HomeMy WebLinkAboutBROOKFIELD SECOND FILING, TRACTS D & E - PDP/FDP - FDP150025 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT1
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
July 13, 2015
RE: Drainage Memorandum
Brookfield Replat A, Lots 1-12, Tract E and a portion of Tract D
Fort Collins, Colorado
Project Number: 374-020
Dear Staff:
Northern Engineering Services is pleased to submit this Drainage Memorandum for
Brookfield for your review. Brookfield Replat A is located within Lots 1-12, Tract E and a portion
of Tract D of the Plat of Brookfield. The Brookfield project is a proposed multi-family residential
site. The site is located in the Northeast Quarter of Section 4, Township 6 North, Range 68
West of the 6th
Principal Median, City of Fort Collins, Larimer County, Colorado. The site is
bounded on the north, east, and west by undeveloped area. The site is bounded on the south
by Brookfield Phase 1, a multi-family site. Fossil Creek Reservoir Inlet Ditch (FCRID) flows north-
west to south and is located east of the site outside of the project boundary. Southeast of the
Site is Detention Pond A constructed as part of Brookfield Phase 1 development that currently
detains flows from Brookfield development Phase 1. Detention Pond A was designed for future
development of Brookfield including this site. The site falls within the East Harmony Portion of
McClellands Creek Master Drainage Basin. This Drainage Memorandum has been prepared in
compliance with technical criteria set forth in the City of Fort Collins Storm Drainage Design
Criteria, Urban Drainage and Flood Control District Criteria Manual (UDFCDCM) and City of Fort
Collins Amendments to the UDFCDCM.
The Brookfield project consists of approximately 6 acres. The development will consist
of 12 multi-family buildings with approximately 68 units. The buildings will be two story with
the possibility of basements. Parking will be provided by on street parking, parking stalls off the
alleys, attached garages, and detached garages. The streets within the multi-family tracts will
be private and will be maintained privately. Public streets bound the tract.
2
The proposed site has been a part of many different drainage plans and studies in the
past. The most recent drainage plan, Final Drainage Report for Brookfield by TST dated
December 14, 2001 covers the entire Brookfield development including already constructed
phases, the proposed site, and future development. The TST drainage report for Brookfield
references two previously completed Drainage Reports that are a part of the McClellands
Drainage Basin: Final Drainage Report for Willow Brook by TST, Inc. Consulting Engineers, dated
October, 2000 and East Harmony Portion of McClellands Creek Master Drainage Plan Update by
ICON Engineering, Inc. dated August, 1999. The proposed site closely matches the original
proposed development as part of the TST Brookfield Utility Plans and TST Brookfield Drainage
Report and is in compliance with the original TST Brookfield Drainage Report. Drainage Basins
are named to match the original Basin names of the TST Drainage Report for ease of
comparison. Several constraints have been identified from the original report including the
maximum runoff allowable into existing inlets and drainageways. Specifically, the Brookfield
Drainage Plan divided the site into two basins. The interior of the site drains to an existing
storm sewer system. The north and east perimeter areas drain offsite to the existing FCRID.
This Drainage Plan incorporated the 100-yr maximum release rate from the Brookfield site as
0.5 cfs per acre.
The site currently sheet flow from west to east with an average slope of 0.5-1.0%. Most
of the Site is discharged into the FCRID. The southern portion of the sites flows into an existing
curb inlet on Precision Drive and into the storm sewer system that flows into Pond A. The
existing land onsite is currently undeveloped and consists of loose soils with no vegetation.
Most of the Brookfield site (constructed and future phases) is graded to Detention Pond
A. This water quality detention pond was sized based on SDDC and Urban Drainage criteria and
has a release rate of 0.5 cfs/ac. The pond was constructed with an orifice plate with multiple
orifices to provide for an extended drainage time. The water quality volume was based on a 2-
yr storm, with the 100-yr storm overtopping the control structure and entering the discharge
pipe. The pond was provided with an orifice plate on the discharge pipe to control the 100-yr
discharge from the pond. If the pond were to overtop, it will spill in a historic eastward
direction and into the FCRID. No structures will be inundated if overtopping occurs. The
required volume of the detention pond is 9.01 ac-ft.
