20200357 Ver 2_Updated Stormwater Calculations_20210209

ISSUED: I 0/ 13/2020
REV. 2/9/202 I
Hilburn Drive 5ubd1v1s1on
STORMWATER CALCULATIONS
APEX, NORTH CAROLINA
PREPARED BY:
JmJES t CNOSSEN ENGINEERING, PLLC
22 I N. SALEM ST., SUITE 00 I, P.O. BOX I OG2
APEX, NC 27502
91 '.3-387-1 I 74 (PHONE)
9 I '.3-387-3375 (FAX)
17G4
Table of Contents
Narrative
B10-Retent1on # I Calculations
Bio-Retention # I Routing
SHWf Documentation
Drainage Map
~
2
2-4
5-10
I 1-14
15
Narrative
The Stormwater Management Plan (MSP) approval for the Hilburn Drive Duplex Lots 1s Condition 4 of the Approval of Minor
Variance under DWR#20-0357. The MSP 1s required for the Neuse River Buffer impacts associate with Lot 2. A b10-retention
device will be used to meet Concl1tion 4, and the device will be sized to accept drainage from the 1mperv1ous surfaces that
include the duplex and driveway. As such, 1t will be the respons1bi11ty of the builder to pipe the duplex roof gutters to the b10-
retention area, and cirade the lot for d1vertinc:i as much runoff to the b10-retention area as practical.
The b10-retent1on device 1s exempt from City of Raleigh stormwater requirements per Section 9.2.2.A.2.b.1. So, the b10-
retention area will be designed to meet the "first flush" only.
B10-Retent1on Area # I -Runoff Calculat1on5
I. Calculate post-development 'C-value' for b10-retenbon dramage area
Descnpt,on !Jc~ {,1c) ~ Qi
Duplex 1mperv1ous surface area
On-site drainage area
TOTAL DRAINAGE AREA
Composite 'C-value' =
Prec1p1tation Intensity for a 5 minute storm:
12 =
l,o =
1100 =
The calculated design flows are:
Qz =
O,o =
O,oo =
Calculate the time to peak by BMP Manual Appendix B:
p =
CN =
5=
Q·=
Time to Peak =
Estimated 1mperv1ous surfaces from above = 0.075 acre
Impervious Percentage = 0.075 ac/0. I 4 ac = 54%
2. Calculate volume from I "storm usmg the Simple Method for total budd-out
Rv = 0.05+0.9(1A) where IA= 54%
Rv = 0.54
Calculated Volume = (3,G30)(Rv)(RD)(A)
Volume = (3,G30)( I ")(0.54)(0. 14 ac)
Volume = 274 cf (required)
Volume = 300 cf (provided)
0.075 1.00
O.OG5 0.25
0.14 acres
O.G5
5.53 1n/hr
7.04 in/hr
8. 7 I in/hr
0.5 cfs
O.G cfs
0.8 cfs
3.9 inches
81
2.37
2.02 inches
19 minutes
98
GI
3. Calculate mm1mum required surface area
Assumed depth of storage area = I '
SA= V / depth
SA= 274 sf (Required)
SA = 300 sf (Provided)
4. Determme depth of SCM and ,f underdrams will be required
Given that we intend to sod the b10-retent1on device, a depth of 2.5' was selected.
Per the recommendations of spec1f1cat1ons from the North Carolina SCM Manual, the following soil
mix will be used 1n this device:
75 -85% Sand
8 - I 0% fines
5 - I 0% Organic
Underdra1ns will be used to assist 1n conveying the water from the filtered soil to the provided riser
strcuture.
5. Calculate water draw through rate
K = I .0 1n/hr
A= 300 sf
H = 4.5'
L = 3.5'
Q = (2 .3e-5 ) ' K ' A ' H/L
Q = 0.09 cfs
6. Calculate time to remove 300 cf@ flowrate Q
T1.s = V1.s / 0
T1.s = 0.9 hrs.
7. Calculate t,me to drawdown water from saturat,on to 2. 5 ft below surface
Assign Porosity -45% (n = 0.45)
V5 _4 = (2.5ft)' (SA)' 0.45
V5 _4 = 338 cf
Ts.2 = Vs.2 I 0
Ts.2 = I .0 hrs.
