**4. Results**

### *4.1. Formulation of Design Criteria*

It should be noted that most of the coastal protective facilities in Taiwan were completed within the last three decades. The design criteria were based on the marine climate, including the impact of waves and surges, for a return period of 50–100 years. However, land use in the protection area was not taken into consideration. It is not economically viable to use the same design criteria to protect shore land areas that have different land uses; for example, residential use should assume a larger protective return period than agricultural use. For sustainable use in a coastal zone, the design criteria should be formulated based on the requirements of coastal protection and hazard management. In this study, shore land areas with different natural and anthropogenic environmental characteristics were objectively investigated and provided with distinct protection strategies and suitable design criteria, upon which subsequent comprehensive protection strategies were planned and designed.

It is noted that ordinary coastal defenses are not designed to withstand tsunami impacts, but normal wave climates and the effects of typhoons. Given the high return period of major tsunami events in the area, it is arguably better to mitigate their potential effects using non-engineering measures.

The design criteria were divided into two categories. One category focused on regulating coastal defenses, and a set of coastal protection structural design criteria were formulated based on the marine climate. These design criteria specified that the protection capabilities of coastal defenses must fulfill the safety standards formulated based on the wave and water level conditions of a certain return period. The other category of design criteria highlighted the need to ensure the safety of coastal social and economic environments, and a set of design criteria for protection was formulated for hazard management.

#### *4.2. Design Criteria for Coastal Defense and Land Use*

Coastal erosion and ground subsidence are long-term hazards which pose no immediate threat to human lives. In contrast, waves, storm surge, and tsunamis are major destructive events that can constitute life-threatening hazards. Consequently, the relevant coastal defenses must be capable of controlling the tides and waves caused by extreme conditions, preventing them from severely affecting protected coastal areas, while reducing coastal hazards. This principle was a crucial factor in the safety validation conducted in the design of coastal defenses. The design criteria proposed here is mainly concerned with the safety of coastal defenses, while against waves and storm surges. However, tsunami forces were not considered when assessing the engineering measures, since these typically have a high design criterion that is probably able to withstand low-level tsunami events. To be on the safe side, possible inundation through tsunami was set as 1 m. Higher than 1 m, it is proposed that tsunami threats be treated with non-engineering measures. Regions with potential inundation through tsunami of less than 1 m are suggested to follow the measures against surges.

The design criteria for different types of land use were selected according to risk levels. It is suggested that building foundations should be elevated above the proposed design criteria to prevent inundation caused by floods or surges. Agriculture and aquafarming areas situated at elevations lower than specified in the design criteria can experience constant inundation, and should be reported. In such cases, we suggest modification of the land use of these areas.

Integrated coastal protection should be realized through a combination of engineering and non-engineering measures. The design criteria for these two categories were flexibly formulated based on actual combinations of protection measures. The proposed principles for formulating coastal protection structural design criteria and land use refer to various risk levels are presented in Table 3 and as follows:

• High risk level (level A): marine conditions (including waves and surges) in a 100-year return period were adopted as the design criteria.


Both coastal defenses and planning of land use can be reevaluated at a later point.


**Table 3.** Grading of risk levels and coastal protection design criteria.
