Tidal flooding

Tides have always been of interest to sailors, clam diggers and others but the Santa Clara Valley Water District also has need for special knowledge of tides in the south San Francisco Bay. Since all creeks and flood control channels eventually discharge into the south bay, it is important to know the magnitude of possible extreme tides for proper design of levee heights. Other unique flood control facilities such as the Palo Alto flood basin requires special knowledge of the tides because it temporarily stores stormwater until the tides recede and the water is released into the Bay. Construction activities, wetlands mitigation design and operation, and water quality testing also require knowledge of tidal action.

TIDE CYCLE
A typical tidal cycle is two high tides (higher high water and lower high water) and two low tides (higher low water and lower low water in a 24-hour interval. The average of a series of lower low water data is called mean lower low water (MLLW) and that of higher high water data is called mean higher high water (MHHW). Both MLLW and MHHW vary with geographic locations and are specific to individual points.

TIDE TABLES
Tide tables used for the south bay estimate the height of the tide relative to MLLW at the National Ocean Survey's station at Golden Gate, which is 2.86 feet below National Geodetic Vertical Datum (NGVD, commonly referred to as mean sea level datum). MLLW at the District's former Gold Street tide Gauge in Alviso was 4.7 feet below NGVD. The tide tables include an adjustment factor to translate the tide information for the Golden Gate to the Alviso location. For high tides, the adjustment factor for Alviso is +3.3 feet, which should be added to the tide table value. The resulting value is the tide height above MLLW at Alviso, but not the height above NGVD or MLLW at the Golden Gate. For example, a prediction of a 3.0 feet tide at the Golden Gate would mean that the tide at Alviso is 6.3 feet (3.0 + 3.3 feet adjustment factor) above MLLW at Alviso. MLLW at Alviso is 4.5 feet below NGVD (-4.5 + 6.3 feet). The following relationship is the result of a tide study that translates the high tide elevations at Golden Gate to Alviso. The tide tables also indicate that high tides at Alviso occur 1.2 hours (72 minutes) after that of the Golden Gate.

More than 90% of the actual observed tides are higher than the predicted tides with 3% more than a foot higher. The National Ocean Survey prepares the tide tales based on astronomic conditions, which account for only 70% of the influence on tidal action. Other factors are meteorological conditions such as barometric pressure or wind speed and direction. A drop of 0.1 inch in barometric pressure will increase the tide height by 0.1 feet. The highest tide recorded at Golden Gate since 1854 was 9.1 feet above MLLW on Jan 27, 1983 when the tide tables predicted only 7.1 feet above MLLW. An example of the possible variation in high tide was Feb 16, 1980, when the tide tables predicted a tide elevation of 6.6 feet at Golden Gate but the observed tide was 8.0 feet above MLLW (5.14 feet, NGVD). The observed tide at Gold Street was 6.86 feet, NGVD.