Functional Relationships of Resistance in Wide Flood Plains with Rigid Unsubmerged Vegetation

The effect of rigid, unsubmerged vegetation on flow resistance in wide flood plains was examined in this study. A series of experiments was run using rigid rods inserted into the flow field of a wide shallow flume. Several configurations of rod diameters with various lateral and longitudinal spacing were tested over a range of discharge values. Head losses across the rods were measured and a resistance-to-momentum-absorbing area concept was employed to relate density parameters to the resistance coefficient. The effects of flow depth, velocity, rod diameter (d), lateral spacing (r), and longitudinal spacing (l) were estimated. The results demonstrated that flow resistance is greatly impacted by both depth and velocity but that the effects are opposite in sign. The friction factor increased in only a slightly nonlinear fashion with depth, but it decreased in a highly nonlinear manner with increasing velocity, and the velocity effects were very sensitive to vegetation density parameters. Of the density parameters tested, the effects of diameter and lateral spacing were far more significant than are those of longitudinal spacing. The results also demonstrated that the effects of all parameters (density and depth) could be effectively combined into one relative density ratio (total vegetation stomatal area/bed cross sectional area) that was linearly related to the Darcy f and the Manning n.     

Riparian Vegetation Mapping for Hydraulic Roughness Estimation Using Very High Resolution Remote Sensing Data Fusion

For detailed hydraulic modeling, accurate spatial information of riparian vegetation patterns needs to be derived in automatic fashion. We propose a supervised classification for heterogeneous riparian corridors with a low number of spectrally separate classes using data fusion of a Quickbird image and LIDAR data. The approach considers nine land cover classes including three woody riparian species, brush, cultivated areas, grassland, urban infrastructures, bare soil and water. The classical “stacked vector” approach is adopted for data fusion, while the nonparametric weighted feature-extraction method and the pixel-oriented maximum likelihood algorithm are used for feature-reduction and classification purposes, respectively. We test the approach over a 14-km stretch of the Sieve River (Tuscany Region, Italy). A one-dimensional river modeling is applied over the study reach comparing the results of a classification-derived hydraulic roughness map and a traditional ground-based approach. Despite the complex study reach, the classification method produced encouraging accuracies (OKS = 0.77) and represents a useful tool to delineate application domains of flow resistance models suited to different hydrodynamic patterns (e.g., stiff/flexible vegetation). Hydraulic modeling results showed that the remotely derived floodplain roughness parameterization captures the equivalent Manning coefficient over 20 test cross sections with uncertainty distributions described by low mean and standard deviation values.     

Hydraulic Radius for Evaluating Resistance Induced by Simulated Emergent Vegetation in Open-Channel Flows

The resistance induced by simulated emergent vegetation in open-channel flows has been interpreted differently in the literature, largely attributable to inconsistent uses of velocity and length scales in the definition of friction factor or drag coefficient and Reynolds number. By drawing analogies between pipe flows and vegetated channel flows, this study proposes a new friction function with the Reynolds number that is redefined by using a vegetation-related hydraulic radius. The new relationship is useful for consolidating various experimental data across a wide range of vegetation density. The results clearly show a monotonic decrease of the drag coefficient with the new Reynolds number, which is qualitatively comparable to other drag coefficient relationships for nonvegetated flows. This study also proposes a procedure for correcting sidewall and bed effects in the evaluation of vegetation drag.     

Flow Resistance Law in Channels with Flexible Submerged Vegetation

In this paper, experimental data collected in a straight flume having a bed covered by grasslike vegetation have been used to analyze flow resistance for flexible submerged elements. At first, the measurements are used to test the applicability of Kouwen’s method. Then, a calibration of two coefficients appearing in the semilogarithmic flow resistance equation is carried out. Finally, applying the Π-theorem and the incomplete self-similarity condition, a flow resistance equation linking the friction factor with the shear Reynolds number, the depth-vegetation height ratio and the inflection degree is deduced.     

Linking Pathogen Sources to Water Quality in Small Urban Streams

Alternative measures of terrestrial pollutant loading were investigated to identify those that are better predictors of water quality in urban streams. Results from 18 watersheds with the same climatic conditions show that the density of terrestrial fecal-coliform loading is a better indicator of median instream concentrations than total terrestrial fecal-coliform loading. Watersheds with fecal-coliform loading densities less than around 2×1011?cfu?km?2?day?1 generally had median instream concentrations less than the reference water-quality standard of 400 cfu/100 mL. Median instream concentrations were also less than the reference water-quality standard for population densities less than around 400?persons?km?2. For any given terrestrial loading intensity or population density, summer conditions of high rainfall and high temperature generally resulted in the greatest water-quality impacts. These results are particularly useful in determining terrestrial loading reductions in support of TMDLs, and in focusing best management practices.     

