No abstract

Runoff attenuation features (RAFs) are low-cost, soft-engineered catchment modifications designed to intercept polluted hydrological flow pathways. They are used to slow, store and filter runoff from agricultural land in order to reduce flood risk and improve water quality, specifically by mitigating diffuse water pollution from agriculture. This study focuses on a sub catchment (30 ha) of the Belford Burn catchment (5.7 km 2 ) where the capacity of two RAFs to reduce concentrations of suspended sediment (SS), phosphorus (P) and nitrate (NO3) in runoff has been investigated. A field bund RAF, designed to intercept overland flow during storm events, has been shown to retain significant volumes of sediment; however, the underlying field drains are still exporting high concentrations of sediment and nutrients, sometimes exceeding 500 mg SS l -1 , 1 mg TP l -1 and 40 mg NO3 l -1 . An on-line sediment pond is accumulating sediment during normal flow conditions, but event sampling has revealed a lack of retention of any pollutants during storm events, which has been attributed to remobilisation of previously deposited material. In order to address these problems and improve the quality of the water leaving the sub catchment, a novel multi-stage RAF has been constructed in the ditch network. A low-cost filter trap, using wood chippings, has been installed and will be the focus of on-going monitoring and investigations. The ability to help tackle flooding and pollution by managing runoff flow pathways does have great potential, despite being somewhat difficult to evaluate.

No abstract

High resolution water quality data has recently become widely available from numerous catchment based monitoring schemes. However, the models that can reproduce time series of concentrations or fluxes have not kept pace with the advances in monitoring data. Model performance at predicting phosphorus (P) and sediment concentrations has frequently been poor with models not fit for purpose except for predicting annual losses. Here, the data from the Eden Demonstration Test Catchments (DTC) project have been used to calibrate the Catchment Runoff Attenuation Flux Tool (CRAFT), a new, parsimonious model developed with the aim of modelling both the generation and attenuation of nutrients and sediments in small to medium sized catchments. The CRAFT has the ability to run on an hourly timestep and can calculate the mass of sediments and nutrients transported by three flow pathways representing rapid surface runoff, fast subsurface drainage and slow groundwater flow (baseflow). The attenuation feature of the model is introduced here; this enables surface runoff and contaminants transported via this pathway to be delayed in reaching the catchment outlet. It was used to investigate some hypotheses of nutrient and sediment transport in the Newby Beck Catchment (NBC) Model performance was assessed using a suite of metrics including visual best fit and the Nash-Sutcliffe efficiency. It was found that this approach for water quality models may be the best assessment method as opposed to using a single metric. Furthermore, it was found that, when the aim of the simulations was to reproduce the time series of total P (TP) or total reactive P (TRP) to get the best visual fit, that attenuation was required. The model will be used in the future to explore the impacts on water quality of different mitigation options in the catchment; these will include attenuation of surface runoff.

It is well known that soil, hillslopes, and watercourses in small catchments possess a degree of natural attenuation that affects both the shape of the outlet hydrograph and the transport of nutrients and sediments. The widespread adoption of Natural Based Solutions (NBS) practices in the headwaters of these catchments is expected to add additional attenuation primarily through increasing the amount of new storage available to accommodate flood flows. The actual type of NBS features used to add storage could include swales, ditches, and small ponds (acting as sediment traps). Here, recent data collected from monitored features (from the Demonstration Test Catchments project in the Newby Beck catchment (Eden) in northwest England) were used to provide first estimates of the percentages of the suspended sediment (SS) and total phosphorus (TP) loads that could be trapped by additional features. The Catchment Runoff Attenuation Flux Tool (CRAFT) was then used to model this catchment (Newby Beck) to investigate whether adding additional attenuation, along with the ability to trap and retain SS (and attached P), will have any effect on the flood peak and associated peak concentrations of SS and TP. The modelling tested the hypothesis that increasing the amount of new storage (thus adding attenuation capacity) in the catchment will have a beneficial effect. The model results implied that a small decrease of the order of 5–10% in the peak concentrations of SS and TP was observable after adding 2000 m3 to 8000 m3 of additional storage to the catchment.