Alan Werritty

FIELD EVIDENCE FOR RAPID DOWNSTREAM FINING OF RIVER GRAVELS THROUGH SELECTIVE TRANSPORT

Doubts have been expressed about the ability of either abrasion or sorting to explain strong downstream fining of river gravels. We describe pronounced fining over a short distance in a Scottish river that has no human disturbance or lateral input of water and sediment. Measured abrasion rates are far too small to explain observed downstream fining, but bed-load trap measurements and the dispersion of magnetic tracer pebbles in six subreaches both show sorting during transport. The observed downstream fining is also simulated well by a numerical sediment routing model using hydraulic and transport laws consistent with our field measurements. The geomorphological cause of the strong fining is slope reduction above a local base-level control. In this common situation the development of downstream fining is part of the river9s tendency to minimize downstream variation in bed-load transport rates, and can proceed far more rapidly than the major aggradation otherwise required for equilibration.

Living with uncertainty: climate change, river flows and water resource management in Scotland

The recent increased variability of Scotlands hydroclimate poses major problems for water resource managers charged with making informed investment decisions given the likely impact of future climate change. Two strategies are developed in this paper to assist managers faced with this environmental uncertainty. The first involves trend analysis of precipitation and runoff since the 1960s and 1970s viewed against longer-term variability reported from instrumental records. The second strategy is based upon current climate change scenarios coupled with GCMs, and downscaling of precipitation and temperature to provide inputs to rainfall-runoff models. The long-term records of precipitation (back to the 1860s) and runoff (back to the 1930s) reveal the late 1980s and early 1990s as the wettest period on record for the west but not for the east. Over the period 1961-1996 the precipitation gradient has intensified across Scotland: wetter west; relatively dry east. Changes in runoff over the period 1970-1996 are also reported with increases in annual flows at 33 out of 38 stations (significantly at 12 stations) and decreases in low flows at 21 out of 38 stations (significantly at one station). The bulk of these flow increases occurred in the south and west especially in the autumn and spring. In terms of high flows over the period 1970-1996, four out of 44 stations reported a change in magnitude and 15 reported an increase in the frequency of POT events. In terms of future climate change, Hulme and Jenkins (1998) predict annual precipitation increases of 6-16% (Scotland) and 6-14% (Scottish Borders) from the 2020s to the 2080s based on the Hadley Centre model (HadCM2) medium-high scenario. Seasonal changes are concentrated in the autumn (SON) and winter (DJF) with increases as high as 24 and 29% for the autumn by the 2080s. (Arnell NW, et al. Institute of Hydrology Report No. 107, Wallingford, 1996), using an earlier transient Hadley experiment (IS92a), predict a 5-15% increase in annual runoff across Scotland by the 2050s, locally rising to 25%. Simulation flow duration curves for the 2050s generate Q95 values up by 5% or less (Rivers Don, Almond and Nith) and Q5 up by 10-24% (Rivers Don, Almond, Nith and Lyne Water). In terms of water resource planning, these predicted changes should be regarded as first order approximations, as they take no account of natural climatic variability, and could generate different absolute values if other scenarios were used. The predictions are, however, broadly consistent with trends in precipitation and runoff for Scotland since the 1970s. Major issues of concern to water resource managers are identified and commented upon in the light of these predictions.

Mobility of river tracer pebbles over different timescales

[1] Tracer pebbles are widely used to learn about gravel transport along rivers. Movement over short times and distances is dominated by factors controlling entrainment: relative particle size and shear stress. Movement at longer scales also involves depositional factors: burial and reexposure and exchange between channels, bars, and other depositional environments. We mapped mixed-size tracers in six reaches of a small Scottish river after 2 and 8 years to investigate differences in relative and absolute mobility and infer the importance of burial and exchange. Patterns of relative mobility according to size and shear stress, both within and between reaches, did not change significantly. Some local bunching of tracers was apparent in both surveys, with redistribution from pools into riffles and bars. The main change was that virtual velocities were ∼50% lower, and estimated gravel fluxes were also lower, in the longer term. This slowdown is attributed to vertical mixing giving decreased mobility as surface-seeded tracers become buried, long-term storage in bars and other less active parts of the system, and in this channel, advection of tracers downstream onto a finer bed giving higher relative size.