Approach to Separate Sand from Gravel for Bed-Load Transport Calculations in Streams with Bimodal Sediment

The bed material found in gravel-bed streams is nonuniform in terms of grain size and can typically be classified as unimodal or bimodal. The latter type of sediment distribution is usually represented by two modes, one of sand size and another of gravel. For this case, the movement of one mode becomes nonlinearly influenced by the other. As a result, the presence of the two modes in a bimodal material complicates the calculation of bed-load transport rates. The present study proposes an approach to separate the calculation of bed-load transport rates for bimodal materials into two independent fractions of sand and gravel, thereby rendering the bed sediment into two unimodal components. This approach is accomplished by decoupling the two fractions through scaling the reference Shields stresses of the sand and gravel modes to match the value of the mode of unimodal materials. Consequently, the contribution of each fraction to bed load can be estimated using a suitable relation derived for unimodal materials. Laboratory and field bed-load data available in the literature are used to examine the validity of the overall approach.     

Hydraulic Resistance of Flow in Channels with Cylindrical Roughness

A laboratory study on the hydraulics of flow in an open channel with circular cylindrical roughness is presented. The laboratory study consists of an extensive set of flume experiments for flows with emergent and submerged cylindrical stems of various sizes and concentrations. The results show that the flow resistance varies with flow depth, stem concentration, stem length, and stem diameter. The stem resistance experienced by the flow through the vegetation is best expressed in terms of the maximum depth-averaged velocity between the stems. Physically based formulas for flow resistance, the apparent channel velocity, and flow velocities in the roughness and surface layers are developed. The formulas are validated with the flume data from the present study as well as those from past studies. A method for calculating channel hydraulic conditions using these formulas is presented.     

Experimental Study of Bed Load Transport through Emergent Vegetation

Vegetation is an important agent in fluvial geomorphology and sedimentary processes, through its influence on the local hydraulics that determine sediment transport. Within stands of emergent vegetation, bed shear is substantially reduced through the absorption of momentum by drag on the stems. This stimulates deposition of sediment and reduces capacity for bed load transport. The effect of emergent vegetation on hydraulic parameters (including equilibrium bed gradient, flow depth, and velocity) and on bed load transport rate has been investigated experimentally for one sediment size, stem diameter, and stem spacing. Bed load transport rate was found to be closely related to bed-shear stress, which must be estimated by partitioning total flow resistance between stem drag and bed shear.     

Hydraulic Resistance in Grass Swales Designed for Small Flow Conveyance

Grass swales, originally used for erosion control in agricultural settings, are now widely employed in urban environments as an effective best management practice for controlling pollutants in stormwater runoff. In particular, vegetated swales are quite successful in removing heavy metal concentrations when the depth of flow is small relative to grass height. However, guidance materials currently available for design of vegetated channels focus on larger depths of flow (large flow conveyance/erosion control), and for such conditions the hydraulic resistance exerted by the vegetation can be significantly different than that observed when the depth of flow is small (remediation). Utilizing a series of laboratory channels, small-flow retardance curves have been developed in the present work for Bluegrass, Centipede, and Zoysia grass species. These “small-flow” curves extend the well-known Stillwater n versus VR diagram by approximately 1 order of magnitude, to smaller values of VR. Experimental results should provide valuable design guidance to those faced with the need to hydraulically design a swale intended for shallow depths of flow.     

Turbulent Flow through Idealized Emergent Vegetation

This paper presents results of several large-eddy simulations (LES) of turbulent flow in an open channel through staggered arrays of rigid, emergent cylinders, which can be regarded as idealized vegetation. In this study, two cylinder Reynolds numbers, RD = 1,340 and RD = 500, and three vegetation densities are considered. The LES of the lowest density and at RD = 1,340 corresponds to a recently completed laboratory experiment, the data of which is used to validate the simulations. Fairly good agreement between calculated and measured first- and second-order statistics along measurement profiles is found, confirming the accuracy of the simulations. The high resolution of the simulations enables an explicit calculation of drag forces, decomposed into pressure and friction drag, that are exerted on the cylinders. The effect of the cylinder Reynolds number and the cylinder density on the drag and hence on the flow resistance is quantified and in agreement with previous experimental studies. Turbulence structures are visualized through instantaneous pressure fluctuations, isosurfaces of the Q-criterion and contours of vertical vorticity in horizontal planes. Analysis of velocity time signals and distributions of drag and lift forces over time reveals that flow and turbulence are more influenced by the vegetation density than by the cylinder Reynolds number.     

