Question

Question 5

A 15-minute-rainfall of 20-year Average Recurrence Interval (ARI) was fallen over a 5ha catchment in Adelaide. Table 1 shows catchment sub-areas covered within 5-minute isochrones. A 3mm of initial loss (IL) and 20mm/hr of continuing loss (CL) is a good representation of the loss model for this catchment. Using this given information, compute;

1. Peak flow rate and
2. Runoff volume at the catchment outlet.

Intensity of rainfall = 17.6

Table 1: Area covered within 5min-isochrones

Isochrones (5 min)	0 -5	5 -10	10-15	15-20
Area covered (ha)	1	1.5	1.5	1

Answer 1

Ans a) We know,

Peak flow rate (Qp) = C i A / 360

where, C = Runoff coefficient

i = Rainfall intensity

A = Area in hecatres = 5 ha

For Adelaide, runoff coefficient = 0.70

According to IDF curve for Adelaide, for 15 min 20 year rainfall, rainfall intensity, i = 72.52 mm/hr

=> Qp = 0.70 x 72.52 x 5 / 360

=> Qp = 0.705 /s

Ans b) We know,

Runoff depth (R) = IL + CL(t) + QF

where, t = duration = 15 min

QF = Quick flow = 17.6 mm (given)

=> R = 3 mm + 20 mm/hr(5min/60 min/hr) + 0

=> R = 25.6 mm or 0.0256 m

Total area = 5 ha or 50000

=> Runoff volume = Runoff depth x area

=> Runoff volume = 0.0256 m x 50000

=> Runoff volume = 1280