HomeMy WebLinkAboutGrading Plans - 02/10/2022Eric Lessard, PE Grading/Stormwater Plan Accepted
304 Smith St. (Building Permit Released)
Accepted by: Dan Mogen
Fort Collins, CO 80524 Date: 02/10/2022
2/7/22
City of Fort Collins — Development Review
281 N. College Ave.
Fort Collins, CO 80524
RE: 304 Smith St. Residential Building Addition Drainage Memorandum
Fort Collins, CO — Old Town Drainage Basin B2200282
S 1/2 of Lot 5 Blk 173 FTC
Dear Staff:
I am pleased to submit this Drainage Memorandum for the property of 304 Smith St. in the Old
Town Basin of Fort Collins, C0. In general, this memorandum serves to document the
stormwater impacts associated with the proposed project on this 9500 square foot property.
Site Location and Project Description
The 304 Smith St. property site (referred herein as "the site") is located in Fort Collins, at Block
173, Lot 5, Section 12, Township 7 North, Range 68 West, 6th PM, Larimer County, Colorado.
The site has a platted area of 0.22 acres and is bounded by Smith Street to the west, Olive
Street to the North, a public alley (Lilac Lane) to the east and an existing residential building to
the south.
The planned development for the site will consist of the following:
• The complete removal of existing 116 sf laundry room from the east side of the building.
• The construction of two-story, residential building addition with a proposed full-buildout
footprint of approximately 534 sf (small addition). The net increase in impervious area will be
418 sf.
• Addition will include a new laundry room, new entrance to existing cellar with a Iower level
shop room and upper level bedroom and % bathroom. The first floor shop will have a doorway
onto an 80 sf ground level deck in the rear and the laundry room will have an exit door onto a
5'x42" landing with stairs.
Existing Conditions
Hydrolo9Y
The site, which is located within the Old Town Basin, is approximately 0.22-
acres in size and is currently developed as a residential property with an existing
percent-imperviousness of approximately 18.8%o. The property is graded from west to east with
a consistent slope. Storm runoff from the site currently partially drains towards the west to
Smith St. or primarily east to the 20' alley where it flows north to Olive St. and intercepted by
curb and gutter. Fencing is present on both the north and south property lines separating the
residences.
Existing building runoff:
Rational Method Calculations:
C coefficient: .95 (roof top)
Intensity: 1.84 in/hr (100-year rain intensity for 2hr storm)
Area: 1785 sf (.041 acres)
Runoff = .95*1.84*.041=.072cfs
�
�e�
��`--.�
a , ��\
A''.I �1 � _ _. ' . .
.' �..�t ' .. - _
�: �i "IV� 11�� �"�_ -
,—,;
; �.,1� �;
� �i ', \ � .
� 1 i.
��_ � ...� I . ' lk.
-_ - Tsr.. '
� '
J
� �
��l -
�
�
,
.�, �
� ;:_�y;.:. _
:.� �
�,� K. �
. �Ii► ' r'r€ � , � + _.� --11�lI �i
I �1
- _ �
' �_ � �
� ,��,�-�_�. �
��s .a�" '
� -
!�
The above photos show the flow path of runoff. The 15t photo is facing east from the proposed
addition. The 2nd photo is the south side property line and 3rd photo is the north property line.
The 4t" photo shows the 20' alley facing north to Olive Street (and the adjacent building).
Floodplain
The site is not located within a FEMA or City-regulatory floodway or floodplain.
Constructed Conditions
Hydrology
The small addition impervious area will increase the percent impervious area of the property to
23.2%. This is an increase of 4.4%. Runoff will be intercepted by roof gutters and directed to
downspouts facing east.
Rational Method Calculations:
The small addition runoff:
C coefficient: .95 (roof top)
Intensity: 1.84 in/hr (100-year rain intensity for 2hr storm)
Area: 418 sf (.0096 acres)
Runoff = .95*1.84*.0096=.017cfs
Total Runoff for all buildings: .017+.072=.089cfs
The attached drawing shows the current building outline, detached garage building outline and
proposed addition outline. In addition, it shows the flowpath (sheet flow) of precipitation
runoff.
' . ,- . . . � � �
` - -_ ". -` - _ ` - �
Detention and Water Quality
Per City code, detention and water quality improvements are not required for this
development. It is less than 5000 sf.
Erasian Control
All runoff from the small addition travels across a mildly sloped grassy lawn. Runoff wifl not
contact any exposed soil. This will prevent erosion from occurring.
Conclusion
The add'rtional runoff created by the proposed small addition will have negligible impacts to the
drainage leaving the site. Drainage will spread out and sheet flow directly east over a
devefoped mildly sloped grass lawn to the adjacent 20' alley. The east side of the alley is
bordered by a building constructed of brick with no doors or openings, and thus is entirely
protected from flooding. Once water enters the alley, there is sufficient capacity in the alley for
drainage to be conveyed to the nearby street and curb & gutter. Street capacity of this small
amount of runoff will not be significantly impacted. No adjacent property owners will receive
or be impacted by any of the resulting runoff fram the proposed new addition.
If you should have any additional questions, please contact me.
Sincerely,
Eric Lessard, PE
Engineer
Certification
I hereby certify that this drainage memorandum for the construction of a 5mall addition at 304
Smith St. was prepared by me in accordance with the provisions of the City of Fort Callins Storm
Drainage Criteria.
Registered Professional Engineer
State af Colorado No. 37562
�
W
W
�
�
CJi
_
�
�
C/7
OLIVE STREET
t
�
�
1 1
1 z T
1 "J 1
1 � 1
% �—
W
� * ¢ *
1 1
L 1 1
� 1 1
a 1 1
ODRAINAGE PLAN _ �
SCALE:1" = 20'-0"
DRAINAG� PLAN NQTES:
The existn9 impervious are.a vddl be reduced by I I 6 so�. �t. by
remov�ng the existmg laundy room or the easc s�de of the
res�derce. T7e propos�d zddition has a io!al foatprint of 534
so,. fc. The ret �ncrease of �mpervious area �s 4 i g sq. �ft,
z
O
��
oz
oJ
¢�
o�
��
a�
��
��
w �
J M
aNu_
�:�.w�
W
c�
a
Z
a
�
0