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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

A distributed simulation model of a reconstructed ancient water harvesting system in the Negev Desert

Lee, Michael David January 1990 (has links)
This thesis addresses the problems of evaluating a water harvesting catchment system in the arid environment of the Negev Desert, Israel. A general interpretation of arid hillslope hydrological response is developed called Contiguous Area Contribution. Whilst agreeing with humid area concepts of partial area contribution, it focusses on the different nature of arid slope contributions to stormflow where frequent and rapid overland flow generation intercepts channels by downslope expansion and extension of flow-lines. The physical system at Avdat is geometrically represented as a flow net of hillslope and channel cascades for kinematic routing. A survey of the micro-morphology and surface materials enables hillslope areas to be classified into six broad units on the basis of their textural characteristics. These units are used to quasi-distribute process data sampled at locations within them. The process of infiltration is discussed and different mathematical models examined using results from a limited number of field measurements with runon/runoff apparatus. The best-fit is provided by the storage model of Green and Ampt and not the Hortonian models of Philip and Kostiakov. Initial infiltration rates vary from 85 to 18 mm hr-1 and steady- state rates from 60 to 6 mm hr-1. The inclusion of detention storage in early time period observations is shown to be a significant problem in modelling. For flow routing, an alternative approach to assuming sheetflow is presented using cross-slope microtopographic profiles to provide an estimate of surface geometry for flow across micro-rough surfaces. Measurements of flow velocity for different discharges are made using the runon/runoff apparatus. By assuming sheetflow, a lower value of Manning's n is predicted if velocity and flow dimensions are known. For a given n, the assumption of sheetflow predicts a lower velocity for a given discharge. The values of n derived at Avdat range from 0.18 to 1.36 with a mean of 0.64 when a micro-rough surface is retained, and 0.12 to 0.62 with a mean of 0.36 if sheetflow is assumed, values considerably higher than those usually adopted from channel studies. A distributed model is developed to handle surface runoff processes at a range of scales from the small plot to the complete catchment. In a detailed sensitivity analysis, the range of physical and process parameters derived for Avdat show sensitivity of the runoff processes to particular parameters and their combination. For the selected range, flow boundary shape is the most significant influence on the shape of flow hydrographs resulting in quicker, higher peaks if a micro-rough flow geometry is assumed. The model is used at the plot scale to examine the problem of including detention storage in infiltration model parameters, at the cascade scale to show the effect of runoff production enhancing and inhibiting slope areas, and at the sub-catchment scale to assess the predictive ability of the model using the limited process parameter data-set. With three sub-catchments, prediction errors were good volumetrically ranging from 6% to 14% for the high intensity, short duration rainstorm, but deteriorated for the long duration, varied intensity storm with predicting high overestimates. Three sub-catchments consistently under-predicted and one over-predicted. In most, the rising and falling limbs were lagged relative to the observed hydrographs.
2

Numerical modelling of wind-induced lake circulation

Hall, Phillip January 1987 (has links)
The main objective of this study was to develop a stable numerical model capable of simulating the three-dimensional steady-state wind-driven circulation in a lake. Firstly, the relevant differential equations were derived from the Navier-Stokes equations by making certain simplifying assumptions. After comparing some standard finite difference schemes the decision was made to base the method of solution upon the alternating direction implicit technique. The depth- and layer-averaged equations were then expressed in a finite difference form before proceeding to solve the resulting sets of simultaneous equations by Gaussian elimination and back substitution. A step-like finite difference grid representation of the lake bed was used initially, before a more refined approach was adopted. The model also developed from a two-dimensional, vertical cross-section of the lake, to three dimensions, thus simulating the circulation throughout the entire lake. A field measurement programme was undertaken on Esthwaite Water, a small lake in the Lake District, and these measurements were used in comparisons with the numerically predicted results, to assess the models performance. Finally, the sensitivity of the results to some of the main model parameters was investigated by several series of simulations of the circulation in Esthwaite Water under various conditions.
3

Quantifying and simulating the impact of flood mitigation features in a small rural catchment

