Cellular modelling of river catchments and reaches: Advantages, limitations and prospects

The last decade has witnessed the development of a series of cellular models that simulate the processes operating within river channels and drive their geomorphic evolution. Their proliferation can be partly attributed to the relative simplicity of cellular models and their ability to address some of the shortcomings of other numerical models. By using relaxed interpretations of the equations determining fluid flow, cellular models allow rapid solutions of water depths and velocities. These can then be used to drive (usually) conventional sediment transport relations to determine erosion and deposition and alter the channel form. The key advance of using these physically based yet simplified approaches is that they allow us to apply models to a range of spatial scales (1–100 km²) and time periods (1–100 years) that are especially relevant to contemporary management and fluvial studies.However, these approaches are not without their limitations and technical problems. This paper reviews the findings of nearly 10 years of research into modelling fluvial systems with cellular techniques, principally focusing on improvements in routing water and how fluvial erosion and deposition (including lateral erosion) are represented. These ideas are illustrated using sample simulations of the River Teifi, Wales. A detailed case study is then presented, demonstrating how cellular models can explore the interactions between vegetation and the morphological dynamics of the braided Waitaki River, New Zealand. Finally, difficulties associated with model validation and the problems, prospects and future issues important to the further development and application of these cellular fluvial models are outlined.     

The role of floodplains in attenuating contaminated sediment fluxes in formerly mined drainage basins

Many upland river catchments in the UK have been historically mined for metals such as lead (Pb) and zinc (Zn), and as part of the mining process large quantities of metal contaminated sediment were released into the river system. The levels of sediment associated heavy metal contamination in river systems are largely controlled by the volumes of contaminated sediment released into the river and fluvial processes (e.g. erosion and deposition). As a consequence, the contamination patterns are often highly variable, which can make it difficult to create accurate assessments of the volumes of contaminated sediment remaining within the system. This paper uses a combination of techniques to establish the volumes of metal contaminated sediment remaining within the River Swale, UK. Firstly, using detailed field sampling and a geographical information system (GIS), it estimates the volumes of sediment remaining within one formerly mined tributary (Gunnerside Beck) which is then extrapolated to represent the contaminant volumes on other tributaries of the River Swale. Secondly, combining fresh field data with a range of existing data, volumes of contaminated sediment on the main stream of the River Swale are established. This two tier approach shows that significant volumes of contaminated sediment remain within the River Swale, with over 32 000 tonnes of Pb within the mined tributaries and 123 000 tonnes within the main channel belt of the River Swale itself. This represents approximately 28% of the Pb produced in the Swale catchment. Given these volumes and present day rates of removal, it may take over 5000 years for all of the metal rich sediment to be removed from the catchment. If the contaminated sediment is used as a tracer, present day rates of reworking of floodplain sediment can be calculated to be 0·02% per year. Copyright © 2008 John Wiley & Sons, Ltd.     

Measuring river velocities using GPS River Flow Tracers (GRiFTers)

Existing methods of measuring flow velocities in natural rivers are largely based on series of point measurements. Acquisition of these data can be time consuming and difficult, especially in high flow conditions. This paper introduces the use of GPS drifters (termed GRiFTers) to measure surface flow velocities in a 400 m reach of the River Swale, UK. Over 10 000 measurements were made in a 3 hour period and aggregated over a 2 m grid to generate a genuine distributed representation of flow across the reach. The technique shows great promise to provide new insights into flow patterns over long reaches of rivers, over a range of flow conditions, and may also provide valuable data for numerical model validation. Copyright © 2007 John Wiley & Sons, Ltd.     

