1. Flood Frequency Analysis
November 16, 2011

2. Flood Response of river to precipitation
An unusually high stage of a river
Peak in a hydrograph
Fills up the stream up to its banks and often spills over to the adjoining flood plain
Poplar creek is perennial and Frio is ephemeral (dry for long periods with occasional large floods) stream.

3. Natural Floodplain Features
Floodplain: Normally dry land area adjoining rivers, streams, lakes, bays or ocean that is undated during flood events
Floodplains carry flow in excess of the channel capacity.
Provides both conveyance mechanism and a temporary storage area for excess water

5. Movement of Flood Wave
Flood may be considered as a wave that propagates downwards
In an ideal channel with frictionless fluid, flood wave may be considered traveling with no change from its point of origin
In natural channel energy is lost due to frictional forces
As a result magnitude of flood wave reduced or attenuated as it travels downstream
But discharge also increases in downstream reaches due to increase in watershed area

6. Flood Estimates
Estimate of extreme flood flow is required for the design of flood control hydraulic structures and Disaster Risk Assessment
Magnitude of flood may be estimated in accordance with the mitigation measures
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7. Flood Estimation: Frequency Analysis
Objective is to develop a frequency curve
Probability or likelihood of having certain events occurring over a specified period is estimated
Frequency curves can be developed to evaluate maximum events
Used for planning water resources structures and for flood mitigation
Relationship between the magnitude of events and either the associated probability or the recurrence interval
Weather systems vary year to year and hence magnitude of future events can not be predicted accurately
Have to rely on statistical analyses of rainfall amounts over certain period
Frequency distribution of past events
Source: Hydrology and the management of watersheds By Kenneth N. Brooks
Reservoirs, waterways, irrigation networks should be planned and designed for future events.

8. Recurrence Interval of a Storm
Number of years within which a given flood may equal or exceed
Also known as return period
Tr = (n+1)/m
Where;
Tr = Return Period
n= Number of years of record
m= Rank of the event (storm) (descending order)
Means this discharge value or more than that occurs m times in n years
Ranking: The serial number of a specific value of precipitation in the descending order
Return Period: Time interval after which a storm of given magnitude is likely to recur.

9. Frequency = p * 100 = m/(n+1)* 100
Probability: Reciprocal of the return period
p = m/(n+1)= 1/ Tr
Frequency: Probability expressed in terms of percentage
Frequency = p * 100 = m/(n+1)* 100
Example 4.13 source: Elementary Engineering Hydrology By Deodhar M. J.
Frequency: Inverse of return period multiplied with 100 and expressed as %. Frequency of a rainfall of a given magnitude means the number of times the given event may be expected to be equaled or exceeded in 100 years

10. Example: Please solve this problem provided in Excel file.
Highest or peak discharges (floods) in each year used for calculation.
If there are 20 years of record for a station we may expect that the largest recorded flood is an approximation to the “20-years flood”….. A flood that occurs, on average” once in 20 years.
Series of the largest flood in each year is termed as “annual series”.

11. Discharge Vs. Recurrence Interval
Extrapolate for 100 year flood

12. The probability of a certain-size flood occurring during any period can be calculated using the following equation:
Pt = 1-(1-p)N
Even though the 100 year flood has a 0.01 or 1% chance of occurring in any year, this does not mean that the 100 year flood has a 100% chance of occurring every 100 years.

13. Prob (no occurrence in N years) = (1-p)N
Explanation
Once the frequency curve is developed, the probability of exceeding certain rainfall amount over a specified period can be determined
The probability that an event with probability p will be equaled or exceeded x times in N years is determined by:
If x=0 (no occurrence in N years) then
Prob (no occurrence in N years) = (1-p)N
Therefore;
Prob (at least 1 occurrence in N years) = 1-(1-p)N

14. Estimate the 100 year Flood?
100 years flood = 0.01 exceeding probability
Please solve above problem?

15. Floodplain Zoning Zoning features of a regulated floodplain
The flood hazard area is generally defined at the 100-year floodplain.

16. Flood Frequency Analysis
Source: Elements of physical hydrology By George M. Hornberger
As records become longer, estimates of extreme events become better.

17. Ref: Flood Estimates
Source: Elements of physical hydrology By George M. Hornberger