HomeMy WebLinkAboutPRAIRIE VILLAGE - MAJOR AMENDMENT - MJA140002 - REPORTS - DRAINAGE REPORTDrainage Memorandum
Date: August 27, 2014
Project: Prairie Village Project No. 1013-001
Project Development Plan/Final Plan
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 Prairie Village project. The existing site
consists of vacant land, situated southwest of the Seneca Center Development. The site is bounded
by an existing parking lot to the northeast, Fromme Prairie Way on the south, and Overlook at
Woodbridge Subdivision lots on the west.
The proposed development will consist of 8 townhome units, sidewalks, paver driveways, and utility
services. The project is expected to disturb approximately 0.73 acres of land.
This project site was originally a part of the Overlook/Seneca Center P.U.D. As such, drainage
facilities were planned for this area and were built with the Seneca Center Development. This letter
is intended to show compliance with the original drainage plan “Final Drainage and Erosion Control
Report – Overlook/Seneca Center P.U.D.” dated April 17, 2000 by North Star Design and the Fort
Collins Amendments to the Urban Drainage and Flood Control District Criteria Manual.
The site is located within the Mail Creek Stormwater Basin. Existing site drainage overland flows to
Fromme Prairie Way. A web soil survey indicates a soils rated as Hydrologic Soil Group B, which
has a moderate infiltration rate.
The proposed site has a percent impervious of 59%, which is lower than the previous
Overlook/Seneca Center P.U.D. at 64% impervious. As such, detention will not be required for this
site. Both the previous study and this study used the most recent Fort Collins Intensity-Duration-
Frequency in their calculations. Rational method calculations were used to determine the proposed
runoff for sizing storm facilities. These calculations are attached to this letter, as is a drainage
exhibit.
There are no regulatory floodplains associated with the project.
Page 2
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 the appropriate and applicable City of Fort Collins standards
for erosion and sediment control. Since the approximately 0.73 acre area of disturbance for the
project is less than one acre, a comprehensive Stormwater Management Plan will not be prepared
for this project. Post construction water quality and erosion control will be achieved by a fully
established and stabilized site. All areas disturbed during construction will receive permanent
hardscape, landscape, or building structure.
There are no existing stormwater BMP’s currently on-site. In order to bring the site into compliance
with the Low Impact Development Standards adopted by the city, bio-retention/rain gardens and
bioswales will be provided on-site. Water quality treatment will occur through a variety of methods.
The first method is the use of a bio-retention area along the north side of the proposed buildings
that intercepts runoff from Basin A. This area will have an engineered section designed to remove
particulates and contaminants from the runoff. While most minor events are expected to infiltrate
into the ground, a subdrain is provided to drain the bio-retention area should it become
overburdened. The underdrain will outfall directly into an existing storm sewer. The runoff from the
south side (Basin B) of the proposed building will be treated within a bioswale. This bioswale will
convey flows to a proposed inlet, outfalling into the existing storm sewer.
The City LID requirements specify that no less than 25% of all new paving shall be treated using
permeable paving technique. The City LID requirements also specify that no less than 50% of any
new impervious areas must be treated using LID techniques. Unfortunately, the site configuration
and drainage patterns for this project do not lend themselves to the use of permeable pavers. In lieu
of permeable pavers, 100% of the new impermeable areas will be treated for water quality. The
combination of treatment provided by the bio-retention area and bioswale far exceeds the amount
required for LID treatment, and provides water quality treatment equal to or better than meeting the
requirement for 25% permeable pavements.
To summarize, the proposed grading concept closely is in compliance with previous drainage
studies. On-site detention is proven to be unnecessary. Stormwater quality has been provided,
and exceeds the city requirements for Low Impact Development treatment. Therefore, it is my
professional opinion that the Prairie Village project satisfies all applicable stormwater criteria.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely,
Stephanie Thomas, PE Nicholas W. Haws, PE
Project Engineer Project Manager
enc.
Prairie Village
CHARACTER OF SURFACE
1
:
Runoff
Coefficient
Percentage
Impervious Project: Prairie Village
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: S. Thomas
Asphalt ……....……………...……….....…...……………….………………………………………………………….0.95 . 100% Date: July 9, 2014
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 Cf = 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.
