The concept of the unit sediment graph is important and useful in the study of non-point source pollutant transport, in the estimation of sediment yield and in the design of sediment basins. At the present time, a physically sound method of deriving unit sediment graphs for ungauged small watersheds is not available. Based on synthetic principles as well as linear and time-invariant principles, applied to the systems approach of hydrology, a synthetic model has been developed to derive the unit sediment graph and to generate the sediment graph for an ungauged watershed. The model is limited to the generation of single peak sediment graphs where the sediment particle sizes of interest range from 0.002 mm to 1.0 mm. Seven small watersheds located in the lower Potomac River Basin were selected for this study. For each watershed about 12 storm events were included in the study. Available hourly rainfall and streamflow data were collected and used for model calibration. Results of both"spatial" and"temporal" verification show that agreement between the synthetic and actual sediment graphs is fairly good. A new rigorous definition regarding the unit sediment graph has been established. The study is based on a one-hour unit sediment graph which is defined as the direct sediment graph resulting from 1 unit of effective sediment yield of a storm of 1-hour duration generated uniformly over the basin at a uniform rate. Thus, the one-hour sediment graph of a storm for a specified watershed can be generated by convolving the one-hour unit sediment graph with the effective sediment erosion of one hour duration provided that the rainfall record and characteristics of that watershed are known. / Ph. D.

published_or_final_version / Civil Engineering / Master / Master of Philosophy

A risk-based approach for addressing several non-structural stream quality management objectives is presented. To estimate risk, the input process, the stream contaminant transport, and the consequence of contamination are modeled mathematically. The transport of soluble contaminant introduced at a point into a turbulent stream medium is modeled as a boundary value problem in which the contaminant satisfies the Kolmogorov forward equation within the medium. Observed properties of turbulence are used to justify the adoption of this equation. The fundamental solution, as the probabilistic response of the stream to an instantaneous unit flux input, is derived and used as the kernel in a stochastic integral representation of the transport problem. The bulk input is used as the forcing function in the integral equation. It is modeled as a sequence of independent pulses with random magnitude and duration and also with random interval between the incidence of adjacent pulses ,. Stochastic simulation is used to construct the moments and the probability distribution of stream concentration and those of several variables associated with the exceedance of the concentration above a specified threshold. The variables include the dosage and the time to the first exceedance. The probability that an observed stream concentration exceeds the threshold within a given interval of time is also constructed. Generalizations of the Chebyshev inequality are extended to the case of a stochastic process. Upper bounds on the constructed probability distributions are calculated using these extensions. Based on previous studies, a rectangular hyperbolic relationship is assumed between dosage and consequence. The relationship is combined with the empirical dosage density function to obtain estimates of value risk of stream concentration for various thresholds. Given an acceptable risk, the corresponding threshold may be used as the stream standard. The reliability function, defined as the complementary density function of exceedance times, may be used as a gauge of the effectiveness of pollution abatement measures. Other illustrated areas of application include the construction of a minimum cost contaminant discharge policy and the determination of the optimal sampling interval for stream surveillance.

Aeolian sediment transport threshold is commonly defined as the minimum wind speed (or shear stress) necessary for wind-driven sediment transport. Threshold is a core parameter in most models of aeolian transport. Recent advances in methodology for field-based measurement of threshold show promise for improving parameterizations; however, investigators have varied in choice of method and sensor. The impacts of modifying measurement system configuration are unknown. To address this, two field tests were performed: (i) comparison of four piezoelectric sediment transport sensors, and (ii) comparison of four calculation methods. Data from both comparisons suggest that threshold measurements are non-negligibly modified by measurement system configuration and are incomparable. A poor understanding of natural sediment transport dynamics suggests that development of calibration methods could be difficult. Development of technical standards was explored to improve commensurability of measurements. Standards could assist future researchers with data syntheses and integration. / xi, 108 leaves : ill. ; 29 cm

