Question

(A) Floods are exceedingly complex natural events. They are resultant of a number of component parameters and are therefore very difficult to model them analytically.
(B) In the regions having same climatological characteristics, if the available flood data are quite insufficient, the enveloping curve technique can be used to develop a relationship between the minimum flood flow and drainage area.
(C) For design purposes, extreme rainfall situations are used to obtain the design storm.
(D) The rational formula is found to be suitable for a peak flow prediction in small catchments upto 75 km\(^2\) in an area.
(E) The rational formula assumes a homogeneous catchment surface.

Choose the correct answer from the options given below:

• A. (A), (C) and (E) only.
• B. (A), (B) and (C) only.
• C. (B), (C) and (D) only.
• D. (C), (D) and (E) only.

Hint:

Check statement (B) for which extreme (maximum or minimum) envelope curves actually represent, and check the numeric limit quoted in (D) against the standard rational method range.

Answer 1

The Correct Option is A

Step 1: Go through each of the five statements about floods and flood estimation and judge them against standard hydrology practice.
Step 2: Statement (A) says floods are complex natural events resulting from many component parameters and are hard to model analytically. This is a standard, widely accepted description of flood behaviour, so (A) is correct.
Step 3: Statement (B) claims the enveloping curve technique, used in regions with similar climate when data is scarce, builds a relationship between the MINIMUM flood flow and drainage area. In reality, envelope curves are drawn through the highest recorded floods of various catchments to bound the MAXIMUM probable flood against drainage area, not the minimum. Because the statement swaps maximum for minimum, (B) is incorrect.
Step 4: Statement (C) says extreme rainfall situations are used to obtain the design storm for design purposes. This matches standard design flood practice, where the most severe realistic rainfall is used to derive the design storm, so (C) is correct.
Step 5: Statement (D) claims the rational formula suits peak flow prediction in small catchments up to 75 km\(^2\). The accepted textbook limit for the rational method is much smaller, around 50 km\(^2\), because beyond that the assumption of a catchment responding uniformly and instantly no longer holds well. Since 75 km\(^2\) does not match the accepted limit, (D) is incorrect.
Step 6: Statement (E) says the rational formula assumes a homogeneous catchment surface, meaning uniform land use and a constant runoff coefficient across the area. This is indeed one of the core assumptions behind the method, so (E) is correct.
Step 7: The true statements are (A), (C) and (E), which is option 1.