Table 6.4 GTSMR summary for Bulahdelah Catchment

The temporal patterns used to derive the probable maximum flood (PMF) should be selected from an ensemble of patterns appropriate for use with the Generalised Probable Maximum Precipitation (PMP).

At present, the best source of ensemble temporal patterns for use with short duration PMF events are those derived by Jordan et al (2005) for durations up to 6 hours. The procedure suggested to derive the design temporal distribution of GSAM and GTSMR patterns for duration of 24 hours or longer were described in the revised edition of Australian Rainfall and Runoff, Book IV, ARR (Nathan and Weinmann, 1999) using the Average Variability Method (AVM) of Pilgrim et al., (1969) (Bureau of Meteorology, 2006) and (Bureau of Meteorology, 2005). The GSDM, GSAM and GTSMR patterns were used for intermediary durations (i.e. 9 hr, 12 hr and 18 hr). The Jordan et al. (2005) patterns were derived specifically from storms associated with thunderstorm or deeply convective events while the GSDM, GSAM and GTSMR patterns are defined in the associated guidelines The ellipse approach from the GSDM was applied to define the areal pattern for the shorter duration events.

These patterns were therefore adopted in this study and applied to the calculated PMP rainfall depth. The critical pattern was determined as per the typical ARR 2019 guidelines applied to the other design events.

6.6 Model results and critical durations

For each design AEP event, 23 different durations were modelled ranging from 10 minutes to 168 hours, except for the PMF which had twenty durations ranging from 15 minutes to 120 hours. Within each duration, 10 specific rainfall events were modelled (as recommended in ARR 2019) which varied the rainfall temporal pattern, though not the magnitude, over that period. This led to 230 individually modelled rainfall events per design event which were then analysed to pick the one event to use as design rainfall.

Critical durations were selected based on the methodology described in ARR 2019. This methodology consists of selecting, for each duration, the rainfall temporal pattern that is the closest to the average flow obtained from the 10 specific patterns provided in the ARR 2019 database. This provides an automated approach that can then be adjusted for consistency in durations between the various events.

Figure 6 9 and Figure 6 10 present box-plots for the 1% AEP for both the river catchment (sub-catchment 1.31) and for the township based on the local sub-catchment (sub-catchment

76.01) Critical durations and temporal pattern (from ARR 2019 Data Hub) based on the hydrologic model for each event are presented in Table 6.5. The design storm for each AEP is the rainfall event which results in the median flow for the critical duration (Figure 6.11). The selection of the critical duration for the overall Myall and Crawford Rivers catchments was based on the peak flow at Bulahdelah Bridge and an example is presented in Figure 6.12 for the 1%AEP event.

Table 6.5 Critical durations for each design event

* Figure 6.10 shows that pattern 112 is the critical duration. This pattern was compared to pattern 114 and the difference in flow is negligible and therefore, pattern 114 was used for all events to improve consistency between events and minimise the risk of having local areas being subject to lower flooding during rarer event due to change in temporal pattern.

** Temporal pattern ID from Jordan et al. (2005) rather than the data hub as recommended for PMF analysis in ARR 2019

6.7 Discussion on Crawford and Myall Rivers flood coincidence

Both the Crawford River and the Myall River catchments are compact and very close. They are therefore expected to typically be subject to similar rainfall patterns and to flood simultaneously. However, the Crawford River catchment was found to be approximately half of the Myall River catchment. This surface area difference would impact on the critical duration of each catchment and the Crawford River would most likely have a shorter critical duration than the Myall River. Application of the typical ARR 2019 approach of analysing all temporal patternsfor all durations would highlight which of the critical durations would lead to the highest flow at Bulahdelah. The temporal pattern and critical duration analysis are described in Section 6.6 The critical durations for the river catchment presented in Table 6.5 include the combined flows of the Myall River and the Crawford River.

The critical duration for the Myall River and Crawford River were then compared separately based on sub-catchments 1.29 for the Myall Riverand 56.11 for the Crawford River and results are presented in Table 6 6 This table compares both rivers’ critical durations and the peak flows from the Crawford River for the Myall River’s critical durations. It was found that both rivers have very similar critical durations. The critical duration of the Myall River only is the same as the combined rivers’ critical duration of 18 hours for most events and 6 hours for the PMF. The critical duration of the Crawford River was found to be between 12 and 24 hours with the 18-hour duration having very similar peak flows. It can also be noted that the flow from the Crawford River catchment is approximately half of the flow from the Myall River only catchment (hence a third of the combined flow at Bulahdelah). Both rivers are therefore likely to peak at the same time except for a very concentrated and localised storm cell.