independent assessment report rainfall and flooding event

Independent Assessment Report Rainfall and Flooding Event of 26 March 2018 in Redlynch Valley Cairns Stage 2 of 2 Report

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Stage 2 Joint Hydrologic/Hydraulic Modelling

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• Peak velocities of greater than 4m/s within Currunda Creek have the potential to cause significant bed and bank scour consistent with the damage captured and observed following the event

• A revised rating curve at the Redlynch Gauge has been developed based on the modelled flows and resultant flood levels at the gauge location as shown below compared to the official DNRM curve

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0 50 100 150 200 250 300 350 400 450

Stag

e (m

AHD

)

Flow (m3/s)

Approximate Rating Curve at Redlynch Gauge (mAHD)

Revised Rating DNRM Rating


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F.3.4 Currunda Creek Crossing Assessment The Redlynch Intake Road crossing of Currunda Creek was subject to significant blockages during the March 2018 event due to large debris load from the upstream catchment. Photos taken the morning after the event show debris along the full length of both guard rails on each side of the road with a section of guard rail on the downstream side torn away during the event. Anecdotal evidence suggested that the debris may have reached up to 2m high above the road surface during the peak of the event. The effect of this debris blockage alone is for flows to be diverted both north and south of the crossing towards residential properties on both banks of the creek.

Similar debris blockage can be seen in images further upstream on Currunda Creek at a local bridge crossing. The effect of debris in this location is to divert some additional flows towards the neighbouring quarry however the majority of the flows do merge again downstream within the main Currunda Creek waterway.

Additional sensitivity scenarios have been included in this Stage 2 assessment to determine the potential impact of blockages on the local flood levels and extents. The sensitivity scenarios undertaken are detailed below:

(1) Currunda Creek culverts/bridge 50% blocked with no guard rails represented

(2) Currunda Creek culverts/bridge 90% blocked with no guard rails represented

(3) Currunda Creek culverts/bridge 90% blocked with 100% blockage of guard rails to 2m above road level

(4) Currunda Creek culverts/bridge 90% blocked with 100% blockage of guard rails to 2m above road level with rapid failure of approximately 20m length just prior to peak of the local flows

The local impacts on peak flood levels for each sensitivity scenario are shown in Figure F-17. Note the Guard Rail Failure Scenario impacts are compared to the 100% Blockage of guard rails up to 2m scenario.

The impact of culvert blockages alone was a localised increase of up to 300mm and 500mm in peak flood levels upstream of the local access bridge on Currunda Creek for the 50% and 90% blockage cases respectively. Impacts extended up to 300m downstream of the local bridge on the northern overbank before returning no blockage levels.

At the Redlynch Intake Road crossing, culvert blockages caused increases of over 140mm and 250mm across the road to the north and over 100mm and 200mm to the south for the 50% and 90% blockage scenarios respectively. Upstream of the crossing, peak flood levels increased by over 250mm (50% blockage) and 450mm (90% blockage).

When 100% debris blockage of the guard rails is considered, peak flood levels increase by over 900mm to the north over the road and over 700mm to the south. The extent of additional inundation extends over 200m further south along the Redlynch Intake Road.

While for each scenario the impacts due to blockages extend up to 300m downstream of the crossing, no additional impacts on peak flood levels propagate further within Freshwater Creek towards the caravan park. There is an actual localised reduction in peak flood levels immediately downstream of the crossing due to the attenuation of flows caused by the restriction to flows caused by the blockages.

The representation of the guard rail failure just prior to the peak of the event resulted in a local increase immediately downstream of the failure location of approximately 700mm before dissipating within Freshwater


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Creek 300m east of the crossing. It can be noted that the peak flood levels downstream of the crossing in the guard rail failure scenario are not higher than the no culvert blockage scenario. The local flood “wave” is effectively offset by the attenuation effects of the debris blockage when considering peak flood levels downstream of the crossing. A comparison of the peak flood level time series immediately downstream of the crossing is shown in Figure F-16 demonstrating the impact of the flood “wave”.

Figure F-16 Guard Rail Failure Impact Time Series

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26/03/2018 18:00 26/03/2018 21:00 27/03/2018 0:00 27/03/2018 3:00 27/03/2018 6:00

Floo

d Le

vel (

mAH

D)

Date

Rail 100% Blocked to2m

Guard Rail Failure with100% Blocked to 2m


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F.4 Design Events The local catchment design hydrographs produced by the validated WBNM model with a Lag Parameter = 1.6, were used as inflows to the TUFLOW model for the 100 year ARI (1% AEP) design event. The hydraulic model was run for a range of durations with the 1080 and 1440 minute events producing the peak flood levels within the main Freshwater Creek floodplain and Currunda Creek for the modelled area extents. As such, it should be noted that higher peak flood levels within the local drainage paths on the periphery of the floodplain may result in the event that high intensity shorter duration local storms occur. The flood extents determined for this flooding assessment are only considered applicable to the larger Freshwater Creek catchment flooding and within Currunda Creek. Local catchment assessments are recommended to define local drainage characteristics where required.

The design event runs have assumed the following catchment conditions:

• Copperlode Dam full to overtopping level at the start of the event

• Catchment topography and landuses current at the start of 2018 with sugarcane crops at full maturity

• Boral quarry catchment self-contained

• No blockages for the culverts represented within the model extents

• Initial losses = 0mm and continuing losses =2.5mm/hr

The resultant 1% AEP peak flood levels and depths are plotted in Figure F-18 and Figure F-19.

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