Many urban streams have vanished from the surface as a collateral effect of urban growth. Often, these buried streams have been forgotten, and only street names remind us of their existence. Reasons for stream burial include the gain of space for road or house construction or the use of stream water to transport wastewater. Today, restoration efforts to bring back fully canalized streams to the surface and to restore their stream bed (so-called daylighting) are being increasingly integrated into urban blue-green space planning, recognizing the high ecological and social value of urban streams, especially to support resilience against climate change impacts in cities. In this paper, we briefly revise the impacts of stream burial, present a series of case studies of daylighting from Europe (France, Switzerland, and Germany), and compare them with case studies from Asia (China, India, Taiwan). We found that high real estate prices, limited buffer riparian zone and resistance by the inhabitants were the greatest obstacles to stream daylighting projects. In contrast, economic gains from separating wastewater from rainwater and revival of cultural linkages with water were the strongest drivers to restore these streams. We then present methods on how to identify buried streams as candidates for daylighting and deliver criteria to select the most promising candidates. Acknowledging that each restoration project requires to be adapted to the local biophysical and local setting, we deliver a preliminary decision support system and a guideline for identifying the best candidate streams for daylighting projects, including the arguments in favor of restoration, the caveats, the social processes of decision-making, and perspectives for the integration of stream daylighting into urban climate change mitigation and adaptation concepts, in a Global North-South comparison.
<p>Fine sediment is a fundamental component of the river system. Fine sediment conditions support good ecological status in different environments since they can affect river habitat and also transport pollutants and nutrients. Moreover, fine sediments can lead to security issues for hydroelectric buildings in the river channel, i.e. sedimentation in reservoirs, turbine erosion, etc. In alpine rivers, a large amount of fine sediments travels over the gravel-bed system as suspension and interacts with the gravel matrix (deposition, infiltration, resuspension). Recent studies highlight that fine sediment stocks in the river bed can be a significant source of suspended load at the event scale, and can have a non-negligible effect on sediment budget estimation. However, there exists no proper estimation of fine sediment stocks in gravel-bed rivers, especially for the sand fraction. One can also question the spatial and temporal variability of these stocks, which makes the estimation of such source of fine sediments challenging.</p><p>In this study, we intend to quantify fine sediment stocks in an Alpine river system (Arc-Is&#232;re in the French Alps) characterized by the presence of alternate bars. We estimate the potentially resuspended fine stocks from the gravel bar matrix for different discharges by coupling field measurements, GIS spatial analysis, and 1D modelling. Fine sediment stocks in the gravel bars are firstly measured using a field protocol optimized from the one proposed by Misset et al. (2021). The evaluation of the total stocks of fine sediments is made by combining these local measurements to GIS spatial analysis based on LiDAR data. Then, in order to predict the resuspended fine stocks, a 1D numerical hydraulic model is used to calculate bed shear stresses on the bar surface and evaluate the thickness of the potential remobilized coarse sediments. Having the volume of sediments remobilized, one can evaluate the potentially re-suspended fine stocks for different discharges. The measured fine stocks show a significant amount of sand present in the river bed, which was rarely if not measured in most studies. The silt-clay part of the calculated re-suspended stocks is found equivalent to around 30% of annual Suspended Sediment Matter (SSM) flux for a 15-year return period flood event, which appears consistent since such a large event may yield up to 50% of the annual SSM flux. However, the silt-clay stocks represent around 20% of the total fine stocks only, 80% corresponding to sand. Therefore, a large amount of sand could be re-suspended from the gravel bar matrix and should not be neglected when estimating the downstream sediment budget.</p>
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