HomeMy WebLinkAbout221 EAST MOUNTAIN AVENUE - BDR180012 - SUBMITTAL DOCUMENTS - ROUND 5 - DRAINAGE RELATED DOCUMENT1218 W. ASH, STE. A, WINDSOR, COLORADO 80550
TEL. 970.674.3300 – FAX 970.674.3303
March 6, 2019
Mr. Wes Lamarque
Fort Collins Utilities – Stormwater
700 Wood Street
Fort Collins, CO 80522
Re: 221 East Mountain
Final 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
Final Storm Drainage Letter
March 6, 2019
Page 2 of 4
PROPOSED DEVELOPMENT
The proposed redevelopment improvements will consist of 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 18,420 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 portion of the proposed roof area and the runoff will be conveyed
through storm drains to the existing Type R inlet on E. Mountain Ave. Basin D1 will be treated by a
FloGard Water Quality Filter system designed by Old Castle prior to being conveyed to the existing inlet.
Basin D2 consists of the northeast portion of the proposed roof area and the runoff will be conveyed
through storm drains to the existing Type R inlet on E. Mountain Ave. Basin D2 will be treated by a
FloGard Water Quality Filter system designed by Old Castle prior to being conveyed to the existing inlet.
Basin D3 consists of the south portion of the proposed roof area and the runoff will be conveyed through
a proposed sidewalk chase to the flowline of Matthews Street where the runoff will be conveyed north to
an existing Type R inlet. Basin D3 will be treated by a FloGard Water Quality Filter system designed by
Old Castle prior to being conveyed to the sidewalk chase.
Basin OS1 consists of the southern alley and several adjacent rooftops and parking areas. The basin was
delineated by survey elevations of the alley visually observing the existing conditions, and buildings in
the vicinity of the project site. In general, the proposed drainage patterns of this basin will not change
from existing, draining along the improved alley east to Mathews Street. This offsite drainage basin can
be viewed in more detail by reviewing the attached Offsite Drainage Basin Exhibit. The runoff associated
with this basin was modeled at key locations along the east-west alley corridor to determine the water
surface elevation of the alley flows in a 100-yr event. The 100-yr peak runoff from this basin is estimated
to be 5.56 cfs. To ensure building protection, the modeled flow depths within the alley were increased by
33% to 7.40 cfs and do not enter the building openings.
Basin OS2 consists of the western DDA owned alley. This alley will be paved and the runoff from this
alley will be collected in an area inlet and directed to the interior of the building where it will be
combined with flows from Basin D1 and D2 prior to being 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. Low Impact Development (LID) code would require that 50% of the site area be treated
with an adopted BMP treatment method. See below for more detail on the proposed FloGard Filters.
All calculations and exhibits are attached for reference.
221 East Mountain
Final Storm Drainage Letter
March 6, 2019
Page 3 of 4
LOW IMPACT DEVELOPMENT TECHNIQUES
In reference to the code requirements for implementation of Low Impact Development (LID) techniques,
the use of 100% treatment of the roof area via three (3) FloGard Filters is proposed to satisfy these
requirements. These filter systems are an emerging technology that has been adopted in other regions of
the country but is not currently on an approved list of approved treatment methods. After extensive
evaluation of alternative methods including Planter Box Rain Gardens, Bio-retention within the public
street parkway, and water quality tree well systems, it was determined that the various site constraints for
the 221 East Mountain project made those alternatives not viable. Therefore, we are proposing the use of
the roof filtering systems to treat 100% of the roof area in lieu of the required 50% treatment area.
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
(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 100% of the roof will be directed to one of three (3) FloGard Filter systems in the building
prior to being released into the adjacent storm drain systems.
Step 2: Implement BMPs that Provide a Water Quality Capture Volume with Slow Release
The roof filter systems do not have a water quality capture volume component.
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.
221 East Mountain
Final Storm Drainage Letter
March 6, 2019
Page 4 of 4
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
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 daily.
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) 674-3300.
Sincerely,
Logan Johnson, P.E. Robert Almirall, P.E.
