Quantifying the hydrological implications of pre- and post-installation willowed engineered log jams in the Pennine Uplands, NW England

Norbury, M., Phillips, H., Macdonald, N., Brown, D., Boothroyd, R. , Wilson, C., Quinn, P. and Shaw, D. (2021) Quantifying the hydrological implications of pre- and post-installation willowed engineered log jams in the Pennine Uplands, NW England. Journal of Hydrology, 603(Part C), 126855. (doi: 10.1016/j.jhydrol.2021.126855)

Abstract

Nature Based Solutions (NBS), including Natural Flood Management (NFM) schemes are becoming an important component of many governmental and organisation responses to increases in flood and aridity risk. NFM structures may take multiple forms to slow, store, disconnect and filter distributed overland flow pathways within a catchment that coalesce to generate a flood-wave downstream and runoff rather than infiltrate groundwaters. To date few studies have conducted observations pre- and post-installation monitoring at river reach-scales, despite widespread and frequent installation, to investigate the efficacy of willowed engineered log jams (WELJs) interventions used in abating flood-flows, through backing-up flood-pulses with consequent reductions in downstream discharges. This paper examines the efficiency, before and after installation of five 1 metre high WELJs incorporating 1,000 Bay willow (Salix pentandra) saplings supporting the dead horizontal timber, across a total of 130 linear metres spanning the floodplain of a decommissioned reservoir. One rain gauge, two fixed point time-lapse wildlife cameras and three water level stations were installed: upstream-of; within, and downstream-of all WELJs. The findings demonstrate a substantial reduction is achieved for most events, with an average of 27.3% reduction in peak discharge being achieved post-installation. The time to peak is little impacted, however there is demonstrable evidence of a longer and higher recessional limb to the events. These findings quantify for the first time the role that WELJs can play in a move towards re-naturalisation of water level regimes, with lower peak water flows achieved, and waters released from the river-reach more slowly. Furthermore, baseflow during dry periods is also elevated by 27.1%, offering greater resilience to dry periods and droughts. Consequently, over the river-reach scale (0-130 m), WELJs play an important role in alleviating flood and drought risk through suppressing flood peaks and increasing baseflow during low flows; steps towards improved hydro-morphological quality overall.

• Item Type: Articles
• Additional Information: This work was in part supported by an intensive 3-month monitoring period made possible by a NPIF Placement with the Mersey Forest supporting the Engineering and Physical Research Sciences Council Funding (EPSRC) PhD studentship to HP (EP/L015927/1 - P90791D). The Natural Flood Management Works were made possible by Environment Agency and Treasury Flood Defence Grant in Aid Funds (ITQ 0219 CW-NFM Smithills Estate) and Heritage Lottery Fund. The engineered log-jams were installed by the Mersey Forest in conjunction with the Environment Agency, Woodland Trust, Heritage Lottery Fund, Pownall Plant Limited, Bolton Council and Natural England. Other partners who have contributed to the project include: Bolton Council, Natural England, Ribble Rivers Trust, the University of Glasgow and Newcastle.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Boothroyd, Dr Richard
• Authors: Norbury, M., Phillips, H., Macdonald, N., Brown, D., Boothroyd, R., Wilson, C., Quinn, P., and Shaw, D.
• College/School: College of Science and Engineering > School of Geographical and Earth Sciences
• Journal Name: Journal of Hydrology
• Publisher: Elsevier
• ISSN: 0022-1694
• ISSN (Online): 1879-2707
• Published Online: 02 September 2021
• Copyright Holders: Copyright © 2021 Crown Copyright
• First Published: First published in Journal of Hydrology 603(Part C): 126855
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher