7/28/2019 1.4 Manual on Project Hydrology (1)
1/28
STANDARDS/MANUALS/GUIDELINES FORSMALL HYDRO DEVELOPMENT
General Works
Manual on Project Hydrology and Installed Capacity
Sponsor:
Ministry of New and Renewable EnergyGovt. of India
Lead Organization:
Alternate Hydro Energy CenterIndian Institute of Technology Roorkee
J uly 03, 2008
7/28/2019 1.4 Manual on Project Hydrology (1)
2/28
CONTENTS
S.No. TITLE Page No.
1. SCOPE AND OBJECTIVE 12. DATE REQUIREMENT 13. RESOURCES OF DATA 14. COLLECTION OF DATA 15. ASSESSMENT OF QUALITY OF DATA 36. FILLING IN MISSING DATA 47. CONSISTENCY CHECKS 48. PROCESSING AND PRESENTATION OF DATA 5
8.1. Rainfall Data 5
8.2. Stream Flow Data 5
9. EXTENSION OF PERIOD RECORD OF STREAM FLOW DATA 69.1.When Short Term Data Is Available At Project Site 6
9.2. When Stream Flow Data To Two Lean Seasons And One Flood 6
Season At Site Are Recorded
10.FLOW ASSESSMENT FOR AN UNGAUGED CATCHMENT 710.1. Long Term Data Of Some Other Site 710.2. Regional Specific Discharge 710.3. Regional Model 8
11.WATER AVAILABILITY ASSESSMENT 811.1. Application of FDC 811.2. Application of NIH Model 911.3. When Only Observed Discharge of Two Non-monsoon 11
7/28/2019 1.4 Manual on Project Hydrology (1)
3/28
Seasons are Available
11.4. Energy Curve 1111.5. Daily Pondage 11
12.ESTIMATION OF FLOOD DISCHARGE 1212.1. Computation of Design Flood 1212.2. Spillway Design Flood and Construction Floods 17
13.SEDIMENTATION 1814.WATER QUALITY 1915.OTHER HYDROLOGICAL AND METEOROLOGICAL 19
DATA REQUIREMENT
15.1. Tail Race Rating Curve 1915.2. Meteorological Data 20
16.REFERENCES 20
Table 1: Regional Flow Estimate For Various Levels of Dependability
Table 2: Values of the parameters of the Regional Models for Mean Flow
Fig. 1: Annual Rainfall Duration Curve
Fig. 2: Regional Hydrographs with 50% dependability
Fig. 3:
Fig. 4: Flow Duration Curve for July Flows
Fig. 5: Flood frequency peak flow vs. return period
7/28/2019 1.4 Manual on Project Hydrology (1)
4/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 1
GUIDELINES ON PROJECT HYDROLOGY
1. Scope and Objective
The scope and objective of these guidelines is limited to hydrological data
collection, estimation / assessment and data analysis to establish a reliable flow
quantity with time variability and the peak flood discharge at the project site
alongwith other hydrological inputs required for project preparation. A large
number of text books (Chow, Mutreja, Varshney etc.) on the subject are available
alongwith the guidelines of CWC and CEA for the preparation of hydrology
chapter of the DPR of the Hydro Power Projects. For specific hydrological issues
I.S. Codes and publications of NIH Roorkee are also of great help. Unfortunately
the available literature is not of much help in Small Hydro Projects where
observed stream flow is either available for a very short period or no data is
available or data from other sources is made use of. For the project report
preparation of a small hydropower project guide lines of CBI&P (Pub. No. 280)
and CEA are available. A publication by J ack J . Fritz is also of help. The purpose
of these guidelines is to familiarize the user with the data requirement, the source
of data and analysis techniques to be used, considering different constraints in
assessing hydrologic inputs, in determining water availability and the peak flooddischarge at the project site. Water availability decides the techno-economic
feasibility of the project and flood discharge is important for safe design of the
structures.
In general the objectives of these guidelines are:
(1) To provide the user the knowledge of data requirement, about the
source of data, and evaluation and extension of data.
(2) To familiarize the user with various methods of synthesizing data at an
ungauged site.
(3) To provide an overview of various analysis techniques available to
work out stream flows with time variability and peak flood discharge.
(4) To provide knowledge and hydrologic considerations for inputs other
than stream flow.
7/28/2019 1.4 Manual on Project Hydrology (1)
5/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 2
2. Data Requirement
The basic requirement is of observed long term stream flow data with
gauge at the project site. Generally such data is not available for the small hydro
project sites. The data requirement to use indirect methods to develop the stream
flow series the following data shall be obtained:
(i) Catchment area with following characteristics:
(a) Altitude,
(b) Raingauge locations and their long term rainfall data
(c) River system
(d) Land use pattern
(e) Snow cover, if any
(f) Details of existing projects in the catchment, if any.
