HomeMy WebLinkAboutDrainage Reports - 05/18/2016 (2)City`of Ft. Collins "Pprov d Plans
Approved By f�
Date
INTlRAlaT CONSULTING .GROU P
April 13, 2016
I
Mr. Wes Lamarque
;
City of Fort Collins
Storm Water Utility
700 Wood Street
Fort Collins, CO 80522
Re: Rigden Farm Filing Nineteen .
(Formerly known as Filing Six Lots M8 & M9 Minor Amendment)
ICG Project No. 1233-136-00 ;
Dear Wes,
Please accept the following letter report on behalf of Schuman Companies, Inc. to demonstrate
the site's compliance with the previously approved Final Drainage Report for Rigden Farm
Filing Six, as prepared by JR Engineering, dated February 8, 2002. -
INTRODUCTION
This proposed development is legally known as Lots M8 & M9 of Rigden Farm Filing Nineteen
and contains 10,995 square feet or 0.25 acres and 12,569 square feet or 0.29 acres, respectively.
Lot M8 is bounded on the north by William Neal Parkway, the west by Tract BD, the south by
Lot 319 and the east by Bryce Drive. Lot M9 is bounded on the north by William Neal Parkway,
on the west by Bryce Drive, on the south by Lot 232 and on the east by Tract AS. Lot M8
generally drains to the east where flows are intercepted by existing curb and gutter and conveyed
to a storm sewer system that discharges into existing detention pond 269. Lot M9 generally
drains to the east'and sheet flows directly into existing detention pond 269. The design for this
pond was included as part of the Filing Six Drainage Report and was constructed with that
Filing. The original runoff coefficient calculations and stage-storag6 table are attached for
reference and all detention and water quality requirements for this site have been met by the
overall Ridgen Farm development.
EXISTING CONDITIONS
The site is currently vacant and contains a stand of grasses. The Overall Drainage Basin Map
that was included in the previously approved Final Drainage and Erosion Control Report shows
Lot M8 draining into sub -basin 146, the western 30 feet of Lot M9 draining into sub -basin 142,
and the remainder of Lot M9 draining into sub -basin 171, as shown on the enclosed map.
Mr. Wes Lamarque
April 13, 2016
Page 2 of 3
PROPOSED DEVELOPMENT
The proposed development continues to direct runoff to the perimeter streets and ultimately to
Pond 269 as anticipated in the Filing Six Final Drainage Report. Pond 269 releases into the
Fossil Creek Reservoir Inlet Ditch (FCRID).
The southern half of Lot M8 will drain south to the proposed flowline of the access drive and
then flow east into the flowline of Bryce Drive. The majority of the runoff will be pulled off a
series of curb cuts that will direct the runoff to a shallow grass lined swale and then. into the
flowline of Bryce Drive via a new 2 foot sidewalk chase. This location has been identified as
design point #1 on the proposed drainage basin map. The northern half of Lot M8 will drain
north toward William Neal Parkway and sheet flow into the flowline of the street. Flows will
then be conveyed east to design point 42 of the Rigden Farm Filing Six report where it will be
collected by an existing sump inlet and conveyed via storm pipe to Pond 269.
The majority of Lot M9 will follow current conditions and drain east toward Tract AS.. The
western most 25 feet of Lot M9 will drain west toward Bryce Drive and sheet flow into the
flowline of the street (as designated in the Filing Six drainage basin map). The northern half of
Lot M9 will drain north toward William Neal Parkway and sheet flow into the flowline of the
street. Similarly to the flows from Lot M8, these flows will be conveyed east to design point 42.
The southern half of Lot M9 will flow south to the proposed flowline of the access drive and
then flow east into a curb cut in the southeast corner of the site. This location has been identified
as design point #2 on the proposed drainage basin map. The concentrated flow at this point will
be dissipated and spread out through the use of a cobble dissipater and a section of ScourStop
Mat that will prevent erosion immediately downstream of the curb cut. The water will then flow
over approximately 25 feet of established turf and finally sheet flow over the existing concrete
trail and into Pond 269. Please refer to the attached Drainage and Erosion Control Plan for flow
direction.
The Filing Six design utilized an "Ultimate Development" condition for the basin that makes up
the site. Per the original report, the area within M8 (sub -basin 146) was assumed to be Multi -
Family with an imperviousness of 65% and a Runoff Coefficient of 0.65. It is noted that Table
3-3 from the City of Fort Collins Design Criteria Manual currently shows a runoff coefficient of
0.65 and percent impervious of 70% for multi -family area. 'Per the weighted average runoff
calculations, the actual imperviousness of Lot M8 is 62%, with a minor storm Runoff Coefficient
of 0.67 as shown on the attached spreadsheet. The small increase in the runoff coefficient of M8
will not have a substantial impact to the overall flow being conveyed to the existing pond.
Per the original report, the area within M9 (sub -basin 171) . was assumed to be single family
development. The single family imperviousness is 55% with a minor storm Runoff Coefficient
of 0.65. However, M9 is zoned multi -family and it is our belief that the assumption that all lots
in sub -basin 171 were single family was a small oversight in the original drainage report. The
actual imperviousness of Lot M9 is 56%, with a minor storm Runoff Coefficient of 0.61 as
shown on the attached spreadsheet. Even though the percent impervious is 1% greater than the
original assumption, the runoff coefficient is less than 0.65 and this'additional impervious area
will not have an adverse impact to the function of the existing pond.
Mr. Wes Lamarque
April 13, 2016
Page 3 of 3
It is requested that the site be deemed in substantial conformance with the previously approved
Final Drainage Reports. Please see the attached excerpts from the Filing Six Final Drainage
Report detailing the assumptions and original flow calculations for these two sub -basins.
