Impacts of River Engineering on River Channel Behaviour: Implications for Managing Downstream Flood Risk

Abstract

Although knowledge of sediment transport has improved over the last 25 years, our understanding of bedload transfer and sediment delivery is still based on a limited set of observations or on models that make assumptions on hydraulic and sediment transport processes. This study utilises repeat lidar survey data of the River Caldew above the City of Carlisle in the UK to investigate the balance of erosion and deposition associated with channel switching from an engineered and managed single thread channel to a naturalising incipient wandering system. Over the 11-year survey period (four bankfull flood events) around 271,000 m3 of sediment were delivered to the river and floodplain and 197,000 m3 eroded suggesting that storage rates of around 7000 m3/annum occurred. The balance of erosion and deposition is influenced by channelisation with very restricted overbank sedimentation and only limited local and transient in-channel bar deposition along the engineered reach (8000 m3 eroded). This contrasts with the activity of the naturalising reach downstream where a developing wandering channel system is acting to store coarse sediment in-stream as large bar complexes and the associated upstream aggrading plane bed reaches and overbank as splay deposits (87,000 m3 stored). Such behavior suggests that naturalisation of channelised systems upstream of flood vulnerable urban areas can have a significant impact on sediment induced flooding downstream. This conclusion must, however, be moderated in the light of the relatively small volumes of material needed to instigate local aggradation in over-capacity urban channels.

Urban flooding in Britain: an approach to comparing ancient and contemporary flood exposure

“…What also remains unknown is the extent to which channel capacities have changed, thus altering the proportion of floodwaters being spilled across floodplain sites. This could well be an important factor (Sofia et al 2020), as well as the post-Roman history of river engineering (Lewin 2010(Lewin , 2013van Dinter et al 2017;Mandarino et al 2020;Heritage and Entwhistle 2020).…”

River science and flood risk management policy in England

“…Historical field evidence in the region (Joyce et al, 2018) has been crucial in understanding how sediment from mountain sources is transported downstream into river reaches identified by farmers and local flood action groups for river channel maintenance. A similar approach was taken to identify catchment-scale flood sediment routing upstream from Carlisle (Heritage and Entwhistle, 2020). By contrast, the Millennium Floods caused few instances of river channel change in the lowlands (Sear et al, 2002).…”

“…The land drainage premise that flood conveyance is impeded by in-channel deposits has been countered by studies demonstrating that floods are mainly throughput events and may actually improve flood conveyance (Heritage and Entwhistle, 2020). Indeed, scientifically based (as opposed to opinion based) hydraulic guidance on conveyance for floodwaters (including sediment transport) has been available for some time (Defra/Environment Agency, 2004; Samuels et al, 2002).…”

Fluvial Geomorphology and River Management

“…The behaviour of Mediterranean fluvial systems was stressed with three distinct approaches by Bentivenga et al [4] and Filocamo et al [5] in southern Italy, and by Sánchez-Donoso et al [6] in southern Spain. Concerning humid continental climatic zone areas, the papers of Heasley et al [7], Zingaro et al [8], and Heritage and Entwistle [9] analyse the fluvial systems of the UK under different point of views, whereas those of Szabó et al [10] and Bihałowicz and Wierzbicki [11] address fluvial systems of central Europe in Hungary and Poland, respectively. The fluvial system in the humid tropical climate of Brazil was analysed by Marinho et al [12], and that in the arid climate of Northern Ethiopia by Annys et al [13].…”

“…In this way, the paper highlights the importance of optimising dam operation and management in order to safeguard downstream riparian users and ecosystems. The assessment of erosion and deposition balance, in association with channel switching in a channel, illustrates their implications for local and downstream flooding, which was proposed by Heritage and Entwistle [9]. They investigated river channel behaviour in the Caldew River of the UK using lidar surveys.…”