No abstract

Stream power can be an extremely useful index of fluvial sediment transport, channel pattern, river channel erosion and riparian habitat development. However, most previous studies of downstream changes in stream power have relied on field measurements at selected cross-sections, which are time consuming, and typically based on limited data, which cannot fully represent important spatial variations in stream power. We present here, therefore, a novel methodology we call CAFES (combined automated flood, elevation and stream power), to quantify downstream change in river flood power, based on integrating in a GIS framework Flood Estimation Handbook systems with the 5 m grid NEXTMap Britain digital elevation model derived from IFSAR (interferometric synthetic aperture radar). This provides a useful modelling platform to quantify at unprecedented resolution longitudinal distributions of flood discharge, elevation, floodplain slope and flood power at reach and basin scales. Values can be resolved to a 50 m grid. CAFES approaches have distinct advantages over current methodologies for reach-and basin-scale stream power assessments and therefore for the interpretation and prediction of fluvial processes. The methodology has significant international applicability for understanding basin-scale hydraulics, sediment transport, erosion and sedimentation processes and river basin management. 286 EARTH SURFACE PROCESSES AND LANDFORMS flood power at an unprecedented spatial resolution for both slope and discharge measurements. This is a significant improvement on earlier approaches (e.g. Lecce, 1997;Abernethy and Rutherfurd, 1998;Knighton, 1999). Many field studies, though useful, have necessarily relied on a limited number of crosssections, often from a single basin, given the time-consuming nature of field surveys. Also, even more recent studies have, at best, made use of finely resolved data in either slope (e.g. Reinfelds et al., 2004;Jain et al., 2006) or discharge, but not both, or have estimated flow at ungauged sites very simply, typically from an area-discharge relation (e.g. Fonstad, 2003). The CAFES methodology, however, integrates within a GIS framework several novel, high-resolution DEMs, digital land use and soil datasets and flood estimation systems to produce digital databases of flood discharge and slope relatively rapidly. These can then be co-registered and combined to generate Figure 3. Longitudinal flood discharge trends for rivers in western Britain in terms of distance downstream and catchment area for (a), (b) River

We demonstrate that the microstructural and mechanical properties of bacterial biofilms can be created through colloidal self-assembly of cells and polymers, and thereby link the complex material properties of biofilms to well understood colloidal and polymeric behaviors. This finding is applied to soften and disassemble staphylococcal biofilms through pH changes. Bacterial biofilms are viscoelastic, structured communities of cells encapsulated in an extracellular polymeric substance (EPS) comprised of polysaccharides, proteins, and DNA. Although the identity and abundance of EPS macromolecules are known, how these matrix materials interact with themselves and bacterial cells to generate biofilm morphology and mechanics is not understood. Here, we find that the colloidal self-assembly of Staphylococcus epidermidis RP62A cells and polysaccharides into viscoelastic biofilms is driven by thermodynamic phase instability of EPS. pH conditions that induce phase instability of chitosan produce artificial S. epidermidis biofilms whose mechanics match natural S. epidermidis biofilms. Furthermore, pH-induced solubilization of the matrix triggers disassembly in both artificial and natural S. epidermidis biofilms. This pH-induced disassembly occurs in biofilms formed by five additional staphylococcal strains, including three clinical isolates. Our findings suggest that colloidal self-assembly of cells and matrix polymers produces biofilm viscoelasticity and that biofilm control strategies can exploit this mechanism.

Biofilms are microbial communities that are characterized by the presence of a viscoelastic extracellular polymeric substance (EPS). Studies have shown that polysaccharides, along with proteins and DNA, are a major constituent of the EPS, and play a dominant role in mediating its microstructure and rheological properties. Here, we investigate the possibility of entanglements and associative complexes in solutions of extracellular polysaccharide intercellular adhesin (PIA) extracted from Staphylococcus epidermidis biofilms. We report that the weight average molar mass and radius of gyration of PIA isolates are 2.01 × 105 ± 1200 g/mol and 29.2 ± 1.2 nm respectively. The coil overlap concentration, c*, was thus determined to be (32 ± 4) × 10−4 g/mL. Measurements of the in situ concentration of PIA (cPIA,Biofilm) was found to be (10 ± 2) × 10−4 g/mL. Thus, cPIA,Biofilm < c* and the amount of PIA in the biofilm is too low to cause polymer chain entanglements. In the pH range 3.0 to 5.5, PIA was found to both self-associate and to form complexes with bovine serum albumin (BSA). By static light scattering, both self-association and complex formation with 0.5 %(w/v) BSA were found to occur at PIA concentrations of 0.30 × 10−4 g/mL and greater, which is about 30 times lower than the measured cPIA,Biofilm. These results suggest that the microscopic origin of EPS viscoelasticity is unlikely to be due to polysaccharide entanglements. Furthermore, the onset of self-association and protein complexation of PIA occurs at concentrations far lower than the native PIA concentration in biofilms. This finding therefore suggests a critical role for those two association mechanisms in mediating biofilm viscoelasticity.

