ENGINEERING HYDROLOGY

CHAPTERS1. INTRODUCTION2. PRECIPITATION3. INFILTRATION AND RUNOFF4. HYDROGRAPH ANALYSIS5. GROUND WATER

UNIT – 1 INTRODUCTION ENGINEERING HYDROLOGY - hydrology means

science in which we study about water, where “Hydro” means water and word “logy” means science, which is directly derived from Greek language

So Hydrology means the science of water. it is the science which deals with the occurrence, distribution and circulation of water in earth and earth’s atmosphere

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SURFACE WATER HYDROLOGY - is a field that encompasses all surface waters of the globe (surface flows, rivers, lakes, wetlands, estuaries, oceans, etc.). This a subset of the hydrologic cycle that does not include atmospheric, and ground waters

GROUND WATER HYDROLOGY - it is a field that includes underground water and water in depth in earth’s surface it is also called sub surface water as water in wells and under ground cavities

APPLICATION IN HYDROLOGY To find out maximum probable flood at proposed

sites e.g. Dams.

The expected flood flows over a spillway, at a highway Culvert, or in an urban storm drainage system can be known by this very subject.

It helps us to know the required reservoir capacity to assure adequate water for irrigation or municipal water supply in droughts condition.

Hydrology is used for city water supply design which is based on catchments area, amount of rainfall, dry period, storage capacity, runoff evaporation and transpiration.

Hydrology is an imperative tool in planning and building hydraulic structures.

HYDROLOGICAL CYCLE

HYDROLOGICAL PROCESS PRECIPITATION: In form of rain, drizzle, snow and hail EVAPORATION : Vaporization of water all sort of fluids INFILTRATION: Process of entering surface water in

to the ground. RUNOFF : after soil saturation remaining

precipitation on the surface is called runoff it has also two phase – a) surface runoff b) subsurface or ground

water runoff CONDENSATION : Formation of water in form of rain

drops

HYDOLOGICAL WATER DISTRIBUTIONCOMPONENT VOLU

ME 1000 KM^3

% OF TOTAL WATER

ATMOSPHERIC WATER 13 0.001SURFACE WATER•SALT WATER IN OCEANS•SALT WATER IN LAKES •FRESH WATER IN LAKES•FRESH WATER IN STREAM CHANNELS•FRESH WATER IN ICE CAPS & GLACERIES•WATER IN BIOMASS

13200001041251.252900050

97.20.0080.0090.00012.150.004

SUBSURFACE WATER• WATER ABSORBED BY SOIL•G/W WITH IN DEPTH OF 0.8 KM•G/W WITH IN DEPH OF 0.8 TO 4 KM

6742004200

0.0050.310.31

TOTAL (ROUNDED) 1360000 100

GLOBAL RESOURCE OF WATER The ocean covers 71% of the surface area of earth

and which have an average depth of 3.8 km hold as large as 97% of earth water while 2% is frozen in ice capes, the deep ground water accounts for 0.31%, these 99.31% of water is of no use to human being. The remaining 0.69% represents fresh water resources which the man has to deal

0.69% of water = 4.374*10^6 km^3 water

 

WATER BUDGET EQUATION A water budget is an accounting of the water stored in and water exchanged between

the atmosphere, land surface, and subsurface. Inflows minus outflows equal the change in storage over a specified period of time within a specified volume.

The quantities of water going through various individual path in the hydrological cycle in a given system can be described by continuity principle also known as water budget equation or hydrologic equation

Water Budget Equation: For a given area ,say a catchment, in an interval of time ∆t , the continuity equation for water in various phases is written as

V1 - V0 = ∆sMass inflow - Mass outflow = change in

mass storage

In applying this continuity equation to the paths of hydrological cycle involving change of state, the volumes considered are the equivalent volumes of water at a reference temperature P – R – G – E – T = ∆SP=precipitation ∆S = ∆Ss+ ∆Ssm+ ∆Sg

