Selecting Between One‐Dimensional and Two‐Dimensional Hydrodynamic Models for Ecohydraulic Analysis

Aquatic habitat assessment and river restoration design require geospatially explicit maps of hydraulic conditions. Diverse mechanistic ecohydraulic models compute spatially explicit depth and velocity results to evaluate habitat suitability spatially as a function of these abiotic conditions. This study compared depth and velocity results from two‐dimensional (2D) and one‐dimensional (1D) hydraulic models with algorithms that laterally discretize 1D velocity and interpolate depth and velocity spatially based on the Laplacian heat mapping approach. These ‘conveyance distributed’ methods constitute ‘best 1D modelling practice’ and were compared with 2D results for the first time. The 1D and 2D models were applied to three morphologically distinct reaches (leveed, meandering, and anastomosing) for three flows (base, bankfull, and flood flows) of the partially regulated, gravel/cobble lower Yuba River in north–central California. The test metrics were the coefficient of determination (R²) and the median absolute residual ( ). These metrics quantified the incremental uncertainty 1D approximation incurs, results which make explicit cost–benefit processes of model selection possible. Finally, velocity residual maps were analysed to identify regions and processes where residuals were high, indicating divergence from the 1D assumptions. Paired data (1D–2D) fell between 0.94 ≥ R² ≥ 1.00 (R²_mean = 0.98 and R²_median = 0.99) for depth and median absolute residuals were all 3.8 ≤ ≤ 7.2% (i.e. 50% of residuals are approximately within ±1.7 to 3.6%). Higher flows and lower gradient reaches had lower residuals and higher R². Velocity diverged more, particularly for base flow in anastomosing reaches (0.42 < R² < 0.58). One‐dimensional, conveyance distributed, assumptions performed better for other channel types, where 0.69 < R² < 0.81 (R²_mean = 0.75 and R²_median = 0.77), with median absolute residuals between 9.6% > > 22.4% (i.e. ~ ± 4.6 to ±11.2%), where _mean = 14.2% and _median = 13% (~ ±7.1 and 6.5%). The conveyance distributed 1D velocity model performed best, where the orthogonal flow assumptions obtained and where side channels did not transition from backwater to conveying area between flows. Copyright © 2015 John Wiley & Sons, Ltd.     

Large wood loads in an urban stream: The role of recruitment limitation versus transport dominance

Large wood (LW) has important physical and ecological functions in streams. Riparian vegetation is extensively removed during urban expansion, and urban streams may experience enhanced fluvial transport of LW due to flashy hydrology. In this study, LW loads were assessed for three reaches on North Buffalo Creek, an urban stream located in Greensboro, North Carolina, United States. These three reaches have similar hydrology but different riparian vegetation densities. We measured the frequencies and sizes of both in-channel LW and riparian vegetation across the three reaches. Our results showed that the recently reforested reach had greater LW volume (22.5 m³/km) compared to the unmanaged forested site (16 m³/km) and the site with low riparian vegetation density (4.78 m³/km). The difference in LW frequency among reaches was statistically significant ($p=.05$). However, the difference in the volume of individual pieces was not significantly different across reaches ($p=.84)$, indicating that a similar size of wood is recruited across the three sites. Our findings also showed that there is a positive relationship between riparian vegetation frequency and in-channel LW frequency, which are significantly related as a power function. Spatial lag models (integrating upstream riparian trees) did not show better results compared to a non-lagged model, suggesting that storage and recruitment were predominantly local and that the LW distribution at our reaches is limited by recruitment rather than dominated by fluvial transport. Our findings suggested that a fully forested watershed is not needed to provide some of the benefits of wood to urban streams.     

