HomeMy WebLinkAbout221 EAST MOUNTAIN AVENUE - BASIC DEVELOPMENT REVIEW - BDR180012 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE RELATED DOCUMENT1218 W. ASH, STE. A, WINDSOR, COLORADO 80550
TEL. 970.674.3300 – FAX 970.674.3303
June 6, 2018
Mr. Wes Lamarque
Fort Collins Utilities – Stormwater
700 Wood Street
Fort Collins, CO 80522
Re: 221 East Mountain
Preliminary Storm Drainage Letter
Dear Wes,
Please accept the following letter describing the storm drainage intent demonstrating the proposed site
improvement’s ability to comply with the City’s stormwater requirements.
INTRODUCTION
The 221 East Mountain is a redevelopment project proposed to construct a mixed-use condominium
building with supporting utility infrastructure and site accesses (pedestrian/vehicular). The site is located
at the southwest corner of the intersection of E. Mountain Ave and Mathews St. There is a 20’ public
alley to the south, and 10’ public alley along the west. The roadway frontages will be improved along E.
Mountain Ave and Mathews St, along with the alleys to the south and west. The legal description is as
follows: Lots 1 thru 6, Block 131, Town of Fort Collins, located in part of the Northwest Quarter of
Section 12, Township 7 North, Range 69 West of the 6th Principal Meridian, City of Fort Collins, County
of Larimer, State of Colorado. The property is addresses as 221 E. Mountain Ave.
A vicinity map is attached for reference.
EXISTING CONDITIONS
The project site is located within the Udall drainage basin. The existing site consists of a vehicle
maintenance shop/showroom, and a parking lot. The site is approximately 0.45 acres and is impervious
except for a 40 sqft gravel area near the existing vehicle entrance off of E. Mountain Ave. The site
generally slopes to the east at 1.0% grade. Approximately 4,250 sqft drains to E. Mountain Ave, and the
remaining drains to Mathew St. All existing improvements will be removed from the site to
accommodate the new construction. The existing southern alley is entirely paved and the existing western
alley appears to be unimproved.
We are not aware of any site-specific drainage studies for this site.
The project is not located within a FEMA mapped 100-yr floodplain and is not within a defined City of
Fort Collins floodplain.
221 East Mountain
Preliminary Storm Drainage Letter
June 6, 2018
Page 2 of 4
PROPOSED DEVELOPMENT
The prosed redevelopment improvements will consist of a constructing a 4-story mixed-use multi-family
condominium building, with below grade parking, and will include the supporting utility infrastructure
and pedestrian/vehicle accesses. The project is planned to be constructed in a single phase.
The project has zero setback requirements, encouraging zero-lot line construction. The proposed building
footprint is approximately 18,365 sqft, consuming 94% of the site. The improvements will generally be
100% impervious, consistent with the existing conditions. The proposed improvements are divided into 3
onsite drainage basins and 2 offsite drainage basins:
Basin D1 consists of the northwest quarter of the proposed roof area and the runoff will be conveyed
through storm drains to the existing inlet on E. Mountain Ave.
Basin D2 consists of the south half of the proposed roof area. Runoff from this basin will be treated by a
roof top vegetation (green roof) and piped to a chase, discharging to the curb and gutter along Mathews
street and conveyed to an existing inlet.
Basin D3 consists of the northeast quarter of the proposed roof area. Runoff from this basin will be piped
to a chase and discharging to the curb and gutter along Mathews street and conveyed to an existing inlet.
Basin OS1 consists of the southern alley. In general, the proposed drainage patterns of this basin will not
change from existing, draining along the improved alley east to Mathews Street. The alley, where it
intersects with Mathews St, will be improved to current standards. An inlet is added at the east end of the
alley.
Basin OS2 consists of the western alley. This alley will be paved and the runoff from this alley will be
collected in an area inlet and conveyed to the existing inlet on E. Mountain Ave.
