4. irrigation engineering and hydraulic structures by …...1. hydrology by h.m. raghunath 2. a text...

Course Title: Hydrology, Irrigation and Flood Management

Course Code: CE 4163

Credit: 3

Reference Books

1. Hydrology by H.M. Raghunath

2. A Text Book of Hydrology - P. Jaya Rami Reddy

3. Engineering Hydrology - K. Subramanya

4. Irrigation Engineering and Hydraulic Structures by S.K. Garg

5. Irrigation: Theory and Practice by A. M. Michael

6. Irrigation and Water Power Engineering by B.C. Punmia

7. Irrigation, Water Resources and Water Power Engineering by P.N. Modi

Course Content

• Hydrologic cycle; hydrologic measurement: precipitation, evaporation and stream flow; hydrographs

• Plant-soil-water relationship; consumptive use and estimation of irrigation water requirements methods of irrigation; quality of irrigation water; problems of irrigated land

• Flood and its management

Hydrology, Irrigation and Flood Management (CE 4163)

Chapter: Introduction

Reference Book: Hydrology by H.M. Raghunath 2nd Edition

Lecture prepared by

Md Nuruzzaman

Lecturer, Department of Civil Engineering

Bangladesh Army University of Engineering and Technology (BAUET)

Introduction

Hydrology

Hydrology is the science, which deals with the occurrence,distribution and disposal of water on the planet earth; it isthe science which deals with the various phases of thehydrologic cycle.

Introduction

Scope of Hydrology

The study of hydrology helps us to know

(i) the maximum probable flood that may occur at a givensite and its frequency;

(ii) the water yield from a basin—its occurrence, quantityand frequency, etc;

(iii) the ground water development

(iv)the maximum intensity of storm and its frequency for thedesign of a drainage project in the area.

Introduction

Hydrologic Data

The basic hydrological data required are:

(i) Climatological data

(ii) Hydrometeorological data like temperature, wind velocity, humidity,

etc.

(iii) Precipitation records

(iv) Stream-flow records

(v) Seasonal fluctuation of ground water table or piezometric heads

(vi) Evaporation data

(vii) Cropping pattern, crops and their consumptive use

(viii) Water quality data of surface streams and ground water

(ix) Geomorphologic studies of the basin, like area, shape and slope of

the basin, (x) Hydrometeorological characteristics of basin:

Introduction

Water Cycle

Introduction


Chapter: Precipitation


(Chapter 2)

Lecture prepared by

Md Nuruzzaman


Bangladesh Army University of Engineering and Technology

(BAUET)

PrecipitationForms of Precipitation

1. Drizzle

2. Rain — the condensed water vapour of the

atmosphere falling in drops (>0.5 mm, maximum

size 6 mm) from the clouds.

3. Glaze.

4. Sleet Snow

5. Snow flakes

6. Hail

7. Dew

8. Frost

9. Fog

10.Mist — a very thin fog

Precipitation

Types of Precipitation

1. Thermal convection (convectional precipitation)

2. Conflict between two air masses (frontal

precipitation)

3. Orographic lifting (orographic precipitation)

4. Cyclonic (cyclonic precipitation)

Precipitation

Measurement of Precipitation

1. Non-Recording Rain Gauge

2. Recording Rain Gauge

There are three types of recording rain gauges

1. Tipping bucket gauge

2. Weighing gauge

3. Float gauge

Precipitation

Precipitation

Optimum Rain-gauge Network Design

Precipitation

Precipitation

GRAPHICAL REPRESENTATION OF RAINFALL

HyetographA hyetograph is a bar graph showing the intensity of rainfall with respect totime and is useful in determining the maximum intensities of rainfall during aparticular storm as is required in land drainage and design of culverts.

Precipitation

GRAPHICAL REPRESENTATION OF RAINFALL

Mass curveA mass curve of rainfall (or precipitation) is a plot of cumulative depth ofrainfall against time. From the mass curve, the total depth of rainfall andintensity of rainfall at any instant of time can be found

Precipitation

ANALYSIS OF RAINFALL DATA

Precipitation


Recurrence intervalRecurrence interval is the average number of years during which astorm of given magnitude (maximum depth or intensity) may beexpected to occur once, i.e.,may be equalled or exceeded.

