20200817 Ver 1_2020-01-31 Farrington SW Narrative_certified_20200619

Engineers & Planners Firm License No. P-1199

PHONE WEB

2755-B Charles Blvd. | Greenville, NC 27858 252-558-0888 www.arkconsultinggroup.com

MEMORANDUM

TO: Daryl Norris, PE, City of Greenville
FROM: Dan Withers, PE
DATE: January 31, 2020
SUBJECT: Farrington Trace - Stormwater Management Narrative
The purpose of this memorandum is to describe the methods proposed to meet City of Greenville’s Stormwater
Management and Control Ordinance (“Ordinance”) for the proposed Farrington Trace project, a development of multi-
family housing on the north side of Firetower Road in Greenville, North Carolina. The project includes a subdivision of
an existing 13 +/- acre parcel. This management plan includes the proposed multi-family parcel and the roadway to
serve the subdivision, but excludes the two outparcels, which total 5.15 acres.
The site of the development is currently used for agriculture but has a significant amount of uncleared woods. The site
drains to Meeting House Branch, a tributary of the Tar River.
The nitrogen and phosphorous export from the proposed development are calculated in the attached worksheets. In
accordance with City development regulations, mitigation of nutrient export from the site will be accomplished by
purchase of nutrient offsets from a qualified mitigation bank. The nutrient offsets are calculated in the following Tables
1 and 2.
Export Rate
(lb/ac/yr)
Target Rate
(lb/ac/yr)
Rate Increase
(lb/ac/yr)
Duration
(years)
Area
(acres)
Total Load
(lb)
Bypassing SCM (Catchment 1) 4.79 4.0 0.79 30 4.75 112.58
Treated by SCM (Catchment 2) 9.23 4.0 5.23 30 3.05 478.55
Table 1. Nitrogen offset calculation.
Export Rate
(lb/ac/yr)
Target Rate
(lb/ac/yr)
Rate Increase
(lb/ac/yr)
Duration
(years)
Area
(acres)
Total Load
(lb)
Bypassing SCM (Catchment 1) 0.64 0.51 0.13 30 4.75 18.53
Treated by SCM (Catchment 2) 0.72 0.51 0.22 30 3.05 20.13
Table 2. Phosphorous offset calculation.
The total nitrogen offset required is 591.13 lb and the total phosphorous offset required is 38.66 lb.
In accordance with the City’s stormwater ordinance, the proposed development will attenuate peak flow from a portion
of the site using a wet pond in order to ensure that there is no net increase in runoff and accomplish the nutrient
removals computed in the offset calculations above. Wet pond design calculations are enclosed, and routing
calculations prepared using Autodesk Hydraflow software are enclosed.
Tar-Pamlico Stormwater Rule 15A NCAC 2B .0258 Last Modified 1/31/2020
Coastal Plain of the Tar-Pamlico River Basin:
Includes Greenville and Washington as well as Pitt and Beaufort Counties
Total Nitrogen and Total Phosphorus Loading Calculation Worksheet (Automated)
Project Name:Farrington Trace
Date:1/31/2020
By:JDW Checked By:STA
Directions (same for pre-development and post-development tables):
Pre-development:
(2)(3)(4)(5)(6)(7)
Area
(acres)
S.M. Formula
(0.51 + 9.1 I)
Average EMC
of TN (mg/L)
Column
(2) * (3) * (4)
Average EMC
of TP (mg/L)
Column
(2) * (3) * (6)
0.00 0.51 2.60 0.00 0.19 0.00
0.00 0.51 1.95 0.00 0.11 0.00
0.00 0.51 1.42 0.00 0.28 0.00
6.13 0.51 4.23 13.23 1.23 3.85
0.00 0.51 2.04 0.00 0.62 0.00
1.66 0.51 0.95 0.81 0.14 0.12
0.00 TN Loading
(lb/yr) =14.03 TP Loading
(lb/yr) =3.96
7.79 TN Exp. Coeff.
(lb/ac/yr) =1.80 TP Exp. Coeff.
(lb/ac/yr) =0.51
Post-development:
(2)(3)(4)(5)(6)(7)
Area
(acres)
S.M. Formula
(0.51 + 9.1 I)
Average EMC
of TN (mg/L)
Column
(2) * (3) * (4)
Average EMC
of TP (mg/L)
Column
(2) * (3) * (6)
2.28 4.39 2.60 26.00 0.19 1.90
1.04 4.39 1.95 8.90 0.11 0.50
3.19 4.39 1.42 19.89 0.28 3.92
1.28 4.39 0.94 5.28 0.14 0.79
0.43 TN Loading
(lb/yr) =60.07 TP Loading
(lb/yr) =7.11
7.79 TN Exp. Coeff.
(lb/ac/yr) =7.71 TP Exp. Coeff.
(lb/ac/yr) =0.91
Note: The nutrient loading goals are 4.0 lb/ac/yr for TN and 0.4 lb/ac/yr for TP. If the post-development nutrient
loading is below these levels, then no BMP is necessary. Otherwise, the next worksheet calculates
post-development TN and TP loadings after BMPs are installed.