The proposed improvements do not create an increase in impervious area over 5,000 sq-ft which would
require onsite detention. Standard water quality is provided downstream within the Udall Basin and is not
required onsite. A combination of a rain garden and tree planters are proposed to provide water quality
enhancement and are designed to satisfy the Low Impact Development (LID) requirements.
All calculations and exhibits are attached for reference.
LOW IMPACT DEVELOPMENT TECHNIQUES
In reference to the code requirements for implementation of Low Impact Development (LID) techniques,
the use of a green roof is proposed to satisfy these requirements. The green roof will promote a reduction
of runoff through evapotranspiration through the roof plantings. Runoff will also be filtered through the
growing media prior to discharging. It is proposed that 50% of the roof, the south half within Basin D2,
will be the green roof. The landscape architect and architect will be detailing this system.
Below is a description of the 4 step process for selecting structural BMPs:
Urban Drainage and Flood Control District (UDFCD) recommends a Four Step Process for receiving
water protection that focuses on reducing runoff volumes, treating the water quality capture volume
221 East Mountain
Preliminary Storm Drainage Letter
June 6, 2018
Page 3 of 4
(WQCV), stabilizing drainageways and implementing long-term source controls. The Four Step Process
applies to the management of smaller, frequently occurring events.
Step 1: Employ Runoff Reduction Practices
To reduce runoff peaks, volumes, and pollutant loads from urbanizing areas, implement Low Impact
Development (LID) strategies, including Minimizing Directly Connected Impervious Areas (MDCIA).
Runoff from 50% of the roof will be absorbed and conveyed through the green roof system. The green
roof will slow runoff, promote evapotranspiration, and filter runoff prior to being released into the
adjacent storm drain system.
Step 2: Implement BMPs that Provide a Water Quality Capture Volume with Slow Release
The green roof system will provide a slow release by infiltration through the growing media.
Step 3: Stabilize Drainageways
Natural Drainageways are subject to bed and bank erosion due to increases in frequency, duration, rate
and volume of runoff during and following development. Because the site will drain to an existing
roadway and storm system, bank stabilization is unnecessary with this project.
Step 4: Implement Site Specific and Other Source Control BMPs
Proactively controlling pollutants at their source by preventing pollution rather than removing
contaminants once they have entered the stormwater system or receiving waters is important when
protecting storm systems and receiving waters. This can be accomplished through site specific needs
such as construction site runoff control, post-construction runoff control and pollution prevention / good
housekeeping. It will be the responsibility of the contractor to develop a procedural best management
practice for the site.
All calculations are attached for reference.
EROSION CONTROL
Erosion and sedimentation will be controlled on-site by use of sediment control logs, inlet protection, a
gravel construction entrance, seeding, mulch, and turf. The measures are designed to limit the overall
sediment yield increase due to construction as required by the City of Fort Collins. During overlot and
final grading the soil will be roughened and furrowed perpendicular to the prevailing winds.
During the performance of the work required by these specifications or any operations appurtenant
thereto, whether on right-of-way provided by the City or elsewhere, the contractor shall furnish all labor,
equipment, materials, and means required. The Contractor shall carry out proper efficient measures
wherever and as necessary to reduce dust nuisance, and to prevent dust nuisance that has originated from
his operations from damaging crops, orchards, cultivated fields, and dwellings, or causing naissance to
persons. The Contractor will be held liable for any damage resulting from dust originating from his
operations under these specifications on right-of-way or elsewhere.
It is unlawful to track or cause to be tracked mud or other debris onto city streets or rights-of-way.
Wherever construction vehicles access routes or intersect paved public roads, previsions must be made to
221 East Mountain
Preliminary Storm Drainage Letter
June 6, 2018
Page 4 of 4
minimize the transport of sediment by runoff or vehicles tracking onto the paved surface. Stabilized
construction entrances are required with base material consisting of 6” coarse aggregate. The contractor
will be responsible for clearing mud tracked onto city streets on a daily basis.