R=surface runoff ∆Ss =change in standing volume of water

G=net ground water flow ∆Ssm =change in ground water volume of unsaturated zone

E=evaporation ∆Sg =change in ground water volume of saturated zone

T=transpirationS=surface water storage

In terms of rainfall runoff relationship above equation can be expressed as

R = P – LWHERE R=Runoff, P=Precipitation, L=losses

PRACTICAL APPLICATION OF HYDROLOGY DESIGN OF HYDRAULIC STRUCTURE : The design of any structure

related to water such as spillway, dam, culvert, highway bridge and railway bridge etc may be considered to consist of these parts

a) hydrologic design-The term hydrology, as used in the design of storm drains, is the study and application of past rainfall records and runoff data to a method of predicting future rainfall and runoff.

these involved in the design of drainage and storm water facilities and consideration for natural passage of water resulting from storm events using instruments as rain gauge

b) hydraulic design-Hydraulic design provides the best suitable shape and section of the structure according to quantity of water estimated.

In the context of hydraulic design, hydrologic analysis provides estimates of flood magnitudes as a result of precipitation. These estimates consider processes in a watershed that transform precipitation to runoff and that transport water through the system to a project’s location.ex-dam hydraulic opening gates.

c) Structural design-the structural design ensures the stability and safety of chosen section against water pressure and other pressures.

Various design of hydraulic components require base foundation and systematic distribution of load and allocation, so this sort of design arranges such system in safe and systematic manner.

2) Municipal and industrial water supply-The availability of water is often the most important factor in locating the major industries & it has considerable effect on growth of municipalities.

3) Irrigation- The irrigation projects are directly based on hydrological variables such as infiltration,runoff and evaporation.

The rainfall data and hydrogarph fixes the storage capacity of any reservoir

4) Hydro power- Hydrological studies are essensial for planning and hydro power development.To determine the hydro power output we have to know first reliable flow duration curve,because minimum flow decides prime capacity of plant.

5) Flood control-flood contol measures become effective when they are associated with flood forecasts.

6) Erosion and sediment control- excessive erosion in catchment feeds sediment to runoff and tons of top fertile soil will be removed.

7) Navigation -

HYDROMETEOROLOGY HYDROMETEOROLOGY-In the hydrologic cycle water

is transported in the air over the land and below the surface of the earth, but meteorology deals with atmospheric portion of hydrological cycle, with the transportation of water in the air, for this reason meteorology always defined as science of atmosphere phenomenon.

  Hydrometeorology is defined as the study of the

atmospheric process which affects the water resources of the earth and are interest to hydraulic engineers.

 

CONSTITUENTS OF ATMOSPHERE CONSTITUENTS OF THE ATMOSPHERE-

The permanent constituents of atmosphere by volume are N2-78%, O2-21% and the rest inert gasses. the variable constituents may be in solid, liquid and gaseous form, it is invisible and referred as humidity and when it is in the form of minute drops, formed up on condensation, it is called fog or cloud.

VERTICAL STRUCTURE OF ATMOSPHERE VERTICAL STRUCTURE OF

ATMOSPHERE - The lowest layer which contains about 75% of mass and almost all the moisture and dust of the atmosphere is called troposphere

The top of the troposphere is called tropopause. The height of the tropopause varies from about 8 km over the poles and 16 km near the equator.

TROPOSPHERE

STRATOSPHERE

MESOSPHERE

IONOSPHEREALTITUDE

LAYERS OF EARTH ATMOSPHERE

LAPSE RATE

In general, a lapse rate is the negative of the rate of temperature change with altitude change, thus:

where γ is the lapse rate given in units of temperature divided by units of altitude, T = temperature, and z = altitude. Average lapse rate is about 2°C/1000ft

The lapse rate is defined as the rate of decrease with height for an atmospheric variable. The variable involved is temperature unless specified otherwise.

The terminology arises from the word lapse in the sense of a decrease or decline; thus, the lapse rate is the rate of decrease with height and not simply the rate of change. While most often applied to Earth's atmosphere.

