503 research outputs found

    Understanding urban rainfall-runoff responses using physical and numerical modelling approaches

    Get PDF
    This thesis provides a novel investigation into rainfall-runoff processes occurring within a unique two-tiered depth-driven overland flow physical modelling environment, as well as within a numerical model context where parameterisation and DEM/building resolution influences have been investigated using an innovative de-coupled methodology. Two approaches to simulating urban rainfall-runoff responses were used. Firstly, a novel, 9 m2 physical modelling environment consisting of a: (i) a low-cost rainfall simulator component able to simulate consistent, uniformly distributed rainfall events of varying duration and intensity, and; (ii) a modular plot surface layer was used. Secondly, a numerical hydroinundation model (FloodMap2D-HydroInundation) was used to simulate a short-duration, high intensity surface water flood event (28th June 2012, Loughborough University campus). The physical model showed sensitivities to a number of meteorological and terrestrial factors. Results demonstrated intuitive model sensitivity to increasing the intensity and duration of rainfall, resulting in higher peak discharges and larger outflow volumes at the model outflow unit, as well as increases in the water depth within the physical model plot surface. Increases in percentage permeability were also shown to alter outflow flood hydrograph shape, volume, magnitude and timing due to storages within the physical model plot. Thus, a reduction in the overall volume of water received at the outflow hydrograph and a decrease in the peak of the flood event was observed with an increase in permeability coverage. Increases in the density of buildings resulted in a more rapid receding limb of the hydrograph and a steeper rising limb, suggesting a more rapid hydrological response. This indicates that buildings can have a channelling influence on surface water flows as well as a blockage effect. The layout and distribution of permeable elements was also shown to affect the rainfall-runoff response recorded at the model outflow, with downstream concentrated permeability resulting in statistically different hydrograph outflow data, but the layout of buildings was not seen to result in significant changes to the outflow flood hydrographs; outflow hydrographs appeared to only be influenced by the actual quantity and density of buildings, rather than their spatial distribution and placement within the catchment. Parameterisation of hydraulic (roughness) and hydrological (drainage rate, infiltration and evapotranspiration) model variables, and the influence of mesh resolution of elevation and building elements on surface water inundation outputs, both at the global and local level, were studied. Further, the viability of crowdsourced approaches to provide external model validation data in conjunction with dGPS water depth data was assessed. Parameterisation demonstrated that drainage rate changes within the expected range of parameter values resulted in considerable losses from the numerical model domain at global and local scales. Further, the model was also shown to be moderately sensitive to hydraulic conductivity and roughness parameterisation at both scales of analysis. Conversely, the parameterisation of evapotranspiration demonstrated that the model was largely insensitive to any changes of evapotranspiration rates at the global and local scales. Detailed analyses at the hotspot level were critical to calibrate and validate the numerical model, as well as allowing small-scale variations to be understood using at-a-point hydrograph assessments. A localised analysis was shown to be especially important to identify the effects of resolution changes in the DEM and buildings which were shown to be spatially dependent on the density, presence, size and geometry of buildings within the study site. The resolution of the topographic elements of a DEM were also shown to be crucial in altering the flood characteristics at the global and localised hotspot levels. A novel de-coupled investigation of the elevation and building components of the DEM in a strategic matrix of scenarios was used to understand the independent influence of building and topographic mesh resolution effects on surface water flood outputs. Notably, the inclusion of buildings on a DEM surface was shown to have a considerable influence on the distribution of flood waters through time (regardless of resolution), with the exclusion of buildings from the DEM grid being shown to produce less accurate results than altering the overall resolution of the horizontal DEM grid cells. This suggests that future surface water flood studies should focus on the inclusion and representation of buildings and structural features present on the DEM surface as these have a crucial role in modifying rainfall-runoff responses. Focus on building representation was shown to be more vital than concentrating on advances in the horizontal resolution of the grid cells which make up a DEM, as a DEM resolution of 2 m was shown to be sufficiently detailed to conduct the urban surface water flood modelling undertaken, supporting previous inundation research

    Evaluation of high quality topographic data for geomorphological and flood impact studies in upland area: North York Moors, UK