All temporary and permanent erosion and sediment control practices must be maintained and repaired as
needed to assure continued performance of their intended function. Silt fence and sediment control logs
will require periodic replacement. Maintenance is the responsibility of the contractor.
All disturbed areas must be seeded and mulched within 30 days of project start. Vegetation shall not be
considered established until a ground cover is achieved which is demonstrated to be mature enough to
control soil erosion to the satisfaction of the City Inspector and to survive severe weather condition.
Thank you in advance for your time and if you have any questions or comments please contact me at
(970) 347-8917.
Sincerely,
Jason T. Claeys, P.E., LEED AP
Interwest Consulting Group
Attachments
SITE
MULBERRY STREET
COLLEGE AVE
LEMAY AVE
RIVERSIDE AVENUE
VINE DRIVE
PROJECT VICINITY MAP
N.T.S.
PROJECT VERTICAL DATUM: NAVD88
CITY OF FORT COLLINS BENCHMARK 1-13: SOUTHWEST CORNER OF COLLEGE AVE. AND
MAPLE ST., ON A CONCRETE TRAFFIC SIGNAL BASE.
ELEVATION = 4979.75 (NAVD88)
FORT COLLINS BENCHMARK 5-00: LOCATED ON THE TOP OF A CURB OF A SIGNAL POLE
BASE AT THE NORTHWEST CORNER OF LINDEN ST. AND JEFFERSON ST.
ELEVATION = 4978.05 (NAVD88)
PLEASE NOTE: THIS PLAN SET IS USING NAVD88 FOR A VERTICAL DATUM. SURROUNDING
DEVELOPMENTS HAVE USED NGVD29 UNADJUSTED FOR THEIR VERTICAL DATUMS.
IF NGVD UNADJUSTED DATUM IS REQUIRED FOR ANY PURPOSE, THE FOLLOWING
EQUATION SHOULD BE USED:
NGVD UNADJUSTED = 4979.75 (NAVD88) - 3.17 = 4976.58
NGVD UNADJUSTED = 4978.05 (NAVD88) - 3.18 = 4974.87
221 East Mountain
DRAINAGE SUMMARY
Design Engineer:
Design Firm:
Project Number:
Date:
IMPERVIOUS SUMMARY:
Description
Impervious
Area
%
Impervious
Existing 19,456 99.9%
Proposed 19,496 100.0%
Difference 40 0.1%
J.Claeys
Interwest Consulting Group
1324-028-00
June 6, 2018
1324-028-00 Rational Calcs (FC).xlsx Page 1 of 6 Interwest Consulting Group
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
100% 0.95 100-year 1.25
90% 0.95
40% 0.50
0% 0.25
C2 to C10 C100
H1 19,496 0.448 11,660 7,795 0 39 0 99.9% 0.95 1.00
Agravel/pavers
(sq feet)
Alawn
(sq feet)
Weighted %
Impervious
COMPOSITE
221 East Mountain
EXISTING IMPERVIOUS AREA CALCULATION
Urban Storm Drainage Criteria Manual by Urban Drainage and Flood Control District, June 2001 (Revised January 2016)
DESIGN CRITERIA:
BASINS:
% Impervious values from Table RO-3 in the Urban Storm Drainage Criteria Manual
Sub-basin
Designation
Atotal
(sq feet)
Atotal
(acres)
Apaved
(sq feet)
Aroof
(sq feet)
Awalk/RAP
(sq feet)
Lawns
(Heavy, 2-7% Slope)
J.Claeys
Interwest Consulting Group
1324-028-00
June 6, 2018
Runoff Coefficients and Frequency Adjustment Factors for City of Fort Collins - Storm Water Criteria Manual
Land Use
Paved
Roof
Walks/RAP
Gravel/Pavers
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
100% 0.95 100-year 1.25
90% 0.95
40% 0.50
0% 0.25
C2
to C10
C100
D1 4,892 0.11 280 4,611 0 0 0 100.0% 0.95 1.00
D2 9,659 0.22 555 9,104 0 0 0 100.0% 0.95 1.00
D3 4,946 0.11 296 4,649 0 0 0 100.0% 0.95 1.00
Overall 19,496 0.45 1,131 18,364 0 0 0 100.0% 0.25 0.31
