A multiple criteria decision‐making approach to estimate minimum environmental flows based on wetted perimeter

Minimum environmental flows in rivers provide a certain level of protection for the aquatic environment. The relationship between wetted perimeter and discharge can be used to define the minimum environmental flows by the slope method (SM), or curvature method (CM), especially for cases with poor understanding of the aquatic ecosystem. SM and CM derived inconsistent values of minimum environmental flows. It was not clear which method better defined minimum environmental flow. Moreover, the computation and optimization procedures are both time consuming and error‐prone, especially for complicated wetted perimeter–discharge relationships. In this study, flow regulation for rivers was regarded as a multiple criteria decision‐making problem, with the objectives of minimum river discharge and maximum wetted perimeter. Ideal point methods (IPMs) with the scaling coefficient r = 1 (IPM1) and r = 2 (IPM2) were used to solve this model to determine optimal environmental flows. IPM was simple in computation, especially when the wetted perimeter–discharge relationship was given as scattered data pairs. Meanwhile, it was applicable to a wider range of wetted perimeter–discharge relationship than SM and CM. Environmental flows estimated by IMP1 are the same as that by SM. The analytical results for environmental flows using SM, CM, IPM1 and IPM2 were compared for wetted perimeter–discharge relationship expressed as power or logarithmic function. It showed that CM is not a good method to define environmental flows. SM with unity slope and IMP1 were recommended. CM, SM and IPM were examined for the determination of environmental flows in a river in North Xinjiang, China. Environmental flows for different transects of the studying river reach were estimated to be 21% of the mean annual flow by SM or IPM1, which provided the satisfactory wetted perimeter, water depth and average velocity for aquatic organisms. Copyright © 2007 John Wiley & Sons, Ltd.     

二维水力模拟在河流生态需水湿周法中的应用

湿周法是河流生态需水量计算中使用较为常见的一种水力学方法,通常采用一维的水力分析结果作为计算依据。二维水力模拟能提供更为细致全面的河道水力参数分布,通过对某水电站坝下13.3 km河道进行平面二维水力模拟,对其在湿周法中的应用进行了探讨。采用弗汝德数(Fr)作为浅滩生境划分标准,把浅滩生境较为集中的河段作为湿周法分析的浅滩河段。随后根据平面二维水力模拟结果,给出了浅滩断面相对湿周-流量关系曲线,采用斜率为1的斜率法计算得到了各浅滩断面的生态需水量。结果表明,采用二维水力模拟结果给出的曲线更为合理。     

基于湿周法的河道最小生态需水量多目标评价模型

以湿周与流量关系为基础,建立了基于多目标评价方法的河道最小生态需水量估算模型,以湿周最大和河道流量最小作为栖息地保护和水资源开发利用的目标,采用理想点法求解,以增江下游麒麟咀站为例计算河道最小生态需水量,并将该模型的计算结果与斜率法和曲率法的计算结果进行了比较分析。结果表明:增江下游河道最小生态需水量阈值范围为20.8～26.3 m3/s,相应的平均流速范围为0.42～0.44 m/s;模型计算结果比传统的湿周法适用性更强,且能较好体现生态用水和经济用水的均衡发展。     

确定河流生态流量的几种湿周法比较

为比较不同湿周法在计算河流生态流量中的适用性,采用幂函数描述无量纲化的湿周～流量关系,利用曲率法、斜率法及多目标评价法中的线性加权和法、理想点法(尺度系数r分别取1,2,∞)、乘除法等7种方法计算河流生态流量。结果表明:曲率法的结果偏小;斜率法、理想点法(r=1)、线性加权和法的结果相同,且位于各方法的中间;另外3种多目标评价方法得到的河流生态流量大于斜率法等的结果,但湿周的增加比例明显小于流量增加比例。在湿周～流量关系符合幂函数关系时,利用斜率法或理想点法(r=1)、线性加权和法确定河流生态流量的方法是适宜的。     

