Goodness-of-Fit Test for Modeling Tracer Breakthrough Curves in Wetlands

Wetland transport models generally either assume plug flow (with or without dispersion) or conceptualize the wetland as a series of continuously stirred tank reactors (CSTRs). To evaluate the CSTR approach, we present a goodness-of-fit test suitable for evaluating breakthrough curves from tracer experiments. The test, which makes use of confidence intervals associated with the multivariate normal distribution, can be used to test the fit of the breakthrough curve model, but requires sampling across a transect rather than from a single point. To test the CSTR assumption, we conducted a pair of two-dimensional tracer experiments within a 9.9?ha wetland constructed to receive effluent from a wastewater treatment plant in San Jacinto, Calif. The wetland operates with five parabolic inlets and a single large parabolic outlet to encourage lateral uniformity. In both experiments tritium oxide (HTO) was used as the tracer. Rhodamine WT dye was also included in the second experiment. Tracer samples were collected along transects installed perpendicular to the direction of flow. Analysis of the results indicates satisfactory lateral mixing and no significant short-circuiting. Rhodamine WT dye performed similarly to HTO when detectable but was too dilute to be observed at the outlet. When tracer movement was modeled as a series of continuously stirred reaction vessels, the parameter associated with the integer number of vessels increased from 2 at the first transect to 8 at the outlet. At each transect, the model was checked with a new goodness-of-fit test. At the α = 0.05 confidence level, all fitted models were rejected, suggesting that while the CSTR assumption may usefully approximate transport processes, it is not statistically valid for this wetland.     

Water Quality and Ecosystem Modeling of Tidal Wetlands

A water quality and ecosystem model is developed to simulate nutrients, heavy metals, and aquatic plants in the Erh-Chung Flood Way wetland in Taiwan. A sediment system was incorporated into the model. The RMA2 and WASP/EUTRO5 models were adopted as the basic framework with modifications and enhancement of kinetics to incorporate ecosystem dynamics and sediment-water interactions. Hydrodynamic results from the RMA2 model were used to quantify mass transport for the EUTRO5 model. The major effort in this study was adding four water quality variables; macrophyte biomass, suspended solids, heavy metals in macrophytes, and heavy metals in the water column and sediment were incorporated into EUTRO5 to form the water quality and ecosystem model. Site-specific water quality data were collected to support the model calibration and verification analyses.     

Data Requirements for Load Estimation in Well-Mixed Tidal Channels

Load estimation is an important component of watershed analysis and management, particularly in light of the Total Maximum Daily Load program set forth by the Clean Water Act. This paper addresses load estimation in tidally driven channels that are dominated by advection and well mixed over the channel cross section. Such systems are common in Southern California, and represent an important link between watersheds and the coastal ocean. The oscillating flow field of tidal channels calls for a time series of data (volumetric flow rate and scalar concentrations) sampled at a sufficiently high frequency to resolve the net load of each scalar. Results of an experiment conducted in the Talbert Channel in Huntington Beach, California, to quantify freshwater loads suggest that sampling should be conducted at a minimum frequency of 0.5/h. In this system, traces of freshwater were detected in the channel by salinity depressions that ranged from 1–7 ppt and lasted 4–6 h. The freshwater load was computed as the integral of the volumetric flow rate multiplied by the freshwater fraction of the water column. Based on the duration of these events and the 0.5/h sampling frequency, these results suggest that loads will be resolved to a reasonable level of accuracy when each event is captured by at least three or four data points. Strategies to estimate the flow rate and scalar concentrations are discussed.     

Dispersion Model for Varying Vertical Shear in Vegetated Channels

A dispersion model for a wide range of depthwise vertical shear is derived by using perturbation analysis and power (m) law velocity profile. For m = 1, the velocity profile provides linear shear, whereas m>1 provides nonlinear shear, and for m>20, the velocity profile resembles the flow through emergent vegetation. The power law represented parametrically simulates well the complex shear profiles involved in emergent and submerged vegetated flows. The proposed model shows reasonable agreement with past data on vegetated flows for a wide range of nonlinear shear velocities.     

Two-Dimensional Approximation for Tidal Dynamics in Coastal Aquifers: Capillarity Correction

Most previous investigations on tide-induced watertable fluctuations in coastal aquifers have been based on one-dimensional models that describe the processes in the cross-shore direction alone, assuming negligible along-shore variability. A recent study proposed a two-dimensional approximation for tide-induced watertable fluctuations that took into account coastline variations. Here, we further develop this approximation in two ways, by extending the approximation to second order and by taking into account capillary effects. Our results demonstrate that both effects can markedly influence watertable fluctuations. In particular, with the first-order approximation, the local damping rate of the tidal signal could be subject to sizable errors.     

