HomeMy WebLinkAbout504 S WHITCOMB STREET, CARRIAGE HOUSE - FDP210006 - SUBMITTAL DOCUMENTS - ROUND 4 - DRAINAGE REPORT
Highland Development Services, Inc.
6355 Fairgrounds Avenue, Suite 100 | Windsor, CO 80550 | 970.674.7550
Memorandum
TO: Robert Kirkpatrick
FROM: Adele Bartel, E.I.T., Jason Claeys, P.E.
DATE: 12/03/21
PROJECT: 504 S. Whitcomb St.
HDS JOB NO: 20-1039-00
SUBJECT: Drainage Summary
This memo has been prepared to describe the existing conditions at 504 S. Whitcomb St, the
adjacent alley, and the drainage impacts of the proposed alley house and change in impervious
area. Below is a vicinity map showing the general location of the property:
The property is approximately 9,000 sqft (50’ wide by 180’ deep, 0.21 acres) and is located along
the east side of S. Whitcomb St and south of W. Mulberry St. A shared alley is located along the
east property line and single-family residences are adjacent to the north and south. The existing
single-family residence is located toward the west side of the lot with a shed adjacent to the alley
to the east and a small garden shed along the south property line. The shed adjacent to the alley
is proposed to be removed, along with portions of the existing sidewalk, to accommodate the
proposed addition of a detached alley house, and additional gravel parking area.
12/3/21
504 S. Whitcomb St.
12/03/21
Page 2 of 3
Based on aerial topography and a couple site visits, it appears there is a highpoint approximately
midway along the east-west property lines of the existing single-family residence. Approximately
1/3rd of the site drains west to Whitcomb St and 2/3rd of the site drains east to the alley. The alley
appears to be paved with asphalt, though the surface is degrading and there are a few potholes.
A topographic survey was completed to confirm the existing alley slopes. The alley has slopes
ranging from 1.5% to 2.0% draining to the north, and an approximate cross slope of 1.5% draining
to the west side of alley. An existing residence is located adjacent to the alley along the east side
of the alley. Based on the survey, drainage is directed away from the existing residence. In its
current condition, during a 100-yr storm event, the runoff depth would inundate the residence’s
existing crawlspace vent (see attached Appendices). Measures have been taken to ensure that
the proposed improvements do not exacerbate the current alley drainage, described on the
following page. Below are photos for reference describing the alley:
504 S. Whitcomb St.
12/03/21
Page 3 of 3
Upon the second site visit after measuring slopes with a digital level, it was discovered that there
is a highpoint at the midpoint of the south side of the single-family residence, as well as a low
point between the sidewalk and the parkway at the front of the lot. The property owner will
modify the parkway landscaping to provide positive drainage relief to S. Whitcomb St and add
roof gutters along the south roofline and east porch cover to redirect roof drainage toward S.
Whitcomb St.
The existing property at 504 S. Whitcomb that drains to the alley is approximately 27.9%
impervious. Accounting for the existing improvements to remain, redirection of the existing
single-family residence roof drainage, removal of the existing shed, and the addition of the alley
house and gravel parking area, the basin draining to the alley reduced an estimated 461 sqft.
There is a reduction of 113 sqft of roof area, and an increase of 283 sqft of gravel parking area
(semi-impervious). There is a total of 170 sqft additional impervious area draining to the alley,
consisting of additional gravel parking. The proposed site is approximately 27.7% impervious, an
approximate decrease of 0.2% imperviousness. See the attached calculations for reference.
Runoff rates were determined utilizing the rational method. The existing peak runoff rate is 0.56
cfs and the proposed peak runoff rate is 0.52 cfs, a reduction of 0.04 cfs.
With the redirection of the existing single-family residence roof drainage toward S. Whitcomb St,
the proposed improvements should not adversely impact the existing drainage conditions within
the shared alley. Please do not hesitate to call (970.674.7550) or email (jclaeys@highland-
ds.com) if there are any additional questions.
W. MULBERRY ST.S. WHITCOMB ST.504 S. WHITCOMB ST.ALLEYHIGHLANDDEVELOPMENT SERVICES6355 FAIRGROUNDS AVENUE, SUITE 100 | WINDSOR, CO 80550PHONE: 970.674.7550 | EMAIL: Info@Highland-DS.com | www.Highland-DS.comOFSHEET
DRAWN BY
CHECKED BY
DATE
SCALE (H)
HDS PROJ #
SCALE (V)504 S. WHITCOMB ST.ALLEY DRAINAGE EXHIBIT10/21/21
1" = 30'
N/A
JTC
JTC
21-1039-00
DR
1 1
N
W E
S
0
SCALE: 1" = 30'
3015
BASIN COEFFICIENT (100-YR)
BASIN DESIGNATION
BASIN AREA
DESIGN POINT
DRAINAGE FLOW ARROW
DRAINAGE BASIN BOUNDARY
LEGEND
EX CONTOUR
(AERIAL TOPOGRAPHY)
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Thursday, Oct 21 2021
Existing Alley @ Crawlspace Vent
User-defined
Invert Elev (ft) = 5.16
Slope (%) = 1.50
N-Value = 0.025
Calculations
Compute by: Known Q
Known Q (cfs) = 2.24
(Sta, El, n)-(Sta, El, n)...
