What Is the “Wind, Storm Surge, Tide, and Flood” Quadruple Threat? Why Are Typhoons Most Dangerous When They Make Landfall on the First or Fifteenth Day of the Lunar Calendar?
Key SummaryThe phrase most feared in flood-control bulletins is: “This typhoon’s landfall coincides with the astronomical high-tide period, creating a high risk of wind, storm surge, high tide, and flooding all occurring together.” What is an astronomical high tide? How can it double the force of a typhoon-driven seawater surge?
Breaking Down the Most Feared Combination in Coastal Typhoon Preparedness: the “Wind, Storm Surge, Tide, and Flood” Quadruple Threat
When a typhoon makes landfall in a coastal estuarine city—such as the Pearl River Estuary, Yong River Estuary, Oujiang Estuary, Min River Estuary, or Huangpu River Estuary—if the following four factors overlap within the same time window, they can cause devastating urban flooding and seawater intrusion:
| Factor | Physical Source | Effect on Its Own | Consequences When All Four Overlap |
|---|---|---|---|
| 1. Gale-force winds (storm surge) | Typhoon winds blowing toward the shore push vast quantities of seawater into bays and estuaries, while the low central pressure “lifts” the sea surface | Sea level rises abnormally by 1–4 meters | Gale-force winds block seawater outside the estuary, preventing inland floodwaters from draining into the sea |
| 2. Torrential rain (urban waterlogging) | The typhoon’s eyewall and spiral rainbands unleash 200–500 millimeters of extreme rainfall within hours | Municipal stormwater drainage networks reach full capacity | High tides submerge sewer outlets, causing rainwater to back up through manhole covers and flood the streets |
| 3. Astronomical high tide (high-tide level) | Within 2–3 days around the first (new moon) or fifteenth (full moon) day of the lunar calendar, the Sun, Earth, and Moon align, producing the strongest tidal forces | The baseline astronomical tide level rises by 1–2 meters above normal | Storm-surge water is added directly to the highest astronomical tide level, overtopping seawalls |
| 4. Upstream flooding (river flood peaks) | Rain from the typhoon converges in mountainous areas into flood peaks that rush downstream through rivers toward estuarine urban areas | River levels surge rapidly | High tides outside block and drive seawater inland, while flash floods pour down from the mountains, flooding the entire city |
Painful Lessons from History: “FITOW” in 2013 and “HATO” in 2017
- October 2013 — Typhoon “FITOW” (astronomical high tide on the third day of the ninth lunar month): Although it was only a typhoon at landfall, it coincided with an astronomical high tide combined with extremely heavy rain driven by cold air. In the Yong River basin, the high tide blocked the floodwaters and prevented them from draining, leaving 70% of Yuyao, Zhejiang, underwater for several days.
- August 2017 — Typhoon “HATO” (astronomical high tide on the second day of the seventh lunar month): When it struck the Pearl River Estuary head-on, it coincided exactly with the midday astronomical high tide. Record-breaking storm surges occurred in Zhuhai, Macao, and Hong Kong, with seawater reaching a depth of 2 meters on Macao’s Inner Harbour streets within half an hour.
How Can You Find Out Whether Your City Will Experience a High Tide?
During the 24 hours before a typhoon makes landfall, pay close attention to the Storm Surge Warnings (Blue/Yellow/Orange/Red) issued by the National Marine Environmental Forecasting Center. If the forecast states that “the highest tide level will occur around [a particular date and time],” residents in low-lying coastal areas and owners of underground shops must complete their evacuation at least 3 hours before the high tide arrives.