The resulting time of concentration is determined as the sum of the travel times from each of the calculations (Sheet Flow, Shallow Flow, Channel Flow).

T_t = T_{sheet flow} + T_{sheet concentrated flow} + T_{channel flow}

Sheet Flow

Parameters

Manning's roughness coefficient - Manning’s n Roughness value.

Flow Length - The length of the Sheet flow path.

Two-year 24-hour rainfall - The depth of rainfall for the 2 year return period 24 hour event.

Land Slope - The slope of the hydraulic grade line.

Calculation

For sheet flow of less than 300 feet, use Manning's kinematic solution (Overtop and Meadows 1976) to compute T_t:

where:

T_t = travel time (hrs)

n = Manning's roughness coefficient

L = Flow length (ft)

P₂ = 2-year, 24-hour rainfall (in)

s = slope of hydraulic grade line (land slope, ft/ft)

 t_c = time of concentration (hrs)

Shallow Concentrated Flow

Parameters

Flow Length - The length of the Shallow flow path.

Watercourse Slope - The slope of the hydraulic grade line.

Average Velocity - The average velocity.

Calculation

Travel time (T_t) is the ratio of flow length to flow velocity:

where:

T_t = travel time (hrs)

L = Flow length (ft)

V = average velocity (ft/s)

Channel Flow

Parameters

Cross sectional flow area - The Cross Sectional Flow Area.

Wetted Perimeter - The Wetted Perimeter for Flow Area. 

Manning's roughness coefficient - Manning’s n Roughness value. 

Flow Length - The length of the Channel flow path.

Calculation

Manning's equation is:

where:

V = average velocity (ft/s)

r = hydraulic radius (ft) and is equal to a/p_w

a = cross sectional flow area (ft²)

p_w = wetted perimeter (ft)

s = slope of hydraulic grade line (land slope, ft/ft)

n = Manning's roughness coefficient for open channel