HomeMy WebLinkAboutMAX FLATS - PDP/FDP - FDP130008 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTDrainage Letter
Date: April 24, 2013
Project: MAX Flats Project No. 860-001
Fort Collins, Colorado
Attn: Mr. Wes Lamarque
City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
Dear Wes:
This letter serves to address the stormwater impacts of the MAX Flats mixed-use redevelopment
located at 203 West Mulberry Street. The existing site includes a two-level concrete block building
with a footprint of approximately 3050 square-feet. It is located along the western edge of the
property and roughly centered north-to-south. The remainder of the site is almost entirely paved as
a parking lot. Existing landscaping is minimal and generally limited to narrow strips of shrubs along
the north and east edges of the property. The site currently surface drains from southwest to
northeast along flat grades of less than one percent. Two existing off-site combination inlets are
located at the southwest corner of Mulberry and Mason Streets, and along the south and west
flowlines of each, respectively. These inlets collect existing developed site runoff and discharge it to
the public storm sewer system. The proposed condition will generally preserve existing drainage
patterns. The total impervious area of the existing site is approximately 28,588 square-feet.
The proposed redevelopment will completely remove all existing above-grade structures and
adjacent hardscape. The project will consist of four stories of residential units over a ground level
containing a commercial/retail flex space and parking. The structures foundation system will consist
of drilled concrete piers and grade beams. The first floor commercial/retail flex space (i.e., non-
residential) foundation will be concrete slab-on-grade. The total impervious area of the proposed
redevelopment is approximately 27,403 square-feet.
Developed runoff from the interior region of the site and roof area will drain to an on-site storm
drain system that will connect to the existing combination inlet in Mulberry Street. The proposed
system will receive developed runoff from Inlet A3 (Neenah R-3403-F), which will be located within
the covered concrete parking area, and Inlet A4, which will be located in the adjacent landscaping.
The roof drains will connect to a subsurface system (i.e., 10" HDPE), which will connect to Inlet
A3. The roof drain system is designed to convey flows from the 10-year storm event. The difference
between the 100 and 10-year flows (1.95 cfs) was added to the concrete parking area surface
runoff (i.e., 1.95 + 2.47 = 4.42 cfs) to size Inlet A3. Peripheral areas along the north and east
property lines will sheet flow into the adjoining street sections. Refer to the attachments for
developed runoff calculations, area inlet sizing and hydraulic grade line calculations.
The project site is located in the Old Town Basin (Old Town Basin Master Drainage Plan, Anderson
Consulting Engineers, Inc., July 15, 2003); therefore, on-site detention is not required so long as
the increase in impervious area is less than 5,000 square-feet. The total impervious area will
decrease with this development by approximately 1,185 square-feet (28,588 - 27,403). A detailed
breakdown of the existing and proposed impervious areas is attached to this letter (see attached
Exhibit 1).
Although stormwater quantity detention is not required, stormwater quality mitigation will be
addressed by both temporary and permanent Best Management Practices (BMPs). During
construction, the contractor will follow appropriate and applicable City of Fort Collins (City)
standards for erosion and sediment control. Post construction erosion control will be achieved by a
fully established and stabilized site. The areas disturbed during construction will be permanently
stabilized with hardscape (i.e., concrete), landscape or building structure. This portion of the Old
Town Basin receives regional water quality treatment in the Udall Natural Area; therefore,
permanent on-site water quality mitigation is not required.
This project is located in the Old Town Basin and impacted by the associated City-designated 100-
year floodway/floodplain. In particular, approximately twenty-seven feet of the north portion of the
project site is located in Zone AE of the City-designated 100-year floodplain. The remaining south
portion of the project site is located in Zone AO of the City-designated 100-year floodfringe. A
Floodplain Use Permit (FUP), which will be submitted at the time of building permit application,
shall be obtained prior to commencing with any construction within the floodplain. The
improvements within the floodfringe of the City-designated floodplain consist of the non-residential
use portion of a mixed-use structure with the residential use on a floor completely above the
Regulatory Flood Protection Elevation (RFPE). Refer to the table below for a summary of pertinent
elevations.
Elevation Designation NGVD 29
(Unadjusted)
BFE 4993.70
RFPE 4995.20
Minimum Floodproofing
Elevation 4995.20
Lowest Floor Elevation 4992.60
HVAC Elevation 5052.60
The non-residential (i.e., commercial/retail flex space) portion of the mixed-use redevelopment shall
comply with requirements for floodproofing in lieu of elevating the structure. Additional details for
the flood doors and elevator entrance(s) will be provided on a Floodproofing Plan with the next
submittal. The floodproofing information (per City Floodproofing Guidelines) will be included with
the utility plan set and completed prior to plan approval.
