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)

P_{2} = 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 (ft^{2})

p_{w }= wetted perimeter (ft)

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

n = Manning's roughness coefficient for open channel

_{t}for the channel segment can be estimated using the equation 2.