Wooded pervious
Type of Land Cover
(1)
Roof impervious
Managed pervious
(pasture)
> Enter the acres of each type of land cover in the green boxes. The spreadsheet will calculate all of the values in light blue.
> Compare total areas of development in pre- and post- tables for consistency (bottom of column (2)), and also for consistency with
the site plans. If all of these values are not the same, there is an error that must be corrected.
> Unless drainage onto the development from offsite is diverted around or through the site, offsite catchment area draining in must be
included in the acreage values and treated.
(1)
Type of Land Cover
Total Area of Development =
Fraction Impervious (I) =
Managed pervious
(lawn/landscaped)
Wooded pervious
Total Area of Development =
Transportation impervious
Fraction Impervious (I) =
Roof impervious
Managed pervious
Managed pervious
(cropland)
Transportation impervious
Tar-Pamlico Stormwater Rule 15A NCAC 2B .0258 Last Modified 5/23/03
Coastal Plain of the Tar-Pamlico River Basin:
Includes Greenville and Washington as well as Pitt and Beaufort Counties
BMP Removal Calculation Worksheet (Automated)
Project Name:Farrington Trace
Date:1/31/2020
By:JDW Checked By:STA
Directions:
TN TP Design Standard
BMP 25 40
NC BMP Manual
Nutrient 40 35
NC BMP Manual
Removal 35 45
NC BMP Manual
Rates 35 45
NC BMP Manual
20 20
NC BMP Manual
20 35
NC BMP Manual
10 10
NC BMP Manual
Catchment 1:
Total acreage of catchment 1 =4.75 ac
First BMP's TN removal rate = % First BMP's TP removal rate = %
Second BMP's TN removal rate = % Second BMP's TP removal rate = %
Third BMP's TN removal rate = % Third BMP's TP removal rate = %
TOTAL TN REMOVAL RATE =0 % TOTAL TP REMOVAL RATE =0 %
(2)(3)(4)(5)(6)(7)
Catchment
Acreage
S.M. Formula
(0.51 + 9.1 I)
Average EMC of
TN (mg/L)
Column
(2) * (3) * (4)
Average EMC of
TP (mg/L)
Column
(2) * (3) * (6)
0.84 3.08 2.60 6.72 0.19 0.49
0.50 3.08 1.95 3.00 0.11 0.17
2.13 3.08 1.42 9.31 0.28 1.84
1.28 3.08 0.94 3.70 0.14 0.55
0.00 3.08 1.95 0.00 0.11 0.00
0.28 Pre-BMP TN
Load (lb/yr) =22.73 Pre-BMP TP
Load (lb/yr) =3.05
4.75 Pre-BMP TN
Export (lb/ac/yr)4.79 Pre-BMP TP
Export (lb/ac/yr)0.64
Post-BMP TN
Load (lb/yr) =22.73 Post-BMP TP
Load (lb/yr) =3.05
Post-BMP TN
Export (lb/ac/yr)4.79 Post-BMP TP
Export (lb/ac/yr)0.64
> It may be advantageous to split the development into separate catchments to be handled by separate BMPs. The tables below allow
the development to be split into as many as three catchments, and can be copied for greater than three. NOTE: Unless runoff flowing
onto the development from offsite is routed separately around or through the site, the offsite catchment area draining in must be
included in the acreage values of the appropriate land use(s) and treated.
> Above each table: Enter the catchment acreage in the top green blank. Based on a comparison of the post-development TN and TP
export coefficients you calculated above to the rule requirements of 4.0 lb/ac/yr TN and 0.4 lb/ac/yr TP, select BMP(s) from the list for
treating the catchment runoff. Enter the chosen BMP(s) nutrient removal rates in the green blanks. If more than one BMP is to be
used in series, the combined removal rates will be calculated automatically in the blue blanks.
