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Natural Disasters Flood Outline

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  • 1. Floods Chapter 11 Streams and Flood Processes: Rising Waters Chapter 12 Floods and Human Interactions Floods Flooding of streams is one of the more common and costly types of natural disasters in the U.S. • Account for one-quarter to one-third of annual disaster dollar losses. • Account for 80% of the annual disaster deaths. The most common cause of flooding is PRECIPITATION Weather patterns determine precipitation.  Area over which the rain falls  Duration of the rain  Intensity The Hydrologic Cycle The Fate of Precipitation Water enters the atmosphere through evaporation. Through precipitation water falls either directly into the ocean or onto the land. Water that falls onto land enters streams by: Infiltration = The movement of water into rocks or soil through cracks and pore spaces Runoff = Water that flows over the land How water gets to a stream: The Drainage Basin = A cup of land bounded by areas of high relief. Precipitation that falls within the cup of land flows to the stream as runoff over the surface or infiltration and Base flow through the groundwater system. Example: The United States is divided by the Appalachian Mountains on the East and the Rockies in the Midwest. All water falling in between those two mountain ranges is in the Mississippi River Drainage basin. Streams A body of running water that is confined in a channel and flows under the influence of gravity. Channel width may vary from a few cm’s to several km’s. Stream Gradient - the vertical drop of a channel over a horizontal distance. The headwaters are the upper part of the stream near its source in the mountains. Upstream regions. – Steep Gradient. Lower reaches of a stream are referred to as downstream regions. – Shallow Gradient. The mouth is the place where a stream channel terminates and enters the sea, a lake, etc. Base level is the theoretical limit to which the stream can erode. It is, in effect, the
  • 2. elevation of the streams mouth. In general, streams begin at higher elevations, and discharge into other streams and lakes (relative base level) that will eventually reach the ultimate base level (sea level). Potential Energy - energy of position; stored energy. Kinetic Energy- energy of motion; energy to do work. As potential energy is converted to kinetic energy, the stream performs the work of erosion. The amount of potential energy is proportional to stream gradient. Erosion by streams has shaped the land surface worldwide over geologic time. The ability of a stream to erode relates to stream Velocity -- the speed of the water, generally measured in feet per second. Discharge -- the total amount (volume) of water carried by the stream. Discharge is generally measured in cubic feet per second, or cfs. Stream Gradation or Equilibrium The volume and velocity of stream flow limit both the size and the amount of sediment that can be carried by the stream. A stream’s COMPETENCE is the MAXIMUM grain size a stream can transport. Faster moving water has a higher competence because it can move larger sized materials. The VOLUME of sediment a stream can carry is called CAPACITY. Larger streams (having a larger volume of water) have a higher capacity to transport stuff. Streams will adjust their cross sections and channel gradient to accommodate the stream flow as well as the volume and grain size of sediment supplied to the channel. Changes are insignificant with normal flow or even small floods, and dramatic with large floods. Upstream - Narrow V-shaped channels - high velocity water cuts into stream bed. -High competence Downstream - wide, deep U-shaped channel - high volumes of water, therefore wide and deep. -High capacity Discharge The volume of water passing a given point in a stream per unit time Q = A*v = cross sectional area of the stream channel (depth x width) x velocity measured in m3 or ft3 per second Upstream - Narrow V-shaped channels -High competence Downstream - wide, deep U-shaped channel -High capacity The Upstream Headwater region • The collecting system (high competence) • Steep gradient - high velocity, therefore high erosion • Consist of a network of narrow v-shaped tributary channels that collect water and
  • 3. sediment to the main stream. The Downstream Region • The transporting system (high capacity) • Shallow gradient: low velocity; major process is to transport water and sediment from upstream to the streams mouth. • Consists of a wide, deep U-shaped channel; called the main trunk stream. • Downstream channels tend to meander more and more over time. The Downstream channel flows along a shallow gradient. On either side of a downstream channel is a floodplain. It is like a larger channel which encompasses the normal downstream channel. As a stream rises prior to flooding, its increased velocity and discharge allow it to carry more sediment. When the water spills out onto the floodplain, water velocity slows down and the sediment is deposited along the banks as levees. The Dispersing System • Consists of a network of distributaries at the streams mouth, where water and sediment are dispersed. • Upon exiting the mouth, the velocity of the stream will decrease to zero, and the sediment is deposited to form a delta (in water) or an alluvial fan (on land) Flooding - an excessive discharge. When water falls within a stream’s drainage basin it enters the stream by infiltration/base flow and by runoff contributing to the streams discharge. The relationship between precipitation and the effect it has on a streams discharge is often shown in graphical form as a hydrograph. The lag time is the time between peak precipitation and peak discharge (flooding). Flooding - Infiltration vs. Runoff High infiltration = high residence time in the soil before entering the stream as base flow: results in a long lag time and low peak discharge. High runoff = water that falls to the ground enters the stream quickly; results in a short lag time and a high peak discharge. Types of Floods Upstream Flash Floods • Caused by locally intense rainfall (covering only one or two tributaries) over a short period of time. • Steep Gradient. • High and Rapid runoff • Narrow V-shaped channels. No floodplains or levees. • Waters rise quickly. • Short lag time and high peak discharge. • Q = A X V. Increased discharge increases water velocity resulting in an increase in erosion. • Unpredictable (because it is a function of weather conditions), high loss of life.
