Module 3
Lecture 2: Watershed and rainfall-runoff
relationship (contd.)
Rainfall-Runoff
 How does runoff occur?
 When rainfall exceeds the infiltration rate at the surface, excess water
begins to accumulate as surface storage in small depressions. As
depression storage begins to fill, overland flow or sheet flow may begin to
occur and this flow is called as “Surface runoff”
 Runoff mainly depends on: Amount of rainfall, soil type, evaporation
capacity and land use
 Amount of rainfall: The runoff is in direct proportion with the rainfall. i.e.
as the rainfall increases, the chance of increase in runoff will also
increases
Module 3
Rainfall-Runoff
Contd….
 Soil type: Infiltration rate depends mainly on the soil type. If the soil is having
more void space (porosity), than the infiltration rate will be more causing less
surface runoff (eg. Laterite soil)
 Evaporation capacity: If the evaporation capacity is more, surface runoff will
be reduced
 Components of Runoff
 Overland Flow or Surface Runoff: The water that travels over the ground
surface to a channel. The amount of surface runoff flow may be small since it
may only occur over a permeable soil surface when the rainfall rate exceeds
the local infiltration capacity.
Module 3
Rainfall-Runoff
Contd….
 Interflow or Subsurface Storm Flow: The precipitation that infiltrates the soil
surface and move laterally through the upper soil layers until it enters a stream
channel.
 Groundwater Flow or Base Flow: The portion of precipitation that percolates
downward until it reaches the water table. This water accretion may eventually
discharge into the streams if the water table intersects the stream channels of
the basin. However, its contribution to stream flow cannot fluctuate rapidly
because of its very low flow velocity
 Data collection
 The local flood control agencies are responsible for extensive hydrologic
gaging networks within India, and data gathered on an hourly or daily basis
can be plotted for a given watershed to relate rainfall to direct runoff for a
given year.
Module 3
Runoff Measurement
Contd….
Rational Method
•
Travel time for open channel flow (Tt)
Tt = L/V
where L = length of open channel (ft, m)
V = cross-sectional average velocity of flow (ft/s, m/s)
Manning's equation can be used to calculate cross-sectional average velocity
of flow in open channels
V = kn / n R2/3 S1/2
where V = cross-sectional average velocity (ft/s, m/s)
kn = 1.486 for English units and kn = 1.0 for SI units
A = cross sectional area of flow (ft2, m2)
n = Manning coefficient of roughness
R = hydraulic radius (ft, m)
S = slope of pipe (ft/ft, m/m)
Module 3
Rational Method
Hydraulic radius (R) can be expressed as
R = A/P
where A = cross sectional area of flow (ft2,m2)
P = wetted perimeter (ft, m)
After getting the value of Tt, the time of concentration can be obtained by
Tc = ∑Tt
Runoff Measurement
Contd….
Rational Method
Values of Runoff coefficients, C (Chow, 1962)
Module 3
Runoff Measurement
Contd….
Rational Method
Calculation of Tc
•
Tc = ∑Tt
where Tt is the travel time i.e. the time it takes for water to travel from
one location to another in a watershed
•
Travel time for sheet flow
where,
n = Manning’s roughness coefficient
L = Flow length (meters)
P2 = 2-yr, 24-hr rainfall (in.) and S is the hydraulic grade line or
land surface
Module 3
Runoff Measurement
Contd….
Rational Method
•
Travel time for open channel flow
•
Where V is the velocity of flow (in./hr)
•
Hence Tt = L/V
•
After getting the value of Tt, the time of concentration can be
obtained by
Tc = ∑Tt
Module 3
Runoff Measurement
Contd….
Rational Method
•
Assumptions of rational method
 Steady flow and uniform rainfall rate will produce maximum runoff when all
parts of a watershed are contributing to outflow
 Runoff is assumed to reach a maximum when the rainfall intensity lasts as
long as tc
 Runoff coefficient is assumed constant during a storm event
•
Drawbacks of rational method
 The rational method is often used in small urban areas to design drainage
systems and open channels
 For larger watersheds, this process is not suitable since this method is
usually limited to basins less than a few hundred acres in size
Module 3