estimation of peak flood, flood frequency, flood design, remedial measures

1. Unit IV
FLOODS
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Prof. Pradeep T. Kumawat
BE Civil, ME Geo-Tech.
(Assistant Professor)
Late G. N. Sapkal College of Engineering, Nashik,
Maharashtra, India.

3. Objective
 Introduction
 Causes of Flooding
 Types of Floods
 Effect of Flooding
 Estimating Floods
 Flood prone areas
 Flood Design
 Flood management

4. Introduction:
 Floods are a common feature in the country that occur
every year in many parts of India.
 Flood is overflow of excess water that submerges land
and inflow of tide onto land.
 Floods in major cities especially during rainy season
are proving to disastrous not only to the environment
but also have serious implications for human life and
property.
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5. A flood occurs when the Geomorphic Equilibrium in the river
system is disturbed because of intrinsic or extrinsic factors or
when a system crosses the geomorphic threshold.
 Flooding in a river due to aggradation of river bed
(intrinsic threshold);
 Flooding in a river due to heavy rainfall (extrinsic
threshold)

6. Causes of flooding
Meteorological Cause
Hydrological Cause
Anthropogenic Cause

7. Tsunami caused by underwater
earthquake
Cyclones / Typhoons
Storms
Meteorological Cause

8. Embankment
Collapse
Snowmelt Runoff
Dam collapse
Hydrological Cause

9. Urbanisation increases surface run-off Deforestation
Global WarmingIntensive Agriculture
Anthropogenic Cause

10. TYPES OF FLOODS
 River floods
 Flash floods
 Coastal Floods/Storm Surge
 Urban Flood

11. River floods

12. Flash floods

13. Coastal floods

14. Urban floods

15. Effects of Flooding
i. Agricultural lands are destroyed due to crops being submerged in water.
This causes a lot of economic loss to farmers.
ii. They cause disruption of all transport by cutting off roads and railway lines
as well as communication systems (e.g. telephone lines are damaged).
iii. They damage houses, particularly kuccha houses in the villages.
iv. There is shortage of drinking water.
v. They harm human, animals and plant life.
vi. They cause a rise in epidemics because of waterlogging. Malaria, Cholera
and diarrhea are common diseases during floods.
vii. They cause decay of organic matter, leading to growth of infection-
spreading germs.
viii. They affect the ecosystem in a negative way.
ix. They cause a lot of harm to the economy of the region.

16. Damage cause to National Highway in Uttarakhand

17. Damage cause to House in Kerala

18. Damage cause to House in Colorado

19. Estimating of Peak Flood
Past Flood records
Rational Method
Empirical Methods
Flood Frequency Studies
Unit Hydrograph Technique

20. Rational Method
The rational method is to be suitable for peak flow prediction in small size
catchments (<50 sq. km). It is most suitable for urban drainage designs and
small culverts or bridges designs.
Qp = C·i·A
Where,
i = Rainfall Intensity
A = Catchment Area
C = Runoff Coefficient
= Runoff / Rainfall

21. Empirical Methods
1. Dicken’s Formula (1865):
Qmax = C·A^(3/4)
Where,
Qmax = Max flood discharge in Cumecs
A= Catchment area in Sq.km
C = 11.4 for North Indianbasins
= 13.9 to 19.5 for Central Indian basins
= 22.2 to 25.0 for the catchments in Western Ghats

22. Empirical Methods
2. Ryve’s Formula (1884): The formula was developed for catchment
areas in the South India.
Q = C·A^(⅔)
Where,
Qmax = Maximum flood discharge in Cumecs
A = Catchment area in Sq.km
C = 6.78 for Catchment lying within 24 km from
coast
= 8.45 for Catchment lying within 24 km to 161
km from coast
= 10.00 for limited places near hills

23. Empirical Methods
3. Inglis Formula (1920):
4. Ali Nawab Jung Bahadur Formula:
Q = C × A^(0.993 – 1/14 log A)
Where, C = 48 to 60

24. Empirical Methods
5. Fuller’s Formula:
Qmax = C × A^(0.8 × (1+0.8 log T))
Where,
C= 0.18 to 1.9
T = Number of year after which such a flood occur again

25. Flood Frequency Studies
 Flood frequency analysis is a technique used by hydrologists to predict
the future maximum flood discharge or peak flow values corresponding
to specific return periods or probabilities along a river.
 Knowledge of flood frequency is necessary also to flood insurance and
flood zoning.
 To estimate the frequency with which floods of a certain magnitude may
occur.
 The flood frequency frequency (F) indicates the possibility of a flood of
given magnitude being equalled or exceeded in an year and is expressed
as percentage i.e. a 5% frequency (F) of a flood denotes that there are 5
out of 100 chances of the flood of a given magnitude being equalled or
exceeded.

26. Flood Frequency Studies
Recurrence Interval (Return Period):
Recurrence intervals or Return period is the time period over which it is
likely that a particular magnitude flood will occur.
For example, if a town has a flood recurrence interval of 100 years, the town
on average will flood one time every 100 years.
Calculate the recurrence interval, which is the number of times in your
record that a flood of a given magnitude occurred.
Recurrence interval, which is the reciprocal of the annual exceedance
probability.
The formula for recurrence interval (Tr) is:
Tr = 100/F

27. Flood Frequency Studies
Then the recurrence interval of a flood having serial number m in above
series can be obtained by any one of the following method:
1. California Formula (USA):
Tr = N/m
2. A. Hazen’s Method:
Tr = (2N/2m-1)
3. Gumbel’s method:
Tr = N/(m+C-1)
4. Weibull’s Formula:
Tr = (N+1)/m

28. Design flood
The design flood for a hydraulic structure may also be defined in a
number of ways like:
 The maximum flood that any structure can safely pass.
 The flood considered for the design of a structure corresponding to
a maximum tolerable risk.
 The flood which a project can sustain without any substantial
damage either to the object which it protects.
 The largest flood that may be selected for design as safety
evaluation of a structure.

29. Design flood
 Probable Maximum Flood (PMF):
This is the flood resulting from the most severe
combination of critical meteorological and hydrological condition
that rare reasonably possible in the region.
 Standard Project Flood (SPF):
This is the flood resulting from the most severe
combination of meteorological and hydrological condition
considered reasonably characteristic in the region.
 Flood of Specific Return Period:
Flood of Specific Return period is the time period over
which it is likely that a particular magnitude flood will occur.

30. Design flood
 Design Flood For Storage Dams
 Design Flood For Barrages and Weirs
 Design Flood For Diversion Works and Cofferdam
 Design Flood For Cross Drainage Works

31. Flood prone zones, areas in INDIA

33. Management and Mitigation of
Floods
Floods cause tremendous destruction of life and property. But there are ways of
protection against floods:
i. If an area is prone to floods, houses in the region should be constructed on raised
platforms. River banks should also be raised.
ii. Administrative authorities should properly map areas that are prone to floods.
iii. Rivers should be mapped and the surrounding areas appropriately prepared for
floods.
iv. Houses should be insured to protect against economic losses. Placing sandbags
around houses can save property.
v. Dams can be constructed to prevent against losses from floods. They control the
water flow.
vi. Afforestation programs should be supported because depletion of forests is
causing a rise in the number of floods witnessed almost every year across the globe.
vii. Rivers should be cleared of harmful garbage like plastics.
viii. Floods should be well forecasted and warnings systems should be in place.