Basin A 0.42 0.01 0.12 0.00 0.16 Clayey | Average 2% to 7% 0.130 0.73 0.73 0.92 62%
Prairie Village
Overland Flow, Time of Concentration:
Prairie Village
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)
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)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
A Basin A No 0.73 0.73 0.92 0 N/A N/A N/A N/A 150 3.40% 2.77 0.9 5.0 5.0 5.0
B Basin B No 0.67 0.67 0.84 0 N/A N/A N/A N/A 302 1.85% 2.04 2.5 5.0 5.0 5.0
Total Basins A and B No 0.71 0.71 0.88 0 N/A N/A N/A N/A 302 1.85% 2.04 2.5 5.0 5.0 5.0
OS OS No 0.28 0.28 0.35 79 2.00% 10.8 10.8 9.9 340 2.00% 2.12 2.7 13.5 13.5 12.6
* Time of Concentrations are calculated for the entire basin and used for both the Impervious and Pervious portions of the basins.
Prairie Village
Rational Method Equation: Project: Prairie Village
Calculations By:
Date:
Rainfall Intensity:
A Basin A 0.42 5 5 5 0.73 0.73 0.92 2.85 4.87 9.95 0.87 1.49 3.81
B Basin B 0.31 5 5 5 0.67 0.67 0.84 2.85 4.87 9.95 0.60 1.02 2.61
Total Basins A and B 0.73 5 5 5 0.71 0.71 0.88 2.85 4.87 9.95 1.47 2.51 6.41
OS OS 0.80 14 14 13 0.28 0.28 0.35 1.95 3.34 7.04 0.43 0.74 1.96
July 9, 2014
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)
S. Thomas
Rainfall Intensity taken from the Fort Collins Stormwater Criteria Manual (FCSCM), Tables RA-7 and RA-8
Q C f C i A
7/9/2014 11:46 AM D:\Projects\1013-001\Drainage\Hydrology\1013-001_Proposed_Rational_Calcs.xlsx\Runoff
Prairie Village
CHARACTER OF SURFACE
1
:
Runoff
Coefficient
Percentage
Impervious Project: Prairie Village
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: S. Thomas
Asphalt ……....……………...……….....…...……………….………………………………………………………….0.95 . 100% Date: August 26, 2014
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 Cf = 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.
Prarie Village 0.73 0.01 0.16 0.00 0.30 Clayey | Average 2% to 7% 0.254 0.71 0.71 0.88 59%
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1. Basin Storage Volume
A) Effective Imperviousness of Tributary Area, Ia Ia = 62.0 %
(100% if all paved and roofed areas upstream of rain garden)
B) Tributary Area's Imperviousness Ratio (i = Ia/100) i = 0.620
C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.19 watershed inches
(WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i)
D) Contributing Watershed Area (including rain garden area) Area = 18,192 sq ft
E) Water Quality Capture Volume (WQCV) Design Volume VWQCV = 295 cu ft
Vol = (WQCV / 12) * Area
F) For Watersheds Outside of the Denver Region, Depth of d6 = in
Average Runoff Producing Storm
G) For Watersheds Outside of the Denver Region, VWQCV OTHER = 0.0 cu ft
Water Quality Capture Volume (WQCV) Design Volume
H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER = cu ft
(Only if a different WQCV Design Volume is desired)
2. Basin Geometry
A) WQCV Depth (12-inch maximum) DWQCV = 9 in
B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 4.00 ft / ft
(Use "0" if rain garden has vertical walls)
Design Procedure Form: Rain Garden (RG)
Stephanie Thomas
Northern Engineering
July 9, 2014
Prarie Village
Harmony and Seneca
C) Mimimum Flat Surface Area AMin = 197 sq ft
D) Actual Flat Surface Area AActual = 460 sq ft
E) Area at Design Depth (Top Surface Area) ATop = 600 sq ft
F) Rain Garden Total Volume VT= 398 cu ft
(VT= ((ATop + AActual) / 2) * Depth)
3. Growing Media
4. Underdrain System
A) Are underdrains provided?
B) Underdrain system orifice diameter for 12 hour drain time
i) Distance From Lowest Elevation of the Storage y = 1.8 ft
Volume to the Center of the Orifice
ii) Volume to Drain in 12 Hours Vol12 = 295 cu ft
iii) Orifice Diameter, 3/8" Minimum DO = 0.41 in
Choose One
Choose One
18" Rain Garden Growing Media
Other (Explain):
YES
NO
1013-001_UD-BMP_v3.02.xls, RG 7/9/2014, 11:47 AM
Sheet 2 of 2
Designer:
Company:
Date:
Project:
Location:
5. Impermeable Geomembrane Liner and Geotextile Separator Fabric
A) Is an impermeable liner provided due to proximity
of structures or groundwater contamination?
6. Inlet / Outlet Control
A) Inlet Control
7. Vegetation
8. Irrigation
A) Will the rain garden be irrigated?
