HomeMy WebLinkAboutGrading Plans - 08/25/2021i2
August 25, 2021
Mr. Dan Mogen, EI
Development Review Engineer
Stormwater Engineering & Development Review Division
City of Fort Collins
281 North College Avenue
Fort Collins, CO 80524
RE: Drainage Letter for 224 2"d Street
Fort Collins, CO
Dear Mr. Mogen:
Consultants, Inc.
16911 Potts PIacE, Mead, CO
970.217.9148
troy@i2-consultants.com
Grading/Stormwater Plan Accepted
(Building Permit Released)
Accepted by: Dan Mogen
Date: 08/25/2021
We are pleased to present this Drainage Letter for 224 2nd Street, Fort Collins, Colorado.
The proposed project will consist of an addition and remodel to the existing residence on
the property.
The subject site is within the Buckingham Place Subdivision, which is north of Lincoln
Avenue and east of Linden Street.
Furthermore, the subject site is within the Buckingham-Lincoln Corridor Master Plan, which
is a subarea of the Poudre River Basin. The assumed imperviousness for this basin is
45%.
The proposed project will consist of a remodel to the existing +/-800 s.f. residence as well
as the addition of a roughly 495 s.f. garage and 625 s.f. addition to the residence. The
project will also include removal of the gravel driveway and gravel area south of the
existing residence. A concrete driveway will be constructed from the proposed garage to
2"a Street.
An existing drainage plan has been included with this letter for comparison to the proposed
improvements. The overall site imperviousness will increase from 34.3 % to 44.3% upon
completion of the proposed improvements. __H__o_we_v_er, there will be a decrease of rouqhly
437 sf of impervious area draininq to the alle�
The overall site impervious area will increase bv approximately 826 sf. Water quality for
this single family lot will be addressed through the downspouts from the roof being directed
to landscaped areas or swales, so runoff from the roof will not be directly connected to
impervious area.
Page 1
224 2"d Street, Fort Collins, CO Drainage Letter
August 25, 2021
A summary and comparison of the existing and proposed basins for the site is provided
below.
Table
� 1: Basin Summar Refer to Basin Im erviousness Calculations
BASIN SUMMARY
Basin ID Area Imperv. Notes
(ac.) %)
EXISTING
A 0.08 68.5 Drains to 2�d St; contains majority of ex. house,
concrete, ravel and 2nd St fronta e
B 0.11 10.7 Drains to alley; contains portion of ex. house(+/-
224 sfl, ravel and alley frontage
Overall 0.19 34.3 Existin Overall site
PROPOSED
A 0.09 81.0 Drains to 2nd St; contains majority of ex. house
and proposed addition, concrete and 2�d St.
fronta e
B 0.10 13.4 Drains to alley; contains portion of ex. house
+/-224 s and ortion of addition +/-205 s
Overall 0.19 44.3 Proposed Overall site (below assumed 45%
im erv.
Table 2: Basin Com arison Refer to Basin Im erviousness Calculations
BASIN COMPARISON
Basin ID Area s Im erv. % Notes
A Developed Net increase of The net increase of 12.5% in Basin A is due to:
Basin A is 12.5% • Larger Basin area;
407 sf imperviousness • Net increase of 241 sf of lawns, clayey soils;
larger than between • Net increase of 978 sf of roof area;
Existing Developed • Net increase of 151 sf of concrete surfaces;
Basin B Basin A and • Net decrease of 963 sf of gravel (963 sf*.4% _
Existing Basin 385.2 sf);
A • Net Change: 985+151-385 = 744 sf impervious
increase
B Developed Net increase of The net increase of 2.7% in Basin B is due to:
Basin B is 2.7% • Net decrease of 304 sf of lawns, clayey soil;
407 sf imperviousness • Net increase of 206 sf of roof area;
smaller between • Net decrease of 519 sf of concrete surfaces;
than Developed • Net decrease of 309 sf of gravel (309 sf*.4°/o =
Existing Basin B and 123.6 sf);
Basin B Existing Basin • Net Change: 206-519-124 = 437 sf impervious
B decrease
Overall No change Net increase of The net increase of 10% in the Overall Site is due to:
in overall 10% • Net decrease of 63 sf of �awns, clayey soils;
basin area imperviousness • Net increase of 1,184 sf of roof area;
between • Net increase of 151 sf of concrete surfaces;
Developed • Net decrease of 1,272 sf of gravel (1272 sf*.4% _
Overall Site 508.8 sf);
and Existing • Net Change: 1184+151-509 = 826 sf
Overall Site �mpervious increase
Page 2
224 2"d Street, Fort Collins, CO Drainage Letter
August 25, 2021
PROPOSED DRAINAGE BASINS
The subject site has been divided into two (2) basins for calculating the impervious area of
the property after the construction of the proposed improvements.
BAS I N A
Basin A consists of the western portion of the site and half of the roadway for 2nd Street
across the lot frontage. This basin consists of the existing residence, proposed garage
and addition, concrete driveway and walks and landscaped area. Runoff will generally
flow from east to west and will reach the existing curb and gutter for 2nd Street.
Downspouts from the roof will be directed to landscaped areas or swales. The calculated
basin imperviousness is 81.0%.
:_ � :
Basin B consists of the eastern portion of the site and half of the existing (unpaved) alley
across the lot frontage. This basin consists of the existing residence, proposed addition,
and landscaped area. Runoff will generally flow from west to east to the existing alley.
The existing lot and alley have gentle/mild slopes. The alley has a high point that directs
runoff to the north and south. The calculated basin imperviousness is 13.4%.