Differences in river ecological functions due to rapid channel alteration processes in two California rivers using the functional flows model,part 2—model applications

This study applies the functional flows model (FFM) that integrates hydrogeomorphic processes and ecological functions to assess physical habitat. Functional flows are discharge values that serve ecological uses. The model was adjusted to evaluate gravel‐bed riffle functionality for fall‐run Chinook salmon with respect to river rehabilitation on the Mokelumne River and flood‐induced channel change on the Yuba River. The goal was to test if differences in ecological performance were traceable to differences in hydrogeomorphic conditions. Ecological functions studied were bed occupation (spawning, incubation and emergence) and bed preparation (river bed reworking periods)‐ both reliant on shear stress dynamics. Model outputs included number of days that have functional flows, ranges of functional flows that provide favourable sediment transport stages and the efficiency of a site to produce functional flows. Statistical significance of results was tested using non‐parametric tests. Functional flows analyses before and after geomorphic alteration indicate that river rehabilitation on the lower Mokelume River increased the number of days with functional flows, while the Yuba River May 2005 flood increased the functional ranges of flows for the test sites. Reach‐scale analyses indicated similar ecological performance at reference sites in both rivers. A comparison between both rivers showed that despite a greater geomorphic potential of the Mokelumne River sites to have functional flows, Yuba River sites actually experienced better ecological performance for fall‐run Chinook salmon freshwater life stages due to greater flow availability. The FFM provided an objective tool to assess changes in ecological functionality at hydrogeomorphically dynamic sites. Copyright © 2010 John Wiley & Sons, Ltd.     

Application of a 2D hydrodynamic model to design of reach‐scale spawning gravel replenishment on the Mokelumne River,California

In‐stream chinook salmon (Oncorhynchus tschawytscha) spawning habitat in California's Central Valley has been degraded by minimal gravel recruitment due to river impoundment and historic gravel extraction. In a recent project marking a new direction for spawning habitat rehabilitation, 2450 m³ of gravel and several boulders were used to craft bars and chutes. To improve the design of future projects, a test was carried out in which a commercial modelling package was used to design and evaluate alternative gravel configurations in relation to the actual pre‐ and post‐project configurations. Tested scenarios included alternate bars, central braid, a combination of alternate bars and a braid, and a flat riffle with uniformly spaced boulders. All runs were compared for their spawning habitat value and for susceptibility to erosion. The flat riffle scenario produced the most total, high, and medium quality habitat, but would yield little habitat under flows deviating from the design discharge. Bar and braid scenarios were highly gravel efficient, with nearly 1 m² of habitat per 1 m³ of gravel added, and yielded large contiguous high quality habitat patches that were superior to the actual design. At near bankfull flow, negligible sediment entrainment was predicted for any scenario. Copyright © 2004 John Wiley & Sons, Ltd.     

THRESHOLD FLOWS FOR THE BREAKDOWN OF SEASONALLY PERSISTENT THERMAL STRATIFICATION: SHOALHAVEN RIVER BELOW TALLOWA DAM,NEW SOUTH WALES,AUSTRALIA

