Unit II Runoff explain the runo.pptx.pdf

Explain all ppt about this

1. Unit II
Runoff
By
Prof Seema Shiyekar
Civil Engg Department GSMCOE
Balewadi, Pune

2. Runoff
•Runoff refers to the flow of water over the
Earth's surface, typically after precipitation like
rain or snowmelt.
• It's a crucial part of the water cycle, carrying
water from land to rivers, lakes, and
eventually the ocean.
•Runoff can also be caused by melting glaciers
or be the result of industrial or agricultural
processes.

3. Unit of Runoff
•The primary unit for runoff is cubic meters per
second (m³/s), representing the volume of
water flowing per unit of time.
• Runoff can also be expressed in other units
depending on the context, such as cubic feet
per second (cfs) or liters per second.
•Another unit, day-second meter, is also used,
which represents the volume of water flowing
at 1 cubic meter per second for one day.

4. Classification of Runoff
•Runoff can be broadly classified based on its
flow path: surface runoff, interflow and base
flow.
•Surface runoff is water flowing over the land's
surface, while interflow is subsurface flow
above the water table
•Base flow is water flowing from groundwater
into surface water.

5. Factors affecting Runoff
•Precipitation characteristics
•Shape and Size of catchment
•Topography of Catchment
•Geological characteristics of basin.
•Metrological characteristics
•Catchment surface characteristics
•Storage characteristics of the catchment.

6. Factors affecting Runoff
Precipitation:
•Intensity:
Higher rainfall intensity leads to more runoff because
the soil cannot absorb water as quickly as it's being
delivered.
•Duration:
Prolonged rainfall saturates the soil, reducing
infiltration capacity and increasing runoff.
•Type:
Snowfall contributes to runoff during snowmelt, which
can result in a large pulse of water.

7. Factors affecting Runoff
Soil Properties:
•Type:
Porous soils (sandy soils) allow for greater
infiltration, reducing runoff, while less porous
soils (clay soils) increase runoff.
•Infiltration Rate:
The rate at which water enters the soil is
crucial. If precipitation exceeds infiltration
capacity, runoff occurs.

8. Factors affecting Runoff
Topography:
•Slope:
Steeper slopes result in faster runoff as water
flows downhill more quickly.
•Drainage Area:
A larger drainage area collects more water,
potentially leading to higher runoff volumes.

9. Factors affecting Runoff
Vegetation:
•Cover:
Vegetation intercepts rainfall, reducing its
impact on the soil and promoting infiltration,
thus decreasing runoff.
•Roots:
Plant roots also improve soil structure and
infiltration capacity.

10. Factors affecting Runoff
Land Use:
•Impervious Surfaces:
Paved areas (roads, parking lots) prevent infiltration
and increase runoff.
•Deforestation:
Removing vegetation increases runoff and soil erosion.
•Urbanization:
Increased impervious surfaces and altered drainage
patterns lead to higher runoff volumes and faster peak
flows.

11. Factors affecting Runoff
Other Factors:
•Geology:
The underlying rock and soil structure can affect
infiltration and groundwater flow, influencing runoff.
•Temperature:
High temperatures can increase evaporation,
potentially reducing runoff.
•Wind:
Wind can influence evaporation and the direction of
storm movement, affecting runoff patterns.

12. Rainfall Runoff relationships & empirical
techniques to determine Runoff
•Linear or exponential regression
•Empirical equations and tables
•Infiltration method
•Unit hydrograph method
•Rational method

13. Empirical Equations
•Binnie percentage
•Barlow’s table
•Inglis and DeSouza formula
•Khosla’s formula

14. Binnie percentage
•Binnie percentage refers to runoff percentages established by
Alexander Binnie based on observations of runoff from rainfall in small
catchments near Nagpur, India. These percentages were used to
estimate runoff from rainfall in the Madhya Pradesh and Vidarbha
regions of Maharashtra, according to document available on runoff
estimation.
•Binnie developed his percentages by measuring runoff and rainfall on a
16 km² catchment area between 1869 and 1872.
•He observed a relationship between cumulative rainfall and cumulative
runoff, expressing this relationship as percentages.
•These percentages were used in the past for estimating runoff volumes
in specific regions of India.
•It's important to note that these percentages are specific to the areas
Binnie studied and may not be universally applicable to all catchments,
especially those with significantly different rainfall characteristics or
catchment sizes.