The overall site imperviousness (including half of 2"d Street and half of the alley)
has been calculated to be 44.3%, which is less than the assumed imperviousness of
45%. Based on the calculated imperviousness, detention is not required as part of this
project. Please refer to the impervious calculations included with this letter for additional
information.
ANTICIPATED IMPACTS
The existing elevations along the northern portion of the existing residence will not be
revised with this project. The proposed porch will match existing elevations at the
northwest corner of the lot.
The grading for the garage and house addition will require elevation changes to the
middle-southern portion of the lot. The grading will tie-in and match existing elevations
along the southern property line and at the back of the existing sidewalk for 2nd Street.
There is an existing high point along the southern property line and runoff will be directed
west and east along the property line. The proposed grading for this project shall not
direct runoff to adjacent properties.
The eastern half (approximately) of the lot will not be impacted by this project and the
elevations will not be revised. This lot does not take access from the existing alley. Due to
the nature of the development and existing conditions, the increase in runoff flowing east is
not expected to be significant. Runoff has been directed to the west to the extent practical,
considering the existing conditions and elevations.
Page 3
224 2"d Street, Fort Collins, CO Drainage Letter
August 25, 2021
The proposed improvements to 224 2nd Street will not have an adverse impact on adjacent
properties or infrastructure.
We look forward to your favorable review of this project and letter. Please do not hesitate
to contact our office if you have any questions.
Sincerely,
i2 Consultants, Inc. oo RE�
� . ...... s
v��.������ .
�:'� 41 1 5 9� : W
o ���. 08/25/2021. � �
F` �'��. �` `
Troy W. Campbell, PE SS/�NA L���
Senior Project Engineer
Page 4
Basin Imperviousness
224 2nd Street
Fort Collins, CO
8/25/2021
EXISTING BASINS
i2
Consultants, Inc.
16911 Potts Place, Mead, CO
970.217.9148
troy@i2-consultants.com
EXISTING BASIN RUNOFF - Basin A
Land Use Area ft2 Area ac I%
Lawns, Cla e Soils 489 0.01 2
Roofs 580 0.01 100
Concrete Surfaces 519 0.01 100
As halt 800 0.02 100
Gravel 963 0.02 40
Wghtd Avg & Total Area 3,351 0.08 68.5
EXISTING BASIN RUNOFF - Basin B
Land Use Area ft2 Area ac I%
Lawns, Cla e Soils 4,097 0.09 2
Roofs 224 0.01 100
Concrete Surfaces - - 100
As halt - - 100
Gravel 539 0.01 40
Wghtd Av & Total Area 4,860 0.11 10.7
EXISTING BASIN RUNOFF - SITE
Land Use Area ft2 Area ac I%
Lawns, Cla e Soils 4,586 0.11 2
Roofs 804 0.02 100
Concrete Surfaces 519 0.01 100
As halt 800 0.02 100
Gravel 1,502 0.03 40
Wghtd Avg & Total Area 8,211 0.19 34.3
DEVELOPED BASINS
DEVELOPED BASIN RUNOFF - Basin A
Land Use Area (ft2) Area (ac) I(%)
Lawns, Cla e Soils 730 0.02 2
Roofs 1,558 0.04 100
Concrete Surfaces 670 0.02 100
As halt 800 0.02 100
Gravel - - 40
Wghtd Avg & Total Area 3,758 0.09 81.0
DEVELOPED BASIN RUNOFF - Basin B
Land Use Area ft2 Area ac I(%
Lawns, Cla e Soils 3,793 0.09 2
Roofs 430 0.01 100
Concrete Surfaces - - 100
Asphalt - - 100
Gravel 230 0.01 40
Wghtd Avg & Total Area 4,453 0.10 13.4
DEVELOPED BASIN RUNOFF - SITE
Land Use Area ft2 Area ac I%
Lawns, Cla e Soils 4,523 0.10 2
Roofs 1,988 0.05 100
Concrete Surfaces 670 0.02 100
As halt 800 0.02 100
Gravel 230 0.01 40
Wghtd Avg & Total Area 8,211 0.19 44.3
BASIN COMPARISON (Developed - Existing)
BASIN A(Developed Basin A is 407 s.f. larger than Existing Basin A)
Land Use Area (ft2) Area (ac) I(%) Notes
Lawns, Cla e Soils 241 0.006 2 Net increase of 241 s.f.
Roofs 978 0.022 100 Net increase of 978 s.f.
Concrete Surfaces 151 0.003 100 Net increase of 151 s.f.
As halt - - 100 No change in asphalt
Gravel 963 0.022 40 Net decrease of 963 s.f.
Net increase of 12.5% impervousness between Developed and Existing Condition due to the
increase in Roofs and Concrete Surfaces
BASIN B(Developed Basin B is 407 s.f. smaller than Existin
Land Use Area ft2 Area ac I%
Lawns, Cla e Soils 304 (0.007
Roofs 206 0.005
Concrete Surfaces - -
As halt - -
Gravel 309 0.007
Net increase of 2.7% impervousness between Developed and E
OVERALL SITE BASIN COMPARISON
Basin B)
Notes
2 Net decrease of 304 s.f.
100 Net increase of 206 s.f.
100 Net decrease of 519 s.f.
100 No change in asphalt
40 Net decrease of 309 s.f.
isting Condition
Land Use Area ft2 Area ac I% Notes
Lawns, Cla e Soils 63 0.001 2 Net decrease of 63 s.f.
Roofs 1,184 0.027 100 Net increase of 1184 s.f.
Concrete Surfaces 151 0.003 100 Net increase of 151 s.f.
As halt - - 100 No change in asphalt
Gravel 1,272 0.029 40 Net decrease of 1,272 s.f.
Net fncrease of 10% fmpervousness between Developed and txistfng Gonditfon
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