8. Total time to draw water through so,!
Time = T5 _1 + Ts-2
Time= I .S hrs.
:3. Calculate underdram pipe capacity
N • D = I G • [Q • n / s 05 )318
N • D = 2.3 1n
Therefore, use two (2) 4" corrugated plastic pipes at 0.5% will work
I 0. Determme the depth of the gravel envelope surroundmg the dramage pipe
Since the underdrains are 4" diameter, the depth of the gravel envelope shall be a rrnnimum of G" deep
(4" pipe diameter + 2" cover).
The routing outflow from the b10-retent1on area are:
02 =
O,o =
O,oo =
0. I cfs
0.2 cfs
0.G cfs
1764 10/13/2020
. Hilburn Road
-------f------------
------~---------+----------------------------------------------------------------------------
Bio-Retention 2 -Yr Storm !
1-------~-----f---------------------------------------------·------------------i
Peak Flow= 0.5 Infiltration Rate 0.1
1--------+------f-----------------------j-----------------i
Time to Peak = 19
---------+-------+------------------j-----------------1
Time (min) Inflow (cfs) Storage (cf) Stage (ft) Outflow (cfs)
1-----~~-+-------+~-f------~~~-----+-----~~-----~~, 0 0.0 0 0 0.00
0
I------------+-----~---------------+---------------+--------l 1.9 0.0 I 0 0.00 0.10
1--------+------f----------------------+-------·--------------i
0 1--------------f---------------------+---------l 3.8 0.0 0 0.00 0.10
-------------------~-------------------------------j
-6 ------------------------------------+--------l 5.7 0.1 -6 0.00 0.10
1-------------------------------------+-----------
1
1-------------f--------------------+----------+-------1 7.6 0.2 -5 0.00 0.10
1----------,------f----------------+--------f-----------i
=====19=/=4=========:=:====:_--::--_--_-= __ --::_-=~~-~;-;_~~~~~~---------jf--: ::--26
0.10
0.10
-------
---------+-------+----------+-----------+-------t 13.3 0.4 46 0.14 0.10
1--------+-----------··-··-------------------------+--------;
I 34
------------------------------------~--------l 15.2 0.5 80 0.24 0.10
-~-----17 __ ._1 ----+ ___ 0_.5 __ --t-__ --_--_---_-_ ---~-~-9 =======l====---0=.3-=-3----=====-=-0_.=1 o====
19 0.5 164 0.42 0.10
-------------------------------------------+--------t 46 I
l--
__ 2_0 __ ·9--+-----o-._5 __ [1--------------22405-449-------------t----0-.5-f__ _______ o_ fcf--
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,____-___ 2_4 ____ 7====-'-i ====0=.4====----' ___ --_-__ ---~=--~2
4
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~:~~~:-::::-----~~:~~~~:~4-:----_-_-:_-____ ~ _____ -_____ I_i__ l _0·-:-:--_-_-_-_--,--:-.-::-----1
! 20 I
1------------------------------------'----------------+-------t 32.3 0.2 407 0.81 0.10 1--------------------------------------------+-------I 16
1---------t-------------~----------------------------+---------
~--=-:-=-:--:::-:~~~:-==:-_-_:~=~-~:=-:=--~=---~---+-+_ -= ___ ------~---====-==-=---:=1_;~-3-:~-=-~------+-,----0
--·_:--:-~----------+--I:~:::_:-:_:~:::
38 I 0.2 444 I 0.85 : 0.10 6 ! ---------+------
1------------------------------., ________ -t------------------~------39.9 0.1 451 o.86 I 0.10
4
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41.8 0.1 455 0.86 0.10
--
. ··---------------------------------·-------·-·---· -----·-----
2
43.7 0.1 457 0.87 0.10
0 I
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45.6 0.1 457 0.87 0.10 ------
I -1
47.5 0.1 456 0.86 0.10 -·-------------
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49.4 0.1 453 I 0.86 0.10 -~--
