West Valley, west of Foothills Parkway. The project team evaluated using conveyance improvements and purchasing the two structures to remove them from the high hazard zone but re-grading the properties was the least costly alternative, Critical facilities include schools, senior housing, and any building with hazardous materials. The majority of the critical facilities in the West Valley are located north of Arapahoe Avenue. No structures along Gaptor Road will be flood proofed or removed from the high hazard zone because Gaptor Road is not included in the study area or scope of work. The majority of the residents along Gaptor Road are going to be annexed into the City of Boulder but do not want improvements to the South Boulder Creek mainstem adjacent to their properties. The residents of Gaptor Road are aware of the risks associated with South Boulder Creek flooding and do not want any changes made. WORKSHOP_3_NOTES AARON.DOC 1 COPYRIGHT 2010 BY CH2M HILL, INC. - COMPANY CONFIDENTIAL SOUTH BOULDER CREEK FLOOD MITIGATION STUDY WORKSHOP #3 NOTES The cost of the High Hazard/ Critical Facilities Protection alternative is approximately $10,682,000 but does not include any costs for non-structural improvements such as a flood warning system. CU Storage with Local West Valley Improvements The detention pond located directly south of US 36 is located partially on CU property and partially on City of Boulder open space. The pond is approximately 750 acre-ft and prevents overtopping of US 36 into the West Valley. Without any grading to the pond area, the surface area of the pond is approximately 115 acres. A berm approximately 8 feet high is required constructed along the west and north edges of the pond to make this detention pond configuration work. The pond must be 750 acre-ft due to the timing of the hydrology and the dead storage at US 36. In the regulatory MIKE FLOOD model, 300 acre-ft overtops US 36 into the West Valley, so 300 acre-ft, in addition to the dead storage at US 36, must be stored. The 750 acre-ft also represents the existing conditions without adding or removing any culverts through US 36. If all of the flow is forced back into the South Boulder Creek mairistem bridge at US 36, a total of 1150 acre-ft of storage is required to prevent overtopping of US 36.. If an additional 6' x 4' concrete box culvert (CBC) is placed adjacent to the existing 6' x 4' CBC at Dry Creek Ditch No. 2, a total of 300 acre-ft of storage is required to prevent overtopping of US 36. The project team discussed the possibility of tlis detention pond configuration with Jeff Lipton from CU. Jeff Lipton suggested that excavation and a tiered approach be used to reduce the surface area of the pond and reduce the impact to the CU property. CH2M Hill will look into re-grading the detention pond to include excavation of the CU property. Re- grading the detention pond will allow CU the same building footprint as what is shown in their Master Plan for this property. The Master Plan currently shows 82 acres dedicated to flood storage. The downside to re-grading the detention pond to include excavation is that the pond at Table Mesa does not have a lot of capacity and flow reaching this pond will quickly overtop US 36. Representatives from the City of Boulder will be in contact with CU to determine what concessions will be required in exchange for this detention pond. The local West Valley Improvements will be common among many of the alternatives in which overtopping of US 36 is prevented. The best alternative for local improvements in the West Valley is improving the Dry Creek Ditch No. 2 alignment with local storage. The diversion to Bear Canyon Creek and the diversion down the West Valley profile were much more costly alternatives, due to extra pipe costs, and were removed from consideration. The first element is a 48" RCP to collect local inflows from US 36 to Baseline Road, west of Foothills Parkway followed by 9 acre-ft detention pond at the Baseline Road Foothills Parkway Intersection. The 9 acre-ft detention pond reduces the peak flow from 150 cfs to 50 cfs and eliminates the need to modify the existing concrete lined channel parallel to Baseline Road which has a capacity of approximately 50 cfs. Along Dry Creek Ditch No, 2, a 78" RCP from South Boulder Road to Illini Way and an upsized open channel from Illini Way to Tenino Avenue are required to convey flows in Dry Creek Ditch No. 2. Improvements along Dry Creek Ditch No. 2 from South Boulder Road to Tenino Ave are not recommended as part of