> Catchment Tables: Enter the acres of each type of land cover in the green boxes. The spreadsheet will calculate all of the light blue
boxes. NOTE: Compare the Total Catchment Acreage for the Development (final table) to the value you established in the pre-BMP
worksheet tables, and also to the site plans, for consistency. All of these values need to be the same
(1)
Sand Filter
Wet Detention Pond
Stormwater Wetland

Grass Swales
Vegetated Filter Strip w/
Level Spreader
Bioretention
Dry Detention
Fraction Impervious (I) =
Total Area of Development =
Wooded pervious
Area taken up by BMP
Type of Land Cover
Transportation impervious
Roof impervious
Managed pervious
Tar-Pamlico Stormwater Rule 15A NCAC 2B .0258 Last Modified 5/23/03
Catchment 2:
Total acreage of catchment 2 =3.05 ac
First BMP's TN removal rate =25 % First BMP's TP removal rate =40 %
Second BMP's TN removal rate = % Second BMP's TP removal rate = %
Third BMP's TN removal rate = % Third BMP's TP removal rate = %
TOTAL TN REMOVAL RATE =25 % TOTAL TP REMOVAL RATE =40 %
(2)(3)(4)(5)(6)(7)
Catchment
Acreage
S.M. Formula
(0.51 + 9.1 I)
Average EMC of
TN (mg/L)
Column
(2) * (3) * (4)
Average EMC of
TP (mg/L)
Column
(2) * (3) * (6)
1.44 5.97 2.60 22.35 0.19 1.63
0.54 5.97 1.95 6.29 0.11 0.35
1.07 5.97 1.42 9.07 0.28 1.79
0.00 5.97 0.94 0.00 0.14 0.00
0.25 5.97 1.95 2.91 0.11 0.16
0.60 Pre-BMP TN
Load (lb/yr) =40.62 Pre-BMP TP
Load (lb/yr) =3.94
3.30 Pre-BMP TN
Export (lb/ac/yr)12.31 Pre-BMP TP
Export (lb/ac/yr)1.19
Post-BMP TN
Load (lb/yr) =30.46 Post-BMP TP
Load (lb/yr) =2.36
Post-BMP TN
Export (lb/ac/yr)9.23 Post-BMP TP
Export (lb/ac/yr)0.72
Catchment 3:
Total acreage of catchment 3 = ac
First BMP's TN removal rate = % First BMP's TP removal rate = %
Second BMP's TN removal rate = % Second BMP's TP removal rate = %
Third BMP's TN removal rate = % Third BMP's TP removal rate = %
TOTAL TN REMOVAL RATE =0 % TOTAL TP REMOVAL RATE =0 %
(2)(3)(4)(5)(6)(7)
Catchment
Acreage
S.M. Formula
(0.51 + 9.1 I)
Average EMC of
TN (mg/L)
Column
(2) * (3) * (4)
Average EMC of
TP (mg/L)
Column
(2) * (3) * (6)
2.60 0.19
1.95 0.11
1.42 0.28
0.94 0.14
1.95 0.11
Pre-BMP TN
Load (lb/yr) =
Pre-BMP TP
Load (lb/yr) =
Pre-BMP TN
Export (lb/ac/yr)
Pre-BMP TP
Export (lb/ac/yr)
Post-BMP TN
Load (lb/yr) =
Post-BMP TP
Load (lb/yr) =
Post-BMP TN
Export (lb/ac/yr)
Post-BMP TP
Export (lb/ac/yr)
Type of Land Cover
Transportation impervious
Area taken up by BMP
Fraction Impervious (I) =
Total Area of Development =
Managed pervious
Wooded pervious
Managed pervious
Wooded pervious
Area taken up by BMP
Fraction Impervious (I) =
Total Area of Development =
Roof impervious
(1)
Transportation impervious
(1)
Type of Land Cover
Roof impervious
Tar-Pamlico Stormwater Rule 15A NCAC 2B .0258 Last Modified 5/23/03
Weighted Average of Nutrient Loadings from the Catchments:
Catchment
Acreage
Post-BMP
TN Loading
(lb/ac/yr)
Post-BMP
TP Loading
(lb/ac/yr)
4.75 4.79 0.64
3.05 9.23 0.72
0.00 0.00 0.00
7.80 6.52 0.67
Note: The nutrient loading goals are 4.0 lb/ac/yr for TN and 0.4 lb/ac/yr for TP. If the post-development nutrient
loading is below these levels, then the BMPs planned are adequate. Otherwise, additional BMPs and/or
modifications in development plans are required.
Catchment 1
TOTAL FOR DEVELOPMENT

Catchment 2
Catchment 3
Ark Consulting Group, PLLC JOB 19045 - Farrington Trace
2755-B Charles Boulevard DATE Jan-20
Greenville, NC 27858 CALCULATED BY JDW