OS1 3,630 0.08 3,630 0 0 0 0 100.0% 0.95 1.00
OS2 1,296 0.03 1,296 0 0 0 0 100.0% 0.95 1.00
221 East Mountain
DEVELOPED IMPERVIOUS AREA CALCULATION
DESIGN CRITERIA:
Urban Storm Drainage Criteria Manual by Urban Drainage and Flood Control District, June 2001 (Revised January 2016)
BASINS:
% Impervious values from Table RO-3 in the Urban Storm Drainage Criteria Manual
J.Claeys
Interwest Consulting Group
1324-028-00
June 6, 2018
Land Use
Paved
Roof
Walks
Runoff Coefficients and Frequency Adjustment Factors for City of Fort Collins - Storm Water Criteria Manual
Gravel/Pavers
Lawns
(Heavy, 2-7% Slope)
Sub-basin
Designation
Atotal
(sq feet)
Atotal
(acres)
Weighted %
Impervious
A COMPOSITE
paved
(sq feet)
Design Engineer:
Design Firm:
Project Number:
Date:
EQUATIONS:
-Equation 6-2 -Equation 6-4
- 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)
Type of Travel
Surface
Cv
D1 D1 100.0% 0.95 0.112 51 0.0100 2.01 32 0.0100 Paved Areas 20 2.00 0.27 5.00 0.0100 5.00 5.00
D2 D2 100.0% 0.95 0.222 53 0.0100 2.04 108 0.0100 Paved Areas 20 2.00 0.90 5.00 0.0100 5.00 5.00
D3 D3 100.0% 0.95 0.114 52 0.0100 2.01 37 0.0100 Paved Areas 20 2.00 0.31 5.00 0.0100 5.00 5.00
OS1 OS1 100.0% 0.95 0.083 15 0.0595 0.60 148 0.0126 Paved Areas 20 2.25 1.10 5.00 0.0169 5.00 5.00
OS2 OS2 100.0% 0.95 0.030 9 0.0180 0.71 88 0.0153 Paved Areas 20 2.47 0.59 5.00 0.0156 5.00 5.00
-Urbanized Check Equation 6-5
June 6, 2018
221 East Mountain
DEVELOPED TIME OF CONCENTRATION
J.Claeys
Interwest Consulting Group
1324-028-00
LENGTH
(ft)
DESIGN CRITERIA:
Urban Storm Drainage Criteria Manual by Urban Drainage and Flood Control District, June 2001 (Revised January 2016)
Overland Flow Equations for City of Fort Collins - Storm Water 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)
OVERALL SLOPE
(ft/ft)
tc
(min)
SLOPE
(ft/ft)
ti
(min)
Design Engineer:
Design Firm:
Project Number:
Date:
EQUATIONS:
-Equation 6-2 -Equation 6-4
- 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)
Type of Travel
Surface
Cv
D1 D1 100.0% 1.00 0.112 51 0.0100 1.34 32 0.0100 Paved Areas 20 2.00 0.27 5.00 0.0100 5.00 5.00
D2 D2 100.0% 1.00 0.222 53 0.0100 1.36 108 0.0100 Paved Areas 20 2.00 0.90 5.00 0.0100 5.00 5.00
D3 D3 100.0% 1.00 0.114 52 0.0100 1.34 37 0.0100 Paved Areas 20 2.00 0.31 5.00 0.0100 5.00 5.00
OS1 OS1 100.0% 1.00 0.083 15 0.0595 0.40 148 0.0126 Paved Areas 20 2.25 1.10 5.00 0.0169 5.00 5.00
OS2 OS2 100.0% 1.00 0.030 9 0.0180 0.47 88 0.0153 Paved Areas 20 2.47 0.59 5.00 0.0156 5.00 5.00
-Urbanized Check Equation 6-5
June 6, 2018
221 East Mountain
DEVELOPED TIME OF CONCENTRATION
J.Claeys
Interwest Consulting Group
1324-028-00
LENGTH
(ft)
DESIGN CRITERIA:
Urban Storm Drainage Criteria Manual by Urban Drainage and Flood Control District, June 2001 (Revised January 2016)
Overland Flow Equations for City of Fort Collins - Storm Water 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)
OVERALL SLOPE
(ft/ft)
tc
(min)
SLOPE
(ft/ft)
ti
(min)
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)
Q (ft3/s) tc (min)
Runoff Coeff
(C2-10)
C(A) (acres)
Intensity
(in/hr)
Q (ft3/s) tc (min)
Runoff Coeff.