Offsite sheet flow from the west in Drainage Basin D-5/9 is proposed to be intercepted
by a drainage swale and into a storm sewer system that goes to Pond A. Offsite flows north of
Le Fever Drive are channelized in a grass swale that flows into the FCRID. Offsite flows from the
eastern portion of the site include the bank above the FCRID, proposed 10’ wide trail, retaining
walls and graded slopes. As this area will be vegetated and closely match undeveloped
conditions it is proposed to release undetained. This undetained release will not be accounted
for in the design of the detention pond since it will be at historic rates. All developed subbasins
are proposed to discharge from the site at a release rate of 0.5 cfs per acre as dictated by the
master plan after passing through the Pond A.
3
The overall stormwater management strategy employed with the Brookfield Replat A is
to closely match the original design while incorporating current LID practices to minimize
adverse impacts of urbanization on receiving waters. Several techniques have been utilized
with the proposed development to facilitate the reduction of runoff peaks, volumes, and
pollutant loads as the site is developed from the current use by implementing multiple Low
Impact Development (LID) strategies including:
Providing permeable paver areas with underground detention area to increase
time of concentration promote infiltration and provide water quality.
Providing Drywells to promote infiltration and water quality throughout the site.
Providing a Rain Garden and Grass Swale to slowly release the flood control
volume in an area and removes pollutants by treatment processes of
sedimentation, filtering, adsorption, evapotranspiration and biological uptake of
constituents.
Snouts and settling basins for water treatment prior to flows entering the
subsurface infiltration galleries and storm systems.
I. NCLUSS
The overall stormwater management strategy employed with the Brookfield Replat A is in
compliance with the following standards:
City of Fort Collins’ Stormwater Criteria Manual.
Final Drainage Report for Brookfield by TST
There are no regulatory floodplains associated with the development.
The drainage plan and stormwater management measures proposed with the
development are compliant with all applicable State and Federal regulations
governing stormwater discharge.
The site achieves the requirements set forth by the City of Fort Collins for Low
Impact Development (LID) by providing 25% of total paved areas as permeable
pavers and providing 51% of total impervious areas as being treated through an
LID treatment.
The drainage design proposed with this project will effectively comply with previous
studies and will limit any potential damage or erosion associated with its stormwater runoff. All
existing downstream drainage facilities are expected to not be impacted negatively by this
development
The proposed development will not have any impact on the Master Drainage Plan
recommendations for Brookfield.
The drainage design is anticipated to be very conservative. We have omitted any runoff
reduction that will manifest due to infiltration from drywells and pervious pavement. This is