Altit

ude

(m)

Temperature (C)

In the troposphere temperature decreases with elevation with an average rate of 6.5 degree C/km. this is known as lapse rate. Above the tropopause is the stratosphere.

Troposphere is the zone in which the weather phenomenon and atmospheric turbulence are most marked. Because of their effect on mass transport and & energy transport

Hydrologist are interested in the troposphere and the process that take place in it. However although the weather is considered as resulting from the interchange of the heat energy within the troposphere process above this layer giving ozone variations and ionization etc may influence the weather conditions to some extent

TROPOSPHERE - The troposphere is where all weather takes place and the thin zone between troposphere and stratosphere is called tropopause.

STRATOSPHERE - Above the troposphere is the stratosphere . The thin ozone layer in the upper stratosphere has a high concentration of ozone, a particularly reactive form of oxygen. This layer is primarily responsible for absorbing the ultraviolet radiation from the Sun

 MESOSPHERE & IONOSPHERE - Above the stratosphere is the mesosphere and above that is the ionosphere (or thermosphere), where many atoms are ionized (have gained or lost electrons so they have a net electrical charge).

The ionosphere is very thin, but it is where aurora(polar light) take place, and is also responsible for absorbing the most energetic photons from the Sun, and for reflecting radio waves, thereby making long-distance radio communication possible.

The structure of the ionosphere is strongly influenced by the charged particle wind from the Sun (solar wind), which is in turn governed by the level of Solar activity. One measure of the structure of the ionosphere is the free electron density, which is an indicator of the degree of ionization.

 

AIR MASS- A vast and deep body of the air in which the temperature and humidity characteristics are relatively homogeneous at any given elevation is called an air mass

There are 4 general air mass classifications categorized according to the source region.

polar latitudes P - located poleward of 60 degrees north and south

tropical latitudes T - located within about 25 degrees of the equator

continental c - located over large land masses--dry

marine m - located over the oceans----moist

   

AIR FRONT- The surface of contact between two adjacent air masses or between an air-mass and the surrounding atmosphere is called air front

There are four types of air fronts –

(a)cold front (b) warm front (c) Occluded front (d) stationary front 

COLD FRONTS -- A cold front is a transition zone where a cold air mass replaces a warm air mass. It is drawn as a blue line with blue triangles pointing in the direction the front is moving.

The cold air mass is more dense than the warm air mass it replaces. As the cold air moves into the warm air region it forces the warm air to rise very quickly, resulting in deeper clouds and heavier precipitation than we saw with a warm front

WARM FRONTS -- A warm front is a transition zone where a warm air mass replaces a cold air mass. It is drawn as a red line with red semicircles pointing in the direction the front is moving. 

The warm air is less dense than the cold air and easily slides up over top of the cold air mass gradually pushing it out of the way. Since warm air easily slides upward in the vicinity of a warm front, we generally do not see convective precipitation.

OCCLUDED FRONT -- An occluded front is a region where the faster moving cold front catches up to the slower moving warm front. As a result the coldest air alters the sloping warm front surface. These separate newer colder air with cool air north of the warm front.

STATIONARY FRONT -- A stationary front is when there is no movement of an actual temperature gradient, but there is still convergence and forced lifting. These front are drawn as alternating small segments of warm and cold front lines.

CYCLONE CYCLONE- A cyclone is a

more or less circular area of low atmosphere

winds blow spirally inwards in counter clockwise direction in the north hemisphere and in clockwise direction in southern hemisphere.

The pressure gradient exists towards the centre of the cyclone.

 .

