Appendix I Intersection Flood Hazard Assessment - 1 of 2

                                                                                         Flood Hazard Assessment
                                                                Intersection of Lake Macdonald Dr and Dianella Crt



EXECUTIVE SUMMARY
This report presents the findings of a flood hazard assessment undertaken for the road intersection
of Lake Macdonald Dr and Dianella Crt in Cooroy.

This report describes the configuration of the local high resolution flood model and summarises the
hydraulic performance of the intersection under a range of design flood conditions with Annual
Exceedance Probabilities of between 50% AEP and 0.2 % AEP.

Detailed ground and feature survey provided by Murray & Associates (see Appendix A) shows three
pipe culvert cells passing beneath Lake Macdonald Dr from the southeast corner of the intersection
with Dianella Ct to the northwest corner of the intersection adjacent to Wilgee Ct. Two of pipe
culverts are shown to be 1050 mm diameter and the third is 1350 mm diameter.

For the 10% AEP design flood event the intersection is not inundated and hence no flood hazard
exists within the trafficable road corridor. For the 2% AEP design flood event, shallow inundation at
the peak of the flood event introduces a low (H1) flood hazard classification across the intersection
with areas of higher classification where flow across the road concentrates and returns to the main
waterway, see Map 5. The flood hazard classifications are similar for the 1% AEP design flood event,
however, a more prominent corridor of higher (H5) flood hazard exists across the intersection, see
Map 6.

The duration of inundation across the intersection for the 2% AEP and 1% AEP design flood events is
anticipated to be less than 1.5 hours. The actual duration of inundation will vary for each major
rainfall event experienced in the local catchment; however, the approximate duration of the design
flood event provides a reasonable indication of the duration of inundation which is also consistent
with the size of the upstream catchment that contributes flood flows to the intersection location.

The intersection is flood free and trafficable for events up to the 10% AEP.

Detailed flood modelling of the catchment and subject intersection has demonstrated the capacity
of the existing culverts provides 10% AEP design flood immunity to the intersection. For larger and
rarer flood events of less than 2% AEP the model results show the intersection will experience water
over the road to depths of less 0.3 m for a 1% AEP flood event for relatively short durations of less
than 1.5 hours.

The flood modelling has also demonstrated that should a significant blockage of the culverts occur
during a major rainfall flood event the depths of inundation across the culvert would not be
expected to significantly change, however, the inundation across the intersection would persist for
longer periods.

The hydraulic performance of the intersection culverts is summarised in Table E -1 below.




                                                  E1