252-558-0888 CHECKED BY STA
Units Symbols & Governing Equations
Site Calculations
Total Site Area (Catchment 1)3.05 ac Asite
Roof Impervious 0.54 ac Aroof
Transportation Impervious 1.44 ac Atrans
Total Impervious Area 1.98 ac Aimpervious
Percent Impervious 65 %(Aimpervious / Asite)*100
Average Depth Calculation
Area of Bottom Shelf 2240 sq ft Abot_shelf
Area of Permanent Pool 3529 sq ft Aperm_pool
Area of Bottom of Pond 177 sq ft Abot_pond
Depth 8.0 ft d
Main pool volume at permanent pool elevation 10023 cu ft Vpp
Main pool area at permanent pool elevation 3529 cu ft Vpp
Average Depth 2.8 ft dav = Vpp / SA (Eq. 2)
Average Depth (Rounded to nearest 0.5 ft)3.0 ft dav
Coastal Region? (Y or N)N
Surface Area/Drainage Area Ratio 2.30 %SA/DA
Permanent Pool Surface Area Req'd 3056 sq ft SA/DA*Asite*43560/100
Permanent Pool Surface Area Provided 3529 sq ft Aperm_pool
Size of BMP (number times minimum size)1.15
Wet Pond Design
Surface Area Requirements
Ark Consulting Group, PLLC JOB 19045 - Farrington Trace
2755-B Charles Boulevard DATE Jan-20
Greenville, NC 27858 CALCULATED BY JDW
252-558-0888 CHECKED BY STA
Units Symbols & Governing Equations
Total Site Area (Catchment 1)3.05 ac Asite
Roof Impervious 0.54 ac Aroof
Transportation Impervious 1.44 ac Atrans
Total Impervious Area 1.98 ac Aimpervious
Percent Impervious 65 %(Aimpervious / Asite)*100
Runoff Volume - Simple Method
Design Storm Rainfall Depth 1.00 in RD
Design Storm Runoff Volume 7022 cu ft V (Simple Method Equation)
Storage Volume Provided 11722 cu ft
Wet Pond Design
Storage Volume Requirements
Site Calculations
Ark Consulting Group, PLLC JOB 19045 - Farrington Trace
2755-B Charles Boulevard DATE Jan-20
Greenville, NC 27858 CALCULATED BY JDW
252-558-0888 CHECKED BY STA
Units Symbols & Governing Equations
7022 cu ft V
Desired Drawdown Time
(2-5 Days)
0.03 cfs Q (V/T/24/3600)
0.60 CD
1.43 ft HO
0.48 ft HO = (TPE - PPE)/3
1.22 in
1.25 in
0.03 cfs Q (Orifice Equation)
2.9 days
Actual Discharge
Actual Drawdown Time
Driving Head
Appoximate Orifice Diameter
Orifice Diameter
Wet Pond Design
Drawdown Time
Discharge
Discharge Coefficient
Design Storm Stage
Orifice Sizing
Design Storage Volume
3 days T
Ark Consulting Group, PLLC JOB 19045 - Farrington Trace
2755-B Charles Boulevard DATE Jan-20
Greenville, NC 27858 CALCULATED BY JDW
252-558-0888 CHECKED BY STA
Main Pool
Stage Side Slope Contour Area
Incremental
Volume
[ft][sq ft][cu ft]
0 711
2 2:1 1227 1938
5 2:1 2240 5201
6 6:1 3529 2885
Forebay 1
Stage Side Slope Contour Area
Incremental
Volume
Percentage of
Permanent Pool
[ft][sq ft][cu ft]
0 195
2 2:1 606 801
3 2:1 854 730 15.3%
* Acceptable range for Forebay Volume is 15%-20% of Main Pool Volume
Cumulative
Volume
Forebay Sizing Calculations
10023
7139
60.00
Contour
[cu ft]
54.00
56.00 1938
59.00
0
Contour
Cumulative
Volume
[cu ft]
57.00 0
59.00 801
60.00 1531
Ark Consulting Group, PLLC JOB 19045 - Farrington Trace
2755-B Charles Boulevard DATE Jan-20
Greenville, NC 27858 CALCULATED BY JDW
252-558-0888 CHECKED BY STA
Contour Contour Area Incremental Volume Cumulative Volume, S ln(S)Stage, Z ln(Z)ln(Z), est.Z, est.
ft MSL ft^2 ft^3 ft^3 ft ft
60 4659 ----0 ----0 --------
61 5836 5247.5 5247.5 8.565507052 1 0 0.01 1.005106885
62 7113 6474.5 11722 9.369222697 2 0.69315 0.68 1.97258572
63 8490 7801.5 19523.5 9.879374147 3 1.09861 1.11 3.02623826
Design Storm Runoff 7961 ft^3 (via simple method from SA/DA spreadsheet)
