Selections from Hurricane Katrina Splay Deposits: Hydrodynamic Constraints on Hyperconcentrated Sedimentation and Implications for the Rock Record, by W.R. Barnhart.
(These selections by Marko Malyj are of the article published in Creation Research Society Quarterly Journal, Volume 42, Number 2, Fall 2011)
(To receive new uMarko posts via a daily email, please click Subscribe)
(On Twitter: FOLLOW uMarko or http://www.twitter.com/uMarko)
Hurricane Katrina made landfall over Florida, regained strength in the Gulf of Mexico, and curved northeast to strike the Louisiana and Mississippi coasts. It crossed New Orleans and the adjacent St. Bernard Parish at about 6:10 am, CDT on 29 August 2005, as a Category 3/4 hurricane, with sustained winds of over 205 kph (125 mph) (Knabb and Rhone, 2005). At 8:14 AM CDT, the National Weather Service issued a flash flood warning for New Orleans and St. Bernard parishes, citing a levee breach on the Industrial Canal (Place of Dead Roads, 2005). This breach appears to have been caused by overtopping of the levee wall by a storm wave up to 7 m (23 ft) high, moving up the Gulf Intracoastal Waterway (Figure 1). The wave pparently struck the levee wall broadside (Seed, 2005). The levee failed, and its breach was quickly enlarged. Within 46 minutes, by approximately 9:00 AM CDT, the Lower Ninth Ward was reported to be inundated by 1.8–2.4 m (6–8 ft) of water (McQuaid, 2005a).
The levee breach eventually widened to 61 m (200 ft), but the initial flow through the narrower outlet can be estimated from a house that was carried some distance.
At that point, a powerful current, which flowed into the neighborhood and began widening the breach, scoured the immediate area near the breach, and deposited sediment as the flow dispersed or was slowed by obstacles. During that time, more energy was added to the system by storm waves surging up the canal. Each one would further erode the scour pit, adding additional, corresponding pulses of sediment to the outflow. Flow was powered by a fall height of up to 4 m (13 ft), with a functional head pressure equal to the total mass of the top 2 m (6.6 ft) of water in Lake Pontchartrain. That was why the initial flow was sufficient to dislodge and move a house.
Splay Sedimentation at the Breach
Flow through the breach resulted in the development of a splay deposit, with characteristics that indicate the mechanics of the water flow. Figure 5 and its accompanying diagrammatic version, Figure 6, show one of the thickest and most complete cross sections through the splay.
 |
| Figure 5 (left). Vertical cross section through nearly the complete deposit, created within minutes. Figure 6 (right). Redrawing of Figure 5 showing the areas where primary composition is pink sand (P), white sand (W), andclay (C) in the cross section. From Nelson and Leclair (2006). |
Katrina Splay Deposits Versus Conventional Thinking
Traditionally, splays are seen as relatively benign accumulations of sediments produced by infrequent, random sedimentary events. Nichols (1999) spent a full chapter describing crevasse splays on various types of deltas, and the expected sequence of strata.
But Walker (2007) showed how facies interpretations could be misleading, when he noted how the virtually-instantaneous Katrina deposits mimic the expected sequence “of different environments over long periods of time.
Mudstones and shales were once thought to represent quiescent sedimentation. Schieber et al. (2007) have demonstrated otherwise. Their work is supported by the Katrina splay deposits: even at high velocity, clay ovoids and floccules will form by cohesion and be deposited as distinct cm-scale laminae in regular sets of flat beds.
Conclusion
Despite features commonly attributed to low, slow flow, the splay deposits were clearly formed by sedimentation in a continuous unidirectional current, overlaid by storm surge waves. Published accounts provide a maximum time of 46 minutes for sedimentation, and a more likely minimum of 2–3 minutes is established, based on the wavelength of the storm surges.
The splay deposits also demonstrated that flat beds can be succeeded by cross beds and then revert to flat beds within a continuous, single, unidirectional current, caused by depth variations under constant flow. They also show that a sequence of wave-deposited bedforms, which could easily be interpreted in the rock record as long periods of time, can be the product of continuously depositing waves, in sequence.
This is another argument for a different approach to the rock record: abandoning the facies mode, system and working out the mechanics of deposition instead. The physics of sediment transport and deposition in moving water are well known and provide hard guidelines for the required physical conditions associated with particular bedforms.
References (selected)
Knabb, R.D., and J.R. Rhome. 2005. Tropical cyclone report: Hurricane Katrina. National Hurricane Center. 20 December 2005 (cited in Timeline, 2006).
McQuaid, J. 2005a. Anatomy of a disaster. Newhouse News Service. 8 September 2005 (cited in Timeline, 2006).
Nelson, S.A., and S.F. Leclair, 2006. Katrina’s unique splay deposits in a New Orleans neighborhood. Geological Society of America Today 16(9):4–10.
Nichols, G. 1999. Sedimentology and Stratigraphy. Blackwell Publishing, Boston, MA.
Place of Dead Roads, The. 2005. NOAA/NWS knew of levee breach morning of
Monday 29th. http://theplaceofdeadroads.blogspot.com/2005/09/Noaanww-knewof-levee-breach-morning.html.
Schieber, J., J. Southard, and K. Thaisen. 2007. Accretion of mudstone beds from migrating floccule ripples. Science 318:1760–1763.
Walker, T. 2007. Katrina’s splay deposits: a small example of the power of flowing water. Journal of Creation 21(3):8–11.
