PDF (53.8 MB) - Urban Drainage and Flood Control District

MEETING
MEMORANDUM
Project
Happy Canyon Creek MDP & FHAD
Sponsors
UDFCD / Douglas County / City of Lone Tree /
Town of Parker / SEMSWA
Meeting Location
SEMSWA
Attendees
See attached list
MULLER ENGINEERING COMPANY, INC.
CONSULTING ENGINEERS
777 SOUTH WADSWORTH BLVD., SUITE 4-100
LAKEWOOD, COLORADO 80226
(303) 988-4939
Meeting Date
September 10, 2012
Issue Date
October 3, 2012
MEC Project No.
12-010.01
Minutes Prepared By
Andy Pultorak, Melanie Chenard
Routing
ASP / MDC / JTW
Purpose
Happy Canyon Creek MDP & FHAD Progress Meeting #3
Action Items
Muller Action Items:
1. Muller will address sponsor comments on the Baseline Hydrology and post the updated report to Dropbox
for sponsor review.
2. Melanie will send a meeting invite for Progress Meeting #4 to all sponsors and stakeholders.
3. Andy will provide Stacey Thompson with a figure showing the draft floodplain delineation near the water
treatment facility south of Jordan Rd. on the Happy Canyon mainstem.
4. Melanie will review the draft Rock Creek MDP for guidance on interactive hydrologic maps.
5. Muller will contact additional stakeholders identified by the project sponsors.
6. Muller will coordinate with Brad Robenstein (Douglas County) to obtain grading plan / LOMR
information at the north end of Chambers Reservoir and to determine the construction time frame of
Meridian Ponds 4A-4C.
UDFCD Action Items:
1. Shea will provide Muller with the updated UDFCD DFHAD Guidelines and checklist.
2. Shea will review the new UDFCD rainfall data (as posted on the UDFCD website) and determine if there
is any impact to the Happy Canyon hydrology.
3. Shea will discuss the following items with Bill DeGroot and instruct Muller accordingly:
a. What starting WSEL should be used at Cherry Creek for 10, 50, and 500-year Happy Canyon
Creek FP?
b. What flowrate/WSEL should be assumed in Green Acres Trib. if the mainstem spills into it in a
100- or 500-year?
c. What steps should be taken (if any) at locations where the meander of the low-flow channel does
not match the floodplain centerline?
d. Where flow spills from the main channel into a side channel, should the spill flow be subtracted
from the main channel if the spill is significant?
e. Should Muller model the proposed channel improvements at I-25 for the Douglas County East-
West trail if construction is not expected until late in 2013?
Happy Canyon Creek MDP & FHAD – Progress Meeting #3 – Meeting Minutes
September 10, 2012
4. UDFCD will review the draft mainstem cross-section locations workmaps provided by Muller and provide
any comments.
5. UDFCD will provide LiDAR mapping for Badger Gulch to Muller once data processing is finished.
Discussion
THE FOLLOWING IS OUR UNDERSTANDING OF THE SUBJECT MATTER COVERED IN THIS
CONFERENCE. IF THIS DIFFERS WITH YOUR UNDERSTANDING, PLEASE NOTIFY US.
1. INTRODUCTIONS
2. DRAFT BASELINE HYDROLOGY COMMENTS
The group discussed the overall results of the baseline hydrology and the calibration effort. The initial
results indicated significantly higher peak flows than the 1993 OSP though the 1993 % impervious was
matched and the subwatershed and conveyance element parameters were very similar to the 1993 model.
Because the Happy Canyon Creek watershed is adjacent to the Cottonwood Creek watershed and should
exhibit similar flow characteristics, the results were compared with the 2010 Cottonwood Creek OSP
hydrology based on pre-development conditions on both watersheds. Ultimately, the Happy Canyon Creek
model was calibrated to match Cottonwood Creek peak flows by adjusting Cp values (see the report for
additional discussion). The resultant peak flows, prior to updates for existing and revised future conditions,
still exceed the 1993 OSP but compare favorably with the 1977 FHAD, which are the regulatory flow
rates. Muller will add the FHAD peak flow rates to Figure B-10 in the report and will provide some
additional discussion of the differences in hydrology between this master plan and the 1977 FHAD.
Bill Ruzzo inquired if the hydrology would be revisited if the increased peak flow rates have detrimental
floodplain impacts. Shea indicated that this is a possibility.
Additional discussion of specific comments is summarized below.
a. SEMSWA has new aerial photography dated Spring 2012. Because the coverage is limited to
Arapahoe County and changes in this portion of the watershed are limited, the 2010 aerial
photography provided by UDFCD will continue to be used for the project.
b. Greg Weeks provided Muller with a conceptual alignment for the future Belford Ave. bridge crossing
over Happy Canyon Creek south of E-470. The area to the east of Happy Canyon, north of Grandview
Estates, is another Town of Parker annexation area (Chambers High Point) – information is available
on the Town’s website.
c. In response to some SEMSWA comments, Muller clarified the peak flow rates at the confluence with
Cherry Creek:
• Arapahoe County FIS published 100-year peak = 3690 cfs for Happy Canyon Creek. The source
of this flow rate is unknown; it does not match any known studies.
• Mapped floodplain appears to be based on the 1993 OSP 100-year future conditions peak of 7303
cfs rather than the FIS published value.
• 1977 FHAD (future conditions) = 6744 cfs
• Current study (future conditions) = 9238 cfs
The discrepancies seem to be limited to the Arapahoe County portion of the channel; the remainder of
the floodplain appears to be based on the 1977 FHAD hydrology.
d. Lone Tree had commented on modeling of piped reaches in SWMM model. The report stated that the
pipe sizes had been artificially enlarged in the SWMM model to convey the entire peak flow, since
overtopping a piped reach could cause instabilities in the model. Actual capacities will be evaluated
for any insufficiencies during the alternatives evaluation.
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