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CY\ 4-2 I I I I I I I I I I I I I I I I I I I 8 City of Littleton Massey Draw and SJCD (South) Major Drainageway Planning Update Conceptual Design Report February 2006 UDFCD and the County constructed culvert improvements at Carr Street with approximately 500 lineal feet of boulder- edged, rectangular channel downstream of Carr Street. During the June 2004 storm, stream banks eroded, undermining detached buildings, and damaging fences and decks downstream of the improved reach. Eleven houses and 10 detached buildings appear to be in the 100-year floodplain. Flooding of three of the houses is due to the Wadsworth culvert headwater depth. Reach 4 - Carr to Garrison: Carr Street is estimated to be overtopped by 2.7 feet during the 100-year event. It is estimated to overtop between the 5- and 10-year storm events. Upstream of Carr Street is Wingate South Park. A sloping concrete drop structure drops the channel approximately 14 feet into a pond upstream of Carr Street. Upstream of the drop structure, the channel slope is approximately 1.0 percent. The drainageway is heavily vegetated and appears to be stable, There is an existing concrete vertical drop structure, approximately 2 feet high; about 650 upstream from the sloping drop structure. Reach 7 - C-470 to Vallev Road: Massey Draw crosses C-470 in a 10-foot by 10-foot box culvert with a side-tapered sloping inlet. The CDOT record drawings show four drop structures that are approximately 3 feet high upstream of the culvert. These structures were not found in the field. This reach is heavily vegetated. The channel has characteristics that are typical of natural drainageways in the foothills. The channel bottom has rock bottom or is naturally armored with gravel and large cobbles. The channel bottom appears to be stable. Along the lower end of the reach, the stream banks have been armored with riprap and grouted riprap. Bank erosion was observed after the June 2004 storm at the upper end of this reach. During that storm, flow was deep enough that the water flowed onto Ken Caryl Avenue. It continued to flow east in the street and flooded the interchange under C-470 to a depth of 2 to 3 feet. Reach 5 - Garrison to KiplinQ: Foothills Park and Recreation District's (FP&RD) Deer Creek Park and Pool are located at the east end of this reach, The west end of this reach is currently privately owned. However, FP&RD is negotiating to acquire the undeveloped property, The average channel slope along this reach is approximately 1.8 percent. The channel is incised 9 to 12 feet at the west end and an encased utility is exposed. Approximately 900 feet upstream of Garrison Street is an old pond embankment. There is a culvert through the embankment and a culvert south of the embankment, through which the low flows in the drainageway typically discharge. A concrete trail crosses the embankment and both culverts. The integrity of the embankment is unknown. There is a concern that the embankment could fail if much water were to pond behind it. During the June 2004 storm, water overtopped the south culvert and trail. Garrison Street is estimated to overtop 0.6 and 2.8 feet during the 5- and 100-year storms, respectively. It is estimated to overtop between the 2- and 5-year storm events. During the 100-year storm, the stream discharges will overflow into the C-470 right-of-way. Reaches 6 and 6,1 - KiplinQ to C-470 and Meadow Ranch Area: This reach flows through the Deer Creek Golf Club. When the golf course was constructed in the late 1990's, the Massey Draw channel was modified, The developers were encouraged by Jefferson County to submit the plan for improvements for review and approval for the UDFCD maintenance eligibility program but they declined. The County did not mandate this course of action. During the June 2004 storm, stream bank erosion occurred throughout this reach and golf cart path bridges were washed out. During this storm, flows from Massey Draw split in the area southeast of the golf course clubhouse. The north flow split flowed into the area between West Coco Place and West Rockland Drive. The culverts under South Oak Street were overtopped and the water flowed east into the detention pond. The detention pond does not appear to have an adequate emergency overflow capable of handling the flows from Massey Draw, Several homes along West Rockland Drive were flooded. The flow split and street overtopping were observed during the August 1999 flash flood event, according to the Deer Creek Golf Course manager. Houses had not been constructed at that time. North Tributarv of Massev Draw Reach 1 - Confluence to Wadsworth: This reach is in Chatfield State Park on land owned by the U,S. Army Corps of Engineers and is the site for the Massey Draw Watershed and Ecosystem Improvement Pilot project. Improvements were designed to stabilize the channel along this reach. Construction was completed in 2005. Reach 2 - Wadsworth to Allison: The drainageway crosses private property along this reach. The houses are 80 to 170 feet apart with the drainageway passing between them. The average slope is 1.9 percent. Wadsworth Boulevard is estimated to overtop 1.2 feet during the 100-year event. It is estimated to overtop between the 5- and 1 O-year storm events, One house and one out building appear to be in the 1 OO-year floodplain Reach 3 - Allison to Carr: UDFCD and the County constructed a rectangular, concrete channel in 1988. The channel begins at Carr Street and ends upstream of Allison Street. It was designed to convey the 1 OO-year flow of 800 cubic feet per second. The design velocity is 20.8 feet per second. The County acquired an easement when the channel was constructed, Allison Street is estimated to overtop 0.9 foot during the 1 OO-year event. It is estimated to overtop between the 50- and 100-year storm events. One house and one out building appear to be in the 1 OO-year floodplain. Reach 4 - Carr to Chatfield: The parcel of land located upstream of Carr Street was platted in 1958 without considering the drainageway. There are platted lots in the regulated floodplain and no drainage easements. The drainageway has an average slope of 1.7 percent, and is incised 3 to 6 feet. In an effort to determine which houses are in the 1 OO-year floodplain, eleven elevation certificates were completed. Four of the houses were determined to be in the 1 OO-year floodplain. Five other houses for which elevation certificates were not completed appear to be in the 1 OO-year floodplain, West Rockland Drive and Oak Street are estimated to overtop more frequently than the 2-year event. Kipling Street is estimated to be overtopped 0.7 feet during the 100-year storm. It is estimated to overtop between the 50- and 100- year storm events. Reach 5 - Chatfield to Garrison: A 30-inch culvert crosses Chatfield Avenue and Dudley Street. From Chatfield to Estes, Massey Draw crosses private property, Upstream of Chatfield, it crosses FP&RD property. The lower end of this segment is heavily vegetated and appears to be stable. Downstream of Garrison, the channel is incised and sanitary sewer and irrigation lines are exposed, Chatfield Avenue is estimated to overtop 0,8 and 1.4 feet during the 5- and 1 OO-year events, respectively, Dudley Street is estimated to overtop 0.7 and 1.3 feet during the 5- and 100-year events, respectively. Both streets are estimated to overtop during storm events smaller than the 2-year event. One house appears to be in the1 OO-year floodplain. Reach 6 - Garrison to KiplinQ: Along this reach the drainageway crosses property owned by FP&RD. In 2002, UDFCD and the County constructed an 8-foot grouted sloping boulder drop structure and two 3-foot vertical drops downstream of Iris, Garrison Street is estimated to overtop 0.9 foot during the 100-year event. It is estimated to overtop between the 10- and 25-year storm events,