To derive a 6-hour unit hydrograph (6-hr UH) from a 3-hour unit hydrograph (3-hr UH), the S-hydrograph method involves:
Choose the correct answer
Doubling all ordinates of the 3-hr UH
Lagging multiple 3-hr UHs by 3 hours, summing them to create an S-curve, then differencing two S-curves lagged by 6 hours, and scaling by (3/6) to get unit response
Averaging ordinates at 6-hour intervals
Multiplying by the ratio of durations
Correct Answer
B. Lagging multiple 3-hr UHs by 3 hours, summing them to create an S-curve, then differencing two S-curves lagged by 6 hours, and scaling by (3/6) to get unit response
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S-hydrograph: result of applying an infinite series of unit storms each of t-hour duration = steady-state hydrograph. To change duration from t1 to t2 hours: (1) create S-curve from t1-hr UH; (2) lag by t2 hours; (3) subtract the two S-curves; (4) multiply by t1/t2 = unit hydrograph for t2-hour duration.
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