    Get PDF
    A flash flood on 19th June 2005 caused more than one hundred landslides in the North-western North York Moors uplands, UK. This project aims to 1) assess digital elevation models (DEMs) in terms of statistical terrain analysis and 2) explore the sensitivity of a 2D FLOWMAP model response to DEMs input data. A variety of topographic data were acquired, generated and processed. These included high-resolution aerial photographs, Ordnance Survey (OS) DEMs, topographic maps, InSAR DEMs, LiDAR data and ground survey data. These DEMs of different horizontal and vertical resolutions were analysed through key topographic parameters calculated using three different software packages. Key topographic attributes such as slope, aspect, profile curvature and the Topographic Wetness Index (TWI) were studied. Results demonstrate that DEMs from different sources or at different resolutions provide different representations of topographic parameters especially in areas where large topographic changes take place. Algorithms used in different packages also had an effect. Degradation in the representation of topographic information is larger between 10 m and 50 m DEMs than between 5 m and 10 m DEMs. Finer resolution and smaller filter size have the same type of impact on slope and aspect. In addition, DEMs at finer horizontal resolutions have smaller minimum profile curvatures and larger maximum values and standard deviations in profile curvature. The TWI is more sensitive to the horizontal resolution than DEM data source and finer DEMs calculate smaller minimum and mean TWI and larger maximum TWI and standard deviations. Modelled hydrological responses are sensitive to both DEM resolution and its data source. Model showed different results when using 5 m LÄ°DAR DEM and 5 m InSAR DEM of the same area, which meant DEM source had impacts on modelling These differences reduced with a larger magnitude flooding. Producing a better representative surface model from the LÄ°DAR data has much larger impact on model response than adjusting a constant roughness coefficient

    Geomorphological connectivity and sensitivity examined in a recently degraded gravel-bed stream: implications for river-floodplain rehabilitation

    Get PDF
    The study of river complexity and sensitivity to future human land-use activities and climate change is a fast growing field within the discipline of fluvial geomorphology. Associated with this is a need to improve river rehabilitation and catchment management approach, design and effectiveness. This study aimed to investigate drivers of the recent geomorphological sensitivity of the Baviaanskloof River-floodplain, an upland system in South Africa, by integrating the concepts of geomorphological connectivity and Panarchy. The understanding generated was used to evaluate the approach of the State agency, Working for Wetlands (WfWet), to river-floodplain rehabilitation in the catchment.The concepts of geomorphological connectivity and Panarchy provide useful frameworks for understanding interactions between geomorphological processes and structure across scales of space and time. Geomorphological connectivity explains the degree to which water and sediment is linked in a river landscape, determined by the distribution of erosional and depositional landforms (Brierley et al. 2006; Fryirs et al. 2007a; Fryirs et al. 2007b). Panarchy attempts to explain lagged response to disturbances, non-linear interactions, and sudden shifts in system state, and has been applied largely to ecological systems. Panarchy theory, when combined with the concept of geomorphological connectivity, provides a guiding framework for understanding river complexity in greater depth. The first results chapter of this study investigated river long-term and recent geomorphological history, towards understanding the nature and timing of river geomorphological cycling between erosion and deposition. Optically Stimulated Luminescence dating of alluvial fan and floodplain sedimentary units was conducted, for analysis of river-floodplain long-term history (100s to 1 000s of years). Interviews with 11 local landowners, combined with analysis of historic aerial imagery and river-floodplain