22-27 September 2019
Trade Fairs and Congress Center (FYCMA)
Europe/Madrid timezone

Groundwater flooding – assessment of different engineered flood relief schemes using semi-distributed modelling of karst system

23 Sep 2019, 17:30
15m
Auditorium 2 ()

Auditorium 2

Oral Topic 5 - Tools, methods and models to study groundwater Parallel

Speaker

Dr Laurence Gill (Trinity College Dublin)

Description

A characteristic of much of the karst in Ireland is its low-lying nature, with many springs discharging at or below sea level. The low hydraulic gradient for such karst networks also promotes a lot of groundwater-surface water interactions with many ephemeral lakes, known as turloughs, appearing over periods of high recharge (normally in the winter) in topographic depressions. These turloughs are designated as groundwater dependent terrestrial ecosystems (GWDTEs) by the Water Framework Directive and are also designated as Priority Habitats in Annex 1 of the European Habitats Directive due to their unique ecology created by the intermittent hydrological environment.
However, extreme rainfall conditions can lead to high levels of groundwater flooding which is disruptive to the local population, flooding road and rail networks, houses and large areas of agricultural land which, unlike fluvial flooding events, normally takes much longer to recede. This paper documents the history of different studies and attempts to understand the hydrogeology and impact of groundwater flooding for an area of lowland karst in south Galway, centred on the town of Gort. This area has experienced several extreme groundwater flood events over the past 25 years (particularly in 1989/90, 1991, 1994, 2009 and 2015/6) which have created much disruption. There have been two major studies on the catchment which have aimed to design flood relief infrastructure for the area, with the latest one currently nearing conclusion.
This paper documents all of the groundwater hydrological studies carried out over the years used to generate insights into the karst plumbing of the area, including anthropogenic and geogenic tracer studies, cave diving, borehole and turlough level monitoring, meteorology and allogenic river flows. These field data have been used to develop a series of conceptual and numerical models ending up with an extensive semi-distributed model based on a pipe network approach (including diffuse recharge) currently being used to design high level flood relief overflow channels. In addition, a new methodology by which specific groundwater flood frequency design events have been defined is detailed. Such statistical analyses of groundwater flooding requires a different approach to design storms typically used for fluvial flooding, as they are more a function of antecedent cumulative rainfall events and surface water levels than just single rainfall events. This process by which climate change forecasts have been incorporated into the target design events is also outlined.
Finally, the process by which different flood relief options were selected to bring down extreme flood levels whilst not interfering with the more natural ecohydrological wetland function of the turloughs is described. In addition, the socio-economic tensions between the pressure from local NGOs who want the scheme built against the central government’s financial responsibility to the wider population is considered.

Primary authors

Dr Laurence Gill (Trinity College Dublin) Dr Patrick Morrissey (Trinity College Dublin) Dr Ted McCormack (Geological Survey Ireland) Dr Owen Naughton (Carlow IT) Mr Paul Johnston (Trinity College Dublin)

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