-4 ----
51.3 0.1 449 i 0.85 0.10 -------
-5
53.2 I 0.0 444 0.85 0.10 i
I -6
55.1 0.0 437 ' 0.84 0.10
-7
-·
57 0.0 430 i 0.83 0.10
·--·----
-8
58.9 I 0.0 422 0.82 0.10
-9
60.8 0.0 414 0.80 I 0.10
-9 ~-------------
62.7 0.0 404 0.79 0.10
·----+-----~---·-------------
I -10
I-----
64.6 i 0.0 395 0.78 I 0.10 -------
I
---
-10 I
66.5 0.0 384
·---1
0.76 0.10 i
1764
f Hilbur!l Road----t-1--__ -_-_-_-_=-=-===i--------_-_-________ +---1011312020
1---------'--------------------j-1 ____________________ ----t
Bio-Retention 10 -Yr Storm ' '
,-------------------------------~-------~------------
Peak Flow= 0.6 Infiltration Rate ,0.1
Time to Peak = 19
l-------~-----!-------------------------+---------1
1--------+-------l-----------------------+-------l
Inflow ( cfs) Time (min) Storage (cf) Stage (ft) Outflow (cfs)
0.00 0.0 0 0 0
I 0
1.9 0.0 0 0.00 0.10
! 0
0.1
0.1
0.2
~----,,--c----------------------------------~------t
3.8 :~ 0.00
-------,-----~----------L-----0.00 ________ 0_. __ 10 _ ___,
-2 i 0.00 I 0.10 ------+-----------
-----+-----' -----+--1--~13_, ~~~-p
---------+--0-.3 ___ ----+-1 ____ ~; -------+-_,I ________ 0_.0_4 ___ -' __ o __ .1 __ 0 ____ _
-----+----_0_.4 _________ 3_3_, ____ _;_, ______ 0.11 _________ 0 __ .1_0 _ ___.
I 33
------------------------------------------------------I ---
0.10
5.7
~
7.6 --------·
9.5
--11.4 ---
~-------+----:-:--+=-----~ ::~l ~·------------+------'---__ 1_09 ___ --+-
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13.3
15.2 --17.1
19
20.9
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22.8
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57 I
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0.71 0.10 1---------------------------------------+--------
i 50
378
47 I
24.7
26.6
0.5
---+--------+-----4-24-------j--,I -------------
, 39
t~10
---
0.78
0.84 0.10
I
28.5
30.4 --
464 0.89 - I
0.10
-
0.93 I 0.10
I ---❖-:----=t=-~---------------i
32.3 ------
-----
34.2
36.1
---
----
38
-~~-------i --0. 96 ===± ___ 0_.1_0_---l
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-5-163_1 ___ ------+------1-._oo ____ t--:-· 1_o --1
__ 5_;_4 ___ --+---1.02 I 0.14
--·-
39.9
------+----·-------·-----·----------+----·----------------------
0.2 I 580 1.02 I 0.15
1
2 I
41.8 0.1 582 1.02 0.15
-1 ! ----I 43.7 0.1 581 ! 1.02 0.14 -·--- -
-3 I
45.6 0.1 578 1.02 0.13 ---·-·
-3 ----
47.5 0.1 575 1.01 0.12
'
-4
~ ----~-
49.4 0.1 571 1.01 0.11 --·-----
-4 ---
51.3 0.1 567 1.00 0.10 -
-4 ---L---
53.2 I 0.1 563 1.00 0.10 I
---------
-5
55.1 i 0.0 558 0.99 0.10 ---
-7
57 0.0 551 0.98 0.10 ----------
-7
58.9 0.0 544 0.97 i 0.10 -····~=r----8 i --
60.8
'
0.0 536 l 0.96 0.10 ----------
-9 :::+-------------
62.7 0.0 527 0.95 0.10 --·------------------------
I -9 -----
0.94 ~ 0.10 64.6 0.0 518 ____ ______J__
--+ -10 ---+----------------~--~
66.5 0.0 508 0.93 I 0.10
Bio-Retention 100 -Yr Storm --------------------------------+-----------
Peak Flow= 0.8 Infiltration Rate 0.1 ------
Time to Peak= 19
.----------+-------+------------~------------:------------<
I I
~-----+--------+------------!----------f---------,
Time (min) Inflow ( cfs) Storage (cf) Stage (ft) Outflow (cfs)
0 0.0 0 0 0.00 ---------+---------------+------------f------------i
0
1.9 0.0 0 0.00 0.10 -------+---------------~------------------
I 0 -3.8 I 0.1 0 0.00 0.10
-3 i
I ------
5.7 -3 i 0.00 0.10
7.6
8
-----f-------------------+----------+------------<
5
[--
0.02 0.10
21 !