FrequencyFrequency F is the percentage of years during which a storm ofgiven magnitude may be equalled or exceeded.

Precipitation


Moving Averages Curve

In order to depict a general trend in the rainfall pattern,the averages of three or five consecutive years arefound out progressively by moving the group averaged,one year at a time known as moving average curve

Precipitation

Probable Maximum Precipitation (PMP)

The probable maximum precipitation (PMP) for a givenregion is the precipitation resulting from the mostcritical meteorogical combinations that are consideredprobable of occurrence.

Precipitation


Chapter: Evaporation


(Chapter 2 – Water Losses)

Lecture prepared by

Md Nuruzzaman



(BAUET)

Evaporation

Water Losses(i) Interception loss-due to surface vegetation, i.e., held by

plant leaves.

(ii) Evaporation:

(a) from water surface, i.e., reservoirs, lakes, ponds, river

channels, etc.

(b) from soil surface, appreciably when the ground water table

is very near the soil surface.

(iii) Transpiration—from plant leaves.

(iv) Evapotranspiration for consumptive use—from irrigated

or cropped land.

(v) Infiltration—into the soil at the ground surface.

(vi) Watershed leakage—ground water movement from one

basin to another or into the sea.

Evaporation from water surfaces (Lake evaporation)

1. Storage reservoirs of more depth and less surface

area, i.e., by choosing a cross section of the reservoir

like a deep gorge

2. By growing tall trees like Causerina on the windward

side of the reservoirs to act as wind breakers.

3. By spraying certain chemicals or fatty acids and

formation of films.

4. By allowing flow of water, temperature is reduced and

evaporation is reduced; i.e., by designing the outlet

works so that the warmer surface water can be

released.

5. By removing the water loving weeds and plants like

Phreatophytes from the periphery of the reservoir.

Evaporation

Methods of Estimating Lake Evaporation

(i) The storage equation

P + I ± Og = E + O ± S ...(3.4)

where P = Precipitation, I = surface inflow, Og =

subsurface inflow or outflow, E = evaporation, O

= surface outflow, S = change in surface water

storage

(ii) Auxiliary pans like land pans, floating pans,

Colarado sunken pans, etc.

(iii) Evaporation formula

Evaporation

Methods of Estimating Lake Evaporation

(iv) Humidity and wind velocity gradients

(v) The energy budget—this method involves too

many hydrometeorological factors (variables) with

too much sophisticated instrumentation and hence

it is a specialist approach

(vi) The water budget—similar to (i)

(vii) Combination of aerodynamic and energy

balance equations—Penman’s equation (involves

too many variables)

Evaporation

Evaporation

Evaporation Pans

(i) Floating pans

(ii)Land pan

(iii)Colarado sunken pan

Land pan

EVAPORATION PANS

Evaporation

Pan coefficient

The small volume of water in the metallic pan is greatly

affected by temperature fluctuations in the air or by

solar radiations in contrast with large bodies of water

(in the reservoir) with little temperature fluctuations.

Thus the pan evaporation data have to be corrected to

obtain the actual evaporation from water surfaces of

lakes and reservoirs, i.e., by multiplying by a

coefficient called pan coefficient and is defined as Pan

coefficient.

Evaporation

Measures to Reduce Lake Evaporation

1. Storage reservoirs of more depth and less surface

area, i.e., by choosing a cross section of the reservoir

like a deep gorge.

2. By growing tall trees like Causerina on the windward

side of the reservoirs to act as wind breakers.

3. By spraying certain chemicals or fatty acids and

formation of films.

4. By allowing flow of water, temperature is reduced and

evaporation is reduced; i.e., by designing the outlet

works so that the warmer surface water can be

released.

5. By removing the water loving weeds and plants like

Phreatophytes from the periphery of the reservoir.

Evaporation

6. By straightening the stream-channels the exposed

area of the water surface (along the length) is

reduced and hence evaporation is reduced.

7. By providing mechanical coverings like thin

polythene sheets to small agricultural ponds and

lakes.

8. By developing undergound reservoirs, since the

evaporation from a ground water table is very much

less than the evaporation from a water surface.

