CC - Storm Drainage Calcs ' ESE Consultants, Inc.
ESE CONSULTANTS -'3103 w. Sheryl Drive
ENGINEERING - PLANNING • SURVEYING ENVIRONMENTAL Meridian, Idaho 83642
TEL: 208-424-0020
STORMWATER MANAGEMENT REPORT
FOR
OAKS NORTH SUBDIVISION, PHASE 9
12449
or-
E. 80 I lagllzi
Justin E. Bozovich
Professional Engineer
Idaho License No. 12449
January 2021
LOCATED IN:
CITY OF MERIDIAN
ADA COUNTY, IDAHO
TABLE OF CONTENTS
SUBJECT SECTION
Project Overview 1
Stormwater Management Design 2
Stormwater Equations 3
Primary Conveyance & Stormwater Facility Calculations 4
Intensity-Duration-Frequency Curve 5
Pipe Flow Calculations 6
Drainage Basin Map 7
SECTION 1 - Project Overview
The Oaks North Subdivision No. 9 is a part of a multi-phase residential development located in
the City of Meridian, Ada County, Idaho. The development is situated along the north side of
West McMillan Road, between North Black Cat Road and North McDermott Road. The applicant
proposes to develop twenty (20) single family residential lots with this phase of the project on
approximately 7.4 acres. The development includes the construction of homes and
infrastructure to include roads, sidewalks, stormwater management facilities, utilities, and other
improvements typical of residential construction. Additional improvements will be constructed
as part of the subdivision in subsequent phases.
Existing Conditions
The project site is currently bounded by single family residential developments to the south
(Oaks South Subdivision) and east (Jump Creek Subdivision). Existing farmland primarily
bounds the project to the north and west.The project site itself is comprised of existing irrigated
farmland with dirt access roads, field drains, and irrigation delivery ditches. There previously
existed several out buildings near McMillan Road that have been razed and removed from the
site by the developer.
The existing terrain is relatively flat with slopes generally less than one percent. The site slopes
down gradient from east to west.
Geotechnical Characteristics
A Geotechnical Engineering Report was prepared by Materials Testing and Inspection (MTI) for
the Oaks North Subdivision, dated July 20, 2018. The soils were generally described as having a
hard pan layer at approximately 3-7 feet below the ground surface overlain by clay material.
Gravel and sands were generally observed below the hard pan layer.
Groundwater
Groundwater monitoring of the project site was commenced by MTI in June 2018. A summary of
the groundwater elevations observed on site to date have been recorded by MTI in the
Groundwater Letter dated, November 20, 2019. The groundwater has been observed at depths
between 3-ft and 10-ft below the ground surface.
Soil Infiltration
MTI performed soil infiltration tests at four locations within the project site and presented their
results in Addendum#2 to the Geotechnical Engineering Report,dated February 8,2019. Results
of the soil infiltration rates ranged from a low of 0.24 in/hr to a high of more than 8 in/hr. An
infiltration rate of 1 in/hr was utilized for design purposes.
SECTION 2 - Stormwater Management Design
Design Criteria
The Stormwater Management Plan described herein has been designed according to the Ada
County Highway District Policy Manual. The proposed Stormwater Management Design has
been prepared to meet the following requirements and guidelines:
♦ The Rational Method (Q=CiA) has been utilized to calculate peak flows. Runoff
coefficients were used per the ACHD Policy Manual. Rainfall intensities were determined
using the ACHD adopted Intensity-Duration-Frequency Curve located in Section 5 of this
report. Calculations for the requisite time of concentration (Tc) value are enclosed. The
overland flow and gutter flow segments of the Tc were calculated based on the equations
provided in the ACHD Policy Manual and enclosed within this report for reference.
♦ Pipe flow calculations are provided to demonstrate that the primary stormwater
conveyance system is sized to adequately handle stormwater flows based on the 25-year
storm event.
