An integrated approach for investigating the correlation between floods and river morphology: A case study of the Saalach River, Germany.

Man-made structures in the Saalach River have changed the hydromorphological characteristics of the river regime. In some river reaches, the Saalach has lost the high morphological versatility and high variation in sediment transport characteristic of a mountain river. Among the negative effects, an extreme flow discharge in combination with riverbed variation could be one of the possible causes of flood disasters along the river. For example, the heavy and long lasting rainfall in June 2013 led to a peak discharge of 1100 m3/s, which was slightly above the 100-year flood return period, inundating a nearby city. However, the influence of the man-made structures on this flood event in this reach is unclear. In this study an integrative hydromorphological model is applied to evaluate this impact by a comparison with a standard clear water model with fixed bed. Moreover, a comparative analysis of a three-and two-dimensional flow model is performed to assess the models suitability representing the flow in this river stretch. The integrative model concept is based on the software TELEMAC-MASCARET, in an enhanced version for better representing graded sediment transport in rivers. In contrast to our integrative model, the standard clear water model with fixed bed overestimates the water elevations as it cannot take the significant changes in morphology into account. Results demonstrate that our proposed model more accurately represents the inundation in the floodplain and could thus be used to provide more reliable predictions to decision-makers for improved flood protection strategy.

Modelling the influence of total suspended solids on E. coli removal in river water.

Following sewer overflows, fecal indicator bacteria enter surface waters and may experience different lysis or growth processes. A 1D mathematical model was developed to predict total suspended solids (TSS) and Escherichia coli concentrations based on field measurements in a large-scale flume system simulating a combined sewer overflow. The removal mechanisms of natural inactivation, UV inactivation, and sedimentation were modelled. For the sedimentation process, one, two or three particle size classes were incorporated separately into the model. Moreover, the UV sensitivity coefficient α and natural inactivation coefficient kd were both formulated as functions of TSS concentration. It was observed that the E. coli removal was predicted more accurately by incorporating two particle size classes. However, addition of a third particle size class only improved the model slightly. When α and kd were allowed to vary with the TSS concentration, the model was able to predict E. coli fate and transport at different TSS concentrations accurately and flexibly. A sensitivity analysis revealed that the mechanisms of UV and natural inactivation were more influential at low TSS concentrations, whereas the sedimentation process became more important at elevated TSS concentrations.

Persistence of fecal indicator bacteria in sediment of an oligotrophic river: comparing large and lab-scale flume systems.

In this study, both a lab and a large-scale flume system were used to investigate the survival of fecal indicator bacteria (FIB) in bed sediments of an alpine oligotrophic river. To determine the influence of substratum on persistence, survival within 3-cm-deep substratum cages versus on thin, biofilm-covered ceramic tiles was tested. Moreover, the impact of bed shear stress on survival in bed sediments was explored. It was seen that in the lab-scale flume having a very low bed shear stress (0.3 N m(-2)), E. coli and enterococci survival in 3-cm-deep substratum cages was nearly the same as in a thin biofilm (200 µm). However, in the large-scale flume system characterized by a bed shear stress of 9 N m(-2), the added protection of the deeper substratum cages promoted considerably longer survival of E. coli and enterococci than the thin biofilm. Additionally, the FIB removal mechanisms in the two flume systems varied. At the lab-scale, enterococci was seen to persist twice as long as E. coli, while in the large-scale flume the two FIB were removed at the same rate. A comparison of qPCR analyses performed in both flumes suggests that bed sediment erosion and the influence of grazers/predators were responsible for FIB removal from the sediments in the large-scale flume, whereas in the lab flume FIB inactivation caused removal. These results indicate that hydraulic parameters such as bed shear stress as well as the presence of macroinvertebrates in a system are both important factors to consider when designing flumes as they can significantly impact FIB persistence in sediments of fast-flowing, alpine streams.

Influence of resuspension on the fate of fecal indicator bacteria in large-scale flumes mimicking an oligotrophic river.

In this study, large-scale flume systems simulating an oligotrophic river were used to explore the fate and transport of the fecal indicator bacteria (FIB) Escherichia coli and enterococci following a combined sewer overflow (CSO). Specifically, the removal pattern of FIB from the water column was examined as well as deposition onto the flume bed. Finally, the impact that a sudden increase in bed shear stress has on FIB in the water column was investigated. The large-scale flumes utilized in this study proved extremely useful for our investigations as they very closely approximated conditions within the Isar River (Munich, Germany). By using both natural substratum and fresh river water, as well as a flow velocity of nearly 1 m s(-1) at a water depth of roughly 0.5 m, shear stresses typical of the Isar River (9 N m(-2)) were achieved. As a result, scaling effects were appreciably reduced. In our flume system, UV inactivation played only a minimal role in overall FIB removal. Therefore, we were able to more precisely investigate other mechanisms which result in FIB removal from the water column. From the two standard FIB removal experiments following a CSO, the removal rate coefficient (k) of 0.2 h(-1) was identified for both E. coli and enterococci in the water column. An increase in the bed shear stress led to more than a 150% rise in total suspended solid (TSS) levels in the water column. These elevated TSS levels (≈ 50 mg l(-1)) increased the persistence of suspended FIB in the water column by 20 h (k = 0.05 h(-1)). This indicates that higher TSS loads resulting from resuspended bed sediments can significantly expand the area that is impacted by a CSO event. At lower TSS loads (<20 mg l(-1)) deposition onto the flume bed did not contribute significantly to FIB removal from the water column. Any deposition which did occur did not result in a net accumulation of culturable FIB in the benthic biofilm.