Notes:
Design Procedure Form: Rain Garden (RG)
Stephanie Thomas
Northern Engineering
July 9, 2014
Prarie Village
Harmony and Seneca
Choose One
Choose One
Choose One
Sheet Flow- No Energy Dissipation Required
Concentrated Flow- Energy Dissipation Provided
Plantings
Seed (Plan for frequent weed control)
Sand Grown or Other High Infiltration Sod
Choose One
YES
NO
YES
NO
1013-001_UD-BMP_v3.02.xls, RG 7/9/2014, 11:47 AM
Project:
Basin ID:
WQCV Design Volume (Input):
Catchment Imperviousness, Ia = 62.0 percent Diameter of holes, D = 0.277 in. Time to Drain the Pond = 40 hours
Catchment Area, A = 0.42 acres Number of holes per row, N = 1
Depth at WQCV outlet above lowest perforation, H = 1 feet OR
Vertical distance between rows, h = 6.00 inches **
Number of rows, NL = 2 Height of slot, H = in.
Orifice discharge coefficient, Co = 0.60 Width of slot, W = in.
Slope of Basin Trickle Channel, S = 0.003 ft / ft
Outlet Design Information (Output):
Water Quality Capture Volume (1.0 * (0.91 * I^3 - 1.19 * I^2 + 0.78 * I)), WQCV = 0.243 watershed inches
Water Quality Capture Volume (WQCV) = 0.009 acre-feet 1.00
Design Volume (WQCV / 12 * Area * 1.2) Vol = 0.010 acre-feet
Outlet area per row, Ao = 0.06 square inches
Total opening area at each row based on user-input above, Ao = 0.06 square inches
Total opening area at each row based on user-input above, Ao = 0.000 square feet
Calculation of Collection Capacity:
Stage Row 1 Row 2 Row 3 Row 4 Row 5 Row 6 Row 7 Row 8 Row 9 Row 10 Row 11 Row 12 Row 13 Row 14 Row 15 Row 16 Row 17 Row 18 Row 19 Row 20 Row 21 Row 22 Row 23 Row 23 6
ft 5144.70 5145.20 Flow
(input)
5144.70 0.0000 0.0000 0.00
5144.80 0.0006 0.0000 0.00
5144.90 0.0009 0.0000 0.00
5145.00 0.0011 0.0000 0.00
5145.10 0.0013 0.0000 0.00
5145.20 0.0014 0.0000 0.00
5145.30 0.0016 0.0006 0.00
5145.40 0.0017 0.0009 0.00
5145.50 0.0018 0.0011 0.00
5145.60 0.0019 0.0013 0.00
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Area Inlet Performance Curve:
Prarie Village - Design Point A
Governing Equations:
At low flow depths, the inlet will act like a weir governed by the following equation:
* where P = 2(L + W)
* where H corresponds to the depth of water above the flowline
At higher flow depths, the inlet will act like an orifice governed by the following equation:
* where A equals the open area of the inlet grate
* where H corresponds to the depth of water above the centroid of the cross-sectional area (A)
The exact depth at which the inlet ceases to act like a weir, and begins to act like an orifice is unknown.
However, what is known, is that the stage-discharge curves of the weir equation and the orifice equation
will cross at a certain flow depth. The two curves can be found below:
If H > 1.792 (A/P), then the grate operates like an orifice; otherwise it operates like a weir.
Input Parameters:
Type of Grate: Neenah R-3409
Shape Rectangular
Length of Grate (ft): 2.4
Width of Grate (ft): 1.4
Open Area of Grate (ft
2
): 1.50
Flowline Elevation (ft): 5145.450
Allowable Capacity: 50%
Depth vs. Flow:
Depth Above Inlet (ft)
Elevation
(ft)
Shallow
Weir Flow
(cfs)
Orifice
Flow
(cfs)
Actual
Flow
(cfs)
0.00 5145.45 0.00 0.00 0.00
0.09 5145.54 0.31 1.21 0.31
0.18 5145.63 0.87 1.71 0.87 Q2
0.27 5145.72 1.60 2.09 1.60
0.36 5145.81 2.46 2.42 2.42
0.45 5145.900 3.44 2.70 2.70
0.54 5145.99 4.52 2.96 2.96
0.63 5146.08 5.70 3.20 3.20
0.72 5146.17 6.96 3.42 3.42
0.81 5146.26 8.31 3.63 3.63
0.90 5146.350 9.73 3.82 3.82 Q100
Inlet at Design Point A is designed to intercept the full 100-yr of 3.81 at 0.90 ft above the inlet
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90
Discharge (cfs)
Area Inlet Performance Curve:
Prarie Village - Design Point B
Governing Equations:
At low flow depths, the inlet will act like a weir governed by the following equation:
* where P = 2(L + W)
* where H corresponds to the depth of water above the flowline
At higher flow depths, the inlet will act like an orifice governed by the following equation:
* where A equals the open area of the inlet grate
* where H corresponds to the depth of water above the centroid of the cross-sectional area (A)
The exact depth at which the inlet ceases to act like a weir, and begins to act like an orifice is unknown.
