SEMINAR (EPRSE-313)HYDRULIC STRUCTURES

FLOODS

WHAT IS MENT BY FLOOD?

• A flood is an unusually high stage in a river normally the level at which the river overflows its banks and inundates the adjoining area.

• The damages caused by floods in terms of loss of life, property and economic loss due to disruption of economic activity are all too well known.

ESTIMATION OF DESIGN FLOOD

A design flood is the flood discharge adopted for design of a hydraulic structure after careful considerations of hydrologic and economic factors.

A design flood observed in most of the cases may be less than MPF (Maximum probable flood).

The estimation of flood can be made by the following methods.

1. By physical indication of past floods.

2. By flood discharge formulae.

3. By flood frequency studies.

4. By unit hydrograph.

BY PHYSICAL INDICATION OF PAST FLOODS.

• The cross section of the river may be plotted and the water line corresponding to the highest flood can be drawn on it.

• From such a cross section the water flow area, wetted perimeter and hydraulic mean depth can be calculated.

• And the procedure should be repeated at several villages or water marks to get consistent results.

EMPIRICAL FORMULA FOR FLOOD DISCHARGE

EMPIRICAL FORMULAE • The empirical formulae used for the estimation

of the flood peak are essentially regional.

1. Dicken’s formula,

2. Ryve’s formula,

3. Inglis formula,

4. Nawab Jang Bahadur formula,

5. Fanning’s formula,

6. Creager’s formula,

7. Fuller’s formula

• Dickens formula:

Q = C A3 / 4

Q= discharge in cumecs

A= area of the basin in sq.km• Ryves formula:

Q = C A2 / 3

Q= discharge in cumecs

A= area of the basin in sq.km

C- depends on catchment

• Inglis formula Q= 123A/√(A+10.4) Ω 123 A½

• Nawab Jang Bahadur Formula Q=CA(0.903-1/14Log A)

• Fannings Formula Q=CA 5/6 • Creagers Form

Q1= 46C1A1(0.8904A1-0.048)

• Fullers formula

Q max= CA0.8(1+0.8 logT) (1+2.67A-0.3)

T=Number of years after which such a flood is to reccur

Q = Maximum flood during any part of the day that could occur in T-years

A= Area of drainage basin in sq.kmC= Constant varying from 0.185 to 1.3

FLOOD FREQUENCY ANALYSIS

• Flood frequency analysis involves the fitting of a probability model to the sample of annual flood peaks recorded over a period of observation, for a catchment of a given region

• Hydrologic processes such as floods are

exceedingly complex natural events. They are the resultants of a number of component parameters of the hydrologic system and are therefore very difficult to model analytically.

BY FLOOD FREQUENCY STUDIES

• FLOOD FREQUENCY:

Flood frequency denotes the likely hood of flood being equalled or exceeded.

• RECURRENCE INTERVAL:

Recurrence interval denotes the number of years in which a flood can be expected once

• RETURN PERIOD:

It is the average recurrence interval for a certain event or flood.

• PROBABILITY OF OCCURRENCE:

The probability of an event being equalled or exceeded in any one year is the probability of its occurrence.

• FREQUENCY:

The probability of occurrence of an event expressed as a percent is known as frequency

UNIT HYDROGRAPH METHOD

• The unit-hydrograph technique can be used to predict the peak-flood hydrograph, if the rainfall producing the flood, infiltration characteristics of the catchment and the appropriate unit hydrograph are available.

• For design purposes, extreme rainfall situations are used to obtain the design storm (viz., the hyetograph) of the rainfall excess causing extreme floods.

GUMBEL DISTRIBUTION•This extreme value distribution was introduced by Gumbel (1941) and is commonly known as Gumbel's distribution. It is one of the most widely used probability-distribution functions of extreme values in hydrological and meteorologic studies for prediction of flood peaks, maximum rainfalls, maximum wind speed, etc. •Gumbel defined a flood as the largest of the 365 daily flows and the annual series of flood flows constitute a series of largest values of flows.

• Gumbel distribution is a statistical method often used for predicting extreme hydrological events such as floods. In this study it has been applied for flood frequency analysis because,

(a) peak flow data are homogeneous and independent.

(b) The river is less regulated, hence is not significantly affected by reservoir operations, diversions or urbanization; and

(c) flow data cover a relatively long record (more than 10 years) and is of good quality.

GUMBEL DISTRIBUTION

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FLOOD ROUTING• It is the process of calculating water levels in

reservoir, the storage quantities and out flow rates corresponding to a particular inflow hydrograph at various instance.

• Flood routing is carried out in a reservoir to determine what will be the maximum rise in its water surface and what will be the discharge in the downstream channel when particular flood passes through it.

• Flood routing is an important technique necessary for the complete solution of a flood control problem and for the satisfactory operation of the flood-prediction service.

• Evaluating the risk of flood requires hydrologic studies.

• Risk evaluation under current landuse,• Prediction of the impact of landuse change.

• Storage and transmission of floodwater • Floodwater moves downstream along a

channel. Flood • occurs when the channel storage capacity is

exceeded

(a) Simple translation:• The flood wave moves without changing its

shape. This tendency is dominant in steep, straight streams. Flow velocities are high and relatively constant.

(b) Attenuation • The wave is attenuated by storage within the

channel and the valley floor. A reservoir is a good example.

(c) Combination • Most natural rivers have both tendencies.

ROUTED FLOOD WAVE

FLOOD PREDICTION

• Important considerations.

(a) Volume of storm runoff

(b) Peak flood discharge

(c) Flood height

(d) Time distribution of storm hydrograph

(e) Area of inundation

(f) Velocity of flow across the valley bottom

METHODS OF FLOOD ROUTING

• Calculus Method• Step by Step Methods

I. Graphical Methods

II. Trial and Error Methods

IMPORTANCE OF FLOOD CONTROL CAUSE

• Low lying flood plains have attracted people to settle for hundreds of years.

• Flood plains provide fertile areas to grow crops • Rivers form navigation routes to carry goods • This all leads to an increase in population in and around floo

d plains over the years. • Many large cities are located in and around an adjacent

river's flood plain. • Rivers flood periodically when the flow increases and

breaches the natural river banks. • Some small floods occur annually from seasonal rain

and or snow melt. Other larger floods occur less often.

IMPORTANCE OF FLOOD CONTROL EFFECT • Data from the U.S. Army Corps of Engineers on flood dam

age in the past 100 years.

• This data shows that when rivers flood, there has been an

increasing trend in the number of lives lost and damaged

property.

• This puts ever more pressure on proper flood control to

help reduce these risks.

• Flood control measures are those measures taken to

reduce the damaging effects flood waters have on human

lives and property damage.

• Each of the different measures vary in cost and

technology and each has specific benefits.

CONCLUSIONS

• Flood controls are not meant to prevent a flood of any size. Thei

r design is meant to attenuate floodwaters and

floodwaves of a certain return period based on a cost

riskanalysis (oftentimes 100‐yr events).

• An increase in channel conveyance provides flood control

by allowing floodwaves to pass more freely downstream.

• Levees create a physical barrier to contain floodwaters and prote

ct floodplain from being flooded.

• Floodways provide floodwave attenuation through diversion

away from the river.

References:

• Pumnia.B.C, Pande. B. B. Lal, A. K. Jain, “Irrigation and Water Power Engineering”,