This document discusses catchment areas and factors that affect runoff. It defines key terms like catchment area, runoff, and runoff coefficient. It describes 3 types of catchment areas and characteristics of catchment areas that can be good, average, or bad. The document lists 7 factors that affect runoff, including pattern of rainfall, catchment surface type, topography, area size and shape, vegetation, geology, and meteorology. It also discusses several methods to estimate runoff, such as empirical formulas, Strange's tables and curves, infiltration method, and the unit hydrograph approach.

1. RUN OFF
Riya Susan
Department of Civil Engineering
AL Azhar College of Engineering and Polytechnic

2. CATCHMENT AREA
The tract of land which contributes water into a stream or a reservoir is called
as catchment basin and its area is called catchment area
The run off of a catchment area in a year is the volume of water discharged by
the stream draining the area or into the reservoir receiving the discharge.
Runoff is measured as cm depth of water
Itcan be measured on daily, monthly or yearly basis depending on the work to
be executed

3. 3 types :
1. Combined catchment area : this is the total catchment area in square
kilometers which contributes water to a stream or a tank.
2. Intercepted catchment area : the catchment area which intercepts the
rate of flow off and diverts a part of it into some other stream or tank if
there are a chain of tanks or more streams in the basin
3. Free catchment area : the catchment area which independently
contributes water after interception by another area is termed as free
catchment area
Intercepted catchment area= combined catchment area- free catchment
area

4. CHARACTERISTICS OF CATCHMENT AREA
● GOOD
● AVERAGE
● BAD

5. FACTORS AFFECTING RUN OFF
1. PATTERN OF RAINFALL
2. CHARACTER OF CATCHMENT SURFACE
3. TOPOGRAPHY
4. SHAPE AND SIZE OF CATCHMENT AREA
5. VEGETATION IN THE CATCHMENT AREA
6. GEOLOGICAL FEATURES OF THE AREA
7. METEOROLOGICAL CONDITIONS

6. 1. PATTERN OF RAINFALL :
● If the intensity of rainfall is more the runoff will be more
● If the duration of rainfall is short it will not produce any runoff because the
quantity of water fallen on the ground will be just sufficient for absorption into
soil
● Runoff will be more if the rain falls over the entire area of catchment
1. CHARACTER OF CATCHMENT SURFACE
● Rocky or compactive surface - more runoff
● Sandy or loose - less runoff
1. TOPOGRAPHY
● Steep slope - more runoff
● Area with depressions or flatter slope - less runoff

7. 4.SHAPE AND SIZE OF CATCHMENT AREA
● Large area - more runoff
● Fan shaped catchment - more runoff
● Fern shaped catchment - less runoff
5. VEGETATION IN THE CATCHMENT AREA
● Great extent of vegetation cover- more runoff
6. GEOLOGICAL FEATURES OF THE AREA
● Hard surface- more runoff
● Area with cracks, flanks - less runoff
7.METEOROLOGICAL CONDITIONS
● High temperature - less runoff

8. RUN -OFF COEFFICIENT
● Ratio of runoff to rainfall.
● k= R/P
● Where R= runoff in cms
k= runoff coefficient
P = rainfall in cms
● Runoff coefficient is always less than one as runoff is always less than
rainfall

9. METHODS OF ESTIMATING RUNOFF -
1. EMPIRICAL FORMULA
● kHOSLA’S FORMULA
R=P-4.18T
R ANNUAL RUNOFF IN MM
P ANNUAL RAINFALL IN MM
T MEAN TEMPERATURE
● INGLI’S FORMULA
R= 0.85P-30.5 FOR GHAT AREAS
R - RUN OFF IN CMS
P - IN RAINFALL IN CMS
R= (P-17.8) X P FOR NON GHAT AREAS
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10. 3. LACEY’S FORMULAE
R= P
1+304.8F
PS
R RUNOFF IN CMS
P RAINFALL IN CMS
S CATCHMENT FACTOR
F MONSOON DURATION FACTOR
CLASS
OF
CATCH
MENT
VALUE
OF S
A 0.25
B 0.60
C 1
D 1.70
E 3.50
CLASS OF
MONSOON
F
VERY
SHORT
0.5
STANDARD
LENGTH
1
VERY LONG 1.5

11. 2. RUNOFF BY STRANGE’S TABLES AND CURVES
● Strange’s tables and curves give the runoff over a catchment based on the
percentage of the total monsoon rainfall
● The available percentage runoff depends mostly on the type of catchment
● It gives runoff for daily and yearly rainfall taking into account three types
of catchment ie. good, average and bad and the surface conditions like
wet damp and dry

12. 3.RUNOFF BY INFILTRATION METHOD
● The movement of water through the surface into the soil is termed as
infiltration
● The capacity of any soil to absorb water from rainfall which is continuous
at an excessive rate decreases with time until a minimum rate of
infiltration is reached.
● The infiltration rate is the rate at which water actually enters the soil
during a storm and id equal to the infiltration capacity or rainfall rate
whichever is less
● The infiltration capacity of a soil can be determined by subjecting an
experimental plot of soil to rainfall rates in excess of infiltration capacity
and by measuring the actual rate of runoff

13. RUNOFF BY UNIT HYDROGRAPH
*hydrograph is a graph showing variations of discharge with time at a particular
point of a stream or a river. It shows the time distribution of total runoff at the
point where measurement is taken.
* UH it is a hydrograph which represents unit runoff resulted from an intense
rainfall of unit duration and specific ariel distribution over the catchment area.
* if a UH results from a 3 hour unit rainfall duration it is known as 3 hour UH
This is a hydrograph produced by surface runoff from a storm lasting for 3
hours and yielding a rainfall excess of 1cm