Leades Farm
Leades farm is a mixed–use farm close to the Boggeragh mountains in Co. Cork. A small 1st/2nd order stream within the Glashagarriff catchment flows through the field before ultimately meeting the River Lee at the Inniscarra reservoir. A range of nature–based solution features were installed within the stream and on adjacent fields at Leades farm to test their effectiveness in mitigating flood peaks and improving water quality.
The plan for NSB features
Diverter-swale-bund
We installed a Diverter-swale-bund suite (Similar to site at Ballygow). The offline bund should temporarily store floodwaters and help to attenuate flood peaks. It will also de-couple the field to stream transport of suspended sediment and particulate nutrients.
The bund is being constructed within a field used primarily as pasture for cattle, situated to the east of Leades House. Here, an engineered land drain (c. 1m wide and 0.3m deep at normal flows) carries water around the northern and eastern border of the field. We will install a diverter to redirect higher flows from the stream via a swale at the northern edge of the field to the field. We will use the field gradient to direct water to the soil bund constructed along its southern and eastern boundary (Fig.: Yellow polygon). At the deepest part, the bund will measure 1m in height, tapering on either side, giving the bund an area of approximately 90m2. The storage volume of the RAF be affected by infiltration rates and water table levels at time of flooding. Field assessments will be carried out prior to and during operation of the bund to help in assessment of water-retention volumes. Flood waters will be gradually released back to the main channel from the RAF via an adjustable outflow drain. The effectiveness of the RAF to attenuate floods will be assessed from data collected by water level monitors upstream of the high-water diversion and at the lowest point of the bund.
Woody dams, diverters and Hyporheic Designs Elements (HDEs)
The downstream reach will comprise of a suite of Woody dams, diverters and Hyporheic Design Elements (HDEs). This is the first field experiment in Ireland that combines woody dams/diverters with HDE’s. The HDE’s will be installed on and within the stream bed to enhance microbially-driven turnover and attenuation of agriculture-derived nutrients at normal flows. Close to and above the HDE’s, Woody Dams and Woody Diverters will be built to span the channel above the height of normal flows so that flood flows can be attenuated (Fig.).
Offline Storage Area (OSA): During flood events a portion of the flood peak was directed into the OSA and stored there until in-stream water levels decreased back to base-flow conditions (Fig. 1). The OSA was installed to help reduce and extend flood peaks, and to test the effects of temporary storage on water quality. Formation of the feature began in February 2020 with the construction of an earthen bund at the lower part of a field bordering the stream. Over the following 12 months, a take-off point in the stream bank at the upper part of the field was formed and reinforced; an adjustable sluice was formed within the take-off point so the level at which water entered the field (OSA) could be controlled; a stone weir was formed in the stream to help divert high flows to the OSA via the sluice; and the bund was shaped and fitted with outlet pipes to allow flood waters fill against the bund and to gradually drain from the field following flood peaks.
Constructing the Bund using soil donated by local county council.
Hyporheic Design Elements (HDEs): Four stone low-head weirs within the stream channel which were installed to enhance transient storage and hyporheic exchange of dissolved water quality parameters during base-flow conditions (Fig. 2). The HDE’s were installed to encourage greater contact with biologically active zones of the stream channel i.e. the stream banks and the hyporheic zone within the streambed. The greater contact between stream water and these zones due to the back-up and head pressure created by each HDE was hypothesized to enhance transformation and attenuation of dissolved water quality parameters in the stream water.
Leaky Dams: Four wooden channel-spanning barriers which were designed to interact with high-flows (Fig. 3). The barriers allow baseflows to flow freely beneath timber logs which span the channel, while at higher flows water is forced to back up against the logs. These leaky dams were installed at c.15m intervals along an incised and linear reach of the stream to increase stream roughness during high-flows with the aim of delaying the onset of the downstream flood peaks.
Cellular series runoff attenuation feature (CS RAF): Amendments were made to narrow (0.5 X 0.5m width x depth) spring-fed drain an area of planted woodland to enhance its potential for attenuation of water quality pressures. The drain is disconnected from the nearest stream and intermittently receives agricultural runoff from adjacent trackways and hard-standings. To encourage contact between the water and the soil-surface, and enhance microbial uptake of water quality pressures, the drain was filled and widened to form a 2 x 0.1 m drain. Wooden logs were embedded within the drain perpendicular to flow at 2m intervals which helped to maximise contact between water and the soil-surface and reduce ponding at the lower end of the drain. A second drain was formed parallel to the wide drain with dimensions 0.5 x 0.5m to emulate the original drain and provide some basis for the real-time effects of the wide cells on water quality mitigation (Fig. 4). The formation of these broad, narrow cells was anticipated improve water quality through providing greater surface area for microbial uptake of dissolved nutrients, greater roughness and reduced flow velocity for sediment retention, and through enhancing oxygen enrichment from the atmosphere to encourage aerobic respiration.
Monitoring
Monitoring at Leades
Hydraulic effects here will be gauged with water level recorders upstream and downstream of in-stream structures. Water-level changes (For location see Map).
High flows diverted to RAF at high-water diversion at northern edge of field and will be retained against the bund spanning the southern and eastern side of the field. RAF-induced changes to flow volumes will be gauged from data collected by water level recorders (U/S RAF and RAF level recorder).
In-stream structures (HDE’s and leaky dams) installed to south of RAF as water quality and flood mitigation measures, respectively. HDE’s will be monitored using water-level sensors upstream and downstream of the bund and woody dam/diverters and HDE’s.
Water quality:
Piezometers will be embedded to known depths in the streambed upstream and downstream of the HDE’s to allow the collection of water samples from different portions of the hyporheic zone. HDE-induced changes in water quality parameters (e.g. DOC, BOD (Biological Oxygen Demand), SRP (Soluble Reactive Phosphorus), NH4+, NH3, Nitrate, Faecal Microbes) can be assessed from piezometer samples at UCC’s Aquatic Services Unit facilities.
Rain gauge:
One rain gauge (tipping gauge) installed to understand the “input” of precipitation and its impact on the stream discharge and flood peaks.
Observations during the study period
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