Flood Frequency Analysis November 16 2011 Flood Response

Flood Frequency Analysis November 16, 2011

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

Natural Floodplain Features Floodplain: Normally dry land area adjoining rivers, streams, lakes, bays or ocean that is undated during flood events

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

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

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

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

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:

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.

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?

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

Flood Frequency Analysis

Ref: Flood Estimates