SW3201003_SAND FILTER CALCS_20201103

SAND FILTER CALCULATIONS
references: NCDENR Stormwater BMP Manual Chapter 11
Date : 7-Oct-20
PROJECT AUTO FINANCE & CAR CARE CENTER
SAND FILTER 1
Base Data
730 Windsor Oak Ct
Charlotte, NC 28241
704-556-1990
YARBROUGH-WILLIAMS & HOULE, INC.
Planning • Surveying - Engineering
Site Area Post = Total Drainage Area (A) =
Site Area Pre = Total Drainage Area (A) =
Measured Impervious Area -
Hydrologic Data
Pre - CN
2.62 ac
4.86 ac
4.86 ac
53.85%
Acreage
Land Use
Soil Type
Hydrologic Group
CN
wei hted CN
0.05
Ex Pvmt
98
1.01
4.81
Woods
ScA
C
70
69.28
0.00
0.00
4.86
Post - CN
CNpre
70
Acrea a
Land Use
Soil T e
H drolo is roup
CN
k4aiahted CN
2.62
Bld /Pvt/SW
98
52.77
2.24
Grass
ScA
C
74
34.15
0.00
0.00
4.86
Pre
CN 70
Post
87
CNp..t
87
Steps 1, 2 Determine if the development site and conditions are appropriate for the use of a
Sand Filter pond and consider any site -specific design considerations.
Step 3 Compute Water Quality Volume (WQv)
Compute Runoff Coefficient, Rv, using (Schueler's Method) Equation 3.1
Rv = 0.05 + 0.009 (1) 1 = percent impervious over basin area =
Total Rainfall amount (P) =
Rv =
0.534618916
RD = 1
Compute Water Quality Volume, WQv, using Equation 3.2
WQv = 1.ORvA/12 = 0.22 ac-ft or
Convert Water Quality Volume, WQv to inches of runoff using Equation 3.3
WQv = 1.0(Rv) = 0.53 in
53.8 %
�in
Design Volume
9432 cf
e`' �`�alH,ereeonta®'og',0'
'9
7537
logl{®Y�dd id 411984
:9.��%" 11` 6��0

Compute Site Hydrologic Input Parameters
Using SCS hydrologic procedures and/or computer models the following data can
be determined for the example development site.
Step 4 Summary of Hydrocad Input and Output
HYDROLOGIC INPUT DATA
Condition
Area ac
CN
tc hours
tc min
Pre-dev
4.86
70
0.08
5
Post dev
4.86
87
0.08
5
Results of Preliminary Hydrologic Calculations
(From Computer Model Results Using SCS Hydrologic Procedures)
Condition
1 YR
Q2- ear
Q10- ear
Runoff
cfs
cfs
cfs
Pre-Dev
3.22
3.27
11.12
w/ out controls
Post Dev
10.04
22.64
23.27
w/ controls
Post Dev
0.47
0.73
9.58
POST MODIFIED CN
Step 5 Compute Water Quality Peak Flow (WQp) using modified CN (adjusted) value
Compute modified SCS curve number, CN, using Equation 3.4
CN mod -post = 1000/[10 + 5P + 10WQv - 10(WQv2 + 1.25 WQVP)0.5]
CN mod -post = 95
Compute WQp using SCS the hydrograph procedure documented in the CMSWDM
using Hydrocad hydrologic model. A 1-inch, 6-hour balanced storm event is required.
See Hydrocad Report WQp = 6.48 cfs
Step 5 Compute Sedimentation Chamber Minimum Surface Area
Per Section 11.3.5 NCDENR Stormwater BMP Manual
Also Min As (ft2� _ [240 * Rv * AD) * Ru (ft) = 623.58 sf
As provided = 1850.0 sf
Forebay Chamber Storage -Elevation Data
Elevation
Area sf
Area ac
Ht ft
Inc. Vol ac-ft
Acc. Vol ac-ft
Acc. Vol cf
685
1000.0
0.023
0.0
0.00
0.00
0.00
686
1400.0
0.032
1.0
0.03
0.03
1200.00
687
1850.0
0.042
2.0
1.00
1.03
44760.00

