Asynchronicity of fine sediment supply and its effects on transport and storage in a regulated river

Abstract

Purpose A disconnected ephemeral tributary was reconnected to the regulated River Ehen (NW England) as part of a river restoration initiative, providing a renewed delivery of sediment to a highly stable and armoured channel. This paper (1) assesses spatial and temporal dynamics of suspended and stored sediments in the Ehen, (2) characterises the composition of stored sediment, (3) develops fine sediment budgets for downstream river reaches, and (4) assesses the controls on the storage of fine sediment in the riverbed. Materials and methods A 3-km study section in the upper part of the River Ehen was divided into two reaches. Suspended sediments were monitored at the downstream limits of each reach over a 2-year period. In-channel storage was measured in three morphological units within the upper reach, on 13 occasions over the same period. Samples were used to assess changes in volumes of stored fine sediment, as well as the grain sizes and organic content of the material. A time-lapse camera facing the confluence of the tributary was used to conceptualise different flow scenarios. These scenarios reflect the degree of synchronicity between flows in the main-stem and those in the tributary. Fine sediment budgets were developed for each reach to assess the relative contribution of different sources of sediment. Results and discussion The reconnection significantly affected suspended sediment loads in the Ehen. Bed storage increased twofold, with changes most evident in the slow-flowing morphological unit. Changes in the composition of stored sediment were less marked than changes in the quantity of material. Changes in bed storage were controlled by the degree of synchronicity between flows in the Ehen and those in the newly reconnected tributary. Results show that three generalised flow scenarios occur, with total asynchronicity between flows in the tributary and the Ehen being responsible for the main episodes of fine sediment deposition. Overall, the estimated sediment budgets provide insights into the importance of non-perennial sources of sediment in supply-limited systems such as the Ehen. Although bed storage values are within the range of those published for UK rivers, the increase observed since the reconnection, together with the persistence of a static pavement, highlights the ecologically critical conditions of the regulated main-stem River Ehen. Conclusions Intermittent sources control fine sediment transport dynamics in the upper River Ehen. In this regulated river, ongoing deposition associated with increased low-and medium-sized flow events exerts more of a control on bed storage than large but rare floods. Management actions to limit delivery of material from lateral sources could help prevent further deterioration of habitat conditions for biota sensitive to fine sediment. Given the ongoing adjustment in the newly reconnected tributary, continued monitoring is needed to capture further morphosedimentary response in the main-stem.

Effects of Hydropeaking on Bed Mobility: Evidence from a Pyrenean River

“…Sediment entrainment and depletion cycles vary not just at-a-section as a function of the frequency and magnitude of the hydropeaks, but also longitudinally in relation to channel geometry, river-bed characteristics (amplitude of grain-size) and the presence of tributaries. Tributaries cause a change in sediment supply that ultimately has a direct effect on sediment availability (e.g., [48]). The range of particles mobilised during hydropeaking ranged between fine gravels to, eventually, medium to large gravels.…”

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

“…Post-reconnection bed stability and texture are compared to these earlier studies. Changes in suspended sediment transport and bed storage were also monitored and have been reported elsewhere (Marteau et al, 2018;. The volumes and characteristics of sediment delivered by Ben Gill following the reconnection are described in detail by Marteau et al (2020).…”

“…The lower limit of this technique is considered to be 8 mm, although particles <8 mm were still picked and counted to know their proportion. The three main morphological units present in the study reach (see Figure 1c No attempt was made to sample subsurface material due to (a) the absence of exposed gravel bars even during low flows, and (b) the risks associated with underwater subsurface sampling and the release of fine sediments, which are known to be present at rather high levels (Marteau et al, 2018) and which on release may impact freshwater pearl mussels.…”

“…These included the construction of a 1.3-m high weir (in 1902) and the diversion of Ben Gill (main headwater tributary of the Ehen) in the 1970s towards the lake. Further details of this initial diversion, along with the reconnection of Ben Gill in 2014, are given by and Marteau, Batalla, Vericat, and Gibbins (2018).…”

Data-driven framework for forecasting sedimentation at culverts

“…Ideally, data-driven investigations of sediment-related issues should rely on the direct measurements of sediment-related variables (i.e., bedload and suspended load) in streams (Marteau et al, 2018). However, direct observations of sediment-related parameters are relatively scarce across Iowa (shown in Table 2.7).…”