Unit - 2

Run off

2.1 Introduction:

• Runoff is a part of the precipitation which appears in a drainage channel as a surface flow either in a perennial or seasonal form. It is also called as "yield of the catchment" i.e. This water can be used for many economic activities such as forming, manufacturing, transport, fishing etc.
• This surface flow is also called as "yield of catchment". Used to produce hydel power so it is rightly known as "Yield of the catchment'.
• Runoff is expressed in terms of volume of water in a given period of time.

Key Takeaways:

Runoff is a part of the precipitation which appears in a drainage channel as a surface flow either in a perennial or seasonal form. It is also called as "yield of the catchment" i.e. This water can be used for many economic activities such as forming, manufacturing, transport, fishing etc.

2.2 Factors affecting runoff:

Following are the sources and components of runoff from any given catchment area.

Direct rainfall on the stream channel:

• When the rainfall is observed over any region some part of it directly falls over the stream water and adds some quantity of water in the flow.
• Considering other sources and their contribution this quantity is very small and so it can be neglected.

Surface runoff:

• The water received by the land surface flows in different directions to meet the main stream. It is known as surface runoff.
• In the area where such sub-streams are more the number to meet the main stream the huge quantity of water reaches the main stream. It is also called as Quick Flow. So larger the number of sub-streams more will be the surface runoff.

Interflows:

• Some part of the water which has been infiltrated does not reach the final ground water-table but due to some local obstruction of a layer of non-porous rock runs parallel to the surface flow and comes out whenever it gets a chance. This addition of water to the surface flow is called as Interflows.
• Depending upon the time taken by the interflow to meet the surface flow we can classify them into two as for a short period of time it is called as Prompt Interflow and for a long period of time it is called as Delayed Interflow.

Ground water flow:

• The water which can infiltrate without any obstruction of a layer of non-porous rock reaches the final layer of impervious rock and gets stored in the underground pool of water.
• It like the surface runoff also starts flowing along the slopes and comes up to join the surface flow in a form of spring. As the underground water has to take a longer route it needs more period of time to came up; may be more than a month or two after the rainfall. So, this water comes up on the surface, when there is no precipitation.

Regeneration:

• When any crop is irrigated a part of it only is used by the plants for their growth the remaining part of the water runs parallel to the surface water after the infiltration and meets the adjoining stream. (Some of the water which can go steel deep adds the level of the ground water.)
• The water which meets the surface flow of the stream is called as 'Regeneration'.

2.3 Rainfall-runoff relationships:

• When the rainwater reaches the surface some of the water gets infiltrated.
• This amount of water getting infiltrated depends upon various factors as the slope of terrain the type of surface rock type of top and sub-soil temperature condition etc.
• Once the sub-soil gets water-saturated the process of infiltration stops and the water received from the rainfall, starts flowing in the downward direction of the catchment.
• It is called as the runoff. So, the ratio between the actual rainfall and runoff depends upon the factors mentioned above and also the amount i.e., intensity and duration of rainfall i.e., the maximum water gets infiltrated from the 1st rainfall of the region as the soil is dry. Once it gets the water the process of infiltration is reduced and runoff increases.

Key Takeaways:

The maximum water gets infiltrated from the 1st rainfall of the region as the soil is dry. Once it gets the water the process of infiltration is reduced and runoff increases.

2.4 Empirical techniques to determine runoff:

• Three different methods are used to estimate the runoff from any catchment area, i.e., from a river basis.

A. Standard Table Method:

• These table are prepared by having the field data from different catchment areas. Having different characteristics (different climatic conditions relief conditions, geological conditions etc.) and the relation between the amount of precipitation and the amount of runoff is established. For explaining the relationship, following tables are used.

Binnie's Table:

• By having the field observations of the rivers in Madhya Pradesh, Sir Binnie has prepared Table wherein the runoff is expressed in percentage to the total amount of precipitation.
• Table: Binnie’s Table

• All the catchment areas receive a moderate rainfall i.e., up to 1100 mm. In a year. So, this Table is not applicable for the areas receiving heavy rainfall.