(C100)
C(A) (acres)
Intensity
(in/hr)
Q (ft3/s)
D1 D1 0.112 5.00 0.95 0.11 2.78 0.30 5.00 0.95 0.11 4.87 0.52 5.00 1.00 0.11 9.95 1.12
D2 D2 0.222 5.00 0.95 0.21 2.78 0.58 5.00 0.95 0.21 4.87 1.03 5.00 1.00 0.22 9.95 2.21
D3 D3 0.114 5.00 0.95 0.11 2.78 0.30 5.00 0.95 0.11 4.87 0.53 5.00 1.00 0.11 9.95 1.13
OS1 OS1 0.083 5.00 0.00 0.00 2.78 0.00 5.00 0.00 0.00 4.87 0.00 5.00 0.00 0.00 9.95 0.00
OS2 OS2 0.030 5.00 0.95 0.03 2.78 0.08 5.00 0.95 0.03 4.87 0.14 5.00 1.00 0.03 9.95 0.30
10-yr Peak Runoff 100-yr Peak Runoff
221 East Mountain
DEVELOPED PEAK RUNOFF
Urban Storm Drainage Criteria Manual by Urban Drainage and Flood Control District, June 2001 (Revised August 2006)
DESIGN CRITERIA:
Design
Point
Sub-basin Area (acres)
J.Claeys
Interwest Consulting Group
1324-028-00
June 6, 2018
2-yr Peak Runoff
Q n = C n I n A n
0.786651)
1
( 10 )
28 . 5
t c
P
I
+
=
1324-028-00 Rational Calcs (FC).xlsx Page 6 of 6 Interwest Consulting Group
LENGTH
(ft)
SLOPE
(ft/ft)
Table RO-2 VELOCITY
(ft/s)
t c = t i + t t 0 . 5
V = C v S w
V
L
tt 60
=
��
= 18 − 15� +
�
60 24� + 12 �
��
=
1.87(1.1 − ��
��
) �
� �
1324-028-00 Rational Calcs (FC).xlsx Page 5 of 6 Interwest Consulting Group
LENGTH
(ft)
SLOPE
(ft/ft)
Table RO-2 VELOCITY
(ft/s)
t c = t i + t t 0 . 5
V = C v S w
V
L
tt 60
=
��
= 18 − 15� +
�
60 24� + 12 �
��
=
1.87(1.1 − ��
��
) �
� �
1324-028-00 Rational Calcs (FC).xlsx Page 4 of 6 Interwest Consulting Group
Aroof
(sq feet)
Awalk
(sq feet)
Agravel/pavers
(sq feet)
Alawn
(sq feet)
1324-028-00 Rational Calcs (FC).xlsx Page 3 of 6 Interwest Consulting Group
1324-028-00 Rational Calcs (FC).xlsx Page 2 of 6 Interwest Consulting Group