National Indicative Fluvial Mapping: applying and updating FSU data to support revised flood risk mapping for Ireland

National Indicative Fluvial Mapping: applying and updating FSU data to support revised flood risk mapping for Ireland.
Brown, E.and O'Loughlin, F. and Galvin, C.
In: Irish National Hydrology Conference 2019, 19 November 2019, Athlone, Ireland. (2019)

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Abstract:A first cycle National Preliminary Flood Risk Assessment (PFRA) for the Republic of Ireland, completed in 2012, identified areas at significant flood risk, and included the production of national indicative fluvial flood maps. The National Indicative Fluvial Mapping (NIFM) project has recently been completed; this has produced second generation indicative fluvial flood spatial data that are of a higher quality and accuracy to those produced for the first cycle PFRA. This project has covered 27,000 km of river reaches, separated into 37 drainage areas, consisting of 509 sub-catchments. The main project goals were to produce higher quality flood maps, improving upon the outputs of the first cycle PFRA, to take account of potential climate change impacts on flooding, and to produce mapping to improve risk assessments for areas not covered by the National Catchment Flood Risk Assessment and Management (CFRAM) Programme. The NIFM approach to this national-scale modelling project has combined intelligent automation of the repetitive processes, such as hydrological calculations and the burning of watercourses into the 2D model mesh, with rigorous quality assurance techniques. The hydrological inputs to the 2D hydraulic models, notably the index flood (QMED) values and hydrograph shape, were derived from Flood Studies Update (FSU) node data. The FSU Physical Catchment Descriptors (PCDs) and methods were developed using a dataset that extended to 2012. Since then, additional recent years of observed data had become available, during which period some significant flood events had occurred (notably in December 2015 – the largest event of the observed record at several locations across Ireland). Furthermore, flood growth factors, which convert the index flood to floods of higher return periods, had also been developed on the dataset to 2012, under the National CFRAM studies. It was therefore necessary and timely to reflect the impact of the additional recent years of data on the index flood and growth curve calculations. Annual maxima data were used to calculate adjustment factors to the QMED values, and these were applied using the pivotal site approach. A pivotal site is the gauging station that is considered most relevant to a particular flood estimation problem at the subject site, ideally, lying a short distance upstream or downstream from the subject site at which the flood estimation is required. In this project, pivotal sites were selected as the nearest downstream gauge on the same river; in the circumstances where no downstream gauge existed, the nearest gauge was used. Traditional alternative approaches to this method, such as the use of analogue catchments, can be potential sources of error; furthermore, the selection of analogue catchments is subjective and therefore difficult to implement on a widespread, automated basis such as the approach being used in this project. CFRAM growth factors had been derived on a range of different bases for different Units of Management (UoM). Growth curves for each UoM were recalculated for the extended data series. The change in the growth factors as a result of the additional years of flow data was calculated for each OPW gauge. The growth curves for each UoM based on CFRAM growth curves were then scaled to account for the additional years of flow data.
Item Type:Conference or Workshop Item (Paper)
Uncontrolled Keywords:Flood Studies Update; flood risk; mapping; index flood; growth factors
Subjects:Floods > Flood risk assessment and mapping
ID Code:1776
Deposited On:09 Oct 2019 14:05
Last Modified:09 Oct 2019 14:05

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