♦ High groundwater observed through the site necessitated the use of Vertical Sand Filters
for the storage and infiltration of the 100-year storm event. An infiltration rate of 1 inch
per hour has been used for the design of the vertical sand filters. There are six vertical
sand filters, located within five drainage areas, proposed in Phase 6; each are located on
common lots.
♦ The residential lot grading was designed to drain approximately the front half of the lot
towards the street and is included in the stormwater basin calculations. Stormwater that
falls within the back of the lots will be retained on site. Yard drains will be installed in the
rear yards during home construction and final landscaping to facilitate infiltration of lot
drainage.
♦ High groundwater observed through the site necessitated the use of a Vertical Sand
Filter for the storage and infiltration of the 100-year storm event. An infiltration rate of 1
inch per hour has been used for the design of the vertical sand filter. The proposed
vertical sand filter known as Basin A is located within Block 28 Common Lot 12. A sand
and grease trap is provided for pretreatment.
SECTION 3 - Stormwater Equations
Stormwater Conveyance ACHD Policy Manual 8011.2)
Q - CIA where:
Q - Discharge rate (cfs)
- Coefficient of Runoff(dimensionless)
i - Rainfall intensity (in1hr) for a duration equal
to the time of concentration (Tc) over the
contribuking area and for the design
frequency.
A - Site area (acres)
Tc - Time of ooncentration
Coefficient of Runoff(ACHD Policy Manual 8011.2.1)
Estimated Ftunoff Coefficients for Various Surfaces
Type ofSUrfaie Runoff Coefficients "C"
Business
Downtown areas 0.70.0.95
Urbannelghborhoods 0.50.0.70
Residential
Single Family 0.35.0.50
Multifamily 0.60.0.75
Residential;rural) 0.25.0.40
Apartment Dwelling Areas. 0.70
Industrial and Commercial
Light areas 0.80
Heavy areas 0.90
Parks,Cemeteries 0.10.0.25
Playgrounds 0.20-0.35
Railroad yaFd areas 0.20.0.40
UnlmpFaved areas 0.10.0.30
StFBBtS
Asphalt 0.95
Gan{Fete 0.95
Brick 0.95
Rook 0.95
gravel 0.75
Fields:Sandy sail Soll Type
Slope A B C
Flat 0.2% 0.04 0.07 0.11 0.1S
Average,2.6% 0.09 0.12 0.15 0.20
Steep' % 0.13 0.18 0.23 0.26
Ada pted from ASCE
Design Storm Runoff Volume (ACHD Policy Manual 8011.3.2)
V=CiA(3600)
V: Design storm runoff volume required for storage
C: Coefficient of Runoff
is Rainfall Intensity per IDF Curve
1-hour 100-year storm event, i=0.96
1-hour 2-year storm event, i=0.26
A: Total catchment area(acres)
Time of Concentration
TC=TC-G+TC-s
Tc : Total time of concentration
LG
Tc-c — 60 • VG
Tc-G : Gutter time of concentration
LG : Length of gutter flow
VG : Gutter flow velocity
VG=K„kSP0.5
K„ : Empirical coefficient(3.281)
k : Intercept coefficient(0.619)
SP : Slope (percent)
0.007(nL)0.8
Tc—S = (p2)0.5So.4
Tc-s : Overland time of concentration
L : Length of overland(sheet) flow
n : Manning's Roughness Coefficient(0.24)
P2 : 2-year 24-hour rainfall(1.2 in.)