壁面微结构流动控制技术的减阻机理研究

为了研究壁面微结构流动控制技术的减阻效应及其产生的原因,利用循环管路系统的方形管道进行了压降测定试验,并利用粒子成像测速仪测量了边界层内部结构和对应的参数。试验采用了沟槽和肋条两种不同类型的微结构壁面,每种形状的微结构各有3种不同的结构尺寸。试验研究结果表明:在一定的无量纲宽度s+范围内,6种不同的微结构壁面都具有减阻效果;减阻率随着s+的增大,呈现先增大后减小的趋势,其中沟槽壁面2的减阻效果最好,最大减阻率为9.90%;壁面微结构通过影响流场内部的涡结构、湍流脉动、雷诺切应力和平均流速等使得不同壁面微结构具有减阻效果。     

Analysis of the relationship for determining the bed load in sand channels

Conclusion The results of the analysis showed that in flatland rivers with a sand channel withu _*u _*0 where movement of the bed load is realized mainly in the form of steep waves and ripples [5], for determining the bed loadq _bd GGI's relationship (1), in whichq _bd is represented as a function of the height of the bed formsh _w, should be used. This relationship gives results substantially differing from the results calculated by relationships in whichq _bd is expressed as a function of the size of the sediment particlesd (8), (9) and valid for conditions not characteristic of sand channels of flatland rivers. Whenu _*t u _*<u _*0, when there is still no mass traction of all fractions of the channel deposits, the Einstein (8) or Paintal (15) relationships are more substantiated for all channels, but for practical calculations for sand channels of flatland rivers one can assume thatq _bd=0 whenu _*<u _*0.Translated from Gidrotekhnicheskoe Stroitel'stvo, No. 5, pp. 47–50, May, 1989.     

Flume simulations of salmon bioturbation effects on critical shear stress and bedload transport in rivers

Nest (redd) construction by female salmonids involves sequences of pit excavation and filling that winnow fines, loosen grains, and moves sediment downstream into a tailspill mound shaped like a dune. Prior research suggests that such bioturbation may destabilize streambeds by reducing friction between grains and converging flow that elevates shear stress on tailspills. Bed stability may alternatively be enhanced by form drag from redds that lowers basal shear stress, an effect that varies with the proportion of the bed that is occupied by redds (P). I used simulated redds and water‐worked (“unspawned”) beds in a laboratory flume to evaluate these competing influences on critical conditions and bedload transport in experiments with P = 0.11 (1 redd), 0.29 (2 redds), and 0.38 (3 redds). Results from competence (largest grain) and reference transport rate estimates of Shields stress indicate that particle entrainment inversely related to P. Bedload transport rates also increased as exponential functions of P and the boundary shear stress that exceeded critical conditions. Therefore, redd form drag did not overcome the destabilizing effects of redd construction. Instead, grain mobility and bedload transport increased with P because larger bed areas were composed of relatively loose grains and redd topography that experiences elevated shear stresses, as suggested in prior research. By winnowing fines and increasing bed surface mobility that exposes small particles in subsurface areas to flow, bioturbation by salmon can mitigate fine sedimentation of streambeds, which suggests an active role for salmon in restoring fish habitat in streams.     

Entrainment of riparian gravel and cobbles in an alluvial reach of a regulated canyon river

Many canyon rivers have channels and riparian zones composed of alluvial materials and these reaches, dominated by fluvial processes, are sensitive to alterations in streamflow regime. Prior to reservoir construction in the mid‐1960s, banks and bars in alluvial reaches of the Gunnison River in the Black Canyon National Monument, Colorado, USA, periodically were reworked and cleared of riparian vegetation by mainstem floods. Recent interest in maintaining near‐natural conditions in the Black Canyon using reservoir releases has created a need to estimate sediment‐entraining discharges for a variety of geomorphic surfaces composed of sediment ranging in size from gravel to small boulders. Sediment entrainment potential was studied at eight cross‐sections in an alluvial reach of the Gunnison River in the Black Canyon in 1994 and 1995. A one‐dimensional water‐surface profile model was used to estimate water‐surface elevations, flow depths, and hydraulic conditions on selected alluvial surfaces for discharges ranging from 57 to 570 m³/s. Onsite observations before and after a flood of 270 m³/s confirmed sediment entrainment on several surfaces inundated by the flood. Selective entrainment of all but the largest particle sizes on the surface occurred at some locations. Physical evidence of sediment entrainment, or absence of sediment entrainment, on inundated surfaces generally was consistent with critical shear stresses estimated with a dimensionless critical shear stress of 0.030. Sediment‐entrainment potential over a range of discharges was summarized by the ratio of the local boundary shear stress to the critical shear stress for d₅₀, given hydraulic geometry and sediment‐size characteristics. Differing entrainment potential for similar geomorphic surfaces indicates that estimation of minimum streamflow requirements based on sediment mobility is site‐specific and that there is no unique streamflow that will initiate movement of d₅₀ at every geomorphically similar location in the Black Canyon. Copyright © 2000 John Wiley & Sons, Ltd.     