It is important to note that the floodway of the City-designated floodplain currently follows the
midpoint of the existing sidewalk along the project's Mulberry Street frontage. The proposed
improvements within the floodway of the City-designated floodplain consist of non-structural
development (i.e., driveway and streetscape modifications). A No-Rise Certification is required for
all work within the floodway. Further details and supporting documentation for the No-Rise
Certification shall accompany the FUP application.
The proposed grading concept closely matches existing elevations and drainage patterns, and only
varies in an effort to improve the existing condition. On-site detention is proven to be unnecessary.
On-site stormwater quality is not required as runoff is treated in the Udall Natural Area water
treatment facility. The project is designed in accordance to the governing floodplain regulations
contained within Chapter 10 of the City Code, and the Floodplain Use Permit and necessary
certifications shall ensure that construction remains compliant with Chapter 10. Therefore, it is my
professional opinion that the MAX Flats mixed-use redevelopment satisfies all applicable stormwater
criteria.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely,
Herman H. Feissner, PE
enc.
MAX Flats
CHARACTER OF SURFACE1:
Runoff
Coefficient
Percentage
Impervious Project: MAX Flats
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: H. Feissner
Asphalt ……....……………...……….....…...……………….………………………………………………………….0.95 100% . Date: April 24, 2013
Concrete …….......……………….….……….………………..….…………………………………………………….0.95 100% .
Gravel (packed) ……….…………………….….…………………………..………………………………………….0.50 40% .
Roofs …….…….………………..……………….……………………………………………………………………… 0.95 100%
Pavers…………………………...………………..……………………………………………………………………… 0.40 22%
Lawns and Landscaping
Sandy Soil
Flat <2% ……………………………………………………………………………………………………………… 0.10 0%
Average 2% to 7% ………………………………………………………………………………………………….0.15 0% .
Steep >7% …………………………………………………………………………………………………………… 0.20 0%
Clayey Soil
Flat <2% ……………………………………………………………………………………………………………… 0.20 0%
Average 2% to 7% ………………………………………………………………………………………………….0.25 0% .
Steep >7% …………………………………………………………………………………………………………… 0.35 0% 2-year Cf
= 1.00 10-year Cf = 1.00 100-year C
f = 1.25
Runoff Coefficients are taken from the Fort Collins Stormwater Criteria Manual, Table RO-11
Basin ID
Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Gravel
(ac)
Area of
Roofs
(ac)
Soil Type and Average
Slope
Area of
Lawns and
Landscaping
(ac)
2-year
Composite
Runoff
Coefficient
10-year
Composite Runoff
Coefficient
100-year
Composite Runoff
Coefficient
Composite
% Imperv.
Existing Conditions 0.70 0.57 0.02 0.00 0.07 Clayey | Average 2% to 7% 0.04 0.91 0.91 1.00 94%
Proposed Conditions 0.70 0.00 0.26 0.00 0.37 Clayey | Average 2% to 7% 0.07 0.88 0.88 1.00 90%
Roof Area 0.37 0.00 0.00 0.00 0.37 Clayey | Average 2% to 7% 0.00 0.95 0.95 1.00 100%
MAX Flats
Overland Flow, Time of Concentration:
Project: MAX Flats
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt
= L / 60V
Tc
= Ti
+ Tt
(Equation RO-2)
Velocity (Gutter Flow), V = 20·S
½
Velocity (Swale Flow), V = 15·S
½
NOTE: C-value for overland flows over grassy surfaces; C = 0.25
Is Length
>500' ?
