Question

Following assumptions are considered related to the Rational method:
(A). The rainfall intensity is uniform over the entire watershed during the duration of storm.
(B). The factors pertaining to losses due to depression storage and initial infiltration were considered during the start of runoff process.
(C). The factors related to retardance to flow caused by the effect of surface detention and channel storage is considered.
(D). The runoff coefficient is dependent only on watershed characteristics.
Choose the correct answer from the options given below:

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

Hint:

The Rational formula is given by \( Q = \frac{C \cdot I \cdot A}{360} \) (in metric units).
Since it assumes uniform rainfall intensity and a constant runoff coefficient, it is only suitable for small watersheds (typically $< 50 \text{ km}^2$).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The Rational method is a widely used empirical equation to estimate peak runoff from small watersheds.
It relies on a set of simplified assumptions to make the hydrologic calculations manageable.
Detailed Explanation:
Let us evaluate each of the given statements according to the standard assumptions of the Rational method:
- Statement (A): It is assumed that the rainfall intensity remains constant and uniform over the entire catchment area for the entire duration of the storm.
This is a standard assumption of the method, so (A) is correct.
- Statement (B): The Rational method does not dynamically model infiltration losses, depression storage, or evaporation during the start of the runoff process.
Instead, it lumps all these losses into a single, constant runoff coefficient ($C$). Thus, Statement (B) is incorrect.
- Statement (C): The method does not consider transient retardance to flow or temporary storage in channels and surface depressions.
The peak runoff is assumed to occur when the entire watershed is contributing, without considering channel storage routing. Thus, Statement (C) is incorrect.
- Statement (D): The runoff coefficient ($C$) is assumed to remain constant during the storm and depends solely on the physical characteristics of the watershed (such as soil type, vegetation cover, and slope). Thus, Statement (D) is correct.
Therefore, only statements (A) and (D) are correct assumptions.

Step 2: Final Answer:
Based on the assumptions of the Rational method, the correct option is (B).