Morphological modeling of seashore with the existance of fishtail groins | |
| Author | Somchai Chonwattana |
| Call Number | AIT Diss. no.WM-05-01 |
| Subject(s) | Coastal engineering Groins (Shore protection)--Mathematical models Sediment transport--Mathematical models |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering, School of Engineering and Technology |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. WM-05-01 |
| Abstract | The present model consists of 4 modules to achieve the aim of study which are wave, waveinduced current, sediment transport and morphological change module. The wave module is based on parabolic approximation equation including energy dissipation due to bottom friction and wave breaking and reflection effect by separating wave to incident and reflected wave employing varyine grid scheme. The wave module has been verified and found satisfactory against the analytical results and laboratory data with beaches in the following conditions; constant slope, uniform depth with seawall, uniform slope with detached breakwater, uniform slope with groin. Wave height in the range of 0.02-1.26 m, bottom slope in the range of 0 - 1:20 and wave angle in the range of 0 - 60° are the limiting scope of wave module verification. The wave module has been found applicable for present study as the verification against the output generated by analytical and physical model results. For waveinduced current module, radiation stress is calculated and used as the driving force including bottom friction and mixing term in the momentum equation. This module is solved by an explicit scheme based on varying grid size. The module has been verified with following condition: cross-shore distribution of longshore current on sloping beach, current field in the presence of offshore detached breakwater and current field in the presence of seawall. It is found that present module can describe wave-induced current in the sloping beach including coastal structure such as detached breakwater and seawall. However in the case of steep bottom slope, 1:10, the wave reflection is higher than laboratory data which is also found to cause the calculated wave-induced current to be too high. For high accuracy, this model is recommended to be applied for mild bottom slope for which good results have been obtained when used to less than 1:20 bottom slope. For the sediment transport module, it takes into account four types of sediment transport: bed load, suspended load, transport due to undertow, and wave runup. Sediment transport due to waves and current combines is used for calculating both bed load and suspended load transport before wave breaking. The method based on balancing the settling of sediment particles with bottom sediment stir-up is used for calculating suspended load transport inside the surf zone. Undertow transport is calculated based on the undertow profile generated from eddy viscosity and used for calculating sediment transport inside the surf zone. The sediment transport rate in the runup region is determined using the method of decay from reference point in the vicinity of shoreline to the limit of runup heights. Inclusion of additional sediment transport due to such effects above enable the present model to calculate longshore and cross-shore sediment transport in- and out-side surf zone which extend to run up zone. Sediment transport module is verified through following conditions: total longshore transport, cross-shore distribution of longshore and cross-shore transport and transport due to undertow effect. Good comparing against the output generated by analytical and physical model results render that present sediment transport module can calculate sediment transport due to such effects above. Finally, a morphological change module has been developed based on varying grid size including bottom slope effect. Gradient of sediment transport is incorporated to calculate 3D morphological change. Present model has been verified with laboratory data from SUPERTANK project. Model results show good comparison with laboratory data. Sensitivity analysis of present model was done for 12 calibration parameters. Among 12 selected parameters, a parameter namely energy dissipation factor of wave breaking is found to be most significantly sensitive to the output. Up to this stage of work, present 3D morphological change model is developed and ready for using as coastal engineering tool for planning and design processes. In the model application, a procedure for calculating representative wave has been developed based on the principle of conservation of wave energy and sediment transport. Two options of sediment transport viz. longshore sediment transport and cross-shore sediment transport have been explored. The 3D morphological model has been used to calculate morphological change between coastal structures based on the proposed representative wave method. The model is applied to the Sangjun beach, Rayong, Thailand, with a study area between two fishtail groins of 212m by 302m. Observed bathymetry before and after the construction of the fishtail groins are used to compare the model results. Classical method and present method were used to calculate morphological change in the study area using observed wave climate and bathymetry data. Computational time of 20 days was required to model the morphological change when using the classical method. By using present method, the computational time was reduced by a factor of 200, while efficiency index showing relative result compare with measured morphology was affected only by 5 percent compared with the classical method. In the application of the model, the number of representative wave to be used is one factor to be selected. Calculation of final morphologies using 1, 2, 3, 4, 6, 8 and 16 representative waves were done and compared with surveyed morphologies. While the accuracy was found to increase with the increased number of the representative wave, time consumed for model run was directly related to the number of representative waves. In the case study, satisfactory calculation results were obtained when two representative waves were used, giving optimum trade-off between the accuracy and computational time. Thus, the proposed representative wave method is effective to reduce the required computation time of the morphological model which can be a great asset in modeling large scale area without having to devote extensive computational resources. However, more applications of the proposed method in diverse study areas will help to strengthen the utility of this method. This study establishes the 3D morphological model including wave, wave-induced current, sediment transport and morphological change modules which are capable to be applied to prototype. Each module is calibrated and verified step by step before proceeding to next module. Finally, representative wave method has been developed and successfully used with the present 3D morphological model to calculate morphological change between fishtail groins at Sangjun beach located at eastern part of Thailand. Present model results show good agreement with the output generated by analytical, physical model results and observed data. Time consumed for computation is acceptable. Both the above reasons prove that present model can be used for the real situation as efficient tools in coastal engineering applications |
| Year | 2006 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertations ; no. WM-05-01 |
| Type | Dissertation |
| School | School of Engineering and Technology |
| Department | Department of Civil and Infrastucture Engineering (DCIE) |
| Academic Program/FoS | Water Engineering and Management (WM) |
| Chairperson(s) | Gupta, Ashim Das;Sutat Weesakul; |
| Examination Committee(s) | Noppadol Phien-wej;Suphat Vongvisessomjai;Shibayama, Tomoya; |
| Scholarship Donor(s) | Japanese Government; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2005 |