 

ANTICYCLONE ANTICYCLONE- Anticyclone are

the high pressure areas generally elliptical in shape

It is covering very large area

the wind blow spirally outward in clockwise direction in the northern hemisphere and in counter clockwise direction in southern hemisphere

Pressure gradient exist towards the circumference of cyclone

CYCLONE & ANTICYCLONE

FORMATION OF PRECIPITATION

There are four basic conditions which are to be satisfied for the precipitation to occur

Accumulation of moisture of sufficient intensity to account for the observed rates of precipitation

Cooling of air to the dew point temperature to produce saturation condition

Condensation Growth of small water droplets to percepitable

size

FORMS OF PRECIPITATION DRIZZLE – It is the fine

sprinkle of very small uniform water drops with diameter between 0.1 to 0.5 mm. the drops are so small that they seems to float in air

  RAIN – it is the precipitation

of liquid water in which the drops are generally larger than 0.5 mm in size

 

GLAZE –The ice coating on land when rain or drizzle, freezes as it comes in contact with cold objects at the ground is called glaze.

FOG - Fog is a visible mass consisting of cloud water droplets or ice crystals suspended in the air at or near the Earth's surface.[Fog can be considered a type of low-lying cloud and is heavily influenced by nearby bodies of water, topography, and wind conditions. In turn, fog has affected many human activities, such as shipping, travel, and warfare.

SLEET – When rain drops are frozen while falling through a layer of subfreezing air near the earth surface it is called sleet.

HAIL – precipitation in the form of irregular balls of ice over 5 mm in diameter is called hail.

METEOROLOGICAL OBSERVATIONS METEOROLOGICAL OBSERVATIONS –

Periodical measurements on meteorological parameters such as atmospheric pressure , wind direction, temperature, humidity, precipitation and evaporation are the essential for the understanding of atmospheric phenomenon.

  Indian Meteorological Department(IMD)

generally take observations twice a day that is at 08:30 and at 17:30

CLIMATE AND WEATHER CLIMATE AND WEATHER – The state of

atmosphere with respect to the temperature, humidity, wind, cloudiness etc at any given time is generally referred to as the weather. The climate of a region is the aggregate of the weather.

 Weather is the condition of the atmosphere at a

particular place over a short period climate refers to the weather pattern of a place over

a long period, usually thirty years and more

In India the year may be divided into 4 weather seasons –

Cold weather season(Dec to Feb) Hot weather season(March to May) South west monsoon season(June to sep) Retreating south west monsoon

season(Oct to Nov)

Cold weather season(winter) -- occurring from December to early April. The year's coldest months are December and January, when temperatures average around 10–15 °C (50–59 °F) in the northwest; temperatures rise as one proceeds towards the equator, peaking around 20–25 °C (68–77 °F) in mainland India's southeast.

Hot weather season(summer) -- Summer or pre-monsoon season, lasting from April to June (April to July in northwestern India). In western and southern regions, the hottest month is April; for northern regions, May is the hottest month. Temperatures average around 32–45 °C in most of the areas.

SOUTH WEST MONSOON SEASON (RAINY) -- Monsoon or rainy season, lasting from June to September. The season is dominated by the humid southwest summer monsoon, which slowly sweeps across the country beginning in late May or early June. Monsoon rains begin to recede from North India at the beginning of October. South India typically receives more rainfall.

RETREATING SOUTH WEST MONSOON SEASON -- Post-monsoon season, lasting from October to December. In northwestern India, October and November are usually cloudless. Tamil Nadu receives most of its annual precipitation in the northeast monsoon season.

MONSOON

MONSOON is referred to a season in which wind system reverses completelyMonsoons are experienced in tropical areas roughly between 200 N to 200 S.Latest atmospheric conditions influence the monsoon wind are-

a) Differential heating and cooling of land and water in which low pressure is created in land areas where high pressure is created in sea zone in day time but this process is reversed in night.

Shift of the position of inter tropical convergence zone (ITCZ)

In summer the equitorial draft that is normally present at 5o north of the equator moves over the Ganga plane as a result monsoon trough(convergence zone) is created in monsoon season.

TIBETAN PLATEAU GETS INSTANTLY HEATED

Tibetan plateau gets instantly heated so a high pressure zone is created above Tibetan plateau up to 9 km above mean sea level.

SOUTHERN OSCILLATIONSouthern oscillation Between Indian ocean and pacific ocean due to pressure difference

Monsoon start to end

RETREATING MONSOON