In case
of reservoirs having flood control as one of the purposes, a separate
flood control storage is to be set apart above the storage meant for
power, irrigation and water supply. Flood control storage is meant
for storing flood waters above a particular return period temporarily
and to attenuate discharges up to that flood magnitude to minimise
effects on downstream areas from flooding. Flood and surcharge
storage between the full reservoir level (FRL), and maximum water
level (MWL) attainable even with full surplussing by the spillway
takes care of high floods and moderates them.
(a)
Flood Control Storage
Storage
space is provided in the reservoir for storing flood water
temporarily in order to reduce peak discharge of a specified return
period flood and to minimize flooding of downstream areas for all
floods IS: 5477 ( Part 1) : 1999 equal to or lower than the return
period flood considered. In the case of reservoirs envisaging flood
moderation as a purpose and having separate flood control storage,
the flood storage is provided above the top of conservation pool.
(b)
Surcharge Storage
Surcharge
storage is the storage between the full reservoir level (FRL) and the
maximum water level (MWL) of a reservoir which may be attained with
capacity exceeding the reservoir at FRL to start with. The spillway
capacity has to be adequate to pass the inflow design flood making
moderation possible with surcharge storage. The methods that are
generally used for estimate of the Design Flood for computing the
Flood Storage are broadly classified as under:
1.
Application of a suitable factor of safety to maximum observed flood
or maximum historical flood.
2.
Empirical flood formulae.
3.
Envelope curves.
4.
Frequency analysis.
5. Rating
method of derivation of design flood from storm studies and
application of the Unit Hydrograph principle.
(c)
Application of a Suitable Factor of Safety to Maximum Observed Flood
or Maximum Historical Flood
The
design flood is obtained by applying a safety factor which depends
upon the judgement of the designer to the observed or estimated
maximum historical flood at the project site or nearby site on the
same stream. This method is limited by the highly subjective
selection of a safety factor and the length of available stream flow
record which may give an inadequate sample of flood magnitudes likely
to occur over a long period of time.
Empirical
Flood Formulae : The empirical formulae commonly used in the
country are the Dicken’s formula, Ryve’s formula and Inglis’
formula in which the peak flow is given as a function of the
catchment area and a coefficient. The values of the coefficient
varywithin rather wide, limits and have to be selected on the basis
of judgement. They havelimited regional application, should be used
with caution and only, when a more accurate method cannot be applied
for lack of data.
Envelope
Curves: In the envelope curve method maximum flood is obtained
from the envelope curve of all the observed maximum floods for a
number of catchments in a homogeneous meteorological region plotted
against drainage area. This method, although useful for generalizing
the limits of floods actually experienced in the region under
consideration, cannot be relied upon for estimating maximum probable
floods for the determination of spillway capacity except as an aid to
judgement.
Frequency
Analysis: The frequency method involves the statistical analysis
of observed data of a fairly long (at least 25 years) period. It is a
purely statistical approach and when applied to derive design floods
for long recurrence intervals, several times larger than the data,
has many limitations. Hence this method has to be used with caution.
(d)
Rational Method of Derivation of Design Flood from Storm Studies and
Application of Unit Hydrograph Principle
The steps
involved, in brief, are:
i.
Analysis of rainfall and run-off data for derivation of loss rates
under critical conditions;
ii.
Derivation of unit hydrograph by analysis (or by synthesis, in cases
where data are not available);
iii.
Derivation of the design storm; and
iv.
Derivation of design flood from the design storm by the application
of the rainfall excess increments to the unit hydrograph.
The
Maximum Water Level of a reservoir is obtained by routing the design
flood through the reservoir and the spillway. This process of
computing the reservoir storages, storage volumes and outflow rates
corresponding to a particular hydrograph of inflow is commonly
referred to as flood routing. The routing is carried out with the
help of the following data,
1.
Initial reservoir stage
2. The
design flood hydrograph
3. Rate
of outflow including the flow over the crest, through sluices or
outlets and through power units, and
4.
Incremental storage capacity of the reservoir.
Source: CE, IIT Kharagpur
