Technical Studies

The purpose of this project is to develop an inundation risk assessment framework for the southwest coastal area, Taiwan. This framework enables the determination of inundation risk with the consideration of hydrologic and topographic uncertainties. Furthermore, based on the results of inundation risk assessment, this project will propose an optimal groundwater pumping strategy which simultaneously considering the inundation risk and water demand.

In the developed inundation risk assessment framework, the inundation risk is defined as the product of inundation hazard and vulnerability. The inundation hazard is the probability density function of inundation depth while the vulnerability is the relationship between damage and inundation depth. The developed framework was applied in the study area (Erlin and Yuliao drainage system in Changhua County and the drainage system in the northern Yunlin County) to assess the inundation risk under present condition. Besides, the spatial distribution of accumulative land subsidence between 2012 and 2021 was predicted by a one-dimensional land subsidence model. The DEM and channel geometry in 2021 were produced based on the predicted land subsidence field along with the present topography data. Thus, the inundation risk in the study area after ten years land subsidence was also predicted.

The effects of land subsidence on the freeboard and inundation depth were analyzed based on the results of inundation risk assessment before and after ten years land subsidence. The analysis showed that except the extremely low-lying area where the elevation is lower than the spring high tide level, the changes of freeboard and inundation depth after ten years land subsidence are mostly influenced by the spatial variation of land subsidence field. According to the above results, a groundwater management model was developed which enables the determination of optimum pumping patterns while minimizing the maximal land subsidence among all the control points outside the extremely low-lying area.

Using the developed groundwater management model, the optimum pumping patterns during 2012 to 2021 among all the pumping wells were obtained. The averaged accumulated land subsidence during the next ten years in Changhua and Yunlin Counties could be reduced 13 and 16 cm, respectively, through the groundwater management.

Finally, the flood hazard and risk under the conditions of with and without the groundwater management were compared. The comparison results show that the groundwater management prevent the flood hazard and risk to to be increased in the future because the spatial variation of land subsidence was significantly reduced.