(ii) Stream flow and rainfall data of adjoining catchments.
3. Sources of Data
(i) Raingauge locations, rainfall and snowfall data and other climatological
information such as temperature, wind velocity, evaporation, cloud cover
etc. are available with the India Meteorological Department (IMD) New
Delhi and its regional offices.
(ii) Major rivers and its tributaries are being gauged regularly by the Central
Water Commission, New Delhi and State Governments and long term
records of water levels, discharges and sediment load at every gauging
site are available. Daily discharges are available and hourly or four hourly
gauges during floods are also recorded. However, the Ganga basin data is
classified and not readily available. It is very rare that the above recorded
data is available for a small hydro project site.
(iii) The information about catchment can be obtained from survey of India
maps.
(iv) Other sources from where some useful information can be obtained are
the concerned.
(a) Irrigation Department
(b) Agriculture Department
7/28/2019 1.4 Manual on Project Hydrology (1)
6/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 3
(c) Forest Department
(d) District Revenue Department.
4. Collect ion of Data
The data from above sources may be obtained after receiving permission
from competent authority or on payment if available on sale.
As stated earlier, the observed stream flow data is generally not available
at the proposed SHP site and in many cases not even a single rain gauge exists
in the entire catchment. In such a situation, a gauging station near the site and a
few rain gauges in the catchment should be established. The existing guidelines
of CEA, CBI&P and IREDA recommend that discharge measurements should be
carried out for a minimum of two years covering two lean seasons and one
monsoon season. Two years discharge data is too short to develop a long term
series but it gives an idea about minimum discharge expected to be available
and can be used with the data from other sources to make use of indirect
methods for estimating flows and flood discharge. In order to have a longer
period observed discharges, the gauging site should be established at the
earliest and the data till the preparation of DPR should be made use of in
hydrological studies. For gauging and discharge measurement techniques
readers may refer guidelines for site investigations.
5. Assessment of Quality of Data
For assessing the quality of data, knowledge of methods of measurement
and observations, the instruments used and the frequency of observations is
essential. For example if discharge measurement is done with the help of floats,
the discharge data should be corrected with a suitable factor which is taken as
0.89. The adequate length of data is essential for any hydrologic analysis. The
longer the length of data more is the confidence on the reliability of the analysis.
Generally data of 25 to 30 years is considered adequate for any statistical
analysis. Quality of data is also adversely affected if there are missing data and
to increase length of data the missing data is filled-in. The normal procedures
adopted are mentioned in following para.
7/28/2019 1.4 Manual on Project Hydrology (1)
7/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 4
6. Filling-in Missing Data
It is generally observed that rainfall and discharge data in many cases are
found missing for some days and even for months. Attempt are made to fill-in
missing links using standard methods to make a continuous record. The
techniques used are:
(i) Using values observed earlier for the missing period
(ii) Interpolation from adjoining values by plotting a smooth curve.
(iii) Using the average proportion with normals for the adjoining stations.
Atleast three stations be used.
Nx =
++
c
c
b
b
a
ax
P
N
P
N
P
NP
3
a , b & c are three adjoining stations. N is normal (mean) precipitation, P is
precipitation during shorter period, and x is station of missing data. Use of
this method is generally limited to precipitation time periods of not less
than a months duration.
(iv) Rainfall run-off correlation may be used. Runoff at a downstream site
should be adjusted for upstream withdrawals before establishing rainfall
runoff correlation. Missing gaps in rainfall data can be conveniently filled-in
by using HEC- 4/6 programme of US Army Corps of Engineers.7. Consistency Checks
The consistency of the precipitation as well as the stream flow data can be
checked by the technique of double mass analysis. In this method a graph is
plotted of cumulative monthly (or any other time period) values at a station to be
checked against those of a reliable or a group of adjoining reliable gauging
stations of the same period. The data of the station in question is consistent if the
above plot is a straight line. The change in the slope of the double mass curve
shall be investigated as it may be caused by change in location of gauging site,
change in measurement techniques, changes in river regime or any other man
made interference.
The consistency of data can also be checked by:
7/28/2019 1.4 Manual on Project Hydrology (1)
8/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 5
(i) A study of stage discharge curve of different periods and outliers
may be examined and corrected.
(ii) Comparing the monthly and annual runoff with corresponding rainfall in
the catchment.
(iii) Comparing monthly specific flows (flow per unit catchment area) with
corresponding figures at other sites on same river or the adjoining
rivers.
