HYDROLOGISTS AND HYDROGRAPHERS HOLDING HANDS
Authors: Ben Tate¹, Jessica Littlejohn² and Rebekah Webb²
¹ Water Technology
This paper outlines work undertaken to upgrade a number of rating curves in Victoria with unreliable extrapolated sections beyond the highest rating curve gauging. The work follows on from a paper presented at the Hydrology and Water Resources Symposium in Perth in 2014 (Tate & Russell, 2014). The previous paper identified significant issues with a number of streamflow records for high flow events as a result of unreliable rating curves. This has further been demonstrated through a number of flood studies in Victoria, some of which are used as case studies in this paper. The paper demonstrates the importance of a reliable rating curve and how Victoria is moving to deliver this. It makes a strong case for why hydrologists and hydrographers need to work closely together to achieve improved rating curves and improved streamflow records.
The importance of a good rating curve has long been recognised (Potter and Walker, 1981/1985; Brown, 1983), so too has the uncertainty in the unreliable extrapolated section of a rating curve. This paper aims to provide current examples demonstrating the importance of a reliable rating curve for floodplain management. The paper identifies some issues with past practices regarding the reliability of rating curves in the extrapolated region and proposes a simple approach to improving those rating curves into the future.
Despite all rating curves and streamflow records in Victoria coming with quality codes, unreliable streamflow is often not treated as an issue until it becomes apparent through model calibration that the streamflow record is in error. This was the case with a number of flood studies that formed the basis of the paper presented at Hydrology and Water Resources Symposium in Perth (Tate & Russell, 2014).
The paper and a number of other flood studies identifying similar issues triggered members of the Victorian floodplain industry to begin to think about how rating curves can be updated when an issue is identified. The problem was related to when an issue with a rating curve is identified during a flood study, what is the mechanism for implementing a revised rating curve. Water Technology had completed many of these flood studies and had been talking with Ventia, who maintain Victoria’s streamflow gauging network. Together with the Department of Environment, Land, Water and Planning it was decided that a pilot project was worthwhile to investigate five streamflow locations where a modelled rating curve review had taken place, and update the rating curves for those gauges.
Water Technology undertook a review of the rating curve for streamflow gauging stations listed in Table 1. The review summarised the detailed review contained in the relevant flood study.
A meeting was held between Water Technology, Ventia and Department of Environment, Land, Water and Planning to discuss the streamflow sites, rating curve recommendations and actions for each gauge. The findings from the review and the rating curve updates (where appropriate) were then implemented in Hydstra. The streamflow record was backdated using the revised rating curve where required. The rating curve revision was then communicated with all partners with an interest in the gauge through the Regional Water Monitoring Partnership.The review and recommendation for any revisions to the rating curves was then provided to Ventia. Ventia compared the current and recommended revised rating curve and where appropriate updated the rating curves in Hydstra.
This paper will not reproduce the detailed results of the five flood studies within which, the rating curve reviews were undertaken. Rather the results are summarised below. Some general discussion on rating curves is also included.
In many flood studies, Water Technology has found that the extrapolated zone of the rating curve can produce flows that are significantly biased, both positively and negatively. This bias is largely attributed to the method by which the curve is extrapolated and the points at which the curve is anchored. Often a simplified method of extrapolating the curve assumes that the trend in the curve remains constant. However, in many instances the floodplain behaviour does not show a constant trend, so it follows that the rating curve may not continue along the same trend. Common instances where this constant trend does not apply includes hydraulic structures like bridges and causeways, where the level-flow relationship changes as a bridge obvert is reached and then when the bridge deck is exceeded, or wide flat floodplains where out of bank overland flows are extensive and may bypass the gauge location. In these situations, it makes sense the standard extrapolation techniques are not applicable, instead two-dimensional hydraulic modelling can better estimate the level-flow relationship.
A summary of the outcomes of the rating curve review for the five streamflow gauges subject to the pilot project is included in Table 2 below. Key findings and discussion points from each of the gauge sites are discussed further below.
Table 2: Summary of Rating Curve ReviewNOTE: Modelled Flow Difference is for the maximum recorded flood level.
* Rating curve gauging upstream in a confined location during January 2011 flood.
#The existing rating curve was based on only a few historic gaugings, and was not maintained.
The pilot study has shown that in Victoria, all the mechanisms required for updating the unreliable section of an extrapolated rating curve using modelled data are in place. All that is required is a recommendation from a flood study that the rating curve be updated, Ventia is then notified of the required update with a summary and CSV file of the revised rating curve, and that through their current contract with DELWP maintaining the streamflow gauge network, the rating curve is officially updated and relevant parties notified.
This process will enable improved gauging at high flows, and over time with more rating curves updated, will facilitate better design flood estimates underpinning improved floodplain management.
- BMTWBM (2014), Charlton Flood and Drainage Management Plan, report prepared for North Central CMA
- Brown, J.A.H. (1983), Australia’s Surface Water Resources – Water 2000: Consultants Report 1,
Australian Government Publishing Service, Canberra.
- Potter, K.W. & Walker, J.F. (1981), A model of discontinuous measurement error and its effects on the probability distribution of flood discharge measurements, Water Resources Research, Vol. 17(5):1505-1509.
- Potter, K.W. and Walker, J.F. (1985), An Empirical Study of Flood Measurement Error, Water Resources Research, Vol. 21(3):403-406.
- Tate, B. & Russell, K. (2014) Improving Rating Curves with 2D Hydrodynamic Modelling. Hydrology and Water Resources Symposium, Perth.
- Water Technology (2014), Murchison Flood Mapping, report prepared for Goulburn Broken CMA
- Water Technology (2014), Quambatook Flood Management Plan, report prepared for North Central CMA
- Water Technology (2016), Regional Flood Mapping Lower Wimmera, report prepared for Wimmera CMA
- Water Technology (2016), Warracknabeal and Brim Flood Investigation, report prepared for Wimmera CMA
Paper Presentation at the 2016 joint Conference for NZ Hydrological Society, Australian Hydrology and Water Resources Symposium and IPENZ Rivers Group on Tuesday 29 Nov at 2:30pm