Sensitivity analysis of urban flood flows to hydraulic controls

Water Qualit

Technical note: Laboratory modelling of urban flooding: strengths and challenges of distorted scale models

Laboratory experiments are a viable approach for improving process understanding and generating data for the validation of computational models. However, laboratory-scale models of urban flooding in street networks are often distorted, i.e. different scale factors are used in the horizontal and vertical directions. This may result in artefacts when transposing the laboratory observations to the prototype scale (e.g. alteration of secondary currents or of the relative importance of frictional resistance). The magnitude of such artefacts was not studied in the past for the specific case of urban flooding. Here, we present a preliminary assessment of these artefacts based on the reanalysis of two recent experimental datasets related to flooding of a group of buildings and of an entire urban district, respectively. The results reveal that, in the tested configurations, the influence of model distortion on the upscaled values of water depths and discharges are both of the order of 10&thinsp;%. This research contributes to the advancement of our knowledge of small-scale physical processes involved in urban flooding, which are either explicitly modelled or parametrized in urban hydrology models.</p

Lessons learned from combined experimental and numerical modelling of urban floods

International audienc

4D GIS for Monitoring River Bank Erosion at Meander Bend Scale: Case of Moselle River

International audienceThe "Wild Moselle" regional nature reserve extends over 13 km at the western foothills of the Vosges Mountains (France). The hydrological regime of the river is characterized by high flow in winter and spring and low flow in summer. Its average slope is 0.12 % and its average bankfull width is 60 m. The coarse sediment load comes mainly from bank erosion. Although this sector is relatively less affected by past or present human activities, the propagation of morphodynamic adjustments initiated by actions carried out both upstream and downstream of this sector impacts the current functioning of the river. These erosion waves converge today towards the central part of the reserve, which led to the collapse of the central pier of the Bainville-aux-Miroirs bridge during a 2-year flood in 2011, and could induce potential risks of defluviation which may destabilize infrastructures. In this context, the study carried out aims to characterize and anticipate the morphodynamic evolutions of the Moselle to be able to propose scenarios of management and restoration of the lateral mobility of the river. For this purpose, a 2D hydro-sedimentary model is being built over the entire reserve, combined with a detailed morpho-sedimentary monitoring. In order to improve the understanding of the lateral migration of the Moselle River, a photogrammetric monitoring was carried out along the concave bank of the most active meander of the studied sector. To follow this morphological evolution more closely, it was decided to establish a 4D GIS. The objective of this monitoring is to compare the rate of bank retreat with hydrodynamic parameters in order to estimate the geotechnical properties of the bank. Comparison of the observed and modelled bank retreat must thus allow these different parameters to be calibrated in the hydro-sedimentary model. As part of this work, this paper aims to highlight the use of 4D GIS to monitor bank retreat at the scale of a meander bend and is divided into three different parts: (i) a state of art to situate the study into the current knowledge and technologies, (ii) a presentation of the study area and the measurements carried out and (iii) a description of the different 3D or 4D data produced and the consequent spatial analyses

Experimental insight for flood flow repartition in urban areas

Modeling floods in urban areas remains a challenge. To understand flow patterns in urban geometries better and constrain models, an experimental rig representing a 1/200 scale urban geometry with various street widths and angles is presented. Measurements of hydraulic variables for flow conditions ranging from moderate to extreme flooding were performed. Over this range, accurate inflow and outflow boundary condition measurements allow the geometry effect on inlet–outlet discharge conservation to be studied for each street. Froude numbers are found to be independent of the total flowrate. Interestingly, the flow distribution among all streets remains comparable over the range of boundary conditions. Moreover, three behaviors have been identified depending on street response as a function of the evolution of the upstream discharge distribution. Future measurements with high spatiotemporal sampling would allow possible coupling of flow features and energy dissipation to be studied at various scales and other flow configurations and district geometries to be characterized

Effective channel and ungauged braided river discharge estimation by assimilation of multi-satellite water heights of different spatial sparsity

Multi-satellite sensing of continental water surfaces (WS) represents an unprecedented and increasing potential for studying ungauged hydrological and hydraulic processes from their signatures, especially on complex ow zones such as multichannel rivers. However the estimation of discharge from WS observations only is a very challenging inverse problem due to unknown bathymetry and friction in ungauged rivers, measurements nature, quality and spatio-temporal resolutions regarding the flow (model) scales. This paper proposes an effective hydraulic modeling approach of sufficient complexity to describe braided river flows from sparse multisatellite observations using the HiVDI inverse method presented in Larnier et al. [42] with an augmented control vector including a spatially distributed friction law depending on ow depth. It is shown on 71km of the Xingu River (braided, Amazon basin) with altimetric water height time series that a fairly accurate upstream discharge hydrograph and effective patterns of channel bathymetry and friction can be infered simultaneously. The coherence between the sparse observation grid and the ne hydraulic model grid is ensured in the optimization process by imposing a piecewise linear bathymetry prole b(x), which is consistent with the hydraulic visibility of WS signatures (Garambois et al. [27], Montazem et al. [46]). The discharge hydrograph and effective bathymetry-friction patterns are retrieved from 8 years of satellite altimetry (ENVISAT) at 6 virtual stations (VS) along flow. Next, the potential of the forthcoming SWOT data, dense in space, is highlighted by infering a discharge hydrograph and dense patterns of effective river bathymetry and friction; a physically consistent definition of friction by reaches enabling to consider more dense bathymetry controls. Finally a numerical analysis of the friction term shows clear signatures of river bottom slope break in low flows and width variations in high flows which is consistent with the findings of Montazem et al. [46] from WS curvature analysis

Uncertainties in laboratory modelling of urban flooding

International audienc

Uncertainties in laboratory modelling of urban flooding

International audienc