Numerical Solution of Fully Developed Flow with Vegetative Resistance

This paper presents a numerical solution of the Reynolds averaged Navier-Stokes and the near-wall k- (turbulent kinetic energy) and ω- (specific dissipation or dissipation per unit kinetic energy) transport equations, which are modified to include vegetative drag terms. For similar treatment of the model coefficients, the use of the near-wall k-ω model produces similar results to previous models that employed the standard k-ε models with wall functions. The study shows that reasonable predictions of streamwise velocity and Reynolds stress profiles can be achieved by adopting universal values for all model coefficients, but the calculated energy gradient can have significant error. The study also indicates that predictions of streamwise turbulence intensity are significantly improved by adopting the universal values of Cfk = 0.05 and Cfω = 0.16 rather than the theoretically based values, Cfk = 1.0 and Cfω = β/αβ?Cfk.     

Use of Aerial Photography to Inventory Aquatic Vegetation

This study demonstrates the feasibility of using low‐altitude aerial photography to inventory submersed macrophytes in the connecting channels of the Great Lakes. For this purpose, we obtained aerial color transparencies and collateral ground truth information about submersed vegetation at 160 stations within four study sites in the St. Clair and Detroit rivers, September 17 to October 4, 1984. Photographs were interpreted by five test subjects to determine with what accuracy they could detect beds of submersed macrophytes, and the precision of delineating the extent of such vegetation beds. The interpreters correctly determined the presence or absence of vegetation 80% of the time (range 73–86%). Differences between individuals were statistically significant. Determination of the presence or absence of macrophytes depended partly on their relative abundance and water clarity. Analysis of one photograph from each of the four study sites revealed that photointerpreters delineated between 35 and 75 ha of river bottom covered by vegetation. This wide range indicates that individuals should be tested to assess their relative capability and be trained before they are employed to delineate plant beds in large‐scale inventories. Within limits, low‐altitude aerial photography, combined with collateral ground truth information, can be used to determine the presence or absence and delineate the extent of submersed macrophytes in connecting channels of the Great Lakes.     

Equivalent Roughness of Gravel-Bed Rivers

The relation between the equivalent roughness and different grain size percentiles of the sediment in gravel-bed rivers was determined under the hypothesis that the vertical distribution of the flow velocity follows a logarithmic law. A set of 954 data points was selected from rivers with gravel size sediment or larger, with a nonsinuous alignment and free of vegetation or obstacles. According to the results, the ks roughness is equivalent to approximately 2.4D90, 2.8D84, and 6.1D50. No correlation was detected between the sediment sorting and the sediment mobility index on one hand and, on the other, the coefficient of proportionality of each grain size percentile.     

Dynamic Routing of Flow Resistance and Alluvial Bed-Form Changes from the Lower to the Upper Regime

On the basis of a Strouhal number and the definition of the control factor, m, a new routing to calculate the energy slope in the lower and upper alluvial regimes is proposed. The control factor, m, representing the interactions in alluvial rivers, is reckoned as a bed-form index: while the flow evolves through transition, the control factor, m, decreases from m = 2, associated with two-dimensional fully developed dunes, to m = 1, associated primarily with in-phase waves. The way to predict the value of the control factor, m, is drawn from a previously published criterion for delineating the upper regime and is calibrated with experimental data. On several data from flumes and rivers, the routing is tested and compared with other methods from the literature. It appears that the new routing is the most robust because it allows researchers to obtain low averages of the discrepancy ratio for a wide range of ratios between the water depth and the median sediment diameter. On a selection of contrasted freshet events, the new routing allows for the capture of the primary dynamic of the flow resistance decrease.     

Numerical Study of the Transition Regime between the Skimming and Wake Interference Flows in a Water Flume by Using the Lattice-Model Approach

A numerical study to describe the transition regime between the skimming and wake interference flows due to the influence of an idealized bed roughness in a water flume was carried out here using the lattice model approach. The model reproduced the skimming, transition, and wake interference regimes for different aspect ratios that determine the bed roughness geometry. The simulated turbulent structures were visualized by drawing the trajectories of a large number of passive tracer particles released in the computational domain, and the results agreed with those reported by the research works. The dimensionless streamwise and vertical turbulent intensities were calculated at five test sections. The results obtained supported the visualized flow patterns permitting us to detect the presence of a shear layer developed at the top of the roughness element, whose strength varied according to the flow regime simulated.     

