Monday, January 23, 2012

Tornadoes On The Beach During Katrina?

As part of my blog, I will take you into heart of Hurricane Katrina where we will look in detail at what structurally happened to a few beachfront properties on the Mississippi Coast. This detail tells a different story than what you might think happened. Here is my first installment of Hurricane Katrina on the Mississippi Gulf Coast.

I will never forget the remains of the first house that I studied that was destroyed by Katrina. At first it looked like everything had been put in a blender and just poured out. Nothing made any sense, but after studying the debris, I started to understand what I was seeing. This was a site that was covered with surge debris that had no relationship with the structure I was evaluating. I estimated that about 60-65% of the debris on the site came from somewhere else other than the site I was trying to evaluate. This had been a brick house in Waveland Mississippi and there was nothing left of the structure but some brick veneer, the slab, and a bird bath. There was some lumber and a roof truss sitting on the slab but these were things that floated in with the surge. The key was not the surge debris but the brick veneer and how the brick was displaced. All of that was under the surge debris. Then I realized that what I was looking at was superposition. A fundamental rule in geology that says that what is on the bottom happened first. All this debris--the lumber, the cars, the sinks, the roofing material, and the bricks, was now, from an analyst's perspective, material that could be studied just like a sedimentary deposit. In addition to superposition, there are other concepts that help show an investigative engineer what happened at a site. For instance, a chimney that just fell over looks totally different from a chimney that had been knocked down by flying debris, and since masonry does not float, it survives the surge and leaves a fingerprint of what really happened. By the time my investigations were completed, I had seen evidence that strong tornadoes had hit the Mississippi coast during Hurricane Katrina and before the surge. But according to the official record there were no tornadoes near the Mississippi coast associated with Katrina. The record shows there were 13 tornadoes on August 29, 2005, none stronger than an F1, all located in central Mississippi.

My job, as a damage investigator was very specific. I needed to determine if each structure was destroyed in part or in totality by wind or by water. This was necessary because specific insurance policies cover each type of damage. With such a daunting task I decided that the best chance for evaluating the demolished properties was to map the debris field from each structure. To my knowledge I am the only engineer that attempted this in the obliterated beachfront zone left by Katrina. For each property an enlarged aerial photo taken after the storm was printed and used on the site. Objects in the photo could then be related with the real objects on the ground. From this, along with field measurements, a scale and a site plan could be established. I worked with the property owners whenever possible, to identify debris that came from their property. Items like masonry that could not float got high priority but the study of wood framing that was anchored so that it could not float was also very beneficial. The tornado evidence I show here is limited, but there is much more and I will post it as I can find time to do so. I will start with one of the most remarkable items of debris that I saw on the Mississippi Coast. Of the eight demolished residences that I investigated from Waveland to the Biloxi Back Bay half had brick chimneys. In Gulfport Mississipi a four foot high segment of one chimney was found 230 feet from where it was built. Because this was not a bouyant item, it really caught my attention.

This story is about another chimney constructed near the northwest corner of a one story beachfront house in Pass Christian Mississippi. The house was located 20 miles west of the where the eye made landfall near the Mississippi-Louisiana border. Here the maximum peak wind gust was determined to be 144 mph using wind speed sensors that did not fail. The maximum sustained wind speed (ARA data) from the hurricane at this location was rated at 100 mph off shore and 90 mph inland.

This residence was estimated to be about 600 feet from the beach. The most remarkable thing about the chimney was it’s pristine condition. It looked as though it could have been lifted with a crane, patched at the top and reused, flashing and all. But it was lying 100 feet from it’s foundation in the driveway behind the house. This segment of the chimney broke off from the lower part just below the roof flashing. The lower part of the chimney was in 2 segments lying about 15 feet from the foundation. Each piece of debris in a hurricane has a history. That history includes forces that were applied to it while it was still attached, its dismemberment, its transportation to its resting place and anything that happened after. The story that this picture tells is that the roof blew off the house bending the flashing up on the south side of the chimney (the back side in the photo). The roof and contents from the house landed in the driveway then the chimney blew off and landed on that debris. The chimney impacted upside down on it’s top and fell over on the debris (without rolling) damaging only the top of the chimney. Later the storm surge cleaned everything from the site that would float away except for the roof deck and shingles that were pinned down by this chimney. The cleanly torn plywood roof deck is also a testament to the kind of forces that were involved. There was no other roofing material from the house found at the site. A 7 minute video of this chimney can been seen on YOUTUBE. The displacement of this chimney by 100 feet appears to indicate wind speeds exceeding the official wind records. Here are my thoughts behind that.

If we use the Enhanced Fujita Scale for guidance we must match our damage with a list of degree of damages (DOD) for residential construction. This scale then gives a range of wind speeds for each level. In this list, 1 is minimal and 10 is described as “destruction of engineered and/or well constructed residence; slab swept clean.” Level 6 is described as “Large sections of roof structure removed; most walls remain standing.” That sounds too low. I think we are at the level where all walls will be collapsed and that takes us up to level 9. At level 9 the wind speeds range from 142 to 198. At level 10 the maximum wind speeds go up to 220. This means that at level 9 at the bottom end we are just touching the maximum wind gusts reported for the area with Katrina. The problem is that I think it would be very unlikely that this chimney would detach and then travel 100 feet under a wind gust. To move this chimney I think we need a longer duration of wind. If that is correct then the sustained wind speeds (90-100) for Katrina alone are not high enough to create this damage. The damage description for these winds speeds is level 4; “uplift of roof deck and loss of significant roof covering (more than 20%);collapse of chimney..” This is a roof that most likely could be repaired if it were not for the higher wind gusts that very well could have destroyed the roof.

Because of the storm surge associated with strong hurricanes, the best way to evaluate wind speeds from damage, is in the storm related displacement of non-buoyant materials like masonry. Unfortunately, I can not find in the literature, debris field studies that include masonry transport from tornadoes. As a result I have started this line of research. There limits to what I can do in this regard, so outside field studies are badly needed. For starters how far does a segment of brick veneer travel during different wind speeds? And when chimneys are present what happens to them?