Design Storm Stage 1.43 ft
Design Storm El.61.43 ft MSL
Surface Area at Design Storm 6371
Slope, m =1.192011934 8.559435075
Intercept, b = 8.559435075
Regression Analysis ("linest" Excel Function):
Stage-Storage Function
3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8
10 0.51 0.47 0.43 0.40 0.37 0.34 0.30 0.29 0.27 0.26 0.25
11 0.54 0.50 0.46 0.43 0.39 0.36 0.32 0.30 0.29 0.28 0.27
12 0.58 0.53 0.48 0.45 0.42 0.38 0.34 0.32 0.30 0.29 0.28
13 0.61 0.56 0.51 0.48 0.44 0.40 0.36 0.34 0.32 0.31 0.30
14 0.64 0.59 0.53 0.50 0.47 0.43 0.38 0.36 0.34 0.32 0.31
15 0.68 0.62 0.56 0.53 0.49 0.45 0.41 0.38 0.36 0.34 0.33
16 0.71 0.65 0.59 0.55 0.51 0.47 0.43 0.40 0.37 0.36 0.34
17 0.74 0.68 0.61 0.58 0.54 0.49 0.45 0.42 0.39 0.37 0.36
18 0.77 0.71 0.64 0.60 0.56 0.52 0.47 0.44 0.41 0.39 0.37
19 0.81 0.74 0.66 0.63 0.59 0.54 0.49 0.46 0.42 0.40 0.39
20 0.84 0.77 0.69 0.65 0.61 0.56 0.51 0.48 0.44 0.42 0.40
21 0.87 0.79 0.72 0.67 0.63 0.58 0.53 0.49 0.46 0.44 0.42
22 0.91 0.82 0.74 0.70 0.66 0.60 0.55 0.51 0.47 0.45 0.43
23 0.94 0.85 0.77 0.72 0.68 0.63 0.57 0.53 0.49 0.47 0.45
24 0.97 0.88 0.79 0.75 0.70 0.65 0.59 0.55 0.51 0.49 0.46
25 1.01 0.91 0.82 0.77 0.73 0.67 0.62 0.57 0.53 0.50 0.48
26 1.04 0.94 0.84 0.79 0.75 0.69 0.64 0.59 0.54 0.52 0.50
27 1.07 0.97 0.87 0.82 0.77 0.71 0.66 0.61 0.56 0.54 0.51
28 1.10 1.00 0.89 0.84 0.79 0.74 0.68 0.63 0.58 0.55 0.53
29 1.14 1.03 0.92 0.87 0.82 0.76 0.70 0.65 0.59 0.57 0.54
30 1.17 1.06 0.94 0.89 0.84 0.78 0.72 0.67 0.61 0.59 0.56
31 1.20 1.09 0.97 0.92 0.87 0.80 0.74 0.68 0.63 0.60 0.58
32 1.24 1.12 1.00 0.95 0.89 0.82 0.76 0.70 0.64 0.62 0.59
33 1.27 1.15 1.03 0.97 0.92 0.85 0.78 0.72 0.66 0.63 0.61
34 1.31 1.18 1.06 1.00 0.94 0.87 0.80 0.74 0.68 0.65 0.62
35 1.34 1.22 1.09 1.03 0.97 0.89 0.82 0.76 0.70 0.67 0.64
36 1.37 1.25 1.12 1.06 0.99 0.91 0.83 0.77 0.71 0.68 0.65
37 1.41 1.28 1.15 1.08 1.02 0.93 0.85 0.79 0.73 0.70 0.67
38 1.44 1.31 1.18 1.11 1.04 0.96 0.87 0.81 0.75 0.71 0.68
39 1.48 1.34 1.21 1.14 1.07 0.98 0.89 0.83 0.76 0.73 0.70
40 1.51 1.38 1.24 1.17 1.09 1.00 0.91 0.85 0.78 0.75 0.71
41 1.54 1.40 1.27 1.19 1.11 1.02 0.93 0.86 0.80 0.76 0.73
42 1.57 1.43 1.29 1.21 1.13 1.04 0.95 0.88 0.81 0.78 0.74
43 1.59 1.46 1.32 1.24 1.16 1.07 0.98 0.90 0.83 0.79 0.76
44 1.62 1.49 1.35 1.26 1.18 1.09 1.00 0.92 0.85 0.81 0.77
45 1.65 1.51 1.38 1.29 1.20 1.11 1.02 0.94 0.87 0.83 0.79
46 1.68 1.54 1.40 1.31 1.22 1.13 1.04 0.96 0.88 0.84 0.81
47 1.71 1.57 1.43 1.34 1.24 1.15 1.06 0.98 0.90 0.86 0.82
48 1.73 1.60 1.46 1.36 1.27 1.18 1.09 1.00 0.92 0.88 0.84
49 1.76 1.62 1.48 1.39 1.29 1.20 1.11 1.02 0.93 0.89 0.85
50 1.79 1.65 1.51 1.41 1.31 1.22 1.13 1.04 0.95 0.91 0.87
51 1.82 1.68 1.54 1.43 1.33 1.24 1.15 1.06 0.97 0.93 0.89
52 1.85 1.71 1.56 1.45 1.35 1.26 1.17 1.08 0.98 0.94 0.90
53 1.88 1.73 1.59 1.48 1.36 1.27 1.18 1.09 1.00 0.96 0.92
54 1.91 1.76 1.61 1.50 1.38 1.29 1.20 1.11 1.02 0.98 0.93
55 1.94 1.79 1.64 1.52 1.40 1.31 1.22 1.13 1.04 0.99 0.95
56 1.97 1.82 1.67 1.54 1.42 1.33 1.24 1.15 1.05 1.01 0.97
57 2.00 1.85 1.69 1.56 1.44 1.35 1.26 1.16 1.07 1.03 0.98
58 2.03 1.87 1.72 1.59 1.45 1.36 1.27 1.18 1.09 1.04 1.00
59 2.06 1.90 1.74 1.61 1.47 1.38 1.29 1.20 1.10 1.06 1.01
60 2.09 1.93 1.77 1.63 1.49 1.40 1.31 1.22 1.12 1.08 1.03
Piedmont and Mountain SA/DA Table
Permanent Pool Average Depth% Impervious
Cover
Items in bold from NCDEQ Stormwater Design Manual; others interpolated linearly.
