Interactions between fine-grained sediment delivery, river bed deposition and salmonid spawning success

Salmonids clean river bed gravels to lay their eggs. However, during the incubation period fine sediment infiltrates the bed. This has been found to limit the success of salmonid spawning, as fine sediment reduces gravel permeability resulting in intra-gravel flow velocities and O2 concentrations decreasing. The success of salmonid spawning is therefore a function of the coincidence of fine sediment delivery and the development of the salmonid eggs. The presence of fine sediment also exerts sub-lethal effects on the rate of egg development with a negative feedback slowing and extending the incubation process meaning the eggs are in the gravels for longer and susceptible to more potential sediment delivery events. The SIDO (Sediment Intrusion and Dissolved Oxygen)- UK model is a physically-based numerical model which simulates the effect of fine sediment deposition on the abiotic characteristics of the salmonid redd, along with the consequences for egg development and survival. This model is used to investigate the interactions and feedbacks between the timing and concentrations of suspended sediment delivery events, and the deposition of fine sediment within the gravel bed, and the consequences of this on the rate of egg development and survival. The model simulations suggest that egg survival is highly sensitive to suspended sediment concentrations, particularly to changes in the supply rate of sand particles. The magnitude, frequency and specific timing of sediment delivery events effects egg survival rates. The modelling framework is also used to investigate the impact of the rate of gravel infilling by sediment. The hypotheses of continual, discrete event and non-linear decline in the rate of infilling are investigated

Criticality in confined ionic fluids

A theory of a confined two dimensional electrolyte is presented. The positive and negative ions, interacting by a $1/r$ potential, are constrained to move on an interface separating two solvents with dielectric constants $\epsilon_1$ and $\epsilon_2$. It is shown that the Debye-H\"uckel type of theory predicts that the this 2d Coulomb fluid should undergo a phase separation into a coexisting liquid (high density) and gas (low density) phases. We argue, however, that the formation of polymer-like chains of alternating positive and negative ions can prevent this phase transition from taking place.Comment: RevTex, no figures, in press Phys. Rev.

Generation of defects and disorder from deeply quenching a liquid to form a solid

We show how deeply quenching a liquid to temperatures where it is linearly unstable and the crystal is the equilibrium phase often produces crystalline structures with defects and disorder. As the solid phase advances into the liquid phase, the modulations in the density distribution created behind the advancing solidification front do not necessarily have a wavelength that is the same as the equilibrium crystal lattice spacing. This is because in a deep enough quench the front propagation is governed by linear processes, but the crystal lattice spacing is determined by nonlinear terms. The wavelength mismatch can result in significant disorder behind the front that may or may not persist in the latter stage dynamics. We support these observations by presenting results from dynamical density functional theory calculations for simple one- and two-component two-dimensional systems of soft core particles.Comment: 25 pages, 11 figure

Understanding the controls on deposited fine sediment in the streams of agricultural catchments

Excessive sediment pressure on aquatic habitats is of global concern. A unique dataset, comprising instantaneous measurements of deposited fine sediment in 230 agricultural streams across England and Wales, was analysed in relation to 20 potential explanatory catchment and channel variables. The most effective explanatory variable for the amount of deposited sediment was found to be stream power, calculated for bankfull flow and used to index the capacity of the stream to transport sediment. Both stream power and velocity category were highly significant (p<<0.001), explaining some 57% variation in total fine sediment mass. Modelled sediment pressure, predominantly from agriculture, was marginally significant (p<0.05) and explained a further 1% variation. The relationship was slightly stronger for erosional zones, providing 62% explanation overall. In the case of the deposited surface drape, stream power was again found to be the most effective explanatory variable (p<0.001) but velocity category, baseflow index and modelled sediment pressure were all significant (p<0.01); each provided an additional 2% explanation to an overall 50%. It is suggested that, in general, the study sites were transport-limited and the majority of stream beds were saturated by fine sediment. For sites below saturation, the upper envelope of measured fine sediment mass increased with modelled sediment pressure. The practical implications of these findings are that (i) targets for fine sediment loads need to take into account the ability of streams to transport/retain fine sediment, and (ii) where agricultural mitigation measures are implemented to reduce delivery of sediment, river management to mobilise/remove fines may also be needed in order to effect an improvement in ecological status in cases where streams are already saturated with fines and unlikely to self-cleanse