Channel movement and erosion response to rainfall variability in southeast Australia

Intrinsic and extrinsic forces on the catchment and stream channel network drive morphological change. Separating individual forcings is difficult given the complexity of such nonlinear systems. Here a modelling approach is used to investigate the sensitivity of channel position and movement under a series of realistic rainfall scenarios for a catchment in southeastern Australia. The results demonstrate the sensitivity of the catchment to different rainfall patterns and how relatively small changes in rainfall can lead to much larger sediment outputs revealing sensitivity to subtle changes in climate. Channel movement occurs as an avulsion. This is the first time such a process has been observed and modelled in an ephemeral stream environment and demonstrates fluvial geomorphic change at human time scales. Human intervention by rock lining channels was demonstrated to prevent the movement of the main channel. Overall the CAESAR landscape evolution and erosion model used in this study is able to replicate both erosion rates and the variation in past channel movement. The modelling suggests that any landscape change is based on both internal and external forcing and that landscape history also plays a significant role. Here, we demonstrate the potential to quantify many of the nonlinearities and thresholds in soil‐mantled catchments using a landscape evolution model. Copyright © 2011 John Wiley & Sons, Ltd.     

Integrating the LISFLOOD‐FP 2D hydrodynamic model with the CAESAR model: implications for modelling landscape evolution

Landscape evolution models (LEMs) simulate the geomorphic development of river basins over long time periods and large space scales (100s–1000s of years, 100s of km²). Due to these scales they have been developed with simple steady flow models that enable long time steps (e.g. years) to be modelled, but not shorter term hydrodynamic effects (e.g. the passage of a flood wave). Nonsteady flow models that incorporate these hydrodynamic effects typically require far shorter time steps (seconds or less) and use more expensive numerical solutions hindering their inclusion in LEMs. The recently developed LISFLOOD‐FP simplified 2D flow model addresses this issue by solving a reduced form of the shallow water equations using a very simple numerical scheme, thus generating a significant increase in computational efficiency over previous hydrodynamic methods. This leads to potential convergence of computational cost between LEMs and hydrodynamic models, and presents an opportunity to combine such schemes. This paper outlines how two such models (the LEM CAESAR and the hydrodynamic model LISFLOOD‐FP) were merged to create the new CAESAR‐Lisflood model, and through a series of preliminary tests shows that using a hydrodynamic model to route flow in an LEM affords many advantages. The new model is fast, computationally efficient and has a stronger physical basis than a previous version of the CAESAR model. For the first time it allows hydrodynamic effects (tidal flows, lake filling, alluvial fans blocking valley floor) to be represented in an LEM, as well as producing noticeably different results to steady flow models. This suggests that the simplification of using steady flow in existing LEMs may bias their findings significantly. Copyright © 2013 John Wiley & Sons, Ltd.     

A 477‐year dendrohydrological assessment of drought severity for Tsable River,Vancouver Island,British Columbia,Canada

Summer streamflow droughts are becoming more severe in many watersheds on Vancouver Island, British Columbia, as a result of climate warming. Small coastal basins that are the primary water source for most communities and essential to Pacific salmon populations have been particularly affected. Because the most extreme naturally occurring droughts are rarely captured within short instrumental records water managers likely underestimate, and are unprepared for, worst‐case scenario low flows. To provide a long‐term perspective on recent droughts on Vancouver Island, we developed a 477‐year long dendrohydrological reconstruction of summer streamflow for Tsable River based on a network of annual tree‐ring width data. A novel aspect of our study is the use of conifer trees that are energy limited by spring snowmelt timing. Explaining 63% of the instrumental streamflow variability, to our knowledge the reconstruction is the longest of its kind in British Columbia. We demonstrate that targeting the summer streamflow component derived from snowmelt is powerful for determining drought‐season discharge in hybrid runoff regimes, and we suggest that this approach may be applied to small watersheds in temperate environments that are not usually conducive to dendrohydrology. Our findings suggest that since 1520, 21 droughts occurred that were more extreme than recent ‘severe’ events like those in 2003 and 2009. Recent droughts are therefore not anomalous relative to the ~400‐year pre‐instrumental record and should be anticipated within water management strategies. In coming decades, worst‐case scenario natural droughts compounded by land use change and climate change could result in droughts more severe than any since 1520. The influence of the Pacific Decadal Oscillation on instrumental and modelled Tsable River summer streamflow is likely linked to the enhanced role of snowmelt in determining summer discharge during cool phases. Copyright © 2015 John Wiley & Sons, Ltd.     

