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.
|ARI (years)||Frequency factor FY|
|Intensity x frequency|
factor FY x It
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 5 – Section 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.
Source: Alderson (2006).
|Qgap = Q100 – QN|
(need to multiply by CA10/0.36)
|Estimate ARI for Qgap|
(from Figure 2.5)
|ARI rounded up|
|0% Blockage||50% Blockage||0% Blockage||50% Blockage||0% Blockage||50% Blockage|
Example of the determination of the ARI for a Qgap:
|from Table 2.3 Q100||=||185|
QN in this example:
|from Table 2.3 Q2||=||64|
Assuming 0% blockage:
|Qgap||=||Q100 – Q2|
|=||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.