NSW Continuing Loss Methodology - ARR tool

[itsgupta]

Introduction

At present, the QGIS ARR tool generates losses for the supplied catchment using default parameters from the ARR data hub. The tool is great for setting up rainfall around Australia. Jurisdiction specific data for NSW was published onto the data hub which changes the loss approach. The current understanding of this approach within the industry is not consistent with the outputs obtained from the TUFLOW plugin.

Guidance from the updated ARR 2019 guidelines suggest that in the absence of calibrated data, for NSW a hierarchy of losses is to be adopted.
The guidelines can be found here: https://data.arr-software.org/nsw_specific

Continuing Loss

>Use default ARR data hub continuing losses with a multiplication factor of 0.4. This is used with the unmodified ARR Data Hub initial losses which requires the application of additional scrutiny to the balance between initial loss and pre-burst to ensure it is reflective of flood history and observations for the catchment being investigated in the lead-up to events. This is particularly important in catchments of 100 km2 or less.

The following concerns are raised:

1. The continuing loss to be adopted is meant to be 0.4 times the default loss, currently the QGIS plugin outputs the full continuing loss in the .trd which may be overestimating the loss. As the plugin does not check any FFA or calibration data at the location, it isnt clear why the 0.4 factor is not adopted.

2. The plugin creates a variable for the continuing loss as "CL_Catchment" in the materials and .trd file. However it outputs the same value over and over again. Current advice from the ARR data hub does not suggest varying the CL loss based on AEP or duration. Hence it is not clear why this variable exists. It is perhaps better for cleaner model setup to simply output the continuing loss directly into the materials.csv file as a number as opposed to a variable.

[TUFLOW-Support]

Hi, thanks for your comments. Please see my responses below:

1. You are correct, this needs to be updated in the tool for catchments within NSW.

2. The variable was originally created to give users the option to vary the continuing loss by catchment should they choose. This may be a useful feature for larger catchments, say in central QLD.. It was also done this way to somewhat future proof the tool as ARR continues to develop and change. However i can see how this may be frustrating if the continuing loss value is constant over all catchments and you would like to change the value. I think a switch in the tool would fix this so both options are available.

I will keep you updated as these points are developed further.
Thanks,
Ellis

[itsgupta]

Thank you Ellis for your prompt response and your continued efforts on this plugin!