OBJECTIVE
To demonstrate the effectiveness of a small network of NSB (bund, swale, sediment trap, temporary pond) in reducing flood peaks and improving water quality at field-scale (~ 1-2 km2) in a typical agriculture landscape in Ireland. To meet the objective a small network of NBS features were built on ‘research ’ sites where stakeholders can visit to gain a better understanding of the measure scale and impact. A suite of environmental sensors were installed to undertake a sampling program. This data will inform on the efficacy of the individual measure; reinforce international programs that seek real data to support models and implementation; demonstrate the wider ecosystem services of NSB measures.
Runoff attenuation feature (RAF) is a soft-engineered flow interception structures which aim to attenuates surface flow pathways. They are used for flood management and can provide multiple benefits including habitat creation, carbon sequestration, and enhanced water quality (Nicholson et al., 2019; Quinn et al., 2013; Wilkinson et al., 2014). In this project we will consider two RAF’s features:
1. Bunds:
Bunds are offline storage areas, located on the floodplain proximal to the river. They are dry for most of the year. Construction involves the building of a soil bund, using locally excavated material. The purpose is to provide temporary water storage (often 300–1,000 m3), and more permanent sediment storage. The offline storage areas become hydraulically active when water levels exceed the channel capacity and flow enters via an armoured inlet. The bunds are gravity-drained via a pipe inserted into the base of the downstream soil embankment, often within 8 hr (Nicholson et al., 2019).
2. Woody dams, diverters and Hyporheic Design Elements:
Timber, often sourced from local woodlands, when placed across the stream can spill water onto the adjacent floodplain, attenuating flow, slowing flood peaks and improving habitat (Welton and Quin, 2011). They are designed to suit the specific location and use local and natural materials. They can be easily and cheaply installed and designed to offer multiple benefits for flooding and wildlife. For example, they are designed to permit fish passage and are documented to trap debris as it floats downstream, thereby reducing blockage of culverts.
Woody dams slow the passage of water downstream, increasing sediment retention (WT, 2016). They span the whole river channel or ditch and are pinned in place behind trees or using cables or large stakes. In strategic low risk locations dams are used to increase out-of-bank water movement, helping to reconnect the floodplain to the river.
Woody Diverters use logs or untreated boards across the full width of ditches and small streams. Their build assists to i. slow and ii. disconnect the drainage network. The structures divert water into preferential storage areas and can be designed to interact with high flows only (ditch top diverters) or be inserted into the stream to restrict flow.
Hyporheic Design Elements (HDE) utilize the fact that a significant proportion of nutrient turnover in streams occurs in the hyporheic zone. Specifically, it is the saturated porous area below the stream bed where fine gravels provide substrate for biofilms, and where mixing of groundwater and surface waters occurs. HDEs work by installing physical structures (e.g. large woody debris, boulders) which induce a hydraulic gradient upstream to downstream of the structure. This encourages greater flow through the hyporheic zone. These structures have been shown to increase the contact of nutrient-rich surface waters with the gravels and the microbial communities of the hyporheic zone. This process enhances microbially-driven attenuation of nutrients to improve water quality (Bakke et al., 2020; Herzog et al., 2019; Peter et al., 2019). HDE’s will be installed with the aim of improving the natural attenuation of water quality pressures such as Dissolved Organic Carbon (DOC), Ammonium/Ammonia and faecal microbes.