Nicholson, Alexander Robert January 2014 (has links)
The management of fluvial flood risk in the UK is undergoing a paradigm shift, with a change in emphasis from structural defences to working with natural processes where possible. Natural Flood Management has been advocated by several interest groups as a potential option for providing a low cost, sustainable solution to catchment flooding. An integrated monitoring, field experimentation and modelling campaign has been undertaken to assess the potential of Natural Flood Management (NFM) to reduce flood risk in the rural Belford Burn catchment, Northumberland (5.7km2). The village of Belford failed to satisfy a risk-based cost-benefit criterion for structural defences, despite a number of floods occurring in recent years. The alternative low cost NFM mitigation approach taken in Belford involves the use of soft-engineered Runoff Attenuation Features (RAFs) that intercept or modify hydrological flow pathways. Within the Belford catchment 35 RAFs have been installed to date, including interception bunds, permeable timber barriers, large woody debris and offline storage ponds. The performance of a number of RAFs has been rigorously assessed using a combination of analyses of in situ observed data and modelling techniques. An innovative ‘Pond’ Model has been developed, which uses in situ observational data and physically-based methods, for evaluating the operational performance of the RAFs and assessing their impact on a number of historical flood events and design storms. In addition, the physical functioning and methodological approach of the Pond Model has been evaluated against a peer-reviewed hydraulic model. Also a hydrological modelling package was modified to also demonstrate the impact of RAF attenuation at the catchment scale, with the aim of creating a methodology for transferring the knowledge gained at Belford to other small catchments. This research has quantified the impacts of individual RAFs in the Belford catchment. From analyses of historical events, the Pond Model reveals that a network of attenuating features has the potential to significantly reduce peak flow (by up to 30%). However, for larger return interval design storms (for example 1:100 year return interval 24 hour duration) it is demonstrated that a certain/threshold of RAF attenuation features are required before the aggregate effects cause reduction in peak flow. The potential transferability of the approach and the methods used could have benefits for other similar small catchments (<10km2). An assessment of cost effectiveness is made that includes the comparison between the original iv cost of the proposed Belford flood alleviation scheme using a traditional structural methods and the RAF based scheme.
4

Remote sensing for the study of ecohydrology in East African Rift lakes

Tebbs, Emma Jayne January 2014 (has links)
This thesis investigates remote sensing as a tool for monitoring the ecohydological sustainability of alkaline-saline lakes in the East African Rift Valley, of critical importance to Lesser Flamingos (Phoenicopterus minor). In Lake Bogoria, an algorithm was developed for retrieving chlorophyll-a as an indicator of cyanobacterial biomass - the Lesser Flamingos' primary food source. Results showed a strong linear relationship between Chl-a and the top-of-atmosphere Landsat ETM+ band ratio R[subscript 835]=R[subscript 660] (R[superscript 2] = 0.801; SE = 70 µg l[superscript -1]); valid for Chl-a up to 800 µg l[superscript -1]. At Lake Natron, the sole breeding site for Lesser Flamingos in East Africa, breeding is hydrologically dependent. Landsat-derived lake surface area estimates and ground-based observations of flamingo breeding showed that breeding takes place on a receding lake level. Upper and lower limits for which breeding is feasible were de fined (700-750 km[superscript 2] and 150-180 km[superscript 2] respectively) based on the presence of islands in Landsat imagery. Extending to a regional scale, a Landsat-based optical classi fication scheme was developed for alkaline-saline lakes; the scheme was able to distinguish six classes with a classifi cation accuracy of 73% when verifi ed against in situ measurements. Classifi ed imagery showed the potential importance to flamingos of the food resources off ered by Lake Logipi. Long-term timeseries of Chl-a and other environment variables for Lake Bogoria, from satellite datasets, showed that direct rainfall and lake levels were both weakly related to Chl-a and between them accounted for 20% of the variance in Chl-a. Examination of Landsat imagery showed common features associated with cyanobacterial bloom collapse in Lake Bogoria, which suggested three plausible explanations for these events. Hence, the results of this thesis have improved understanding of the connections between ecological and hydrological processes in alkaline-saline lakes and the role these lakes play in supporting the Lesser Flamingo species.
5

Generation of daily rainfall time series using a hybrid stochastic model

Alajarmeh, Ramiaah Mohammad Saleh January 2014 (has links)
Rainfall occurrence and intensity are the most important drivers of the surface runoff process. The knowledge of the occurrence and intensity of rainfall events is a crucial concern for water resources planners and designers. Stochastic rainfall generators are considered a robust tool that can generate the rainfall intensity of any time length at the interested locations. The validity of the stochastic daily rainfall generators for the Middle East in general and Jordan in specific has not been well researched. The aim of the present research is to estimate daily rainfall time series in different climates with particular focus on semiarid and arid regions. As an important result of the present research, a hybrid single-site stochastic daily rainfall generator (the hybrid model) has been developed using both a two-state first-order Markov chain based model and enhancing the simulation of long dry series using a modified serial approach. Long-term daily rainfall time series from rainfall stations under two different precipitation-hydrological regimes; namely, a frontal dominated precipitation-hydrological regime (Northern Ireland) and a semiarid to arid regime driven predominantly by convective rainfall (Jordan) have been used in testing and evaluation of the hybrid model, in addition to the two original approaches (Markov chain based model and Serial approach represented by LARS-WG). Standard statistical analysis and tests have been used to evaluate the performance of the single-site stochastic daily rainfall generators in both regimes. These single-site stochastic daily rainfall generators then have been assessed at multiple sites using a network of daily rainfall stations within the Lough Neagh basin in Northern Ireland and the Mujib basin in Jordan in order to evaluate their ability to correlate the generated time series of the neighbouring stations. According to the present research results of the comparative performance of the single-site stochastic daily rainfall generators, the hybrid model performance in both regimes was superior.
6