S : Slope(ft/ft)
Manning's Roughness Coefficient(ACHD Policy Manual 8011.5.3)
Typical Range of Manning's Coefficient (n} for Channels and Pipes
COnd4iil rrl t6ri01 f4l nrlirrg'S '
Closed Conduits
concrete pipe 0-010-0.015
CRAP 0-011 -0.037
Plastic (smooth) 0-009-0.015
Plastic carru etedl 0.018-0.025
PavalvanUgunor seceions 0.012-0-016
Small Open Channels
Concrete 0-011 -0.015
Rubble or riprap 0-020 - 0.035
Vegetation 0.020 - 0.150
Bate Soil 0.016- O.Q25
Rock Cut 0.025- 0.045
Natural onannals{minor streams. lop wpdth at flood stage <30 m(100 ft)I
Fairiy regular section O.Q25- 0.050
Irr"ular were d'1 with pools 1 0.040- 0.150
-Lower values are usually For well-constructed and maintained(smoother)pEpes
and charnels
Table 3-2. Manning's Roughness Coefficient(n)for OVerland Sheet Flow.:
Surface Description n
Smooth asphalt O,011
Smooth concrete 0.012
Ordinary concrete lining 0.013
Good wood O.D14
Brick with cement mortar 0.014
Vitrified Clay O.D15
Cast iron O.D75
Corrugated metal pipe 0.024
Cement rubble surface 0.024
Fallow (no residue) 0-05
Cultivated soils
Residue cover s 20% 0.06
Residua cover� 20% 0.17
Range(natural) O.13
Grass
Short grass prairie 0.15
Dense grasses 0.24
Bermuda grass 0.41
woods'
Light underbrush 0.40
Dense underbrush 0.80
When selecting n, consider cover to a heig ht of about 30 mm. This is only part of
the plant cover that will obstruct sheet flow.
SECTION 4
Primary Conveyance & Stormwater Facility Calculations
BASIN CATCHMENT AREA A
TIME OF CONCENTRATION
Overland Flow Gutter Flow
Sub-Catchment eng ope,s TC-5 Elev I Elev 2 Length,LG Slope,Sp Gutter Tc_G Total TC
(ft) M (min.) (ft) (ft) (ft) M (ft/sec) (min.) (min.)
Al 72.0 1.u" 23.6 2533.5 2531.4 520 0.40 1.29 6.7 30.4
A2 72.0 1.00 23.6 2533.7 2531.4 560 0.41 1.30 7.2 30.8
A3 72.0 1.00 23.6 2534.7 2533.4 335 0.39 1.27 4.4 28.1
A4 72.0 1.00 1 23.6 2534.4 2533.4 270 0.35 1 1.20 3.7 1 27.4
CONVEYANCE
Sub-Catchment C Area,A i2 i25 iioo Q2 Q25 Qioo
4 (acres) (in/hr) (in/hr) (in/hr) (cfs) (Cfs) (cfs)
Al 0.50 1.20 0.4 1.07 1.49 0.24 0.64 0.89
A2 0.50 1.89 0.4 1.05 1.47 0.38 0.99 1.39
A3 0.50 0.74 0.42 1.1 1.54 0.16 0.41 0.57
A4 0.50 1 0.42 1.11 1.55 0.24 0.63 0.88
Total 4.96 1.01 2.67 3.73
SAND&GREASE TRAP
No.of S&G Baffle Throat Velocity, Velocity
Vault Size Traps Spacing Throat Width Throat Area Flow Rate, Q25 V Flow Rate Check Check
(galon) (inch) (inch) (ft) (cfs) (fps) Q25<3.33 cfs V<0.5 fps
1000 20 48 6.67 2.67 0.40 TRUE TRUE
STORAGE VOLUMES
Chamber 1
Req'd Volume, Rock Void Volume, Storage Volume,