currently unable to be calculated with available soils data.
If you have any questions, please feel free to contact me at your earliest convenience.
4
Sincerely,
NORTHERN ENGINEERING SERVICES, INC.
Amber Morse, PE
Project Engineer
374-020 Brookfield
CHARACTER OF SURFACE1:
Runoff
Coefficient
Percentage
Impervious Project: 374-020 Brookfield
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: A. Morse
Asphalt ……....……………...……….....…...……………….…………………………… 0.95 100% Date: July 14, 2015
Concrete …….......……………….….……….………………..….……………………… 0.95 90%
Gravel (packed) ……….…………………….….…………………………..…………… 0.50 40%
Roofs …….…….………………..……………….………………………………………… 0.95 90%
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
Sub-Basin ID
Sub-
BasinBasin
Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Pavers
(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.
D-2A 0.27 0.15 0.07 0.00 0.00 Clayey | Average 2% to 7% 0.049 0.82 0.82 1.00 79%
D-2B 0.53 0.12 0.09 0.00 0.07 Clayey | Average 2% to 7% 0.241 0.63 0.63 0.79 51%
Brookfield
Overland Flow, Time of Concentration:
374-020 Brookfield
Gutter/Swale Flow, Time of Concentration:
Tt = L / 60V
Tc = Ti + Tt (Equation RO-2)
Velocity (Gutter Flow), V = 20·S½
Velocity (Swale Flow), V = 15·S½
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)
Up Stream
Elevation
Down
Stream
Elevation
Slope,
S
(%)
Ti
2-yr
(min)
Ti
10-yr
(min)
Ti
100-yr
(min)
Length,
L
(ft)
Up Stream
Elevation
Down
Stream
Elevation
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
(min)
10-yr
Brookfield
Rational Method Equation: Project: 374-020 Brookfield
Calculations By:
Date:
Rainfall Intensity:
D2A OF-1 & D-2A 0.27 23 23 12 0.82 0.82 1.00 1.49 2.55 7.29 0.33 0.57 1.97
45 D-2B 0.53 5 5 5 0.63 0.63 0.79 2.85 4.87 9.95 0.94 1.61 4.12
42
D-2B & D-2C(not
a part) 0.77 7 7 7 0.80 0.80 1.00 2.60 4.44 9.06 1.61 2.75 6.98
D3A D-3A 0.44 5 5 5 0.74 0.74 0.93 2.85 4.87 9.95 0.93 1.59 4.07
43 D-3B 1.27 6 6 6 0.77 0.77 0.96 2.67 4.56 9.31 2.60 4.45 11.35
D3C D-3C 0.61 64 64 44 0.62 0.62 0.78 0.79 1.34 3.54 0.30 0.51 1.67
44 D-5/92 9.94 49 49 45 0.28 0.28 0.36 0.95 1.61 3.46 2.67 4.55 12.22
46 D-7 0.30 5 5 5 0.71 0.71 0.89 2.85 4.87 9.95 0.61 1.05 2.68
51 D-7.1 0.33 5 5 5 0.74 0.74 0.93 2.85 4.87 9.95 0.70 1.19 3.04
52 D-7.2 0.33 5 5 5 0.85 0.85 1.00 2.85 4.87 9.95 0.80 1.37 3.27
47 D-8 0.33 5 5 5 0.77 0.77 0.97 2.85 4.87 9.95 0.72 1.23 3.14
48 D-8.1 0.33 5 5 5 0.84 0.84 1.00 2.85 4.87 9.95 0.79 1.34 3.25
54 D-10 0.36 5 5 5 0.76 0.76 0.95 2.85 4.87 9.95 0.78 1.33 3.40
56 D-11 0.36 5 5 5 0.87 0.87 1.00 2.85 4.87 9.95 0.89 1.52 3.58
60 OF-12 0.44 51 51 46 0.31 0.31 0.39 0.91 1.56 3.41 0.12 0.21 0.58
July 14, 2015
Intensity,
i10
(in/hr)
C10
Area, A
(acres)
Intensity,
i2
(in/hr)
100-yr
Tc
(min)
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)
Sub-Basin(s)
A. Morse
374-020 Brookfield
LID ID
LID Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Pavers
(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.
Impervious
Area (ac)
Basin
Treated thru
LID?
LID Treated
Impervious Area
(ac)
LID Treated Type
1 0.22 0.00 0.08 0.00 0.11 Clayey | Average 2% to 7% 0.03 0.86 0.86 1.00 78% 0.17 YES 0.17 Curb Inlet to Dry Well