topographic surveys, provided a means of describing recent (last few decades) geomorphological dynamics. The results indicated that the Baviaanskloof is naturally a cut- and-fill landscape over scales of several hundred to thousands of years, characterized by the alternation between phases of high fluvial energy and alluvial fan expansion, and low energy conditions associated with floodplain accretion. Recent and widespread river-floodplain degradation was compressed into a short period of approximately 30 years, suggesting that one or more drivers have pushed the system beyond a threshold, resulting in increased water and sediment connectivity. The second results chapter investigated the role of human land-use activities and flooding frequency and magnitude, as drivers of recent river-floodplain degradation. Human impacts were investigated by describing land-use activities for the preceding 80 years, and relating these activities to changes in river-floodplain form and behavior. Temporal trends in flood events of different frequency and magnitude were investigated by analyzing rainfall data, integrated with landowner reports of flood-inducing rainfall magnitudes. The findings indicated that human land-use activities have been an important driver of recent river- floodplain degradation, through the enhancement of water and sediment connectivity across spatial scales of the catchment. Episodic and high magnitude floods synergized with human driven increased connectivity, precipitating stream power and geomorphological threshold breaches, resulting in a shift in river behaviour. The third results chapter investigated the influence of tributary-junction streams and fans on the geomorphological form, behavior and sensitivity of the Baviaanskloof River. Local- scale topographic impacts of tributary fans and streams were described using topographic surveys and geomorphological mapping techniques. Tributary streams form a major control on the behaviour of the river, by influencing the degree of coarse sediment connectivity with the main channel. Although tributary fans buffer the river from disturbances occurring in the wider catchment, they initiate topographic variations along the floodplain, influencing local-scale patterns of deposition and erosion along the river. The main river responds to water and sediment inputs from tributary junction streams by locally adjusting longitudinal slope, maintaining an overall constant slope of 0.0066 m/m. The response of the Baviaanskloof River to tributary junction fans and streams is however variable, and is fashioned by complex interactions between geomorphological and anthropogenic factors. The final two chapters of the thesis evaluate the findings of the study within the context of river-floodplain rehabilitation approaches in South Africa, and within the theoretical, philosophical and methodological context of the research. The first of these two chapters evaluates the approach of the WfWet programme to river-floodplain rehabilitation in the Baviaanskloof. The chapter indicates that the present practice of WfWet is to reinstate a pre-degradation state, which is not suited to the Baviaanskloof River-floodplain, since the river-floodplain has passed a geomorphological threshold, resulting in a new set of interacting processes and landforms. The author presents a conceptual model illustrating the existence of geomorphological adaptive cycles interacting across spatial and temporal scales, thereby attempting to explain a river Panarchy specific to the Baviaanskloof. From this conceptual model, a hierarchical rehabilitation framework, targeting geomorphological processes and structure situated at different spatial and temporal scales of the landscape is suggested. The final chapter discusses the implications of integrating the concepts of geomorphological connectivity and river Panarchy theory in studies of river complexity and sensitivity to geomorphological change. The author suggests that there is scope for further investigation of the application of the two concepts within the discipline of fluvial geomorphology, particularly with regard to developing quantitative approaches to measuring and describing connectivity and Panarchy