8. Processing and Presentation of Data
8.1 Rainfall Data
It is, generally, used to develop rainfall runoff correlation. The rain gauge
records point rainfall and the areal distribution is worked out from the rain-gauge
records of the rain-gauges located in side or around the catchment. The average
catchment rainfall is estimated on 10-daily or monthly or annual basis by using
one of the following methods.
(i) Arithmatic mean method used commonly when large number of
rain-gauges are uniformly distributed in catchment.
(ii) Thiessen Polygon method used when a few rain-gauges are
located in and around the catchment.
(iii) Isohyetal. method
used when large number of rain-gauges arelocated in the catchment. It is time consuming.
HEC 4/6 package can be used to compute average rainfall in the
catchment. If long term data is available then 50%, 75% and 90% dependable
rainfall can be worked out and used for further hydrological analysis.
This analysis of rainfall data is very useful if the raingauges in the
catchment and their long term record is available but generally it is rarely
available in the catchments of small hydro projects, which harness flow of small
catchments.
8.2 Stream Flow Data
The stream flow data shall be processed and compiled in suitable formats
/ tables for appropriate time units generally as 10 day average, monthly and
annual runoff, annual maximum flow.
7/28/2019 1.4 Manual on Project Hydrology (1)
9/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 6
9. Extension of Period of Record of Stream Flow Data
9.1 When Short Term Data (5 to 10 years) is available at Project Site
Stream flow data of long duration is generally, not available at the
proposed project site. The short term data is normally extended with the help of
long term data of other sites on the same stream or in the adjoining catchments.
Sometimes the rainfall data is also used to extend the short term discharge data.
The correlations are generally developed through regression analysis of short
term data of the site with the corresponding period data of other sites where long
term data is available. Generally following two regression models are used:
Bivariate linear y = a + bx
Bivariate curvilinear y = axm (log y = log a + m log x)
A correlation is considered good when correlation coefficient is near unity.
In a discharge discharge correlation x and y are discharges of two sites
and by using the correlation the short term discharge data is extended with the
help of long term data. In a rainfall runoff correlation x may be for rainfall values
and y the runoff.
Standard computer programmes for statistical relations can be used to
determine the best fit regression model. This can also be done manually using
least square method (any standard book on hydrology, CBI&P manual etc. may
be referred for the computations).
9.2 When stream flow data of two lean seasons and one flood season at
site are recorded:
In small hydro project sites even the short term data is generally not
available and project reports are based on the minimum requirement of
discharge observations of two lean seasons and one flood season. In such
situations the following approximate method of determining discharges of
different dependability utilizing the rainfall data may be used:
(i) An annual rainfall duration curve for the catchment of the project is
developed (a typical curve is shown in fig. 1).
(ii) Find annual rainfall of different dependability (R50, R75, R90) from the
curve.
7/28/2019 1.4 Manual on Project Hydrology (1)
10/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 7
(iii) Find mean annual rainfall of the catchment for the period of discharge
observation (Rm).
(iv) Find the ratios of annual rainfalls of different dependabilities (as
determined in (ii) to the mean annual rainfall of the catchment (as
determined in (iii)).
mmm R
R
R
R
R
Rr 9090
7575
5050 and; ===
(v) Mean monthly discharges of 50%, 75% and 90% can be computed by
multiplying the mean monthly discharges of the observed period by the
corresponding ratios (as determined in (iv)).
10.0 Flow Assessment for an Ungauged Catchment
Many times situation arises when the discharge observations are available
and flow assessment has to be made for the preparation of project report.
Depending on the availability of data of other sites or basins one of the following
method may be adopted.
10.1 Long Term Data of some other site:
When long term flow measurement data of a site on the same stream or
adjoining stream is available. It can be transposed to the proposed site in
proportion to the catchment areas of the two sites.
2
2
11
2
1
2
1 Qi.e. QA
A
A
A
Q
Q==
1. Denotes ungauged site and 2 the site for which flow data is available.
10.2 Regional Specific Discharge
In this method the discharge data of hydro meteorologically similars river
basin is used. The data is converted with specific discharges which is defined as
discharge per unit catchment area. Generally monthly hydrographs of specific
discharges of a specific dependability of a number of adjoining basins are plotted
on a graph. Typical 50% dependable specific discharge hydrograph of a number
of basins are shown in Fig. 2. A mean hydrograph may be drawn which will
approximately represent the specific discharges of 50% dependability for the
ungauged catchment in the region. The monthly specific discharge values of the
7/28/2019 1.4 Manual on Project Hydrology (1)
11/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 8
mean hydrograph when multiplied by the area of the ungauged catchment will
give the 50% dependable discharge series. Similarly the discharge series of
other dependabilities such as 75%, 90% can be generated.
