HWRS 2014: Improving Rating Curves with 2D Hydrodynamic Modelling

HWRS 2014: Improving Rating Curves with 2D Hydrodynamic Modelling

Improving Rating Curves with 2D Hydrodynamic Modelling

Authors: Ben Tate; Kathy Russell

Abstract: The  use  of  gauged  flows  for  flood  hydrology  depends  on  rating  curves,  relating  flow  rate  to  gauge  height.  Rating  curves  are  developed  by  undertaking  a  series  of  discrete  flow  measurements  during  flood  events.  When  a  flow  occurs  that  exceeds  previous  gauging  records,  particularly  widespread  overbank flow conditions, an extrapolation of the rating curve is required to allow flow rate estimation.

At numerous gauges across Victoria, the January 2011 gauge height exceeded the highest rated level  by a significant margin. In these cases, extrapolated rating curves are developed by Thiess to allow  flow estimation for these extreme events. However, in a number of flood studies undertaken by Water  Technology,  calibrated  hydraulic  models  have  shown  that  rating  curve  extrapolation   does  not  accurately predict peak flows for the January 2011 event for many gauge locations. For some gauges  the extrapolated rating curve underestimated flow and on others (such as Murchison) significantly over  estimated flow.

The peak flows of these significantly large events within the extrapolated range of rating curves are  critical  in  the  estimation  of  design  flood  peak  flows  using  flood  frequency  analysis.  Taking  the  Murchison gauge on the Goulburn River as an example, a recent revision to the extrapolated section  of the rating curve by Thiess resulted in a change in the estimated peak flow of the largest flow on  record  from  195,000  ML/d  to  311,000  ML/d,  a  60%  increase.  This  significant  change  in  peak  flow  means that the revised 1% AEP design peak flow would be larger than the previous 0.2% AEP design  peak flow. This extrapolated section of the rating curve therefore could result in dramatic changes in  design flood levels, influencing planning decisions.

This   paper   discusses   three   examples   of   current   rating   curves,   with   verification,  revision   and  extrapolation of the curves undertaken to estimate peak flows at Murchison, Quambatook and Skipton.  For each of these gauges, a 2D hydrodynamic model was calibrated to known flood levels and flows,  and used to verify, revise or extend the high-flow or extrapolated section of the rating curve. The 2D  modelling approach is superior to traditional 1D modelling, Manning’s equation or extrapolation based  methods  of  extending  a  rating  curve.  The  2D  hydraulic  modelling  is  less  sensitive  to  model  schematisation  and  parameterisation  and  takes  into  account  the  spatial  variability  of  overbank  flow  conditions.

This paper highlights the need to consider uncertainty in rating curves, particularly in their extrapolated  section,  as  a  standard  component  of  the  flow  estimation  process.  It  also  demonstrates  the  role  2D  hydrodynamic modelling can play in the verification, revision and extrapolation of rating curves. The  method  used  in  this  study  is  widely  applicable  to  the  verification  of  other  gauges  with  extrapolated  rating curves.

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