LOW IMPACT DEVELOPMENT REQUIREMENTS
City Staff has stated that the use of porous pavers is not applicable on the residential driveways
providing access to the units on M8 and M9,' however, Low Impact Development (LID) criteria
should be achieved where readily available. The sites are constrained in both size and relative to
storm drain outfall options as there are no storm pipes adjacent to the property that could be used .
to serve a variety of LID options such as rain gardens. Therefore, the project is meeting the
intent of runoff reduction practices through the use of two LID techniques: grass buffers and a
grass Swale.
The southern half of the roof area of M8 in addition to the driveway runoff will be conveyed to a
grass swale that runs along the south property line. The surface area of the swale is 472 sf
(105'x4.5') and the new impervious area contributing to the swale is 3,945 sf as shown as Basin
Area 1 on the attached LID Exhibit. Therefore, the ratio of new impervious area to the grass
swale is 8.4 which is less than the allowable 10:1 ratio.
Runoff from the northern half of the roof area and the access walks. for both M8 and M9 will
flow over A grass buffer prior to sheet flowing over the sidewalk adjacent to William Neal
Parkway. As previously mentioned, runoff from the southern half of the roof area of M9 in
addition to the driveway runoff will be conveyed to a curb cut. This concentrated runoff -will be
spread out through the use of a cobble dissipater and segment of ScourStop Mat and then flow
through a previously established grass buffer located in the open space directly east of the site
before ultimately entering the regional pond.
Please refer to the attached LID Exhibit for a breakdown of the proposed impervious area and
LID treatment areas.
If you have any questions or comments please contact me at (970) 460-1091.
Sincerely,
p0 I
Skyl r Brower, P.E.
Colorado Professional Engineer 44248 "248 v
Attachments
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SS�ONA� E�
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LOT M8 II
FF=98.93 FF=97.93
GFF=97.60 GFF=96.60
BFF=89, 09 BFF=B3.09
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DRAINAGE SUMMARY TABLE
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SCALE: I"- 20
QQ1 BASIN DESIGNATION
1.25 OSO BASIN COEFFICIENT II ODYRI
BASIN AREA
QDESIGN POINT
-1111- DRAINAGE FLOW ARROW
- - DRAINAGE BASIN
BOUNDARY
v O u SILT FENCE
O \I INLET PROTECTION
O GRAVEL BAG
r yIC CONSTRUCTION ENTRANCE
W/ VEHICLE TRACKING CONTROL
O ® CONCRETE WASHOUT AREA
NOTES:
1. CONCRETE WASHOUT LOCATION TO BE DETERMINED IN
THE FIELD.
2. CONTRACTOR TO ESTABLISH PERIMETER PROTECTION
ALONG THE LIMITS OF DISTURBANCE TO LIMIT ACCESS TO
THE ACTIVE CONSTRUCTION AREA. CONTRACTOR TO LIMIT
SEDIMENT FROM ENTERING THE PUBLIC RIGHT OF WAY
THROUGH THE USE OF A VEHICLE TRACKING PAD OR OTHER
APPROVED METHOD.
3. THE DOWNSTREAM INLET ON THE SOUTH SIDE OF
W ILLIAM NEAL PARKWAY JUST WEST OF THE ZIEGLER
INTERSECTION MAY BE REQUIRED TO RECEIVE INLET
PROTECTION IF GRAVEL BAG AT PROPERTY LINE IS NOT
ADEQUATELY MAINTAINED.
4. THE SITE CURRENTLY HAS EXISTING PERIMETER SILT
FENCE INSTALLED. CONTRACTOR TO INSTALL NEW SILT
FENCE AS NEEDED AND/OR MAINTAIN THE EXISTING SILT
FENCE IN PLACE.
CALL UnUTY NOTIFICATION
CENTER OF COLORADO
811
FAIL 2-BUSINESS DAYS IN ADVANCE
BEFORE YOU DIG, GRADE. OR EXCAVATE
FOR THE MARKING OF UNDERGROUND
MEMBER UTILITIES.
CITY OF FORT COLLINS. COLORADO
UTILITY PLAN APPROVAL
APPROVED:
OTT ENGINEER
CHECKED BY:
WATER h WASTEWATER UTUTY
CHECKED BY:
STORMwAMR UTIUTY
CHECKED BY:
PARKS 4 REOREATION
CHECKED BY:
TRAFFIC ENGINEER
CHECKED BY:
NATURAL RESWRCES
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GRASS BUFFER
M8 AREA TO-
-GRASS BUFFER
WILLIAMNEAL PARKWAY
VD0�04��O�Oa�0�0�0�6
OpOpO6-zzz o�o�0 >0
GRASS BUFFER
M9 AREA TO
GRASS BUFFER
i M9 AREA TO
GRASS BUFFER
ON TRACT 'AS'
232
LID BASIN AREA EXHIBIT
NEW IMPERVIOUS AREA:
15,090
sf
MIMMi7MAREA TO BE TREATED (5056)r
sf
BASIN AREA I-MBGRASS SWALE
3,945
Sf
BASIN AREA 2 - M9 GRASS BUFFER
(TRACT 'AS)
4,252
sf
TOTAL AREA TREATED:
sf
25
RIGDEN FARM M8 & M9
121e W.A B WC LID AREA EXHIBIT
P. ceweao eases
PMre:(970)e7�
Fez: (970) 61d3303
DATE: 03/13/16
I M T E N W E! T C O N S U L T I N G G R O U P
TRACTAS
I
GRASS BUFFER
W/ SCOUR STOP
I I
I I I
III
O f 25
SCALE: 1" = 50'
50