Restoring fluvial forms and processes by gravel augmentation or bank erosion below dams: A systematic review of ecological responses

Sediment Augmentation for River Rehabilitation and Management—A Review

Sediment management is an important part of river rehabilitation and management. Global case studies provide a growing number of examples of successful sediment augmentation measures that can counter the adverse effects of disturbed sediment regimes. The initial river state and the objectives of the reported measures can vary largely, however. In this review, a summary of selected case studies is presented, and an objective-focused classification of sediment augmentation measures is introduced. Case-specific restrictions, design approaches and assessment methods based on the literature review and our own experience from working in the field are presented. This summary aims to provide an overview on up-to-date knowledge for applied river rehabilitation and management.

Design Criteria for Process-Based Restoration of Fluvial Systems

Process-based restoration of fluvial systems removes human constraints on nature to promote ecological recovery. By freeing natural processes, a resilient ecosystem may be restored with minimal corrective intervention. However, there is a lack of meaningful design criteria to allow designers to evaluate whether a project is likely to achieve process-based restoration objectives. We describe four design criteria to evaluate a project's potential: the expansion of fluvial process space and connectivity lost because of human alterations, the use of intrinsic natural energy to do the work of restoration, the use of native materials that do not overstabilize project elements, and the explicit incorporation of time and adaptive management into project design to place sites on recovery trajectories as opposed to attempts to “restore” sites via a single intervention. Applications include stream and infrastructure design and low-carbon construction. An example is presented in California's Sierra Nevada foothills.

Gravel replenishment and active-channel widening for braided-river restoration: The case of the Upper Drac River (France)

Massive gravel replenishment combined with active-channel widening could theoretically improve the morphological recovery of altered braided rivers but this restoration strategy was not yet tested in the field. A recent braided restoration project based on this principle was set up to restore a 4.2 km long reach in the Upper Drac River (French Alps) using 355.000 m³ of gravels to rise the bed level and to design a 100-m wide trapezoidal cross-section. The aim of this paper is to capture the morphological trajectory after restoration in order to evaluate efficiency and sustainability of this strategy. A Before and After Control Impact monitoring design has been used by combining a repetitive topographic survey (using airborne LiDAR data and terrestrial topographic surveys along cross-sections), an assessment of bedload supply to the restored reach using sediment tracing and active-layer surveys, and a systematic analysis of historical aerial photographs. In a particular context of low hydrological forcing after restoration, the restored reach adjusts with local braiding reference that highlights the efficiency of the restoration strategy. Despite this spontaneous braiding recovery, scouring processes are observed locally along the restored reach and the sustainability remains uncertain even if a good connection to sediment sources was observed. Feedbacks make it possible to propose recommendations to river managers who plan to use similar strategy of braided river restoration. This field study demonstrated for the first time that sediment replenishment combined with channel widening can be an efficient solution for the spontaneous recovery of braiding conditions in altered alpine gravel-bed rivers.