    Ecohydrological characterisation of Whangamarino wetland

    Get PDF
    The Whangamarino wetland is internationally recognised and one of the most important lowland wetland ecosystems in the Waikato Region. The wetland’s hydrology has been altered by reduced river base levels, the installation of a weir to raise minimum water levels and the Lower Waikato Waipa Flood Control Scheme, which is linked via the (hypertrophic) Lake Waikare and affected by varying catchment land use practices. When water levels exceed capacity, the overflow is released into the Whangamarino wetland, which also receives flood waters from Whangamarino River. Water levels in the wetland are also affected at high stage, by a control structure near Meremere at the confluence of Waikato and Whangamarino Rivers, and at low stage by a weir a short distance upstream. The ecohydrology of a representative part of the wetland was studied to assess the linkage between wetland ecology and the natural and anthropogenic modification of the flood regime and land use. The primary goal of this research was to characterise the present state of the wetland, which will aid in developing future goals and approaches for restoration. The study focused on a 2.3 km transect extending from the Whangamarino River, through the wetland to adjacent farmed hillsides. Hydrological and meteorological data were retrieved and analysed from an automated weather station and seven water level sites along the transect. Historical water level records (over 46 years) were used to determine changes in the hydrological regime and the impact of the flood control scheme, through a flood inundation and frequency analysis. During a winter flood event, river water quality was assessed. Peat surface oscillation in the restiad bog was examined. Vegetation patterns were assessed and classified through ordination and statistical techniques. Peat, soil and foliage physical and chemical quality were measured. Atmospheric ammonia (NH3) deposition rates of N into the wetland were measured. Water levels in the inland 0–1.1 km of the transect line (restiad bog) were relatively stable and consistent, rising and falling through winter and summer. This area had rainfall as the primary water input and was independent from the Whangamarino River, except during large flood events where the fringe of the restiad bog was inundated. Closer to the Whangamarino River water levels were more variable and strongly responsive to the river’s hydrological regime. A flood inundation event in September 2010 impacted on wetland water level regimes up to 1.4 km from the river and had a return period of 3.3 years. Frequency analysis showed sites up to 500 m from the river will likely be inundated by floods every year. A 100 year flood was estimated to inundate 1.75 km from the river, but would not cover the entire wetland. River water samples collected during a flood event showed total suspended solids within the Whangamarino River peaking at 260 mg L-1, double the concentration from Pungarehu Canal (86 mg L-1). Nutrient concentrations (such as dissolved reactive phosphorus) followed a similar pattern to the flood hydrograph. Minimum water levels have increased since the development of the artificial weir, but before this occurred water table lowering may have encouraged manuka invasion towards the restiad bog. Increased flood inundation is now the most likely threat to continued wetland degradation and manuka invasion into the restiad bog, due to the change in water levels and the deposition of sediment and nutrients. Nutrients, heavy metals, isotopes (ή15N) and physical soil characteristics (such as bulk density) increased from the start of the manuka belt (1100 m) and were greatest near the Whangamarino River (2300 m). A gradient was observed in peat and soil chemistry patterns, with increasing fertility and a change from bog to swamp-type environments along the transect line towards the river. A mineralised swamp fringe belt was present next to the farmland (0–50 m). From 50–1100 m a restiad bog (dominated by Empodisma minus) was present and changed to a manuka transition zone from 1100–1500 m. From 1500–1900 m, a swamp environment was present with a dominant canopy of manuka changing to Coprosma tenuicaulis closer to the river. C. tenuicaulis appears to be acting as a buffer zone over 150 m, removing a large amount of nutrients and sediment from flood waters. The remaining 400 m (1900–2300 m) of the transect line was a marshland, with the highest nutrient and sediment abundances and the most variable water level patterns. This area was colonised primarily by Polygonum persicaria (willow weed). The major risk to the wetland is from continued flood inundation with nutrient and sediment rich waters. Recommendations for future management include restoring catchment water quality and better management of the flood control regime

    Microhabitat Effects on Nitrous Oxide Emissions, Production Pathways, and Reduction in Floodplain Soils