2 0.35 0.00 0.07 0.00 0.19 Clayey | Average 2% to 7% 0.16 0.81 0.81 1.00 66% 0.23 YES 0.23 Dry Well
3 0.39 0.00 0.10 0.05 0.15 Clayey | Average 2% to 7% 0.18 0.77 0.77 0.97 63% 0.24 YES 0.24 Permeable Pavement, Dry Well
4 0.46 0.00 0.07 0.00 0.19 Clayey | Average 2% to 7% 0.20 0.65 0.65 0.81 51% 0.23 YES 0.23 Grass Swale
5 0.82 0.00 0.24 0.00 0.36 Clayey | Average 2% to 7% 0.44 0.84 0.84 1.00 66% 0.54 NO N/A N/A
6 0.32 0.00 0.02 0.00 0.15 Clayey | Average 2% to 7% 0.15 0.62 0.62 0.78 48% 0.15 NO N/A N/A
7 0.36 0.00 0.02 0.05 0.20 Clayey | Average 2% to 7% 0.11 0.70 0.70 0.88 59% 0.21 YES 0.21 Permeable Pavement
8 0.31 0.00 0.06 0.05 0.11 Clayey | Average 2% to 7% 0.15 0.70 0.70 0.87 55% 0.17 YES 0.07 Permeable Pavement
9 0.23 0.00 0.02 0.00 0.13 Clayey | Average 2% to 7% 0.08 0.70 0.70 0.87 58% 0.14 NO N/A N/A
10 0.15 0.00 0.02 0.00 0.08 Clayey | Average 2% to 7% 0.06 0.69 0.69 0.87 57% 0.09 YES 0.09 Rain Garden
11 0.37 0.00 0.13 0.00 0.14 Clayey | Average 2% to 7% 0.23 0.85 0.85 1.00 66% 0.24 NO N/A N/A
12 0.13 0.00 0.01 0.00 0.07 Clayey | Average 2% to 7% 0.04 0.70 0.70 0.88 58% 0.07 NO N/A N/A
13 0.52 0.00 0.12 0.07 0.18 Clayey | Average 2% to 7% 0.26 0.73 0.73 0.91 58% 0.30 YES 0.30 Permeable Pavement
14 1.55 0.00 0.15 0.00 0.08 Clayey | Average 2% to 7% 1.32 0.35 0.35 0.44 13% 0.21 NO N/A N/A
TOTAL: 6.17 3.00 1.54
LID Requirements
Permeable Pavement LID Treatment
X X
UD
UD
BROOKFIELD DRIVE
MORNINGSIDE
VILLAGE CONDOS
FOSSIL CREEK RESERVOIR INLET DITCH
WETLAND
BLDG
31
BLDG
23
BLDG
22
POND A
1
2
3
4
5
6
7
8
9
10
11
12
13
14
RAIN GARDEN
PERMEABLE
PAVEMENT
PERMEABLE
PAVEMENT
PERMEABLE
PAVEMENT
PERMEABLE
PAVEMENT
GRASS SWALE
DRY WELL
KEYMAP
LEGEND:
5013
PROPOSED CONTOUR 93
EXISTING CONTOUR
EXISTING STORM SEWER
LID AREA 1
LID BOUNDARY
PROPOSED STORM SEWER
FOR LID REVIEW ONLY
NOT FOR CONSTRUCTION
( IN FEET )
0
1 INCH = 70 FEET
70 70 140 210
N O R T H E RN
2015.06.23
D:\PROJECTS\374-020\DWG\EXHIBITS\374-020_LID.DWG
LOW IMPACT DEVELOPMENT (LID) AREAS
(1) Private Drive Pavement (ac)= 0.94 Total Proposed Impermeable Area(ac)= 2.70 3.00
Total Permeable Pavement (ac)= 0.23 Total Impermeable Area Treated with LID(ac)= 1.24 1.54
Percent Permeable Pavement= 25% Percent Treated with LID= 46% 51%
LOW IMPACT DEVELOPMENT (LID) CALCULATIONS
7/14/2015 1:42 PM D:\Projects\374-020\Drainage\Hydrology\374-020_Proposed_Rational_Calcs.xlsx\LID
Rainfall Intensity taken from the Fort Collins Stormwater Criteria Manual (FCSCM), Tables RA-7 and RA-8
Q = C f (C )( i )(A )
7/14/2015 1:35 PM D:\Projects\374-020\Drainage\Hydrology\374-020_Proposed_Rational_Calcs.xlsx\Runoff
Tc
(min)
100-yr
Tc
(min)
D2A OF-1 & D-2A Yes 0.82 0.82 1.00 700 4.56 1.47 0.44% 18.0 18.0 6.5 412 100.60 98.84 0.43% 1.31 5.3 23.2 23.2 11.8
45 D-2B No 0.63 0.63 0.79 0 0.00 0.00 N/A N/A N/A N/A 395 98.84 96.42 0.61% 1.57 4.2 5.0 5.0 5.0
42
D-2B & D-2C(not a
part) No
0.80 0.80 1.00 0 0.00 0.00 N/A N/A N/A N/A 525 98.34 96.00 0.45% 1.34 6.6 6.6 6.6 6.6
D3A D-3A No 0.74 0.74 0.93 0 0.00 0.00 N/A N/A N/A N/A 375 100.96 98.23 0.73% 1.71 3.7 5.0 5.0 5.0
43 D-3B No 0.77 0.77 0.96 0 0.00 0.00 N/A N/A N/A N/A 626 99.63 95.60 0.64% 1.60 6.5 6.5 6.5 6.5
