Calculation of Overland Sheet Flow Travel Time

Using the Manning Kinematic Equation - S.I. units

Inputs Calculations

Manning Roughness Overland Flow Time

0.15 32.9 min

25 m

120 mm

0.0004

Calculation of Shallow Concentrated Flow Travel Time

Using the NCRS Method - S.I. units

Inputs Calculations

30 m For unpaved surface:

0.0004 0.098356 m/s

5.1 min

For paved surface:

0.12392 m/s

4.0 min

Coefficient, n = Travel, t1 =

Length of Flow Path, L =

2 yr, 24 hr rainfall, P =

Ground Slope, S =

Length of Flow Path, L =

Ground Slope, S = Flow Velocity, V =

Travel time, t2 =

Flow Velocity, V =

Travel time, t2 =

Calculation of Channnel Flow Travel Time

Using the Manning Equation - S.I. units

For a Trapezoidal Channel Cross-section

Inputs Calculations

0.7 m 0.76

0.4 m 3.2 m

3 0.24 mm

0.23

0.022

0.300 m/s

0.0003 m/m

1.4 min

25 m

Calculation of Time of Concentration

Inputs ( values from above) Calculations

32.9 min 37.5 min

Bottom width, b = Cross-Sect. Area, A = m2

Depth of flow, y = Wetted Perimeter, P =

Side Slope, z = Hydraulic Radius, R =

( H:V = z:1 )

Discharge, Q = m3/s

Manning roughness, n =

Ave. Velocity, V =

Channel bottom slope, S =

Channel travel time, t3 =

Length of Flow Path, L =

( tc = t1 + t2 + t3 )

t1 = tc =

5.1 min

1.4 min

t2 =

t3 =

n = Manning roughness coefficient, dimensionless

L = length of the flow path, m (Max. L should be 100 m)

P = 2 year, 24 hr rainfall, mm

S = ground slope, m/m

Equations for NCRS Method for Shallow Concentrated Flow

where:

L = length of the flow path, m

V = shallow concentrated flow velocity, m/sec

S = surface slope, m/m

t1 = overland sheet flow runoff travel time, min

t2 = L/(60V)

for unpaved surface: V = 4.9178S0.5

for paved surface: V = 6.1960S0.5

t2 = shallow concentrated flow runoff travel time, min

V = Q/A

R = A/P

Where:

V = average velocity of flow, m/s

P = wetted perimeter of channel, m

S = channel bottom slope, m/m

n = Manning roughness coefficient for channel

L = Length of flow path, m

t3 = L/(60V)

Q = channel flow rate, m3/s

A = channel cross-sectional area, m2

t3 = travel time for channel flow, min