    Get PDF
    The potential of river floodplains to emit nitrous oxide (N2O), a powerful greenhouse gas and ozone-depleting compound, considerably reduces the climate regulation function of these dynamic transition zones between aquatic and terrestrial ecosystems. However, the assessment of N2O emissions from floodplain soils is challenging, due to the inherently high spatial heterogeneity and the characteristic occurrence of sporadic inundation phases. Such short-term flood events can temporarily alter the conditions for nitrogen (N) transformation processes taking place within distinct microhabitats, which can lead to the local formation of transient hot spots of enhanced N2O emissions. This situation emphasizes the urgent need to understand how characteristic factors of microhabitat formation in river floodplains control the balance between major microbial N2O source processes and N2O reduction to N2 that determine the magnitude and duration of N2O emissions. Therefore, the main objective of this thesis project was to systematically assess the relative importance of microhabitat effects related to soil aggregate size, organic matter accumulation, and plant-soil interactions on the microbial N2O production and consumption processes controlling the spatiotemporal emission patterns of N2O under changing pore water saturation. To achieve this objective, a mesocosm experiment under controlled climatic conditions, and a study within the framework of a field manipulation experiment were conducted. In the mesocosm experiment, presented in chapters 2 and 3, two model soils with equivalent structure and texture, comprising macroaggregates (4000–250 ÎŒm) or microaggregates (< 250 ÎŒm) from a N-rich floodplain soil were used. These model soils were planted either with basket willow (Salix viminalis L.), mixed with leaf litter, or left unamended. The resulting six aggregate size / amendment factor combinations were exposed to a flood of 48 hours and then left to dry. Emission rates of N2O during the experiment were determined using the closed-chamber method. The relative contributions of different N transformation processes to the production of N2O and the degree of N2O reduction to N2 were assessed using a novel approach based on the isotopic and isotopomeric composition of the emitted N2O. In addition, the procaryotic and fungal soil microbiomes were characterized by sequencing of DNA and qPCR of functional genes related to potentially N2O producing and consuming processes. N2O production during the 48-hour flood phase originated almost entirely from heterotrophic bacterial denitrification and/or nitrifier-denitrification in all experimental treatments, yet most of the produced N2O was further reduced to N2 resulting in low N2O flux rates. In the drying phase, a period of enhanced N2O emissions occurred in all treatments, however with the unamended and litter added model soils with macroaggregates emitting significantly more N2O than in all other treatments. During this period, most of the N2O production continued to derive from bacterial denitrification in anoxic micro-sites. However, the aeration of the inter-aggregate pore space led to additional contributions by oxidative N2O production, the magnitude of which depended on treatment and time point within the drying phase. Also here, aggregate size emerged as a key parameter. Unamended macroaggregates seemed to prolong anoxia within microsites when compared to microaggregates. Litter addition further enhanced soil anoxia but also altered soil structure and nutrient availability. This increased soil heterogeneity modulated the temporal pattern of the N2O emission, leading to short-term peaks of high N2O fluxes at the beginning of the period of enhanced N2O emissions. These maximum N2O emissions were the result of rapid changes in N2O source partitioning of nitrifying and denitrifying processes in combination with a temporary partial disruption of N2O reduction. By contrast, the presence of S. viminalis prevented the occurrence of strong N2O emissions from both model soils, attenuating any effect of flooding and aggregate size on N2O production pathways and the degree of N2O reduction. Root respiration and the decomposition of root exudates likely promoted the formation of anoxic microsites that support complete denitrification, resulting in the low emission rates observed in the planted model soils. Irrespective of treatment and throughout the experiment, nitrogen-cycling gene abundances revealed a higher potential for bacterial denitrification and for N2O reduction than for ammonia oxidation, thus supporting the implications of the isotopic data on the dominance of denitrification in N2O production and on the generally high degree of N2O reduction. DNA sequencing data and functional gene abundances further revealed that large and small soil aggregates represent distinct microhabitats with a different potential for both denitrifying and nitrifying processes, thus suggesting that in addition to structure-related physical effects, differences in the microbial community composition contribute to aggregate size effects on N2O emission rates and N2O production pathways. Litter accumulations strongly altered the soil microbial community composition of the aggregate size fractions, whereas the presence of willow had little respective effects. Both soil amendments affected the abundance of ammonia-oxidizing bacteria and archaea, but not the one of denitrifying microorganisms. The field study, presented in chapter 4, took place in the hydrologically most dynamic floodplain zone of a re-naturalized section of the Thur River in NE Switzerland. In a randomized complete block design, experimental plots where the dominant vegetation, the pioneer plant canary ryegrass (Phalaris arundinacea L.), was constantly removed were compared to unmanipulated plots. During a three-week drying-phase after a major flood, the dynamics of efflux, source partitioning and reduction of N2O were assessed using the same methods as in the mesocosm experiment. In addition, temporal changes in the activity of specific groups of N transforming soil microorganisms were analyzed using extracted RNA. It became evident, that young, sandy sediments under a dense plant cover experienced longer periods of elevated N2O emissions, whereas emissions from bare sediments gradually decreased after initial peak rates. Nitrification and/or fungal denitrification contributed consistently about 20-30 % to gross N2O production in plots covered by P. arundinacea, whereas this process group contributed only to the beginning of the post-flood phase to N2O emissions from bare plots. N2O reduction was temporarily interrupted at the beginning of the post-flood phase in bare plots, whereas N2O reduction in the Phalaris plots was stable during the entire drying phase. The detection of denitrifying and nitrite oxidizing microorganisms as the most active N transforming microorganisms in this part of the river floodplain further supported the results from source partitioning and reflected the adaptation of the microbial community to fluctuating redox conditions. Overall, the results of this thesis (i) demonstrate the importance of soil aggregation, litter accumulation and plant-soil interactions in floodplain soils in governing the production, consumption, and emission of N2O during flood-induced hot moments, (ii) present evidence for the formation of related specific microhabitats and indications of explanatory physical effects, (iii) highlighted the role of microbial N2O reduction as a major controlling factor of N2O emissions (iv) and confirm the dominance of denitrifying processes as source processes. Our findings thus should help to predict the location of temporary hotspots of N2O emissions, and to improve the estimations of local N2O budgets of river floodplains in a world of global climate change

