| Summary: | <p>Large wood is an integral part of freshwater ecosystems that provides a plethora of ecosystem services and affects river hydraulics, channel morphology and catchment hydrology. However, large wood has been historically removed from rivers and channels. Moreover, rivers have been straightened, channelized and dredged. The main reason for these modifications was to provide a constant output of water for agriculture and farming, increase channel conductance, and decrease the local flood risks by lowering water levels and increasing water flow velocities locally. However, the importance of large wood for riverine ecosystems has been recently re-discovered.</p>
<p>The newly developed interest in large wood presence and the understanding of the benefits it provides on channel hydraulics, sediment retention and biodiversity made the reintroduction of large wood a sought-after restoration technique. More recently, in-channel large wood has been introduced in form of Engineered Log Jams (ELJs) as part of Natural Flood Management (NFM) projects. ELJs are deployed in channels as part of NFM projects with the aim of managing flood risk by increasing in-channel hydraulic resistance at the reach scale and impacting the peak flow magnitude and timing at the catchment scale.</p>
<p>This research was undertaken in a large rural catchment situated in South East England, the Evenlode catchment (431 km2). In recent years, Thames Water and the Evenlode Catchment Partnership developed a 5 years NFM project (2016-2021) in a sub-catchment of the Evenlode: the Littlestock Brook (16.3 km2). The Littlestock Brook NFM project has involved the design of several NFM measures, including the installation of 42 high water ELJs.</p>
<p>This thesis addressed the effects of high water ELJs in the Littlestock Brook catchment from both a flood management and an ecological perspective. Moreover, this work considered the different scales at which ELJs can exert an effect on channel and floodplain hydraulics, and on the hydrology of small and large catchments.</p>
<p>The effect of the installation of high water ELJs on channel conveyance at the reach scale was measured in the field by quantifying hydraulic resistance with Manning’s n values. The data collected in the field showed Manning’s n values between 0.09 and 0.6 for a short reach (30 m) impacted by one ELJ. The Manning’s n values measured for the ELJ were in line with values found in the literature for naturally occurring large wood. The increase in channel roughness that follows ELJs installation was shown to cause a decrease in water flow velocity of up to 40% upstream of the ELJs for a 25-year flood event. Since the increase in channel roughness has the potential to lead to an increase in habitat heterogeneity at the local channel scale, the response of macroinvertebrate communities to the presence of ELJs was also analysed. Macroinvertebrate communities at ELJs sites showed a higher taxonomical diversity compared to control sites. This higher taxonomical diversity was due to higher species richness. This higher diversity at ELJ sites might be correlated to an increase in habitat heterogeneity as a result of the presence of ELJs.</p>
<p>The effects of the widespread installation of ELJs in the Littlestock Brook catchment were quantified by using a hydrological-hydraulic modelling cascade. The effect of a wide installation of ELJs across the catchment was remarkable, with a reduction of 10-20%, depending on the number of ELJs deployed, of the peak flow height and a delay in the peak flow timing of 5-6 hours for a 1-year flood event. The effects of catchment-wide installation of ELJs were instead negligible for the 25-year flood event.</p>
<p>The sensitivity of a large catchment to the widespread change of the in-channel roughness was tested by simulating the hydrology of the Evenlode catchment with a hydrological model. The widespread change of the in-channel roughness gave a big reduction in peak flow height (12-17%) for the 25-year flood event. However, such a big change in channel roughness would be really difficult to achieve with river restoration measures that influence the roughness, such as ELJs. In conclusion, high water ELJs represent an important NFM technique with the potential to restore innate hydraulic and morphological characteristics, impact small catchment hydrology, re-connect channels to floodplains and provide additional environmental co-benefits.</p>
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