Interwest Consulting Group 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
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 6,086 0.14 294 5,792 0 0 0 100.0% 0.95 1.00
D2 6,086 0.14 376 5,709 0 0 0 100.0% 0.95 1.00
D3 7,324 0.17 497 6,826 0 0 0 100.0% 0.95 1.00
Overall 19,496 0.45 1,168 18,328 0 0 0 100.0% 0.95 1.00
OS1 24,323 0.56 11,309 8,950 0 4,064 0 90.0% 0.87 1.00
OS2 1,296 0.03 1,296 0 0 0 0 100.0% 0.95 1.00
Weighted %
Impervious
A COMPOSITE
paved
(sq feet)
Aroof
(sq feet)
Awalk
(sq feet)
Agravel/pavers
(sq feet)
Alawn
(sq feet)
Gravel/Pavers
Lawns
(Heavy, 2-7% Slope)
Sub-basin
Designation
Atotal
(sq feet)
Atotal
(acres)
Land Use
Paved
Roof
Walks
Runoff Coefficients and Frequency Adjustment Factors for City of Fort Collins - Storm Water Criteria Manual
% Impervious values from Table RO-3 in the Urban Storm Drainage Criteria Manual
L. Johnson
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 0.140 53 0.0100 2.05 44 0.0100 Paved Areas 20 2.00 0.36 5.00 0.0100 5.00 5.00
D2 D2 100.0% 0.95 0.140 53 0.0100 2.04 44 0.0100 Paved Areas 20 2.00 0.36 5.00 0.0100 5.00 5.00
D3 D3 100.0% 0.95 0.168 34 0.0100 1.65 126 0.0100 Paved Areas 20 2.00 1.05 5.00 0.0100 5.00 5.00
OS1 OS1 90.0% 0.87 0.558 149 0.0100 5.15 308 0.0143 Paved Areas 20 2.39 2.15 7.29 0.0129 6.50 6.50
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
tt
(min)
OVERALL SLOPE
(ft/ft)
tc
(min)
SLOPE
(ft/ft)
ti
(min)
LENGTH
(ft)
SLOPE
(ft/ft)
Table RO-2 VELOCITY
(ft/s)
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)
-Urbanized Check Equation 6-5
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 0.140 53 0.0100 1.36 44 0.0100 Paved Areas 20 2.00 0.36 5.00 0.0100 5.00 5.00
D2 D2 100.0% 1.00 0.140 53 0.0100 1.36 44 0.0100 Paved Areas 20 2.00 0.36 5.00 0.0100 5.00 5.00
D3 D3 100.0% 1.00 0.168 34 0.0100 1.10 126 0.0100 Paved Areas 20 2.00 1.05 5.00 0.0100 5.00 5.00
OS1 OS1 90.0% 1.00 0.558 149 0.0100 2.29 308 0.0143 Paved Areas 20 2.39 2.15 5.00 0.0129 6.50 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
tt
(min)
OVERALL SLOPE
(ft/ft)
tc
(min)
SLOPE
(ft/ft)
ti
(min)
LENGTH
(ft)
SLOPE
(ft/ft)
Table RO-2 VELOCITY
(ft/s)
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)
-Urbanized Check Equation 6-5
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 0.140 5.00 0.95 0.13 2.78 0.37 5.00 0.95 0.13 4.87 0.65 5.00 1.00 0.14 9.95 1.39
D2 D2 0.140 5.00 0.95 0.13 2.78 0.37 5.00 0.95 0.13 4.87 0.65 5.00 1.00 0.14 9.95 1.39
D3 D3 0.168 5.00 0.95 0.16 2.78 0.44 5.00 0.95 0.16 4.87 0.78 5.00 1.00 0.17 9.95 1.67
OS1 OS1 0.558 6.50 0.87 0.49 2.58 1.26 6.50 0.87 0.49 4.40 2.15 5.00 1.00 0.56 9.95 5.56
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)
L. Johnson
Interwest Consulting Group
1324-028-00
September 12, 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 4 of 4 Interwest Consulting Group
Channel Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Tuesday, Sep 11 2018
221 E Mountain - Southern Alley Flow Depth
User-defined
Invert Elev (ft) = 4977.54
Slope (%) = 0.40
N-Value = 0.013
Calculations
Compute by: Known Q
Known Q (cfs) = 7.40
(Sta, El, n)-(Sta, El, n)...
( -10.00, 4979.12)-(1.00, 4977.59, 0.013)-(10.00, 4977.92, 0.013)-(14.00, 4978.00, 0.013)
Highlighted
Depth (ft) = 0.36
Q (cfs) = 7.400
Area (sqft) = 3.29
Velocity (ft/s) = 2.25
Wetted Perim (ft) = 18.92
Crit Depth, Yc (ft) = 0.36
Top Width (ft) = 18.91
EGL (ft) = 0.44
-5 0 5 10 15 20
Elev (ft) Depth (ft)
Section
4977.00 -0.54
4977.50 -0.04
4978.00 0.46
4978.50 0.96
4979.00 1.46
4979.50 1.96
4980.00 2.46
Sta (ft)
Interwest Consulting Group
1218 W. Ash, Suite A
Windsor, CO 80550
Inlet Flow Calculation for Area Inlet (24" Drop ADS Inlet)
Project: 221 E. Mtn
Job Number : 1324-028-00
Calculations by : laj
Date : 3/6/2019
Objective: to find the number of inlet grates required
Geometry at inlet :
Square Feet Open (A): 1.145 sf
Reduction Factor (F): 50%
Grate Flow:
Use the orifice equation Qi
= C*A*SQRT(2*g*H) to find the ideal inlet capacity.*
*See Hydraulic Design Handbook by McGraw-Hill for verificaiton of equation use and C-value
C = Orifice discharge coefficient= 0.67
A = Orifice area (ft
2
) - open area of grate
g = gravitational constant = 32.2 ft/s
2
H = head on grate centroid, ponding depth (feet)
Then multiply by the reduction factor for the allowable capacity.