C*Cf
(2-yr
Cf
=1.00)
C*Cf
(10-yr
Cf
=1.00)
C*Cf
(100-yr
Cf
=1.25)
Length,
L
(ft)
Slope,
S
(%)
Ti
2-yr
(min)
Ti
10-yr
(min)
Ti
100-yr
(min)
Length,
L
(ft)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
Length,
L
(ft)
MAX Flats
Rational Method Equation: Project: MAX Flats
Calculations By:
Date:
Rainfall Intensity:
Proposed Conditions 0.70 5.0 5.0 5.0 0.88 0.88 1.00 2.85 4.87 9.95 1.76 3.00 6.97
Roof Area 0.37 5.0 5.0 5.0 0.95 0.95 1.00 2.85 4.87 9.95 0.99 1.70 3.65
Concrete Parking Area 0.25 5.0 5.0 5.0 0.83 0.83 1.00 2.85 4.87 9.95 0.59 1.01 2.47
Intensity,
i2
(in/hr)
100-yr
Tc
(min)
Intensity,
i10
(in/hr)
PROPOSED RUNOFF COMPUTATIONS
C100
Design
Point
Flow,
Q100
(cfs)
Flow,
Q2
(cfs)
10-yr
Tc
(min)
2-yr
Tc
(min)
C2
Flow,
Q10
(cfs)
Intensity,
i100
(in/hr)
Basin(s)
H. Feissner
Rainfall Intensity taken from the Fort Collins Stormwater Criteria Manual (FCSCM), Tables RA-7 and RA-8
April 24, 2013
C10
Area, A
(acres)
Q C f C i A
4/24/2013 10:02 AM D:\Projects\860-001\Drainage\Hydrology\860-001_Rational_Calcs\Runoff
MAX Flats
Area Inlet Capacity Calculations
Grate: Neenah R-3402-E
Number of Grates: 1
Length: 24 in 2.000 ft
Width: 24 in 2.000 ft
Open Area: 302.40 sq in 2.1 sq ft
Weir Perimeter, P 96.00 in 8.00 ft
Open Area, A 302.40 sq in 2.10 sq ft
Clogging Factor, c 50%
Stage Interval, ¨d 0.050 ft
Weir Calculation: Orifice Calculation:
Qi = C
wPd
1.5
Qi = C
oAg
(2gd)
0.5
Cw 3.00 C
o 0.61
cP 4.00 ft CoA
g 1.05 ft
2
Water Depth, d Elevation Qw-INLET Q
o-INLET Inflow
ft ft cfs cfs cfs
0.00 4992.25 0.00 0.00 0.00 **Proposed top of grate elevation
0.05 4992.30 0.13 1.15 0.13
0.10 4992.35 0.38 1.63 0.38
0.15 4992.40 0.70 1.99 0.70 ĺQ2
=0.59 cfs
0.20 4992.45 1.07 2.30 1.07 ĺQ10
=1.01 cfs
0.25 4992.50 1.50 2.57 1.50
0.30 4992.55 1.97 2.82 1.97
0.35 4992.60 2.48 3.04 2.48
0.40 4992.65 3.04 3.25 3.04
0.45 4992.70 3.62 3.45 3.45
0.50 4992.75 4.24 3.63 3.63
0.55 4992.80 4.89 3.81 3.81
0.60 4992.85 5.58 3.98 3.98
0.65 4992.90 6.29 4.14 4.14
0.70 4992.95 7.03 4.30 4.30
0.75 4993.00 7.79 4.45 4.45
0.80 4993.05 8.59 4.60 4.60
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80
Inlet Capacity, cfs
Depth of Flow, Feet
Channel Report
Hydraflow Express Extension for AutoCAD® Civil 3D® 2012 by Autodesk, Inc. Wednesday, Apr 24 2013
Roof Drain | 10 inch Diameter HDPE
Circular
Diameter (ft) = 0.83
Invert Elev (ft) = 5280.00
Slope (%) = 0.50
N-Value = 0.012
Calculations
Compute by: Known Q
Known Q (cfs) = 1.70
Highlighted
Depth (ft) = 0.70
Q (cfs) = 1.700
Area (sqft) = 0.49
Velocity (ft/s) = 3.49
Wetted Perim (ft) = 1.93
Crit Depth, Yc (ft) = 0.59
Top Width (ft) = 0.60
EGL (ft) = 0.89
0 1
Elev (ft) Section
5279.75
5280.00
5280.25
5280.50
5280.75
5281.00
Reach (ft)
Qw-INLET
Qo-INLET
Inflow
4/23/201310:28 AM
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
Proposed Conditions No 0.95 0.95 1.00 239 0.81% 4.6 4.6 3.1 0 0.00% N/A N/A 0 0.00% N/A N/A 5.0 5.0 5.0
Roof Area No 0.95 0.95 1.00 0 0.00% N/A N/A N/A 0 0.00% N/A N/A 0 0.00% N/A N/A 5.0 5.0 5.0
Concrete Parking Area No 0.95 0.95 1.00 0 0.00% N/A N/A N/A 0 0.00% N/A N/A 0 0.00% N/A N/A 5.0 5.0 5.0
* Time of Concentrations are calculated for the entire basin and used for both the Impervious and Pervious portions of the basins.
PROPOSED TIME OF CONCENTRATION COMPUTATIONS
H. Feissner
April 24, 2013
Design
Point
Basin
Overland Flow Gutter Flow Swale Flow Time of Concentration
(Equation RO-4)
3
1
1 . 87 1 . 1 *
S
Ti C Cf L
4/24/2013 10:02 AM D:\Projects\860-001\Drainage\Hydrology\860-001_Rational_Calcs\Tc
Concrete Parking Area 0.25 0.00 0.21 0.00 0.00 Clayey | Average 2% to 7% 0.04 0.83 0.83 1.00 83%
1. Table RO-11 | Rational Method Runoff Coefficients for Composite Analysis
COMPARISON OF COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
Composite Runoff Coefficient with Adjustment
4/24/2013 10:01 AM D:\Projects\860-001\Drainage\Hydrology\860-001_Rational_Calcs\Composite Runoff Coeff