Impact of Foliage on the Drag Force of Vegetation in Aquatic Flows

The objective of this study was to determine the contribution of a plant’s foliage to the total plant’s hydrodynamic drag. Experiments were conducted in a laboratory flume using samples of vegetation with different physical forms and biomechanical properties: Branches of pine (Pinus sylvestris) and ivy stipes (Glechoma hederacea). The drag force was measured directly using a strain gauge technique and determined for a series of velocities for each vegetation species with and without foliage. Experimental results revealed a distinct contribution of foliage to the total plant drag. For both plant types, this was particularly marked at lower velocities where the foliage is not streamlined and compressed and, hence, the frontal projected area of the plant is at a maximum. It was found that the flexibility of the plant’s foliage and its ability to streamline with the flow may reduce the overall drag considerably. There was a distinct difference in the CdAP parameter-velocity squared relationship between the “with” foliage plants and nonfoliage counterparts due to the streamlining effect of the foliage with the flow and, hence, the reduction in overall drag associated with the new compressed plant form.     

Rate and Volume of Stream Flow Depletion due to Unsteady Pumping

Analytical but approximate methods are developed for obtaining pumping induced rate and volume of stream flow depletion, which can account for unsteady (any variation) pumping discharge and are also applicable for intermittent pumping and recovery. Exact analytical solutions for a sinusoidal variation in the pumping discharge are proposed; the proposed methods are verified using these solutions. The proposed methods use ramp kernels that give results superior to the conventional convolution. These ramp kernels assume the linear variation in pumping discharge between the two consecutive discretized points as opposed to the uniform variation assumed in the conventional convolution. The proposed solutions are applicable for homogeneous and isotropic aquifers hydraulically connected to streams.     

Case Study: Bed Resistance of Rhine River during 1998 Flood

Detailed field measurements during the 1998 flood of the Rhine River in The Netherlands show that both Manning n and Darcy–Weisbach friction factor f increase with discharge. The changes in bedform roughness height and friction factors are attributed to the increased dune height during floods. There is a near-peak hysteresis in the dune height measurements. At a given discharge, dunes are significantly larger after than before the peak discharge. The trend is most apparent for the Bovenrijn with weaker variations for the Waal. The methods of Engelund and Vanoni–Hwang provide similar estimates of form drag. When combined with van Rijn’s method to estimate grain resistance, both methods tend to overpredict the measured bed friction factor after the peak discharge. These methods perform best when field bedform measurements are available to estimate form drag. The composite effect of primary and secondary dunes should be considered in the analysis of resistance to flow.     

Flow Distribution Parameters in Relation to Flow Resistance in an Upflow Anaerobic Sludge Blanket Reactor System

The use of flow resistance in the distribution of flows is well known in traditional hydraulics. To evenly distributed flows, flow resistance forms the basis of flow distribution in pipes connected in parallel. Flow distribution in different zones of upflow anaerobic sludge blanket (UASB) reactors is well documented in existing literature, and so far modeling of flow distribution parameters, i.e., the fraction of inflow entering into the bed, the fraction of flow bypassing over the bed and entering into the blanket, and the fraction of inflow to the bed entering into the blanket, has remained empirical in nature. The role of flow resistance in the distribution of flows in UASB reactor systems is still unexplained. In this study, some of the available data on flow distribution parameters are analyzed to assess if there is any correlation between these parameters and flow resistance. It is found that with an increase in flow resistance in the UASB reactor system, the magnitude of short-circuiting flows at the reactor bed increases. Also, the flow distribution at the blanket and settler levels of UASB reactor systems is related to parameters influencing flow resistance. Some of the functional forms derived in this study are expected to form the basis for representing flow distribution in the simulation studies of UASB reactor performance.     

Rapid Calculation of Oxygen in Streams: Approximate Delta Method

The “approximate delta method” is a simple procedure for simultaneous calculation of the stream reaeration coefficient, primary production rate, and respiration rate from a single-station stream diurnal profile of dissolved oxygen (DO). It approximates the exact graphs of results for the “delta method” reported in 1991 by Chapra and Di Toro by means of simple logistic curve-fitting approximations. The necessity of reading graphs or of obtaining numerical solutions is thereby avoided, so making it suitable for inclusion in a decision support system, particularly for streams reaeration coefficients less than 10?day?1 and for moderate photoperiods (10–14?h). Worked examples are given for streams in the USA and in New Zealand. Results are used to show that the constellation of parameters for the three fundamental processes is much more important than their individual values in calculating diurnal DO profiles. Independent measurement of the reaeration coefficient enhances the utility of the method, by enabling separate calculation of production and respiration rates.