9. If the reservoir is surrounded by huge trees and

forest, the evaporation loss will be less due to cooler

environment.

Measures to Reduce Lake Evaporation

Evaporation

Soil Evaporation

The evaporation from a wet soil surface immediately

after rain or escape of water molecules with more

resistance when the water table lies within a meter

from the ground is called soil evaporation.

Evaporation opportunity

Soil evaporation is expressed as a percentage of

evaporation from free water surface, which is called

evaporation opportunity.

Evaporation

Evapotranspiration

Evapotranspiration (Et) or consumptive use (U) is

the total water lost from a cropped (or irrigated) land

due to evaporation from the soil and transpiration by

the plants or used by the plants in building up of

plant tissue.

Evaporation

Estimation of Evapotranspiration

(i) Tanks and lysimeter experiments

(ii) Field experimental plots

(iii) Installation of sunken (colarado) tanks

(iv) Evapotranspiration equations as developed by

Lowry-Johnson, Penman, Thornthwaite, Blaney-

Criddle, etc.

(v) Evaporation index method, i.e., from pan

evaporation data as developed by Hargreaves and

Christiansen.

Evaporation

EvaporationBlaney-Criddle Formula

Example 3.2 Determine the evapotranspiration andirrigation requirement for wheat, if the water applicationefficiency is 65% and the consumptive use coefficient forthe growing season is 0.8 from the following data :

Evaporation

Evaporation

CE 4127

Chapter: Hydrographs


(Chapter 5)

Lecture prepared by

Md Nuruzzaman



(BAUET)

HYDROGRAPHS

Hydrograph

A hydrograph is a graph showing discharge (i.e.,stream flow at the concentration point) versustime.

HYDROGRAPHS

HYDROGRAPHS

Recession curveAfter the peak of hydrograph, GWT declines and thehydrograph again goes on depleting in the exponentialform called the ground water depletion curve or therecession curve.

HYDROGRAPHSBaseflow Separation


(i) Simply by drawing a line AC tangential to both the

limbs at their lower portion.

(ii) Extending the recession curve existing prior to the

occurrence of the storm up to the point D directly

under the peak of the hydrograph and then drawing a

straight line DE, where E is a point on the hydrograph

N days after the peak, and N (in days) is given by

N = 0.83 A0.2

where A = area of the drainage basin, km2


(iii) Simply by drawing a straight line AE, from the

point of rise to the point E, on the hydrograph, N days

after the peak.

(iv) Construct a line AFG by projecting backwards the

ground water recession curve after the storm, to a

point F directly under the inflection point of the falling

limb and sketch an arbitrary rising line from the point

of rise of the hydrograph to connect with the projected

base flow recession.

HYDROGRAPHS

Unit Hydrograph

The hydrograph of direct surface discharge measured

at the outlet of drainage area, which produces a unit

depth of direct runoff (i.e., a Pnet of 1 cm over the

entire area of the catchment) resulting from a unit

storm of specified duration (called unit period) is

called a unit hydrograph of that duration.

HYDROGRAPHS

HYDROGRAPHS

Elements of unit hydrographBase width (T)—The period of direct surface runoff (due to a unit storm) of the unit hydrograph is called the time base or the base width.

Unit storm—The storm of unit duration (i.e., duration of the unit hydrograph) regardless of its intensity is called unit storm.

HYDROGRAPHS

Elements of unit hydrographLag time (tp)—The time from the centre of a unit storm tothe peak discharge of the corresponding unit hydrographis called lag time.

Recession time (Tr)—The duration of the direct surfacerunoff after the end of the excess or net rainfall, is calledrecession time in hydrograph analysis.

HYDROGRAPHS

Assumptions of Unit Hydrograph

(i) The net rainfall is of uniform intensity within its

duration (i.e., unit period).

(ii) The net rainfall uniformly occurs over the entire

area of the drainage basin.

(iii) For a given drainage basin, the base period of the

hydrographs of direct runoff corresponding to net

rains of different intensities but same unit duration, is

constant.

(iv) The ordinates of direct runoff hydrographs due to

net rains of different intensities (but same unit

duration) are proportional.

(v) A unit hydrograph reflects all the physical

characteristics of the basin.

