9 Model sensitivity

A number of factors require some sensitivity analysis prior to completing the design runs. These factors include:

• Tailwater level: impact of reduced and increased water level at Bombah Broadwater was investigated.

• Losses: impact of reduced and increased losses were investigated.

• Roughness: impact of reduced and increased losses was investigated.

• Blockage: AR&R 2019 recommends running two blockage sensitivity scenarios including double design blockage and no blockage.

Sensitivity results are provided in Appendix F

9.1 Tailwater level sensitivity

In order to analyse the influence of the tailwater level on the flood behaviour, the following scenarios were modelled:

• Scenario with a 500 mm increase in tailwater level for the 1% AEP flood event.

• Scenario with a 500 mm decrease in tailwater level for the 1% AEP flood event.

Each of these scenarios was modelled for both the 45-minute and 18-hour critical durations. The following observations can be made:

• For the 18-hour critical duration event, the tailwater level has negligible impact on the flood level at the Bulahdelah Township and only increases the flood level at the downstream end of the catchment near Bombah Broadwater.

• For the 45-minute critical duration, the short duration does not allow the river to respond significantly to the rainfall and the tailwater has therefore a more direct change in level due to the low-lying floodplain. However, the majority of the additional flooding would occur at a lower level than for the long-duration flooding and remain predominantly along the river and vegetated areas.

9.2 Losses and roughness sensitivity analysis

In order to analyse the influence of losses and roughness on the flood behaviour, the following scenarios were modelled:

• No loss scenario with neither continuing losses nor initial losses in the pervious areas for the 1% AEP flood event.

• High loss scenario with 2.5 mm/hr continuing losses and 20 mm initial losses in the pervious areas for the 1%AEP flood event.

• Low roughness scenario with roughness reduced by 20% for the 1% AEP flood events.

• High roughness scenario with roughness increased by 20% for the 1% AEP flood events.

Each of these scenarios was modelled for both the 45-minute and 18-hour critical durations. The following observations can be made:

• The removal of all losses would generate increases in levels in the order of up to 0.100.15 m around both the township and the river catchment.

• High losses would result in decrease in flood level by up to 0.30-0.40 m within the river catchment and up to 0.05-0.10 m in the township catchment.

• Changes in material roughness by 20% impact river levels by up to +/- 0.15-0.25 m for the 1% AEP event around the river catchment for the 18-hour duration event but has minimal impact on the flood level for the 45-minute duration event

9.3 Blockage sensitivity analysis

In order to analyse the influence of blockage on the flood behaviour, the following scenarios were modelled:

• No blockage scenario for the 1% AEP event. These scenarios assumed that all the pits and pipes were free of blockages.

• Double design blockage scenario for the 1% AEP flood event. These scenarios consider the double design blockage for the cross-drainage structures as recommended by AR&R 2019 and the severe blockage value of 100% blockage for the pits and pipes system as described in AR&R 2019 Project 11.

Based on the results of this analysis, the following observations can be made:

• The no blockage scenario for the 18-hour duration event for the 1% AEP event results in local changes in flood level in the order of up to 0.10 m. Some areas would have reduced levels directly upstream of major culverts/pipes and some other areas would have increased levels downstream of major structures. There are a number of locations where there is a significant reduction in flood levels directly upstream of Bulahdelah Way and the Bulahdelah Bypass.

• The no blockage scenario for the 45-minute duration event for the 1% AEP event results in similar outcomes as the 18-hour duration. However, the increases in flood levels in the township are slightlyexacerbated with some local increasesalong drainage channels of up to approximately 0.30 m and further reduction in flood levels upstream of the Bulahdelah Bypass upstream of the township.

• The double blockage scenario for the 1% AEP event typically results in increases of up to 0.10 m in the township catchment with some local higher increase of up to 0.20-0.40 m along some of the main drainage channels and upstream of major culverts The 45minute duration event also highlights that the flooding would be diverted from the main drainage channels should there be high blockage of the various culverts that intersect these drainage channels at regular interval.

• In general, afflux is worse in the township under shorter duration events and upstream of major culverts where the local area acts as a detention basin (e.g. upstream of the Bulahdelah Bypass) for longer durations as a greater volume of water is unable to drain from this basin