However, what is known, is that the stage-discharge curves of the weir equation and the orifice equation
will cross at a certain flow depth. The two curves can be found below:
If H > 1.792 (A/P), then the grate operates like an orifice; otherwise it operates like a weir.
Input Parameters:
Type of Grate: Nyloplast 12" H-20
Shape Circular
Length of Grate (ft): 1
Width of Grate (ft): 1
Open Area of Grate (ft
2
): 0.39
Flowline Elevation (ft): 5142.230
Allowable Capacity: 50%
Depth vs. Flow:
Depth Above Inlet (ft)
Elevation
(ft)
Shallow
Weir Flow
(cfs)
Orifice
Flow
(cfs)
Actual
Flow
(cfs)
0.00 5142.23 0.00 0.00 0.00
0.05 5142.28 0.05 0.24 0.05
0.10 5142.33 0.15 0.33 0.15
0.15 5142.38 0.27 0.41 0.27
0.20 5142.43 0.42 0.47 0.47
0.25 5142.480 0.59 0.53 0.53
0.30 5142.53 0.77 0.58 0.58 Q2
0.35 5142.58 0.98 0.62 0.62
0.40 5142.63 1.19 0.67 0.67
0.45 5142.68 1.42 0.71 0.71
0.50 5142.730 1.67 0.75 0.75
Inlet at Design Point B is designed to intercept the full 2-yr of 0.60 cfs at 0.33 feet above the inlet
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
Discharge (cfs)
Stage (ft)
Stage - Discharge Curves
Weir Flow
Orifice Flow
Q 3 . 0 P H 1 . 5
Q 0 . 67 A ( 2 gH ) 0 . 5
Stage (ft)
Stage - Discharge Curves
Weir Flow
Orifice Flow
Q 3 . 0 P H 1 . 5
Q 0 . 67 A ( 2 gH ) 0 . 5
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Central Elevations of Rows of Holes in feet
Collection Capacity for Each Row of Holes in cfs
STAGE-DISCHARGE SIZING OF THE WATER QUALITY CAPTURE VOLUME (WQCV) OUTLET
Prairie Village
A
UD-Detention_v2.33.xls, WQCV 7/9/2014, 11:47 AM
Seneca Center P.U.D. 0.73 0.11 0.13 0.00 0.28 Clayey | Average 2% to 7% 0.219 0.74 0.74 0.92 64%
1. Table RO-11 | Rational Method Runoff Coefficients for Composite Analysis
PROPOSED COMPOSITE % IMPERVIOUSNESS
Composite Runoff Coefficient with Adjustment
8/26/2014 8:49 AM D:\Projects\1013-001\Drainage\Hydrology\1013-001_Impervious_Calcs.xlsx\Composite C
PROPOSED TIME OF CONCENTRATION COMPUTATIONS
S. Thomas
July 9, 2014
Design
Point
Sub-Basin
Overland Flow Channelized Flow (Swale or Gutter) Time of Concentration
(Equation RO-4)
3
1
1 . 87 1 . 1 *
S
Ti C Cf L
7/9/2014 11:46 AM D:\Projects\1013-001\Drainage\Hydrology\1013-001_Proposed_Rational_Calcs.xlsx\Tc
Basin B 0.31 0.00 0.04 0.00 0.15 Clayey | Average 2% to 7% 0.124 0.67 0.67 0.84 54%
Total Basins A and B 0.73 0.01 0.16 0.00 0.30 Clayey | Average 2% to 7% 0.254 0.71 0.71 0.88 59%
OS 0.80 0.00 0.00 0.00 0.03 Clayey | Average 2% to 7% 0.767 0.28 0.28 0.35 4%
1. Table RO-11 | Rational Method Runoff Coefficients for Composite Analysis
PROPOSED COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
Composite Runoff Coefficient with Adjustment
7/9/2014 11:44 AM D:\Projects\1013-001\Drainage\Hydrology\1013-001_Proposed_Rational_Calcs.xlsx\Composite C