Network Scale Sediment Transport Modelling with the Perspective of Improved Sediment Connectivity and Delivery: A case of small dam removal.

Abstract Sediment movement from the headwater region (source) to the catchment outlet (sink) constantly transforms the river geomorphology and produces various geomorphic features. The geomorphic diversity in the river supports hydro-morphic sediment conditions that formulate suitable physical habitat for the riverine ecosystem. Water, sediment, and organic matter must flow uninterrupted for a healthy ecosystem. However, humans have raised civil engineering structures on many rivers. River fragmentation (dam/weirs) in the European river has crossed the 630,000 marks, and the projected number is 1 million. Fragmentation restricts the free movement of discharge, sediment, organic matter, and riverine species. The impacts of dams on river geomorphology, discharge and sediments are well documented. At the same time, the declining riverine species trend is a significant cause of concern. Recent studies revealed a 95% decline since 1970 (in the living planet index). Though, dam removal is gaining momentum to reverse the negative impacts on the riverine ecosystem. Dam removal studies have shown positive outcomes on river geomorphology and ecology. However, it would not be admissible to remove all the obstacles present on the river course; for example, there is a considerable cost involved and the economic benefits offered during the lifespan of a dam. Political will is another obstacle in river restoration projects beyond the cost and benefits analysis of the dams and weirs. The previous dam removal studies addressed site-specific geomorphological and ecological recovery, though case studies often lack long term monitoring. Thus, a few cases reported no significant differences in the river. Still, the presence of a dam or weirs footprint impacts the upstream and downstream river course, which turns a river's lotic environment into a lentic. In addition, the existence of multiple dams or weirs further deteriorates the river's condition by restricting water and matter movement. Thus, dam or weir removal impact assessment must be studied for the catchment and river network scale. The current study is applied to the Eamont river catchment (396.2 km2), located in the lake district region, which receives maximum rainfall in the U.K. region. Hard rocks dictate the regional geology, and rivers are confined. Thus, no significant changes in river form have occurred in the Eamont catchment rivers. However, rivers of the Eamont catchment have high stream power and sediment continuity. 2 major and 20 small weirs interrupt the natural sediment transport. Additionally, enormous Ullswater and other lakes provide local sinks in the catchment. The catchment area and the number of obstacles present in the Eamont river catchment offers a suitable catchment setting to address the European river fragmentation case. Two models and other tools are employed to quantify the impacts of multiple obstacles (dam and wires) on a river network. In hierarchical processes and quantification bases, the tools can be briefly summarised; First, a semi-distributed hydrological model SWAT that has quantified the distributed discharge for 166 reaches, at daily frequency, over 55 years (1960 – 2015). Second, SWAT model calibration and validation in SWAT-CUP, using the SUFI algorithm. Third, the DJI-4 drone's images were captured for gravel bars at low altitudes for higher ground resolution. Forth, the Metashape application processed orthophotos of gravel bars, which are further processed in BASEGRAIN, to perform optical-granulometry and develop grain size distribution at the river network scale. Fifth, integration of hydrologic and grain size distribution information in the CASCADE framework for sediment connectivity, dam and weir removal impact analysis on sediment flux, at the network scale. The CASCADE framework provided sediment entrainment, transport, and deposition pattern on the Eamont network. The study's key findings have highlighted that the Lowther River contributes more sediment than the Eamont river, despite the high entrainment in the headwater regions. The entrained sediment gets deposited in the Brothers water and Ullswater lake on the Eamont river course. Whilst the Lowther river's significant deposition takes place in Haweswater reservoir. The main reason why Lowther River provide high sediment flux is that it has a high gradient and transport power than Eamont. Moreover, based on the geomorphological difference between the Eamont river and its tributary Lowther, the presence of multiple weirs with approximately similar sediment trap efficiency, the CASCADE simulation showed higher sediment flux would be released when weir removal activity was performed on the Lowther River. The current study had presented an opportunity to analyse the multiple river obstacle (dam/ weirs) impacts on sediment flux and sediment connectivity at a network scale. Such an experiment can be applied to formulate the dam removal planning for a complex river catchment. However, future integration of the CASCADE framework with ecological modelling would improve the analysis of riverine ecosystem benefits