Step 6 Compute Sand Filter Area to Treat Water Quality Volume
Af = (WQv)(df)/[(k)(hf+df)(tf)]
where:
Af = surface area of filter bed (ft2)
WQv = Treatment Water Quality Volume (cf)
9431.64 cf
df = filter bed depth (ft)
1.50
k = coefficient of permeability of filter media (ft/day)
3.5
hA = average height of water above filter bed (ft)
0.96
tf = design filter bed drain time (days)
1.67
Af = 981.9
sf required min. area of sand filter
Est. Sand filter area 31.3
foot by 31.3
Filter Surface Area (Af=)
982 sf for calculations
NOTE: top sand filter provided is 43' X 43'
bottom of sand filter (41' X 41) is used to calculate outflow through the filter
Actual Sand Surface Area Provided =
1600 sf
Step 7 Verify the Volumes
[At (St) + AS (Sf)] x hMa.Filter = 14909.25 cf >_
Filter area provided = 6100 sf
Forebay area provided = 1625 sf
hMa.Fiitar= 1.93 ft max head interpolated
Step 8 Develop Water Quality Storage -Elevation Table
Water Quality Stnra F=1evati- flat. /i-i... un- v.,..,ti_..�
9432
40 hrs
cf minimum
Elevation
Area sf
Area ac
Ht ft
Inc. Vol ac-ft
Acc. Vol ac-ft
Acc. Vol cf
684.5
1000.0
0.023
0.00
0.00
0.00
0.00
685
4450.0
0.102
0.50
0.03
0.03
1362.50
686
5500.0
0.126
1.50
0.11
0.15
6337.50
687
6700.00
0.154
2.50
0.14
0.29
12437.50
688
7950.00
1
0.183
3.50
0.17
0.45
19762.50
689
9050.00
0.208
4.50
0.20
0.65
28262.50
690
10150.00
0.233
1 5.50
0.22
0.87
37862.50
Top of Sand Filter Elevation
684.50
100 yr WSE
688.65
WQv Elevation =
686.43
WQv Required =
9431.61
cf
0.217
ac-ft per sand filter
WQv treated =
9440.01
Jcf
0.217
ac-ft per sand filter
Forebay Weir Elevation =
687
Top of Box Weir Elevation =
688.30
WQv release in (max 40 hrs) = IYES

Step 9 Develop Stage -Discharge for Sand Filter Media
Af = (WQv)(df)/[(k)(hf+df)(tf)]
WQv/tf = Qo = Af (k)(hf+df)/(df))
where:
Af = surface area of filter bed provided (ft2)
df = filter bed depth (ft)
k = coefficient of permeability of filter media (ft/day)
hA = average height of water above filter bed (ft)
At Elevation 687 2.5 feet above sand filter
Qo = 9333 cf/da
= 0.108 cfs
At Elevation 686 1.5 feet above sand filter
Qo = 7000 cf/da
= 0.081 cfs
At Elevation 685 0.5 feet above sand filter
Qo = 4667 cf/da
= 0.054 cfs
At Elevation 684.5 sand filter surface
Qo = 0 cf/da
= 0.000 cfs
Bottom of Sand Filter Elevation =
Time to Drain WQV through sand filter =
SHWT elevation =
1000.00 sf
1.50 ft
3.5
0.96 ft
682.00
40 hr
NOT ENCOUNTERED AT ELEVATION 677.5

Step 10 Design Underdrain System
The underdrain capacity must be greater than the filter media capacity.
The capacity must drain the runoff volume from the system within design duration of 40 hours.
The design must assume that 50 percent of the underdrain system (perforations and pipe system capacity)
is lost due to clogging.
Design specifications require the underdrain system to be a 8-inch perforated PVC pipe with 3/8-inch
perforations spaced 3 inches on center, along 4 longitudinal rows spaced 90 apart. Minimum underdrain
slope is 0.5 percent.
Sand Filter Area = 981.92 sf
The filter media is the controlling outflow condition.
Compute minimum drawdown discharge
Water Quality Volume =
Drawdown =
Compute Performation Capacity
The maximum underdrain spacing is 10 feet on center.
The dimension of the sand filter bottom is
Actual length of pipe containing holes is 29 feet.
Number of perforations =
50 percent of perforations =
Capacity of one hole = CA(2gh)0.5
Total Capacity
Filter Media Capacity =
Total Capacity of
Compute underdrain pipe capacity
9431.64 cf
9431.64 cf /(120 hrs)(3600 sec/hour)
0.022 cfs
33'x33'
1 i e 3 rows)((29) fUpipe)(4 holes/ft)
3481 holes
174 holes
_(0.6)(3.1416)[(3/8in)(1/24)]2[(64.4)(5.0ft)]0.s
0.0083 cfs
= 0.083 x 174
1.44 cfs
0.068 cfs at 1 foot of head
1.44 cfs is greater than Filter Media Capacity and
Minimum Drawdown Discharge
Underdrain Pipe Size = ©in = Area=
For 6-inch PVC underdrain pipe at 0.005 fUft slope:
Capacity of pipe = (1.49/n)(A)(A/P)o.67(S)o.5
_=(1.49/0.013)(0.196)(0.125)0 67(0.005)0.5
0.39 cfs
Fifty percent assuming clogging = 0.20 cfs
The underdrain pipe capacity (0.20 cfs) is greater than the filter media capacity of 0.068 cfs,
and greater than the minimum drawdown capacity requirement (0.020 cfs computed in this step). Therefore
the design is acceptable.
Step 11 Design Overflow Weir
A 16 foot emergency spillway overflow weir is designed with a weir elevation of 688.3 design elevation
Step 12 Access Maintenance Access and Safety Features
All areas of both chambers must be within 25 feet of an access location from the surface that is large enough
for personnel to enter in order to remove trash, debris, sediment, etc.
In addition, the sand filter media must be accessible in order to facilitate complete replacement when the
sand filter ponds runoff without filtration.
Check 50 year storm elevation - The 50 year storm events bypass the sand filters. The partial flow
from these storms flowing to the sand filter stays within the sand filters with overflow over the outlet structure.
0.196 sf
0.125

