Test: Rational Method - Civil Engineering (CE) MCQ

According to the Rational method, the storm water or runoff is defined by Q = K*A*I*R where, ‘K’ is a constant, ‘A’ is the catchment area in hectares,’I’ is the impermeability factor and ‘R’ is the intensity of rainfall in mm per hour.

Impermeability factor is defined as the percentage of rain water in the form of runoff. The storm water flow depends on the imperviousness of the surface and if it is more, more runoff takes place.

The water tight roof surface has maximum Kuichling’s impermeability factor with a magnitude of 0.7 to 0.95 while gravel road, unpaved surface and macadamized road ways have < 0.7.

The value of Kuichling’s impermeability factor for a surface of the most densely populated portion of the city lies between 0.7 to 0.9.

The frequency of storm for which the sewer is to be designed for a peripheral residential area is twice a year.

The intensity of rainfall is inversely proportional to the duration period, so the shorter the duration of rainfall, greater will be the rainfall intensity.

According to Kuichling, when the storm occurs once in 10 years, the rainfall intensity R is given by R = 260/(t + 20) and when it occurs once in 15 years, it is given by R = 3048/(t + 20) where t is the duration of storm in minutes.

It is because if duration of storm is shorter than time of concentration, then whole catchment will not be the contributing water, resulting in lesser runoff whereas if duration of storm exceeds the time of concentration, then rainfall intensity will be less, resulting in lesser runoff.

According to the Rational method, the storm water or runoff is defined by
Q = K*A*I*R where, K is a constant and K = 1/360,
So, Q = A*I*R/360.

The impermeability factor is also called as runoff coefficient and it includes Kuichling’s impermeability factor and Fruhlings factor.

Extremely suburban area having 20% to 60/5 parking and widely detached houses has a minimum Fruhlings factor with a magnitude of 0.35 while business area, closely built up area and area with 50% attached and detached house have magnitude > 0.35.

If the type of locality is suburban area having detached houses, the value of Fruhlings factor lies in the range of 0.45 to 0.55.

The commercial and higher priced area more subject to less storm frequency than the residential area and its storm frequency is once in 2 years.

When the storm occurs once a year, the rainfall intensity R is given by R = 150/ t^0.625 Where, t is the duration of storm in minutes.

The time of concentration is defined as the longest time required for a drop of water to flow from the farthest point of drainage area to the point of concentration.