Poverty, sustainability and human wellbeing: A social wellbeing approach to the global fisheries crisis

The purpose of this paper is to explore the extent to which a social wellbeing approach can offer a useful way of addressing the policy challenge of reconciling poverty and environmental objectives for development policy makers. In order to provide detail from engagement with a specific policy challenge it takes as its illustrative example the global fisheries crisis. This crisis portends not only an environmental disaster but also a catastrophe for human development and for the millions of people directly dependent upon fish resources for their livelihoods and food security. The paper presents the argument for framing the policy problem using a social conception of human wellbeing, suggesting that this approach provides insights which have the potential to improve fisheries policy and governance. By broadening the scope of analysis to consider values, aspirations and motivations and by focusing on the wide range of social relationships that are integral to people achieving their wellbeing, it provides a basis for better understanding the competing interests in fisheries which generate conflict and which often undermine existing policy regimes.     

Modelling differential catchment response to environmental change

The CAESAR (Cellular Automaton Evolutionary Slope And River) model is used to demonstrate significant differences in coarse sediment transfer and alluviation in medium sized catchments when responding to identical Holocene environmental changes. Simulations for four U.K. basins (the Rivers Swale, Ure, Nidd and Wharfe) shows that catchment response, driven by climate and conditioned by land cover changes, is synchronous but varies in magnitude. There are bursts of sediment transfer activity, generally of rapid removal but with some sediment accumulation ‘spikes’, with longer periods of slow removal or accumulation of sediment in different valley reaches. Within catchments, reach sensitivity to environmental change varies considerably: some periods are only recorded in some reaches, whilst higher potential sensitivity typically occurs in the piedmont areas of the catchments modelled here. These differential responses appear to be highly non-linear and may relate to the passage of sediment waves, by variable local sediment storage and availability, and by large- and small-scale thresholds for sediment transfer within each catchment. Differential response has major implications for modelling fluvial systems and the interpretation of field data. Model results are compared with the record of dated alluvial deposits in the modelled catchments.     

Quantifying fluvial non linearity and finding self organized criticality? Insights from simulations of river basin evolution

A numerical study was undertaken to investigate non linearity and the potential for self-organized criticality (SOC) in the evolution of river basins. Twenty-three simulations were carried out, using the authors' CAESAR landscape evolution model, in which the magnitude of storm events, variability of storm events, sediment heterogeneity, sources of sediment supply, and catchment morphology are systematically varied to evaluate their importance as possible drivers for non linear behavior and SOC.Temporal fluctuations in simulated sediment yield show notable non linear behavior. Storm magnitude and occurrence of landslides appear to have little impact on variability of the sediment yield, when compared to the impacts of sediment heterogeneity, rainfall variability and catchment morphology. Particularly, it appears that the non linearity of sediment yields results from the manner in which the catchment processes the variable rainfall, rather than just the rainfall variability itself.The variations in sediment yield show a power law magnitude–frequency distribution, which is a possible, but inconclusive, indicator of SOC. However, several other, more qualitative arguments can be made to support the case for SOC in these simulations. Specifically, we identify the nature of the critical state and suggest two cascade mechanisms by which the system can organize itself around this critical state. Combined, these arguments indicate that simulated evolution of river basins indeed exhibits SOC, at least with respect to sediment yield. The critical state appears to be an indicator of the connectivity of the drainage network. Thus, the simulations indicate that, unlike traditional SOC systems, the critical state of the system can vary in time, as sudden changes in drainage network connectivity may result in sudden changes in the SOC behavior of the system.