通航保证率是进行航道设计、整治和评估航道效益的一项重要指标，一般采用航道水位高于设计最低通航水位的天数来确定。由于钱塘江河口河床冲淤变化剧烈，其低潮位的年际年内变化与河床冲淤变化关系密切，采用上述方法不能确切反映该河口的通航保证率。为此，利用1980年以来历年的实测水下地形分析了钱塘江河口七堡段的年际年内河床冲淤规律，讨论了低潮位变化与河床冲淤的关系，提出了采用逐日低潮位下的实际水深计算通航保证率的方法。并针对该河口河床冲淤主要取决于径流大小的特点，选取典型丰、平、枯水文年分析了七堡河段的通航保证率。结果表明，对应于丰、平、枯水文年，钱塘江河口七堡段的通航保证率分别达到100%，100%，87%，与实际较为吻合。该方法对我国其他冲淤变化显著、低潮位变幅较大的潮汐河口的通航水深保证率计算具有借鉴意义。

The cumulative frequency of low tidal level of navigation is an important index for navigation channel design, improvement and benefit evaluation. Normally, the cumulative frequency could be calculated by the number of days that the water level is higher than the designed lowest navigable water level. In the Qiantang river estuary of Zhejiang Province, the bed erosion and deposition is drastic, and the low tidal level is closely related to the bed elevation changes. Hence the aforementioned method could not count definitely the cumulative frequency in Qiantang estuary. In this study, the historical bathymetries since 1980 were used to analyze the bed evolution patterns in the annual years at Qibao section in Qiantang estuary, and the relationships between the variations of low tidal level and the bed erosion/deposition were discussed. On this basis, a new method based on the actual water depth under daily low tidal level was proposed for the calculation of cumulative frequency. According to the hydrological characteristics of Qiantang estuary, three typical hydrological years representing high flow year, normal flow year and low flow year were chosen to analyze the cumulative frequency of water depth for navigation in Qibao section. The results show that in the three years, the cumulative frequencies at Qibao section could reach 100%, 100% and 87%, respectively, which agrees with the reality. This method could be also applicable to other tidal estuaries where changes of bed elevation and low tidal level are remarkable.