Generation and evolution characteristics of the mushroom-like vortex generated by a submerged round laminar jet

This paper studies the formation mechanism and the evolution characteristics of the mushroom-like vortex generated by a submerged laminar round jet based on experiments, CFD simulation and a theoretica...     

Description of mechanical response including detachment using a novel particle model of biofilm/flow interaction.

Bacterial biofilms, while made up of microbial-scale objects, also function as meso- and macroscale materials. In particular, macro-scale material properties determine how biofilms respond to large-scale mechanical stresses, e.g. fluid shear. Viscoelastic and other constitutive properties influence biomass structure (through growth and fluid shear stresses) by erosion and sloughing detachment. In this paper, using the immersed boundary method, biofilm is modelled by a system of viscoelastic, breakable springs embedded in a fluid flow, evolving according to the basic physical laws of conservation of mass and momentum. We demonstrate in the context of computer simulation biofilm deformation and detachment under fluid shear stress.     

Assessing channel geometry in response to land use disturbance in a low-relief,glacially conditioned setting

Bankfull stage, the highest flow elevation contained by a river channel before overbank flooding occurs, is the presumed threshold for channel morphological change. At bankfull, the channel boundary experiences the highest shear stress, producing somewhat predictable hydraulic relationships and a cross-sectional form. However, land use and glacial conditioning can profoundly impact a channel's geomorphic responses. Two common methods characterize bankfull flow: one based on flow frequency and other based on channel form. This study considers a simplified approach to identify upstream land use relative to estimates of bankfull flow versus a channel's geometric form. The approach compares archived geomorphic surveys of 140 river reaches in southern Ontario, Canada, to 2-year flood quantiles modelled from historical flood data of 207 gauge stations. Flood frequency analysis determines that annual maximum series (AMS) datasets, fitted to optimized probabilistic distributions, underestimate discharge for low-magnitude, high-frequency flood events compared to partial duration series (PDS) datasets. For smaller drainage areas (<100 km²) associated with an extensive agricultural activity and/or urbanization, the estimates of bankfull discharge (Q_bf) generated by cross-sectional channel geometry are greater than the gauge-derived Q₂ values. Channels impacted by high levels of upstream land use disturbance show statistically significant lower width-to-depth ratios (p < 0.001) and a trend towards a finer D₅₀ bed material, suggesting enhanced surface runoff delivering more mobile finer sediments and channel confinement or incision. This research quantifies the formative bankfull stage to better understand the link between land use and a channel's ‘natural’ hydrogeomorphic response in a low-relief, glacially conditioned setting.     

Effects of stream hydraulics and other environmental variables on density of Narapa bonettoi (Oligochaeta) in the Paraná River system

Hydraulic and substratum conditions have been identified as two stream features which affect the benthic community composition, abundance and distribution. However, little attention has been given to the influence of hydraulic variables in large river beds. The aim of this study was to analyse the incidence of the near‐bottom hydraulic conditions and other environmental variables on the density of Narapa bonettoi (Oligochaeta, Narapidae), a typical and dominant species found in sandy bed rivers of diverse hierarchy of the Paraná River basin. A large amount of existing and available benthic data were used. The highest correlations among N. bonettoi densities and environmental variables were obtained with the hydraulic variables of friction Reynolds number (R_*) and shear stress (τ₀); the substratum type (sand, silt and clay) and organic matter content. The results show that N. bonettoi density would be related with the hydraulic variables following a ‘bell‐shaped’ tendency, e.g. with the friction Reynolds number, N. bonettoi would have a preference for transition values of turbulence (～40 < R_* < ～50), which is equivalent to τ₀ between ～0.6 and ～0.8 kg ^?2. Densities show a slight decreasing tendency toward the maximum R_* values, suggesting that the species does not support a completely developed turbulence (R_* > ～70). Significant but negative correlations were reported in the literature between benthic macroinvertebrates typical of smaller and steeper streams and R_* values far beyond the threshold 70 value. N. bonettoi shows a clear preference for a completely sandy substratum and for sand particles sizes around 300 µm. Finally, considering the important role played by the hydraulic variables on the benthic community studied herein, the contagious distribution pattern of N. bonettoi, reported by other authors, would be principally linked with the interactions between current and sediment near the bottom rather than with the mobility of these organisms. Copyright © 2008 John Wiley & Sons, Ltd.     

