QUANTIFYING THE IMPA CT OF NATURAL FLOOD MANAGEMENT DURING STORM EVENTS A comparison of in - channel storage attenuation and gauged flow upstream of Flimby, Cumbria (UK) Daniel Crewe | MSc. Sustainable Water Management, Lancaster University ACKNOWLEDGEMENTS & FURTHER INFORMATION Thank you Professor Nick Chappell (Lancaster University) and Dr Caitlin Pearson (West Cumbria Rivers Trust) for your support and encouragement. Contact : d.crewe@lancaster.ac.uk OVERVIEW • This project uses water level timeseries and bathymetric survey data to es- timate the storage volumes contained within a series of nine runoff atten- u a t i o n f e a t u r e s ( R A F s ) l o c a t e d a l o n g t h e c h a n n e l o f a s m a l l b e c k • A 16 month timeseries, collected in collaboration with the West Cumbria Rivers Trust, will be analysed in the context of further data covering this time period. EXPERIMENTAL SETUP • Hydrometrics: Recorded from a telemetry system downstream of the RAFs. In- cludes rain gauge (net rainfall) and a flume recording discharge. • Water level: Nine ‘online’ RAFs (semi - engineered ‘leaky’ woody dams) in series along Penny Gill each with two HOBO pressure transducers logging upstream and downstream water level at a 15 minute timestep, and a central ‘atmospheric’ pres- sure logger to use as a barometric compensation. Every 4 months observations made of the RAF morphology to ensure appropriate offset, and logger data down- loaded before being redeployed. • Storage area: Each RAF surveyed using total station at start of the project (see be- low). After data processing and removal of outliers, each storage area was interpo- lated to create a DTM. See below for an example of how one dam looks when in- terpolated using the Kriging method and plotted as a 3D surface. NEXT STEPS Chappell and Kennedy (2018) identify field - observed hydro- graph reductions as the 'gold standard' in meeting the EA's objective to improve NFM data availability. Through the immediate objectives of the project achieved to date, it has been demon- strated that the chosen methodological approach is capable of contributing to this broader goal. Beyond this, it will be interesting to explore water level data from the culvert outside Flimby - as this is essentially a fixed outlet for the beck above which flooding is guaran- teed. This will be particularly interesting with respect to the March 2021 event. STUDY SITE: PENNY GILL • Nearest town : Flimby (pop. 2,000). Village on the Cumbrian west coast. • Catchment size : c.4.5km 2 • Local hydrology : Four watercourses draining into the Irish Sea. High annual rainfall (1,000mm) and long record of flooding – 90 homes flooded in 2015. • Land use : Woodland and improved pasture. ‘Flashy’ due to impermeable/ waterlogged soils • NFM : Riparian planting, soil aeration, and 49 in - channel features (‘kerplunks’ and barriers) installed by West Cumbria Rivers Trust in partnership with the EA INTRODUCTION • Natural Flood Management (NFM) complements traditional approaches to flood risk management while also offering a suite of wider environmental benefits. However, evidence for its effectiveness is lacking (Dadson et al ., 2017). • Survey findings suggested that in 2016 as few as 6% of NFM schemes had intensive hydrological monitoring in place (Hankin et al ., 2017). Furthermore, in a recent re- view of the literature, the author finds that just 7 of 14 pilot NFM studies utilising in - channel ‘leaky’ barriers had attempted to quantify the storage attenuated by said features. • The EA (2018) identified improving the available data for in - channel RAFs as a prior- i t y o b j e c t i v e f o r r e d u c i n g f l o o d r i s k , a s t h i s w i l l f a c i l i t a t e d i r e c t c o m p a r i s o n w i t h o t h- er approaches to flood risk management. This project therefore aims to confirm whether leaky dams are effective in attenuating peak flows. • The immediate objectives of the project are: 1. To derive a time - series of volume behind each dam 2. To derive the difference in volume at each time step CASE STUDY: MARCH 2021 EVENT The hydrograph above shows a storm event recorded dur- ing the evening of March 10th 2021. This was a particularly intense rainfall event (14.9 mm in 1 hr) and reports indi- cate that the village was on the verge of flooding. RESULTS • Storage volume over time: By calculating volume as a function of water level it is possible to derive a timeseries of storage volumes (as per Magliano et al , 2019). • At the time of writing, a 16 month timeseries has been derived for one of the nine RAFs, demonstrating the methodological approach is appropriate. • These values provide a good estimate for the real storage attenuated by the RAF. • Chappell, N. and Kennedy, D. (2018) Strategy for observing NFM - related reductions in flood peaks. • Dadson, S., Hall, J.W., Murgatroyd, A., Acreman, M., Bates, P., Bevan, K., Heathwaite, L., Holden, J., Holman, I.P., Lane, S.N., O’Connell, E., Penning - Rowsell, E., Reynard, N., Sear, D., Thorne, C., and Wilby, R., (2017) “A restatement of the natural science evidence concerning catchment - based ‘natural’ flood management in the UK” in Proceedings of the Royal Society A. 473. (2199) • EA (2018c) Monitoring and evaluating the DEFRA funded Natural Flood Management projects. • Hankin, B., Metcalfe, P., Johnson, D., Chappell, N., Page, T., Craigen, I., Lamb, R., and Bevan , K. (2017) “Strategies for testing the impact of natural flood management measures” in Hromadka, T. and Rao, P. (Eds) Flood Risk Management • Magliano, P., Mindham, D., Tych, W., Murray, F., Nosetto, D., Jobággy, E., Niorski, M., Rufino, M., and Chappell, N. (2019) “Hydrological functioning of cattle ranching impoundments in the Dry Chaco rangelands of Argentina” in Hydrology Research . 50. REFERENCES Left : An online RAF located on Penny Gill during a storm event. Below : Schematic diagram of different flow modes for an online RAF and the key parameters (Hankin et al., 2019). Right: Maximum storage volume estimate during the March 2021 storm event (linear interpolation) Below : Total station survey points plotted in 3D (Kriging in- terpolation method)