River habitat mapping using Airborne Imaging Spectrometry (CASI-3)

A Compact Airborne Spectrographic Imager (CASI-2) is used to map the depth of the River Test, a chalk stream in southern England. Lyzenga’s (1981) method is used to decouple variations in water depth from changes in bed material type. The results show that atmospheric correction improves the relationship between water depth and the logarithm of CASI reflectance, especially in visible wavelengths. The estimated accuracy of water depth was ± 20 cm, but validation was difficult due to the need to precisely co-locate the ground samples and the CASI data

Event bed load yield measurement with load cell bed load traps and prediction of bed load yield from hydrograph shape

One of the goals of sediment transport research is to predict the bed load yield of ungauged catchments. However, what little field data exist are characterized by temporal fluctuations in the record of event yield. Attempts to model the process of transport typically involve the derivation of empirical relationships between commonly measured variables such as stream discharge and the rate of bed load transport. Such approaches fail to account for the effect of sediment supply on the process of transport, which is known to contribute to the variability in records. This paper sets out a conceptual model of sediment transport that establishes a link between the form of the stream hydrograph and the effectiveness of an event as indexed by sediment yield. The model is then tested against a dataset of 60 bed load transport events recorded using a new type of load-cell pit trap. Total event power is shown to be capable of explaining 70% of the variance in event yields for a small woodland stream

Sediment transport processes in riffle-pool sequences and the effects of river regulation for hydro-electric power within the North Tyne

SIGLEAvailable from British Library Document Supply Centre- DSC:DX179107 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Environmental change in river channels: a neglected element. Towards geomorphological typologies, standards and monitoring

Rivers integrate the impacts of change in atmospheric and terrestrial systems; they then deliver these to the coast. En route geomorphological processes create dynamic and diverse habitats, both in-stream and in riparian/floodplain ecotones. The dynamics of channel change conflict with human resource development, the outcome is that many river and riparian environments have been significantly modified, complicating the interpretation of change. Collection of geomorphological data on both form and process has to date been overwhelmingly an academic pursuit; standard measurement networks and long-term monitoring have, as a result been largely absent-as in the Environmental Change Network (ECN), despite the emerging requirements of legislation such as the EU Water Framework Directive. In this paper, we utilise a unique set of repeat channel surveys and long-term bed-load sediment yields to provide guidance on both definitions of change and those variables and survey techniques which might form the basis, in future, of improved national-scale monitoring. The Environment Agency's River Habitat Surveys suggest the basis for channel typologies that could structure a sampling framework and rationalise the variables to be monitored. We also point to the value of more detailed geomorphological procedures in use at the catchment/project scale-Catchment Baseline Surveys and Fluvial Audits-as a standardised basis for monitoring the detail of change in the fluvial sediment system. A perfect opportunity to lay foundations for such monitoring activity has been provided in England and Wales by the winter floods of 2000/2001

Impacts of river restoration on small-wood dynamics in a low-gradient headwater stream (in Special Issue: Wood In World Rivers)

Wood is an important element in many river systems, interacting with channel and floodplain geomorphology, hydrology and ecology. Restoration practices are increasingly re-introducing wood into streams and researchers have started to study the geomorphic and ecological effects of re-introducing wood into rivers. However, little research has attempted to quantify the impacts of river restoration (including the addition of wood jams) on wood retention. Based on tracing dowels to simulate small wood (ranging in length from 0·184 to 1·06 m and diameter from 0·006 to 0·035 m) in three study reaches before and after restoration, this study provides a detailed representation of the influence of restoration on small-wood transport and the relative importance of different trapping sites within a low-order meandering stream in the New Forest, UK. The research specifically addresses the following questions. (i) Does restoration reduce transport of small wood? (ii) Does restoration increase the frequency and type of small-wood trapping sites? (iii) Do wood jams trap more small wood than other trapping sites? (iv) Do shorter pieces of wood travel further than long pieces? The study has demonstrated that (i) different types of restoration have different effects on the frequency and type of small-wood trapping mechanisms, and hence also on small-wood transport; (ii) wood jams were the most effective structures for trapping small wood in this environment; (iii) shorter pieces of wood travelled further than long pieces. Channel-floodplain interactions were also found to be important, allowing the floodplain to function as a trapping site