( 0.00, 6.11)-(5.91, 5.19, 0.025)-(10.95, 5.16, 0.025)-(21.09, 5.25, 0.025)-(27.73, 5.22, 0.025)-(29.02, 5.17, 0.025)-(29.12, 6.17, 0.025)
Highlighted
Depth (ft) = 0.13
Q (cfs) = 2.240
Area (sqft) = 1.96
Velocity (ft/s) = 1.14
Wetted Perim (ft) = 23.88
Crit Depth, Yc (ft) = 0.12
Top Width (ft) = 23.76
EGL (ft) = 0.15
-5 0 5 10 15 20 25 30 35
Elev (ft)Depth (ft)Section
4.50 -0.66
5.00 -0.16
5.50 0.34
6.00 0.84
6.50 1.34
7.00 1.84
Sta (ft)
Crawl space vent
flooded in existing
100-yr event.
504 S. Whitcomb Street
DRAINAGE SUMMARY
Design Engineer:
Design Firm:
Project Number:
Date:
IMPERVIOUS SUMMARY:
Description Basin Area
(sqft)
Roof Area
(sqft)
Gravel Area
(sqft)
Total
Impervious
Area (sqft)
%
Impervious
100-yr Peak
Runoff Rate
(cfs)
Existing 5,818 761
723 1,484 27.9% 0.56
Proposed 5,357 648 1,006 1,654 28.8% 0.52
Difference -461 -113 283 170 0.9% -0.04
A.McKenna
Highland Development Services
21-1039-00
November 15, 2021
This table is to document the existing and proposed impervious areas, percent impervious, and
associated peak runoff rates, related to the areas draining to the alley from 504 S. Whitcomb St.
504 S. Whitcomb St - Rational Calcs.xlsx Page 1 of 5 Highland Development Services
Design Engineer:
Design Firm:
Project Number:
Date:
%
Impervious
Runoff
Coefficient C
Return
Period
Frequency
Adjustment
Factor (Cf)
100% 0.95
2-year to
10-year 1.00
90% 0.95 100-year 1.25
90% 0.95
40% 0.50
15% 0.35
* Lawn area assumed to be 15% imperv and 0.35 coeff to account for misc walks
C2 to C10 C100
Ex Alley 22,098 0.507 718 3,458 0 1,958 15,964 31.7% 0.48 0.60
Ex 504 5,818 0.134 0 761 0 723 4,334 27.9% 0.45 0.56
Prop 504 5,357 0.123 0 648 0 1,006 3,703 28.8% 0.45 0.56
Weighted %
Impervious
COMPOSITEApaved
(sq feet)
Aroof
(sq feet)
Awalk
(sq feet)
Agravel/pavers
(sq feet)
Alawn
(sq feet)
Gravel/Pavers
Lawns*
(Clayey, Flat < 2% Slope)
Sub-basin
Designation
Atotal
(sq feet)
Atotal
(acres)
% Impervious values from Table 3.2.2 in the Fort Collins Stormwater Criteria Manual
Land Use
Paved
Roof
Walks
Runoff Coefficients and Frequency Adjustment Factors for City of Fort Collins - Storm Water Criteria Manual
504 S. Whitcomb Street
DEVELOPED IMPERVIOUS AREA CALCULATION
DESIGN CRITERIA:
Fort Collins Stormwater Criteria Manual
BASINS:
A.McKenna
Highland Development Services
21-1039-00
November 15, 2021
504 S. Whitcomb St - Rational Calcs.xlsx Page 2 of 5 Highland Development Services
Design Engineer:
Design Firm:
Project Number:
Date:
EQUATIONS:
-Equation 5-3 -Equation 5-4 -Urbanized Check Equation 3.3-5
- CoFC Overland Flow
CONSTRAINTS:
300 ft - Overland flow shall not exceed for developed condition
500 ft - Overland flow shall not exceed for undeveloped condition
Final t c = minimum of t i + t t and urbanized basin check
recommended minimum t c = 5 min for urbanized basins
Time of Concentration (2-yr to 10-yr)
Ex Alley Ex Alley 31.7% 0.48 0.507 191 0.0063 18.79 24 0.0078 0.14 0.025 1.42 0.28 19.07 215 11.19 11.19
Ex 504 Ex 504 27.9% 0.45 0.134 150 0.0047 19.24 13 0.0174 0.14 0.025 2.12 0.10 19.34 163 10.90 10.90
Prop 504 Prop 504 28.8% 0.45 0.123 70 0.0076 11.11 101 0.0040 0.14 0.025 1.02 1.65 12.76 170 10.95 10.95
OVERALL LENGTH
(ft/ft)
tc
(min)
SLOPE
(ft/ft)
ti
(min)
LENGTH
(ft)
SLOPE
(ft/ft)
VELOCITY
(ft/s)
ROUGHNESS
COEFFICIENT
HYDRAULIC
RADIUS
(ft)
LENGTH
(ft)