  • 4. • Flood damage mainly from the force of the rapidly moving water and deposition of sediment. • Floods rapidly, recedes quickly. • Downstream unaffected. • Covers a smaller area of land. Flash Floods – Rise rapidly and appear unexpectedly. – The force of the water can wash cars downstream, and damage roads and bridges. – The turbulence of the water makes them capable of transporting large debris. – High velocity waters scour out new channels. – Trigger landslides and debris flows. Downstream Regional Floods • Caused by large amounts of rainfall over an extended period of time over a large portion of the drainage basin. • Shallow gradient. • High infiltration (Natural) • Wide, deep U-shaped channels with floodplains and levees. • Water rises slowly. • Long lag time and broad discharge curve. • Q = A x V. Waters rise as soil becomes saturated. Water spreads out onto the floodplain (large cross-sectional area). • Predictable. Low loss of life. • Flood damage mainly from extensive wetting and deposition of sediment. Great property damage because it covers a large area of land and flood waters remain high for long periods of time. • Recedes slowly. • Floods affect tributaries. Regional Floods The Great Midwestern Flood of 1993  Abnormally high rainfall in many Midwest states  Record flood stage levels recorded  Covered nine states and 400,000 square miles.  The preceding winter and the first half of 1993 were 50 to 100 percent wetter than average, so most of the ground was saturated with water.  Flood waters lasted 200 days  Over 1,000 levees were topped or failed (Levees fail by overtopping, breaching, bank erosion, slumps or seepage).  Fifty deaths  Over $22 billion in damage  Only 5.2% of households had flood insurance. Societal Responses to Flood Hazards Response to flood hazards can be attempted in two main ways:
  • 5. • An engineering approach to control flooding. • A regulatory approach designed to decrease vulnerability to flooding. Artificial Levees Levees form naturally along the banks of a stream as a result of flooding. By building natural levees up it increases the area of the channel so the channel can accommodate a larger discharge. Problems: • Floodwater is confined between the levees, causing the flood flow to be deeper and faster. • Flooding upstream and downstream of the levees is increased. • Levees fail by overtopping, breaching, bank erosion, undercutting and slumping, or seepage. Avulsion When the floodplain lies lower than the streambed, a breach in the levee may cause the stream to abandon its channel and form an entirely new one, a process called avulsion. During a major flood, the river breaches the levee and its water heads down a steeper, shorter path to the sea and gradually taking over to become the new main channel. New Orleans Example Avulsion formed the Atchafalaya River. The Army Corps of Engineers built the Old River control structure to permit 30% of the flow to the Atchafalaya. Avulsion of the Mississippi above New Orleans could be disastrous for the city, destroying its key role as a shipping center for the Mississippi basin. Channel Modifications Involve measures such as straightening the channel, deepening and widening the channel, or lining the channel with concrete. All are attempts to increase the amount of discharge the stream can accommodate. Q = A X V. – Straightening and smoothing reduce friction and increase velocity. – Widening and deepening increase area. Problem: Result in increased flood water levels and increased water velocity. Floodwalls Reinforced concrete structures parallel the river banks and prevent floodwaters from inundating the settled areas behind them. Problem: If floods overtop the walls, waters take longer to recede. Result in increased flood water levels and increased water velocity. Multipurpose Dams • electrical power production • recreation (boating and fishing on reservoirs)
  • 6. • water storage for times of drought • flood control - Dams can be used to hold water back so that excess water can be release through spillways at a controlled rate so that discharge can be regulated downstream. The main drawbacks of dams are their environmental impacts: 1. Drown vast areas of land. 1. Stop the seasonal cycle of flooding, 1. Water downstream of dams is generally much colder and clearer. Many species of plants and animals cannot adapt to this new artificial environment. 