D3C D-3C No 0.62 0.62 0.78 160 1.00 0.99 0.01% 61.5 61.5 41.6 210 100.86 99.63 0.59% 1.53 2.3 63.8 63.8 43.9
44 D-5/92 Yes 0.28 0.28 0.36 660 104.10 99.21 0.74% 43.3 43.3 39.5 440 99.85 98.00 0.42% 1.30 5.7 49.0 49.0 45.2
46 D-7 No 0.71 0.71 0.89 0 0.00 0.00 N/A N/A N/A N/A 135 98.25 96.72 1.13% 2.13 1.1 5.0 5.0 5.0
51 D-7.1 No 0.74 0.74 0.93 0 0.00 0.00 N/A N/A N/A N/A 68 98.68 98.07 0.90% 1.89 0.6 5.0 5.0 5.0
52 D-7.2 No 0.85 0.85 1.00 0 0.00 0.00 N/A N/A N/A N/A 225 100.14 97.50 1.17% 2.17 1.7 5.0 5.0 5.0
47 D-8 No 0.77 0.77 0.97 0 0.00 0.00 N/A N/A N/A N/A 150 100.14 98.67 0.98% 1.98 1.3 5.0 5.0 5.0
48 D-8.1 No 0.84 0.84 1.00 0 0.00 0.00 N/A N/A N/A N/A 50 0.37 0.18 0.38% 1.23 0.7 5.0 5.0 5.0
54 D-10 No 0.76 0.76 0.95 0 0.00 0.00 N/A N/A N/A N/A 210 98.45 97.09 0.65% 1.61 2.2 5.0 5.0 5.0
56 D-11 No 0.87 0.87 1.00 0 0.00 0.00 N/A N/A N/A N/A 105 98.45 97.93 0.50% 1.41 1.2 5.0 5.0 5.0
60 OF-12 Yes 0.31 0.31 0.39 700 4.56 1.47 0.44% 51.4 51.4 46.3 0 0.00 0.00 N/A N/A N/A 51.4 51.4 46.3
OF2 OF-2 No 0.70 0.70 0.88 70 98.97 97.40 2.24% 4.8 4.8 2.7 0 0.00 0.00 N/A N/A N/A 5.0 5.0 5.0
PROPOSED TIME OF CONCENTRATION COMPUTATIONS
A. Morse
July 14, 2015
Design
Point
Sub-Basin(s)
Overland Flow Gutter/Pipe Flow Time of Concentration
(Equation RO-4)
()
3
1
1 . 87 1 . 1 *
S
Ti C Cf L
−
=
7/14/2015 1:34 PM D:\Projects\374-020\Drainage\Hydrology\374-020_Proposed_Rational_Calcs.xlsx\Tc
D-2C2 0.77 0.11 0.29 0.00 0.22 Clayey | Average 2% to 7% 0.160 0.80 0.80 1.00 73%
TOTAL BASIN D2 1.57 0.38 0.45 0.00 0.29 Clayey | Average 2% to 7% 0.450 0.75 0.75 0.94 67%
D-3A 0.44 0.12 0.08 0.00 0.11 Clayey | Average 2% to 7% 0.131 0.74 0.74 0.93 66%
D-3B 1.27 0.36 0.30 0.00 0.28 Clayey | Average 2% to 7% 0.335 0.77 0.77 0.96 69%
D-3C 0.61 0.06 0.07 0.00 0.19 Clayey | Average 2% to 7% 0.287 0.62 0.62 0.78 49%
TOTAL BASIN D3 2.32 0.54 0.45 0.00 0.58 Clayey | Average 2% to 7% 0.753 0.72 0.72 0.90 63%
D-5/92 9.94 0.41 0.07 0.00 0.00 Clayey | Average 2% to 7% 9.456 0.28 0.28 0.36 5%
D-7 0.30 0.00 0.06 0.05 0.11 Clayey | Average 2% to 7% 0.138 0.71 0.71 0.89 57%
D-7.1 0.33 0.00 0.02 0.05 0.20 Clayey | Average 2% to 7% 0.079 0.74 0.74 0.93 65%
D-7.2 0.79 0.00 0.24 0.00 0.36 Clayey | Average 2% to 7% 0.421 0.85 0.85 1.00 68%
D-8 0.39 0.00 0.10 0.05 0.15 Clayey | Average 2% to 7% 0.180 0.77 0.77 0.97 63%
D-8.1 0.33 0.00 0.07 0.00 0.19 Clayey | Average 2% to 7% 0.141 0.84 0.84 1.00 70%
D-10 0.49 0.00 0.12 0.07 0.18 Clayey | Average 2% to 7% 0.234 0.76 0.76 0.95 61%
D-11 0.36 0.00 0.13 0.00 0.14 Clayey | Average 2% to 7% 0.221 0.87 0.87 1.00 68%
TOTAL BASIN D 16.82 1.33 1.71 0.23 2.19 Clayey | Average 2% to 7% 12.074 0.48 0.48 0.60 29%
OF-12 0.44 0.03 0.01 0.00 0.00 Clayey | Average 2% to 7% 0.399 0.31 0.31 0.39 8%
OF-2 0.15 0.00 0.02 0.00 0.08 Clayey | Average 2% to 7% 0.053 0.70 0.70 0.88 58%
1. Table RO-11 | Rational Method Runoff Coefficients for Composite Analysis
2. Sub-Basin outside of Project Boundary
Composite Runoff Coefficient with Adjustment
PROPOSED COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
7/14/2015 1:32 PM D:\Projects\374-020\Drainage\Hydrology\374-020_Proposed_Rational_Calcs.xlsx\Composite C