Enveloping curve is a method: 

Explanation:

Enveloping Curve:

• Envelope curves, obtained by plotting the highest observed floods of various rivers versus their drainage areas, are useful for designing dams and flood control structures on the rivers in those areas where flood records or hydro-meteorological data are not sufficient. 
• This method is commonly used in flood hydrology to estimate the probable maximum flood or other extreme flood events for the purpose of designing hydraulic structures such as dams and levees.

Hence option (3) is the correct answer.