1. Dams failure causes great destruction and loss of life downstream. Dam Disasters Due to floods, seepage and erosion under the dam, poor design and construction, and major landslides. The Teton Dam, Idaho, June 5, 76’ Within only a few hours of the initial leak, the dam breached obliterating two small towns, killing 11 people and 13000 livestock, and causing $3.2B in damage. Pecola Dam, S. Dakota, June 10, 72’ Intense rainfall resulted in major flooding dramatically raising the level of the reservior. The dams failure killed 238 people, destroyed 1,335 homes and 5K vehicles in Rapid City, and caused $690M in damage. Retention Ponds Serve a similar purpose as dams. Water can be trapped in a retention pond and then released at a controlled discharge to prevent flooding downstream. Urbanization Increases Flooding Paved streets, parking lots, buildings, and storm sewer systems decrease infiltraton, carrying rainwater as runoff quickly to streams channels. After urban development, floods tend to peak earlier after a rainstorm, and have higher peak discharge! Flood Frequency and Recurrence Intervals To predict flooding for a particular stream, geologists make use of long-term records of previous floods on that stream. The recurrence interval (also called the flood frequency) is the average length of time, in years, between floods of a particular size, or magnitude. The recurrence interval (T) is calculated by this formula: T = (N + 1) M N = the number of years for which flood records are available for the stream M = the rank of a particular size of flood during those N years of record EXAMPLE: Lets say you have 140 years of flood records for a particular stream, and during those 140 years a flood 20 feet above normal was the fourth largest flood (i.e. during those 140 years there were three floods larger than 20 feet).
  • 7. The recurrence interval for a flood 20 feet high would be: T = (N + 1) / M = (140 + 1) / 4 = 35 years. This means that for this stream you could expect to have a 20 foot flood once every 35 years, on average. The “on average” part is important! It does not mean such a flood will occur exactly every 35 years. It just means that the average time between such floods is 35 years. In this example, we would call a 20 foot flood a “35 year flood”. People commonly refer to “50 year floods” or “100 year floods”. Again, it is important to remember that this wording reflects the average time interval between floods of a particular size. A “100 year flood” could occur two years in row! The probability that a particular size flood will occur in any single year is simply the inverse of T (1/T). In our example, the chance of a 20 foot flood coming in any particular year is 1 / 35 = 0.028 or 2.8%. In other words, there is a 2.8% chance of a 20 foot flood happening each year on this stream. The recurrence interval reflects the probability of future flooding, based on past flooding of a stream. The probability of a “50 year flood”occurring in any single year is 1/50 = 2%. The probability of a “100 year flood”occurring in any single year is 1/100 = 1%. As you might expect, large floods happen less frequently (or have longer recurrence intervals) than small floods. The relationship between flood size and recurrence interval can be shown on a flood frequency curve. Problems with Recurrence Interval 1. The data must cover a long enough period of time to be representative. See Sidebar. 2. The conditions that affect stream flow must be similar through time. 3. All the floods should have resulted from similar causes. FLOODPLAIN MANAGEMENT In response to flood disasters, the U.S. government established the National Flood Insurance Program which provides federally subsidized insurance to property owners in designated flood-prone areas. The following areas of the floodplain are recognized: The regulatory floodway – the channel and its banks. No new construction permitted. The regulatory floodway fringe -- areas below the 100 year floodplain. Construction permitted only if protected by: - raising the land level with fill
  • 8. - building walls and/or levees - using water-resistant building materials  In many places, floodplains are not adequately zoned.  Flood insurance premiums are set too low to cover the actual cost of flood insurance.  Few people whose property is damaged by flooding have flood insurance.  Billions of tax dollars are spent every year to fight and control floods and provide relief.  Human alterations to the landscape have significant effects on the magnitude and frequency of future floods. Monitoring the Progress of Storms If factors such as amount of rainfall, degree of ground saturation, degree of permeable soil, and amount of vegetation can be determined, then these can be correlated to forecast possible floods. For a flood with a long lag time. a flood warning is issued about the possible extent of the flood to give people time to move out of the area. In areas known to be susceptible to flash floods, a flash flood warning is often issued any time heavy rainfall is expected.

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