Statement I): Collin’s method for the determination of the unit hydrograph for a mutli-period storm considers residuals as attributable to the unit graph of the period of the largest rain. Statement II): Collin’s method is ab initio premised on the Bernard method where a unit graph is necessarily to be proportional to the net rain.

Statement I): Collin’s method for the determination of the unit hydrograph for a mutli-period storm considers residuals as attributable to the unit graph of the period of the largest rain. Statement II): Collin’s method is ab initio premised on the Bernard method where a unit graph is necessarily to be proportional to the net rain. Correct Answer Both Statement I) and Statement II) are individually true and Statement II) is the correct explanation of Statement I)

Collin’s method for the determination of the unit hydrograph considers two unit graphs, namely, the assumed unit hydrograph and the residual unit hydrograph . It is a successive approximation method.   Unit graph is necessarily to be proportional to the net rain.

The basic steps in the Collin's Method of Unit Hydrograph Determination  are:

1. Assume a unit hydrograph by considering  all rains  combined into one average rain and apply it to all the effective rainfall   blocks of the ERH excepting  largest rainfall excess.

2. after that find out the resulting hydrograph by subtracting  its ordinates from the corresponding ordinates of the actual direct runoff Hydrograph.

3. Divide the ordinates of the residual hydrograph by the largest block of effective rainfall to get the unit hydrograph.

4. Compute the weighted average of the two unit graphs, namely, the assumed unit hydrograph and the residual unit hydrograph and use it as the revised approximation for the next trial.