V2-YR Ll Wl d1 Pipe Volume,Vpl VRl V1
(ft) (ft) (ft) (ft) (ft) (ft)
2,321 170 6.5 5 298 2,091 2,389
Chamber 2
eq Volume, ioo- Rock Void Volume, Infiltration Rate, Bed Infiltration StorageTime to
Ya M2W2 d2 Pipe Volume,VP2 VR2 I Volume,V2i Volume,V2 Drain 90%
(ft3) (ft) (ft) (ft') (in/hr) (ft') (ft') (hrs)
8,571 170 24 5 133 8,107 1.0 340 8,580 22.69
SECTION 5
Intensity-Duration-Frequency Curve
ACHD Intensity-Duration-Frequency
Intensity(inches per hour)
Design Storm 2 5 10 25 50 100
Tc(hr) Tc(min)
0.17 10 0.69 1.15 1.48 1.85 2.20 2.58
0.25 15 0.59 0.97 1.22 1.56 1.86 2.18
0.33 20 0.49 0.81 1.01 1.30 1.54 1.81
0.42 25 0.43 0.71 0.89 1.14 1.35 1.58
0.50 30 0.41 0.67 0.85 1.08 1.29 1.51
0.58 35 0.34 0.56 0.70 0.90 1.07 1.25
0.67 40 0.31 0.51 0.64 0.82 0.98 1.15
0.75 45 0.29 0.48 0.60 0.77 0.91 1.07
0.83 50 0.27 0.45 0.56 0.72 0.85 1.00
0.92 55 0.26 0.43 0.54 0.69 0.82 0.96
1 60 0.26 0.43 0.54 0.69 0.82 0.96
2 120 0.16 0.25 0.31 0.39 0.46 0.54
3 180 0.13 0.19 0.23 0.29 0.34 0.40
6 360 0.09 0.12 0.14 0.18 0.21 0.25
12 720 0.06 0.08 0.10 0.12 0.14 0.16
24 1440 1 0.04 0.06 1 0.06 0.08 1 0.09 0.10
Intensity Duration Frequency
------ 2
3
-)K-5
-x-10
2.5
25
t 50
2
---0-100
L
H
u 1.5
c
c
c �X�X
X----X_X\X_X-
--~-- \X-_)K X- �X-X
0.5 ------ ---------X-X-X- \ _
0 - - ----
10 15 20 25 30 35 40 45 50 55 60 120 180 360 720 1440
Duration(minutes)
SECTION 6
Pipe Flow Calculations
Channel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk, Inc. Thursday,Jan 14 2021
CBA4toMHA6
Circular Highlighted
Diameter (ft) = 1.00 Depth (ft) = 0.30
Q (cfs) = 0.630
Area (sqft) = 0.20
Invert Elev (ft) = 100.00 Velocity (ft/s) = 3.17
Slope (%) = 0.50 Wetted Perim (ft) = 1.16
N-Value = 0.010 Crit Depth, Yc (ft) = 0.33
Top Width (ft) = 0.92
Calculations EGL (ft) = 0.46
Compute by: Known Q
Known Q (cfs) = 0.63
Elev (ft) Section Depth (ft)
102.00 2.00
101.50 1.50
101.00 1.00
100.50 0.50
v
100.00 0.00
99.50 -0.50
0 1 2 3
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk, Inc. Thursday,Jan 14 2021
CB A3 to M H A6
Circular Highlighted
Diameter (ft) = 1.00 Depth (ft) = 0.24
Q (cfs) = 0.410
Area (sqft) = 0.15
Invert Elev (ft) = 100.00 Velocity (ft/s) = 2.82
Slope (%) = 0.50 Wetted Perim (ft) = 1.02
N-Value = 0.010 Crit Depth, Yc (ft) = 0.27
Top Width (ft) = 0.85
Calculations EGL (ft) = 0.36
Compute by: Known Q
Known Q (cfs) = 0.41
Elev (ft) Section Depth (ft)
102.00 2.00
101.50 1.50
101.00 1.00
100.50 0.50
100.00 0.00
99.50 -0.50
0 1 2 3
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk, Inc. Thursday,Jan 14 2021