    Quantitative Modelling of Climate Change Impact on Hydro-climatic Extremes

    Get PDF
    In recent decades, climate change has caused a more volatile climate leading to more extreme events such as severe rainstorms, heatwaves and floods which are likely to become more frequent. Aiming to reveal climate change impact on the hydroclimatic extremes in a quantitative sense, this thesis presents a comprehensive analysis from three main strands. The first strand focuses on developing a quantitative modelling framework to quantify the spatiotemporal variation of hydroclimatic extremes for the areas of concern. A spatial random sampling toolbox (SRS-GDA) is designed for randomizing the regions of interest (ROIs) with different geographic locations, sizes, shapes and orientations where the hydroclimatic extremes are parameterised by a nonstationary distribution model whose parameters are assumed to be time-varying. The parameters whose variation with respect to different spatial features of ROIs and climate change are finally quantified by various statistical models such as the generalised linear model. The framework is applied to quantify the spatiotemporal variation of rainfall extremes in Great Britain (GB) and Australia and is further used in a comparison study to quantify the bias between observed and climate projected extremes. Then the framework is extended to a multivariate framework to estimate the time-varying joint probability of more than one hydroclimatic variable in the perspective of non-stationarity. A case study for evaluating compound floods in Ho Chi Minh City, Vietnam is applied for demonstrating the application of the framework. The second strand aims to recognise, classify and track the development of hydroclimatic extremes (e.g., severe rainstorms) by developing a stable computer algorithm (i.e., the SPER toolbox). The SPER toolbox can detect the boundary of the event area, extract the spatial and physical features of the event, which can be used not only for pattern recognition but also to support AI-based training for labelling/cataloguing the pattern from the large-sized, grid-based, multi-scaled environmental datasets. Three illustrative cases are provided; and as the front-end of AI study, an example for training a convolution neural network is given for classifying the rainfall extremes in the last century of GB. The third strand turns to support decision making by building both theory-driven and data-driven decision-making models to simulate the decisions in the context of flood forecasting and early warning, using the data collected via laboratory-style experiments based on various information of probabilistic flood forecasts and consequences. The research work demonstrated in this thesis has been able to bridge the knowledge gaps in the related field and it also provides a precritical insight in managing future risks arising from hydroclimatic extremes, which makes perfect sense given the urgent situation of climate change and the related challenges our societies are facing

    Elements at risk

    Get PDF

    Spatio-temporal rainfall estimation and nowcasting for flash flood forecasting.

    Get PDF
    Thesis (Ph.D.Eng.)-University of KwaZulu-Natal, Durban, 2007.Floods cannot be prevented, but their devastating effects can be minimized if advance warning of the event is available. The South African Disaster Management Act (Act 57 of 2002) advocates a paradigm shift from the current "bucket and blanket brigade" response-based mind set to one where disaster prevention or mitigation are the preferred options. It is in the context of mitigating the effects of floods that the development and implementation of a reliable flood forecasting system has major significance. In the case of flash floods, a few hours lead time can afford disaster managers the opportunity to take steps which may significantly reduce loss of life and damage to property. The engineering challenges in developing and implementing such a system are numerous. In this thesis, the design and implement at ion of a flash flood forecasting system in South Africa is critically examined. The technical aspect s relating to spatio-temporal rainfall estimation and now casting are a key area in which new contributions are made. In particular, field and optical flow advection algorithms are adapted and refined to help predict future path s of storms; fast and pragmatic algorithms for combining rain gauge and remote sensing (rada r and satellite) estimates are re fined and validated; a two-dimensional adaptation of Empirical Mode Decomposition is devised to extract the temporally persistent structure embedded in rainfall fields. A second area of significant contribution relates to real-time fore cast updates, made in response to the most recent observed information. A number of techniques embedded in the rich Kalm an and adaptive filtering literature are adopted for this purpose. The work captures the current "state of play" in the South African context and hopes to provide a blueprint for future development of an essential tool for disaster management. There are a number of natural spin-offs from this work for related field s in water resources management
    • 

    corecore