Prof. Rajesh BhagatAsst. Professor

Civil Engineering Department

Yeshwantrao Chavan College Of Engineering

Nagpur

B. E. (Civil Engg.) M. Tech. (Enviro. Engg.)

GCOE, Amravati VNIT, Nagpur

Mobile No.:- 8483003474 / 8483002277

Email ID:- rajeysh7bhagat@gmail.com

Website:- www.rajeysh7bhagat.wordpress.com

ENGINEERING HYDROLOGY

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Unit-III

1) Runoff: Runoff, sources and component, classification of streams, factors

affecting runoff, Estimation Methods. Measurement of discharge of a stream by Area-

slope and Area-velocity methods.

2) Hydrograph: Flood hydrographs and its components, Base flow & Base flow

separation, S-Curve technique, unit hydrograph, synthetic hydrograph. Instantaneous

Unit hydrograph.

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Runoff:-

Flowing off of precipitation from catchment area through a surface channel.

It is normally expressed as volume per unit time.

It is defined as that portion of precipitation which is not absorbed by the deep

strata but find its way into the stream after meeting the demands of losses.

Types of runoff:- (Based on time delay between precipitation and runoff)

1) Direct runoff: part of runoff, which enters the stream immediately after the

precipitation. Includes surface runoff, prompt interflow and precipitation on

channels surface. Also called storm runoff.

2) Interflow: the delayed flow that reaches a stream essentially as ground

water flow. Includes ground water flow & delayed interflow.

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Runoff:-

Runoff may be referred to as a stream flow, river discharge or catchment yield.

Types of runoff:-

1) Surface runoff: the flow of water over land or surface before joining open

channel. Open channel finally reaches the catchment outlet. Laminar &

turbulent regime.

2) Interflow: part of precipitation that infiltrates and moves laterally through

upper crust of the soil or above GWT and returns to the surface at some

location. Through flow or storm seepage or subsurface flow or quick return

flow.

3) Ground water flow: part of precipitation that undergo deep percolation,

reaches the ground after some time (usually delayed flow)

Surface water

• Watershed – area of land draining into a stream at a given location

• Streamflow – gravity movement of water in channels

– Surface and subsurface flow

– Affected by climate, land cover, soil type, etc.

Sources of runoff or stream flow

1) The infiltrated water which percolates deeply and reaches the

ground water storage in the soil which follows a complicated and

long path of travel and ultimately reaches the surface is called

ground water run-off.

2) The ground water movement is very slow.

3) Based on the time delayed, runoff is classified as direct runoff

and base flow.

4) Base flow consists of delayed interflow & ground water runoff.

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Groundwater accreditation

Interflow

Overland flow

EvaporationTranspiration

Direct runoff

Evapotranspiration

Soil water

Wate

r Flo

w

Runoff Processes:

Horton overland flow

Subsurface storm flow,

Return flow

Groundwater flow

Water balance of drainage basins

Factors affecting distribution of runoff in time:

1) Type of precipitation: rain contributes directly to runoff. Hail & sleet takes

time to melt.

2) Rainfall intensity: heavy rainfall contribute directly & rapidly towards

stream than lower intensity rainfall

3) Rainfall duration: if rainfall duration is equal to or greater than time of

concentration, runoff will be more. Also at the starting infiltration capacity is

more.

4) Rainfall distribution: a uniformly distributed rainfall volume may lead to

delayed runoff.

5) Catchment factors: shape, roughness, storage, topography & soil type.

6) Slope: if slope is more, water moves faster.

7) Geology: fined grained compacted soil will yield more surface water.

8) Vegetation: vegetation slow down the movement of water.

9) Drainage network: closely spaced stream allow efficient drainage of

precipitated water.

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Types of Streams:

1) Perennial stream: always carries some flow, ground water flow occurs

during non precipitation period.

2) Intermittent: streams remain dry for most of dry month, limited contribution

from the ground water.

3) Ephemeral: does not have any base flow contribution, streams becomes dry

soon after the end of storm flow. Annual hydrograph shows series of short

duration spikes marking flash flow.

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Types of Streams:

1) Influent stream : If the ground water below the bed of stream, the seepage

from the stream feeds the ground water such a streams are called influent

streams. Ephemeral streams.

2) Effluent stream : When GWT is above the water surface elevation in the

stream , ground water feeds the streams such a stream is called effluent stream.

Perennial streams.

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Antecedent Precipitation:

1) If due to previous rain, soil is already saturated, runoff will be more due to next

rainfall. The previous rainfall is called Antecedent precipitation.

2) Antecedent Precipitation Index (API) is taken as a measure of the soil moisture

conditions existing on the day of storm under consideration.

It = K x I t-1 + Pt

It = API of any Day t.

K = constant known as Recession Factor.

I t-1 =API of the day (t-1).

Pt = the precipitation of t th day.

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Determination of Runoff :-

1) Using Empirical Formulae such as Lacey, Inglis, Khosla, etc.

2) Using Curves & Tables.

3) Using Runoff Coefficient Method.

4) Using Infiltration Curves.

5) Using Water Shed Simulation Method.

6) Using Rainfall Runoff Correlation.