61 2.13 1.97 1.80 1.66 1.52 1.43 1.34 1.24 1.14 1.09 1.04
62 2.17 2.00 1.83 1.69 1.55 1.46 1.36 1.26 1.16 1.11 1.06
63 2.22 2.04 1.87 1.72 1.58 1.48 1.39 1.29 1.19 1.13 1.07
64 2.26 2.08 1.90 1.76 1.61 1.51 1.41 1.31 1.21 1.15 1.09
65 2.30 2.12 1.93 1.79 1.65 1.54 1.44 1.33 1.23 1.17 1.10
66 2.34 2.15 1.96 1.82 1.68 1.57 1.46 1.36 1.25 1.18 1.11
67 2.38 2.19 1.99 1.85 1.71 1.60 1.49 1.38 1.27 1.20 1.13
68 2.43 2.23 2.03 1.88 1.74 1.62 1.51 1.40 1.30 1.22 1.14
69 2.47 2.26 2.06 1.91 1.77 1.65 1.54 1.43 1.32 1.24 1.16
70 2.51 2.30 2.09 1.95 1.80 1.68 1.56 1.45 1.34 1.26 1.17
71 2.55 2.34 2.12 1.97 1.83 1.71 1.59 1.48 1.37 1.28 1.19
72 2.59 2.37 2.15 2.00 1.85 1.73 1.61 1.50 1.40 1.31 1.22
73 2.63 2.41 2.19 2.03 1.88 1.76 1.64 1.53 1.42 1.33 1.24
74 2.67 2.45 2.22 2.06 1.91 1.79 1.66 1.56 1.45 1.36 1.26
75 2.72 2.48 2.25 2.09 1.94 1.81 1.69 1.59 1.48 1.38 1.29
76 2.76 2.52 2.28 2.12 1.96 1.84 1.72 1.61 1.51 1.41 1.31
77 2.80 2.56 2.31 2.15 1.99 1.87 1.74 1.64 1.54 1.43 1.33
78 2.84 2.59 2.35 2.18 2.02 1.89 1.77 1.67 1.56 1.46 1.35
79 2.88 2.63 2.38 2.21 2.04 1.92 1.79 1.69 1.59 1.48 1.38
80 2.92 2.67 2.41 2.24 2.07 1.95 1.82 1.72 1.62 1.51 1.40
81 2.95 2.69 2.43 2.26 2.09 1.97 1.84 1.74 1.64 1.53 1.42
82 2.99 2.72 2.46 2.29 2.12 1.99 1.86 1.76 1.66 1.55 1.44
83 3.02 2.75 2.48 2.31 2.14 2.01 1.89 1.79 1.69 1.57 1.46
84 3.05 2.78 2.50 2.33 2.17 2.04 1.91 1.81 1.71 1.59 1.48
85 3.09 2.81 2.53 2.36 2.19 2.06 1.93 1.83 1.73 1.61 1.50
86 3.12 2.83 2.55 2.38 2.21 2.08 1.95 1.85 1.75 1.63 1.51
87 3.15 2.86 2.57 2.40 2.24 2.11 1.97 1.87 1.77 1.65 1.53
88 3.18 2.89 2.59 2.43 2.26 2.13 2.00 1.90 1.80 1.67 1.55
89 3.22 2.92 2.62 2.45 2.29 2.15 2.02 1.92 1.82 1.69 1.57
90 3.25 2.95 2.64 2.48 2.31 2.18 2.04 1.94 1.84 1.72 1.59
91 3.28 2.97 2.66 2.49 2.33 2.20 2.07 1.97 1.86 1.73 1.61
92 3.31 2.99 2.67 2.51 2.35 2.23 2.10 1.99 1.88 1.75 1.62
93 3.34 3.01 2.69 2.53 2.37 2.25 2.13 2.02 1.90 1.77 1.64
94 3.37 3.04 2.70 2.55 2.39 2.28 2.16 2.04 1.92 1.79 1.65
95 3.40 3.06 2.72 2.57 2.42 2.30 2.19 2.07 1.94 1.81 1.67
96 3.43 3.08 2.73 2.58 2.44 2.33 2.22 2.09 1.96 1.82 1.69
97 3.46 3.10 2.75 2.60 2.46 2.35 2.25 2.12 1.98 1.84 1.70
98 3.49 3.13 2.76 2.62 2.48 2.38 2.28 2.14 2.00 1.86 1.72
99 3.52 3.15 2.78 2.64 2.50 2.40 2.31 2.17 2.02 1.88 1.73
100 3.55 3.17 2.79 2.66 2.52 2.43 2.34 2.19 2.04 1.90 1.75
Pre-Development
Overland Flow
Hydraulic length of watershed, L 180 ft
Height of watershed, H 2 ft
Flow Path Multiplier, K 2
Channelized Flow
Hydraulic length of watershed, L 1190 ft
Height of watershed, H 16 ft
Flow Path Multiplier, K 0.2
Time of concentration, tc 6.7 min
Time of concentration, tc 6.7 min
Post-Development - Bypass Area
Overland Flow
Hydraulic length of watershed, L 157 ft
Height of watershed, H 0.3 ft
Flow Path Multiplier, K 2
Channelized Flow
Hydraulic length of watershed, L 1664 ft
Height of watershed, H 17.7 ft
Flow Path Multiplier, K 0.2
Time of concentration, tc 11.3 min
Time of concentration, tc 11.3 min
Post-Development - Wet Pond DA
Overland Flow
Hydraulic length of watershed, L 135 ft
Height of watershed, H 1 ft
Flow Path Multiplier, K 2
Channelized Flow
Hydraulic length of watershed, L 235 ft
Height of watershed, H 2 ft
Flow Path Multiplier, K 0.2
Time of concentration, tc 5.2 min
Time of concentration, tc 5.2 min
Time of Concentration per City of Greenville MSDD
1 2 3
4
5
1
Watershed Model Schematic Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020
Project: 19045 Farrington Trace.gpw Friday, 01 / 31 / 2020
Hyd.Origin Description
Legend
1 SCS Runoff Pre-Development
2 SCS Runoff Post-Dev Bypass
3 SCS Runoff Post-Dev Treated
4 Reservoir Pond Route
5 Combine Post-Dev Total