Rainfall -Runoff Correlation:

• The runoff can be corrected to the rainfall, by using the following equation.

Where, K is constant = KX (annual rainfall).

• The value of K for different types of catchment areas is shown in Table
• Table: Value of K in different catchment areas

Strange Coefficient:

• It has been observed that the rainfall-runoff relationship depends mainly on the nature of the catchment area and also on the soil condition. i.e., Thin-thick, fine-broad etc.
• The data has been collected from the old state the Bombay to prove the rainfall-runoff relationship.
• On the basis of the conditions of the catchment areas they have been classified into three groups. Such as dry, damp and wet. For the correlation daily average rainfall in mm has been used [Refer Table]
• Table: Strange's Coefficient Table

Barlow's Method:

• In this method the runoff is calculated in percentage to the total rainfall received in a small catchment area having variations in the type of slope, type of land use, type of catchment area and percentage of runoff.
• Table: Coefficient of Rainfall runoff in a Small Catchment area (On the basis of land use)

• The percentages depending up on the type of land and level utilization, can be further modified, by multiplying them by a coefficient which depends upon the nature of rainfall.
• Table: The Coefficient Depending Upon the Rainfall conditions in a catchment area

Where in, Light Rainfall – up to 10 mm

Average Rainfall - 10 mm to 50 mm

Heavy Rainfall- 50 mm to 150 mm and above

B: Empirical Formulae

• These formulae are based on the actual field data wherein following parameters are considered,
• Catchment area [Area size]
• Annual Rainfall [Total Receipt of Rainfall]
• Nature of Catchment [ Relief type of Rocks no. Of streams etc.]
• Average temperature [Rate of evaporation]
• Following formulae are used to calculate the rainfall-runoff relationship.

(1) Sir Inglis Formula

• He has suggested two formulae, based on the field observations of two different geographical locations, in the old Bombay State, they are as follows-

• Ghat fed Catchments [i.e., The western slopes of western ghats receiving heavy rains from the south west monsoon winds]

R = 0.85 P- 30.5

• Plain areas in water-shadow regions [i.e., The Eastern slopes of Western Ghats, receiving less rainfall as they are on the Lee-ward side of the Western Ghats).

R = PX(P-17.8)/ 2.54

Where R= Runoff, (in mm)

P= Precipitation (in mm)

• If a catchment area has some parts in Ghat region and some in plain rain shadow region use both the formulae separately to calculate the runoff and then add these two figures to get the total runoff of that catchment area.

(2) Khosla's Formula

• In his formula, Dr. Khosla has added one more aspect of temperature to see the effect of evaporation on the total water received through the precipitation. The formulae read as,

R = P-5T

Where R = Runoff (in mm);

P = Average Precipitation (in mm)

T = Average temperature (in 0°)

• Using the above formula for each month, monthly average runoff is calculated and all the values of twelve months are added together to get annual runoff.
• If the average temperature is less than 10°C, the formula is modified.

(3) Rational Formula

• This formula is useful to calculate the runoff of small catchment.
• If the catchment is large, for the accuracy the catchment is divided into small sub sections (based on the topography land use, type of soil etc.),
• The results of the sub-sections are added finally to get the total run-off. So, this method is called as a Rational method.
• The formula reads as,

R = C.A.P.

Where, R = Run off (in million m³),

C = Constant

A = Total catchment areas (in km³)

P = Average Annual precipitation (in meters)

• The value of 'C' varies as the surface area type of land use and type of soil changes. The following table will give you, the changes in the value of 'C' as the type of catchment changes.
• Table: Value of "C

Deducting Abstractions from precipitation:

• The runoff of any catchment area can be calculated by deducting all the abstractions i.e., rates evaporation, surface relation and infiltration.
• The only problem, this method has is that, it is very difficult to collect the correct data of these losses of water through the abstraction.

