Arboretum VillageApril 11, 2001
Randall Hedlund
Hedlund Engineering
2005 Pin Oak Drive
Eagan, MN 55122
Re: Arboretum Village
Dear Sir,
This note is to follow-up what was discussed at Monday's meeting about pond sizing and bounce.
Many of the following issues have been addressed at pervious meetings since the original Puke
plan, but I would like to clarify these items.
It is Mn/DOT Water Resources Engineering policy that existing drainage rates and patterns must
be perpetuated for any drainage impacting Mn/DOT right-of-way. In the case of the PuRe
development, the existing drainage rates and patterns are being changed. The existing condition
is a farm field / prairie grass area that will be turned into a residential development. The change
to a residential development greatly increases the runoff coefficient from existing conditions and
thus the development must provide ponding to perpetuate existing drainage rates and patterns.
For this project, Mn/DOT is allowing Pulte to expand an existing Mn/DOT pond at the junction
of T.H. 5 and Century Blvd. The proposed drainage rates, patterns and pond properties must be
perpetuated for the following reasons:
The Mn/DOT design HWL (high water level) of the pond is 962.6 ft. In looking at the invert
elevation of the apron draining the area to the east of Century Blvd to the Mn/DOT pond
(structure #5030), the invert is at an elevation of 962.20 ft. This means that under current
conditions built with the 1002-61 plan, water will backup through this pipe into the area east
of Century Blvd. The proposed Pulte HWL elevation as of April 6th is 966.2 ft. This
proposed elevation is 4.0 ft higher than the invert of the apron on the east side of Century
Blvd. This will make it very difficult to drain any future developments on this parcel.
,
It is Mn/DOT practice to not allow private developments to pond on Mn/DOT right-of-way.
However, in certain circumstances like this project, Mn/DOT is allowing PuRe to expand the
existing Mn/DOT pond for their ponding needs. The existing Mn/DOT pond characteristics
must be perpetuated, as it would be improper and unfair use of taxpayer money to allow
private developments to change existing drainage properties on Mn/DOT right-of-way. If
Mn/DOT were to allow Pulte to change the existing drainage properties of the Mn/DOT
pond, we would be setting precedent for other private developments to do the same
throughout the Metro. Mn/DOT has taken a firm stand on this issue for other developments
affecting Mn/DOT right-of-way throughout the Metro, and this development is not being
treated any different.
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It is a design goal to keep the maximum pond depth from the bottom of the pond to the HWL
under 10 ft. In the case of the pond Pulte is proposing to expand, this design standard should
be maintained.
Other issues that should be addressed with this project are as follows:
o
The proposed Pulte plan does not show the proposed Mn/DOT catch basins and storm sewer
on T.H. 41. It was my understanding from previous meetings that. Pulte would tie this apron
outlet into their system and route the flow to the Mn/DOT pond.
,
Mn/DOT only mitigated for the wetland impacts within the construction limits for the 1002-
61 project. It appears that with the most current Pulte plan that the proposed contours may
impact the existing wetland at the junction of T.H. 5 and Century Blvd. Pulte will be
responsible for mitigating any wetland impact associated with expanding the Mn/DOT pond.
3,
The parcel to the east of Century Blvd has a drainage divide approximately in the middle.
Half of the parcel drains to the west and half to the east. Both of the Mn/DOT ponds at this
location were designed for existing conditions and not a proposed developed condition. It
will again be the responsibility of any developer in this parcel to perpetuate existing drainage
rates and patterns.
If you have any questions or need more information, please feel free to contact me at
patrick.mclarnon@dot.state.mn.us or (651) 634-2400.
Sincerely,
Patrick McLarnon, H.I.T.
Water Resources Engineering
Mn/DOT Metro Division
Cc~
Benjamin Timerson - Mn/DOT Water Resources Engineering (email)
Jane Krebsbach - Mn/DOT Design (email)
Susan Klein - Mn/DOT Design (email)
Teresa Burgess - City of Chanhassen (email)
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