Hydrograph Return Period Recap
2
Hyd. Hydrograph Inflow Peak Outflow (cfs)Hydrograph
No. type hyd(s)Description
(origin) 1-yr 2-yr 3-yr 5-yr 10-yr 25-yr 50-yr 100-yr
1 SCS Runoff ------ 34.35 45.51 ------- 64.21 80.25 104.32 125.07 148.23 Pre-Development
2 SCS Runoff ------ 29.44 37.07 ------- 49.62 60.28 76.22 89.96 105.18 Post-Dev Bypass
3 SCS Runoff ------ 11.18 13.94 ------- 18.49 22.34 28.12 33.10 38.63 Post-Dev Treated
4 Reservoir 3 4.433 6.725 ------- 9.213 11.36 19.53 26.90 33.06 Pond Route
5 Combine 2, 4 33.44 43.47 ------- 58.48 70.93 95.59 116.86 138.24 Post-Dev Total
Proj. file: 19045 Farrington Trace.gpw Friday, 01 / 31 / 2020
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020
Hydrograph Summary Report
3
Hyd. Hydrograph Peak Time Time to Hyd. Inflow Maximum Total Hydrograph
No. type flow interval Peak volume hyd(s) elevation strge used Description
(origin) (cfs) (min) (min) (cuft) (ft) (cuft)
1 SCS Runoff 34.35 2 720 85,264 ------ ------ ------ Pre-Development
2 SCS Runoff 29.44 2 720 85,316 ------ ------ ------ Post-Dev Bypass
3 SCS Runoff 11.18 2 716 25,287 ------ ------ ------ Post-Dev Treated
4 Reservoir 4.433 2 724 25,278 3 61.62 9,191 Pond Route
5 Combine 33.44 2 722 110,594 2, 4 ------ ------ Post-Dev Total
19045 Farrington Trace.gpw Return Period: 1 Year Friday, 01 / 31 / 2020
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020
Hydrograph Summary Report
4
Hyd. Hydrograph Peak Time Time to Hyd. Inflow Maximum Total Hydrograph
No. type flow interval Peak volume hyd(s) elevation strge used Description
(origin) (cfs) (min) (min) (cuft) (ft) (cuft)
1 SCS Runoff 45.51 2 720 113,934 ------ ------ ------ Pre-Development
2 SCS Runoff 37.07 2 720 108,700 ------ ------ ------ Post-Dev Bypass
3 SCS Runoff 13.94 2 716 32,003 ------ ------ ------ Post-Dev Treated
4 Reservoir 6.725 2 722 31,994 3 61.87 10,757 Pond Route
5 Combine 43.47 2 722 140,694 2, 4 ------ ------ Post-Dev Total
19045 Farrington Trace.gpw Return Period: 2 Year Friday, 01 / 31 / 2020
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020
Hydrograph Summary Report
5
Hyd. Hydrograph Peak Time Time to Hyd. Inflow Maximum Total Hydrograph
No. type flow interval Peak volume hyd(s) elevation strge used Description
(origin) (cfs) (min) (min) (cuft) (ft) (cuft)
1 SCS Runoff 64.21 2 720 163,261 ------ ------ ------ Pre-Development
2 SCS Runoff 49.62 2 720 147,893 ------ ------ ------ Post-Dev Bypass
3 SCS Runoff 18.49 2 716 43,228 ------ ------ ------ Post-Dev Treated
4 Reservoir 9.213 2 722 43,219 3 62.24 13,488 Pond Route
5 Combine 58.48 2 720 191,112 2, 4 ------ ------ Post-Dev Total
19045 Farrington Trace.gpw Return Period: 5 Year Friday, 01 / 31 / 2020
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020
Hydrograph Summary Report
6
Hyd. Hydrograph Peak Time Time to Hyd. Inflow Maximum Total Hydrograph
No. type flow interval Peak volume hyd(s) elevation strge used Description
(origin) (cfs) (min) (min) (cuft) (ft) (cuft)
1 SCS Runoff 80.25 2 720 206,598 ------ ------ ------ Pre-Development
2 SCS Runoff 60.28 2 720 181,695 ------ ------ ------ Post-Dev Bypass
3 SCS Runoff 22.34 2 716 52,891 ------ ------ ------ Post-Dev Treated
4 Reservoir 11.36 2 722 52,882 3 62.55 15,910 Pond Route
5 Combine 70.93 2 722 234,577 2, 4 ------ ------ Post-Dev Total
19045 Farrington Trace.gpw Return Period: 10 Year Friday, 01 / 31 / 2020
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020
Hydrograph Summary Report
7
Hyd. Hydrograph Peak Time Time to Hyd. Inflow Maximum Total Hydrograph
No. type flow interval Peak volume hyd(s) elevation strge used Description
(origin) (cfs) (min) (min) (cuft) (ft) (cuft)
