We are now into November, traditionally the last month of the official hurricane season that sees very little action based on trends in the past years. Until now, the hurricane season has been very uneventful, nary a whimper, which is a kind of anti-climax given the ferocious 2004 and 2005 seasons. This may sound callous but people do get conditioned and may become fatalistic to past calamities, if they happen in succession.
However, we have been blessed since we arrived at Tampa three years ago. For the past three years, Tampa has been spared the full brunt of a land-falling hurricane. The most we felt was some windy condition and elsewhere some local flooding due to a combination of storm-induced high bay water level and rainfall-driven high flow from the hinterland.
One such vulnerable area is the Bayshore Boulevard that skirts along Hillsborough Bay located on the upper part of Tampa Bay. The bay area provides sufficiently long over-water distance (called fetch) for the wind to whip up the water mass such that it piles up against the shoreline. In addition to inundating low-lying areas, the high water elevation also impedes inland drainage through storm drains that connect to the bay, a phenomenon termed the backwater effect since water will always flow to a lower spot by gravity. I managed to capture a manifestation of this effect during a post-storm excursion that I took along the Bayshore Boulevard.
The following image seems to indicate the upward issuance of a bubbling stream through the middle of the road. The resulting flow then gravitates toward the Bayshore Boulevard as seen in the image.
The frothing water actually is located at a manhole where the metal cover has been forced open by the water pressure underneath. The buildup in water pressure is in turn caused by water backing up from the underground drainage conduit that drains into the Hillsborough Bay. The manhole acts as a relief valve that dissipates the pent-up pressure.
I can think of a couple of reasons for this infrastructure being put under stress. One is related to hinterland development resulting in more paved/hardened surface. Hence, rain that previously could have soaked into the ground is now captured as a surface runoff that collects in the road-side drains that empty into the storm drain that in turn debouches into the bay. That increase in the volume of surface runoff within a short time overwhelms the design capacity of the storm drain as an effective conveyance.
Then there is a more regional effect that has been the subject of much debate: global warming, which induces a rise in sea level due to a combination of glacier melting and heat-induced expansion of the ocean water as a consequence of a rise in ambient temperature. When that storm drain was designed some years back, it was based on a certain downstream water level in the bay that controls its discharge capacity. When this downstream control level goes up, the discharge capacity of the same storm drain decreases, thus reducing its effectiveness in removing water that flows into it.
I will blog more about global warming and my take of the entrenched positions of the proponents on both sides of the controversy in another post. For now, I'm glad that the hurricane season of 2006 has not lived up to its full potential, but I'm too much of a realist to think that the lull will continue into next year. However, that's next year and I will savor every moment that this hurricane-free season offers.
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