Question

2. In the Rational Method the peak runoff resulting from a rainfall event depends on which of the following factors

1. The frequency and duration of the rainfall event.
2. The size of the area on which the rainfall lands.
3. The characteristics of the surface over which the rainwater flows.
4. The time that has elapsed since the last rainfall event.

1. I, II, III and IV.
2. I, II and III only.
3. I and III only.
4. II and III only.

3. Which of the following are assumptions of the Rational Method?

1. Peak flow occurs when the entire watershed is contributing to the flow.
2. Rainfall intensity is the same over the entire drainage area.
3. Rainfall intensity is uniform over a time duration equal to the time of concentration.
4. Frequency of the computed peak flow is the same as that of the rainfall intensity.

1. I, II, III and IV.
2. I, II, and III only.
3. I and III only.
4. II and IV only.

Answer 1

1) d) II and III only

Factors affecting runoff

•Climatic factors

1. –Type of precipitation
2. –Rainfall intensity
3. –Duration of rainfall
4. –Rainfall distribution
5. –Direction of prevailing wind

•Physiographic factors

1. Size of watershed
2. Shape of watershed
3. Orientation of watershed
4. Landuse
5. Soil moisture
6. Soil type
7. Topographic characteristics
8. Drainage density

2) ii) I,II and III only

The Rational Method is limited to drainage basins 160 acres (64 hectares) or smaller. This is a result of the assumptions associated with the Rational Method,which include

•Recurrence interval (TR) used for estimating peak flow is the same as that for determining rainfall intensity(i.e., a 50 year storm is assumed to produce a 50 year peak flow). Peak flow is assumed to occur when the entire watershed is contributing to flow.

•Rainfall intensity is the same over the entire drainage area and is uniform over a time duration equal to the time of concentration (Tc).