Seasonality of flooding: a case study of North Britain

Abstract The seasonality of river flooding in North Britain displays considerable spatial variation. This paper identifies the geographical patterns of flood seasonality, using a database of events exceeding modest flood-flow thresholds at each of 156 gauging stations, and seeks to explain them in terms of climatological and catchment characteristics. Floods are found to occur at all times of year, but most rivers register at least 7846 of events in the October-March half-year, and these generally occur later in the year with distance from west to east. However, notable exceptions are superimposed upon this general pattern and, in particular, two areas of less pronounced seasonality occur on north-facing parts of the east coast. Seasonality is characterized using three complementary methods, including a four-fold seasonal classification which summarises the patterns found. In order to explain these patterns, reference is made to the seasonality of storm rainfall, soil moisture deficits, catchment size and lake storage. Seasonality class is correctly explained by reference to these catchment characteristics in 7446 of cases using discriminant analysis. The work is presented as an advance in the understanding of flood generation and, ultimately, in the assessment of flood risk.

River channel planform change: software for historical analysis

Abstract ER Mapper™, an image processing system, was used to manipulate scanned maps, plans and aerial photographs to enable detailed analysis of historical river channel planform change. The ER Mapper™ package offers the opportunity to spatially reference each item of planform data into the same scale and co-ordinate space as a base map, using a warping algorithm and operator selected ground control points. Once all the data are in the same co-ordinate space and at the same scale, a given point can be returned to over time, and a number of indices can be easily calculated to examine past channel behaviour. Here, lateral shift, channel width, sinuosity and area of sedimentation within the reach were used. As the maps and aerial photographs themselves are used, the errors associated with digitising vectors are avoided, and a more flexible approach towards analysis and the identification of important indices can be adopted at the outset. Results obtained for the Cleekhimin Burn, a site in southern Scotland, are briefly presented and discussed, and the advantages of the package and the limitations of the historical database are also outlined.

Quantitative determination of the activity of within-reach sediment storage in a small gravel-bed river using transit time and response time

Abstract Reach-scale sediment storage is rarely quantified in sediment budget studies, yet it has a considerable effect on the sediment delivery ratio at the basin scale, and on the accuracy of morphological methods of bedload estimation at the reach scale. Deployment of magnetic tracer particles allows accurate characterisation of sediment fluxes in gravel-bed rivers and provides the opportunity to quantify storage activity using reservoir theory. Activity was quantified at reach and sub-reach scales in two reaches of a small gravel-bed river and the possibility of quantifying the activity of smaller-scale sediment stores is explored. Reach-scale transit time functions were derived from the cumulative output of sediment against age or time. The shape of the functions in both reaches was determined by flood incidence and magnitude (hydraulics), sediment (tracer) availability, grain size and local morphology. Accurate transit time functions were difficult to determine due to reliance upon tracer output and the associated problems of tracer exhaustion over time and imperfect tracer recovery rates. In addition, if tracers of different ages are allowed to mix (for example where upstream input is possible), then the resultant transit time distributions (age) are not comparable with the timing of the hydraulic processes responsible unless the hydraulic conditions are constant. These results suggest that transit time is difficult to determine from tracer studies in gravel-bed rivers. Consequently, a refinement of transit time, the response time, is introduced and is defined as the time, after the initial input of tracer sediment, when cumulative tracer output exceeds the amount of tracer sediment remaining in storage. Whilst still based upon tracer output, the calculation of response time also utilises storage data and is expressed relative to time since the start of the study rather than age (it is therefore directly comparable with the incidence of hydraulic conditions). This provides a more informative measure of activity which is readily available from tracer data in gravel-bed rivers and allows evaluation of the importance of flood incidence and magnitude, grain size, morphology and reach characteristics/stability. Like transit time, response time estimates are hindered by mixing of sediment of different ages at the sub-reach scale. However, it does account for errors from less than 100% recovery and tracer exhaustion more effectively than transit time. Quantification of absolute and relative size effects in transport is also possible from response time.

