Hazards Planning and Risk Management Flood Frequency Analysis Lecture No. 13 Flood Frequency Analysis Fall 2016 Picture source: Prof. Stephen A. Nelson (Tulane University) notes. US – Pakistan Center for Advanced Studies in Water

Learning Objective How to calculate floods with different recurrence intervals using historical data Picture from Internet.

Reading Material Elements of Physical Hydrology By George M. Hornberger, Patricia L. Wiberg, Jeffrey P. Raffensperger, Paolo D'Odorico A limited preview available on Google Books.

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 Fills up the stream…….: when river is in its natural state that’s no hydraulic structures. Peak in a hydrograph regardless of whether river actually leaves its banks and causes damages. Flood hydrographs are characterized by a steeply rising section followed by a less steeply falling segment as the flood passes. Flood occur throughout the year but the magnitude of peak discharge varies seasonally in response to changing weather conditions. Poplar creek is perennial and Frio is ephemeral (dry for long periods with occasional large floods) stream.

Mean Rainfall Mean Annual Rainfall: determined by averaging the total rainfall of several consecutive years at a place Mean monthly Rainfall: by averaging the monthly total rainfall for several consecutive years

Natural Floodplain Features Floodplain: Normally dry land area adjoining rivers, streams, lakes, bays or ocean that is undated during flood events http://www.utdallas.edu/~brikowi/Teaching/Environ_Geology/LectureNotes/riversFlooding.pdf Floodplains carry flow in excess of the channel capacity. Provides both conveyance mechanism and a temporary storage area for excess water

http://www.utdallas.edu/~brikowi/Teaching/Environ_Geology/LectureNotes/riversFlooding.pdf

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 2104

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.

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.

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

Example: Solution: 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. Depends on both type of structure and damage or loss of life expected if the design flow exceeded. Typically 20 years flood for road culverts and 10,000 for major dams. Series of the largest flood in each year is termed as “annual series”.

Discharge Vs. Recurrence Interval

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.

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

Estimate the 100 year Flood? 100 years flood = 0.01 exceeding probability Source:?

Floodplain Zoning Zoning features of a regulated floodplain The flood hazard area is generally defined at the 100-year floodplain. http://www.utdallas.edu/~brikowi/Teaching/Environ_Geology/LectureNotes/riversFlooding.pdf

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

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

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