Nutrient Discharges from a Typical Watershed in the Hilly Area of Subtropical China

Agriculture is still the biggest contributor of non-point source (NPS) pollution to water bodies andrunoff discharges of nutrients and other chemicals are one of the most important pathways. This studywas conducted during 1998～1999 in a typical watershed with complex agriculture and forestry systems aswell as dotted farmer villages, in a transitional region between the mid- and northern subtropical zones ofChina. Continuous sampling of stream water was performed regularly at a weekly frequency, with additionalsampling after all major rainfall events. The discharges of N and P nutrients and suspended materials weremeasured and the total and area-averaged annual discharges of all components were calculated. The resultsshowed an uneven seasonal distribution of nutrient discharges with summer storms contributing most tothe total fluxes. This study demonstrated a high dependence of runoff volume on rainfall but the overallrunoff coefficients were dependent on land use type and watershed size. The area-averaged annual dischargesvaried greatly among the sub-watersheds with different sizes and land use structures. This is the first studyestimating the area-averaged annual discharges of N and P in the hilly areas of subtropical China, which were1.5 g m-2 a-1 and 0.1 g m-2 a-1, respectively, providing important reference values for the assessment ofregional agricultural non-point source pollution.

Effect of Plant Roots on Penetrability and Anti-Scouri-bility of Red Soil Derived from Granite

Distribution of plant roots in a red soil derived from granite was investigated to study the effect of plantroots on intensifying soil penetrability and anti-scouribility by the double-cutting-ring and the undisturbedsoil-flume methods, respectively. The plant roots system consisting mostly of fibrils, < 1 mm in diameter,was mainly distributed in the upper surface soil 30 cm in depth. It can remarhably increase the penetrabilityand anti-scouribility of the red soil derived from granite. When the root density was > 0.35 root cm-2, theintensifying effect of roots on both the penetrability and the anti-scouribility could be described by exponentequations, △ Ks = 0.0021RD1.4826 (R2 = 0.9313) and △ As = 0.0003RD1.8478 (R2 = O.9619), where △ Ks isthe value of intensified soil penetrability, a As the value of intensified soil anti-scouribility and RD the rootdensity, especially in the top soils within 30 cm in depth where plant roots were conceotrated.

Mechanism of Downcutting Erosion of Debris Flow over a Movable Bed

The phenomenon of debris flow is intermediate between mass movement and solid transport. Flows can be sudden, severe and destructive. Understanding debris flow erosion processes is the key to providing geomorphic explanations, but progress has been limited because the physical-mechanical properties, movement laws and erosion characteristics are different from those of sediment-laden flow. Using infinite slope theory, this research examines the process and mechanism of downcutting erosion over a moveable bed in a viscous debris flow gully. It focuses specifically on the scour depth and the critical slope for viscous debris flow,and formulas for both calculations are presented.Both scour depth and the critical conditions of downcutting erosion are related to debris flow properties(sand volume concentration and flow depth) and gully properties(longitudinal slope,viscous and internal friction angle of gully materials,and coefficient of kinetic friction). In addition, a series of flume experiments was carried out to characterize the scouring process of debris flows with different properties. The calculated values agreed well with the experimental data. These theoretical formulas are reasonable, and using infinite slope theory to analyze down cutting erosion from viscous debris flow is feasible.

An Enhanced Physically Based Scour Model for Considering Jet Air Entrainment

Based on systematic experiments on the influence of air entrainment on rock block stability in plunge pools impacted by high-velocity jets,this study presents adaptations of a physically based scour model.The modifications regarding jet aeration are implemented in the Comprehensive Scour Model(CSM),allowing it to reproduce the physical-mechanical processes involved in scour formation concerning the three phases;namely,water,rock,and air.The enhanced method considers the reduction of momentum of an aerated jet as well as the decrease of energy dissipation in the jet diffusive shear layer,both resulting from the entrainment of air bubbles.Block ejection from the rock mass depends on a combination of the aerated time-averaged pressure coefficient and the modified maximum dynamic impulsion coefficient,which was found to be a constant value of 0.2 for high-velocity jets in deep pools.The modified model is applied to the case of the observed scour hole at the Kariba Dam,with good agreement.

