Bendway weirs: Could they create habitat for the endangered Rio Grande silvery minnow

Rehabilitation of the Middle Rio Grande in central New Mexico has become necessary to mitigate the effects of over a century of water and land development. The primary driving force behind rehabilitation efforts is the federally endangered Rio Grande silvery minnow (Hybognathus amarus). Bendway weirs, erosion control and channel‐stabilization structures placed transverse to the channel flow, have been used to prevent river migration while enhancing aquatic habitat. Habitat improvement plans on the Middle Rio Grande include the installation of bendway weirs, but the potential benefits of these structures for Rio Grande silvery minnow are unknown. We conducted a theoretical study on the flow conditions created by bendway weirs to determine if it is possible to create physical habitat for Rio Grande silvery minnow while simultaneously protecting the riverbank. Our study suggested that bendway weir installation could lead to the reduction of downstream displacement of Rio Grande silvery minnow eggs, the creation of Rio Grande silvery minnow feeding and refugia habitat, and the creation of drought or low flow habitat through scour hole formation. We also noted that the weirs could also serve as potential habitat for predators, and suggest further studies to better quantify the possible role of bendway weir installation. Copyright © 2009 John Wiley & Sons, Ltd.     

Drift and retention of pelagic spawning minnow eggs in a regulated river

Long‐distance drift of eggs and larvae has been identified as a possible cause of downstream displacement and poor recruitment of the endangered Rio Grande silvery minnow (Hybognathus amarus; silvery minnow). Seven experiments were conducted using artificial eggs to estimate silvery minnow egg drift and retention in the Albuquerque and Isleta reaches of the regulated Middle Rio Grande, New Mexico, USA over a range of flows during expected spawning times. Bead retention varied by reach, discharge, and shape of the hydrograph. Highest retention (6.9 and 9.7% per km in the Albuquerque and Isleta reaches, respectively) occurred on the ascending limb of a high flow in areas where there was substantial floodplain inundation. Retention was maximized at different flows in each reach (97 and 140 m³/s, respectively), possibly associated with reach‐specific floodplain inundation thresholds. Lowest retention in each reach (2.1 and 1.7%, respectively) occurred on the descending limb of low and high flows, respectively. Of the silvery minnow eggs produced in the combined Albuquerque and Isleta reaches in 2005, 8–14% are predicted to have been retained in the Albuquerque Reach (67 km) and 49–83% in the Isleta Reach (86 km) based on the distribution of adult fish and measured bead retention rates. Although silvery minnow propagules are capable of drifting long distances, our study suggests that considerable retention occurs in the Middle Rio Grande. Habitat restoration to increase channel habitat complexity, and flow management to promote floodplain inundation should help to retain a greater proportion of propagules in upstream reaches. Copyright © 2010 John Wiley & Sons, Ltd.     

Predicting joint frequency distributions of depth and velocity for instream habitat assessment

Stream ecosystem structure and function are strongly influenced by patterns of velocity and depth. Simple methods for predicting the spatial distributions of these two variables, as functions of one‐dimensional reach and discharge characteristics, have been recently reported in the literature. These studies have provided valuable insight into the fundamental factors influencing stream behaviour and represent a practical alternative to multi‐dimensional hydrodynamic models. However, these previous studies have handled velocity and depth separately, while there is evidence that meso‐habitats and stream biota are associated with distinct combinations of the two variables. Therefore, we used survey data from 92 stream reaches in New Zealand to develop a model for the joint distribution of depth and velocity. We found that, for each reach, the bivariate distribution of relative velocity and relative depth could be described by a mixture of two end‐member distributions, one bivariate normal and the other bivariate lognormal, each with fixed parameters. The relative contribution of each shape for a particular reach at a particular discharge could then be related to the reach mean Froude number, the reach mean relative roughness, and the ratio of the survey discharge to the mean discharge. As these inputs can be readily estimated for changed channel morphology, our model should provide a useful approach for linking river rehabilitation strategies to hydraulics and ecology. Copyright © 2007 John Wiley & Sons, Ltd.     

考虑河流流速和横向扩散系数变化的污染混合区理论分析及其分类

宽阔河流的中部深槽和远岸地形变化,对排放岸附近污染混合区范围的影响很小。因此,河流断面平均流速和横向扩散系数不能很好地反映排放岸附近污染物的移流扩散特性。通过对长江黄沙溪排污混合区平水期同步观测资料的分析,提出了深度平均流速的横向指数分布和横向扩散系数的二维变化关系式。在宽阔河流顺直岸边稳定点源条件下,求解了变系数二维移流扩散方程浓度分布的解析解,进行了浓度分布的特性分析。在此基础上,推导了污染混合区最大长度、最大宽度和对应纵向坐标以及面积的理论公式,给出了污染混合区边界归一化(等浓度)曲线方程。讨论了深度平均流速和横向扩散系数分布的特性参数(m,n和α)对污染混合区形态的影响规律,提出了河流岸边排放污染混合区的类别、形状特征、分类条件和变横向扩散系数的估算方法。实例表明,α-Ⅰ型污染混合区、边界曲线方程和几何特征参数,能够较好地表征长江黄沙溪排污混合区平水期COD_(Cr)现场观测等值线形状。     

在水力设计中提高水轮机稳定性的几点措施

从水力设计的角度出发,结合CFD技术的具体应用,分析讨论了混流式水轮机转轮压力脉动与转轮出口环量、尾水管中的流速分布、转轮泄水锥及导叶等之间的关系,提出了减轻压力脉动的具体措施,并对利用水力设计来预测转轮压力脉动进行了一些探索。