Rainfall-Runoff correlation: 

• This relationship between the rainfall and runoff can be expressed by using the last few decades’ record of rain fall and runoff in a given catchment, by using a graph based on the following equation.

R = m (P-x)

Where in R = Run off (in million m²)

m= A constant.

P = rain fall (in man)

x= Interception of a straight line showing the relationship on x axis

• The relation between run off rainfall will be applicable only for the catchment area from where the past decade's data has been used.
• This relation also does not remain same due to human interference in the natural conditions of the catchment and it is needed to be modified, every year. This show, rough estimates of the relationship.
• For the rainfall runoff analysis computer models have been introduced.

Fig: 1

2.5 Runoff hydrograph-Introduction:

• The Direct Runoff Hydrograph (DRH) can be defined as "It is an effective rainfall occurring uniformly over a water-shed at a uniform rate over a unit period of time". So, it is also known as unit Hydrograph.
• The shape of the hydrograph is influenced by the factors, such as climate, i.e., temperature conditions and relief and geological factor of the basin.

Uses of Hydrograph:

• The hydrographs have the following uses:

1. They help to know the magnitude of the extreme rainfall.
2. They help to decide the design of the hydraulic structure.
3. They are useful in extension of flood flow records (based on rainfall records)
4. They are very useful for the development of flood forecasting and flood warning system to reduce the losses due to floods.

Key Takeaways:

The Direct Runoff Hydrograph (DRH) can be defined as "It is an effective rainfall occurring uniformly over a water-shed at a uniform rate over a unit period of time". So, it is also known as unit Hydrograph.

2.6 Factors affecting flood hydrograph:

• Factors affecting flood hydrograph are as follows:

Shape

• A round formed drainage basin results in speedy drainage while an extended drainage basin will take time for the water to attain the river.

Topography & relief

• The steeper the basin the extra speedy it drains. Indented landscapes will acquire water and decrease runoff rates, decreasing the quantity of water attaining the river channel.

Heavy Storms

• Runoff will boom after soil discipline capability is met because of this that water will attain the channel quicker.

Lengthy rainfall

• This results in the floor being saturated and runoff will boom because of this that water will attain the channel extra speedy as soon as soil capability has been reached.

Snowfall

• Until the snow melts, the water is held in garage however while the snow melts this will cause flooding.

Vegetation

• This can lessen discharge because it intercepts precipitation. Roots of flowers also can absorb water that is going into the soil. Seasonally, with inside the UK the flora will lessen discharge with inside the summer time season while with inside the wintry weather it'll have much less of an effect because of much less foliage being gift on trees.

Rock type

• The underlying geology varies inside drainage basins and may be permeable (permitting water through) or impermeable (now no longer permitting water through). Impermeable rocks inspire more quantities of floor runoff and an extra speedy boom in discharge than permeable rocks.

2.7 Components of Hydrograph:

• It has three components as shown below

1. The Rising curve
2. The crest (The Peak point)
3. The falling curve.

• Hydrograph is a graph that shows the discharge i.e., the rate of flow of surface water versus the time past a specific point in a stream or a river.
• The rate of flow is expressed in cubic meters per second (cms).
• As shown in Fig the hydrograph has rising curve / rising limb peak discharge / the crest and the falling curve/falling limb.

Fig. 2: A single peaked hydrograph

1. The rising curve/Rising limb: It is also called as concentration curve. It reflects the prolonged rise in the discharge, from the catchment; with the result of the rainfall.
2. The peak discharge / The crest: It is 42 the highest point of the Hydrograph to indicate the maximum discharge per sec.
3. The falling curve / Falling limb: It extends from the crest to indicate the end of the storm flow. This curve represents the withdrawal of water from the storage built up in the basin during the earlier phases of the hydrograph.

Key Takeaways:

It has three components as shown below

1. The Rising curve
2. The crest (The Peak point)
3. The falling curve.

2.8 Base flow Separation:

• The base flow is a portion of the stream for which is not directly generated from the excess rainfall during the storm event.
• To separate the base flow the straight-line method is commonly used. It is useful for an individual steam i.e., a single storm only.