Cellular clustering and separation of Staphylococcus epidermidis surface adherent biofilms were found to depend significantly on both antibiotic and environmental stress present during growth under steady flow. Image analysis techniques common to colloidal science were applied to image volumes acquired with high-resolution confocal laser scanning microscopy to extract spatial positions of individual bacteria in volumes of size ~30 × 30 × 15 μm3. The local number density, cluster distribution, and radial distribution function were determined at each condition by analyzing the statistics of the bacterial spatial positions. Environmental stressors of high osmotic pressure (776 mM NaCl) and sublethal antibiotic dose (1.9 μg/mL vancomycin) decreased the average bacterial local number density 10-fold. Device-associated bacterial biofilms are frequently exposed to these environmental and antibiotic stressors while undergoing flow in the bloodstream. Characteristic density phenotypes associated with low, medium, and high local number densities were identified in unstressed S. epidermidis biofilms, while stressed biofilms contained medium- and low-density phenotypes. All biofilms exhibited clustering at length scales commensurate with cell division (~1.0 μm). However, density phenotypes differed in cellular connectivity at the scale of ~6 μm. On this scale, nearly all cells in the high- and medium-density phenotypes were connected into a single cluster with a structure characteristic of a densely packed disordered fluid. However, in the low-density phenotype, the number of clusters was greater, equal to 4% of the total number of cells, and structures were fractal in nature with df =1.7 ± 0.1. The work advances the understanding of biofilm growth, informs the development of predictive models of transport and mechanical properties of biofilms, and provides a method for quantifying the kinetics of bacterial surface colonization as well as biofilm fracture and fragmentation.

We studied the interaction between capsule production and hydrodynamic growth conditions on the internal and macroscopic structure of biofilms and spontaneously formed aggregates of Klebsiella pneumoniae. Wild-type and capsule-deficient strains were studied as biofilms and under strong and mild hydrodynamic conditions. Internal organization of multicellular structures was determined with a novel image-processing algorithm for feature extraction from high-resolution confocal microscopy. Measures included interbacterial spacing and local angular alignment of individual bacteria. Macroscopic organization was measured via the size distribution of aggregate populations forming under various conditions. Compared with wild-type organisms, unencapsulated mutant organisms formed more organized aggregates with less variability in interbacterial spacing and greater interbacterial angular alignment. Internal aggregate structure was not detectably affected by the severity of hydrodynamic growth conditions. However, hydrodynamic conditions affected both wild-type and mutant aggregate size distributions. Bacteria grown under high-speed shaking conditions (i.e., at Reynolds' numbers beyond the laminar-turbulent transition) formed few multicellular aggregates while clumpy growth was common in bacteria grown under milder conditions. Our results indicate that both capsule and environment contribute to the structure of communities of K. pneumoniae, with capsule exerting influence at an interbacterial length scale and fluid dynamic forces affecting overall particle size.

Abstract. The Focused Rainfall Growth Extension (FORGEX) method produces rainfall growth curves focused on a subject site. Focusing allows the incorporation of rainfall extremes observed regionally while respecting local variations in growth rates. The starting point for the analysis is an extensive set of annual maximum rainfalls, with values at each gauged site standardized by the median. Following the philosophy of the earlier FORGE method, a strongly empirical approach is adopted. The rainfall growth curve is represented by linear segments on a Gumbel scale, and is fitted by a least-squares criterion. The selection of data points is intricate and includes both the traditional pooling of regional extremes and the incorporation of network maximum events. The latter comprise the largest events from successive hierarchical networks of gauges, focused on the site for which estimates are requires. Their treatment takes account of interdependence using the Dales and Reed model of spatial dependence in rainfall extremes.

The paper compares results from two approaches to the quantification of river flood frequencies applicable nationally in Britain. One approach uses both the flood peak and event‐based methods of the Flood Estimation Handbook (FEH) of current water industry practice and the other approach is a recently developed set of continuous simulation techniques using parameter‐sparse modelling of catchment flood runoff response. The methods were applied to over a hundred sites in Britain, treated as if without flow data, although such observations existed and were used only for testing purposes. Errors of ≤20% in peak flows at ungauged sites are currently very good in this hydrologically challenging context; errors of up to around 35% may have to be contended with in flood management practice. The results from the FEH statistical method reinforce its established role in peak‐flow estimation. On the basis of the aspects that have been tested here, the emerging continuous simulation approaches show considerable potential to offer good performance for peaks and flow time series. The errors associated with the FEH unit hydrograph approach reflect the additional challenge it incorporates of ungauged rainfall estimation in addition to ungauged discharge.