20170949 Ver 4_Memo-Calcs_20170808

D&A WOLVERINE PLLC
6120 BROOKSHIRE BLVD., Unit R
CHARLOTTE NC, 28216
TEL: [phone]
WWW.DNAWOLVERINE.COM

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5ğƷĻʹ June 26, 2017
ƚʹ Mr. Richard McMillan, PE
Town of Midland, NC
CƩƚƒʹ Christopher Fleck, PE/Sean Coldren, PE
D&A Wolverine, PLLC
{ǒĬƆĻĭƷʹ Brentwood (Saddlebrook) Bypass Design (D&A Project #172012)
___________________________________________________________________

True Homes tasked D&A Wolverine PLLC (D&A) with providing an engineering resolution for
USACE/NCDEQ permitting violations associated with the construction of the Saddlebrook (aka Brentwood)
subdivision in the Town of Midland, NC (Cabarrus County). D&A performed a GIS analysis of the offsite
drainage area draining to the project site from the north. GIS data was obtained from Cabarrus County
for use in this analysis. D&A used the results of this analysis as the basis for sizing and designing the
proposed bypass storm system.

The upstream drainage area consists of two sub areas which drain to two distinct points along the
northern property line of the project site. Drainage Area 1 is the more western of the two drainage areas
and flows to the existing wetlands (Analysis Point 1). Drainage Area 2 is the more eastern of the two
drainage areas and flows to an existing low point along the property line (Analysis Point 2). Two hydrologic
methods were used in calculating the runoff from these drainage areas for the 10-yr storm event, the
Rational Method and the SCS method. Approximate percentages of different land covers were determined
by using recent aerial photography of the drainage areas. The drainage area land cover was broken into
three categories: impervious, grass and woods. See Figure 1 for a map of the two drainage areas in relation
to the project site.

Drainage Area 1 is 19.3 acres and consists of 10% impervious, 13% grass and 77 % wooded surfaces. The
Rational Method calculations resulted in a 10-year peak flow of 36 cfs at Analysis Point 1. See Table 1 for
the Rational Method input parameters and calculations. The SCS method requires the calculation of times
of concentration (Tc) based on the hydrologic parameters of the drainage area. The Tc for Drainage Area
1 was calculated as 33 minutes. See Table 2 for the Drainage Area 1 SCS Tc calculations. The SCS Method
parameters were input into Hydraflow Hydrographs to determine the peak outflow for the 10-year storm
event. The Drainage Area 1 SCS peak flow for the 10-year storm is 19 cfs. See Table 4 for the Drainage
Area 1 SCS input parameters and peak flows.

Drainage Area 2 is 18.9 acres and consists of 11% impervious, 5% grass and 84% wooded surfaces. The
Rational Method calculations resulted in a 10-year peak flow of 34 cfs at Analysis Point 2. See Table 1 for
the Drainage Area 1 Rational Method input parameters and calculations. The Tc for Drainage Area 2 was
calculated as 31 minutes. See Table 3 for the Drainage Area 1 SCS Tc calculations. The SCS Method
parameters were input into Hydraflow Hydrographs to determine the peak outflow for the 10-year storm
1

June 26, 2017
Brentwood (Saddlebrook) Bypass Design
D&A Project #172012
event. The Drainage Area 1 SCS peak flow for the 10-year storm is 19 cfs. See Table 4 for the Drainage
Area 1 SCS input parameters and peak flows.

The total flows from both drainage areas were calculated as 70 cfs (Rational) and 38 cfs (SCS). The average
total peak flow is 54 cfs. D&A used a flow of 60 cfs for sizing and laying out the proposed 42 bypass storm
system on the plans.

