Abstract
In the meandering riverbank of the Upper Yellow River (UYR), the native alpine swamp meadow (AS) has continuously degenerated into an alpine meadow (AM) due to climate change and intensified grazing. Its implication on river morphology is still not well known. This study examined this effect by in situ measurings of (1) physical properties of roots and their distribution in the soil-root mixture of the upper bank layer, and (2) the tensile strength in terms of excavating tests for triggering cantilever collapses of AS and AM riverbanks. The results showed that the root number in AS was significantly greater than that in AM, though the root distribution in both was similar. Also, the average tensile strength of individual roots in AS was 31,310 kPa, while that in AM was only 16,155 kPa. For the soil-root mixture, it decreased from 67.39 to 21.96 kPa. The weakened mechanical property was mainly ascribed to the lessened root number and the simpler root structure in the soil-root mixture of AM that reduces its ability to resist the external force. These findings confirmed that healthy AS can enhance bank stability and delay the development of tensile cracks in the riverbank of the meandering rivers in the UYR.
Highlights
Banks of meandering rivers are often composed of silts and sand that have significantly higher compressive strength than tensile strength or cohesion [1,2,3,4]
Our results demonstrated that the composition and physical properties of the two layers in the vertical profile of the riverbank are significantly different between the two sites (Table 2)
The upper soil-root layer was composed of silt, while the lower layer consisted of silty sand with some poorly graded fine gravel
Summary
Banks of meandering rivers are often composed of silts and sand that have significantly higher compressive strength than tensile strength or cohesion [1,2,3,4]. There has been a consensus that riparian vegetation can reinforce bank strength [5,6,7,8,9] and stability through the interaction between soil and roots [10,11,12,13,14,15,16]. Mechanisms of riverbank failure are quite different from those found on hillslopes because steeper and shorter riverbanks tend to have a more variable profile and relatively small size of the failed block [17]. Vegetation types and their root distributions throughout the bank profile play a critical role in resisting riverbank failure. The lower soil layer of the composite riverbank is subject to fluvial erosion, often resulting in a cantilever upper layer that includes the mixture of cohesive soil and vegetation. Once the gravity moment generated by the upper cantilever
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