HomeMy WebLinkAboutWATERGLEN PUD PDP - 71 93A - CORRESPONDENCE - CITY STAFF- Some of the swales are shown to be crossing the garden plots,
please correct this conflict.
- Please show all cross -sections for the swales on the drainage
plan.
- Please use a larger scale for the next submittal, in order to
make the drainage plan more readable.
- Please provide all necessary drainage details.
- Please document all off -site flows (Anheuser Busch) and include
them in .the appendix.
At final the following requirements should be met:
- outlet controls for the detention ponds, as well as a grading
plan showing the required 1 foot freeboard should be provided.
- A SWMM analysis of the pond system should'be prepared.
- A grading plan with building footprints and elevations, as well
as existing and proposed contours at two foot intervals isN
required.
A plat showing all necessary drainage easements is needed.
- Rip rap sizing calculations need to be provided where necessary,
- If you have any questions about this review, or what is required
for the final submittal, please do not hesitate to contact me at
the Stormwater Utility at (303) 221-6589
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This part of the review is concerned with the water quality issues
associated with the Cooper Slough.
The approximate wetland boundaries drawn on the drainage plan show
some encroachment by development along the South end of proposed
Elgin Court on the outlined wetland area. The alignment of the
road will need to be adjusted to prevent that.
It is further suggested in order to enhance water quality in the
slough that the following alternatives be explored.
- The bottom of the detention ponds along the slough should be kept
close to the groundwater level in order to create an additional
wetland buffer area in the slough (wet bottom pond). This will
create a wetland pond that would in turn enhance the habitat
quality in the corridor around the slough.
- The flow path in the detention ponds should be made as long as
possible in order to enhance water quality benefits to the slough
and minimize the amount and effect of pollutants associated with
urban runoff entering the stream ("first -flush" effects). .Please
reference Urban Drainage and Flood Control manual Volume 3 for
design criteria for extended detention pond design criteria.
- Within the development area itself the system can be designed in
a manner as to minimize "Directly Connected Impervious Area"
(DCIA). This can be done through the use of grass lined swales,
irrigated grass buffer strips, berms, etc...
- The outlet structure from the detention area into the Slough
should be designed in a way to minimize sediment loading of the
stream.
.The groundwater investigation provided seems to suggest that the
stream is fed by groundwater recharge from the irrigation return
flow drain tiles and from the canal. It is agreed that these
sources are probably the two main sources of water to the Slough,
however it is suggested that this analysis is preliminary and that
more longitudinal data (in time) should be collected to ascertain
this point.
This part of the review deals with the rest of the non -water
quality related comments:
- All grass swales with slopes less than 2% are required to have
concrete pans or underdrains. (A trickle channel may be needed for
larger swales. (Side slopes should be a maximum of 4:1 unless some
sort of stabilization is utilized.)
- Where is the 2.65. Ac. ft. of detention for Basin K being
provided? Please label correctly all detention ponds.
Utility _ rvices
Stormwater
From: Basil Hamdan, Stormwater Utility
To: Steve Olt, Planning and Environmental Services
Love and Associates.
Date: 4/20/9X4
Subject: Waterglen, Preliminary submittal comments
It is noted that this review was done using plans that were
submitted to the Stormwater Utility on April 4 th.which are
different from more recent plans submitted to the Planning
Department.
The Boxelder overflow swale that conveys flows along I-25 is shown
to be in the state Right Of Way. Formal approval from the Colorado
Department of Transportation will be needed in that case before
this preliminary submittal is approved.
Cross -sections showing the swale, the berm, and the Interstate
highway are needed to verify how the flows are currently being
accommodated. Please show on these cross -sections the property
line and the Right of Way line. These cross -sections should be
done at least on three locations: one at the northernmost part of
the property where Q100 is around 250 cfs, in the middle section
and another one after the additional overflow from Boxelder Creek
(Q100= 1450 cfs). Two cross -sections along East Vine Drive also
should be shown.
Two Existing buildings are shown to be in the revised floodplain.
Since this reconfiguration of the floodplain will concentrate the
'flows and thus cause higher velocities, these buildings will need
to be flood -proofed. Note again that the floodplain boundaries
shown on the Drainage plan submitted to the Stormwater Utility on
April 4 th is different from the one shown on the updated plan
submitted to the Planning and Zoning Board. Please clarify which
plan is the correct one.
135 Mathews • P.O. Box 580 • Fort Collins, CO 80522-0580 • (303) 221-6589 • FAX (303) 221-6239