Fig. 3: The methods of base flow separation

• Other method of Base flow separation is

1. Constant-slope method
2. Concave method
3. Constant discharge method

Key Takeaways:

The base flow is a portion of the stream for which is not directly generated from the excess rainfall during the storm event.

2.9 Effective rainfall:

• An Effective Rainfall or Effective Precipitation (EP) can be defined as, "It is the amount of precipitation which is actually added and which is stored in the soil".
• If in summer, on any day the total amount of rainfall is less than 5 mm, it cannot be considered as the effective rainfall because, the amount of rainfall would get evaporated, before it would soak into the ground to be considered as an effective rainfall or an effective precipitation.
• So when, within 24 hours the rainfall received at a given station, is more than the amount of water which could be get evaporated, would get percolated into the topsoil.
• It would be considered as the effective rainfall or the effective precipitation (E.P.): This amount would vary seasonally and regionally also.

Key Takeaways:

An Effective Rainfall or Effective Precipitation (EP) can be defined as, "It is the amount of precipitation which is actually added and which is stored in the soil".

2.10 Unit Hydrograph theory:

• Unit hydrograph can be defined as, "a method to express the relation; between time and the discharge by graphical method".
• The units of time are hours, days or months which are expressed on the X-axis as per the requirement i.e., the unit of time depends upon the purpose and nature of study
• e.g., If the study is confined to the discharge of water through floods the unit will be hours and in case of discharge through run off is to be calculated the unit will be either a month or a year.
• The discharge is expressed by m³/s. In some cases, the discharge is expressed in cm/s i.e., depth of the case of water per unit are of the catchment, per second le.g. Cm/s/m².

Theory of Unit Hydrograph:

• To develop, the theory of unit hydrograph, the following definitions are taken as the base,

Excess Rainfall unit:

• It is total precipitation after all the abstractions like evaporation, infiltration, surface storage and interception.
• Excess rainfall is 1 unit. It may be 1 cm or 4 cm. Let is imagine that the excess rainfall unit is 1 cm, so the excess rainfall will be 1 cm / n for 1 hour or 1/2 cm / h for 2 hours; or 2 cm / n for 1/2 hour.

Duration of Excess Rainfall:

• This duration must be lesser than the period of concentration i.e.

t = t0/4

• [The base period of T is the total time of flood hydrograph at a given gauging site.]

Specifications of Unit Hydrograph:

• Both the, unit of precipitation and the intensity of the excess precipitation are the controlling parameters.
• So, in case of unit hydrograph, it is specified as 1 cm 1 h hydrograph (in this unit if precipitation is 1 cm and period of precipitation is 1 hour so, intensity of the precipitation will be 1 cm / n). e.g., The surface runoff a catchment area of A km² will be;
• Surface Runoff = (1 cm / 100) × A × 106 m³ = (A/100) x 106 m² = A x 104 m²

Key Takeaways:

Unit hydrograph can be defined as, "a method to express the relation; between time and the discharge by graphical method".

2.11 S-curve hydrograph:

• A 'S' curve hydrograph is nothing but a hydrograph generated by a continuous effective rainfall, occurring at a uniform rate for an indefinite period of time.
• It is known as 'S' hydrograph because its shape is like the English alphabet 'S', may be slightly deformed.

Construction of 'S' curve Hydrograph:

• 'S' hydrograph is a hydrograph generated by continuous effective rainfall occurring at an uniform rate for an indefinite period.
• It is known as 'S' hydrograph as the shape of this hydrograph looks like the English alphabet 'S' but slightly deformed. [Refer Fig.]

Fig: 4

• If the unit Hydrograph is plotted successively with offset equal to unit duration and the overlapping ordinates are summed up, we get the ordinate of 'S' Hydrograph.
• At this stage, it is necessary to know. The number of times the unit hy graph have s to be plotted.
• As a thumb rule, u. Unit hydrograph may be added 'T' it times with offset equal to unit duration of the limit Hydrograph.
• Where in, T = the time base of unit hydrograph
• t = the unit duration of the unit hydrograph
• The 'S' hydrograph ordinate at the equilibrium stage has maximum value, as after this point the ordinate has same value which is equal to the rate of effective rainfall. [Refer Fig.]