Suspension flows in varied settings and at different concentrations of particles are studied theoretically using various modeling techniques. Particulate suspension flows are dispersion of particles in a continuous medium and their properties are a consequence of the interplay among hydrodynamic, buoyancy, interparticle and Brownian forces. The applicability of continuum modeling techniques to suspension flows at different particle concentration was assessed by studying systems at different time and length scales. The first two studies involve the use of modeling techniques that are valid in systems where the forces between particles are negligible, which is the case in dilute suspension flows. In the first study, the growth and progradation of deltaic geologic bodies from the sedimentation of particles from dilute turbidity currents is modeled using the shallow water equations or vertically averaged equations of motions coupled with a particle conservation equation. The shallow water model provides a basis for extracting grain size and depositional history information from seismic data. Next, the Navier-Stokes equations of motion and the convection-diffusion equation are used to model suspension flow in a biomedical application involving the flow and reaction of drug laden nanovectors in arteries. Results from this study are then used prescribe the best design parameters for optimal nanovector uptake at the desired sites within an artery. The third study involves the use of macroscopic two phase models to describe concentrated suspension flows where interparticle hydrodynamic forces cannot be neglected. The isotropic form of both the diffusion-flux and the suspension balance models are solved for a buoyant bidisperse pressure-driven flow system. The model predictions are found to compare fairly well with experimental results obtained previously in our laboratory. Finally, the power of discrete type models in connecting macroscopic observations to structural details is demonstrated by studying a system of aggregating colloidal particles via Brownian dynamics. The results from the simulations match experimental shear rheology and also provide a structural explanation for the observed macroscopic behavior of aging. / text

Identifying areas of the hillslope that are most sensitive to soil erosion and contribute significantly to sediment yield is a primary concern in environmental protection and conservation. Therefore the ability to predict the magnitude and variability of soil erosion and sediment yield is important to catchment managers in order to select the appropriate conservation practices that keep soil erosion and sediment yield within the tolerable limits. A number of models have been developed for simulating soil erosion and sediment yield from a catchment. However, none of them are universally applicable and most of them require extensive data which are extremely costly, time consuming and sometimes not available except in research catchments. Hence it was concluded that the combined use of an empirically based soil loss model, RUSLE, Geographic Information Systems (GIS) techniques, and a Sediment Delivery Ratio (SDR) concept would be a candidate modelling tool, which would be a compromise between the advantages of simplicity, data availability, the complex spatial variability of hydrological and geomorphological characteristics of a catchment and the economic limitation of field data measurements in sediment yield studies. Such a modelling tool was developed in this research and was able to identify sediment source areas and predict annual sediment yield from catchments. Data from the Henley catchment, South Africa have been used for demonstrating the potential use of the model in soil erosion and sediment yield studies. Arcview GIS grid functions were used to define the flow direction, accumulation, pathways, and velocity in a catchment as a function of topography and land use and to describe spatially variable input and output information. In addition the Arcview GIS grid function was used to discretise the catchment into hydrologically homogeneous grid cells to capture the catchment heterogeneity. The gross soil erosion in each cell was calculated using the soil loss model RUSLE while a distributed topography based SDR parameter was used to determine the mass of eroded sediment that would be transported to the nearest stream and ultimately to the catchment outlet. The average annual soil loss and sediment yield values were 26 t. ha-1.yr -1 and 1.6 t. ha-1.yr -1 respectively. High soil erosion and sediment yield rates are evident in the residential and agricultural areas, which are characterised by degradation due to overgrazing and traditional and peri-urban settlements with mixed crops. The average annual SDR value was 0.19 for the Henley catchment and large SDR values are associated with areas adjacent to the channel system. This can be explained by recognizing that the SDR is significantly influenced by characteristics of the drainage system. Comparison of event based simulations of sediment yields to those estimated from measurements demonstrated that the proposed model predictions ranged between 13 % and 60 % of the measured estimates, consistently over predicting. This is because the SDR component of the model is developed as a mean annual parameter, assuming that over a long period a stream system must intimately transport all the sediments delivered to it. Hence the channel network sediment delivery parameters would have to be considered at short temporal scales. Comparing the results of the model prediction against other sediment modelling techniques in South Africa demonstrated the usefulness of the model as an effective catchment management tool. The model has advantages over these other techniques since it includes a distributed grid based component, which enables the identification of sediment source areas in the catchment. The sensitivity analysis shows that the model was highly sensitive to parameters derived from topography and land use of the catchment. Future research with the model should include further testing and analysis of its components on different catchments. The topography based SDR concept which is a key component in sediment routing for prediction of either long term average sediment yield or isolated storm event simulation from a catchment warrants specific attention. Effort in future should focus on identifying parameters which affect the sediment delivery within a catchment. This may be achieved by incorporating processes describing the movement of sediments in the channel network of the catchment. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2004.