Reduced mixing of deep pools attributable to river regulation and downstream flow suppression can lead to an increase in the magnitude, frequency and duration of thermal stratification in riverine pools over summer. This study monitored hourly temperature profiles with five thermistor loggers in a 15 m deep natural pool over 12 months from May 2005. Detailed bathymetric and topographic survey data and HEC‐RAS hydraulic modelling of layer Richardson numbers were used to extend thermistor observations of flow‐related stratification breakdown in this single deep pool to a 20 km long pool‐riffle dominated river reach below the dam. Reach‐wide breakdown of persistent thermal stratification in deep pools over spring and summer was likely to be achieved by a flow rate of 3000 ML day^?1. This flow rate approximates the long‐term mean annual natural flow (2860 ML day^?1) and the 16th flow duration percentile (mean daily flows equalled or exceeded for 16% of time), indicating that thermal stratification of the deepest pools in the Shoalhaven River is a common, natural phenomenon not solely attributable to river regulation. Should reasonably consistent hydraulic geometry relationships exist between low salinity rivers in similar climatic, hydrologic and geomorphic settings, then we suggest that the mean annual natural flow is likely to achieve widespread breakdown of thermal stratification across lengthy reaches of similar pool‐riffle sequence rivers elsewhere. Hourly mean wind speeds of up to 65 km h^?1 recorded at an automated weather station 25 km from the study site were found to suppress of the degree of thermal stratification in the study pool but did not achieve deep mixing of persistent seasonal thermoclines. Large, rapid and sustained air temperature decreases associated with the passage of cold fronts across southeastern Australia in summer were found to be more effective than wind and achieved mixing to depths of at least 4.2 m. Copyright © 2011 John Wiley & Sons, Ltd.     

Instream flow methods: a comparison of approaches

Minimum flows in rivers and streams aim to provide a certain level of protection for the aquatic environment. The level of protection is described by a measure such as a prescribed proportion of historic flows, wetted perimeter or suitable habitat. Conflicting minimum flow assessments from different instream flow methods are arguably the result of different environmental goals and levels of protection. The goals, the way in which levels of protection are specified, and the relationship between levels of protection and the aquatic environment are examined for three major categories of flow assessment methods: historic flow, hydraulic geometry and habitat. Basic conceptual differences are identified. Flow assessments by historic flow and hydraulic methods are related to river size and tend to retain the ‘character’ of a river. Habitat-based methods make no a priori assumptions about the natural state of the river and flow assessments are based primarily on water depth and velocity requirements. Flow and hydraulic methods assume that lower than natural flows will degrade the stream ecosystem, whereas habitat methods accept the possibility that aspects of the natural ecosystem can be enhanced by other than naturally occurring flows. Application of hydraulic and habitat methods suggests that the environmental response to flow is not linear; the relative change in width and habitat with flow is greater for small rivers than for large. Small rivers are more ‘at risk’ than large rivers and require a higher proportion of the average flow to maintain similar levels of environmental protection. Habitat methods are focused on target species or specific instream uses, and are useful where there are clear management objectives and an understanding of ecosystem requirements. Flow and hydraulic methods are useful in cases where there is a poor understanding of the ecosystem or where a high level of protection for an existing ecosystem is required. © 1997 John Wiley & Sons, Ltd.     

EFFECTS OF FLOW DYNAMICS ON THE AQUATIC‐TERRESTRIAL TRANSITION ZONE (ATTZ) OF LOWER MISSOURI RIVER SANDBARS WITH IMPLICATIONS FOR SELECTED BIOTA

Sandbars are an important aquatic terrestrial transition zone (ATTZ) in the active channel of rivers that provide a variety of habitat conditions for riverine biota. Channelization and flow regulation in many large rivers have diminished sandbar habitats and their rehabilitation is a priority. We developed sandbar‐specific models of discharge‐area relationships to determine how changes in flow regime affect the area of different habitat types within the submerged sandbar ATTZ (depth) and exposed sandbar ATTZ (elevation) for a representative sample of Lower Missouri River sandbars. We defined six different structural habitat types within the sandbar ATTZ based on depth or exposed elevation ranges that are important to different biota during at least part of their annual cycle for either survival or reproduction. Scenarios included the modelled natural flow regime, current managed flow regime and two environmental flow options, all modelled within the contemporary river active channel. Thirteen point and wing‐dike sandbars were evaluated under four different flow scenarios to explore the effects of flow regime on seasonal habitat availability for foraging of migratory shorebirds and wading birds, nesting of softshell turtles and nursery of riverine fishes. Managed flows provided more foraging habitat for shorebirds and wading birds and more nursery habitat for riverine fishes within the channelized reach sandbar ATTZ than the natural flow regime or modelled environmental flows. Reduced summer flows occurring under natural and environmental flow alternatives increased exposed sandbar nesting habitat for softshell turtle hatchling emergence. Results reveal how management of channelized and flow regulated large rivers could benefit from a modelling framework that couples hydrologic and geomorphic characteristics to predict habitat conditions for a variety of biota. Published in 2011 by John Wiley & Sons, Ltd.     

