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Reach-scale river restoration or environmental water allocation (EWA) exercises typically address the magnitude and temporal dynamics (frequency, duration, timing, rate of change) of flows required to sustain desirable ecological conditions along a river. The role of geomorphology in this process is to broaden the gaze beyond flows to consider larger and longer-terminteractions between valley lithological structure, and the feed and fate of flow-sediment mixtures. This paper proposes the integration of numerical morphodynamic modelling in evaluations of environmental water requirements for non-perennial riverscapes (channel--riparian--floodplain environments). The paper presents a methodological framework, and proof of concept case study from the Touws River, South Africa, for the application of morphodynamic modelling in EWA. The paper illustrates operational approaches to modelling the complexity of dryland mixed bedrock-alluvial (and mixed-load) riverscapes with highly variable non-perennial flow regimes, including an approach to generating initial bed conditions for numerical experiments by 'morphodynamic spin-up', and approaches to synthesising and presenting numerical experiment output in the formof a dynamic range of potential variability inmetrics of physical habitat suitability and diversity, and disturbance/renewal regimes. Such efforts can assist in enhancing field observations and testing field-based hypotheses of flow-sediment regime--physical habitat associations, extending the timescales of analysis beyond field observation, and constraining uncertainty about the dynamic range of variability in responses to predicted future flow-sediment regime modifications. Further research is needed to develop growth models appropriate for key non-perennial river vegetation types, to support bio- morphodynamic modelling of geomorphology--vegetation interactions, and to determine or predict appropriate inlet sediment concentrations for historical and future modification scenarios. [ABSTRACT FROM AUTHOR]

The co-efficient of variation for inter-annual streamflow of the Mfolozi River is extremely high at 79%. An analysis of flow frequency indicated that streamflow is skewed towards low-flow values, with a number of extremely large flood events occurring as outliers on the histogram. Streamflow variability in the Mfolozi River may be linked to multiple factors including a large catchment size, a seasonal climate of a dry winter and wet summer, evergreen vegetation in the catchment, variable precipitation and the occurrence of regionally pervasive climatic oscillations. This research aimed to address how streamflow variability impacted upon sediment transport and thus, geornorphology. It was found that sediment transport variability occurred at the intra- and inter-annual scale. Analysis of mean monthly sediment concentration and discharge showed a hysteresis effect, such that sediment concentration peaked prior to discharge in the early wet season. During the late wet season, peak discharges often had unexpectedly low sediment concentrations. Furthermore, data suggested the existence of long-term hysteresis that may be related to decadal-scale climatic oscillations that alter sediment availability and stream capacity, resulting in discharge peaking in 2000 and sediment concentration in 2005. However, more data are required to confirm this relationship. Variability in streamflow appears to share a causal relationship with sediment transport variability, as both are linked to variation in precipitation and the resultant impacts on vegetation growth and evapotranspiration rates. The variability of streamflow and sediment transport has implications for stream and floodplain geomorphology, and the hydrology of variable rivers should be considered when interpreting their geomorphology. [ABSTRACT FROM AUTHOR]

Sea-level rise associated with climate change presents a major challenge to plant diversity and ecosystem service provision in coastal wetlands. In this study, we investigate the effect of sea-level rise on benthos, vegetation, and ecosystem diversity in a tidal wetland in west Wales, the UK. Present relationships between plant communities and environmental variables were investigated through 50 plots at which vegetation (species and coverage), hydrological (surface or groundwater depth, conductivity) and soil (matrix chroma, presence or absence of mottles, organic content, particle size) data were collected. Benthic communities were sampled at intervals along a continuum from saline to freshwater. To ascertain future changes to the wetlands' hydrology, a GIS-based empirical model was developed. Using a LiDAR derived land surface, the relative effect of peat accumulation and rising sea levels were modelled over 200 years to determine how frequently portions of the wetland will be inundated by mean sea level, mean high water spring and mean high water neap conditions. The model takes into account changing extents of peat accumulation as hydrological conditions alter. Model results show that changes to the wetland hydrology will initially be slow. However, changes in frequency and extent of inundation reach a tipping point 125 to 175 years from 2010 due to the extremely low slope of the wetland. From then onwards, large portions of the wetland become flooded at every flood tide and saltwater intrusion becomes more common. This will result in a reduction in marsh biodiversity with plant communities switching toward less diverse and occasionally monospecific communities that are more salt tolerant. While the loss of tidal freshwater wetland is in line with global predictions, simulations suggest that in the Teifi marshes the loss will be slow at first, but then rapid. While there will be a decrease in biodiversity, the model indicated that at least for one ecosystem service, carbon storage, there is potential for an increase in the near future., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Eroding farm tracks represent important spatially distributed features in many agricultural landscapes and there is concern over their role in catchment sediment problems. It is, however, important to place eroding farm tracks in the context of catchment sediment sources more generally, especially since the former afford potential for targeted sediment mitigation. A sediment source tracing procedure was therefore used to assess the importance of eroding farm track surfaces as a contemporary primary suspended sediment source relative to inputs from pasture or cultivated topsoils and channel banks/subsurface sources, in the upper River Piddle catchment (~100km(2)), in southern England. The study provided a timely opportunity to assess the performance of both local and global (genetic algorithm; GA) optimisation techniques in the sediment geochemistry mass balance modelling used to apportion sources. Over the duration of the study, average median source contributions for individual time-integrated suspended sediment samples collected from three sub-catchments ranged between 1±1 and 19±3% for farm track surfaces, 31±3 and 55±2% for pasture topsoils, 1±1 and 19±1% for cultivated topsoils and 23±2 and 49±1% for channel banks/subsurface sources. Comparison of the local and GA optimisation techniques demonstrated that GA with random initial values improved the minimisation of the objective functions compared to local searching by 0.01-0.04% of 5000 repeat Monte Carlo iterations. GA informed by the outputs of the local optimisation as initial values improved corresponding performance by 0.05-0.20%. These findings increased confidence in the outputs from the local optimisation mass balance modelling, but fingerprint property datasets should be treated on an individual basis. Future sediment source tracing studies should always endeavour to combine local and global search tools to avoid the risk of using localised solutions for source apportionment estimates., (Copyright © 2010 Elsevier B.V. All rights reserved.)