The spatial and temporal patterns of aggradation in a temperate, upland, gravel‐bed river

Abstract

Intensive field monitoring of a reach of upland gravel-bed river illustrates the temporal and spatial variability of in-channel sedimentation. Over the six-year monitoring period, the mean bed level in the channel has risen by 0·17 m with a maximum bed level rise of 0·5 m noted at one location over a five month winter period. These rapid levels of aggradation have a profound impact on the number and duration of overbank flows with flood frequency increasing on average 2·6 times and overbank flow time increasing by 12·8 hours. This work raises the profile of coarse sediment transfer in the design and operation of river management, specifically engineering schemes. It emphasizes the need for the implementation of strategic monitoring programmes before engineering work occurs to identify zones where aggradation is likely to be problematic. Exploration of the sediment supply and transfer system can explain patterns of channel sedimentation. The complex spatial, seasonal and annual variability in sediment supply and transfer raise uncertainties into the system's response to potential changes in climate and land-use. Thus, there is a demand for schemes that monitor coarse sediment transfer and channel response. AGGRADATION IN A TEMPERATE, UPLAND, GRAVEL-BED RIVER 1187 Note: The table shows the 12 cross-sections which have experienced the highest rates of erosion and deposition since December 2002 alongside the slope and curvature rank for that location. Large slope rank values indicate the greatest increase in slope whilst small slope rank values correspond with reduction in slopes. Low curvature rank values indicate curved reaches whilst high values indicate straight reaches. For both the slope and curvature, values range from 1-34. Italic typeface values indicate higher importance.Figure 6. Seasonal width-averaged bed level change. Individual data points indicate seasonal change whilst the solid line is the cumulative bed level change over time. Vertical error bars in (a) represent the bed level change uncertainty.

Sediment budget monitoring of debris-flow and bedload transport in the Manival Torrent, SE France

“…Cross-sections were used for quantifying volumes of erosion and deposition in the channel and back-calculating bedmaterial sediment transport using the morphological method (Ashmore and Church, 1998), widely applied for balancing sediment budgets in gravel-bed rivers (Ferguson and Ashworth, 1992;Martin and Church, 1995;Raven et al, 2009;Reid et al, 2007). Volumes of deposition (V D ) and erosion (V E ) between cross-sections are obtained by the following:…”

Local perception of infrequent, extreme upland flash flooding: prisoners of experience?

“…Such views clearly result from reliable observations and personal experience, and these opinions are common in the UK after floods (see, for example, Environment Agency, 2007). Importantly, other research in North Yorkshire has found that the aggradation of sediment in an upland river (the River Wharfe near Buckden; see Figure 1a) influences changes in flooding (Lane et al, 2007;Raven et al, 2009), and modelling based on participatory research in Pickering found that vegetation and sediment build-up around river channels can increase flood risk (Whatmore, Ward, and Lane, 2008). It is debatable whether recent reports of public frustration regarding the lack of river dredging and blaming of the Environment Agency after floods in England (Merrill, 2014;Smith, 2014) suggest a tendency to blame others in the face of flood risk (Harries, 2013) or reflect views of 'ignorant but arrogant experts' disregarding local knowledge and expertise (Wynne, 1992, p. 295).…”

Quantifying River Channel Stability at the Basin Scale

“…To overcome this potential problem and maintain a progressive increase in flow with rising stage, it is prudent to further partition the cross-section when compositing discharge and investigate its impact. Inevitably, though, flow partitioning to generate a channel discharge (and thus specific stream power) over the active bed will be subject to uncertainties in selecting Manning n coefficients for the different flow areas in Equation (9).…”

“…Conversely, if sediment supply is greater than the transporting capacity, then sedimentation processes are more likely to predominate with potential for channel instability through aggradation and/or channel narrowing. Deposited sediment can be a critical factor in flooding [8][9][10], reducing in-channel conveyance and the standard of protection afforded by defences, resulting in maintenance problems [11,12]. Additional complex responses to channel instability include tributary rejuvenation through headcutting, accelerated rates of lateral mobility in meandering rivers [13,14] and channel widening as flow is deflected around maturing sediment bars, both of which could threaten linear flood defences not set back from the channel.…”