Experimental study on influence of different patterns of an emergent vegetation patch on the flow field and scour/deposition processes in the wake region

Flume experiments were conducted to comprehend the impact of different patterns of an emergent vegetation patch on the flow field and the scour process in natural rivers. Velocity measurements, flow visualization, and scour tests were undertaken around different vegetation patch patterns, which were simulated inspired by the expansion process of a typical instream vegetation. The patch expansion process was idealized with an initially a circular patch of rigid emergent stems becomes elongated due to positive and negative feedbacks. The expansion of the vegetation patch was considered to occur in three stages, in which the density of the patch from the previous stage was increased while the patch was also elongated by connecting at its downstream side with another sparser vegetation. These stages were replicated individually by only increasing in density and elongating the patch. In this way, two processes (i.e.elongation and decrease in permeability), which usually have hydrodynamically opposite effects on flow fields, were simulated at the same obstruction. Despite generally elongated obstacles being streamlined bodies, the morphometric analysis of obtained by laser scanner revealed that streamlined elongation of permeable patches amplifies global scour and enhances localization of the local scour hole. This situation implies that as the patch expands, in the wake region, the steady-wake region becomes shorter, turbulence diminishes, lateral shear stress enhances, and deposition cannot occur far from the patch. Consequently, as the patch expands, the hydrodynamic consequences may restrict further patch expansion after a certain length/density

Exchange between drainage systems and surface flows during urban flooding: Quasi-steady and dynamic modelling in unsteady flow conditions

The accurate modelling of urban flooding constitutes an integral part of flood risk assessment and management in residential and industrial areas. Interactions between drainage networks and surface runoff flows are commonly modelled based on weir/orifice equations; however, this approach has not been satisfactorily validated in unsteady flow conditions due to uncertainties in estimating the discharge coefficients and associated head losses. This study utilises experimental data of flow exchange between the sewer flow and the floodplain through a manhole without a lid to develop two alternate approaches that simulate this interaction and describe the associated exchange flow. A quasi-steady model links the exchange flow to the total head in the sewer pipe and the head losses in the sewer and the manhole, whilst a dynamic model takes also into account the evolution of the water level within the manhole at discrete time steps. The developed numerical models are subsequently validated against large-scale experimental data for unsteady sewer flow conditions, featuring variable exchange to the surface. Results confirmed that both models can accurately replicate experimental conditions, with improved performance when compared to existing methodologies based only on weir or orifice equations

A Hybrid Fuzzy Regression-Based Methodology for Normal Distribution (Case Study: Cumulative Annual Precipitation)

Part 12: FuzzyInternational audienceAn advantage of the probabilistic approach is the exploitation of the observed probability values in order to test the goodness-of-fit for the examined theoretical probability distribution function (pdf). Since in fact, the interest of the engineers is to determine the hydrological variable which corresponds to a selected return period, a fuzzy linear relation between the standardized normal variable Z and the examined hydrologic random variable is achieved in condition that the hydrological variable is normally distributed. In this work, for the first time, the implementation of the fuzzy linear regression of Tanaka is proposed, to achieve a fuzzy relation between the standardized variable Z and the annual cumulative precipitation. Thus, all the historical data are included in the produced fuzzy band. The proposed innovative methodology provides the opportunity to achieve simultaneously a fuzzy assessment of the mean value and the standard deviation based on the solution of the fuzzy linear regression. The suitability test of the examined theoretical pdf is founded on the comparison of the spread of the fuzzy band and the distance between the achieved central values of the mean value and the standard deviation with the unbiased statistical estimation of the same variables

Survival of the thickest? Impacts of extreme wave-forcing on marsh seedlings are mediated by species morphology

Although tidal marshes are known for their coastal defense function during storm surges, the impact of extreme wave forcing on tidal marsh development is poorly understood. Seedling survival in the first season after germination, which may involve exposure to extreme wave events, is crucial for the natural establishment and human restoration of marshes. We hypothesize that species-specific plant traits plays a significant role in seedlings survival and response to wave induced stress, i.e., through stem bending and uprooting. To test this hypothesis, seedlings of pioneer species (Bolboschoenus maritimus, Schoenoplectus tabernaemontani, Spartina anglica, and Puccinellia maritima) with contrasting biophysical traits were placed in the Large Wave Flume in Hannover (Germany) and exposed to storm wave conditions. Seedlings of P. maritima and S. anglica experienced a lower loss rate and bending angle after wave exposure compared to S. tabernaemontani and especially B. maritimus. The higher loss rates of B. maritimus and S. tabernaemontani result from deeper scouring around the stem base. Scouring depth was larger around stems of greater diameter and higher resistance to bending. Here, B. maritimus and S. tabernaemontani have both thicker and stiffer stems than S. anglica and P. maritima. Our results show that especially seedlings with thicker stems suffer from erosion and scouring, and have the highest risk of being lost during extreme wave events. This implies that for successful seedling establishment and eventually the establishment of a mature tidal marsh vegetation, the species composition and their capacity to cope with storm wave disturbances is crucial