Sunday, September 18, 2005
Rim Gravels, Paleohydraulic Analysis, and the Flood/Post-flood boundary
Deposits Remaining from the Genesis Flood: Rim Gravels in Arizona
Here they examine the gravels in Arizona, and determine that the flood/post-flood boundary is shortly after the deposition of the gravels, in the "late Cenozoic" period in the standard geologic column for that area. The discuss that the gravels include large, rounded quartzite rock with many percussive marks. The roundness of the stones indicate large volumes of water, while the percussive marks indicate very energetic transport.
They used paleohydraulic analysis to examine the rocks, and came up with the following results:
- Estimated minimum depths range from 3.3 meters (11 feet) to 9.9 meters (32.5 feet). Actual depths may have been greater.
- Estimated minimum current speeds range from 11.5 m/s (26 mph) to 21.5 m/s (48 mph). Actual peak current speeds may have been greater. These are far in excess of the recommended maximum allowable current speed for channels excavated in hard rock, which is 3 to 4.5 m/s (6.7 to 10 mph) (Julien, 1995), indicating that very rapid erosion would have taken place. Peak current speeds in excess of 30 m/s (67 mph) may result in cavitation and extremely rapid destruction of rock masses (Holroyd, 1990a,b).
- Estimated discharge per meter width range from 38 to 198 m3/s per meter width (410 to 2,130 ft3/s per foot width). Actual peak unit discharge may have been greater. The estimated unit flows exceed historic peak flood unit flows for the Colorado River at Bright Angel. Unit discharge estimates indicate a very different environment of deposition for the Rim Gravels from current environments.
- Paleocurrents were supercritical (Fr>1.0). To reduce the Froude number to 1.0 (critical flow) would require a flow depth of 4.6 kilometers (2.86 miles)! Flow, therefore, was almost certainly rapid, not tranquil.
- Estimated minimum Reynolds numbers are near the boundary between laminar and transitional flow. If actual peak depths and current speeds exceed the minimums estimated here, Reynolds numbers would have been higher, and flow would have been turbulent.
Minimum paleocurrents would have been very energetic, capable of eroding hard rock, planing off obstructions, rounding clasts, and transporting large amounts of sediment.
It's a very interesting article, and very well displays how creationists view geologic features of the earth.