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

DRAFT ALTERNATIVE ANALYSIS COMMENTS
Draft alternative analysis was submitted to the project sponsors on March 1, 2013 for review.
Comments were received on March 27, 2013 in the form of an annotated PDF copy of the report.
Additional comments were provided by CCBWQA on April 9, 2013. Select comments have been
reproduced or paraphrased below; responses are italicized. Comments/corrections not requiring
additional clarification were addressed in the revision but are not listed here.
Urban Drainage and Flood Control District (UDFCD) Comments:
Section 5.6.2 “Costs for this reach include excavation required to lower the embankment at CPNMD
Reach 1 Pond #12 to provide overtopping capacity without flooding the adjacent sanitary lift
station.” Weren't these costs already included in the detention alternative?
We considered where to include this cost and determined that it is best reflected in the
channel costs rather than the detention costs. Though this work is related to a detention
facility, it is not a detention improvement as it results in reduced storage capacity.
Instead, we consider it a channel improvement that addresses a flooding issue. Once we
move on to conceptual design of the selected plan, there will not be a desire to
differentiate between the channel alternatives and detention alternatives, and we may
reevaluate how we present these costs.
Douglas County Comments:
Section 5.6.1 “Meridian Village includes 319 acres tributary to the Grandview Pond; detention for
most of this area will be provided in private onsite facilities, with two small publically
owned facilities providing a combined 100-year volume of 5.5 acre-feet.” Grandview
Pond design was based on assumption that Meridian ponds would be metro district
maintained. Douglas County to investigate.
Additional information was provided on two proposed detention ponds in Meridian’s
Stepping Stone development. These ponds have sufficient contributing area and capacity
to warrant including in the master plan and will be maintained by the Meridian Metro
District. Subwatershed boundaries and land use have been revised to better reflect the
proposed development, and the ponds have been incorporated into the model. Because the
ponds are currently under construction, they have been reflected in the baseline
hydrology. With these additions, the Grandview Pond release now corresponds with the
design flow rate for the downstream improvements.
(re: Charter Oaks pond) There are a couple of existing on-site ponds just upstream that
could potentially be converted to Full Spectrum ponds. Would CPNMD consider taking
those over? Do the benefits of all the additional detention on this tributary exceed the
costs to implement the detention?
Plans for the existing pond adjacent to the Montessori school were provided and reviewed.
The existing pond reflects minimal downstream benefit when added to the SWMM model
(though the model’s basins do not correspond with the pond’s contributing area). We had
planned to abandon the idea of a pond at this location, but were then made aware of
downstream flooding and crossing capacity concerns on the Beverly Hills Tributary. To aid
in these situations, the existing pond could potentially be connected with the low area to
the west of the Montessori school under construction; the school site will include an offsite
pond adjacent to the road, which could form the center of the “U” shaped pond. Though
such a facility would provide some benefit to Happy Canyon Creek, the benefit would
mostly be recognized on the Beverly Hills Tributary channel. Because conveyance and
floodplain improvements on the tributary are not a part of this scope, we have not
associated a channel cost savings with these improvements, though initial analysis
indicates that the planned culvert replacement at Oakwood Lane may not be necessary.
Section 5.6.2 Do these costs reflect that the lower portion of Reach 2b has been stabilized by Douglas
Reach 2b County and the UDFCD or are the typical costs applied to the entire reach length?
Drop structure costs are based on the number of vertical feet of drop needed based on
upstream and downstream invert elevations, a design slope, and the total vertical feet of
existing drop structures. Other improvement items such as riprap and earthwork are
applied along the entire reach based on average cross sectional dimensions. These
assumptions can be refined for the conceptual design if appropriate/desired.
Figure 5-1
(re:Reach 3) Is Alternative D2 adequate considering the close proximity of I-25 and the
need for that bank to remain very stable?
The improvements assumed for Alternative D2 are still quite substantial, representing
85% of the D1 costs; we feel the per linear foot costs are appropriate for a reach of this
nature. This alternative is intended to be paired with the recommended detention
improvements upstream; if the timing of such improvements would significantly lag
channel improvements, Alternative D1 may be needed to provide additional protection in
the near term.
Town of Parker Comments
Section 5.6
Some clarification is requested regarding the E-470 Detention Pond…..The Town
requests revising the language in the document related to modification of the E-470
Detention Pond (Compark South).
The report and models have been revised to reflect a single proposed configuration for the
E-470 pond, with sizing based on the upstream contributing area within Compark South
and Grandview Estates. The resulting volume for Alternative C decreases slightly due to
the addition of the Airport 320 pond (the higher volume was used for cost estimating).
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