RORB modelling
Familiarisation with the RORB model:RORB Familiarisation Activity – South Creek
This activity is assessable and is included as a way for you to trial the RORB software before embarking on the full calibration and application exercise (See Sections 3 and 4). Refer to Section 5 on the submission requirements for this part of the assignment. The familiarisation activity is based on the South Creek worked example outlined in the Introduction to RORB guide.
To demonstrate your familiarisation of the RORB model, complete the following:
- Rerun the calibration March 1956 storm using m = 0.75 instead of m = 0.8. Retain kc = 9.65 and IL = 0 mm. Plot the hydrograph. Describe the effect of reducing the m value. Why has this occurred?
- Rerun the AR&R 1987 single design storm analysis with Unfiltered temporal patterns. Plot the predicted hydrographs and report the estimated 1 in 100 AEP design discharge. Are the results significantly different to the run using Filtered patterns?
RORB Model – Caboolture River
This part of the assignment involves the setup up and calibration of a RORB model for Caboolture River at Upper Caboolture (GS 142001A). Details of the streamgauge can be found at the Qld Government Water Information Monitoring Portal website. The RORB model will be validated to flood hydrograph data obtained for two selected historical events.
The validated RORB model will then be applied to estimate design discharges at a proposed road crossing close to the stream gauge site (refer Section 4). These design discharges will be used to provide a flood risk assessment at the road crossing as part of Assignment 3.
Setup the RORB model
Prepare a RORB catchment file (*.cat or *.catg) for the Caboolture River catchment upstream of the stream gauge. It is recommended that the catchment be split into at least 8 to 12 sub-areas of roughly equal shape and size. The centroid of each sub-area can be located by visual estimation. If the centroid is located substantially from the main stream, the additional overland flowpath from the centroid must be included in the reach length estimate. Provide a catchment plan showing nodes and subarea boundaries, using the QTopo map as a base.
The purpose of the RORB model is to estimate design discharges at a proposed road crossing located close to the streamgauge. You will be assessing a preliminary design of the crossing in Assignment 3.
Select Floods
Streamflows and relevant gauging information for the Caboolture River station can be downloaded from the Qld Government Water Information Monitoring Portal:
https://water-monitoring.information.qld.gov.au/
Access the Portal and find the streamgauge details. Tabulate the following basic gauge information: location (latitude and longitude), the catchment area, when streamgauging started, when rainfall measurements started, the maximum observed instantaneous discharge and when this occurred, and the maximum observed daily total rainfall and when this occurred.
The hydraulic control and associated rating curve are also of interest. Include in your report a photograph of the control weir and an image of the creek cross section at the streamgauge.
Provide an image of the rating curve and the flood gaugings that have been undertaken at the site. How many gaugings have been done? What is the highest gauging (level and discharge) and how does this compare with the highest recorded flood? Make a comment of the adequacy of the rating curve based on this information.
Download the monthly discharge data from the full streamflow record. Generate a timeseries plot of monthly peak discharges. What percentage of the data is missing (code 255)? Rank these discharges from highest to lowest to identify the significant events that have been observed at the streamgauge.
The RORB model is to be calibrated against one of the high-ranking floods and the verified by another independent flood. Select two historical flood for analysis, noting that:
- Use the pluviograph data recorded at the streamgauge or other available sites to define rainfall temporal patterns for historical flood events.
- Daily-read rain gauge data can be used to identical the spatial distribution of total storm rainfall across the catchment. The location and availability of local rain gauges can be obtained from the BOM Climate Online website. Comment on the data quality and whether you consider the spatial coverage of rainfall is adequate.
- Streamgauge data recorded for each flood has been quality coded so you should take this into account
After selecting the historical floods, undertake the following checks on the data suitability for use in RORB: - Prepare timeseries plots overlaying the rainfall hyetograph and the observed streamflow hydrograph (this can be done directly in the Qld Government Water Information Monitoring Portal). Visually check for any unrealistic data values or potential timing problems within the data.
- Estimate the average total rainfall over the catchment and the equivalent runoff depth expressed as mm (from the streamflow hydrograph, alternatively daily streamflow volumes can be extracted from the Water Portal) – the volumetric runoff coefficient based on the ratio of runoff to rainfall should be a realistic figure.
Prepare a storm file (*.stm) for each of the selected storm events. Linear interpolation of the observed rainfall depths may be used to estimate the total storm rainfall for each sub-area. Alternatively, prepare a rainfall isohyetal map for the storm as a way to determine the rainfall at each subarea centroid.
Calibrate and verify the RORB model:
For the calibration flood, utilise the FIT run feature of RORB to obtain a suitable kc value that produces the best fit between predicted and observed hydrographs. Note the calibrated kc may be very different to the suggested ‘initial guess’ estimates provided by RORB. The m parameter is normally kept at the default 0.8, but test whether changing m improves the fit.
Use an initial loss that represent the catchment conditions at the start of the storm and provides a reasonable match to the shape of the hydrograph, in particular the rising limb when discharges start to occur. The FIT run will determine a continuing loss that minimises the difference between recorded and predicted flood event volume at the streamgauge.
For the verification flood, again use the FIT run feature but adopt the calibrated kc value. As initial losses vary substantially between flood events, a different loss to the calibration analysis can be used.
If the predicted hydrograph poorly fits the observed, you may need to trial a different kc and then revisit the calibration flood to see if this kc value compromises the fit achieved in the calibration analysis.
RORB Sensitivity Analysis
A ‘one-at-a-time’ sensitivity analysis should be done to check the predicted change in 1 in 100 AEP discharge estimate in response to the following:
- A +10% and -10% change in the calibrated kc and m values
- A +20% and -20% change in the design storm loss rates (20% was selected as the magnitude of loss rates can easily vary by at least this amount and loss rates are a major source of uncertainty in design flood estimation)
Make adjustments to the design discharge estimates based on the outcomes of the sensitivity analysis.
Sanity Check of RORB outputs
Do you think that the Minor and Major design discharges predicted by the RORB model are reasonable estimates? It is recommended to use an alternative method of analysis to check the discharges. One such method is the Regional Flood Frequency Estimation (RFFE) approach to estimate design peak discharges up to 1% AEP for Australian small to medium sized rural catchments developed as part of 2016 AR&R (refer to Book 3, Chapter 3 Regional Flood Methods). The RFFE method supersedes the Probabilistic Rational Method recommended in past versions of AR&R. A raft online tool has been developed to generate RFFE estimates that depend on catchment location and this website can be found at http://rffe.arr.org.au/
As part of the sanity check, state the aspects of the RORB analysis that you consider would contribute most to uncertainty in the design discharge estimates.
Submission
Your submission for Assignment 2 should be in the form of a single file report. The purpose of a file report is to provide a concise record of your work that (hypothetically) can be put on file/archived so relevant information can be recovered at a later date. It is acceptable to use dot points to describe the analyses.
A marking scheme is provided as Table 2. Use this marking scheme to check that you have addressed the full scope of the work. If an element of the assignment has not been documented in the file report than no marks will be given for that element. It is recommended that you structure your report in such a way that each element is clearly and easily identified. Key information such as the methodology that was used, assumptions about analysis inputs and parameters, outputs and results, interpretation of results and recommendations should be included in the file report.
A portion of the available marks has been allocated to reward reporting that is well set out and easy to follow. Submissions that are untidy and/or poorly structured and thus difficult to assess will attract less marks for this element.
Electronic submission of this assignment is preferred, including:
• A single pdf of the report incorporating all appendices
• RORB files
The following filename convention shall be used: *Ass2.pdf and *Ass2.xlsx, where * is your student number.