HYDROGRAPHS

Propositions of the Unit Hydrograph

(i) Same runoff duration. For all unit storms of differentintensities, the period of surface runoff (i.e., time base,base width or base period) is approximately the same,although they produce different runoff volumes.

(ii) Proportional ordinates. For unit storms of differentintensities, the ordinates of the hydrograph at any giventime, are in the same proportion.

HYDROGRAPHSPropositions of the Unit Hydrograph

(iii) Principle of superposition. If there is a continuousstorm and/or isolated storms of uniform intensity net rain,they may be divided into unit storms and hydrographs ofrunoff for each storm obtained, and the ordinates addedwith the appropriate time lag to get the combinedHydrograph.

(iv) Same distribution percentages. If the total period ofsurface runoff (i.e., time base or base width) is divided intoequal time intervals the percentage of surface runoff thatoccurs during each of these periods will be same for allunit storms of different intensities.

HYDROGRAPHSAPPLICATION OF UNIT HYDROGRAPHThe application of unit hydrograph consists of two aspects:(i) From a unit hydrograph of a known duration toobtain a unit hydrograph of the desired duration,either by the S-curve method or by the principle ofsuperposition.(ii) From the unit hydrograph so derived, to obtain theflood hydrograph corresponding to a single storm ormultiple storms.

HYDROGRAPHS

Limitation of the Unit Hydrograph

1. The design storm continuing for several unit periodsmay not have the same areal distribution for each timeincrement.

2. Storm movements also affect the proportions of theunit hydrograph if the basin is large. Hence, the unithydrograph can not be applied for basins larger than5000 km2.

HYDROGRAPHS

Synthetic Unit HydrographsThere are many drainage basins (catchments) for which nostreamflow records are available and unit hydrographs maybe required for such basins. In such cases, hydrographsmay be synthesised directly from other catchments, whichare hydrologically and meteorologically homogeneous, orindirectly from other catchments through the applicationof empirical relationship. This is known as Synthetic unithydrograph.

HYDROGRAPHS

Example 5.1 (a) The runoff data at a stream gaugingstation for a flood are given below. The drainage area is 40km2. The duration of rainfall is 3 hours. Derive the 3-hourunit hydrograph for the basin and plot the same.

HYDROGRAPHSExample 5.1 (a)

HYDROGRAPHSExample 5.1 (a)- Solution

2 5 8 11 14 17 20 23 2 5 8 11 14 17 20 23

50 47 46 45 45 45 46 48 50 53 54 57 60 55 51 50

HYDROGRAPHSExample 5.1 (a)- Solution

HYDROGRAPHS

HYDROGRAPHS

S-CURVE

S-curve or the summation curve is the hydrograph

of direct surface discharge that would result from a

continuous succession of unit storms producing 1

cm in certain duration of the unit graph (tr–hr).

HYDROGRAPHS

Example 5.4 The ordinates of a 4-hour unit hydrographfor a particular basin are given below. Derive theordinates of (i) the S-curve hydrograph, and (ii) the 2hour unit hydrograph, and plot them, area of the basin is630 km2.

HYDROGRAPHS

HYDROGRAPHSExample 5.11 The design storm of a water shed hasthe depths of rainfall of 4.9 and 3.9 cm for theconsecutive 1-hr periods. The 1-hr UG can beapproximated by a triangle of base 6 hr with a peak of50 cumec occurring after 2 hr from the beginning.Compute the flood hydrograph assuming an averageloss rate of 9 mm/hr and constant base flow of 10cumec. What is the area of watershed and itscoefficient of runoff?

HYDROGRAPHSExample 5.11 (Solution)

HYDROGRAPHSExample 5.13 The successive three-hourly ordinates ofa 6-hr UG for a particular basin are 0, 15, 36, 30, 17.5,8.5, 3, 0 cumec, respectively. The flood peak observeddue to a 6-hr storm was 150 cumec. Assuming aconstant base flow of 6 cumec and an average stormloss of 6 mm/hr, determine the depth of storm rainfalland the streamflow at successive 3 hr interval.