Modelling the impacts of a changing climate on flood risk

Smith, Andrew M. January 2014 (has links)
In recent decades there has been a significant increase in reported flood events and flood losses. Although these losses may be attributed to improvements in reporting and increased exposure in flood prone areas, a perception now exists that flood risk is increasing as a direct result of anthropogenic global warming. In response to this perceived risk, research focused on producing projections of future flood risk has been receiving considerable attention. Indeed, climate impact studies are now being used to guide and test government policy. However, there are significant uncertainties associated with the application of climate model output in flood impact studies. Moreover, there is a disparity between current impact studies and the information required by decision makers, with studies typically focussing on changing river flows. This thesis aims to bridge this gap, cascading climate model output through to impacts at the building scale under an uncertainty framework. The overall aim is to explore the feasibility of using future flood projections as a decision making tool and ultimately to better inform decision makers. The first component of this research was focussed on investigating current climate models and exploring the suitability of their application in flood impact studies. It was found that poor model performance currently precludes their application in assessing flood risk in some regions. The results also provided recommendations for future flood impact studies; these were then used to inform the cascade modelling framework. The rest of the thesis details the development of the modelling framework, driving ensemble climate projections through hydrological, hydraulic and damage models. The modelling framework was also structured to enable uncertainty under current climate conditions to be explored. For the first time, this work presents uncertain climate projections in terms of damage at the building scale. The results reveal that there is an increase in flood magnitude under future climate conditions, however there is significant variability between projections. In fact, the results reveal that there is significant uncertainty under current climate conditions with the potential for exceptional flooding regardless of any future change. The results also emphasise the need to model damage in impact studies as the assumption of linearity between changing hazard and changing risk is invalid. The research presented here has proposed suitable methods for informing decision makers and demonstrated that there is significant scope for improvement in climate impact studies.
7

Modelling the dynamic interaction between hydrology, slope stability and wave run-up processes in the soft-sea cliffs at Covehithe, Suffolk, UK

Chapman, Neil January 2014 (has links)
Soft-rock coastal cliff retreat progresses by an intermittent and discontinuous series of slope mass movements, generally accepted to be concentrated during phases of strong wave attack or heavy rain. One of the fundamental limitations to improving understanding of these processes is a lack of accurate quantitative data on the hydrological and geotechnical behaviour of the cliff slope. In this study, high-resolution terrestrial surveys of coastal change over a fifteen year period have been analysed and combined with hydrological and geotechnical simulations of cliff behaviour under rainfall stress. The input parameters for the simulations have been established from site survey, cross-checked with data from a range of published literature. The numerical model has been applied to typical hydrological, climatic and geotechnical conditions at Covehithe, Suffolk. In addition, analyses of water levels and beach elevations have subsequently been included using archive observation data, to further investigate the mechanisms governing the nature of change at the study site. Key findings include: (a.) high-resolution modelling of rainfall-infiltration processes combined with slope stability analysis provides a unique insight into the complex interaction between slope morphology and dynamic hydrology in soft sea cliffs. (b.) detailed analysis of daily factors of safety related to specific daily rainfalls is significant in reproducing failure conditions at the study site, and elucidates the complex interaction between cliff stratigraphy, cliff hydrology and rainfall. (c.) The results of the water level and beach elevation analyses show that marine processes are significant to the generation of cliff instability, consistent with the field observations and with the Sunamura (1983) model. These findings suggest that the instability of soft sea-cliffs results from complex and interacting controls that require an approach utilising a fully integrated transient hydrology and slope stability modelling. These results have significant implications for current coastal management practice.
8

