Table of Contents

2.7.1 Example 1: Estimation of Gap Flow

For a location in the northern (tropical) part of Australia the rainfall intensities in Table 2.3 apply for a storm with duration of 45 minutes, previously determined as the critical storm duration. It is desired to determine what ARI should be used to calculate Qgap for the design of the major system elements.

Table 2.3: Rainfall Intensity (mm/h) for a 45 minute duration storm in tropical Australia

ARI (years)Frequency factor FY
(Table 2.1)
Intensity It
(mm/h)
Intensity x frequency
factor FY x It
10.806552
20.857564
50.959489
101.00105105
201.05120126
501.15139160
1001.20154185

For the particular location and situation it has been decided that the major event occurs with an ARI of 100 years (taken from AGRD Part 5Section 4.6). The data in Table 2.3 is plotted in Figure 2.5. The Qgap can be determined for each of the varying ARIs applicable to the minor drainage system and these are shown in Table 2.4 (divided by [CA10/0.36]) for an assumption of, zero blockage and with 50% blockage. These values can then be used to estimate the design ARI for Qgap using Figure 2.5. The final two columns of Table 2.4 are simply rounding up of the design ARI which provides some conservatism in the procedure.

Figure 2.5: Intensity x frequency factor for various ARI plotted on a logarithmic scale

Source: Alderson (2006).

Table 2.4: Estimation of Qgap for ARI of 100 years

N
(years)
Qgap = Q100QN
(need to multiply by CA10/0.36)
Estimate ARI for Qgap
(from
Figure 2.5)
ARI rounded up
0% Blockage50% Blockage0% Blockage50% Blockage0% Blockage50% Blockage
113315921502550
212115314411545
5961406261030
1080132320520
2059122214215
50251050.58110

Example of the determination of the ARI for a Qgap:

from Table 2.3 Q100=185

QN in this example:

N=2 years
from Table 2.3 Q2=64

Assuming 0% blockage:

Qgap=Q100Q2
 =185 – 64 = 121

Reading from Figure 2.5, Intensity x Frequency aligns with an ARI of 15 years.

If 50% blockage assumed:

Qgap=185 – 50% x 64 = 153

Reading from Figure 2.5, Intensity x Frequency aligns with an ARI of 35 years.

Using the data in Table 2.4 it is possible to design the major drainage system to cater for Qgap based on a minor drainage system designed to cope with storms with varying ARI and blockage levels.