Question....is the storm surge at its highest when the eye crosses the coastline, or has it already abated some as the seabottom rises towards the coastline?
The storm surge combined with the heavy rain that comes with the hurricane can cause dangerous flooding in low-lying coastal areas, especially when a storm surge coincides with a high tide. This flooding can be the most dangerous part of a hurricane, potentially causing many deaths.
The height of the storm surge is the difference between the level of the ocean and the level that would have occurred normally. A storm surge is usually estimated by subtracting the regular high tide level from the observed storm tide - it can be 15 feet tall or more in a very strong storm.
from that, I extrapolate the storm surge is at its highest as the eye makes landfall. If anything, the shallow depths as the storm approaches land push the water up higher, no? Maybe someone here knows more.
I think tides figure into the equation.
Off the top of my head this is how I would approach the problem.
Storm surge is a complex multivariate problem. Variables which effect its computation and modeling include: topology of the shoreline, topology of the seafloor gradients and inflexion points, air pressure, storm geometry (radius, distribution of wind velocities over the radii, velocity of rotation), storm velocity, tide levels, basin size, water currents, wave heights, and wind current distributions.
As a simple rule of thumb for Texan coast hurricanes, a storm with about 90 mph wind velocity, with 20 ft wave heights 100 miles out in the gulf might produce a storm surge of about 5 feet upon hitting land.
Other rules of thumb include one centimeter of storm surge for every millibar in reduced barometric pressure.
People should not be decieved by the wind velocities and relative comparisons they identify those wind speeds with during normal storms of less than 50mph winds/gusts.
Most structures built on the coast for hurricane codes may be designed for 100 mph winds or roughly 25 lbs/SF of pressure against an exterior wall. Inland structures are frequently built for about 80 mph winds or about 16 lbs/SF pressure from wind loads. Structurally, the design process typically multiplies the surface area of a building times that wind load, then multiplies that total times the height of the building 'center of mass' (centroid more accurately) to obtain a total moment for the design. The foundation of the structure is frequently designed to include any soil over the concrete foundation combined with the foundation to provide an overturning moment.
When posed with large winds, the wind forces vary proportionally with the square of the wind velocity. That means a 160mph wind doesn't merely double the wind forces' load on a structure from 80 mph, it quadruples the forces.
The forces of 70mph winds quadruple from 45 mph winds. Likewise they quadruple again to 135 mph.
Additional problems arise in that most structures, signs utilities, concrete sidewalks, you name it,,..really aren't deigned for much over 100mph (some specific designs might get designed to 120, but whether or not construction actually places them that rigorously is rarely tested, high rises for example usually get designed for higher wind loads).
For example, the increased water flows from storm water, rain, surf, greatly erodes soil conditions. Typically during hurricanes, grounds become saturated with water and the structural properties of the soil can change. Couple this with simple erosion where soil is physically removed, many designed structures might not even have the strength during storm conditions to remain intact in lessor wind loads.
Since these conditions aren't normal, they are rarely tested,..when they are, failures are expected as 'acts of God' and frequently lessons are too costly to find or redesign to overcome.
Additionally, people tend to tie things down to structures and previous design assumptions might not be obeyed by actual loading conditions. For example, somebody designs a signpost to withstand 125 mph winds, you get 100 mph winds and somebody ties down a trailer bewtween two signs,..the trailer might rip out the two signs along with the trailer flying,...or debris, not calculated for the impulse it renders when it strikes the signs, takes out everything that is tied together and the whole mess goes flying to strike something else.
Bottom line, is if you anticipate a large storm over 90mph winds,..don't hang around. There might not be anything to protect you from the elements.
There are a lot of variables involved. The centroid of low pressure at the eye's center lifts the surface of the ocean in a large bulge. The surge effect attributable to this source reaches maxima when the eye's center makes landfall.
The tangential winds tend to drive water into the shore, piling it up. The surge effect attributable to this source reaches maxima when the winds are perpendicular to the shoreline.
Coastal geometery can funnel high water to a specific point, increasing the levels. The surge effect attributable to this source will reach maxima when the windspeed, direction and shoreline geometry combine to fullest effect.
You also have to factor in tides and wave heigth.
The following is cross posted from another thread here:
This is EARLY analysis, but it is a useful illustration of what happened and where:
http://users.in-motion.net/~jefft/tech/Mapping/afghanistan/4msurgetext.jpg
This is a complex image. First, a pair of one degree coverage 30 meter per pixel digital elevation models (DEMs) were merged, and displayed with normal sea levels and coastlines.
The sea level data was extracted, and saved in a temporary file.
Next, the same two DEMs were merged and displayed with an increase in sea level of 4 meters, or 13.2 feet. The temporary file with the normal sea level data was then overlaid onto the "Plus 4 meter
Merge", and then vector data including county borders and city locations added.
Finally, windfield data from
http://www.boatus.com/hurricanes/
was analyzed to generate the eye, eyewall, and 100 MPH boundary, and used to generate the maximum surge indicators to complete the composite image. The general location of eyewall landfall and radial
wind velocity maxima were additionally supported by Nexrad radar data from Mobile and Eglin.
Note that the dark blue surge overlay was applied to the entire image, while in reality, ONLY the portion within "Maximum Surge" area would have actually received the highest levels of surge.
Note also that I used an expected 4 meter surge, while some news agencies are reporting surges up to 16 feet, or 2.8 feet higher. While an additional 2.8 feet will make a difference to some areas, analysis of
the DEMs indicates that such a difference would not affect a significantly larger area. This part of the Florida coast rises in elevation more quickly than the landfall areas of Charley and Frances, and hopefully,
was more resistant to surge damage.
It looks to me from this analysis, that the barrier islands and a limited number of non-barrier areas were subject to storm surge, however, the additional 2.8 feet, plus variations in high tide and wave heigth
are/were not available and were not used for this analysis.
