---
title: "TR-55 Methods"
canonical: "https://help.innovyze.com/space/infodrainage2021v1/16547880/TR-55%20Methods"
format: markdown
---
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***<sub>***t***</sub>*** = T***<sub>***sheet flow***</sub>*** + T***<sub>***sheet concentrated flow***</sub>*** + T***<sub>***channel flow***</sub>


## 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<sub>t</sub>:


![image](media://8220edca-3571-49f7-82c9-8c59153d8cc6)

where*:*

T<sub>t</sub> = travel time (hrs)

n = Manning's roughness coefficient

L = Flow length (ft)

P<sub>2</sub> = 2-year, 24-hour rainfall (in)

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

<span style="color: #707070"> </span><span style="color: #707070">t</span><sub>c </sub><span style="color: #707070">= time of concentration (hrs)</span>


> ℹ️ 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 (T<sub>t</sub>) is the ratio of flow length to flow velocity:


![image](media://5b325379-20a6-42eb-b068-27cc302d7fa7)

where*:*

T<sub>t</sub> = 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:


![image](media://1a23354a-4ab3-4501-8df2-55c5dc238c6e)

where*:*

V = average velocity (ft/s)

r = hydraulic radius (ft) and is equal to a/p<sub>w</sub>

a = cross sectional flow area (ft<sup>2</sup>)

p<sub>w </sub>= wetted perimeter (ft)

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

n = Manning's roughness coefficient for open channel


> ℹ️ <span style="color: #5f6062">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, T</span><sub>t</sub><span style="color: #5f6062"> </span><span style="color: #5f6062">for the channel segment can be estimated using the equation 2.</span>