DESIGN CRITERIA:
City of Fort Collins Stormwater Criteria Manual
Overland Flow Equations for City of Fort Collins Stormwater Criteria Manual
SUB-BASIN DATA INITIAL/OVERLAND
TIME (ti)TRAVEL TIME (tt)
tc=ti+tt
(min)
Urban Check Final
tc
(min)DESIGN
POINT Sub-basin %
Impervious C2-10
AREA
(acres)
tt
(min)
November 15, 2021
504 S. Whitcomb Street
DEVELOPED TIME OF CONCENTRATION
A.McKenna
21-1039-00
Highland Development Services
ticttt+=
V
Ltt60
=1.871.1
10
180
+=LtcV.R2/3 S 1/2
504 S. Whitcomb St - Rational Calcs.xlsx Page 3 of 5 Highland Development Services
Design Engineer:
Design Firm:
Project Number:
Date:
EQUATIONS:
-Equation 5-3 -Equation 5-4 -Urbanized Check Equation 3.3-5
- CoFC Overland Flow
CONSTRAINTS:
300 ft - Overland flow shall not exceed for developed condition
500 ft - Overland flow shall not exceed for undeveloped condition
Final t c = minimum of t i + t t and urbanized basin check
recommended minimum t c = 5 min for urbanized basins
Time of Concentration (100-yr)
Ex Alley Ex Alley 31.7% 0.60 0.507 191 0.0063 15.20 24 0.0078 0.14 0.025 1.42 0.28 15.48 215 11.19 11.19
Ex 504 Ex 504 27.9% 0.56 0.134 150 0.0047 15.95 13 0.0174 0.14 0.025 2.12 0.10 16.04 163 10.90 10.90
Prop 504 Prop 504 28.8% 0.56 0.123 70 0.0076 9.18 101 0.0040 0.14 0.025 1.02 1.65 10.83 170 10.95 10.83
OVERALL LENGTH
(ft/ft)
tc
(min)
SLOPE
(ft/ft)
ti
(min)
LENGTH
(ft)
SLOPE
(ft/ft)
VELOCITY
(ft/s)
HYDRAULIC
RADIUS
(ft)
ROUGHNESS
COEFFICIENT
LENGTH
(ft)
DESIGN CRITERIA:
City of Fort Collins Stormwater Criteria Manual
Overland Flow Equations for City of Fort Collins Stormwater Criteria Manual
SUB-BASIN DATA INITIAL/OVERLAND
TIME (ti)TRAVEL TIME (tt)
tc=ti+tt
(min)
Urban Check Final
tc
(min)DESIGN
POINT Sub-basin %
Impervious C100
AREA
(acres)
tt
(min)
November 15, 2021
504 S. Whitcomb Street
DEVELOPED TIME OF CONCENTRATION
A.McKenna
21-1039-00
Highland Development Services
ticttt+=
V
Ltt60
=1.871.1
10
180
+=LtcV.R2/3 S 1/2
504 S. Whitcomb St - Rational Calcs.xlsx Page 4 of 5 Highland Development Services
Design Engineer:
Design Firm:
Project Number:
Date:
EQUATIONS:I = rainfall intensity (in/hr)
Q n = n -yr peak discharge (cfs)P 1 = one-hour point rainfall depth (in)
C n = n -yr runoff coefficient t c = time of concentration (min)
I n = n -yr rainfall intensity (in/hr) P 1-2yr =0.82 in
A n = Basin drainage area (ac)P 1-10yr =1.40 in
P 1-100yr 2.86 in
BASIN SUMMARY:
tc (min)
Runoff
Coeff
(C2-10)
C(A)
(acres)
Intensity
(in/hr)
Q2
(ft3/s)
tc (min)
Runoff
Coeff
(C2-10)
C(A)
(acres)
Intensity
(in/hr)
Q10
(ft3/s)
tc (min)
Runoff
Coeff
(C100)
C(A) (acres)Intensity
(in/hr)
Q100
(ft3/s)
Ex Alley Ex Alley 0.507 31.7% 11.19 0.48 0.24 2.12 0.51 11.19 0.48 0.24 3.61 0.87 11.19 0.60 0.30 7.38 2.24
Ex 504 Ex 504 0.134 27.9% 10.90 0.45 0.06 2.14 0.13 10.90 0.45 0.06 3.65 0.22 10.90 0.56 0.07 7.46 0.56
Prop 504 Prop 504 0.123 28.8% 10.95 0.45 0.06 2.14 0.12 10.95 0.45 0.06 3.65 0.20 10.83 0.56 0.07 7.48 0.52
10-yr Peak Runoff 100-yr Peak Runoff
504 S. Whitcomb Street
DEVELOPED PEAK RUNOFF
DESIGN CRITERIA:
Design
Point Sub-basin Area
(acres)
A.McKenna
Highland Development Services
21-1039-00
July 1, 2021
2-yr Peak Runoff
City of Fort Collins Stormwater Criteria Manual
%
Impervious
nnnnAICQ=
(0.786651)
1
)10(
5.28
ct
PI
+
=
504 S. Whitcomb St - Rational Calcs.xlsx Page 5 of 5 Highland Development Services