Geomorphological response to system‐scale river rehabilitation I: Sediment supply from a reconnected tributary

Abstract

This paper is the first of a pair that report the findings of a river rehabilitation project centred on the reconnection of a formerly diverted headwater tributary (Ben Gill) to its main-stem river (the River Ehen). The present paper describes the geomorphic evolution of the tributary in the 2 years following its reconnection, with a particular focus on assessing the volumes of sediment now being supplied to the main-stem Ehen. Structure-from-Motion photogrammetry was used to produce Digital Elevation Models (DEMs) of the newly connected tributary, with successive DEMs compared to assess topographic changes in the channel and quantify volumes of material exported. 3D errors in the DEMs were small relative to the scour and fill observed in the channel (error 0.016-0.056 m compared to up to 1.7 m vertical change between consecutive surveys). Erosion was the dominant process in the tributary channel, though this varied spatially and temporally. Over the 2-year period, an estimated minimum of 384 m 3 of coarse sediment was exported from Ben Gill and delivered to the confluence zone, where a new bar feature developed as a result. This estimate is twice as high as earlier ones. Analysis of the growth of this bar suggested that much of the material supplied by Ben Gill remains here temporarily, with onward conveyance constrained by the competence of the regulated main-stem. The work shows that, thanks to the reconnection, this small (0.55 km 2) ephemeral tributary (flowing for only around 20% of the time) has become a key source of sediment for the main-stem Ehen. The second in the pair of papers focuses on the geomorphic responses of the main-stem to this renewed supply of sediment.

Geomorphological response to system‐scale river rehabilitation II: Main‐stem channel adjustments following reconnection of an ephemeral tributary

“…Changes in suspended sediment transport and bed storage were also monitored and have been reported elsewhere (Marteau et al, 2018; Marteau, Batalla, et al, 2017). The volumes and characteristics of sediment delivered by Ben Gill following the reconnection are described in detail by Marteau et al (2020). Only summary information on this source material is presented here, to provide a context for changes observed in the Ehen.…”

“…On each occasion (December 2014, March 2015, July 2015, Jan 2016, May 2016 and July 2016, see Figure 2 and Table S1), transects were resurveyed 2–4 times, with data used to determine the average bed elevation at points and a measure of uncertainty (i.e., standard deviation of bed elevations) at a spacing of 0.2 m (> D 84 of coarsest unit). Similar to the procedure used to determine the minimum level of detection (minLoD) for successive Digital Elevation Models(DEMs, see the application of this in Marteau et al, 2020), difference in elevation (topographic change) was defined as certain only if change was higher than the minLoD: with SD i and SD i+1 the standard deviation of surveys i and i+1 respectively (based on repeated surveys), and t = 1 (confidence interval = 64%).…”

“…Analyses presented in Marteau et al (2020) indicated that Ben Gill provided a minimum volume of 200 m 3 y −1 of bed material during the study period, of which only approximately 24% was readily available for transport in the Ehen. The tributary is capable of delivering a large variety of sediment clasts, but only pebbles to fine cobbles are present at the surface of the newly developed confluence bar (excluding fine sediments), and thus readily available for transport in the Ehen.…”

“…According to field observations, most particles deposited in response to the reconnection are smaller than 64 mm. Changes in bed material fluxes from Ben Gill during the period September 2014 to October 2016 are fully described in Marteau et al (2020).…”

“…Initial monitoring of the Ehen (i.e., in the first 2 years post‐reconnection) demonstrated how the small increase in catchment size (1.2%) stemming from the reconnection resulted in a 65% increase in main‐stem suspended sediment yield (Marteau, Batalla, Vericat, & Gibbins, 2017). Other studies developed and applied methods to assess changes in the newly reconnected channel (Marteau, Vericat, Gibbins, Batalla, & Green, 2017) and the amount of material being supplied to the Ehen (Marteau, Gibbins, Vericat, & Batalla, 2020). The current paper focusses on assessing how this new situation affects main‐stem morpho‐dynamics.…”