“Hurricane” Hal Needham is a storm surge scientist who specializes in data-driven storm surge analysis. He is the founder and president of Marine Weather and Climate.
Tropical Cyclone Chapala slammed into the Yemini coast on Tuesday. Chapala was the first tropical cyclone in recorded history to make landfall in Yemen.
At its peak intensity, Chapala's maximum sustained winds reached 155 mph, according to Jeff Masters and Bob Hansen at Weather Underground. That made it the second most intense tropical cyclone in the recorded history of the Arabian Sea, with only Tropical Cyclone Gonu in 2007 produced higher sustained winds.
Chapala's greatest impact was flash flooding from torrential rains, which could reach 3-4 times the average annual rainfall, according to an article posted by The Weather Channel on Monday.
People in lush landscapes sometimes are surprised to hear about the impact of heavy rains in arid regions. In subtropical climates with plentiful rainfall, dense foliage and deep organic matter on the ground serve to break the fall of rain and soak up much water. In arid climates, vegetation is sparse and soil is thin, so much of the rainfall lands on rock and immediately runs off.
Such is the case in Yemen, where photos and videos of flash flooding are depicting catastrophic impacts, particularly near the city of Mukalla.
Historically, heavy rainfall has produced the most severe tropical cyclone impacts in this region. Heavy rainfall from a tropical cyclone in 1890 led to 727 deaths — the most storm-related deaths recorded in the region — and the loss of approximately 100,000 date trees in Oman.
In regards to coastal flooding, the Yemeni coast fortunately has deep bathymetry, or water depth. Deeper bathymetry minimizes storm surge levels while elevating wave levels. This explains why coastal flooding videos coming out of Yemen today depict large waves striking the coast, but we have seen little in the way of serious coastal flood impacts.
The coastal profile of Yemen can be contrasted with flatter locations in the Northern Indian Ocean region like Bangladesh, which contains flat topography both onshore and offshore. Storm surges in Bangladesh have reached the highest level in recorded history anywhere on Earth.
Along the Arabian Peninsula, topography and bathymetry is flatter in Oman, the country northeast of Yemen, which enables storm surge to reach higher levels in that country. Tropical Cyclone Gonu generated maximum water levels (storm tide wave runup) of approximately 16.5 ft (5 m) in Oman in the year 2007, which is the highest coastal inundation level on record for the Arabian Peninsula.
A good rule of thumb, which is generally true, is that topography onshore tends to mirror topography offshore. This is not true in all cases, but in general, "flat" coastal regions, like Bangladesh and Louisiana in the United States, tend to observe shallow bathymetry offshore and experience high surges. However, mountainous areas like Hawaii or Taiwan, contain deeper bathymetry and observe lower storm surges (but higher waves).
Such generalizations can be helpful, but can also lead us to underestimate storm surge potential. The Philippines stands out as a mountainous region that has observed catastrophic storm surges, mostly because extremely intense tropical cyclones tend to push water rapidly into the numerous bays and coastal inlets.