Table 15: Selected Critical Durations and Storms

ARR2016 recommends that the critical storm is selected based on the storm that produces the median peak flood level. As such, a critical storm was selected for each event and duration.

For each critical storm a range of durations was then tested. The critical storm duration that produced the peak flood level was then selected as the critical duration. Table 15 summarises the critical storms and durations selected for each design event.

Table 15: Selected Critical Durations and Storms

Event (AEP) Critical Durations (min) Critical Storm 1 EY 90 Storm 5 20% 90 Storm 5 10% 60 & 90 Storm 7 (60 & 90) 5% 60 & 180 Storm 6 (60) & Storm 4 (180)

2%

1% Envelope1

45 & 180 5% Tailwater 25, 45 & 120 1 % Tailwater 60 & 180 0.5% 25, 45 & 120 Storm 6 (45) & Storm 7 (180)

Storm 7 (25), Storm 6 (45) & Storm 2 (120)

Storm 6 (60) & Storm 4 (180) Storm 7 (25), Storm 6 (45) & Storm 2 (120)

0.2% 25, 45 & 120 Storm 7 (25), Storm 8 (45) & Storm 2 (120)

1 Note: Assessment of the 1% AEP envelope found that peak flood levels are governed by 1% rainfall and 5% tailwater results (25, 45 and 120 min durations). As such, the 5% rainfall and 1% tailwater results have not been considered further. Please see Section 7.3.2 for more information on tailwater levels.

A detailed explanation of the assessment of the critical storms and durations is provided in Section 9.2.1.

9.2.1 Selection of Critical Storm and Duration

For each design event ensemble and duration, a median flood level result was generated. For example, for the 10 results (10 temporal patterns for ensemble) in the 1% AEP 25 min event, a median flood level result was calculated for each grid cell throughout the study area. This was repeated for every design event and duration. Thus, producing ten results for comparison for the 1% AEP event for example (see the six durations that were modelled in Table 14).

For each design event, the median flood level results from each duration modelled were compared, and a maximum flood level result was generated from these inputs. For example, for the 1% AEP event, ten median flood level results (six durations as per Table 14) were used to generate an overall maximum flood level result. Therefore, compiling one hundred 1% AEP flood level results into one result.

Since it is impractical to adopt this overall maximum result generated due to time and data allowances, analysis of these results then followed to determine the critical storm duration. This analysis compared the median flood level result for each duration to the overall maximum flood level output and calculated the mean difference between the two results across the entire catchment. Analysis was also undertaken to determine the proportion of the median results that were within 2%, 5% and 10% of the overall maximum result. The durations that were found to be closest to the maximum result were selected as critical (see selected durations in Table 15).