2

Table 1: Rational Method Input Parameters and Calculations
Drainage AreaTotal AreaQ (cfs)
119.335.5
218.934.2
Total38.2
69.8
Area 1
SurfacePercentageArea (ac)CIntensity (in/hr)Q (cfs)
Impervious10%1.90.957.3813.5
Lawn13%2.50.37.385.6
Woods77%14.90.157.3816.5
35.5
Area 2
SurfacePercentageArea (ac)CIntensity (in/hr)Q (cfs)
Impervious11%2.10.957.3814.6
Lawn5%0.90.37.382.1
Woods84%15.90.157.3817.6
34.2

Table 2: Drainage Area 1 SCS Method Time of Concentration Calculations
TYPESHEETDATE:6/21/2017
Flow Length (ft)100PROJECT:Saddlebrook
Upstream Elev (ft)567PROJECT #:172012
Downstream Elev (ft)566DESIGNER:SMC
0.400Drainage Area 1
Mannings nCATCHMENT:
0.010
Slope (ft/ft)
3.12
P (in)
2
Travel Time, T (min)28.7
t
SHALLOW CONCENTRATED
TYPE
SECTION12345
Flow Length (ft)395
Upstream Elev (ft)567
Downstream Elev (ft)555
Surface TypeUnpavedN/AN/AN/AN/A
Slope (ft/ft)0.030N/AN/AN/AN/A
V (ft/s)2.81N/AN/AN/AN/A
2.35N/AN/AN/AN/A
Travel Time, T (min)
t
Travel Time, T (min)2.35
t
OPEN CHANNEL
TYPE
SECTION12345
Channel TypeCircularTrapezoidalN/AN/AN/A
Width, b (ft)2
Flow Depth, h (ft)1
Top Width, T (ft)8
Side Slope, z3
Pipe Dia., D (ft)24
2
452.395.00N/AN/AN/A
Area (ft)
Wetted Perimeter (ft)75.408.32N/AN/AN/A
Hydraulic Radius (ft)6.000.60N/AN/AN/A
Flow Length (ft)50537
Upstream Elev (ft)555553
Downstream Elev (ft)553541
Mannings n0.0240.040
Slope (ft/ft)0.0400.022N/AN/AN/A
Velocity, V (ft/s)41.003.96N/AN/AN/A
0.022.26N/AN/AN/A
Travel Time, T (min)
t
Travel Time, T (min)2.28
t
TOTAL Tc (min)33
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Table 3: Drainage Area 2 SCS Method Time of Concentration Calculations
TYPESHEETDATE:6/21/2017
Flow Length (ft)100PROJECT:Saddlebrook
Upstream Elev (ft)564PROJECT #:172012
Downstream Elev (ft)562DESIGNER:SMC
Mannings n0.400CATCHMENT:Drainage Area 2
Slope (ft/ft)0.020
3.12
P (in)
2
Travel Time, T (min)21.7
t
SHALLOW CONCENTRATED
TYPE
SECTION12345
Flow Length (ft)527165375
Upstream Elev (ft)562548544
Downstream Elev (ft)548544536
Surface TypeUnpavedPavedUnpavedN/AN/A
Slope (ft/ft)0.0270.0240.021N/AN/A
V (ft/s)2.633.172.36N/AN/A
3.350.872.66N/AN/A
Travel Time, T (min)
t
Travel Time, T (min)6.87
t
OPEN CHANNEL
TYPE
SECTION12345
Channel TypeCircularTrapezoidalN/AN/AN/A
Width, b (ft)2
Flow Depth, h (ft)1
Top Width, T (ft)8
Side Slope, z3
Pipe Dia., D (ft)24
2
452.395.00N/AN/AN/A
Area (ft)
Wetted Perimeter (ft)75.408.32N/AN/AN/A
Hydraulic Radius (ft)6.000.60N/AN/AN/A
Flow Length (ft)50469
Upstream Elev (ft)536534
Downstream Elev (ft)534526
Mannings n0.0130.040
Slope (ft/ft)0.0400.017N/AN/AN/A
Velocity, V (ft/s)75.693.46N/AN/AN/A
0.012.26N/AN/AN/A
Travel Time, T (min)
t
Travel Time, T (min)2.27
t
TOTAL Tc (min)31
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Table 4: SCS Method Time of Concentration Calculations
Drainage AreaTotal AreaQ (cfs)
1
19.319
2
18.919
Total
38.238
CURVE NUMBER CALCULATION
Drainage Area 1
AreaHSGCNCN*Area
Impervious1.93C98189.14
Grass2.51C79198.211
Woods14.86C731084.853
1472.204Sum
76CN Weighted
Drainage Area 2
AreaHSGCNCN*Area
Impervious2.08C98203.742
Grass0.95C7974.655
Woods15.88C731158.948
1437.345Sum
76CN Weighted