River basin models for operational forecasting of flow in real-time

Powell, Sian M. January 1985 (has links)
This thesis examines the potential role of transfer function models in the real-time forecasting of the rainfall-runoff and flow routing processes. The theory of the transfer function model structure and the recursive least-squares estimator is described. The use of a sampling rule to reduce the order of large transfer function models is discussed. Traditional sampling theories and control engineering sampling rules are examined. An hydrological sampling rule is developed and tested, with the conclusion that accurate and parsimonious transfer function models can be calibrated for most of the hydrological systems considered in this thesis. Accepted methods of flow routing and river-basin modelling are introduced. Synthetic simulations of the flow routing process by transfer function models are compared to linear and non-linear, hydrological and hydraulic methods of flow routing. The transfer function model structure adequately simulates simple and more complex synthetic systems which exhibit tributary inflow, varying roughness parameters and out-of-bank flooding. A sampled cascade model structure is developed as an alternative model order reduction technique. Use is made of the Rosenbrock parameter optimisation method. The transfer function model structure is used to forecast flows in three case studies. The first two applications concentrate on the ability of transfer function models to approximate tributary inflows and the use of a dual-model structure to simulate out-of-bank flooding. Finally, an accurate and parsimonious river-basin model structure is proposed for the real-time forecasting of flows in river catchments
9

The hydrography and ecology of an estuary polluted by acid mine drainage : Dulas Bay, Anglesey, U.K

Jones, Andrea Parry January 1995 (has links)
The behaviour of Fe, Cu, Zn and Mn and their ecological impact have been examined in Dulas Bay, Anglesey, UK. Dulas Bay receives metal-rich run-off from a disused copper mine via the Afon Goch. Variations in stream discharge conditions affect the concentration and partitioning of metals entering the estuary. Metal concentrations in the Afon Goch (eg. Fe 752, Cu 659, Zn 1810, Mn 526 ~g/l) are one to two orders of magnitude higher than those found in other Welsh streams draining mineralised catchments. During estuarine mixing, a large proportion of the Fe was removed from solution by flocculation, occurring over the salinity range 0-8%0. There was also removal of Cu, and Zn to a lesser extent. The behaviour of Mn was more variable, ranging from conservative to non-conservative. Analysis of the estuarine sediments has revealed gross contamination with metals (Fe 5.08%, Cu 2007, Zn 1200, Mn 427 ~g/g), such that Dulas Bay is among the UK's most metal-polluted sites. High freshwater flows appear to bring about resuspension of estuarine sediments, with the release of porewater enriched in trace metals. Under these conditions, elevated levels of heavy metals will be released to the coastal waters of the Irish Sea. Thus Dulas Bay can act as both a source and sink for metals, depending on the prevailing hydrological conditions. The concentrations of dissolved eu and Zn entering the estuary exceed levels known to produce toxic effects in marine organisms. Analysis of the sediments has revealed a very restricted infaunal community in comparison with nearby unpolluted sites; only two species were present in any number - the polychaete N ereis diversicolor and the amphipod Corophium v olutator. The distribution of rocky shore fauna also appears to be limited. There is evidence for metal tolerance in N. diversicolor and C. volutator from Dulas Bay, and this study has demonstrated tolerance in the gastropod Littorina saxatilis. The only other abundant invertebrate species present at Dulas Bay are barnacles (Semibalanus balanoides and Elminius modestus) and they appear to survive by virtue of their naturally efficient detoxification systems.
10

An assessment of uncertainties in the analysis of the impact of climate change on flooding

Ledbetter, Ralph James January 2012 (has links)
This thesis aims to address the role of uncertainty in climate change impact studies, with particular focus on the impacts of climate change on UK flooding. Methods are developed to quantify the uncertainty associated with climate variability, hydrological model parameters and flood frequency estimation. Each is evaluated independently, before being combined to assess the relative importance of the different sources of uncertainty in the 'top down' impact study framework over multiple time horizons. The uncertainty from climate variability is addressed through the creation of a resampling methodology to be applied to global climate model outputs. Through resampling model precipitation, the direction of change for both mean monthly flows and flood quantiles are found to be uncertain with large possible ranges. Hydrological model parameter uncertainty is quantified using Monte Carlo methods to sample the model parameter space. Through sensitivity experiments, individual hydrological model parameters are shown to influence the magnitude of simulated flood quantile changes. If a larger number of climate scenarios are used, hydrological model parameter uncertainty is small only contributing up to 5% to the total range of impacts. The uncertainty in estimating design standard flood quantiles is quantified for the Generalised Pareto distribution. Flood frequency uncertainty is found to be most important for nearer time horizons, contributing up to 50% to the total range of climate change impacts. In catchments where flood estimation uncertainty is less important, global climate models are found to contribute the largest uncertainty in the nearer term, between 40% and 80% of the total range, with emissions scenarios becoming increasingly important from the 2050s onwards.

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