HYDROGRAPHSExample 5.13 (Solution)


Chapter: Stream Gauging and Runoff


(Chapter 4 – Runoff and Chapter 6 – Stream Gauging)

Lecture prepared by

Md Nuruzzaman



(BAUET)

Runoff

Overland flowWhen a storm occurs, a portion of rainfall infiltrates intothe ground and some portion may evaporate. The restflows as a thin sheet of water over the land surface whichis termed as overland flow.CatchmentThe entire area of a river basin whose surface runoff (dueto a storm) drains into the river in the basin is consideredas a hydrologic unit and is called drainage basin,watershed or catchment area of the river flowing.Drainage divideThe boundary line, along a topographic ridge, separatingtwo adjacent drainage basins is called drainage divide.

Runoff

Concentration pointThe single point or location at which all surface drainagefrom a basin comes together or concentrates as outflowfrom the basin in the stream channel is calledconcentration point or measuring point.

Concentration timeThe time required for the rain falling at the most distantpoint in a drainage area (i.e., on the fringe of thecatchment) to reach the concentration point is called theconcentration time.

FACTORS AFFECTING RUNOFF

RUNOFF

Runoff Estimation

(i) Empirical formulae, curves and tables

(ii) Infiltration method

(iii) Rational method

(iv) Overland flow hydrograph

(v) Unit hydrograph method

(vi) Coaxial Graphical Correlation and API

RUNOFF

Empirical formulae, curves and tables

RUNOFF

Rational Method

A rational approach is to obtain the yield of a catchmentby assuming a suitable runoff coefficient.

Yield = CAP

where A = area of catchment

P = precipitation

C = runoff coefficient

RUNOFF

RUNOFF

STREAM GAUGING

Stream gauging

Measuring the discharge of the stream drainingis termed as stream gauging.

Gauging station

A gauging station is the place or section on astream where discharge measurements aremade.

STREAM GAUGING

METHODS OF MEASURING STREAM FLOW

1. Venturiflumes or standing wave flumes

2. Weirs or anticuts

3. Slope-area method

4. Contracted area methods

5. Sluiceways, spillways and power conduits

6. Salt-concentration method

7. Area-velocity methods❖ Float

❖ Rod

❖ Current meter

STREAM GAUGING

Velocity Profiles

STREAM GAUGING

STREAM GAUGING

Current meterThe current meter is an instrument, which has a rotatingelement which when placed in flowing water, the speedof revolutions has a definite relation with the velocity offlow past the element.

STREAM GAUGING

Rating of the Current MeterThe relationship between the revolutions per second (N,rps) of the meter and the velocity of flow past the meter(v, m/sec) has to be first established, or if the ratingequation is given by the maker, it has to be verified. Thisprocess of calibration of the meter is called rating of thecurrent meter.

The rating equation is of the formv = aN + b

where a and b are constants (determined from rating ofthe current meter).

STREAM GAUGING

Rating of the Current Meter (Method)

STREAM GAUGINGSELECTION OF SITE FOR A STREAM GAUGING STATION(i) The section should be straight and uniform for a length of about10 to 20 times the width of the stream.

(ii) The bed and banks of the stream should be firm and stable so asto ensure consistency of area-discharge relationship, i.e., the crosssection should not be subjected to change by silting or scouring,during different stages of flow; a smooth rock, shingle or clay bed isfavourable, while a fine sandy bed is unfavourable.

(iii) The bed and banks should be free from vegetal growth,boulders or other obstructions like bridge piers, etc.

(iv) There should be no larger overflow section at flood stage. Thebest cross section is one with V-shape, so that there is sufficientdepth for immersing the current meter without being affected bythe bed roughness of the stream.

STREAM GAUGING

SELECTION OF SITE FOR A STREAM GAUGING STATION(v) The part of the reach having the most regular transversesection and steady flow with the current normal to the meteringsection and velocities in the range of 0.3–1.2 m/sec should beselected.

(vi) To ensure consistency between stage and discharge, thereshould be a good control section far downstream of the gaugingsite. This control may be in the form of steep rapids, large rockyboulders, restricted passages, crest of weirs or anticuts etc.

(vii) The sites above the confluence of rivers are best avoided if theflow is affected by back water conditions due to the varyingdischarges in the tributaries.

(viii) The stream gauging station should be easily accessible.