Flow structure through pool‐riffle sequences and a conceptual model for their sustainability in gravel‐bed rivers

Detailed field measurements and simulations of three‐dimensional flow structure were used to develop a conceptual model to explain the sustainability of self‐formed pool‐riffle sequences in gravel‐bed rivers. The analysis was conducted at the Red River Wildlife Management Area in Idaho, USA, and enabled characterization of the flow structure through two consecutive pool‐riffle sequences, including: identification of jet concentration and dissipation zones, and the development of local turbulence features (i.e. vertical and horizontal eddies) under different flow conditions. Three‐dimensional hydraulic simulations were used to evaluate how the flow structure varies across a range of flow conditions and with different degrees of sediment aggradation within the upstream pool. The analysis demonstrated a significant influence of the residual pool depth on the flow structure, with reduced residual depth causing a shift in the orientation of the jet and a reduction in the influence of vertical eddies and the size and intensity of horizontal eddies. The proposed conceptual model seeks to explain the sustainability of pools in terms of the flow structure in pool‐riffle morphology and how this flow structure will change as a result of altered external forcing, such as upstream sediment delivery or changes in bank stability. Copyright © 2010 John Wiley & Sons, Ltd.     

Tradeoffs associated with sediment‐maintenance flushing flows: a simulation approach to exploring non‐inferior options

Sediment‐maintenance flushing flows designed to mimic the action of natural floods in removing the accumulated fine sediments from the channel and loosening the gravel bed have been increasingly proposed as an effective alternative in dam management and a required component of riverine restoration programmes. However, reservoir releases are generally associated with financial and environmental costs, thus it is highly desirable to specify flushing flows as accurately as possible. In this paper we present a simulation approach to evaluating flushing flows and exploring the tradeoffs associated with non‐inferior flushing options. A two‐fraction sediment routing model is used to simulate the gravel‐sand bed response to flushing flows. The results reveal that the sand cleansing effect propagates from upstream to downstream and from surface to subsurface. Under a steady gravel supply from upstream, an equilibrium state of gravel transport and bed degradation is eventually reached in the simulation reach. The flushing flow and sediment transport system investigated in this study involves a transient state variable (bed sand content), a decision variable (flushing flow discharge), a flushing goal (ultimate bed sand content), and three outcomes to be minimized (flushing duration, released water volume, and total gravel loss). A series of numerical simulations are carried out with a range of flows and pre‐flushing bed sediment conditions. The results reveal that the flushing efficiency is higher for the larger flow. However, for flows greater than ～100 m³/s the flushing duration is less sensitive to the flow discharge, thus the system may be simplified as a bi‐objective one. The gravel loss and water volume are two conflicting outcomes within the non‐inferior flow region. Under a worse bed sediment condition, the feasible flushing options are constrained in a narrower range and also associated with higher costs. The tradeoffs between the conflicting outcomes are quantitatively displayed with the transformed feasible solutions in the objective space. We provide here a general and practical approach permitting a quantitative evaluation of the different flushing options that is appropriate to the level of data typically available. Copyright © 2004 John Wiley & Sons, Ltd.     

