SW5221001_Design Calculations_20230609

UmsteadState Park
Campground Improvements
Impervious Surface Area Calculations
January 24, 2023
IMPERVIOUS SURFACES
Existing:
Asphalt: 117,100sf
Gravel: 61,800sf
Concrete: 700sf
Buildings: 1100sf
Total: 180,700sf
Existing to Remain:
Asphalt: 117,100sf
Proposed:
Asphalt: 8600sf
Gravel: 48,600sf
Concrete: 11,300sf
Buildings: 3000sf
Total: 71,500sf
TOTAL POST CONSTRUCTION IMPERVIOUS:
117,100+ 71,500= 188,600sf
IMPERVIOUS AREA PERCENTAGE CALCULATION
DrainageArea: 6,473,100sf
Total Impervious: 188,600sf
% Impervious = 188,600/ 6,473,100x 100% = 3%
309 N.BoylanAvenue
Raleigh, NC27603
919833-1212
nch-associates.com

Summary

Calculations for the capacity and velocity of the projects two major drainage swales are provided
below. The required capacity for swales #1 and #2 is 2.1 cfs and 2.7 cfs, respectively.

Based on these required capacities, channel parameters were determined through iterations. The
results the calculations are summarized in the Swale Capacity/Velocity Summary Chart. Multiple
results are provided for each swale to represent multiple conditions in which the swales must function.

Velocity and capacity values are provided for bare soil and grass channel linings for both swales.
Additionally, because the slope of swale #2 varies, multiple values are provided to represent the
values at the minimum and maximum slope.

As shown in the Capacity/Velocity chart, the maximum depth required to carry the flow volume of
the design storm is 0.73 for swale #1, and 0.75 for swale #2. Also, the calculated velocity for swale #2
with a bare soil lining exceeds the maximum allowable value. Therefore, swale#2 shall be reinforced
with a geotextile channel lining while the grass lining becomes established.

The maximum calculated velocity of neither channel is sufficient to require a stone velocity dissipator.

Capacity

Diversion Swale 1 Diversion Swale 2

Contributing Drainage Area: 0.74 ac Contributing Drainage Area: 0.94 ac
Impervious area: 0.27 acres Impervious area: 0.32 acres
Runoff coefficient (C) Runoff coefficient (C)
Impervious area: 0.95 Impervious area: 0.95
Lawn/Woodland: 0.15 Lawn/Woodland: 0.15
Composite C = 0.4 Composite C = 0.4
Time of Concentration: 5 min Time of Concentration: 5 min
Intensity (I): 7.06 in/hr Intensity (I): 7.06 in/hr
1010

Runoff Volume: Runoff Volume:
C = 0.4 C = 0.4
I = 7.06 I = 7.06
1010
A= .74 A = 0.94

Q = CIA = 2.1 cfs; Q = CIA = 2.7 cfs

Swale Capacity/Velocity Summary Chart
(values determined by iteration)

* 4.O fps allowable velocity for grass lined swales
* 3.0 fps allowable velocity for silt loam

Diversion Depth S R n A M V Q Q
required
1 (grass) 0.73 .007 .346 .045 1.60 3 1.4 2.2 2.1
1 (bare soil) 0.55 .007 .261 .055 0.91 3 2.5 2.3 2.1

2 (grass) 0.7 .03 .332 .05 1.47 3 2.5 3.6 2.7
2 (grass) 0.75 .015 .356 .045 1.69 3 2.0 3.4 2.7
2 (bare soil) 0.7 .03 .237 .06 0.75 3 4.9 3.7 2.7
2 (bare soil) 0.55 .015 .261 .06 0.91 3 3.7 3.4 2.7

S = slope
M = side slope
2
A = M x depth
21/2
R = A ÷ \[2 x depth x (1 + M)\]
2/31/2
V= velocity = (1.486 ÷ n)(R)(S)
Q = flow = VA
he NC Erosion and Sediment Control
Planning and Design Manual 2013.

Stormwater Outlet Calculations

Outlet #1
Contributing Drainage Area (A): 1.1 ac
Runoff coefficient (C)
Area (Ac) C
Impervious area: 0.6 0.9
Forest: 0.4 0.1
Open Areas: 0.1 0.2
Composite: 0.5
Time of Concentration: 5 min
Rainfall Intensity ((I): 7.06 in/hr
10
Runoff Volume (Q) = C x I x A = 0.5 x 7.06 x 1.1 = 3.4 cfs

Pipe parameters: 15 RCP @ 3.5%
Flow Depth: 5.4 in
Velocity: 8.5 ft/s

See Stone Dissipator Sizing Note below

Outlet #2
Contributing Drainage Area (A): 1.4 ac
Runoff coefficient (C)
Area (Ac) C
Impervious area: 0.7 0.9
Forest: 0.3 0.1
Open Areas: 0.4 0.2
Composite: 0.5
Time of Concentration: 5 min
Rainfall Intensity ((I): 7.06 in/hr
10
Runoff Volume (Q) = C x I x A = 0.5 x 7.06 x 1.4 = 4.9 cfs

Pipe parameters: 15 RCP @ 2.4%
Flow depth: 7.4 in
Velocity: 8.13 ft/s

Stone Dissipator Sizing
Stone Dissipators sized using Figure 8.06a from the May 2013 Edition of the NC Erosion and Sediment
Control Planning and Design Manual. Velocity and Flow calculated for the existing outlets is less than
the minimum amounts shown on Figure 8.06a for 15 diameter pipe. Therefore, the minimum dissipator
length of 8 and stone diameter of 6 as provided in the figure for a dissipator pad receiving
discharge from a 15 diameter pipe have been applied.