the City of Boulder Stormwater Master Plan. The local West Valley Improvements also include a 25 acre-ft detention pond at WORKSHOP-3-NOTES AARON. DOC 2 COPYRIGHT 2010 BY CH2M HILL, INC. • COMPANY CONFIDENTIAL I SOUTH BOULDER CREEK FLOOD MITIGATION STUDY WORKSHOP #3 NOTES Manhattan Middle School. The detention pond at Manhattan Middle School discharges to the existing 48" RCP at along Dry Creek Ditch No. 2 and eliminates the need to upsize that pipe. With the two detention ponds upstream of Baseline Road, no improvements are required along Dry Creek Ditch No. 2 from Baseline Road to Arapahoe Avenue. At Arapahoe Avenue, a 58 acre-ft detention pond/ collection system is recommended to collect and store flows from along Dry Creek Ditch No. 2 and flows spilling across Flatirons Golf Course from South Boulder Creek. To create the storage at Arapahoe Avenue, a 3' barrier will be constructed along Arapahoe Avenue to separate vehicle traffic from bike traffic while also providing additional storage capacity. The outlet to the detention pond is a 12'x 10' CBC to an existing channel downstream of Arapahoe Avenue. The CBC also serves as a bike crossing. Currently, the City of Boulder Parks Department is updating the Master Plan for the Flatirons Golf Course and are on board with this improvement. This detention pond is included in all alternatives because each alternative has a need to collect flows at Arapahoe Avenue and it removes structures below Arapahoe Avenue from the floodplain, providing a large damage reduction, and protects critical facilities. A drainage easement must be purchased for the existing open channel directly downstream of Arapahoe Avenue but no other improvements are necessary downstream. Improvements to the Wellman Canal are recommended for all alternatives. These improvements allow the Wellman Canal to convey the decreed flows. A 36" RCP is recommended along the Wellman Canal through Foothills Parkway to prevent additional flows from Bear Canyon Creek to be conveyed through the Wellman Canal during flooding events. The total cost for this alternative is $30,058,000 although the costs of the concessions to CU are unknown at this point. Storage South of CU with Local West Valley Improvements The detention pond south of CU is located entirely on City of Boulder Open Space. The detention pond is 400 acre-ft and has large. outlet pipes so that low flows are conveyed through the pond and only the peak flows are stored within the pond. This pond has to be greater than the 300 acre-ft that overtop US 36 into the West Valley because it is located higher in the watershed. Kurt Bauer and Annie Noble met with Don D'Amico from City of Boulder Open Space to discuss this detention pond alternative. Don said that no excavation can occur and that a berm cannot be constructed on this site, The site is a wetland mitigation site and is being purchased by CDOT. During the meeting, Don suggested an alternative detention site located adjacent to Marshall Road. The site adjacent to Marshall Road is not on City of Boulder Open Space but is a smaller site than the open space site. There would be no limits to excavation or building a berm on this site although the site would have to be purchased. If this alternative continues forward, wetlands can be incorporated into the detention pond. Representatives from the City of Boulder are planning on meeting with Don again next week and will report to the rest of the project team. The local improvements in the West Valley are the same as the improvements recommended in the CU storage plan. The total cost for this alternative is $28,220,000. The WORKSHOP 3_NOTES AARON.DOC 3 COPYRIGHT 2010 BY CH2M HILL, INC. • COMPANY CONFIDENTIAL SOUTH BOULDER CREEK FLOOD MITIGATION STUDY WORKSHOP #t3 NOTES total cost assumes the cost to purchase the open space land to construct a detention pond is $36,000 per acre as was given in the Urban Drainage and Flood Control District Master Planning cost estimating spreadsheet. Mainstem Flow Containment with Local West Valley Improvements This alternative forces all flow above US 36 back to the South Boulder Creek mainstem by making improvements to the CU levee and extending the levee along US 36. It is likely that this levee will become a jurisdictional levee due to the height, volume stored upstream of the levee, or surface area of the storage upstream. From a comparison of the regulatory MIKE FLOOD model and the levee MIKE FLOOD model, the levee forces approximately an additional 200 acre--ft of volume and an increase in