riverflooding - River Flooding EENS 204 Tulane University...

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This page last updated on 07-Apr-2004 EENS 204 Natural Disasters Tulane University Prof. Stephen A. Nelson River Flooding River Flooding Having covered the basics of stream systems we now turn our attention to the details of flooding associated with rivers and streams. Flood Stage z The term stage refers to the height of a river (or any other body of water) above a locally defined elevation. This locally defined elevation is a reference level, often referred to as datum. For example, for the lower part of the Mississippi River, reference level or datum, is sea level (0 feet). Currently the Mississippi River is at a stage of about 3.5 feet, that is 3.5 feet above sea level. Other river systems have a reference level that is not sea level. Most rivers in the United States have gaging stations where measurements are continually made of the river's stage and discharge. These are plotted on a graph called a hydrograph, which shows the stage or discharge of the river, as measured at the gaging station, versus time. z When the discharge of a river increases, the channel may become completely full. Any discharge above this level will result in the river overflowing its banks and causing a flood. The stage at which the river will overflow its banks is called bankfull stage or flood stage. For example, the flood stage of the Mississippi River at New Orleans 17 feet. Discharge that produces a stage over 17 feet will result in the water nearing the top of the levee with potential flooding of the city of New Orleans (the top of the levee is actually at 23 feet above sea level). (Note that for the Mississippi River and other large rivers in Louisiana, the current stage and flood stage are published on a daily basis in the weather section of the Times-Picayune newspaper). z Discharge is not linearly related to stage because discharge depends on both the depth and width of the stream channel, or more precisely, on the cross-sectional shape of the channel. Stage refers only to the height of the water above some reference level. For example, the graph below is a hydrograph of the Mississippi River at St. Louis, Missouri during the time period of the 1993 flood. Discharge is plotted on the Y-axis, and dates are plotted on the x-axis. Note that stages corresponding to various discharges are shown on the left-hand y-axis, and that the spacing between equal units of stage are not equal along the y-axis. River Flooding 4/7/2004 Page 1 of 6
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z Note that for the 1993 Mississippi River Flood, the river reached flood stage of 30 feet above datum on about June 26 and peaked (or crested) at just under 50 feet above datum on August 1. The sudden drops seen in discharge around July 15 and July 20 corresponded to breaks in the levee system upstream from St. Louis that caused water to flow onto the floodplain upstream, thus reducing both the stage and discharge measured at St. Louis.
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riverflooding - River Flooding EENS 204 Tulane University...

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