Modelling sea dike toe erosion during storms

Toe erosion, especially in stormy conditions, is one of the common mechanism causing the failure and instability of the sea dikes and revetments. The erosion intensity becomes more serious at the beaches which is under the impacts of typhoons. Reliable forecasts about the intensity of toe erosion of sea dikes in stormy conditions have important ecnomomic and technical meaning in the design and construction of sea dikes.
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Modelling sea dike toe erosion during stormsModelling sea dike toe erosion during storms Thieu Quang Tuan1, Nguyen Quang Luong1, Le Ngoc Anh1Abstract: Toe erosion, especially in stormy conditions, is one of the common mechanism causing the failureand instability of the sea dikes and revetments. The erosion intensity becomes more serious at the beacheswhich is under the impacts of typhoons. Reliable forecasts about the intensity of toe erosion of sea dikes instormy conditions have important ecnomomic and technical meaning in the design and construction of seadikes. This study considers and evaluate the extent of the scour in front of the dike toe during the typhoonusing the numerical model WADIBE-TC. The protective structures for dike toes consist of buried toes,cylinders and coarse rock apron.Keywords: sea dike, toe erosion, storm, numerical model, WADIBE1. IntroductionIn the North and Central provinces of Vietnam, toe erosion or foreshore loss is a dangerousand common mechanism causing the failure of the sea dikes, especially when the dikes areconstructed in the area having strong erosion development. During storms, the cross-shoresediment transport due to the impacts of waves and storm surges are main causes of theformation of scours in front of the toes and foreshore sink. There are differences withrespect to the phenomena, process as well as the training solution between erosionoccurring in stormy conditions (due to cross-shore sediment transport processes) anderosion cause by the deficiency of supplementary sediment for the longshore sedimenttransport. The latter process causes the chronic erosion and it is very expensive to controlwhile the first process is the cause of acute erosion occuring only during stormyconditions. Up to now, in the design of sea dikes, toe erosion calculations all have beenbased on the empirical formulas set up for vertical walls. Field observations have shownthat these formulas have not taken all the influence of the parameter into consideration, andthey often produce overestimated results when applied to sea dikes.For the reasons above, this study considers and evaluate the extent of the scour in front ofthe dike toe during the typhoon using the numerical model WADIBE-TC developed byFaculty of Marine and Coastal Engineering, Water Resources University, which simulatesthe time-dependent development of the scour in front of the toes of the structures based onthe cross-shore sediment transport modelling. The model is calibrated using the measureddata from the wave flume experiment belonging to the Sea Dike Research Project No.3carried out by Marine and Coastal Engineering Faculty. It is also applied to compute andverify a case study of erosion of Thinh Long dike in Hai Hau, Nam Dinh in Damreytyphoon in 2005.2. Simulations of some typical toe erosion structures with WADIBE-TC2.1. Main scenariosThe model simulates 3 types of common toe protection structures: buried toes, cylindersand combination of cylinders and coarse rock apron. Detailed simulation cases are shownin Table 1.1 Faculty of Marine and Coastal Engineering, Water Resources University; E-mail: tuan.t.q@wru.edu.vn 235 Table 1. Different scenarios executed in the model SWL Wave Dimension Duration Type of protection (m) Hs (m) Tp (s) S (m) L(m) Crown (hour) wall1. Buried toe 0.65 0.3 3 0 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 3 0.25 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 3 0.15 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 3 0 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.5 0 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.5 0.1 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.2 0 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.2 0.1 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.2 0.25 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.5 0 0.15 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.5 0 0.25 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.2 0.1 0.25 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.5 0.1 0.25 2.0- 3.0-4.0-6.0-8.02. Cylinder 0.65 0.25 2.5 0 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.5 0.1 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.2 0 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.2 0.1 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.5 0.25 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.2 0.25 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2.5 0.3 2.0- 3.0-4.0-6.0-8.0 0.65 0.25 2. ...