Key Takeaways:

A 'S' curve hydrograph is nothing but a hydrograph generated by a continuous effective rainfall, occurring at an uniform rate for an indefinite period of time.

2.12 Uses and limitations of Unit hydrograph:

Uses of hydrographs:

• The hydrographs have the following uses:

1. They help to know the magnitude of the extreme rainfall.
2. They help to decide the design of the hydraulic structure.
3. They are useful in extension of flood flow records (based on rainfall records)
4. They are very useful for the development of flood forecasting and flood warning system to reduce the losses due to floods.

Limitations of Unit Hydrograph

• The basic assumption of uniform distribution of the excess rainfall over. The entire catchment is far away from the reality.
• The principle of linearity is assumed in this theory, which, in reality is not correct.
• This theory is not applicable to the surface runoff originated form show and ice.
• The theory is applicable for the floods in bank only i.e., the flood water is contained to the river channel only. It is not applicable in the areas where the floods over cross the banks. The theory is applicable to the catchment area which is less than 5000 km², only.
• The theory is not applicable to the narrow-elongated catchments because in such cases the uniform distribution of the perception is not possible over the entire catchment area.
• The theory becomes unapplicable in the catchment having surface storages in the upstream areas of the gauging station.

2.13 Synthetic hydrograph:

• If observed data is not enough for derivation of a unit hydrograph then the unit hydrograph is prepared on the basis of the characteristics of the catchment area i.e., size, shape, slope, type of rock surface, climate, land use etc.

Synthetic unit Hydrograph-Snyder's method:

• F. F. Snyder analyzed a number of Unit Hydrographs from the Appalachian maintain region which runs almost parallel to the east coast of U. S. A.
• He has presented a set of equations to develop the synthetic unit hydrograph. They are based on the following three catchment areas characteristics i.e. A, L and Lc.

Where A= Catchment area (in km²);

L = Length of the main stream (in km)

Le = Distance (in km) along the main stream

• From the outlet up to a point nearest to the centre of gravity. i.e. (c.4.) from the catchment area.

t = Duration of unit hydrograph in hours.

Tp = Time between the c 4 of effective rainfall to the peak discharge in hours (it is known as basic lag in hours)

Qp = Peak discharge in m3/s

T = Base period in hours.

W75 = Width of unit hydrograph for 75% Qp

W50=Width of unit hydrograph for 50% of Qp

C₁ = A constant depending upon the slop an SD of the catchment area. (The value of the constant between 1.35 and 1.65)

C₂ = A coefficient ranging between 0.56 and 0.69.

• Following Equations are suggested to calculate and plot the Synthetic Unit Hydrograph.

Equation I:

Equation II:

Equation III: T=5.455tp

Equation IV:

Equation V:

Equation Vi:t=2

• The width of unit hydrographs W50 and W75, may be divided into two parts in such way that 1/3 part will lie in rising curve and 2/3 part will lie in the recession curve.
• The unit hydrograph can be framed based on the above-mentioned equations. As a check the area of the unit hydrograph may be checked and corrected for a value of unit precipitation [Refer Fig.]

Fig. 5: A Sketch of Synthetic Unit Hydrograph

Key Takeaways:

If observed data is not enough for derivation of a unit hydrograph then the unit hydrograph is prepared on the basis of the characteristics of the catchment area i.e., size, shape, slope, type of rock surface, climate, land use etc.