QG
= Qi
* (1-F)
DP L
Q = 0.30 cfs Q100
H = 0.01 ft
A= 1.15 ft
2
Qi
= C*A*SQRT(2*g*H)
= 0.62 cfs
QG
= Qi
* F
= 0.31 cfs OK
Q= 0.30 cfs
Req'd Head d= 0.01 ft
Available Head Prior to Overtopping= 1.48 ft
Grate Open Area and information:
WSEL
Page1
Low Impact Development
Design Engineer:
Design Firm:
Project Number:
Date:
DESIGN CRITERIA
Low Impact Development Summary
Developed Area Summary
18,420 sqft
Total Roof Area required for Treatment (50%): 9,210 sqft
18,420 sqft
100.0%
221 East Mountain
Total Roof Area:
L. Johnson
Interwest Consulting Group
1324-028-00
March 6, 2019
Urban Storm Drainage Criteria Manual, Urban Drainage and Flood Control District, January 2010
City of Fort Collins - Ordinance No. 007, 2016
- Treat at least 75% of any newly developed or redeveloped impervious area using one or a combination of LID
techniques, or
- Treat at least 50% of any newly developed or redeveloped impervious area using one or a combination of LID
techniques when 25% of private driveable surfaces are permeable.
This project is required to treat 50% of any newly developed or redeveloped impervious area using one or a
combination of LID techniques.
Total Area Treated by FloGard Roof Filters:
Percent Area Treated by LID:
INLET FILTRATION
STORM
WATER RECONDITION
Rooftop Runoff
Installation & Maintenance
is easy and economical
by design.
Custom Downspout
adapter shapes and sizes
are available.
Filter Medium
can be customized depending
on the pollutants of concern.
Removes Non-Soluble Solids
such as sediment, debris, metals
and hydrocarbons.
Corrosion-Resistant
stainless steel accepts standard
diameter downspout pipes.
Variable Design
can be flush mounted or
recessed.
The Flogard Downspout Filter is typically installed on downspout
pipes of commercial buildings for the removal of non-soluble
pollutants normally found on building roofs and parking decks.
The inline downspout filter is an ideal solution for urban sites
with little-to-no property area outside of the building perimeter.
It is designed to provide effective filtering at low flows and
incorporates a high-flow bypass to ensure conveyance is not
impeded.
FLOGARD DOWNSPOUT FILTER
Inline Filter Specially Designed for Downspouts
®
Storage capacity reflects 80% of maximum solids collection prior to impeding filtering bypass.
Filtered flow rate includes a safety factor of 2 to 1.
Available with standard Fossil Rock or other custom absorbants.
Should be used in conjunction with a regular maintenance program.
Refer to manufacturer’s recommended guidelines.
(800) 579-8819
oldcastlestormwater.com
RECONDITION
Rooftop Runoff
SPECIFICATIONS
Model No.
FG-DS4 4 14 x 29 x 7.5 0.35 30 145
FG-DS6 6 14 x 29 x 7.5 0.35 85 425
FG-DS8 8 22 x 33 x 17.5 1.70 185 915
FG-DS10 10 22 x 33 x 17.5 1.70 325 1,650
Inlet ID
(dia, in)
Box ID
(in x in x in)
Solid Storage
(cu ft)
Filtered Flow
(gpm)
Bypass Capacity
(gpm)
Standard round
downspout inlet
connection
Waterproof access
door and gasket
assembly
Removable slide-in
collection basket
with geotextile liner
Standard round
downspout outlet
connection
CITY OF LOS ANGELES
* Research report #5584
* Filter is approved for use in
the city of Los Angeles.
Fully Scalable Solution for Stormwater Treatment
The Flogard Downspout Filter can be installed as a stand-alone treatment device
or used in conjunction with other Oldcastle products as part of a total stormwater
management solution.
September 12, 2018
221 East Mountain
DEVELOPED TIME OF CONCENTRATION
L. Johnson
Interwest Consulting Group
1324-028-00
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 3 of 4 Interwest Consulting Group
September 12, 2018
221 East Mountain
DEVELOPED TIME OF CONCENTRATION
L. Johnson
Interwest Consulting Group
1324-028-00
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 2 of 4 Interwest Consulting Group
Interwest Consulting Group
1324-028-00
September 12, 2018
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:
1324-028-00 Rational Calcs (FC).xlsx Page 1 of 4 Interwest Consulting Group