MACROINVERTEBRATE RESPONSE TO FLOW CHANGES IN A SUBALPINE STREAM: PREDICTIONS FROM TWO‐DIMENSIONAL HYDRODYNAMIC MODELS

Two‐dimensional hydrodynamic models are being used increasingly as alternatives to traditional one‐dimensional instream flow methodologies for assessing adequacy of flow and associated faunal habitat. Two‐dimensional modelling of habitat has focused primarily on fishes, but fish‐based assessments may not model benthic macroinvertebrate habitat effectively. We extend two‐dimensional techniques to a macroinvertebrate assemblage in a high‐elevation stream in the Sierra Nevada (Dana Fork of the Tuolumne River, Yosemite National Park, CA, USA). This stream frequently flows at less than 0.03 m³ s^?1 in late summer and is representative of a common water abstraction scenario: maximum water abstraction coinciding with seasonally low flows. We used two‐dimensional modelling to predict invertebrate responses to reduced flows that might result from increased abstraction. We collected site‐specific field data on the macroinvertebrate assemblage, bed topography and flow conditions and then coupled a two‐dimensional hydrodynamic model with macroinvertebrate indices to evaluate habitat across a range of low flows. Macroinvertebrate indices were calculated for the wetted area at each flow. A surrogate flow record based on an adjacent watershed was used to evaluate frequency and duration of low flow events. Using surrogate historical records, we estimated that flow should fall below 0.071 m³ s^?1 at least 1 day in 82 of 95 years and below 0.028 m³ s^?1 in 48 of 95 years. Invertebrate metric means indicated minor losses in response to modelled discharge reductions, but wetted area decreased substantially. Responses of invertebrates to water abstraction will likely be a function of changing habitat quantity rather than quality. Copyright © 2011 John Wiley & Sons, Ltd.     

Low‐Head Dam Impacts on Habitat and the Functional Composition of Fish Communities

The natural flow regime of many rivers in the USA has been impacted by anthropogenic structures. This loss of connectivity plays a role in shaping river ecosystems by altering physical habitat characteristics and shaping fish assemblages. Although the impacts of large dams on river systems are well documented, studies on the effects of low‐head dams using a functional guild approach have been fewer. We assessed river habitat quality and fish community structure at 12 sites on two rivers; the study sites included two sites below each dam, two sites in the pool above each dam and two sites upstream of the pool extent. Fish communities were sampled from 2012 to 2015 using a multi‐gear approach in spring and fall seasons. We aggregated fishes into habitat and reproductive guilds in order to ascertain dams' effects on groups of fishes that respond similarly to environmental variation. We found that habitat quality was significantly poorer in the artificial pools created above the dams than all other sampling sites. Fast riffle specialist taxa were most abundant in high‐quality riffle habitats farthest from the dams, while fast generalists and pelagophils were largely restricted to areas below the downstream‐most impoundment. Overall, these dams play a substantial role in shaping habitat, which impacts fish community composition on a functional level. Utilizing this functional approach enables us to mechanistically link the effects of impoundments to the structure of fish communities and form generalizations that can be applied to other systems. Copyright © 2017 John Wiley & Sons, Ltd.     

Evaluation of Steelhead Passage Flows Using Hydraulic Modeling on an Unregulated Coastal California River

Passage and habitat connectivity flows for steelhead Oncorhynchus mykiss through depth sensitive natural, low gradient, critical riffle sites were investigated in the unregulated Big Sur River, California. The River2D two‐dimensional hydraulic habitat model, along with quantitative passage metrics and species‐specific and lifestage‐specific depth criteria, were used to evaluate and compare predicted fish passage flows with flows derived by a traditional empirical critical riffle fish passage method. Passage flows were also compared with historical unimpaired natural hydrology patterns to assess the frequency and duration of suitable passage flows under the naturally variable flow regimes characteristic of Central California coastal rivers. A strong relationship (r² = 0.93) was observed between flows predicted by hydraulic modeling and flows identified by the empirical critical riffle method. River2D provided validation that the flows derived using the traditional critical riffle methodology provided for contiguous passable pathways of suitable hydraulic (depth and velocity) conditions through complex cobble‐dominated riffle sites. Furthermore, steelhead passage flows were spatially and temporally consistent between lagoon and upstream riffles for adults, and were generally indicative of a river system in equilibrium with a naturally variable flow regime and associated intact ecological processes. An analysis of 25 years of continuous flow data indicated sufficient flows for upstream passage by young‐of‐year and juvenile steelhead were produced between 37% and 100% and between 1% and 95% of the time, respectively. September and October are the most challenging months for natural flows to meet young‐of‐year and juvenile passage and habitat connectivity flows. Careful consideration of seasonal and interannual flow variability dynamics, therefore, are critical components of an effective flow management strategy for the maintenance and protection of passage and habitat connectivity flows between lagoon and upriver habitats. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

