Flood Attenuation Studies

This report presents the findings of a study that investigates floodplain attenuation effects in Irish catchments. The work, undertaken by the Centre for Water Resources Research in UCD Civil Engineering, comprised Work-package 3.3 of the Flood Studies Update Programme (FSU). Flood flows in river channels in Ireland are commonly influenced by the effects of floodplain storage. This influence tends to be greater than that experienced in UK catchments and may, in part, explain why many growth curves in Ireland are mildly graded. This has significance for statistical methods of flood estimation recommended in the Flood Studies Report (FSR) for Ireland. Methods include single site flood frequency analysis and regional flood frequency analysis, or the Index Flood Method. The Index Flood Method is generally carried out in two stages, the first involving an estimate of the index flood and the second using a regional growth curve to extract a multiplier for this index flood to estimate floods of required return period. The index flood recommended in the FSR is the mean annual flood, Q , but in the Flood Estimation Handbook (FEH) which superseded the FSR for UK catchments and in the FSU that will supersede the FSR for Irish catchments, the median annual flood will be the index flood. For ungauged catchments, the index flood is determined from equations developed from a multivariate regression model and related to catchment descriptors that have a statistically important influence on rainfall runoff. However, these equations do not include a parameter that accounts for floodplain effects. For gauged catchments the index flood is obtained from the annual maximum series, should a record of good quality hydrometric data be available at the site in question. Failure to include floodplain attenuation effects in either single site or regional flood frequency analysis will potentially result in errors in estimated peak flows. Floodplain attenuation effects are inherently included in single site or regional flood frequency estimation procedures that use Annual Maximum series, resulting in calculated flows that are potentially underestimated. This presents a problem when these flows are used as inputs in river models where the flows are further attenuated. Therefore, the ability to properly account for floodplain effects in the hydrological analysis of catchments is essential to unravel this ‘double accounting’ of floodplain attenuation, particularly in the context of the growing desire to combine hydrological and hydraulic models in a manner that provides a detailed and spatially coherent representation of flood risk. Furthermore, in the context of using groups of similar catchments or ‘pooling groups’ to determine growth factors that can be applied to index floods for estimating peak flows of required probabilities (return periods), data from floodplain-affected (FPA) areas has the capacity to contaminate growth curve estimates at non FPA sites. WP 3.3 of the Flood Studies Update developed simple indices that allows floodplain effects to be either ‘switched on’ or switched off’ depending on the application. The approach adopted involved generating flood hydrographs of specified return periods and routing these hydrographs through a generalised river reach using a flood routing model capable of simulating a variety of channel geometries and roughnesses, thus providing a downstream flow record. The index developed from this approach was validated with prototype data in the case study of the River Suir, Co. Tipperary, Ireland. Results, however were unsatisfactory with peak flow attenuations being grossly underestimated. Moderate improvements were obtained by including a scaling factor which was specific to the River Suir catchment between New Bridge and Caher Park but further work is required to a scaling factor that could reasonably predict floodplain attenuations for catchments throughout Ireland. Results from the equation for predicting the relative delay in flood wave propagation, while better, were still variable.