Numerical Modeling of Local Scour Around Hydraulic Structure in Sandy Beds by Dynamic Mesh Method

Local scour, a non-negligible factor in hydraulic engineering, endangers the safety of hydraulic structures. In this work, a numerical model for simulating local scour was constructed, based on the open source code computational fluid dynamics model Open FOAM. We consider both the bedload and suspended load sediment transport in the scour model and adopt the dynamic mesh method to simulate the evolution of the bed elevation. We use the finite area method to project data between the three-dimensional flow model and the two-dimensional(2D) scour model. We also improved the 2D sand slide method and added it to the scour model to correct the bed bathymetry when the bed slope angle exceeds the angle of repose. Moreover, to validate our scour model, we conducted and compared the results of three experiments with those of the developed model. The validation results show that our developed model can reliably simulate local scour.

Characteristics of the Nujiang River runoff for a long term and its response to climate change

Nujiang 河流量每年，和流量从 5 月显然增加从 5 月在时期期间统治了 81.9% 粗野的年度列到 10 月，它是在这个区域上的雨季。吝啬的流量因此在雨季期间在每个月内说明 13.65% 年度列但是仅仅 3% 在旱季期间。在另一个词，在雨季的月经流量在另外的月内像乘那的 4.5 一样。它证明流量与季节极大地变化，在雨季的流量在年度列起主导的作用。在河流量和降水之间的关系的分析结果因为同步关联系数从 1 月是重要的到 4 月但是与 1 个月的关联除了在 7 月，在另外的月内落后。基于在河流量上完成的降水的分析，在 5 月的降水极大地影响年度流量并且有一种靠近的关系到旱季，这被注意。年度，雨季和旱季流量和降水被小浪分析变换。结果证明降水和流量的变化与低频率变化(大约 24 年) 展出主导的时期，十(大约 8 年) 并且 interdecadal (大约 16 年) 摆动。变化年度，雨季和旱季流量与一个增加的趋势一致，除了在 6 月和 8 月，它与 0.05 信心水平是明显的。数据显示平均流量被 57.6， 28.1 和 85.7 m3/s 为年度列，旱季和雨季分别地在每 10 年里增加。最明显的变化在 10 月，它每 10 年增加了 111.3 m3/s。在 Nujiang 河排水盆的流量，温度和降水的变化主要是一致的。气候变化在 Nujiang 导致了流量变化，这被揭示。它在 10 月最大地被增加，第二，在 5 月和每年的增加数量，流量能完成大约 60 m3/s。在未来， 20 年，在早阶段的流量愿望减少，而是以后的增加分阶段执行。

Effects of submerged sheet pile vanes on mobile river beds

Submerged vanes are low-height flow-training structures emerging from the riverbed with a suitable angle of attack to the incoming flow. These structures redirect the stream flow and modify erosion and depositional rates in the bottom and in the banks of a river as a result of the secondary currents generated by their installation. For this reason they have many applica- tions in river hydraulics for controlling river bed morphology. An experimental investigation is carried out to compare the effi- ciency of sheet-piling vanes versus thin plane ones in controlling sediment redistribution in the channel bed. In particular, exper- imental tests were carried out within a straight water channel, in conditions of bed load motion. The morphology of the river bed both in the area close to the structure and in the far field was examined at different angles of attack of the vane to the incoming flow and at different values of the submergence parameter, which is the ratio between the height of the water above the structure and the water level. The experimental results show that both the shape of the vanes as well as the angle of attack affect their per- formance in terms of the effects on the bed morphology, especially for greater submergence parameters. Specifically, plane and sheet-piling vanes produce comparable remodelings of the channel bed in the downstream region, but when the attack angle is increased, the thin plane vane causes deeper scour holes close to the structure. This last effect is probably due to the increased erosive capacity of the horseshoe vortex associated with the plane vane, while the uneven surface of the sheet-piling vane miti- gates the erosive strength of that vortex.