Natural Flood Management, in context Evaluating and enhancing the impact. Peter Metcalfe (1), Keith Beven(1)(2), Barry Hankin (3) and Rob Lamb (4) (1) Lancaster Environment Centre, Lancaster University (2) Department of Earth Sciences, Uppsala University (3) JBA Consulting, Warrington, UK (4) JBA Trust, Skipton, UK 1

Target catchment Agricultural catchment North Yorkshire, UK, ~ 26km² History of severe flooding, most recently in 2012 Intensive arable farming: heavily modified channel network, extensive subsurface drainage; high land-channel connectivity Apply Natural Flood Management to mitigate flood risk w/o hard- engineered structures? Regulatory, land-use and access constraints = in-channel features only allowed, in one subcatchment Water End, Nov 26th 2012 (Jan Hodgson) 2

Runoff / routing model Depth averaged 1D diffusion wave formulation Hillslope runoff and base flows = r using semi-distributed hydrological model (Dynamic TOPMODEL) Trapezoidal, prismatic composite channel sections, flow area = A In-channel features represented using stage-discharge (h - Q) relationship for structure Underflow barriers significantly attenuate open channel flow and add channel storage Ditch barriers Underflow sluice: Swamee gives discharge coefficient Swamee (1992), Figure 1. Free discharge through underflow sluice 3

Testing model response Double-peaked storm event November 2012 Hydrological parameters calibrated against discharges reconstructed from stage gauge at outlet. Hydraulic parameters calibrated to match observed timings and magnitudes of flood peaks: NSE > 0.9 Underflow barriers applied in batches from highest reaches downstream until network filled Examine effect on flood wave timing and attenuation 4

Most barriers in upper reaches never used…. Results - run 1 All barriers same geometry: hmax=1.6m, hb=0.4m. Separation = 300m Most barriers in upper reaches never used…. ... whilst many of those further downstream are overflowing. How can such a large intervention reduced the peak by only 0.16mm/hr? 5

Results – run 2 Adding 40 barriers has synchronised the flood peaks! Consider confluence of un-named tributary with main channel of Ing Beck Main channel peak arrives after tributary Adding 40 barriers has synchronised the flood peaks! 6

Alternative approach Marginal land identified upstream of railway viaduct Storage potential enhanced by small bund Divert flow into area by afflux from large feature ~ 5m high Or use the viaduct as the barrier! Halstead flood storage basin, Essex, UK 7

Results with larger barrier We have attenuated the main peak sufficiently to just avoid flooding in Water End At the cost of a large area flooded for 36 hours…. 8

Interventions may cause subcatchment flood peaks to synchronise. Conclusions? NFM may be difficult to implement effectively in some types of catchment. Interventions may cause subcatchment flood peaks to synchronise. Fewer, larger features may be more beneficial, and cost-effective, than many smaller features. 9