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2. David discussed the regional detention alternatives and noted that the peak flows and storage volumes shown in the matrix were based on 1 OO-year storm criteria, Bob asked whether information from the 10-year storm was available. The10-year storm had not been evaluated with the detention but will be in the draft report. 8. Ken Caryl and UDFCD boards have authorized additional funding to include the area west of the hogback. The intergovernmental agreement is being reviewed by Ken Caryl. The masterplanning activities in this area will be limited to channel stabilization. I I . I I I I I I I I I I . I I I . . 0\. Massey Draw and SJCD Update Project Meeting Minutes April 27, 2004 Page 2 of 3 OLSSON ASSOCIATES ENGINEERS' PLANNERS' SCIENTISTS' SURVEYORS MEETING MINUTES 3. David discussed potential locations for regional water quality detention basins and confirmed that the estimated basin surface areas listed in the alternatives matrix did not include allowances for freeboard or maintenance. The WQCV were based on the sizing criteria for extended dry detention basins in the criteria manual. Ben noted that many retrofit projects won't be able to meet the design criteria but that they should provide the maximum possible benefit. David also pointed out that the standard basin design calls for storage in addition to the 1 OO-year storm event storage. On-line water quality ponds may result in an increased flood plain, Bob observed that the heavy vegetation in the ponds on SJCD (South) Tributary #2 likely provides water quality benefits. He suggested looking at ways to formalize the benefits. He also mentioned that the Arapahoe County might have construction plans on the existing pond on the North Tributary of SJCD (South) west of S, Platte Canyon Road, which was suggested as a potential water quality location. A water quality basin is included in the PMF channel design on the North Tributary of SJCD (South), at the confluence. The PMF channel project also includes a sedimentation basin at the east end of the PMF channel. A question was raised regarding the design basis for these two facilities. David will contact Tetra Tech to obtain the design criteria for the water quality and the sedimentation basins, Ben noted that the wetland bottom of the PMF channel should provide great water quality benefits, 4. Ben stated that it will be important to document with the Corps of Engineers the design of wetland bottom channels, detention and water quality basins so that the wetlands in these facilities can be maintained and rehabilitated under the 404 permit program to the original design as originally approved under that program. 5, The results of a "first-cut" HEC-RAS model of Massey Draw showing the flood plain were shown. The results appeared to indicate water was being backed up at certain crossings. Ben wondered if there was inadvertent detention occurring since some of the street crossings aren't large enough to convey the 100-year event. David noted that the PMF channel design, rather than the existing topographic information will be used for SJCD (South) downstream of South Platte Canyon Road, The datum for the design will be checked to make sure it's the same as the project mapping. 6. Paula stated that the North Tributary of Massey Draw crossing at Chatfield had reported overtopping problems and that Massey Draw at Carr overtopped downstream of the pond during the 1999 storm. Paula also noted that the Platte Canyon crossing is in poor condition. 143 Union Blvd. Suite 700 Lakewood, CO 80228720-962-6072 Massey Draw and SJCD (South) Update OA Project No. 02-2003-1321 April 27, 2004 Attendees: Ben Urbonas - Urban Drainage and Flood Control District (UDFCD) Paula Lundquist - Jefferson County Bob Deeds - City of Littleton Bruce Buttner - Olsson Associates (OA) David Krickbaum - Olsson Associates Deb Ohlinger - Olsson Associates Minutes Additions/Corrections: A correction to the March 31, 2004 meeting minutes was noted. The following sentence replaced the second sentence of item 6: "The North Tributary of SJCD 6200 South channel upstream of the Centennial Reservoir property to S. Platte Canyon Road is being considered for acceptance into the UDFCD maintenance program." Items Distributed: . The Hydr%gy Report for the Massey Draw and SJCD (South) Drainageway Planning Update, dated April 2004. . Updated matrix of preliminary alternatives, . Observations from the April 19, 2004 watershed tour. Discussion Items: 1. David discussed new observations from the watershed tour. Incised channels were observed west of Wadsworth on SJCD (South) and Massey Draw. He pointed out that we were unable to access the channel on North Tributary Massey Draw between Wadsworth and Allison Street and it might be incised as well. Ben suggested that viewing the aerial photos with a stereo viewer might be helpful to look at some of the inaccessible areas, if there isn't too much vegetation. 7. The alternatives matrix shows the number of 3-foot high check high structures needed to obtain a 0.4% slope in a given channel reach and a general cost associated with that number. Ben noted that in places where the channel is deeply incised, drop structures may be required, which would be more expensive than the check structures.