Shallow

Early versions of TR-55 used the following velocity equation for slopes greater than 0.005 ft/ft:

Where

• Tt is travel time (hr)
• L is flow length (ft)
• V is average velocity (fps)
• 3600 is a conversion factor from seconds to hours

For slopes less than or equal to 0.005, the following equations for shallow flow were used:

Unpaved:

Paved:

Where

• V=average velocity (fps)
• s=slope of hydraulic grade line (ft/ft)

The two equations are based on Manning’s equation with different assumptions for n and hydraulic radius. For unpaved areas, n is 0.05 and r is assumed to be 0.4; for paved areas n is 0.025 and r is 0.2.

The current version of TR-55 now does away with the velocity equation and removed the 0.005 limit on paved and unpaved shallow flow. In fact, shallow flow is now separated into paved and unpaved for any slope. StormShed3G™ now computes shallow flow velocity based on the paved and unpaved equations. It bases the determination by the Manning’s value. Values greater than 0.013 are considered unpaved.

The remainder of this discussion on Shallow flow is considered obsolete and remains only as a reference source.

StormShed3G™ uses the following relationship to compute an average velocity.

Where

• V is velocity in (fps)
• k is time of concentration velocity factor (fps)
• s is the slope of flow path

“k” has been computed for various land covers and channel characteristics with an assumed hydraulic radius using the relationship:

Where

• R is hydraulic radius
• n is Manning’s roughness coefficient for open channel flow

StormShed3G™ provided the following lookup values based on the above equation with an assumed hydraulic radius of 0.1.

Ks values for Shallow Concentrated Flow (R=0.1)
Description	Ks Value
Forest with heavy ground litter and meadows (n=0.10)	3
Brushy ground with some trees (n=0.060)	5
Fallow or minimum tillage cultivation (n=0.040)	8
High grass (n=0.035)	9
Short grass, pastures and lawns (n=0.030)	11
Nearly bare ground (n=0.025)	13
Paved and gravel areas (n=0.012)	27