9.5
---
11.4
13.3
15.2
~"-----+-------~6-18 ______ --_-+-___ 0.4_4 __ =F_----~-~-=---o-_=1-0=--------~~
17.1 239 0.57 0.10 ------------__________________ ____,
78
19
20.9
22.8
24.7
26.6
28.5
30.4
-
32.3
34.2 ----
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---+-----_-_ -----+---____ :_!_: ____ L ~: :~=f_--==-:-:=::===---·
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0.4
0.3 ~ -I
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--
0.3 0.22
38
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7 ___ _L ____ _
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--=t== 0.2
I
-------------------
39.9 0.2 598 1.02 0.16
41.8 0.2 1.06 0.33
43.7
45.6
-16 t -
-----+-----0-.2--+----------58_9_____ I ------1. -0-2----+-----o._1_ 3 _____ _
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582 1.01 0.12 0.1 -----------------------------------------~ ----------<
0 --------------------------------------------1
49.4 0.1
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51.3
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S&
EC
Soil & Environmental Consultants, PA
8412 Falls of Neuse Road, Suite 104, Raleigh, NC 27615 • Phone: [phone] • Fax: [phone]
sandec.com
Marlowe & Moye, LLC
Attn: Howard Moye
PO Box 20667
Raleigh, NC 27619
February 9, 2021
Job # 10842.S 1
Re: Detailed storm water soils evaluation at 7401 Hilburn Road, Raleigh, NC.
Dear Mr. Moye:
Soil & Environmental Consultants, PA (S&EC) performed a detailed soil evaluation within
the targeted area of the potential storm water BMP on the site mentioned above (see attached
map for the soil boring location). The purpose of this evaluation was to provide additional
information for the proper design of the proposed bio-retention area to treat the on-site storm
water. A soil morphological profile description was performed at the specified location to
determine depth to seasonal high water table (SHWT). The following is a briefreport of the
methods utilized in this evaluation and the results obtained.
Soil/Site Evaluation Methodology
The site evaluation was performed by advancing hand auger borings to sufficient depths to
estimate seasonal high water table. S&EC navigated to the soil boring location to describe
soil morphological conditions using standard techniques outlined in the "Field Book for
Describing and Sampling Soils, Version 3" published by the Natural Resources Conservation
Service (NRCS, 2012).
Soil/Site Conditions
This site is located in the Acid Crystalline geological area of Wake County. Soil Boring 1,
located in bio-retention area, was advanced 65 inches from the top of existing ground until
hand auger refusal. Field investigation revealed seasonal high water table indicators at 30
inches below top of existing ground at the Soil Boring 1 location.
A soil profile description for Soil Boring 1 is shown below.
Horizon Depth Matrix Color Texture Mottles Other notes
(inches)
A 0-llin Brown Sandy Loam
Bt 11 -30 in Light Reddish Clay
Brown
BC 30-65 in Gray Fine Sandy Black, SHWT@30"
Loam with Light Gray
clay lenses
AR 65 in Auger
Refusal
Each storm water device has different requirements with regard to in-situ soils. Under current
regulations, the bottom of any bio-retention area must be a minimum of two feet above the
seasonal high water table. Current NC-DWR regulations also stipulate that an underdrain
must be installed in a bioretention basin if soils underlying the structure have a drainage rate
ofless than 2 in/hr. S&EC made no attempt to measure soil saturated hydraulic conductivity
(Ksat) in the underlying soil at this time but, due to the underlying clay horizons, S&EC
believes that the hydraulic conductivity would be less than 2 in/hr.
Soil & Environmental Consultants, PA is pleased to be of service in this matter and we look
forward to assisting in the successful completion of the project. Ifrequested, S&EC can meet
on-site with NC-DWR to discuss our findings and recommendations. Please feel free to call
with any questions or comments.
Sincerely,
Soil & Environmental Consultants, PA
Ricky Pontell
NC Licensed Soi
Page 2 of 2
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