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Stormwater Runoff Designs and Considerations Relating to on-Site Wastewater Systems

Bill Hunt

Extension Specialist, Urban Stormwater Engineering,

NCSU Dept. of Biological and Agricultural Engineering


Routing of stormwater runoff away from septic and well head systems aids in protecting water quality. The effects of urbanization on water quality are many fold. One primary impact is that water that once was able to infiltrate into the ground water by falling on forested or other rural land now becomes a part of stormwater runoff. Vegetated areas are replaced with impervious areas such as rooftops, sidewalks, and driveways that serve to increase the total amount of stormwater runoff and the severity of its effects. High flows carry debris and erode the land. it is necessary to divert runoff away from sensitive areas such as on-site wastewater systems. Designers must not only consider runoff emanating from their own sites but also stormwater which enters from adjacent properties.

Traditional means have been used to divert stormwater runoff from sensitive areas such as septic fields. These devices include tying gutter systems into vegetated and rock-lined swales and concrete, plastic or metal culverts. Example designs of each of these practices are important to designers of septic systems and will be reviewed. The equations used in simple design are given in a following section.

there are also non-traditional means of reducing, storing, and conveying runoff. These practices include the use of alternative pavements, rain gardens, roof rainfall catchments, and vegetation-reinforced swales. Examples of each will be reviewed.

Important Runoff Calculations:

Rational Method:
Calculation of Peak Flow from site

Q = C * I * A

Where Q = Flow (cfs), C = Watershed Cover Coefficient, I = Rainfall Intensity (in/hr), A = Watershed Area (acres)

Sample Watershed Cover Coefficients (Rational Runoff Coefficients)1

Roof 1.00-0.90 (depending upon inclined or flat)
Driveway-paved 0.95
Apartment/School 0.60
Residence – 2 du/acre 0.40
Lawn – clay 0.35-0.17 (depending upon slope)
Lawn – sand 0.15-0.10 (depending upon slope)
Wooded 0.20-0.10 (depending upon ground liter)

Rainfall Intensities for Raleigh, North Carolina1
(assuming Time of Concentration <5 min)


2 Year Storm 5.74 in/hr
5 Year Storm 6.50 in/hr
10 Year Storm 7.22 in/hr
25 Year Storm 8.29 in/hr
50 Year Storm 9.00 in/hr
100 Year Storm 9.67 in/hr

Manning’s Equation:
Traditional Equation used to calculate flow through a channel or pipe

Q = 1.486/n * A * R.667 * S.5

Where Q = Flow (cfs), n = Manning’s coefficient, A = cross-sectional area (ft), R = Hydraulic Radius (Area/Wetted Perimeter)(ft), S = Slope (NOT in %)

Common Manning’s Coefficients:

HDPE (smooth plastic) =0.0102
Reinforced Concrete Pipe =0.0131
Corrugated Metal Pipe =0.0241
Grass Lined Swale =0.0301
9″ Stone Lined Channel =0.0321
15″ Stone Lined Channel =0.0351
Wetland-like Channel =0.153

Continuity Equation:
Serves as a Velocity Check for channel lining.

Q = V * A Where Q = Flow, V = Velocity, A = Cross- section Area

Allowable Velocities for Channel Lining1,2

Bare Soil – Sand 2.5 fps
Bare Soil – Clay 4.0 fps
Grass Lining 4.0 – 5.0 fps (depending upon maintenance)
9″ Stone 7-11 fps (depending upon side slope)
15″ Stone 9-14 fps (depending upon side slope)
Reinforced Grass 9-15 fps (depending upon Turf Reinforcement Mat)
Human Safety 8-10 fps (do you like the person or not?)

1. Elements of Urban Stormwater Design, H.R. Malcom
2. W.F. Hunt, Conversations with practicing engineers
3. NRCS presentation