100-year flood
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A one-hundred-year flood is calculated to be the level of flood water expected to be equaled or exceeded every 100 years on average. The 100-year flood is more accurately referred to as the 1% flood, since it is a flood that has a 1% chance of being equaled or exceeded in any single year. Based on the expected flood water level, a predicted area of inundation can be mapped out. This floodplain map figures very importantly in building permits, environmental regulations, and flood insurance.
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[edit] Probability
A 100-year flood has approximately a 63.4% chance of occurring in any 100-year period, not a 100 percent chance of occurring. The probability of a certain-size flood occurring during any period can be calculated using PT = 1 – (1-Pf)n where PT is the probability of occurrence for the entire period; Pf is the probability of occurrence in any single year; and n is the number of years. Ten-year floods have a 10% chance of occurring in any given year (Pf =0.10); 500-year have a 0.2% chance of occurring in any given year (Pf =0.002); etc. The percent chance of an X-year flood occurring in a single year can be calculated by dividing 100 by X.
The field of extreme value theory was created to model rare events such as 100-year floods for the purposes of civil engineering.
[edit] Upslope Factors
The amount, location, and timing of water reaching a drainage channel from natural precipitation and controlled or uncontrolled reservoir releases determines the flow at downstream locations. Some precipitation evaporates, some slowly percolates through soil, some may be temporarily sequestered as snow or ice, and some may produce rapid runoff from surfaces including rock, pavement, roofs, and saturated or frozen ground. The fraction of incident precipitation promptly reaching a drainage channel has been observed from nil for light rain on dry, level ground to as high as 170 percent for warm rain on accumulated snow.[1]
Most precipitation records are based on a measured depth of water received within a fixed time interval. Frequency of a precipitation threshold of interest may be determined from the number of measurements exceeding that threshold value within the total time period for which observations are available. Individual data points are converted to intensity by dividing each measured depth by the period of time between observations. This intensity will be less than the actual peak intensity if the duration of the rainfall event was less than the fixed time interval for which measurements are reported. Convective precipitation events (thunderstorms) tend to produce shorter duration storm events than orographic precipitation. Duration, intensity, and frequency of rainfall events are important to flood prediction. Short duration precipitation is more significant to flooding within small drainage basins.[2]
The most important upslope factor in determining flood magnitude is the land area of the watershed upstream of the area of interest. Rainfall intensity is the second most important factor for watersheds of less than approximately 30 square miles or 80 square kilometers. The main channel slope is the second most important factor for larger watersheds. Channel slope and rainfall intensity become the third most important factors for small and large watersheds, respectively.[3]
[edit] Downslope Factors
Water flowing downhill ultimately encounters downstream conditions slowing movement. The final limitation is often the ocean or a natural or artificial lake. Elevation changes such as tidal fluctuations are significant determinants of coastal and estuarine flooding. Less predictable events like tsunamis and storm surges may also cause elevation changes in large bodies of water. Elevation of flowing water is controlled by the geometry of the flow channel.[3] Flow channel restrictions like bridges and canyons tend to control water elevation above the restriction. The actual control point for any given reach of the drainage may change with changing water elevation, so a closer point may control for lower water levels until a more distant point controls at higher water levels.
Effective flood channel geometry may be changed by growth of vegetation, accumulation of ice or debris, or construction of bridges, buildings, or levees within the flood channel.
[edit] Prediction
Statistical analysis requires all data in a series be gathered under similar conditions. A simple prediction model might be based upon observed flows within a fixed channel geometry.[4] Alternatively, prediction may rely upon assumed channel geometry and runoff patterns using historical precipitation records. The rational method has been used for drainage basins small enough that observed rainfall intensities may be assumed to occur uniformly over the entire basin. Time of Concentration is the time required for runoff from the most distant point of the upstream drainage area to reach the point of the drainage channel controlling flooding of the area of interest. The time of concentration defines the critical duration of peak rainfall for the area of interest.[5] The critical duration of intense rainfall might be only a few minutes for roof and parking lot drainage structures, while cumulative rainfall over several days would be critical for river basins.
Extreme flood events often result from coincidence such as unusually intense, warm rainfall melting heavy snow pack, producing channel obstructions from floating ice, and releasing small impoundments like beaver dams.[6] Coincident events may cause flooding outside the statistical distribution anticipated by simplistic prediction models.[7] Debris modification of channel geometry is common when heavy flows move uprooted woody vegetation and flood-damaged structures and vehicles, including boats and railway equipment.
[edit] References
- ^ Babbitt, Harold E. and Doland, James J., Water Supply Engineering, McGraw-Hill Book Company, 1949
- ^ Simon, Andrew L., Basic Hydraulics, John Wiley & Sons, 1981, ISBN 0-471-07965-0
- ^ a b Simon, Andrew L., Practical Hydraulics, John Wiley & Sons, 1981, ISBN 0-471-05381-3
- ^ Linsley, Ray K. and Franzini, Joseph B., Water-Resources Engineering, McGraw-Hill Book Company, 1972
- ^ Urquhart, Leonard Church , Civil Engineering Handbook, McGraw-Hill Book Company, 1959
- ^ Abbett, Robert W., American Civil Engineering Practice, John Wiley & Sons, 1956
- ^ United States Department of the Interior Bureau of Reclamation, Design of Small Dams, United States Government Printing Office, 1973
- "What is a 100 year flood?". Boulder Area Sustainability Information Network (BASIN). URL accessed 2006-06-16.
