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).


Tt = Tsheet flow + Tsheet concentrated flow + Tchannel 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 Tt:


            

where:

Tt = 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= time of concentration (hrs)


This simplified form of the Manning's kinematic solution is based on the following: (1) shallow steady uniform flow, (2) constant intensity of rainfall excess (that part of a rain available for runoff), (3) rainfall duration of 24 hours, and (4) minor effect of infiltration on travel time.


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 (Tt) is the ratio of flow length to flow velocity:


            

where:

Tt = 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/pw

a = cross sectional flow area (ft2)

pw = wetted perimeter (ft)

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

n = Manning's roughness coefficient for open channel


Manning's n values for open channel flow can be obtained from standard textbooks such as Chow (1959) or Linsley et al. (1982). After average velocity is computed using equation 3, Tt for the channel segment can be estimated using the equation 2.