HABITAT USE OF AN ENDEMIC MOLLUSC ASSEMBLAGE IN A HYDROLOGICALLY ALTERED REACH OF THE SNAKE RIVER,IDAHO, USA

Major waterways have been dramatically altered by dams some of which have operated with consistent and significant flow alterations annually since the early 1900s. Re‐operation strategies are increasingly being considered for such dams in regions where biodiversity hotspots of sensitive taxa remain. Understanding the impacts of current dam operations on sensitive species is vital to designing re‐operations that effectively promote species conservation. Our objectives were to examine the effects of consistent and dramatic flow alterations from a major dam on the distribution of a highly diverse endemic mollusc assemblage in the middle Snake River, Idaho, USA. Quarter metre plots were surveyed along transects with a Venturi suction dredge and classified as permanently or seasonally wetted habitat by depth. Species were classified by degree of endemism. Chi‐square analysis was used to compare species presence between habitat types and analysis of variance was used to assess the significance of habitat type as a predictor of mollusc abundance, species richness and Shannon's H. Multivariate analyses were used to investigate mollusc assemblage composition and correlations with environmental parameters. The majority of mollusc species, including species of conservation concern, were found more frequently and in greater abundance in permanently wetted habitat. In addition, the mollusc assemblage in permanently wetted habitat was more diverse. Species presence and abundance patterns were consistent with source sink population dynamics with permanently wetted habitat serving as a source for seasonally wetted habitat. Results indicate that most mollusc species do not use seasonally wetted habitat even four months after it becomes available despite viable environmental conditions, highly mobile species and colonization sources. Our findings suggest that dam re‐operations which reduce the magnitude and duration of seasonal low flows may improve habitat availability for sensitive benthic assemblages. Copyright © 2013 John Wiley & Sons, Ltd.     

资料短缺平原河流的生态需水量计算——以通顺河武汉段为例

各类计算生态需水量的方法多需要长序列实测的水文或生境资料,无法直接适用于资料短缺的河流。在实测资料短缺的平原河流通顺河武汉段上布置10个典型断面,利用人为设定的多级试算流量来替代长序列实测流量,利用MIKE11软件模拟推求河道典型断面水力参数(河宽、水深、流速和湿周等)随流量的变化关系;在此基础上,依据平原河流滩槽明显的特点,选用水力学法中基于水力参数与流量间相关关系的湿周法和生态水力学法分别计算研究河段的生态需水量。计算结果表明,通顺河武汉段的河道基本形态得以维持和生物基本栖息地得以保障时的生态需水量应为26 m³/s。所提出的计算方案能较好地推求资料短缺地区平原河流的生态需水量,也可为类似河流的生态需水计算提供一定的参考。     

Estimating discharge in gravel‐bed river using non‐contact ground‐penetrating and surface‐velocity radars

