Table 14: Design events and durations assessed

9. DESIGN FLOOD MODELLING

9.1 Overview

The following sections present the design flood behaviour in Exile Bay using the calibrated hydrologic/hydraulic modelling system. Design flood behaviour has been derived for the 1 EY, 20%, 10%, 5%, 2%, 1%, 0.5%, 0.2% AEP and PMF events. This modelling has been undertaken utilising the methodologies outlined in ARR2016 for mainstream and overland flow flood affectation.

Please note that all figures for Design Flood Modelling are presented at the end of the report.

9.2 Application of ARR2016

ARR2016 provides guidance on the industry best practise approach for design flood estimation across Australia. Key changes to design flood estimation utilising the ARR2016 methodology include:

• Design rainfall data (i.e. intensity-frequency-duration data) across Australia has been updated utilising an additional three decades of data (see Section 3.4.3); • Where previously a single temporal pattern was used for a particular design event and duration, now an ensemble of 10 temporal patterns is modelled per storm duration (see

Section 3.4.3); • Use of the pre-burst rainfall incorporated prior to the design storm burst (see Section 6.5); and • Update to the Initial and Continuing Loss values which better reflect local conditions (see

Section 6.4).

Since the nature of overland flow flooding in an urban catchment is typically relatively distributed (multiple flow paths as opposed to one in mainstream flooding), determining the critical storm and duration for design events can be difficult and inaccurate using the hydrologic model. As such, the ensemble of 10 temporal patterns for a range of durations were modelled in both the hydrologic (DRAINS) model and hydraulic (TUFLOW) model. Table 14 presents the durations assessed for each design event.

Table 14: Design events and durations assessed

Event (AEP) Durations Assessed (min) No. Model Iterations 1 EY 20, 25, 30, 45, 60, 90, 120, 180 80 20% 20, 25, 30, 45, 60, 90, 120, 180 80 10% 20, 25, 30, 45, 60, 90, 120, 180 80 5% 20, 25, 30, 45, 60, 90, 120, 180, 270 90 2% 20, 25, 30, 45, 60, 90, 120, 180, 270 80 1% 10, 15, 20, 25, 30, 45, 60, 90, 120, 180 100 0.5% 20, 25, 30, 45, 60, 90, 120, 180 80 0.2% 20, 25, 30, 45, 60, 90, 120, 180 80 Total 670