Land use and a low-carbon society

Land use and the management of our natural resources such as soils and water offer great opportunities to sequester carbon and mitigate the effects of climate change. Actions on forestry, soil carbon and damaged peatlands each have the potential to reduce Scottish emissions in 2020 by hundreds of thousands of tonnes. Most actions to reduce emissions from land use have beneficial effects on other ecosystem services, so if we can cut emissions we can in many circum- stances improve the environment. The cost of reducing emissions through land use change can be low in relation to other means of cutting emissions. The Scottish Land Use Strategy and the Eco- system Approach it calls for, employing the concept of ecosystem services, offers a way of balancing environmental, social and economic demands on the land. Scotlands land, soils, forests and waters are all likely to be significantly altered by future climate change. Each of these components of the land-based environment offers opportunities for mitigation and adaptation to climate change. The emerging new imperatives for securing food, water and energy at a global level are equally impor- tant for Scotland, and interact with the need for environmental security and for dealing with climate change.

Climate change and Scotland: recent trends and impacts

This paper reviews the key evidence for global climate change and outlines the trends of climate change in Scotland, the potential impacts and the implications for policy makers. Human activity is causing a rise in atmospheric CO2 concentrations and there is little doubt that this is contributing to global warming. There is greater uncertainty about how this global trend will play out at a regional scale and also how close we are to climatic tipping points. Instrumental records document the overall trends and variability in Scotlands climate since 1914. These show that since the 1960s, Scotlands average climate has proved to be wetter (especially in the west) and warmer. This trend is expected to continue throughout the 21st Century with, on average, hotter and drier summers and milder and wetter winters. However, extreme events will continue to affect Scotland, as they have always done, and the severity and frequency of these events may increase. Sea levels will continue to rise modestly, especially in the Outer Hebrides and the Northern Isles. Some of the uncertainty in climatic predictions is captured in the probabilistic outputs of Defras UK Climate Projections 2009 programme. An initial attempt to assess the likely impacts of climate change is provided in Defras 2012 Climate Change Risk Assessment, which includes a report specific to Scotland. Whilst most of the risks involve negative impacts, with increased flooding and loss of biodiversity being especially adverse, there are also positive impacts with associated opportunities, especially in terms of increased agricultural production and larger numbers of tourists. The report on Scotland will allow different groups of policy makers to refine the risks associated with specific activities. But given the fragile nature of many of the metrics underpinning the report, caution should be exercised in using it to frame climate adaptation strategies.

Hydrology in Scotland: towards a scientific basis for the sustainable management of freshwater resources—foreword to thematic issue

The physical background to hydrology in Scotland is briefly reviewed. The main scientific events associated with understanding the science of hydrology in Scotland are summarized and major contemporary research themes are outlined. A brief overview of each paper in the current volume is given.

Hydrological science, society and the sustainable management of Scottish freshwaters resources in the 21st century.

Hydrology in Scotland has emerged as a diverse and maturing discipline in recent years following its origins in engineering and the environmental sciences. Despite significant progress in understanding the physical, chemical and biological aspects of the hydrological cycle in Scotland, hydrologists face a number of significant challenges. These include: improved basic process understanding and modelling of catchment functioning; increased understanding of climatic variability and change; the collection of more extensive and well-integrated data sets; improved understanding of the role of hydrology in maintaining good ecological status in managed rivers; and a rapidly evolving policy agenda both within Scotland and the EU. So far, the response of the scientific community to these challenges has been encouraging. However, it is concluded that in the future, hydrologists need to be increasingly engaged in interdisciplinary research projects and communicate better with environmental planners and various stakeholder groups if the discipline is going to make its full contribution to sustainable water resource management in Scotland.