Conflict of interest

subsidence over a 10-year management period of with and without GWM are, respectively, 9.833 and 4.712 M\$NT. The benefit of GWM in Year 2021 associated with the expected FDR in the study area is E Bð Þ¼ j2021 9:833 � 4:712 ¼ 5:121 M\$NT. Assume that the interest rate is 4.5% and the annual expected benefit by GWM follows a linear increasing pattern from 0 (in Year 2011) to 5.121 M\$NT (by Year 2021), the value of the uniform gradient present worth factor in Eq. (12) is UGPW nð Þ ¼ 10; i ¼ 4:5% = 32.74. The corresponding present worth of the total benefit by GWM accrued in the study area over the 10-year GWM, by Eq. (11), is 16.768 M\$NT or equivalent to an annual benefit of 2.119 M\$NT amortized in 10 years. By comparing the total amount of inundation damage amount in the study area (in the order of 200 M\$NT annually), the GWM benefit associated with FDR does not appear to be very impressive. This might be due to a relatively short management period of 10 years. For sustainable GWM, the period of management would generally be longer and it can be easily shown, by a similar analysis described above, that the economic benefit of GWM in terms of flood damage reduction would grow

Recent Advances in Flood Risk Management

with the management period. Furthermore, Figure 8 clearly shows that

mandated freeboard can be upheld or even improved by GWM.

4. Conclusions

Acknowledgements

34

implementing GWM in the land subsidence prone area can sustain the design flood protection level of drainage systems by preventing the freeboard from decreasing. This implies that potential huge saving in the capital cost can be realized because the lower levee height in many parts of the study area would be sufficient if an effective GWM policy is in the place. Also, the maintenance cost for levee systems could be reduced as fewer existing levee segments require height upgrading because the

Groundwater is an important source of water supply, especially in regions where surface water supply is insufficient or not stable. However, the lack of proper management for groundwater extraction and usage in land subsidence prone areas could create a number of undesirable consequences such as damaging building structures, aggravating flood inundation hazards, and diminishing effectiveness of flood control facilities. This chapter presents a methodological framework demonstrating how a subsidence-focused GWM model can be formulated and applied to obtain an optimal pumping strategy that reduces the negative impact of land subsidence in a coastal region in western Taiwan which is experiencing serious land subsidence and associated flood hazards. Numerical results clearly show that, through the use of an optimal GWM model with an explicit consideration given to subsidence control, one is able to ease off uneven land surfaces and reduce seriousness of land subsidence and flood damage as well as sustain the flood protection level of drainage systems by maintaining a suitable freeboard. All these features provide strong evidence that GWM can play an important role, along with other engineering measures, in providing a sustainable solution to flood inundation problem in land subsidence prone areas.

This study was support by the Water Resources Planning Institute, Water

Resources Agency, Ministry of Economic Affairs of Taiwan.

No potential conflict of interest is present in this chapter.
