The effects of river restoration on catchment scale flood risk and flood hydrology

TLDR

Increasing development and climate‑driven extreme weather are raising flood risk, making reliance on point‑based engineered defenses unrealistic, while soft engineering such as riparian forests has shown promise at small scales but remains untested at catchment scale. The study investigates how river restoration—through land‑use and channel changes—affects catchment‑scale flood flows using reduced‑complexity hydrological modelling. Reduced‑complexity hydrological models were employed to evaluate the influence of restoration‑induced land‑use and channel modifications on catchment flood dynamics. Engineered logjams produce variable effects, whereas riparian and floodplain forest restoration—especially at sub‑catchment scales covering 20–40% of the area—consistently reduce peak flood magnitude by up to 19% via wave‑synchronization, with smaller forested floodplain patches yielding 6% reductions after 25 years. © 2016 John Wiley & Sons, Ltd.

Abstract

Abstract A rising exposure to flood risk is a predicted consequence of increased development in vulnerable areas and an increase in the frequency of extreme weather events due to climate change. In the face of this challenge, a continued reliance on engineered at‐a‐point flood defences is seen as both unrealistic and undesirable. The contribution of ‘soft engineering’ solutions (e.g. riparian forests, wood in rivers) to integrated, catchment scale flood risk management has been demonstrated at small scales but not larger ones. In this study we use reduced complexity hydrological modelling to analyse the effects of land use and channel changes resulting from river restoration upon flood flows at the catchment scale. Results show short sections of river‐floodplain restoration using engineered logjams, typical of many current restoration schemes, have highly variable impacts on catchment‐scale flood peak magnitude and so need to be used with caution as a flood management solution. Forested floodplains have a more general impact upon flood hydrology, with areas in the middle and upper catchment tending to show reductions in peak magnitude at the catchment outflow. The most promising restoration scenarios for flood risk management are for riparian forest restoration at the sub‐catchment scale, representing 20–40% of the total catchment area, where reductions in peak magnitude of up to 19% are observed through de‐synchronization of the timings of sub‐catchment flood waves. Sub‐catchment floodplain forest restoration over 10–15% of total catchment area can lead to reductions in peak magnitude of 6% at 25 years post‐restoration. Copyright © 2016 John Wiley & Sons, Ltd.