水沙动力学适用牛顿流体的基本方程

通过改进蔡树棠两相流基本方程的相间阻力项,建立能够进行二维解析研究的水沙动力学基本方程.该研究给出沙相压力和沙相粘性切应力的学科定义,以及它们的本构关系.对于牛顿流体,当输沙稳定时,挟沙水流的能坡与清水并无差别.沙相压力的二阶导数不为零是泥沙扩散条件,也是凹岸冲、凸岸淤自然现象的力学本质.蔡树棠方程隐含着流体的粘性切应力按运动组成可以分解或叠加的物理性质.正是这个结构性特点,给了解答浑水粘性这个古老的力学难题的方法,使我们能够完整地推导出牛顿流体沙相运动学粘滞系数vs公式;推导并证明了浑水综合粘滞系数μm=εv+ svνs,而不是习惯使用的μm=v+svvs.探讨了影响vs值的一些复杂因素,如颗粒表面束缚水的影响.本文研究能够证明:对分散相应用连续介质原理不仅可行,且是遵从压力传递规律惟一的正确方法.而揭示粘性切应力的结构性,在多相流和流变学领域具有深远的意义.     

The influence of coarse particle mobility on scour depth in salmonid spawning habitat

This study examined the influence of flow hydraulics and coarse particle mobility on bed scour adjacent to coho salmon (Oncorhynchus kisutch) redds in a coastal California watershed for a bankfull flood. It was theorized that coarse particle mobility (i.e., mobility of particles larger than the median bed particle size, D₅₀) exerts a strong control on bed scour depth. Maximum scour depth at the study sites was found to be negatively correlated with flow shear stress, which is dissimilar to findings from previous scour studies in spawning reaches. This resulted from a relatively similar coarse particle size (D₈₄) for all study sites and a negative relationship between shear stress and coarse particle exposure to flow (or the D₈₄/D₅₀ ratio), which together caused sites with low shear stress to have a high degree of localized coarse particle mobility and an associated high maximum scour depth. This study provides new insights into the vulnerability of spawning reaches with low flow energy to redd scour and highlights the need to consider the mobility of coarse particle sizes explicitly when examining the dominant controls on redd scour.     

Relations between complex hydraulics and the localized distribution of mussels in three regulated rivers

A stratified random sampling design was used to examine the relations between selected hydraulic variables and the density of unionids at five sites on the Green, Licking, and Rough Rivers of Kentucky. We located the strata to ensure that samples occurred in a wide range of hydraulic conditions at each site. Eight 0.25 m² quadrat samples were collected from each 25 m² stratum. We measured mean water column velocity, depth, and substrate roughness before we sampled each quadrat. ‘Fliesswasserstammtisch’ (FST) hemispheres were used to estimate shear stress. In all, we collected 798 individuals of 28 species of freshwater mussels. Simple hydraulic characteristics of our study sites were not correlated consistently with mussel density. For instance, water depth and mussel density were positively correlated in the Green River, negatively correlated in the Rough River, and not significantly correlated in the Licking River. In contrast, we found consistent negative correlations in all rivers between mussel density and complex hydraulic variables, such as shear velocity and FST hemisphere number. We believe that the limited recruitment observed in these rivers may have resulted from operation of upstream flood control dams that altered flow regimes seasonally. We suspect that the increased discharge during spring and early summer resulted in shear forces sufficiently high on mussel beds to prevent settlement of newly metamorphosed juveniles. Copyright © 2001 John Wiley & Sons, Ltd.     

