presented. For further reading, the reader may refer to Chow et al. (1988), Mays and
Tung (1992), and Linsley et al. (1992).
Rational Method
The rational method is the most prominently used method for estimating storm
runoff in the basin. Although this method has some merits and demerits, it is still
widely used because of its simplicity. The following assumptions are made in a
rational formula. Maximum flood flow is produced by a certain rainfall intensity that
lasts for a time equal to or greater than the period of concentration time. When a
storm continues beyond concentration time, every part of the catchment would be
contributing to the runoff at the outlet and therefore it represents the condition of
peak runoff. The concentration time is the time required for the surface runoff from
the remotest part of the catchment area to reach the basin outlet. The runoff rate to
this condition is given by Eq. (3.1):
Q p ¼ CIA
ð3:1Þ
where A is the area of the catchment in km
2 , and I is the intensity of rainfall in cm/h.
The maximum rainfall intensity depends on duration and frequency. The intensity
of rainfall should therefore be corresponding to a duration equal to concentration
time and desired return period. The rational method is found to give good results for
small areas up to about 50 km
2 . It is used to estimate the peak flood in the design of
urban drainage systems, storm sewers, airport drainage, and in the design of small
culverts and bridges.
Structural and Nonstructural Measures
Structural measures are involved in the construction of hydraulic structures such as
levees, dykes, flood divide walls, and detention and retention ponds to reduce the
peak discharge in the basin. The nonstructural measures used to reduce flood risks
include adopting master drainage plans, identifying flood-prone areas, early flood
forecasting and warning, and proper land use plans in urban regions. For accurate
flood forecasting, flood modeling studies are required to estimate the flood spread
and depth by using integrated flood modeling tools. In such cases, remote sensing
data combined with GIS have been found to provide valuable tools for flood risk
modeling and defining possible areas of flood extent in the basin. The relationship
between flood conditions and the spatial distribution of urban development with an
optimization-based approach is used for investigations by flood modeling studies.
Catchment treatments such as terracing and afforesting to reduce water velocities
and rainwater harvesting to reduce peak runoffs are also commonly proposed, but so
far have not attracted the importance they deserve in India.
42
S. Natarajan and N. Radhakrishnan
Tung (1992), and Linsley et al. (1992).
Rational Method
The rational method is the most prominently used method for estimating storm
runoff in the basin. Although this method has some merits and demerits, it is still
widely used because of its simplicity. The following assumptions are made in a
rational formula. Maximum flood flow is produced by a certain rainfall intensity that
lasts for a time equal to or greater than the period of concentration time. When a
storm continues beyond concentration time, every part of the catchment would be
contributing to the runoff at the outlet and therefore it represents the condition of
peak runoff. The concentration time is the time required for the surface runoff from
the remotest part of the catchment area to reach the basin outlet. The runoff rate to
this condition is given by Eq. (3.1):
Q p ¼ CIA
ð3:1Þ
where A is the area of the catchment in km
2 , and I is the intensity of rainfall in cm/h.
The maximum rainfall intensity depends on duration and frequency. The intensity
of rainfall should therefore be corresponding to a duration equal to concentration
time and desired return period. The rational method is found to give good results for
small areas up to about 50 km
2 . It is used to estimate the peak flood in the design of
urban drainage systems, storm sewers, airport drainage, and in the design of small
culverts and bridges.
Structural and Nonstructural Measures
Structural measures are involved in the construction of hydraulic structures such as
levees, dykes, flood divide walls, and detention and retention ponds to reduce the
peak discharge in the basin. The nonstructural measures used to reduce flood risks
include adopting master drainage plans, identifying flood-prone areas, early flood
forecasting and warning, and proper land use plans in urban regions. For accurate
flood forecasting, flood modeling studies are required to estimate the flood spread
and depth by using integrated flood modeling tools. In such cases, remote sensing
data combined with GIS have been found to provide valuable tools for flood risk
modeling and defining possible areas of flood extent in the basin. The relationship
between flood conditions and the spatial distribution of urban development with an
optimization-based approach is used for investigations by flood modeling studies.
Catchment treatments such as terracing and afforesting to reduce water velocities
and rainwater harvesting to reduce peak runoffs are also commonly proposed, but so
far have not attracted the importance they deserve in India.
42
S. Natarajan and N. Radhakrishnan