Hello readers, it was long time i didn't write in my blog. So, sorry guys. There are a lot of deadline from Otake Sensei (supervisor) in Hokkaido University. Now, i have a little time to share my knowledge. In Hokkaido University, i have already taken 5 subjects that related to my background education. One of them is Environmental Geology. Hello guys, if you know that talking about geology is not only mining and oil and gas. There is most important thing in the world that we need everyday. It is water. Yeah! But in this case, i want to share about Hydrologic Hazards at the earth's Surface. It was my article as assignment of Environmental Geology's task. Happy reading ^^
Water, Water, Everywhere
New Orleans, the Crescent City, has the flattest, lowest, and youngest geology of all major U.S. cities. The city's "alps," with a maximum elevation of about 5 meters (16 ft) above sea level, are natural levees built by Mississippi River, and its average elevation is just 0.4 meters (1.3 ft). No surface deposits in the city are older than about 3.000 years. The city was established in 1717 on the natural leeves along the river. The leeves' sand and silt provided a dry, firm foundation for the original city, which is now called Vieux Carre (French "old square"), or "the French Quarter." Father from the river the land remained undeveloped, because it was mainly water-saturated cypress swamp and marsh formed between distributaries of the Mississippi River's ancient delta. There areas are underlain by as much as 5 meters (16 ft) of peat and organic muck (Figure 1). When high-volume water pumps became available about 1900, drainage canals were excavated into the wetlands to the north. Swamp water was pumped upward into Lake Pontchartrain, a cutoff bay of the Gulf of Mexico. About half the present city is drained wetlands lying well below sea and river level. The lowest part of the city, about two meters below sea level. The lowest part of the city, about two meters below sea level, is in the lively, historic French Quarter, noted for its music, restaurants, and other tourist attractions.
Figure 1. High-altitude false-color image of New Orleans a city almost completely surrounded by water. The light spot in the lower center is the city. Note that the Mississippi River meanders through it, Lake Pontchartain is to the north, and smaller Lake Borgne is to the northeast. Low, swampy delta lands appear brownish to the east and south. The land width in the photo is approximately 200 kilometers (125 ml).
Subsidence is a natural process in the Mississippi River delta due to the great volume and the sheer weight of the sediment laid down by the river. The sediment compacts, causing the land surface to subside. The region's natural subsidence rate is estimated to have been about 12 centimeters (4 in) per century for the past 4.400 years. The estimate is based upon C-14 dating of burried peat deposits and does not take into account any rise of sea level during the period.
Urban development in the 1950s added to the subsidence. Compaction of peaty soils in reclaimed cypress swamps coincided with the period's construction of drainage canals and planting of trees, both of which lowered the water table. Peat shrinks when it is dewatered, and also oxidation of organic matter and compaction contribute to subsidence. Most homes constructed on reclaimed swamp and marsh soils in the 1950s were built on raised-floor foundations. These homes are still standing, but they require periodic leveling. Unfortunately, homes built on concrete-slab foundations, a technique that has just been introduced, sank into the muck and became unlivable. Other homes' foundation were constructed on cypress-log piles sunk to a depth of 10 meters or more (at least 30 ft). These house have remained at their original level, but the ground meet the house level. It is very common to see carports and garages that have been converted to extra rooms when subsidence has cut off the driveway access to them (Figure 2). One also sees many houses with an inordinate number of porch steps; the owners have added steps as the ground has sunk. In 1979, Jefferson and Orleans parishes (counties are called "parishes" in Louisiana) passed ordinances requiring 10- to 15-meter-deep wooden-pile foundations for all houses built upon former marsh and swamp land. This has been beneficial, but differential subsidence continue to damage New Orleans' sewer, water, and natural-gas lines and streets and sidewalks.
Figure 2. Differential subsidence around this pile-supported house has left the carport high and dry. The carport was converted to a family room, and fill was imported to bring the yard surface up to the previous level.
"Rain added to a river that is rank perfoce will force it overflow the bank". (William Shakespeare)
Refference : Environmental Geology (D. D. Trent, 2003)
20:47
December 18th, 2013
Laboratory of Sustainable and Resources Engineering, Hokkaido University
