Question

3. O-index method. You are given a hydrograph. The 15 hour the rain intensity was 4mm/hr, the 2nd hour it was 6mm/hr, the 3rd hour it was 10mm/hr, the 4th hour it was 8mm/hr, the 5th hour it was 7mm/hr, the 6th hour it was 3mm/hr, and the 7th hours it was 1mm/hr. If the total runoff was 11mm find the O-index for this rain.

Answer 1

Solution => t Ø- Index Ø-index is the average rainfall above which stainfall volume is equal to runoff-volume, Determination of -inder :- Stebli- initial = Pt-R where R = Runoff Pt= Total Rainfall tt = total time Step 2: compare the o initial with rainfall intensity of each interval Step 3: If all intervals do not contribute to the runoff then & modified Pm Modified Rainfall ton = hodified time Step 4: Repeat step-2 onwards until the condition satisfied. Pan-R tm

6+ Lo + 8 +7=31 wn According to question :- Interval thel Rainfall (mm Rainfall intensity (mm certain 0-1 ar 4 mm 4 1-2 6 mm 2-3 6 10 min 10 3-4 8 mm ♡ 4.5 7 mm 5-6 3 mm 3 6-7 1 mm 1 Total Rainfall 4+6+10+8+7+3+1 = 39 mm Given total runoff (R= 11 mm total time = 7 howes Pinitial 39 - 11 7 - Z 4 mm hr As rainfall of 1st hour, 6th hour and 7th news do not contribute to the runoff. Runoff (R = 11 mm Modified time (tzn = 7-3=4 hours Omodified) = Pm-R 31-11 tm 4 &- Index for this rain: 5 myne 5 mm hr Answoi
For the modified fi index only 2nd ,3rd ,4th and 5th hour rain is considered because only they are contribute to the runoff and modified value satisfied the 2nd, 3rd, 4th and 5th hour rainfall which contributes to runoff. Thanks