HomeMy WebLinkAboutSEVEN GENERATIONS (FORMERLY EASTBROOK) OFFICE PARK - PDP - 4-06 - CORRESPONDENCE - DRAINAGE REPORT (3)Steve Olt - Re: Drainage Issues at Easth ,k Office Park M __ Page 2
I have reviewed the comments that Glen provided at this morning's meeting on the Eastbrook project.
There were a few comments that are of concern, I want to go through my rational and get your feedback
in preparation for a meeting next week.
The comments state that detention to the 2 year historic rate is required for the site. In the drainage
report, it shows that the proposed site (to the centerline of Eastbrook) is 64% impervious which is less
than the master plan of 65%. There was also a note that I felt the model's 65% was high because of the
railroad right of way and drainage area to the north - this was just a comment and was not meant to say
that we were counting on using other pervious land to increase our site imperviousness. The site meets
the master plan model, and for that reason, we were not proposing detention.
Glen provided for me a copy of the URS report (sheet 3-1). In Section 3.1, third paragraph, last sentence,
it states, "It is recommended that developers provide on site detention for future development ..... utilizing
a 2-year existing rate ..." I am assuming this is the basis for the comment requiring detention.
In the first paragraph of the URS Section 3.1, second sentence, it states "This holds true unless otherwise
specified in the master plan or it can be shown that the development peak discharge is conveyed prior to
the main drainage peak discharge." The Eastbrook site will have its peak flow at 5 to 10 minutes. The
SWMM model that I have shows the peak in the channel occurring at 3 hours 11 minutes. If meeting the
master plan imperviousness is not sufficient to relieve this site of having detention, I would ask that the
argument that we will "beat the peak" be used as the reason for not requiring detention.
I also have questions regarding the comment about the flows from the south - there are no development
plans or drainage report on file at the City for this lot, so I do not know how you want me to provide you
with those rates - we can discuss this at the meeting.
Last, the comments about the floodplain and that it may be changing are a little confusing. We do not
plan to do any grading off of our site because of the Jurisdictional Wetlands and the fact that the area is
railroad right of way. We are elevating the buildings, but at this time, the floodplain is off of our site, so we
have not addressed it. I do not know what cross sections I would provide. The floodplain maps indicate
that these flows stay off of our site in the existing channel.
am free Monday and Tuesday morning to meet with you at your office. I think we need a meeting to
discuss these issues, but if I could get some feedback on the detention issue prior to the meeting, it would
be helpful.
Thank you.
Mike
Michael Oberlander, PE, LSI
North Star Design, Inc.
970-686-6939
CC: Dave Sitzman; Steve Olt; Tricia Kroetch
S`eve ,k Office Park Page 1
Oft_.Re Drainage Issues at Eastt
From: Glen Schlueter
To: Basil Hamdan; Michael Oberlander; Sue Paquette
Date: 03/08/2006 3:48:53 PM
Subject: Re: Drainage Issues at Eastbrook Office Park
Mike,
Could you please send by email or FAX the table you showed me where you calculated the
imperviousness to be 64%. 1 gave the report to Engineering. I also read the paragraph where you
discussed the 65% and I thought you were talking about basin 50 as a whole and not just the site. Only a
portion of the railroad ROW is in basin 50 so it didn't appear to us that with the proposed intensity of
development that the basin would be 65%. 1 sitll look at the site plan and wonder how the site
imperviousness could be at 64% so that is why I would like Basil to take a look at the numbers. We may
need a site plan or maybe you have a working copy showing the impervious areas and their square
footages.
The floodplain comment is primary to show the limits of the floodplain to document that it is not on the site.
The cross section idea was mine and it is not really needed if the floodplain is indeed offsite. The north
floodplain should also be indicated on the drainage plan since we require information at least 50 feet
offsite and more if necessary. I have found out that the water surface elevation of the floodplain along the
west side of the site is determined by the ponding elevation, not a conveyance elevation so forget the
cross section idea. However as I mentioned, the water surface elevation may change since the existing
modeling will be corrected due to finding the additional two pipes that bring more water under the railroad.
The initial modeling for the improvements west of the railroad shows a reduction in the peak flow rates
after the improvements are completed, but we won't really know until the project as -build modeling is
completed.
You are correct about the basis for requiring the 2 year historic release rate. The idea of using the basin
imperviousness is something that got started with trying to justify no detention for infill sites, primarily in
Old Town. I have extended that to other basins thinking it was a way to justify no detention and to make
the drainage design simpler as well as to eliminate small individual ponds that don't really have an affect in
the basin as a whole. That being said, other stormwater staff think that rationale is just plain wrong. The
master plan models consider the basin's imperviousness with detention on remaining undeveloped sites to
be released at the 2 year historic rate. Thinking about it that way, even the "beat the peak" idea doesn't
fly. I guess it would be more acceptable if the site imperviousness were included in the SWMM and then
the peak rates could be compared. I really think we should look at a 2 year release rate and see how
difficult it would be to meet that requirement. The main thing is, we don't want to make it worse
downstream. There is a building to the northeast that could be getting water into it in the 100 year event
so we don't want to make that problem worse. Looks like I need to stop saying the statement about using
the master plan imperviousness and use that only as a variance justification.
The comment about flows from the south is like any other offsite flow that would be calculated for a
develoment proposal. If there isn't a report then you need to calculate the offsite flow that will pass
through the site.
Basil will be back Monday so we can set up a meeting. I don't think Sue needs to be there. I have stated
above where master planning is coming from and it's not just Sue. Matt has been giving me s--- about it
for a few years. I think you, Basil and I can workout something. I do not have conceptual review Monday
so give Basil or myself a call Monday morning. I do have a meeting at 11.
Glen
>>> "Michael Oberlander" <mike@northstardesigninc.com> 03/08/2006 1:01:34 PM >>>
Glen, Basil and Sue-