Bond Enhancement for FRP Pile Repair in Tidal Waters

Vacuum bagging and pressure bagging are established techniques used by the composites industry for fabricating components. This paper describes a study that explored the adaptation of these techniques for improving the FRP-concrete bond in the repair of partially submerged piles. Prototype vacuum bagging and pressure bagging systems were developed and bond improvement assessed from results of pullout tests on full size piles repaired under simulated tide in the laboratory. Pressure bagging gave better bond and was found to be simpler because it did not require an airtight seal. A field demonstration project was conducted in which pressure bagging was used in combination with two different GFRP systems to repair two corroding piles supporting the Friendship Trails Bridge across Tampa Bay. Inspection of the postcured wrap showed no evidence of air voids. The study demonstrates that techniques developed by the composites industry may be readily adapted to provide effective and inexpensive means for improving FRP-concrete bond.     

Evaluation of Storm-Water Wetlands in Series in Piedmont North Carolina

Three storm-water wetlands in series were monitored in a heavily urbanized 12.5 ha watershed in Mooresville, North Carolina. Monitoring of this system allowed an examination of the diminishing returns provided by three successive best management practices (BMPs) of a similar type. At least 80% of the total concentration reduction for all pollutants occurred within the first wetland cell. Only the first wetland cell significantly (p<0.05) reduced all pollutants tested. No pollutant was significantly reduced from the outlet of Wetland Cell 2 to the outlet of Wetland Cell 3 (p<0.05). Median complete system (outlet of Wetland Cell 3) effluent concentrations for total suspended solids, total phosphorus, total nitrogen, and turbidity were 8, 0.09, 0.73 mg/L, and 10 NTU, respectively, which compared favorably to published results. Organic nitrogen generated from wetland vegetation seemed to result in a background source of nitrogen in the wetlands, supporting the idea of an irreducible concentration for nitrogen in these systems. The results indicate that the successive BMPs in a series do not perform as well as the first when each BMP uses similar removal mechanisms.     

Optimal Self-Scheduling of a Tidal Power Plant

This paper considers the optimal operation of a tidal power producer. Different generating cycles for the tidal power plant are considered. The paper provides a tool for self-scheduling of the tidal power producer so that the maximum profit from selling energy in an electricity market is achieved, while meeting all technical operating constraints of the tidal power plants. The formulation provided is also valid in a centralized operation framework; the only change needed is to define a different cost-related objective function. The resulting optimization problem, which is mixed-integer and linear, is solved using commercially available software. Results from realistic simulations are reported and analyzed.     

Reducing Dispersion of Linear Triangular Elements for the Helmholtz Equation

The Galerkin/least squares (GLS) modification improves the performance of finite-element computations of time-harmonic acoustics at high wave numbers. The design of the GLS resolution-dependent method parameter for two-dimensional computation in previous work was based on dispersion analysis of one-dimensional and square bilinear elements. We analyze the dispersion of linear triangular finite elements, and define method parameters that eliminate dispersion on a hexagonal patch. Numerical tests compare the performance of the proposed method with established techniques on structured and unstructured triangular meshes. Based on this work, we propose a method parameter that may be used for computation with both linear triangular and bilinear quadrilateral elements.     

Persistence of Skewness in Longitudinal Dispersion Data: Can the Dead Zone Model Explain It After All?

Field studies reporting coefficients of temporal skewness that do not decrease in the main flow direction have cast doubt on the transient storage (TS) or dead zone model of longitudinal dispersion in rivers and streams. In this study, the conditions under which the TS model predicts persistent or growing skewness coefficients are investigated. The findings clearly show that, though not outright impossible, an instantaneous slug release into a uniform channel reach is, indeed, extremely unlikely to result in persistent or growing skewness coefficients. In contrast, the passage of a tracer or pollutant along a sequence of (hydraulically) different subreaches may easily give rise to nondecaying skewness coefficients, the occurrence of which is governed by the parameter sets of the subreaches concerned. Thus, the TS model does show a certain potential to explain the persistence of skewness. The findings reported here are expected to be useful in guiding future field studies on the subject. An application of the newly derived criterion to stream tracer data has been successful.     

Analytical Solution for Tidal Propagation in a Leaky Aquifer Extending Finite Distance under the Sea

This paper focuses on groundwater dynamics in response to the tidal fluctuation in a coastal aquifer system. An analytical solution is derived to describe groundwater level fluctuation in a leaky aquifer extending finite distance under the sea. Based on this solution, the joint effects of various parameters, such as the dynamic effect of water table fluctuation and the leakages of the inland and offshore, on the behavior of the groundwater level fluctuations in the inland part of the leaky confined aquifer can be thoroughly analyzed. When the roof length is increased, the dynamic effect of the water table fluctuation on the dimensionless groundwater amplitude, intrusion distance, and fixed phase shift in the unconfined aquifer become more important and the water table fluctuation approaches constant values when the roof length is greater than a threshold value. However, given the same values of dimensionless leakage and roof length, the dimensionless groundwater amplitude, intrusion distance, and fixed phase shift in the leaky aquifer with considering the dynamic effects are always larger than those of neglecting such effects.     