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Rainfall Runoff Relationship:-

1) In hydrological analysis & design, it is necessary to develop relations between

precipitation & runoff.

2) Such relations are important & useful for extrapolation or interpolation of

runoff records from the precipitation records which are generally available for

longer periods.

3) These relations may estimates runoff of ungauged catchments.

The equation between runoff & precipitation is

R = a x P + b

The values of coefficient a & b are given by

a = ( N ( ∑ P x R ) - ( ∑ P ) ( ∑ R ) ) / ( N ( ∑ P2 ) - ( ∑ P2 ))

b = ( ( ∑ R ) - ( a x ∑ P ) ) / N

N = number of observation sets for R & P

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Rainfall Runoff Relationship:-

The equation between runoff & precipitation is

R = a x P + b

The values of coefficient a & b are given by

a = ( N ( ∑ P x R ) - ( ∑ P ) ( ∑ R ) ) / ( N ( ∑ P2 ) - ( ∑ P)2 )

b = ( ( ∑ R ) - ( a x ∑ P ) ) / N

N = number of observation sets for R & P

The coefficient of correlation ‘r’ can be calculated by

r = ( N ( ∑ P x R ) - ( ∑ P ) ( ∑ R ) ) / √((( N ( ∑ P2 )) - ( ∑ P)2 ) x ( N ( ∑ R2 ) – ( ∑R)2))

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Empirical Equation:

Empirical runoff estimation formulae:-

1) Binnie’s Percentages: developed for small catchment near to Nagpur.

2) Barlow’s Tables: developed for small catchment in UP.

3) Strange’s Table : developed for border areas of maharashtra and karnataka.

4) Inglis’s formula : developed for western India.

5) Khosla’s formula: developed for time period of the month.

Khosla’s relationship for runoff estimation is given by

Rm = Pm – Lm

Lm = 0.48 Tm ( Tm > 4.5OC )

Rm = monthly runoff in cm

Pm = monthly rainfall in cm

Lm = monthly losses in cm

Tm = monthly temp. of catchment

Annual runoff coefficient = annual runoff / annual rainfall.

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Empirical Equation:

1) Khosla’s formula: developed for time period of the month.

Khosla’s relationship for runoff estimation is given by

Rm = Pm – Lm

Lm = 0.48 Tm ( Tm > 4.5OC )

Rm = monthly runoff in cm

Pm = monthly rainfall in cm

Lm = monthly losses in cm

Tm = monthly temp. of catchment

Annual runoff coefficient = annual runoff / annual rainfall.

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If Tm < 4.5OC

TOC 4.5 -1 -6.5 -12 -18

Lm (cm)

2.17 1.78 1.52 1.25 1.0

1) For a catchment in UP, India, the monthly rainfall and temperature are

given. Calculate the annual runoff coefficient by Khosla’s Formula.

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Month Temp. OC Rainfall in cm

January 12 4

February 16 4

March 21 2

April 27 0

May 31 2

June 34 12

July 31 32

August 29 29

September 28 16

October 29 2

November 19 1

December 14 2

the annual runoff coefficient by Khosla’s Formula.

Rm = Pm – Lm & Lm = 0.48 Tm ( Tm > 4.5OC )

If the loss Lm is higher than Pm then Rm will be zero.

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Month Temp. OC(Tm)

Rainfall (Pm) in cm

Monthly Loss (Lm) in cm

Runoff (Rm) in cm

January 12 4 5.76 0

February 16 4 7.68 0

March 21 2 10.08 0

April 27 0 12.96 0

May 31 2 14.88 0

June 34 12 16.32 0

July 31 32 14.88 17.1

August 29 29 13.92 15.1

September 28 16 13.44 2.6

October 29 2 13.92 0

November 19 1 9.12 0

December 14 2 6.72 0

Annual Runoff coefficient = (annual runoff / annual rainfall) = 34.8 / 106.0 = 0.328

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Month Temp. OC(Tm)

Rainfall (Pm) in cm

Monthly Loss (Lm) in cm

Runoff (Rm) in cm

January 12 4 5.76 0

February 16 4 7.68 0

March 21 2 10.08 0

April 27 0 12.96 0

May 31 2 14.88 0

June 34 12 16.32 0

July 31 32 14.88 17.1

August 29 29 13.92 15.1

September 28 16 13.44 2.6

October 29 2 13.92 0

November 19 1 9.12 0

December 14 2 6.72 0

∑ 106 34.8

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Type Of Drainage Area

Runoff

Coefficient,

C

Steep, bare rock

Rock, steep but wooded

Plateaus lightly covered, ordinary ground bare

Densely built up areas of cities with metal led roads & paths

Residential areas not densely built up, with metal led roads

Residential areas not densely built up, with unmetalled roads

Clayey soils, stiff and bare

Clayey soils lightly covered

Loam, lightly cultivated or covered

Loam, lightly, largely cultivated

Suburbs with gardens, lawns and macadamized roads

Sandy soil, light growth

0.90

0.80

0.70

0.70-0.90

0.50-0.70

0.20-0.50

0.60

0.50

0.40

0.30

0.30

0.20

Runoff Coefficients for the Rational Method

Area Velocity Method of Flood Measurement in River:-

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Classification of Catchment:-

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