Designing mountain rivers for restoration with artificial step-pool sequences

The degradation of river ecosystem is one of the major problems facing the world today. With the increased human activities in the mountainous areas and growing awareness of the importance of maintaining aquatic biodiversity, there has been a significant increase in restoration projects for mountain rivers. Typically, river restoration focuses on improving the environmental health, functionality, and resilience of rivers and streams of water sources that have been negatively impacted by human activities. It also aims to restore and preserve the negative consequences of natural hazards like landslides, debris flows and flash floods.

Steep mountain channels would experience large morphological changes due to increased flow energy and sediment transport. This often damages the local stream habitats and riparian zones, posing a great risk to infrastructure and populated areas. Thus, river restoration methods that improve channel stability and local ecology in steep mountain streams are highly desirable. Step-pools are one of the most common morphological features in mountain streams and offer effective flow energy dissipation for enhanced flow resistance and channel stability. Artificial step-pools that mimic the natural step-pool features are used worldwide as nature-based and environmentally friendly approaches for river restoration in mountain channels.

Artificial step-pool channels have been identified as a promising approach owing to their advantages, including preserving longitudinal riverine connectivity and creating a diverse and sustainable habitat for aquatic life. To this end, artificial step-pool sequences have been widely used for ecological improvement and channel stabilization. Unfortunately, existing artificial step-pools design methods are problematic and fail to meet desired restoration objectives. Additionally, the existing design follows two methods: maximum resistance theory and empirical relationships for the morphological dimensions, which also face numerous limitations.

To address the issue, Dr. Chendi Zhang, Prof. Mengzhen Xu, and Prof. Zhaoyin Wang from China (Chinese Academy of Sciences and Tsinghua University) in collaboration with Prof. Marwan Hassan, Dr. Matteo Saletti and Prof. André Zimmermann from Canada (University of British Columbia and Simon Fraser University) proposed a new modular framework for designing artificial step-pool sequences. This framework combined different approaches to quantify step-pool dimensions at the unit scale. It also consisted of five modules based on step layouts, longitudinal profiles, hydraulics, energy dissipation morphological evolution, and stability of step-pools. Their work is currently published in the peer-reviewed, Journal of Hydraulic Engineering.

In the new paper, the authors first discussed all the modules before summarizing the design flow into step-by-step procedures. Based on an iterative procedure in the workflow, several standards for improving and evaluating the design were recommended. Compared with the previous design approaches, the present design method considered all the restoration objectives and stability requirements of individual units instead of subreaches. As a result, it enhanced design flexibility, enabling step-pool units to acquire different grain sizes and geometries within a sequence. This also allowed the adjustments only localized at several units in the sequence in the designing process, instead of redesigning the entire sequence.

In order to validate its design feasibility and practical application, the new framework was applied to three mountain streams to evaluate the existing natural and artificial step-pool sequences. The results were remarkable and show how the framework worked to link the design to restoration objectives. The framework exhibited numerous advantages over the previous approaches, including enhanced step-pool stability and design flexibility, allowing for unit scale adjustments.

In summary, a unit-scale modular framework is presented to overcome the inherent challenges associated with the design of step-pool sequences. From the results, the present framework was superior to previous approaches in terms of robustness, flexibility, stability and general performance. Additionally, the framework could be modified to accommodate new design experiences and scientific findings. In a statement to Advances in Engineering, first author Chendi Zhang said: “we aim to offer a powerful tool for robust and comprehensive step-pool design to achieve river restoration objectives for river engineers and scientists. The new framework is also open for new knowledge on step-pools in future.”