15. Barlow’s table

•Barlow, the first Chief Engineer of the
Hydro-Electric Survey of India (1915)
•Barlow studied small catchments (area ~ 130
km
2
) in Uttar Pradesh and expressed runoff R
as:
R = K
b
P ,
•Where K
b
= Runoff coefficient which depends
upon the type of catchment and nature of
monsoon and P is the rainfall.

16. Barlow’s table

17. Inglis and DeSouza formula
•The Inglis and DeSouza formula provides two
empirical relationships for estimating annual runoff
(R) from annual rainfall (P) in specific regions of
India,
R = 0.85P - 30.5 for ghat areas
R = (1/254) P(P - 17.8) for Deccan Plateau areas
(where R and P are in centimeters).
•These formulas are designed to be used in different
geographical contexts, reflecting varying
hydrological characteristics.

18. Khosla’s formula

•Khosla's formula estimates runoff using monthly rainfall and temperature data. It's
based on the concept that monthly runoff is the difference between monthly rainfall
and monthly losses, where monthly losses are a function of the mean monthly
temperature.

K
b
= P
m
– L
m


–Where:
•K
b
: is the monthly runoff (cm)
•P
m
: is the monthly rainfall (cm)
•L
m
: is the monthly losses (cm)
•Calculating Monthly Losses:
L = 0.48 *
T: when the mean monthly temperature (T) is greater than 4.5°C
•If the mean monthly temperature (T) is less than or equal to 4.5°C, the monthly
losses are calculated using a different formula, often provided as a table or graph
based on temperature.

19. Runoff Hydrograph
•A runoff hydrograph is a graph that visualizes
the discharge rate of water (runoff) over time
at a specific point in a river or channel.
•It's a crucial tool in hydrology, representing the
integrated effects of rainfall, watershed
characteristics, and runoff processes.
•It consists of flow in all the three phases of
runoff namely Surface runoff, interflow and
base flow.

20. Components of Hydrograph
•Rising Limb:
The portion of the hydrograph where
discharge increases due to rainfall and
surface runoff.
•Peak Discharge:
The highest point on the hydrograph,
representing the maximum flow rate
during the event.
•Falling Limb (or Recession Limb):
The part of the hydrograph where
discharge decreases as the runoff event
subsides.
•Base flow:
The sustained, low flow in a river, often
from groundwater discharge, that
contributes to the overall hydrograph.
•Time to Peak:
The time elapsed between the start of
rainfall and the peak discharge.

21. Uses of hydrograph
•Response to Rainfall:
A hydrograph shows how a watershed responds to a rainfall event. The
shape and characteristics of the hydrograph (e.g., peak flow, time to peak,
duration) are influenced by factors like rainfall intensity, duration,
watershed size and shape, soil type, land cover, and drainage network.
•Flood Prediction and Management:
Hydrographs are essential for flood forecasting, helping to predict
potential flood risks and manage floodplains.
•Infrastructure Design:
They are used in the design of drainage systems, such as storm sewers and
culverts, to ensure adequate capacity to handle runoff.
•Water Resources Management:
Hydrographs help in understanding water availability, managing water
resources, and assessing the impact of land use changes on runoff
patterns.

22. Factors affecting flood hydrograph
•Physiographic factors
Basin characteristics
✔Shape
✔Size
✔Slope
✔Nature of valley
✔Elevation
✔Drainage density

23. Factors affecting flood hydrograph
•Physiographic factors
Infiltration characteristics
✔Land use and cover
✔Soil type and geological conditions
✔Lakes, swamps & other storage

Channel characteristics
✔Cross section
✔Roughness and storage capacity

24. Factors affecting flood hydrograph
•Climatic factors
Storm characteristics
✔Precipitation
✔Intensity
✔Duration
✔Magnitude and movement of storm
Initial loss
evapotranspiration

25. Base flow separation
•It is the process of partitioning a stream flow
hydrograph into its base flow and direct runoff
components.
• Base flow is the portion of stream flow sustained
by delayed pathways, primarily groundwater,
between precipitation events.
•Separating base flow from direct runoff is
essential for understanding hydrological
processes, managing water resources, and
calibrating hydrological models.

26. Base flow separation methods
1. Straight Line Method:
A straight line is drawn connecting the start and end points of the
direct runoff on the hydrograph.
2. Fixed Base Method (Two-Line Method):
The base flow curve before the direct runoff is extended to intersect
the ordinate at the peak, and then connected to the end of direct
runoff with a straight line.
3. Curve extension method
This method uses the master recession curve concept to retain
physical relevance in the separated base flow components.