peak flow of approximately 500 cfs to the mainstem. Without any improvements along the mainstem, the levee increases the depth of flow along the mainstem. by less than 0.5 meters in most locations. Other improvements along the South Boulder Creek mainstem are also required to mitigate the additional flow that is being forced to the mainstem. The first improvement is additional storage in Baseline Reservoir. It is unknown at this time what the additional capacity of the reservoir is, the price of storage in the reservoir, or what other concessions would have to be made to store stormwater flows in the reservoir. For comparison, Pine Brook Hills paid $50,000 per acre-ft of permanent storage. The other improvement recommended along the mainstem is 2 - 8' x 5' CBCs through Baseline Road to allow some of the water ponding behind Baseline Road into open space land downstream of Baseline Road, reducing the flow in the mainstem. The improvements in the West Valley are the same as those recommended in the detention alternatives with one exception. The detention pond at Arapahoe Avenue would need a secondary outlet, an 8' x 8' CBC with a new open channel back to South Boulder Creek. This secondary outlet is required because, unlike the detention alternatives where flow was being stored upstream, this alternative forces additional flow to the detention pond at Arapahoe Avenue. This alternative was mentioned as a possible alternative at the first public meeting. According to UDFCD, levees are acceptable as long as the prevent flooding, but detention above US 36 would be better. The total cost for this alternative is likely greater than either detention option but does provide a possible alternative should the upstream detention options be eliminated due to constraints. No Storage above US 36, Diversion to Bear Canyon Creek This alternative includes no detention upstream of US 36 and large pipelines in the West Valley. In the West Valley there are two primary flow paths, one to Bear Canyon Creek and the other along Dry Creek Ditch No. 2. No local detention is recommended because the cost for local detention is greater than the decrease in cost due to decreasing the pipe size with local detention in place. Bath the diversion to Bear Canyon Creek and improvements to Dry Creek Ditch No. 2 are still being considered because the cost of each alternative is similar. The first element of this alternative is a 114" RCP extending from US 36 to Baseline Road, west of Foothills Parkway. The 114" RCP crossing the Foothills Parkway - Baseline Road intersection and continues parallel to Foothills Parkway to Bear Canyon Creek. WCRKSHOP 3_NOTESAARON.DOC 4 COPYRIGHT 2010 BY CH2M HILL, INC. - COMPANY CONFIDENTIAL I SOUTH BOULDER CREEK FLOOD MITIGATION STUDY WORKSHOP #3 NOTES An 8' x 5' CBC is required along the New Anderson Ditch to prevent overtopping into the West Valley and Dry Creek Ditch No. 2. Along Dry Creek Ditch No. 2, all pipes must be upsized to 90" RCPs, and open channels must have their capacity increased to convey the 100-year regulatory flow. With large pipes, right-of-way constraints may become an issue. The section of open channel through the golf course will not be upgraded. This alternative also includes improvements to the Wellman Canal. Similar to the other alternatives, detention/ collection at Arapahoe Avenue must occur and the detention pond must have two outlets. The first outlet is a 12' x 10' CBC to an existing open channel downstream of Arapahoe Avenue and the second, an 8' x 8' CBC, to a new open channel back to South Boulder Creek. A drainage easement must be purchased for the existing open channel downstream of Arapahoe Avenue. No other improvements are necessary downstream of Arapahoe Avenue. The total cost for this alternative is $36,533,000, higher than the detention alternatives but a low enough number to still provide a positive benefit to cost ratio. No Storage above US 36, Improvements to Dry Creek Ditch No. 2 This alternative does not include any detention upstream of US 36 and conveyance in the West Valley must convey the 100-year regulatory flows. Siinilar to the previous alternative, this alternative does not include local detention in the West Valley. This alternative includes a 114" RCP from US 36 to Baseline Road in the West Valley and a 132" RCP to replace the existing concrete lined channel parallel to Baseline Road. Along Dry Creek Ditch No. 2, pipes are to be replaced with 90" RCPs upstream of Baseline Road and with a 132" RCP from Baseline Road to the Golf