Sylvie Van Emelen

Impact of sediment transport formulations on breaching modelling

ABSTRACT This paper analyses the ability of a classical shallow-water–Exner model to simulate the complex process of breaching of a non-cohesive earthen embankment, with a special focus on the sediment transport modelling. Two experimental test cases are presented: the overtopping of a sand embankment over its whole width, involving only vertical erosion, and a full three-dimensional (3D) breaching problem, with lateral erosion of the banks. Various sediment transport formulations are tested, including sheet-flow and classical bed-load, with and without accounting for a steep slope correction factor. It is shown that the steep slope correction does not play significant role in the simulated results. Variations are nevertheless observed in the results obtained with the different sediment transport formulations. These variations are amplified for the full breaching problem, where a local bank-failure operator is used. This highlights the importance of a good sediment transport modelling to accurately simulate a full breaching process.

Simulations of the New Orleans 17th Street Canal breach flood

The City of New Orleans is surrounded by an intricate network of floodwalls and dikes built to protect it from flooding from the Mississippi River and Lake Pontchartrain. In 2005, following Hurricane Katrina, New Orleans experienced one of the most devastating urban floods in recent history caused by several dike breaches. This work presents an idealized case study of a part of this event on a scale model. It concerns the flow through one of these breaches and flooding in the surrounding urban area. The scale model reproduces part of the remaining dike, the complex morphology caused by the breach and a number of houses around the area. The flow field computed by a two-dimensional depth-averaged finite-volume numerical model is compared with measurements on the scale model. As the transient part of the flood is quite short, the numerical and experimental results were compared under steady-state flow, showing satisfactory agreement.

COMPARISION BETWEEN MODELS USED TO SIMULATE DAM BREAK FLOWS OVER A MOBILE BED MADE OF DIFFERENT MATERIALS

The hydrodynamic and morphodynamic processes are very important aspects in the study of dam break flow over the mobile bed. It is possible to simulate and thus to forecast the water and sediment movements thanks to appropriate models. The hydrodynamics is well represented by the Saint–Venant shallow water equations that constitute a simple hyperbolic system and for which different numerical models exist. The inclusion of the morphodynamics introduces additional difficulties, so different solutions have been developed in order to better represent the movement of sediments and the associated changes in the morphology. Several models with different approaches have been developed and are available in the literature: (i) the clear water layer model, representing one layer of clear water flowing over a mobile sediment bed; (ii) the mixture layer model, considering one layer of a variable density mixture of sediment and water; (iii) the two-layer model, which distinguishes the upper clear water layer from the lower moving sediment layer, where the bed-load transport takes place and (iv) the two-phase model, which considers the water and the sediments as two immiscible phases characterized by different velocities. This paper aims at assessing the performances of these models using two different test cases of dam-break flow over the mobile bed, for which experimental data exist. The results from the different models are compared to data measured in dam-break flow experiments over mobile bed composed by (i) coarse sand for the first case and (ii) light PVC pellets for the second case.

Closure to“Impact of sediment transport formulations on breaching modelling” by S. VAN EMELEN, Y. ZECH and S. SOARES-FRAZÃO,J. Hydraulic Res. 53(1), 2015, 60−72

Cestero, J. A. F., Imran, J., & Chaudhry, M. H. (2015). Experimental investigation of the effects of soil properties on levee by overtopping. Journal of Hydraulic Engineering, 141(4), 04014085, 1–14. Coleman, S. E., Andrews, D. P., & Webby, M. G. (2002). Overtopping breaching of noncohesive homogeneous embankments. Journal of Hydraulic Engineering, 128(9), 829–838. Frank, P.-J., & Hager, W. H. (2015, June). Spatial dike breach: Sediment surface topography using photogrammetry. Paper presented at 36th IAHR World Congress, The Hague, The Netherlands. Savitzky, A., & Golay, M. J. E. (1964). Smoothing and differentiation of data by simplified least squares procedures. Analytical Chemistry, 36(8), 1627–1639. Schmocker, L., Halldórsdóttir, B. R., & Hager, W.H. (2011). Effect of weir face angles on circular-crested weir flow. Journal of Hydraulic Engineering, 137(6), 637–643. Schmocker, L., & Hager, W. H. (2012a). Plane dike-breach due to overtopping: Effects of sediment, dike height and discharge. Journal of Hydraulic Research, 50(6), 576–586. Schmocker, L., & Hager, W. H. (2012b, September). Effect of sediment diameter on plane dike breach shape and breach discharge. Paper presented at River Flow 2012, San Jose, Costa Rica. Spinewine, B., Delobbe, A., Elslander, L., & Zech, Y. (2004, June). Experimental investigation of the breach growth process in sand dikes. Paper presented at River Flow 2004, Naples, Italy. Wallner, S. (2014). Influence of reservoir shape and size on the flood wave caused by progressive overtopping dam failure. Doctoral thesis, TU Wien, Vienna, Austria.