明渠柱体上游马蹄涡的运动学特征研究

马蹄涡是水下柱体结构周围床面冲刷的主要动力。为掌握马蹄涡的运动学特征,引入了基于旋转强度的马蹄涡识别方法,提出了基于椭圆涡拟合的马蹄涡特征提取方法,其中,椭圆涡由Oseen涡和倾斜单向剪切流叠加而成。基于上述方法,通过开展明渠圆柱绕流PIV实验,对柱体上游对称面时均流场中马蹄涡的运动学特征进行了研究。结果表明,在柱体雷诺数大于104的紊流柱体绕流中,马蹄涡上游的流动分离点随柱体雷诺数的增大而缓慢向下游移动,但马蹄涡的位置保持不变,涡中心距柱体中轴约0.67D(D为柱体直径或宽度),距床面约0.06D;随着柱体雷诺数的增大,马蹄涡的半径不变而强度增加,使得马蹄涡作用于床面的剪切应力增大。研究结果建立了客观提取马蹄涡运动学特征的数学方法,得到了流动分离点、马蹄涡位置等运动学特征随水流条件的变化规律,揭示了明渠柱体绕流与其它流动中柱体绕流的差异,初步构建了马蹄涡的动力作用概念模型。     

基于修正立方定律对单裂隙辐射流剪切耦合的研究

修正了辐射流立方定律,并研究了裂隙的力学性能和水力特性。进行了不同恒定法向压力和剪切速率下的高液压水头剪切渗流实验。发现峰值剪切强度随剪切速率的增加呈现先增大后减小的趋势。线性系数A和非线性系数B都随剪切位移先增大后减小,且减小后的稳定值小于剪切破坏前的值。但是系数B比系数A大5个数量级,变化幅度更大。临界雷诺数经剪切破坏后大于剪切破坏前的值,流态将变好。此外,剪切过程中的水力特性也表现出与力学性能相似的趋势。     

掺气挟沙对高速水流边界剪切力特性影响的分析

在高速水流循环系统中，用自行研制的剪切力传感器分别量测了清水、掺气水流及挟沙水流壁面上的剪切力；实验表明，含沙掺气高速水流壁面剪切力的脉动值的概率密度函数分布基本符合正态分布规律；文中根据实测资料，分析了剪切力的时均值和脉动值与水流速度的关系；讨论了掺气及含沙量对壁面边界剪切力的影响；在同一流速下，剪切力的时均值及脉动值随着含沙量的增加而略有增加，随着掺气浓度的增加而降低。     

Biofilm early stage development in two nutrient‐rich streams with different urban impacts

The aim of this study was to describe the colonization of the biofilm during its early stages under different concentrations of nutrients and organic matter, specifically in urban streams influenced by agriculture and urbanization. We hypothesized that in a stream with higher concentrations of nutrients and organic matter, the initial biomass growth would be faster, and the changes in the structure of the community would be greater. Sterile glass substrates were placed in 2 urban streams that differed in nutrient and organic matter concentrations; samples were collected during their first week of colonization to measure total biomass, bacterial biomass, chlorophyll a, activity of the electron transfer system, and the community composition. Results show that biofilm development in both streams began within a few hours and differed under different conditions of nutrients; in the stream with a better water quality, the colonization dynamics consisted of 2 increments of bacterial biomass linked with an increase of algal biomass. In the urban stream with higher nutrient and organic matter concentrations, biofilm development was slower and consisted of a simultaneous increase of bacteria and algae, consistent with a lower electron transfer system activity. Therefore, the dynamics of the colonization process in addition to those characteristics of the fully developed biofilm could have potential applications in water monitoring of urban streams.     