Course. There are no improvements recommended to Dry Creek Ditch No. 2 in the Golf Course. Similar to the other alternatives, a detention pond/collection at Arapahoe Avenue is required to reduce the peak flow and collect flows. Two outlets to the pond are required. The first is a 12' x 10' CBC to the existing channel north of Arapahoe Avenue. The 12' x 10' CBC also serves as a bike crossing. The second outlet to the pond is an 8' x 8' CBC to a new open channol to convey flows back to South Boulder Creek. A drainage easement is required for the new open channel north of Arapahoe Avenue. This alternative also includes improvements to the Welhnan Canal. The total cost for this alternative is $36,092,000. No Storage above US 36, Stormwater Master Plan Improvements This alternative does not convey the 100-year peak flows from the regulatory model but does solve the flooding issues associated with the 5--year peak flows from the City of Boulder Stormwater Master Plan. There is no detention associated with the alternative, upstream of US 36 or local detention in the West Valley. For local inflows, a 48" RCP is recommended from US 36 to Baseline Road in the West Valley. No improvements are necessary for the existing concrete lined channel parallel to Baseline Road or along Dry Creek Ditch No. 2 from South Boulder Road to Arapahoe Avenue. For local inflows from Baseline Road to the Wellman, a 36" RCP is necessary along Foothills Parkway. A 36" RCP is recon-unended along 55th Street from approximately Tenino Avenue to Baseline Road and another 36" RCP is recommended west of 55th Street WORKSHOP _3_NOTES AARON,DOC 5 COPYRIGHT 201013Y CH2M HILL, INC. - COMPANY CONFIDENTIAL SOUTH BOULDER CREEK FLOOD MITIGATION STUDY VORKSHOP #3 NOTES connecting to the Wellman Canal. Downstream of Arapahoe Avenue, a 48" x 30" RCP is needed along Dry Creek Ditch No. 2 to the railroad and a 54" x 36" RCP is needed from Arapahoe Avenue to South Boulder Creek west of 55t1, Street. The total cost for this alternative is $13,004,000. Damage Estimates CH2M Hill has reproduced the damage estimates from HDR with a few modifications. It appears that HDR double counted approximately 30 properties in the West Valley so the CH2M Hill damage estimates in the West Valley are less than those produced by HDR. The following is a summary of the HDR approach to damage estimates and the FEMA BCA. HDR Approach • Used Hazus Curves • Depth/ damage relationship • Stepwise function • Assessed values FEMA BCA • Possible change to Hazus Curves • Uses curve rather than stepwise function s • Uses flow rate versus damage relationship • Replacement values rather than assess values Because of the difference in approach, the FEMA BCA and the approach used by HDR may lead to differing results. CH2M Hill suggests the HDR approach with a few modifications. • Use updated Hazus Curves • Use replacement value The project team must speak with representatives from the State to determine if this approach will work because the State has stated previously that, in order to get grant money, the City of Boulder must use the FEMA BCA approach. Next Steps • Water Resources Advisory Board Meeting - June 21, 2010 • Second Public Meeting July, 2010 • No conference call on Wednesday, May 12, 2010 Action Items • CH2M Hill will run the best alternative plans in MIKE FLOOD on the 8 meter grid. WORKSHOP 3_NOTESAARON.DCC 6 COPYRIGHT 2010 BY CH2M HILL, INC. -COMPANY CONFIDENTIAL SOUTH BOULDER CREEK FLOOD MITIGATION STUDY WORKSHOP #3 NOTES • CH2M Hill will determine the residual floodplain for each alternative input into MIKE FLOOD using the 8 meter grid. Using the results of the residual floodplain, the damage associated with each property will be determined and a benefit to cost ratio for each alternative can be established. • CH2M Hill will evaluate excavation at the CU detention site • CH2M Hill will evaluate the possibility of additional storage in Baseline Reservoir • CH2M Hill will evaluate how the stormwater master plan improvements impact critical facilities and facilities within the high hazard zone • CH2M Hill will provide the City of Boulder with a figure for detention at Manhattan Middle School. The City of Boulder will discuss this alternative with representatives from Manhattan Middle School. • CH2M Hill will send a copy of the PowerPoint presentation to the project team • CH2M Hill will send a link to the FEMA BCA software • CH2M Hill is working on sections of the report WORKSHOP-3-NOTES AARON.DOC 7 COPYRIGHT 2010 BY CH2M HILL, INC. • COMPANY CONFIDENTIAL