2.14 Stream Gauging:

• Through precipitation we receive the water on the land surface which divided into three as evaporation percolation and surface runoff. This water which flaws along the slopes is called a stream.
• It is necessary to measure the amount of water which flows in the form of a stream, because it is basic cause of floods. So, it is necessary know the amount of surface water, its frequency and also its velocity.
• Here we are going to discuss the methods used to measure these stream flows. These methods are called as gauging methods.
• These methods are of two types-

1. The direct methods of measurement.
2. The Indirect methods of measurement.

Direct Method of Measurement

• Three methods are used to have the direct measurement of the surface flow they are as given below,

1. The Area-Velocity Method
2. The moving Boat Method

A: Area-Velocity Method

• In this, the cross-sectional area i.e., a and the mean velocity of the flow i.e., V, are considered to know the discharge passing through which will be equal to (a Va).
• The total cross-section of the given stream is plotted by observing the soundings. By doing so the relation between the stage and area of cross-section is established.
• In the next step, the mean velocity of the flow of the stream is calculated. On the basis of the variation in the velocity in the cross-section of the stream, the stream is sub-divided into small section.
• The velocity of each of these sub-sections is measured. This is done either by using surface floats, velocity rods or by current meters.

Surface floats Methods:

• They are made up of the wooden disc and is allowed to float on the surface of water upto a given distance and the time required for each of the float to reach the distance is calculated so the surface velocity can be calculated as

Where in Vs= surface velocity;

D = fixed distance

t = the travel time (of the float): using this equation, the average velocity can be calculated i.e.

Total time taken by floats /Total number of floats

• As the floating condition of all the float are not same due variation in the wind velocity waves, debris etc. the average velocity calculated by this method may to be very accurate.

Fig 6: Surface Floats

The velocity Rods Method:

• These roads are made up of wood and a lead weight is attached at their one end. So, they remain upright is the water with the lead part under the surface of the water.
• They are used to calculate the velocity of the flow by using the same method used for the floats i.e.

Where in D =The distance;

t = The travel time of the rod

The current meter method:

• It is a sample instrument having a rotating element, which rotates due to the flow velocity each time the element complete one rotation the electric current is developed and make a sound.
• If the frequencies of such sound is recorded per given period of time the velocity of the stream flow can easily be calculated. (These current meters can be of both the types i.e., one with vertical axis and another with horizontal axis.) Between these two the performance of horizontal type current meter is better than that of vertical axis.
• It is necessary to have the proper calibration of this current meter, to establish the relationship between the rotation per second and velocity of the steam flow.

B: Moving Boat Method

• The methods we have discussed above are not convenient if the stream has high velocity and during flood. When amount of the surface runoff is also huge. It becomes difficult to keep the boat at one point i.e., keeping it stationary.
• In such cases, the moving boat method is most suitable. The current meter is mounted on the boat, keeping the meter at right angle to the direction of the stream flow.
• On the basis of the velocity of flow (Vs) and the velocity of the boat (Vp). The resultant velocity (Vr) is calculated.

C: Salt Concentration or Dilution Technique Method

• The method is based on "The continuity Equation Principle". In this the chemical is used (Generally Common Salt is used) to calculate the stream discharge.
• Let us assume that the chemical of concentration (C1) is Injected at a constant rate (Q1) at a given section into the stream flow (Q) of concentration C. After proper mixing of the chemical with the stream flow, at some distance in downstream. Let us assume the concentration of chemical became C2.

By applying the equation of continuity

So,

So,

will be the stream flow.

• If the stream flow is turbulent this method is very convenient because the proper mixing of the chemical is easily achieved.
• It is necessary to note that the chemical to be used to calculate the stream flow, should not be toxic it should not get lost it should not be very expensive.
• Generally, the radioactive elements like Bromine, iodine or sodium etc. can be used for this purpose.

Indirect Measurement Methods:

• In these methods the discharge of the stream is measured by calculating.
• The stage-discharge relationship. Once this relationship is calculated at a given location, then the depth of the flow (al that location) is measured and by referring the stage discharge curve, the discharge can be computed.
• Following are some of the indirect methods which are applied to calculate the discharge of the flow.

(A) Notches Weirs Venturi Flumes and the Spillways:

The sharp-crested notches and weirs:

• Following equation is used to calculate the discharge through the notches and weirs.