Please complete the yellow shaded items.
�� OF W A TF9
" Oa PG
m- O Y
NCDENR STORMWATER MANAGEMENT PERMIT APPLICATION FORM
401 CERTIFICATION APPLICATION FORM
SAND FILTER SUPPLEMENT
This form must be filled out on line, printed and submitted with all of the required information.
Make sure to also fill out and submit the Required Items Checklist (Section Ill) and the 1&M Agreement (Section IV)
!. PROJECT INFORMATION
Project name AUTO FINANCE & CAR CARE CENTER
Contact name Vincent G Keene, PE
Phone number 704-556-1990
Date October 2, 2020
Drainage area number # 1
!l. DESIGN INFORMATION
Site Characteristics
Drainage area (AD)
211,701.60 ftZ OK
Impervious area
113,994,00 ft'
% Impervious (IA)
53.8%
Design rainfall depth (RD)
1.00 in
Peak Flow Calculations
1-yr, 24-hr runoff depth
1.38 in
1-yr, 24-hr intensity
0.11 in/hr
Pre -development 1-yr, 24-hr runoff
3.22 ft3/sec
Post -development 1-yr, 24-hr runoff
0.47 ft3/sec
Storage Volume
Design volume (WQV)
9,431.64 ft3
Adjusted water quality volume (WQVAdi)
9,431.64 ft3
Volume contained in the sedimentation basin and on top of the sand filter
9,440.00 ft3
Top of sand filter/grate elevation
684.50 ft amsl
Weir elevation (between chambers)
687 ft amsl
Maximum head on the sedimentation basin and sand filter (hMaxFilter)
1.93 ft
Average head on the sedimentation basin and sand filter (hA)
0.96 ft
Runoff Coefficient (Rv)
0.53 (unitless)
Type of Sand Filter
Open sand filter?
Y Y or N
NOT
SHWT elevation
ENCOUNTERED ft amsl
AT ELEVATION
677.5
Bottom of the sand filter elevation
682.00 ft amsl
Clearance (dsHwr)
>2'
Closed/pre-cast sand filter?
N Y or N
SHWT elevation
ft amsl

Bottom of the sand filter elevation
ft amsl
Clearance (dsHWT)
If this is a closed, underground closed sand filter: The clearance between
the surface of the sand filter and the bottom of the roof of the
ft
underground structure (dsp...
Sedimentation Basin
Surface area of sedimentation basin (As)
1,850.00 ft2
Sedimentation basin/chamber depth
2.00 ft
Sand Filter
Surface area of sand filter (AF)
10,150.00 ft2
Top of sand media filter bed elevation
684.50 It amsl
Bottom of sand media filter bed/drain elevation
682.00 ft amsl
Depth of the sand media filter bed (dF)
1.50 ft
Coefficient of permeability for the sand filter (k)
3.50 (ft/day)
Outlet diameter
6.00 in
Outlet discharge/flowrate
0.20 ft3/sec
Time to drain the sand filter (t)
40.08 hours
Time to drain the sand filter (t)
1.67 days
Additional Information
Does volume in excess of the design volume bypass the sand filter?
Y Y or N
Is an off-line flow -splitting device used?
N Y or N
If draining to SA waters: Does volume in excess of the design volume flow
evenly distributed throuqh a veqetated filter?
N/A Y or N
What is the length of the vegetated filter?
ft
Does the design use a level spreader to evenly distribute flow?
Y or N
Is the BMP located at least 30ft from surface waters (50ft if SA waters)?
Y Y or N
If not a closed bottom, is BMP located at least 100ft from water supply
wells?
Y Y or N
Are the vegetated side slopes equal to or less than 3:1
Y Y or N
Is the BMP located in a recorded drainage easement with a recorded
access easement to a public Right of Way (ROW)?
Y Y or N
What is the width of the sedimentation chamber/forebay (Wsed)?
60.00 ft
What is the depth of sand over the outlet pipe (dpipe)?
1.50 ft