Simulation of Bed-Load Dispersion Process

Two general dispersion models suitable for nonequilibrium bed-load transport were constructed. The first one, called the P model, is based on the probability of migration for specific groups of sediment particles. The second one, called the D model, is derived from the advection equation discretized in finite-difference form, which is equivalent to the general dispersion equation. By comparing these models, it is found that the D model can be treated like the P model in some respects. The Courant number, Cr, in the D model has the same physical meaning as the probability of migration, P, in the P model. Although the D model and P model were based on different concepts, the simulated bed-load transport rates, which result from their application, are the same. Therefore, the dispersion equation was replaced by the numerical algorithm of the advection equation (D model) to examine several dispersion phenomena of bed-load transport. To explore further the nonequilibrium dispersion process, a series of flume experiments was conducted by using color-painted fine gravels. Having compared model simulation results and experimental data, it is shown that the models derived in this study have a reasonably good agreement with the experimental results. In summary, this study has indirectly proven that the D model, which is equivalent to the dispersion equation, is capable of simulating the nonequilibrium bed-load transport.     

Undular Tidal Bores: Basic Theory and Free-Surface Characteristics

The present study examines the free-surface properties of undular tidal bores observed for 1相似文献   

Stage–Discharge Relations for Low-Gradient Tidal Streams Using Data-Driven Models

Development of stage–discharge relationships for coastal low-gradient streams is a challenging task. Such relationships are highly nonlinear, nonunique, and often exhibit multiple loops. Conventional parametric regression methods usually fail to model these relationships. Therefore, this study examines the utility of two data-driven computationally intensive modeling techniques namely, artificial neural networks and local nonparametric regression, to model such complex relationships. The results show an overall good performance of both modeling techniques. Both neural network and local regression models are able to predict and reproduce the stage–discharge multiple loops that are observed at the outlet of a 28.5?km2 low-gradient subcatchment in southwestern Louisiana. However, the neural network model is characterized with higher prediction ability for most of the tested runoff events. In agreement with the physical characteristics of low-gradient streams, the results indicate the importance of including information about downstream and upstream water levels, in addition to water level at the prediction site.     

Modeling of Cadmium Removal from Domestic Wastewater in Constructed Wetlands Using STELLA Simulation Program

A mathematical model was developed to simulate cadmium removal from wastewater in free water surface (FWS) and subsurface flow (SF) constructed wetlands using STELLA simulation program. The model simulated the accumulation of cadmium in soil (Cds), uptake by plants (Cdp), and residual concentration in effluent (Cdww_eff). The model was calibrated using one-half of the experimental data for the adjustment of the coefficients and the remaining data for model verification. The comparison of simulated and experimental values of Cds, Cdp, and Cdww_eff showed good agreement. The results indicated that the developed mathematical model could be useful for predicting the fate of cadmium when treating domestic effluents in constructed wetlands.     

关于降低本钢氧气放散率的探讨

介绍了国内特大型钢铁企业近年来氧气放散率情况及本钢氧气放散的经济损失．闸述了本钢氧气放散率高的原因，指出了降低氧气放散应采取的措施．     

Bed Drag Coefficient Variability under Wind Waves in a Tidal Estuary

In this paper we report the results of a study of the variation of shear stress and the bottom drag coefficient CD with sea state and currents at a shallow site in San Francisco Bay. We compare shear stresses calculated from turbulent velocity measurements with the model of Styles and Glenn reported in 2000. Although this model was formulated to predict shear stress under ocean swell on the continental shelf, results from our experiments show that it accurately predicts these bottom stress under wind waves in an estuary. Higher up in the water column, the steady wind-driven boundary layer at the free surface overlaps with the steady bottom boundary layer. By calculating the wind stress at the surface and assuming a linear variation of shear between the bed and surface, however, the model can be extended to predict water column shear stresses that agree well with data. Despite the fidelity of the model, an examination of the observed stresses deduced using different wave–turbulence decomposition schemes suggests that wave–turbulence interactions are important, enhancing turbulent shear stresses at wave frequencies.     

Analytical Solutions for Unsteady Transport Dispersion of Nonconservative Pollutant with Time-Dependent Periodic Waste Discharge Concentration

Analytical solutions have been obtained by the Fourier transform method for the case of unsteady transport dispersion of nonconservative pollutant/biochemical oxygen demand with first-order decay under each of the sine and cosine variation of waste discharge concentration at upstream boundary and nonzero initial condition throughout the river. The solutions give correct results along the whole length of the river for all times in contrast to those available in the literature which yield sensible results under quasisteady-state assumption and for large times only. Appropriate expressions for memory length and memory time have been derived so as to include the effect of decay rate of the pollutant in terms of the Thomann number.