Severe hydraulic conditions which are the key technique problems in reclamation engineering often occur in the process of cofferdam blocking-off. In order to ensure successful implementation of blocking-off

it is necessary to know the hydraulic characteristics. The current methods of design tidal type have many problems which are single and lack of uniform standard. So

a method based on risk analysis was proposed. As an example in Chiwan sea area

the design tidal type with different recurrence periods for the blocking-off period were performed .The main achievements are summarized as follows: ① The results show that there is risk problem for the current method of amplifying the measured tidal level process curve by the same multiple to the design tidal level in Chiwan sea area;② Differences in the design tidal type with different recurrence periods for the blocking-off period are small in Chiwan sea area

so

the uniform design standards(5% frequency) can be conformed;③ The new method is reasonable and will provide reference for the computation of design tidal type in the process of cofferdam blocking-off.