Where Q = Discharge

Cd = The coefficient of discharge of the notch or the weir

L= The total length of the notch or weir

H = The head of the water measured above the crest of the notch or weir

For A Board Crested Weir:

• The following equation is used

For,

∴

For A Venturi Flumes:

• In case of these, to measure the discharge a critical section is created by the construction of the width of the channel, whose discharge is to be measured.
• To produce the control section a hump is provided by raising the bottom of the channel.
• For the channel have a large discharge capacity the control meters or the venturi flumes are used.

For the spillways:

• The overflow section of the dam i.e., the spillways are used to measure the discharge, passing through the large streams.

(B) Slope-Area Method

• In this method the discharge of the stream (Q) is measured by using the following equation.
• Q = (Area of cross-section) x (Velocity of the flow)
• In this required area of the cross-section would be the area enclosed between the bed-level of the channel and high flood level (H.F.L). To know the bed level of the channel, the soundings (the measurement of the depth of the water from the surface level) are taken at required intervals and these figures are plotted with a given scale to get the profile.
• For calculating the slope of this channel, the difference in the elevation of the water surface at two sections (may be h) is divided by the total length of the channel (1) between two sections. So, the slope of the channel would be

S=

• The velocity of the flow is calculated by using either chezy's formula or Manning's formula

(1) Chezy's formula

V =

(2) Manning's formula

In the above formula of chezy

C= Chezy's constant:

R = Mean hydraulic radius=A/P

Where A= Area of the cross-section of the channel flow.

P= The wetted parameter;

S= The slope of the water surface

The value of chezy's constant (c) and manning's 'n' should be fixed from the known bed, bank and the vegetation cover in the channel.

2.15 Selection of site:

• The circulation route is directly for approximately three hundred ft upstream and downstream of the gage web page.
• At all ranges, the full glide is constrained to an unmarried channel. There is likewise no subsurface or groundwater glide that bypasses the web page.
• The streambed withinside the region of the web page isn't always concern to scour and fill. It is likewise freed from aquatic plants.
• The banks of the circulation channel are permanent. There are freed from brush and excessive sufficient to incorporate floods.
• The circulation channel has unchanging herbal controls. These controls are bedrock outcrops or solid riffle for low glide conditions. During excessive flows, the controls are channel constrictions or a cascade or falls this is unsubmerged in any respect ranges.
• At extraordinarily low ranges, a pool is gift upstream from the web page. This will make sure the recording of extraordinarily low flows and keep away from the excessive velocities related to excessive streamflow’s.
• The gaging web page is some distance sufficient eliminated from the confluence with some other circulation or from tidal consequences to keep away from any feasible affects at the size of circulation stage.
• Within the proximity of the gage web page, a attain for the size of discharge in any respect ranges is available.
• The web page is obtainable for set up and operation and protection of the gaging web page. The choice of a gaging web page is once more a compromise among those criteria.

2.16 Discharge measurement by velocity-area method:

• In this, the cross-sectional area i.e., a and the mean velocity of the flow i.e., V, are considered to know the discharge passing through which will be equal to (a Va).
• The total cross-section of the given stream is plotted by observing the soundings. By doing so the relation between the stage and area of cross-section is established.
• In the next step, the mean velocity of the flow of the stream is calculated. On the basis of the variation in the velocity in the cross-section of the stream, the stream is sub-divided into small section.
• The velocity of each of these sub-sections is measured. This is done either by using surface floats, velocity rods or by current meters.

