HomeMy WebLinkAboutMAX FLATS - PDP - PDP120034 - REPORTS - DRAINAGE REPORTDrainage Letter Report
Date: February 20, 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 curb inlets are located at the southwest corner of Mulberry and Mason Streets, 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 flex space and parking. The structure's foundation system will consist of drilled concrete piers
and grade beams. The first floor (non-residential) will be concrete slab-on-grade. The total impervious area
of the proposed redevelopment is approximately 27,403 square-feet.
The site is located in the Old Town drainage master plan basin, and 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.
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 drainage master plan 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 drainage master plan 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 floodplain. 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
flood fringe of the City-designated floodplain consist of mixed-use redevelopment 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 at U.S. end of structure 4993.70
RFPE 4995.20
Lowest Floor Elevation 4992.60
HVAC Elevation 5052.60
The mixed-use redevelopment shall comply with requirements for floodproofing in lieu of elevating the
structure. The floodproofing information will be submitted at the time of the building permit application
and approved before the Certificate-of-Occupancy (CO) is issued.
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: February 19, 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 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 0.70 0.00 0.26 0.00 0.37 Clayey | Average 2% to 7% 0.07 0.88 0.88 1.00 90%
1. Table RO-11 | Rational Method Runoff Coefficients for Composite Analysis
COMPARISON OF COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
Composite Runoff Coefficient with Adjustment
2/19/2013 12:42 PM D:\Projects\860-001\Drainage\Hydrology\860-001_Existing_Rational_Calcs\Composite Runoff Coeff