The term "100-year flood" is used to describe the recurrence interval of floods. The 100-year recurrence interval means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100.
Discharge increases, as noted above, because water is added to the stream from tributary streams and groundwater. As discharge increases, the width, depth, and average velocity of the stream increase. The gradient of the stream, however, will decrease.
about 30,000 cfsThis method would result in a 100-year discharge of about 30,000 cfs – essentially the same as the 2009 flood (Figure 3).
Discharge is the volume of water that passes through a given cross section per unit time, usually measured in cubic feet per second (cfs) or cubic meters per second (cms).
Discharge = V x D x W If length is measured in feet and time in seconds, Discharge has units of feet3/sec or cubic feet per second (cfs). Depth times Width gives the cross-sectional area. The shape of the channel is important because of the reduction in velocity due to friction along the channel margins.
Discharge is the volume of water moving down a stream or river per unit of time, commonly expressed in cubic feet per second or gallons per day.
Example Calculations A 100 year flood has a return period of T = 100, so the probability of a flood of equal or greater magnitude occurring in any one year period is p = 1/T = 1/100 = 0.01. Thus there is a probability of 0.01 or 1 in 100 that a 100 year flood will occur in any given year.
The 100-year flood level is that gage height that corresponds to the discharge at RI = 100, which has a probability of being met or exceeded of 1%. Similarly, the 500-year flood level is that gage height corresponding to the discharge extrapolated at RI = 500, which has a probability of being met or exceeded of 0.2 %.
What is the probability that a 100 year flood will occur along the Mississippi River this year? 1% The term 100-year flood means that regardless of the river, there is a 1% chance every year of a flood whose magnitude is so large, we expect to see it only once in a 100 years.
The magnitude of the 10-year flood has been determined through statistical analysis to be approximately 31,100 cubic feet per second (ft3/s). You can see from the graph that the actual interval between floods greater than this magnitude ranged from 4 to 28 years, but the average of these intervals is about 10 years.
The flood discharge shall preferably be the maximum of 100 years' recorded value for important bridges and 50 years' recorded value for less important bridges. The terms “100 years' value” and “50 years' value” are defined as momentary peak discharge which occur “on the average” once in 100 years or once in 50 years.
Heavy storms result in more water entering the drainage basin which results in a higher discharge. The type of precipitation can also have an impact. The lag time is likely to be greater if the precipitation is snow rather than rain. This is because snow takes time to melt before the water enters the river channel.