2.17 Introduction to advance techniques/equipments used in gauge discharge measurements such as radar, Current meter, ADCP (acoustic Doppler current profiler):

Radar:

• It is a new technology which is used to measure the depth of the stream. It is best method to measure the water-level. These Radar water level-sensors need less construction work to be installed than the traditional "Control water level sensors".
• This radar level gauge has an antenna, which generates millions of very short "one nano second microwave pulses" every second. Each one of these pulses is directed and transmitted to and reflected from a product surface.
• The elapsed time period between the transmission and reception of the signal at the speed of light (300000 km/per sec) is measured and is calculated as the distance.
• The radar level sensors work with safe, low emitted power in the C and K-band frequency range. The proven ECHOFOX signal processing. Selects the correct level echo reliably.
• No adjustment through fill and emptying the vessel, is necessary. It is carried out by the input of vessel dimensions. Two emitting frequencies are available for these applications. In case of the need of high accuracy, the compact and high frequency sensors are applied.
• The low frequency c-band sensors can penetrate, the form and the strong condensation and so they are suitable for such arduous process condition. These sensors, which remain unaffected by steam gas composition pressure and temperature changes can detect the product surface of different products very reliably.

Current Meter:

• One technique that has been used for many years through the USGS for measuring discharge is the mechanical contemporary-meter technique. In this technique, the flow channel pass phase is split into several vertical subsections.
• In every subsection, the place is received through measuring the width and intensity of the subsection, and the water pace is decided the usage of a contemporary meter.
• The discharge in every subsection is computed through multiplying the subsection place through the measured pace. The overall discharge is then computed through summing the release of every subsection.
• Numerous kinds of gadget and techniques are utilized by USGS employees to make contemporary-meter measurements due to the huge variety of flow situations all through the United States. Subsection width is typically measured the usage of a cable, metal tape, or comparable piece of gadget.
• Subsection intensity is measured the usage of a wading rod, if situations permit, or through postponing a sounding weight from a calibrated cable and reel machine off a bridge, cableway, or boat or via a hollow drilled in ice.
• The pace of the streamflow may be measured the usage of a contemporary meter. The maximum not unusual place contemporary meter utilized by the USGS is the Price AA contemporary meter.
• The Price AA contemporary meter has a wheel of six metallic cups that revolve round a vertical axis. A digital sign is transmitted through the meter on every revolution permitting the revolutions to depend and timed. Because the price at which the cups revolve is without delay associated with the rate of the water, the timed revolutions are used to decide the water pace.
• The Price AA meter is designed to be connected to a wading rod for measuring in shallow waters or to be installed simply above a weight suspended from a cable and reel machine for measuring in rapid or deep water.
• In shallow water, the Pygmy Price contemporary meter may be used. It is a two-fifths scale model of the Price AA meter and is designed to be connected to a wading rod. A 1/3 mechanical contemporary meter, additionally a variant of the Price AA contemporary meter, is used for measuring water pace below ice.
• Its dimensions permit it to match without difficulty via a small hollow withinside the ice, and it has a polymer rotor wheel that hinders the adherence of ice and slush.

ADCP (acoustic Doppler current profiler):

• This ADCP technology has been proved to be successful for gauging at low flows in a shallow river. It is an accurate technology and is very cost effective. It is a very fast method so it also saves time (it needs about 10% the time required for other conventional methods).
• The equipment is small so can be deployed from a small manned boat on can be fitted on any floating device from a bridge.
• This can be well-operated by two men team' by attaching a rope to both the banks and fitting the equipment on a boat to which this rope is attached.

Advantages of ADCP Technology:

1. This method needs a very short period of time (10% of the time required for the other traditional methods.)
2. It can measure the velocity of the flow and also the bed profile at the same time.
3. It provides a detailed bed profile of the river stream.
4. The uncertainty in the velocity is totally reduced.
5. It can be used for safe measurement of the peak floods.
6. It can be linked to the GPS i.e.; geographical position system and entire river reach can be surveyed to generate real time data. (The up-to-date data)

Fig. 7: AVDM Schematic diagram

References:

1. A textbook of Hydrology, Dr. P. Jaya Rami Reddy, USP Publisher

2. Irrigation, Water Resources and Water Power Engineering, P.N. Modi.

3.Irrigation and Water Power Engineering, Dr. Purnima and Dr. Pande

4. Irrigation Engineering, Bharat Singh, Nem Chand & Bros. India

5.Irrigation Engineering, H.M Raghunath, Wiley