### 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 *Q _{gap}* for the design of the major system elements.

ARI (years) | Frequency factor F_{Y}(Table 2.1) | Intensity I_{t}(mm/h) | Intensity x frequencyfactor F x _{Y}I_{t} |
---|---|---|---|

1 | 0.80 | 65 | 52 |

2 | 0.85 | 75 | 64 |

5 | 0.95 | 94 | 89 |

10 | 1.00 | 105 | 105 |

20 | 1.05 | 120 | 126 |

50 | 1.15 | 139 | 160 |

100 | 1.20 | 154 | 185 |

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 *Q _{gap}* 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 [

*CA*/0.36]) for an assumption of, zero blockage and with 50% blockage. These values can then be used to estimate the design ARI for

_{10}*Q*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.

_{gap}*Source: Alderson (2006).*

N(years) | Q = _{gap}Q – _{100}Q_{N}(need to multiply by CA_{1}_{0}/0.36) | Estimate ARI for Q_{gap}(from Figure 2.5) | ARI rounded up | |||
---|---|---|---|---|---|---|

0% Blockage | 50% Blockage | 0% Blockage | 50% Blockage | 0% Blockage | 50% Blockage | |

1 | 133 | 159 | 21 | 50 | 25 | 50 |

2 | 121 | 153 | 14 | 41 | 15 | 45 |

5 | 96 | 140 | 6 | 26 | 10 | 30 |

10 | 80 | 132 | 3 | 20 | 5 | 20 |

20 | 59 | 122 | 2 | 14 | 2 | 15 |

50 | 25 | 105 | 0.5 | 8 | 1 | 10 |

Example of the determination of the ARI for a *Q _{gap}*:

from Table 2.3 Q_{100} | = | 185 |

*Q _{N}* in this example:

N | = | 2 years |

from Table 2.3 Q_{2} | = | 64 |

Assuming 0% blockage:

Q_{gap} | = | Q – _{100}Q_{2} |

= | 185 – 64 = 121 |

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

If 50% blockage assumed:

Q_{gap} | = | 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 *Q _{gap}* based on a minor drainage system designed to cope with storms with varying ARI and blockage levels.