27. Effective rainfall
•Surface runoff hydrograph obtained after
base flow separation is known as direct
runoff hydrograph (DRH).
•In the fig typical storm hyetograph of
storm is shown. The initial loss and
infiltration losses are subtracted from it.
•Hence resulting hyetograph is known as
Effective rainfall hyetograph (ERH).
•Both DRH & ERH represent the same
total qty. but in diff units.
•Area of ERH multiplied by catchment area
gives total volume of direct runoff which
is same as that of DRH.
•Initial loss & infiltration losses are
estimated based on available data of
catchment.

28. Unit hydrograph (UHG)theory
•Was first presented by L.K. Sharman(1932) which
was initially known as unit graph.
•A unit hydrograph (UH) represents the runoff
response of a watershed to a specific amount
(unit depth) of rainfall excess, applied uniformly
over the entire basin for a specific duration.
• It's a fundamental concept in hydrology used to
estimate the stream flow resulting from rainfall
events. The unit depth is typically 1 cm or 1 inch
of rainfall excess.

29. Assumptions made in theory of unit
Hydrograph
•Time Invariance:
This assumption means that the shape of the unit hydrograph (the
direct runoff response to a unit of rainfall excess) remains the same
over time. In other words, if you have a 1-hour unit hydrograph, it
will be the same shape regardless of whether the rainfall occurs at
1 PM or 1 AM.
•Linear Response:
This assumption states that if you double the amount of rainfall
excess, you will double the direct runoff volume, and the shape of
the hydrograph will remain the same. This allows for the
application of the principle of superposition, where hydrographs
from multiple rainfall events can be combined by simple addition
of their ordinates.

30. S curve hydrograph
•An S-curve hydrograph, also known as an
S-hydrograph, represents the cumulative
runoff from a catchment area due to a
continuous, uniform rainfall of infinite
duration and constant intensity. It's a useful
tool in hydrology, particularly for deriving unit
hydrographs of different durations.

31. Synthetic hydrograph
•A synthetic unit hydrograph retains all the
features of the unit hydrograph, but does not
require rainfall-runoff data. A synthetic unit
hydrograph is derived from theory and
experience, and its purpose is to simulate
basin diffusion by estimating the basin lag
based on a certain formula or procedure.

32. Stream gauging
•Stream gauging is the process of
measuring water flow (discharge) in a
stream or river. This involves determining
the volume of water passing a specific
point over a period of time. It's a crucial
practice for understanding water
resources, forecasting floods, and
managing irrigation.

33. Importance of stream gauging
•Flood forecasting:
Accurate stream flow data helps predict flood events,
allowing for timely warnings and evacuations.
•Water resource management:
Monitoring stream flow is essential for managing water
supplies for irrigation, drinking water, and other uses.
•Hydrological studies:
Stream gauging provides data for understanding river
behavior, water availability, and climate change impacts.
•Infrastructure design:
Data from stream gauging is used in the design of bridges,
dams, and other hydraulic structures.

34. Methods of stream gauging
•Area velocity method
This is a common method where the stream's cross-section is
divided into smaller areas, and the average velocity and area of
each subsection are measured. The discharge is then calculated by
summing the discharges of all subsections.
•Hydraulic structure
Weirs and flumes are structures with known shapes and sizes that
can be used to measure flow based on the water level (stage).
•Floats
Floats can be used to measure surface velocity, which can be
related to average velocity.
•Bubbler system
These systems use pressurized gas to measure water depth,
allowing for remote monitoring.

35. Methods of stream gauging
•Advanced techniques
Radar
Acoustic Doppler current profiler (ADCP)
These instruments use sound waves to measure
water velocity at different depths and locations in
the stream cross-section.
Current meter
These devices measure the velocity of the water
flow.

36. Current meter
•A current meter is a device
used to measure the
velocity of flowing water,
typically in rivers, streams,
canals, and other open
channels. It works by
sensing the water's
movement and converting it
into a measurable value,
usually through a rotating
component like a propeller
or cups.
•

37. Acoustic Doppler current profiler
(ADCP)

•An Acoustic Doppler Current Profiler (ADCP)
is a hydroacoustic instrument used to
measure water current velocities over a
depth range using the Doppler effect of
sound waves.
•It's essentially a sonar system that measures
how fast water is moving at different depths
within a water column.
•Applications:
•Oceanography: Studying ocean currents,
tides, and circulation patterns.
•Coastal Engineering: Monitoring coastal
erosion, sediment transport, and wave
patterns.
•River Discharge
Measurement: Determining the flow rate of
rivers and streams.
•Marine Research: Investigating marine
ecosystems, pollution dispersal, and
underwater construction.
•

38. Thank You