STREAM GAUGING

Problem

Following Velocities Were Recorded In A Stream

With A Current Meter. Find The Discharge Per Unit

Width Of Stream Near The Point Of Measurement

By Two-point Method. Depth Of Flow At The Point

Was 3.5 Meter

DEPTH ABOVE BED (M) 0 0.7 1.4 2.1 2.8

VELOCITY (M/SEC) 0 2.62 2.94 3.16 3.28


Chapter: Flood and its management


Lecture prepared by

Md Nuruzzaman



(BAUET)

Flood routing

Flood routing is the process of determining the reservoirstage, storage volume of the outflow hydrographcorresponding to a known hydrograph of inflow into thereservoir.

Design flood

The maximum flood that any structure can safely pass iscalled the ‘design flood’ and is selected after considerationof economic and hydrologic factors.

FLOOD ESTIMATION &ROUTING

Standard Project Flood (SPF)

This is the estimate of the flood likely to occur from themost severe combination of the meteorological andhydrological conditions, which are reasonablycharacteristic of the drainage basin being considered, butexcluding extremely rare combination.

Maximum Probable Flood (MPF)

This differs from the SPF in that it includes the extremelyrare and catastrophic floods and is usually confined tospillway design of very high dams. The SPF is usuallyaround 80% of the MPF for the basin.


ESTIMATION OF PEAK FLOOD

(i) Physical indications of past floods—flood marks andlocal enquiry

(ii) Empirical formulae and curves

(iii) Concentration time method

(iv) Overland flow hydrograph

(v) Rational method

(vi) Unit hydrograph

(vii) Flood frequency studies


ESTIMATION OF PEAK FLOOD (Empirical formula)


ESTIMATION OF PEAK FLOOD


Return period

The return period or recurrence interval (T) is the averagenumber of years during which a flood of given magnitudewill be equalled or exceeded once.


Return periodThe return period or recurrence interval (T) is the averagenumber of years during which a flood of given magnitudewill be equalled or exceeded once.

METHODS OF FLOOD CONTROL(i) by confining the flow between high banks byconstructing levees (flood banks) , dykes, or flood walls.(ii) by channel improvement by cutting, straightening ordeepening and following river training works.(iii) by diversion of a portion of the flood through bypassesor flood ways. In some cases a fuse plug levee is provided.It is a low section of levee, which when once over topped,will

FLOOD ESTIMATION &ROUTINGMETHODS OF FLOOD CONTROL

(iv) by providing a temporary storage of the peak floods byconstructing upstream reservoirs and retarding basins(detention basins).

(v) by adopting soil conservation measures (landmanagement) in the catchment area.

(vi) by temporary and permanent evacuation of the floodplain, and flood plain zoning by enacting legislation.

(vii) by flood proofing of specific properties by constructing aring levee or flood wall around the property.

(viii) by setting up flood forecasting—short term, long term,rhythm signals and radar, and warning centres at vulnerableareas.


METHODS OF FLOOD CONTROL

❖ Flood Control by Reservoirs

❖ Retarding Basins

❖ Construction of Levees

❖ Channel Improvement


Steps involved in the design of a flood control

project

(i) Determination of the project design flood and flood

characteristics of the basin.

(ii) Assessment of flood damages for different flood

stages and socio-economic problems involved.

(iii) Field survey of flood-prone areas and defining

areas to be protected.

(iv) Determination of the possible methods of flood

protection.

(v) If flood control is feasible by construction of

reservoirs and levees, selection of suitable sites and

their physical characteristics.



project

(vi) Design details of the flood control structures

proposed and preparation of cost estimates.

(vii) For a combination of flood control measures,

selection of a flood peak that offers the desired

protection at minimum cost.

(viii) Making the cost-benefit analysis of the project

and its economic justification.

(ix) Development of a scientific flood forecasting and

warning system.



project

(x) Preparation of a detailed project report,

indicating the alternative flood control measures

explored, the combination finally selected with the

economic justification for the same, and the degree

of flood protection offered.

(xi) Construction of the project proposed after

approval and sanction of the budget from

the authority concerned.

4. irrigation engineering and hydraulic structures by …...1. hydrology by h.m. raghunath 2. a text...

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