Design Flood Estimation

Instructional Objectives
The student, after completion of this lesson, shall know:
1. How is a design flood described fo
r a particular hydraulic structure?
2. How are design floods designated fo
r: a) Storage Dams; b) Barrages and
weirs; c) Diversion works and Coffer dam
s; and d) Cross drainage works,
according to the Indian Standard guidelines.
3. What are the methods to calcul
ate design floods, viz, a) The hydro-
meteorological approach, and b)
The statistical approach
4. The steps followed in finding out design flood by the hydro-meteorological
approach, which is generally adopt
ed for large and intermediate sized
dams.
5. The steps followed in finding out desi
gn flood by the statistical approach,
either by probability distributions or
by the method of plotting positions.
2.4.0 Introduction
A flood is commonly consider
ed to be an unusually high
stage of a river. For a
river in its natural state,
occurrence of a flood usually
fills up the stream up to its
banks and often spills over to
the adjoining flood plains.
For a hydraulic structure
planned within the river (like a dam or a
barrage) or on an adj
oining area (like
flood control embankments), due
consideration should be given to the design of
the structure so as to pr
event it from collapsing an
d causing further damage by
the force of water released from behind t
he structure. Hence an estimate of
extreme flood flow is required for the des
ign of hydraulic structures, though the
magnitude of such flood may
be estimated in accordan
ce with the importance of
the structure. For example, the design flood for a large dam like the
Bhakra
(Figure 1)
or the
Hirakud
(Figure 2) would be estima
ted to be more than a
medium sized dam like
Chamera
(Figure 3). It must be remembered that proper
selection of design flood va
lue is of great importance. While a higher value
would result in an increase in the cost
of hydraulic structures, an under-estimated
value is likely to place the struct
ure and population involved at some risk.
Version 2 CE IIT, Kharagpur