Previous studies of overbank hydraulics and sedimentation have adopted a variety of methods to investigate overbank processes, using either field based, physical or numerical modelling approaches. In this study a two-dimensional depth-averaged model of overbank hydraulics (Hydro2de) is tested and then used to develop a two-dimensional model of suspended sediment transport and deposition. A model grid was used with a higher resolution than those employed in most equivalent model applications in order to represent the topographic complexity of the natural floodplain in detail. The models were calibrated and rigorously tested using a varied and detailed set of spatially and temporally distributed field data. Models were applied to a short reach of the River Culm, Devon, UK. A wide range of field and laboratory data were used to enable model implementation, calibration and validation. Continuous monitoring of stage and sediment concentration were carried out at the site for the period of study and existing data were used from gauging stations located up- and downstream of the site. A large data set of spatially distributed sediment concentration, velocity and flow depth data were collected across the floodplain over a series of flood events. Inundation boundaries were identified using GPS surveys supplemented by field observations and both ground and aerial photography. The amount of overbank deposition was measured for individual floods using astroturf sedimentation traps. Medium-term sedimentation rates were estimated at selected locations using the Caesium-137 whole core technique. Both models were shown to yield good results which, when compared with field measurements were within the limits of uncertainty associated with these data. Simulation results allow a number of conclusions to be drawn about floodplain processes and their representation by numerical models. First, representation of complex, small-scale floodplain topography (features with dimensions <10 m) is of considerable importance when modelling flow and sedimentation on natural floodplains. Second, sediment transfer from the channel to the floodplain is dominated by advection rather than diffusion. Third, low magnitude events are more important than infrequent high magnitude events in terms of total conveyance losses, despite the lower sediment load of the former. Fourth, high resolution distributed models may be used to develop simple conceptual statistical models that capture the complexity of hydraulic conditions on natural floodplains. Finally, further work on floodplain modelling should concentrate on the development of a physically-based understanding of deposition processes in the presence of vegetation.

<p>Threats from man-made radionuclides include waste issues, increasing number of power plants, underground bomb testing, nuclear weapons, and “dirty bombs”. Until recently the ionizing radiation protection system focused on protecting humans with an implied protection of biota. However, goals of sustainable development and precautionary principles for human activity are leading to an inclusion of plant and animal populations in the protection system.</p><p>From this perspective, the present thesis examines wetlands that function as sinks for the radionuclide 137Cs, and describes calculated and measured radiation doses to residing biota. Also, multi-level effects from exposure to low-dose rate ionizing radiation were studied. Accumulation of 137Cs after the Chernobyl accident fallout was studied in a riparian wetland with a mean activity concentration of 1 200 kBq m-2 in Sweden (paper I). A mass balance budget of 137Cs showed that the sedimentation of new material was balanced by the decay process of 137Cs in parts of the wetland (paper I).</p><p>Frogs were identified as organisms of concern in this wetland. Internal radiation doses, based on whole body measurements of frogs, were estimated to be lower than external doses based on soil samples (paper II). Current dose models for biota resulted in a wide range of doses depending on different levels of conservatism in the models. Therefore, in situ measurements with frog-phantoms were found to provide valuable dose information (paper III). Measured doses using frog-phantoms were lower than calculated doses using several dose models. Although a dose conversion factor by FASSET was found to be useful for comparison with measurements in the field. A higher dose was measured to the phantom surface in comparison to inner parts, i.e. the sensitive skin of frogs receives the highest dose. Estimated and measured radiation doses to frogs were below suggested dose rate limits.</p><p>Low-dose rate 137Cs exposure of eggs and tadpoles from three amphibian species, Scaphiopus holbrookii, Bufo terrestris, and Rana catesbeiana, showed no increased levels of strand breaks in red blood cells, and no effects on development, survival or growth up to metamorphosis (paper IV). The ecological factor larval density had a stronger effect on metamorphic traits than low-dose rate radiation. Higher levels of strand breaks were detected after an acute dose in R. catesbeiana than after a chronic dose supporting a dose rate limit for protection of amphibians rather than a dose limit (paper IV).</p><p>Based on current knowledge, frogs in the contaminated wetland are probably not exposed to radiation doses from 137Cs that are harmful for the population. However, variations in sensitivity between populations and species, and adaptive responses have been shown for amphibians exposed to other stressors. This supports further research on effects of chronic low-dose rates of ionizing radiation on amphibians.</p>

Threats from man-made radionuclides include waste issues, increasing number of power plants, underground bomb testing, nuclear weapons, and “dirty bombs”. Until recently the ionizing radiation protection system focused on protecting humans with an implied protection of biota. However, goals of sustainable development and precautionary principles for human activity are leading to an inclusion of plant and animal populations in the protection system. From this perspective, the present thesis examines wetlands that function as sinks for the radionuclide 137Cs, and describes calculated and measured radiation doses to residing biota. Also, multi-level effects from exposure to low-dose rate ionizing radiation were studied. Accumulation of 137Cs after the Chernobyl accident fallout was studied in a riparian wetland with a mean activity concentration of 1 200 kBq m-2 in Sweden (paper I). A mass balance budget of 137Cs showed that the sedimentation of new material was balanced by the decay process of 137Cs in parts of the wetland (paper I). Frogs were identified as organisms of concern in this wetland. Internal radiation doses, based on whole body measurements of frogs, were estimated to be lower than external doses based on soil samples (paper II). Current dose models for biota resulted in a wide range of doses depending on different levels of conservatism in the models. Therefore, in situ measurements with frog-phantoms were found to provide valuable dose information (paper III). Measured doses using frog-phantoms were lower than calculated doses using several dose models. Although a dose conversion factor by FASSET was found to be useful for comparison with measurements in the field. A higher dose was measured to the phantom surface in comparison to inner parts, i.e. the sensitive skin of frogs receives the highest dose. Estimated and measured radiation doses to frogs were below suggested dose rate limits. Low-dose rate 137Cs exposure of eggs and tadpoles from three amphibian species, Scaphiopus holbrookii, Bufo terrestris, and Rana catesbeiana, showed no increased levels of strand breaks in red blood cells, and no effects on development, survival or growth up to metamorphosis (paper IV). The ecological factor larval density had a stronger effect on metamorphic traits than low-dose rate radiation. Higher levels of strand breaks were detected after an acute dose in R. catesbeiana than after a chronic dose supporting a dose rate limit for protection of amphibians rather than a dose limit (paper IV). Based on current knowledge, frogs in the contaminated wetland are probably not exposed to radiation doses from 137Cs that are harmful for the population. However, variations in sensitivity between populations and species, and adaptive responses have been shown for amphibians exposed to other stressors. This supports further research on effects of chronic low-dose rates of ionizing radiation on amphibians.