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I I I I I I .' I I I I I I I I I I I I Massey Draw and SJCD (South) Major Drainageway Piannlng Update Conceptual Design Report February 2006 City of Littleton 6.0 Conceptual Design of Master Plan 6.2 Master Plan Description 6.1 Master Plan Overview Table 7 presents a summary of the Master Plan recommendations, costs, benefits, and priority, Figure 2 shows the Master Plan Recommended Improvements, Although the majority of improvements represent those described in the Selected Plan, more detailed examination during the conceptual design process and additional information from project sponsors led to variances between the improvements recommended in the Selected Plan and the Master Plan. The Notice to Proceed and Selected Plan for Massey Draw and SJCD (South) were issued on April 11 , 2005, and are contained in Appendix D. The Selected Plan essentially followed the recommendations made in the Alternatives Evaluation Report for Massey Draw and SJCD (South) Major Drainageway Planning Update, dated February 2005 (Reference 14). Specific direction was given to Massev Draw . Reexamine the need for a low flow channel downstream of the diversion levee on Massey Draw intended to divert the 100-year flows to the south side of West Rockland Drive, Reach 1 - Chatfield Reservoir to C-470: Upstream of the park access road, low flow check structures are proposed to control headcutting. The structures would have 3-feet maximum drop, with a predicted 0.4% slope between the structures. The existing culverts at the park access road are proposed to be replaced with a double cell 10-foot x 10-foot reinforced concrete box culvert to reduce the 1 OO-year overtopping to 0.4 feet. Reach 2 - C-470 to Wadsworth: A 10-foot x 10-foot reinforced concrete box culvert is proposed to be installed parallel to the existing structure at Wadsworth. The culvert is intended to reduce the headwater depth and eliminate flooding of three houses. . Comply with the minimum FEMA levee requirements for structural integrity and freeboard (Le" minimum of 3-feet), . Add or enlarge the low flow channel, as needed, so as to not increase the floodplain or floodway elevations, that exists without the levee, with the added flows. The Master Plan takes into account four existing regional detention basins in the study area, and includes construction of a new regional detention basin on the North Tributary of Massey Draw upstream of Carr Street. Inadvertent detention that may occur throughout the study area was not taken into account. Specific onsite detention and BMP requirements were not identified that would supercede those of the local jurisdictions, Reach 3 - Wadsworth to Carr: Improvements were evaluated to reduce or eliminate overtopping at Carr Street and to convey the 100-year storm. Due to the proximity of houses to the channel, there is not enough room to construct a 1 OO-year channel downstream of Carr Street without removing houses. No improvements are proposed at Carr Street, but channel improvements are proposed between Carr Street and Wadsworth Boulevard. Between Stations 66+10 and 73+50, the channel is proposed to be realigned and reshaped with a bottom width of 20 feet and 3:1 (horizontal to vertical) side slopes. The channel would be lined with double-stacked grouted boulders. A 5-foot high sloping boulder drop structure is proposed in this segment. The channel between Stations 51+70 and 61+10 would also be realigned and reshaped with a bottom width of 20 feet, 3:1 right side slopes and 4:1 left side slopes. The channel would be lined with double-stacked grouted boulders and would contain a 4-foot high sloping boulder drop structure. The Master Plan improvements are intended to provide a reasonable level of protection while adhering to the Jefferson County drainage criteria, where possible, Because development has occurred along most reaches, the land requirement to construct improvements that fully conform to the criteria was considered to be excessive. Channel stabilization is recommended along most of the reaches. The stabilization will help to minimize future channel erosion, improve public safety, protect utilities and improve water quality. Culvert improvements are recommended for arterial streets to eliminate overtopping and maintain general and emergency access. On minor streets, culvert improvements are recommended if structure flooding can be eliminated or if the overtopping can be significantly reduced, At some locations, culvert improvements for minor street crossings are not recommended even though the overtopping depth exceeds the Jefferson County's drainage criteria because alternate access routes are available and the minor reduction in overtopping depth did not justify the cost of improvements. Regional detention is recommended on the North Tributary of Massey Draw, upstream of Carr Street. This parcel of land is currently undeveloped and is in the floodplain, The flow reductions that will result from the detention basin will reduce the street overtopping at Allison Street, eliminate downstream structure flooding and will reduce stream erosion. Low flow check structures are proposed to control headcutting, The structures would have a 3-foot maximum drop, with a predicted 0.40% slope between the structures, Acquisition of a drainage easement is recommended. Reach 4 - Carr to Garrison: Low flow check structures are proposed to control headcutting, if it becomes a problem, The structures would have a 3-foot maximum drop, with a predicted 0,4% slope between the structures. Select channel improvements are recommended to improve capacity and/or to eliminate vertical banks. In the Meadow Ranch area, a levee is recommended to divert the discharges in Massey Draw to the south side of West Rockland Drive, eliminating structure flooding on the north side of West Rockland Drive. In some locations, using the 2-foot contour interval project mapping, it appears that structures are located in the floodplain. It is recommended that elevation certificates be prepared for structures to determine whether they are located in the 1 OO-year floodplain. Reach 5 - Garrison to Kiplinq: The existing culvert at Garrison Street is proposed to be removed and replaced with a double cell 1 O-foot x 1 O-foot reinforced concrete box culvert. The culvert improvements will reduce street overtopping to 1.0 foot during the 100-year storm, and will eliminate the 1 OO-year overflow into the C-470 right-of-way. Low flow check structures, an 8-foot high sloping drop structure and a 5-foot high sloping drop structure are proposed to control headcutting. The check structures would have a 3-foot maximum drop, with a predicted 0.4% slope between the structures. Between Stations 125+90 and 135+20, both channel banks are proposed to be regraded to 4: 1 (horizontal to vertical) side slopes. A maintenance access and recreation trail is proposed on the north side of Massey Draw along this segment. ~ 6-1