*3.2. Inundation Area Comparison of Each Case*

The total inundation area (areas with maximum inundation depth above 0.3 m) of the current situation and of the five proposed cases is shown in Table 3. The total inundation areas of case A when under the 25-year and 100-year return periods were 0.12 and 5.2 hectares, respectively, which is less than the total inundation area of the current situation and demonstrates that case A has good flood mitigation. Cases B and C are better in terms of total inundation area when compared with the current situation, but their flood mitigation effects were not as profound as case A. Case D increased the capacity of the main channel through dredging, but as this action has little effect when under the steady flow condition, the total inundation area remained unchanged from the current situation. Case E increased the maximum river capacity through land expropriation, dredging, and land grading. When under the 25-year and 100-year return periods, the total inundation areas were 4.3 and 13.96 hectares, respectively. The flood mitigation effect of case E was second only to case A when under the 25-year return period, and roughly equivalent to case C when under the 100-year return period.



Unit: Hectare.

#### *3.3. Case Selection*

As can be seen from the results of the simulations, under all four return periods case A surpassed all other cases in the reduction of inundation area size, i.e., case A had the best flood mitigation in all simulated situations. The rest of the cases are ranked in descending order as case E, case C, case B, and case D in terms of flood mitigation. Case D had little effect under the steady flow condition.

All NbS cases consider benefitting the protection of resident life and property in the field side of the levee as the main objective; the expected co-benefits are as shown in Table 4. According to the simulation results and statistics on inundation area sizes of each case, all cases except case D are able to reduce agricultural losses caused by inundation.

Other factors to consider are cost, ecosystem service efficiency, and recreational value. Case A and case E will be more costly than the other cases, as case A employs high-strength levees and case E employs land expropriation. Case C can increase ecosystem service efficiency via the setting up of a wildlife corridor, and case E via the planting of local tree species in the floodplain area to enrich the landscape and the ecology. In case E, floodplains can also be planned as parks, detention wetlands, stadiums, or promenade facilities to serve recreational purposes.

Taking all benefits and co-benefits into account, this study suggests case E for the case area Nangang River; not only does case E provide good flood mitigation for low return period floods, but also increases ecosystem service efficiency and provides recreational areas for locals.


**Table 4.** Expected co-benefits of NbS case.

#### **4. Conclusions**

This study focused on the Nangang River area and developed and proposed five NbS cases. Cases A and B can be condensed as the act of extending the existing levees, case C as the act of adding discontinuous levees, case D as the act of dredging the main channel, and case E as the act of implementing "Room for the River". The PHD model was used to simulate the steady flow of each return period and compare the results of the simulations with the current situation.

Overall, case A has the best flood mitigation under all return period conditions, and the flood mitigation effects of the other cases are ranked in descending order as cases E, C, B, and D.

The core value and principle of NbS is to solve social challenges through nature-based means and increase biodiversity and ecological services at the same time. Cases C and E are good in this regard, as they can bring benefits to ecological services, while the co-benefits of cases A and B are relatively small. Taking both the expected benefits and co-benefits into account, this study believes that case E is most suited for improving flood disaster mitigation in the Nangang River area. It is still necessary to discuss with local stakeholders and obtain their consensus and support before the actual implementation of the case, so as to achieve the goal of "Living with nature, booming with water".

There may be negative impacts to the environment after the implementation of NbS; constant monitoring and assessment of the environment will be required to establish the level of impact. It should be noted that uncertain negative impacts to the environment may be caused by the implementation of NbS; constant monitoring and assessment of the environment will be required to establish the level of impact, if any. Another point to take note of is that the PHD model uses fewer cells to describe physiographic conditions in order to rapidly assess the flood mitigation benefits of each case. For areas with drastic changes to the elevation, water depth cannot be accurately described by a PHD model with fewer cells. If one wishes to increase the accuracy of simulations, this study recommends increasing cells in areas that fulfill the aforementioned criteria.

**Author Contributions:** Conceptualization, W.L. and D.-J.D.; methodology, M.-H.W.; software, M.-H.W.; validation, C.-T.H. and M.-H.W.; formal analysis, C.-T.H. and M.-H.W.; investigation, M.-H.W. and D.-J.D.; resources, L.-H.T.; data curation, Y.-J.C.; writing—original draft preparation, C.-T.H.; writing—review and editing, M.-H.W.; visualization, M.-H.W.; supervision, W.L. and M.-H.W.; project administration, C.-H.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Water Resources Planning Institute, Water Resources Agency, Ministry of Economic Affairs, ROC(MOEAWRA1090267).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are available from the corresponding author.

**Acknowledgments:** This study was supported by grants from the Water Resources Planning Institute, Water Resources Agency Ministry of Economic Affairs, ROC(MOEAWRA1090267). The authors gratefully acknowledge the support.

**Conflicts of Interest:** The authors declare no conflict of interest.
