New Flood Growth Curve for Dublin

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The Centre for Water Resources Research at UCD was commissioned to carry out a study of the Irish Growth Curve and it’s applicability to the Dublin Region.

The main recommendations of the study are as follows:

  1. The Flood Studies Method growth curve method, applied to a known QBAR, is likely to lead to an underestimation of the flood flows for high return periods in the Mid-Eastern side of Ireland, and especially in the Dublin area.
  2. Comparison of QBAR estimated from the FSR regression equation with measured data shows a large range of differences for most catchments tested in the Mid-Eastern part of Ireland. There are similar numbers of over and underestimates. There are some catchments in the Dublin area for which the FSR equation seems to overestimate. While there is insufficient data to draw firm conclusions from this, the large variability in estimating QBAR from the FSR regression equation indicates the need for further study if this variability is to be reduced.
  3. In the light of these findings, I consider it imperative that the question of design flood estimation, particularly in the Dublin area, be urgently addressed. It is of critical importance to enhance the flow data sets being collected by OPW, EPA and Local Authorities, so that long term high quality data sets are available for this type of analysis. In particular, the rating curves for many sites do not extend to include some of the higher flows and this should be addressed by direct measurement and hydraulic modelling.  

In brief, the study has conclusively shown that the Irish Growth Curve underestimates extreme flows in the Dublin Region. This has serious impacts for design of bridges and culverts and for calculating Greenfield Runoff for attenuation schemes. The OPW are aware of problems with existing flood prediction methods for Ireland and hope to have completed a Flood Studies Update by the end of the decade. In the meantime, the UCD report recommends an interim growth curve for the Dublin Region.

Dublin City Council recommends that the following position be taken as regards the assessment of flood flows in rivers:

Given the degree of uncertainty demonstrated in the QBAR equation, this equation should not be used in isolation for estimating flows for smaller Dublin Rivers. Good engineering practice would say that this should not be done in any case. Checks should always be made using other methods such as ADAS 345, IH 124 and Prudhoe and Young.
Ideally, growth curves based on actual gauge data should be used for the design of culverts, and bridges in the Dublin region. Where this data is not adequate, the new Dublin Growth Curve derived by this study should be used in preference to the Irish FSR curve.
These curves can be used to estimate Greenfield runoff for different return periods for carrying out attenuation design. The Dublin Region local authorities will accept Greenfield runoff calculations based on the new curve in most cases but designers should note that this may be unconservative in a small number of cases. The use of river gauge data may be more appropriate where adequate data exists.

Click on the link below to download the UCD report.