Achieving Flood Reduction with Natural Water Retention Measures in Agricultural Catchments in Ireland&#160;
Authors: Laue Pia , Quinn Paul , Bourke Mary , Murphy Darragh , Wilkinson Mark , Harrison Simon , Weatherill John 
Published: March 2021

&lt;p&gt;In recent decades, land-use and climate change&amp;#160;have dramatically&amp;#160;altered&amp;#160;catchment runoff rates.&amp;#160;For example, agriculture intensification has led to increased flood risks by decreasing&amp;#160; soil permeability and reducing channel-floodplain&amp;#160;connectivity. Natural Water Retention Measures (NWRM)&amp;#160;is an approach that has been adopted European-wide for the attenuation of peak floods and the provision of wider ecosystem services. A reduction in peak flow is achieved&amp;#160;by&amp;#160;increasing&amp;#160;water storage potential in the landscape and by modifying natural flow pathways. In agricultural&amp;#160;areas (~70% of Irish land use), &amp;#160;runoff attenuation features such as offline ponds, earthen bunds, sediment traps and leaky dams&amp;#160;are frequently deployed natural&amp;#160;retention measures.&lt;/p&gt;&lt;p&gt;Despite the growing evidence across Europe of their&amp;#160;efficacy for flood peak reduction, water quality enhancement and&amp;#160;biodiversity on the local scale, NWRM features have not been&amp;#160;adopted in Ireland as a flood mitigation approach. In order to build a case that will help address this, this presentation will detail a NWRM demonstrate site in Ballygow, Co. Wexford. &amp;#160;The construction and instrumentation of a network of features developed at&amp;#160;the&amp;#160;field-scale (~1km&lt;sup&gt;2&lt;/sup&gt;) is shown.&amp;#160;This site is an&amp;#160;intensive pasture, small-hold farm.&amp;#160;We aim to quantify&amp;#160;the effectiveness of&amp;#160;these&amp;#160;NWRM features to demonstrate their potential&amp;#160;to&amp;#160;attenuate flood peaks&amp;#160;on agricultural areas using temporary storage, whilst minimising the impact on farming.&lt;/p&gt;&lt;p&gt;The&amp;#160;constructed&amp;#160;measures&amp;#160;consist of&amp;#160;a&amp;#160;flood swale that connects the channel to the floodplain&amp;#160;during high flows, an&amp;#160;earthen bund, an offline&amp;#160;pond with a sediment trap,&amp;#160;that can retain the water from the channel and contributing field&amp;#160;slopes, for &lt;12&amp;#160;hours. On-site video footage and eyewitnesses confirm that&amp;#160;the flood water flows along the field without draining back into the stream. At approximately 800m across the field, the water is retained temporarily, permitting water storage and the opportunity for suspended sediment to settle out of the water column. Flood water is returned to the channel via a perched 20 cm diameter pipe in the bund. Four&amp;#160;automated water level&amp;#160;recorders&amp;#160;(In-Situ Rugged Troll 100) continuously&amp;#160;monitor water levels&amp;#160;in the stream&amp;#160;and the offline pond at 5 min intervals. In addition,&amp;#160;local&amp;#160;rainfall&amp;#160;(EML Event Logger)&amp;#160;is&amp;#160;monitored. These data are used to identify the hydrograph characteristics of&amp;#160;several&amp;#160;storm events&amp;#160;and are used&amp;#160;to determine the effectiveness of the NWRM structures for flood attenuation. The quantification of the&amp;#160;effectiveness&amp;#160;of NWRM features will use the observed time series combined with hydraulic and&amp;#160;hydrological modelling.&amp;#160;&lt;/p&gt;&lt;p&gt;The&amp;#160;quantitative evidence provided by our findings will contribute to&amp;#160;establishing&amp;#160;vital evidence for the implementation of local and national NWRM schemes in Ireland.&lt;/p&gt;