1 SCS Runoff 104.32 2 720 272,985 ------ ------ ------ Pre-Development
2 SCS Runoff 76.22 2 720 232,827 ------ ------ ------ Post-Dev Bypass
3 SCS Runoff 28.12 2 716 67,490 ------ ------ ------ Post-Dev Treated
4 Reservoir 19.53 2 722 67,481 3 62.85 18,112 Pond Route
5 Combine 95.59 2 720 300,308 2, 4 ------ ------ Post-Dev Total
19045 Farrington Trace.gpw Return Period: 25 Year Friday, 01 / 31 / 2020
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020
Hydrograph Summary Report
8
Hyd. Hydrograph Peak Time Time to Hyd. Inflow Maximum Total Hydrograph
No. type flow interval Peak volume hyd(s) elevation strge used Description
(origin) (cfs) (min) (min) (cuft) (ft) (cuft)
1 SCS Runoff 125.07 2 720 331,260 ------ ------ ------ Pre-Development
2 SCS Runoff 89.96 2 720 277,312 ------ ------ ------ Post-Dev Bypass
3 SCS Runoff 33.10 2 716 80,180 ------ ------ ------ Post-Dev Treated
4 Reservoir 26.90 2 720 80,171 3 63.00 19,474 Pond Route
5 Combine 116.86 2 720 357,483 2, 4 ------ ------ Post-Dev Total
19045 Farrington Trace.gpw Return Period: 50 Year Friday, 01 / 31 / 2020
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020
Hydrograph Summary Report
9
Hyd. Hydrograph Peak Time Time to Hyd. Inflow Maximum Total Hydrograph
No. type flow interval Peak volume hyd(s) elevation strge used Description
(origin) (cfs) (min) (min) (cuft) (ft) (cuft)
1 SCS Runoff 148.23 2 718 396,609 ------ ------ ------ Pre-Development
2 SCS Runoff 105.18 2 720 326,917 ------ ------ ------ Post-Dev Bypass
3 SCS Runoff 38.63 2 716 94,324 ------ ------ ------ Post-Dev Treated
4 Reservoir 33.06 2 720 94,314 3 63.13 20,701 Pond Route
5 Combine 138.24 2 720 421,231 2, 4 ------ ------ Post-Dev Total
19045 Farrington Trace.gpw Return Period: 100 Year Friday, 01 / 31 / 2020
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020
Hydraflow Rainfall Report
10
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020 Friday, 01 / 31 / 2020
Return Intensity-Duration-Frequency Equation Coefficients (FHA)
Period
(Yrs) B D E (N/A)
1 112.0000 18.0000 1.0000 --------
2 127.0000 18.0000 1.0000 --------
3 0.0000 0.0000 0.0000 --------
5 181.0000 21.0000 1.0000 --------
10 199.0000 22.0000 1.0000 --------
25 245.0000 23.0000 1.0000 --------
50 0.0000 0.0000 0.0000 --------
100 310.0000 25.0000 1.0000 --------
File name: Greenville.IDF
Intensity = B / (Tc + D)^E
Return Intensity Values (in/hr)
Period
(Yrs) 5 min 10 15 20 25 30 35 40 45 50 55 60
1 4.87 4.00 3.39 2.95 2.60 2.33 2.11 1.93 1.78 1.65 1.53 1.44
2 5.52 4.54 3.85 3.34 2.95 2.65 2.40 2.19 2.02 1.87 1.74 1.63
3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
5 6.96 5.84 5.03 4.41 3.93 3.55 3.23 2.97 2.74 2.55 2.38 2.23
10 7.37 6.22 5.38 4.74 4.23 3.83 3.49 3.21 2.97 2.76 2.58 2.43
25 8.75 7.42 6.45 5.70 5.10 4.62 4.22 3.89 3.60 3.36 3.14 2.95
50 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
100 10.33 8.86 7.75 6.89 6.20 5.64 5.17 4.77 4.43 4.13 3.88 3.65
Tc = time in minutes. Values may exceed 60.
Rainfall Precipitation Table (in)
Precip. file name: D:\Dropbox (Ark)\03 - Support\Software\Hydraflow\Greenville.pcp
Storm
Distribution 1-yr 2-yr 3-yr 5-yr 10-yr 25-yr 50-yr 100-yr
SCS 24-hour 3.11 3.78 0.00 4.89 5.84 7.27 8.51 9.89
SCS 6-Hr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Huff-1st 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Huff-2nd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Huff-3rd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Huff-4th 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Huff-Indy 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Custom 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00