Discharge measurement is a critical task for gravel‐bed channels. Under high‐flow conditions, the elevation of the riverbed changes significantly by intensive torrential flow. The stage–discharge relations commonly used for stream discharge estimation may no longer be adequate. The contact‐type velocity measuring is also subject to measurement errors and/or instrument failures by the high‐flow velocities, driftwood, stumps, and debris. This study developed a new real‐time method to estimate river discharge in gravel‐bed channels. A systematic measuring technology combining ground‐penetrating radar and surface‐velocity radar was employed. The rating curves representing the relations of water surface velocity to the channel cross‐sectional mean velocity and flow area were established. Stream discharge was then deduced from the resulting mean velocity and flow area. The proposed method was examined in a steep gravel‐bed reach of the Cho‐Shui River in central Taiwan. The estimated stream discharge during three flood events were compared to the prediction by using the stage–discharge relation and the index‐velocity method. The proposed method of this study is capable of computing reasonable values of discharge for an entire flood hydrograph, whereas the other two methods tend to produce large extrapolation errors. Moreover, when the computed discharge is used in 2D flood flow simulation, the proposed method demonstrates better performance than the commonly used stage–discharge and index‐velocity methods.     

USE OF HYDRAULIC MODELLING TO ASSESS PASSAGE FLOW CONNECTIVITY FOR SALMON IN STREAMS

The maintenance of hydrologic connectivity in river networks has become an important principle for guiding management and conservation planning for threatened salmon populations, yet our understanding of how fish movement is impaired by spatial and temporal variation in connectivity remains limited. In this study, a two‐dimensional hydraulic modelling approach is presented to evaluate flow connectivity in relation to passage requirements of adult steelhead trout (Oncorhynchus mykiss) in coastal California streams. High‐resolution topographic data of stream reaches with distinct channel morphology were collected using terrestrial light detection and ranging surveys and linked with water surface measurements to calibrate hydraulic model simulations. Quantitative metrics of longitudinal flow connectivity were developed to assess fish passage suitability in relation to stream discharge. Measured flow data from the 2008–2009 winter season and simulated long‐term records indicated that suitable passage flows occur with relatively low frequency and duration at all sites, suggesting that instream flow protections for fish passage are warranted. Results from the hydraulic modelling simulations were then compared with two alternative methods for assessing passage flows. A regional formula used by the State of California to identify minimum instream flow needs provided conservative estimates of passage flow requirements, whereas an approach based on riffle crest water depths underestimated flow needs. The hydraulic modelling approach appears well suited for simulating flows for fish passage studies and may be particularly useful for testing alternative environmental flow assessment methods and evaluating habitat–flow relationships in stream reaches of importance, such as critical habitat for threatened fish species. Copyright © 2011 John Wiley & Sons, Ltd.     

基于水力水文学法的大渡河上游生态流量确定

利用湿周法、Tennant法、Tessman法、逐月最小径流法分别计算大渡河上游的最小生态流量,并通过鱼类繁殖产卵所需的流量验证4种方法的合理性.结果表明,Tennant法、Tessman法、逐月最小径流法能保证足木足断面鱼类繁殖产卵所需的最低流量,大金断面4种方法均能满足.推荐3-8月采用Tennant法,9月至次年...     

Three‐dimensional flow dynamics around deflectors

Many river rehabilitation projects to enhance the aquatic habitat focus on the creation of pool and riffle habitat by the implementation of flow deflectors, with various degrees of successes and failures. A more comprehensive understanding of the complex three‐dimensional flow dynamics that induces scour around instream structures is required for a more effective design. The objective of this study is to examine the three‐dimensional mean and turbulent flow characteristics around paired flow deflectors for various types of deflector design in a laboratory flume. Three deflector angles (45°, 90° and 135°) and two deflector heights (with flow under and over the deflector height) were tested over a smooth (plexiglas) bed and a sand bed. Three‐dimensional velocity measurements were taken with an acoustic Doppler velocimeter at several planform positions at two heights above the bed. Results show that the 90° deflectors create the most important disturbance in the mean flow field, in turbulence intensity and bed shear stress. There is, however, a marked difference in the spatial distribution of the mean and turbulent parameters over a mobile bed and over a smooth, fixed bed. This stresses the importance of understanding the feedback between bed topography and flow dynamics and limits the applicability of conclusions drawn from plane bed experiments to natural rivers. Copyright © 2005 John Wiley & Sons, Ltd.