Evaluation of Owabi Reservoir (Ghana) water quality using factor analysis

The Owabi Reservoir receives water inflows from several streams that drain through the rapidly urbanizing towns in the Kumasi metropolis. The reservoir catchment had been encroached upon by various human activities, resulting in the generation of large volumes of waste water being discharged into streams draining to the reservoir. This study presents a comprehensive evaluation of the water quality of Owabi Reservoir. Water samples from 10 sampling sites within the reservoir were analysed for various water quality parameters; namely, pH, electrical conductivity (EC), sulphate (), phosphate (), chloride (Cl^?), lead (Pb), arsenic (As), zinc (Zn), iron (Fe), copper (Cu), total dissolved solids (TSS), turbidity, faecal coliform and E. coli. The measured water quality parameters were compared to the guidelines proposed by the WHO for drinking water and natural background levels. Mean values for Pb, As, turbidity, TDS, faecal coliform and E. coli were greater than the WHO guidelines. Factor analysis for the measured parameters resulted in the extraction of five factors accounting for 83.13% of the total variance. Factor 1 showed strong loadings for , and TDS, reflecting a negative influence on the water quality by agricultural discharges. Factor 2 showed high loadings for EC, Cl^? and apparent colour, while Factor 3 registered high loadings for colour and faecal coliform, suggesting domestic wastewater discharges and the presence of decaying organic matter. Factors 4 and 5 showed high loadings for copper, lead, turbidity and E. coli, an indication of both organic and inorganic pollution. Accordingly, periodic monitoring of Owabi Reservoir water quality, and the streams draining into it, is recommended as a means of ensuring good water quality, as well as facilitating the identification of the possible types and sources of water pollutants entering the reservoir.     

Growth, structure and oxygen penetration in particle supported autotrophic biofilms.

Particle supported autotrophic biofilms were cultivated in external-loop airlift reactors at two different pumice concentrations. Oxygen microelectrodes were used to investigate substrate transport and conversion. A special flow cell was designed for the measurement of oxygen concentration profiles in the particle supported biofilms under defined hydrodynamic conditions. The oxygen concentration profiles inside the biofilms were found to be steeper at high flow velocities in the bulk phase of the flow cell compared to those at low flow velocities. Furthermore, the oxygen flux increased and the thickness of the concentration boundary layer decreased with increasing flow velocity. This dependence was found to be more pronounced in less dense biofilms out of airlift reactors with lower pumice concentrations. In addition confocal laser scanning microscopy (CLSM) was used to visualize the biofilm structure. The volume fractions of bacteria and extracellular polymeric substances (lectin-specific EPS-glycoconjugates) were measured in living fully hydrated biofilms. Both the microelectrode and CLSM measurement showed the influence of shear stress on particle supported biofilms. A higher particle concentration led to dense biofilms with a homogeneous surface, lower thickness of the concentration boundary layer and steeper oxygen concentration profiles. The combination of both techniques allows a detailed and quantitative characterisation of particle associated biofilm structure and function.     

Simulated recruitment of riparian trees and shrubs under natural and regulated flow regimes on the Wisconsin River,USA

Models that link ecological responses to hydrologic changes are important for assessing the effects of flow regulation on aquatic and riparian ecosystems. Based on the Recruitment Box Model, a graphical model used to prescribe environmental flows for cottonwood (Populus spp.) recruitment, we designed a simulation model to represent the influence of river flow dynamics on seedling recruitment of riparian pioneer woody plants. The model simulates the influence of temporal patterns of river stage on dispersal, germination, initial recruitment and over‐winter survival of first‐year seedlings of riparian pioneer shrubs and trees. We used the model to simulate seedling recruitment patterns for five species (Acer saccharinum, Betula nigra, Populus deltoides, Salix nigra and Salix exigua) on the Wisconsin River (Wisconsin, USA) under three flow scenarios: historic (1935–2002), simulated natural (1915–1975) and simulated regulated flows (1915–1975). Simulation results agreed well with field‐observed relative differences among years (1997–2000) in seedling densities for the five focal species. Simulated successful recruitment years were highly synchronous among species, but species differed in their sensitivity to flows at different times during the growing season, consistent with among‐species differences in seed dispersal timing. Comparison of simulated natural and regulated flows for 1915–1975 showed that flow regulation decreased monthly flow variability, increased late summer to winter baseflow and reduced the magnitude of spring peaks. Simulated recruitment and over‐winter survival of tree seedlings of all species was enhanced under the regulated flow scenario, likely due to increased summer baseflow and reductions in peak flood magnitude. Our analyses show the utility of extending the Recruitment Box Model to include multiple species of riparian shrubs and trees, and the effects of post‐colonization flows on their recruitment success. However, some key functional relationships between flow patterns and woody seedling demography (e.g. shear stress thresholds for seedling mortality) have not been adequately quantified and merit further study. Copyright © 2006 John Wiley & Sons, Ltd.