A document by Zijing Wang. Click on the document to view its contents.

High-density attachment of Limnoperna fortunei (LF) would lead to the increase of flow resistance, which has posed big challenges to the normal operation of water conveyance projects. It is very necessary to quantify the flow resistance caused by the attachment of LF. In this study, a 3D geometric model of LF was generated based on real images. Attachment models of LF were generated with different densities and mussel size distributions, whose geometric characteristics were evaluated by some fundamental physical quantities, including attachment thickness, bed coverage, surface vertical roughness, and roughness concentration. Furtherly, a 3D numerical model with specific boundary conditions was established in OpenFOAM to simulate the flow over the LF attachment. Body-fitted mesh was generated using snappyHexMesh based on the LF attachment model. The results show that in high-density scenarios, a big wake zone formed inside LF attachment by the combined effects of each individual LF. Turbulence kinetic energy distribution indicated that LF attachment would cause viscous dissipation thus leading to more energy loss. The flow structure inside LF attachment was controlled by the size and spacing between each individual LF. Manning’s n values were calculated based on the CFD results at different densities. The results show that the flow resistance of LF attachment also followed the classic flow regimes, where in the skimming flow regime, the mussel size distribution played a non-negligible role. Higher flow speed resulted in larger flow resistance, and n could increase more than 90% compared to the scenario without LF attachment.

An investigation on 152 gullies along the Daheba River in the Tongde sedimentary basin was performed. Debris flows develop in gullies with an excess topography ZE, which represents the sediment availability, above a critical threshold value. Debris-flows in the Daheba watershed are supply-unlimited, i.e sediment is abundantly available from the steep erodible gully banks. Debris flows consist of a head and a body. The body propagates faster than the head and constantly supplies it with sediment. The body and head propagate in an intermittent way through the transient storage of sediment on the riverbed and its subsequent remobilization. Although the main sediment supply is provided by bank collapse, debris-flow events also incise the gully bed. The growth and incision of debris-flow gullies in supply-unlimited watersheds is mainly controlled by the frequency of occurrence of debris flows, which is closely related to ZE. With growth of the gully drainage area, ZE and the debris-flow frequency initially increase, until they reach maximum values in gullies with a drainage area of intermediate size, which are assumed to be the morphologically most active gullies. With further growth of the gully drainage area, ZE and the debris-flow frequency decrease, which opposes the development of debris flows and leads to a more stable gully morphology. The observations indicate and explain the upstream migrating incision of the Daheba watershed. The lack of available sediment in the mountain reach is supposed to limit the further upstream migration of the reach of most active debris flows.

Anthropogenic impacts and climate change modify instream flow, altering ecosystem services and impacting on aquatic ecosystems. Alpine rivers and streams on the Qinghai-Tibet Plateau (QTP), are especially vulnerable to disturbance due to a limited taxonomic complexity. The effects of variations in flow have been studied using specific taxa, however, the flow-biota relationships of assemblages are poorly understood. A multi-metric habitat suitability model (MM-HSM) was developed, using biological integrity measures of macroinvertebrate assemblages to substitute for habitat suitability indices (HSI) derived from individual taxa. The MM-HSM was trained using macroinvertebrate data from three representative alpine rivers (the Yarlung Tsangpo, the Nujiang, and the Bai Rivers) on the QTP, and was verified using data from the Lanmucuo River. The model produced reliable predictions using the training dataset (R2 = 0.587) and the verification dataset (R2 = 0.489), and was robust to inter-basin differences and changes in dataset size. By coupling the MM-HSM with hydrodynamic simulations, the relationship between weighted usable area (WUA) and flow variations (0.11–1.99 m3/s) for macroinvertebrates was established, and a unimodal response pattern (optimal flow Q = 1.21 m3/s) was observed for macroinvertebrate assemblages from the Lanmucuo River. This was in contrast to the skewed unimodal or monotonically increasing relationships observed for individual indicator taxa, supporting our hypothesis that biological integrity varies with changing flow and conforms to the intermediate disturbance hypothesis. The MM-HSM provides a novel framework to quantify species-environment relationships, which may be used for integrated river basin management.