The Science:
Tide levels are affected by multiple factors. All tide charts are calculated using a “static”
barometric pressure of 29.91” or 1013mb.
This is the average barometric pressure.
As the barometric pressure rises, this additional pressure actually
pushes the water down. The opposite is
true as well. As the pressure drops, the
water levels rise. In a simple, static
situation this is fairly simple to calculate.
I have this tide impact value in real time on my site. For every millibar change in pressure, the
sea level will change by 1 centimeter.
In the case below, the barometric pressure is 1019mb. That is 6mb above the average 1013. Because the pressure is higher than average
it is pushing the water down. This
results in the level being 6 centimeters or approximately 2 inches below the
forecast level.
This highlighted value above is hyperlinked to the chart below that shows
tide impact at multiple barometric pressures.
So this covers the "easy" calculations for tide level. Barometric pressure is just one of the
elements that can affect predicted tides.
The other factors are:
Storm Surge – This is caused by the wind pushing
the water onto the shoreline. It can be a
VERY significant effect during storms. This is what we
hear about all the time with hurricanes.
The same physics apply to our wind storms, just at lesser level.
Heavy Rainfall and run off can increase sea
level close to shore. Rainfall alone can
cause shoreline flooding.
Shoreline Contours and restricted bays can also affect
sea levels.
Enough science…. Now
for a story….
A Little History and a Story
In the early 80’s we had a storm I will never forget. I was Fire Chief at the time and I saw an
incredible amount of destruction. This
was pre-internet and forecasts were still done on TV with a chalkboard. I didn’t even have a weather station. I woke up in the middle of the night and the
wind was howling. It also sounded a
little different too. The power was out as
well. At 5:00am I got a frantic phone
call from one of our volunteer fire fighters.
He said that his house, garage and car were under water. Everybody in his area was experiencing flooding. I jumped in my car and took off right away toward his
place. When I got close to the Nature
Conservancy on Twin Spits road, it was flooded…. With
salt water..... Rawson Swamp was overflowing onto the road. I looked at it carefully and took a chance to traverse this area that had floor board level water. I was informed that shortly after I went
through it was impassable. When I got to
the firefighters home it was exactly as he described. Houses, garages and cars flooded…. I really couldn’t believe what I was
seeing. There was nothing we could do
but wait for the wind and tide to subside.
I ended up spending that time with an elderly woman that lived
alone. The waves were crashing directly
into her home. The whole house shook violently with every wave that struck it. When the tides subsided
we saw that her 12” concrete bulkhead had been totally destroyed.
This is just one of many stories of destruction from that
day. Many homes were flooded and roads
were impassable. Cars were
destroyed. There were stories of folks
finding their chest freezer at the other end of their garage after the
flood. I could go on and on about this
horrific experience. Yes…. Peaceful Hansville can have catastrophic
storms.
So… What happened
with this storm? We were watching our
analog barometer drop like a rock the night before, so I knew something was
coming. As I said, it sounded different…. That is because this incredible wind was out
of the East or ESE. This is a rare direction
for us. I would estimate the wind gusts
at 50+ mph. This all happened at the exact
same time of a king tide. Low barometer,
high winds and high tides are a formula for disaster. Are we prepared?
Worst Case Scenario:
I would like to share some video and then create a case
study for a worst case scenario. First,
let’s look at the video. This was taken
on January 18, 2017. We had a very high
tide and low barometer and I drove around Hansville to see what areas were
impacted.
Now I have some data to lay over
this video. It was taken after high tide and the water had already receded about 12". The forecast high tide was 11.45
feet at 8:35am that day. At high tide, the wind gusts were 16 mph. The barometer was 29.26” or 991mb. This would impact the tide by +9”. The net result would be a high tide of 12.2
feet. Since the video is after the high,
it is probably around 11.2 or so. Of the
3 elements that can really cause flooding, it was a “so-so” event. The forecast tide was high, but not real
high. The barometer was low, but not
real low. The wind was essentially not a
factor. I’ll share one more observation
from that day. The Driftwood Key launch
ramp was completely under water. In
fact, half of the parking lot was submerged too.
So, let’s put together the perfect
trifecta for disaster. Think of the
video you just watched and we’ll adjust a few factors. First, let’s start with changing the forecast high tide to 13.5 feet. Yes, it’s
possible.... The lowest barometric
pressure I have recorded on my site is 28.65” or 970mb. Let's use that number.... The high winds my site recorded over that
period were 46mph. I had a shorter anemometer
mast back then, so realistically the winds would have been 50+ mph. Fortunately the high tides that day were "only" about 9.75 feet.
So, let’s run the numbers. Barometric pressure of 28.65” results in an additional 17” to the high tide. If the predicted high is 13.5 feet, the actual theoretical high tide would be 14.9 feet. For good measure, let’s add some 50mph wind. This could easily add another foot or two.
In the video, sea level was about 11.2 feet. This worst case scenario would result in a 14.9 ft. tide…. plus another 2 foot storm surge. Watch the video again and visualize sea level about 5 feet or more above what you see…. With 50mph winds.
Are We Prepared?
This is an extreme depiction of
what "could" happen. More than likely we
will never see this, but something in between is definitely not out of the question. Are we prepared? It’s not just waterfront properties. Roads would be blocked in this scenario due
to flooding. Do you know where the
flooding will be and do you have an alternate route? Floods
happen fast.
I put together a chart showing all remaining high tides above 11 feet this year.
I put together a chart showing all remaining high tides above 11 feet this year.