M H A6 to M H A3
Circular Highlighted
Diameter (ft) = 1.00 Depth (ft) = 0.39
Q (cfs) = 1.040
Area (sqft) = 0.28
Invert Elev (ft) = 100.00 Velocity (ft/s) = 3.66
Slope (%) = 0.50 Wetted Perim (ft) = 1.35
N-Value = 0.010 Crit Depth, Yc (ft) = 0.43
Top Width (ft) = 0.98
Calculations EGL (ft) = 0.60
Compute by: Known Q
Known Q (cfs) = 1.04
Elev (ft) Section Depth (ft)
102.00 2.00
101.50 1.50
101.00 1.00
100.50 0 0.50
100.00 0.00
99.50 -0.50
0 1 2 3
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk, Inc. Thursday,Jan 14 2021
CBA2toMHA3
Circular Highlighted
Diameter (ft) = 1.00 Depth (ft) = 0.38
Q (cfs) = 0.990
Area (sqft) = 0.27
Invert Elev (ft) = 100.00 Velocity (ft/s) = 3.61
Slope (%) = 0.50 Wetted Perim (ft) = 1.33
N-Value = 0.010 Crit Depth, Yc (ft) = 0.42
Top Width (ft) = 0.97
Calculations EGL (ft) = 0.58
Compute by: Known Q
Known Q (cfs) = 0.99
Elev (ft) Section Depth (ft)
102.00 2.00
101.50 1.50
101.00 1.00
100.50 0.50
NZ
100.00 0.00
99.50 -0.50
0 1 2 3
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk, Inc. Thursday,Jan 14 2021
CB Al to MH A3
Circular Highlighted
Diameter (ft) = 1.00 Depth (ft) = 0.30
Q (cfs) = 0.640
Area (sqft) = 0.20
Invert Elev (ft) = 100.00 Velocity (ft/s) = 3.22
Slope (%) = 0.50 Wetted Perim (ft) = 1.16
N-Value = 0.010 Crit Depth, Yc (ft) = 0.34
Top Width (ft) = 0.92
Calculations EGL (ft) = 0.46
Compute by: Known Q
Known Q (cfs) = 0.64
Elev (ft) Section Depth (ft)
102.00 2.00
101.50 1.50
101.00 1.00
100.50 0.50
v
100.00 0.00
99.50 -0.50
0 1 2 3
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk, Inc. Thursday,Jan 14 2021
MHA3toSGAl
Circular Highlighted
Diameter (ft) = 1.00 Depth (ft) = 0.69
Q (cfs) = 2.670
Area (sqft) = 0.58
Invert Elev (ft) = 100.00 Velocity (ft/s) = 4.60
Slope (%) = 0.50 Wetted Perim (ft) = 1.96
N-Value = 0.010 Crit Depth, Yc (ft) = 0.70
Top Width (ft) = 0.92
Calculations EGL (ft) = 1.02
Compute by: Known Q
Known Q (cfs) = 2.67
Elev (ft) Section Depth (ft)
102.00 2.00
101.50 1.50
101.00 1.00
v
100.50 0.50
100.00 0.00
99.50 -0.50
0 1 2 3
Reach (ft)
SECTION 7
Drainage Basin Map
v IVI F
Z
W
f
0 N
Q ' 00
z
c
m
t9 -o
cFN
W> ya
o
a
CBA2 H a
_,nCb
W
z Wg
a t
CB Al
• M
1.20ac \ ' I z o
I� v�/
CB A3 ZCB A4 �/ FUTURE PHASE �� z
0.74ac
� 1.13ac W
c
� f
ff
o
0
Q
z
0
OAKS NORTH SUBDIVISION NO. 6 o
U
~ O<
w OZa
:5 Z Z
z
U NOw
Q YN�
c' aO s m
w 0 '
Q Wm�
z �M
Q
� N �
o
U
L1
(n
FEET DATE: SCALE:
/ 120 0 60 120 240 1/29/2021 AS NOTED
I N: WN:
'Ell MOF
JOB NO.: FILE NAME:
GRAPHIC SCALE 1 inch = 120 ft. S 4468 468 S-SM
HEEP
NO.: OF