10.3 Regional Model
A regional model for generating flows of different dependabilities has been
developed by AHEC and Department of Earth Sciences of I.I.T. Roorkee and NIH
Roorkee under UNDP GEF Program (MNES, Govt. of India) which is useful for
generating the flow duration curve of an ungauged catchment in Himalayas.
11.0 Water Availabili ty Assessment
Water availability with time variability at the proposed project site is
essential to estimate the power potential and annual energy generation on which
depends the financial variability of the project. Since small hydro project is a run-
of-river scheme the flow duration curve (FDC) is used to know the time variability
of flow at a location. The FDC is a simple depiction of flow at a location against
percentage of time. It shows a discharge which has equaled or exceeded certain
percentage of time out of the total time period which is generally taken as one
year. Typical FDCs are shown in Fig. 3. The shape of FDC reflects the
hydrological characteristics of the stream. FDC of shape A in Fig. 3 belongs to a
flashy stream in which high floods occur for a very short duration and shape C
reflects the characteristics of a stream in which variation between high and low
flows throughout the year is not large.
11.1 Application of FDC
For water availability studies for a SHP the FDC is drawn for 90%
dependable years. The 90% dependable year is calculated by arranging in
descending order, the annual runoff of all the years for which observed or
extrapolated / extended discharge data is available and using Weilbuls formula:
100x1+
=N
mP
P is dependability percent, m is the rank of runoff of the desired
dependability, N is the number of data. If P is 90% N = 19, m works out as
7/28/2019 1.4 Manual on Project Hydrology (1)
12/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 9
181)(19x100
90=+ . Thus 90% dependable flow year will correspond to the runoff
which is at rank 18 from the top.
For working out the FDC for 90% dependable year, the 10-day discharge
series of that year is considered. These 36 discharges are arranged in
descending order and percentage of time each has exceeded or equaled is
worked out using the above Weilbuls formula. Discharge of rank first will be
equaled or exceeded by 100x136
1
+i.e. 2.7% of time. Similarly discharge of rank
2 will be equaled or exceeded by 5.4% of time. In this manner percentage of time
equaled or exceeded by all the 36 discharges can be worked out and potted. A
typical FDC is shown in Fig. 4. From this curve discharges of variousdependability such as Q90, Q75, Q50 etc. may be obtained. The energy
corresponding to Q90 will be the firm energy from the project. Secondary energy
can be generated if the system is designed for a higher discharge. This will,
however, be generated for a part period of the year. To decide the design
discharge cost of generation is worked out for four or five discharges of
dependability less than 90% and the discharge which gives the minimum cost of
generation is selected for further planning and design of project.
This procedure of developing and using FDC is possible when long term
discharge data at site observed or extended is available.
11.2 Application of NIH Model
Generally long term discharge data at the site of SHP is not available, the
model developed by NIH for ungauged catchments based on
hydrometereologically similar regional catchments can be used. In this model
FDC for an ungauged catchment is derived using regionalization procedure. The
regions are identified as below:
7/28/2019 1.4 Manual on Project Hydrology (1)
13/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 10
Regions States covered
A&B J ammu and Kashmir
C Himachal PradeshD Uttarakhand
E J harkhand
F Sikkim
F West Bengal
G Assam
G Arunachal Pradesh
H Meghalaya
I Manipur, Nagaland, Tripura, Mizoram
For each region based on available data of gauged catchments a mean
FDC of that region in terms of Q/Qman and percentage of time is developed. The
regional flow estimated values for (Q/qmean)D for various dependability levels (D)
are given in Table 1. Qmean for each gauged catchment is related with catchment
area (A) in the following form.
Qmean = CAm
Where C is coefficient and m is exponent. The values of C and m for each region
(A to I) are given in Table 2.
Knowing the area of ungauged catchment Qmean can be worked out using
the values of C & m of the region in which the ungauged catchment lies. This
value of Qmean multiplied by the factor (Q /Qmean )D for that region from table 1 will
give the required D% dependability flow (QD) for that ungauged catchment. After
obtaining QD for different value of D the FDC of the ungauged catchment can be
plotted for further planning purposes.
7/28/2019 1.4 Manual on Project Hydrology (1)
14/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 11
11.3 When Only Observed Discharge of Two Non-monsoon Seasons areavailable:When no other information except the discharges of two non-monsoon
season at the proposed site is available and no analysis for extension of data is
possible, the design discharge for the feasibility stage planning may be taken asthree times the minimum observed discharge at site during two non-monsoon
seasons.
11.4 Energy Curve
The FDC on a different scale represents the energy curve because the
energy is power multiplied by time and the power is directly proportional to
discharge. It is given by
P = 9.81 Q H
Where,
P is power in kW
Q is discharge is cumec
H is net head to be utilized in generation in metre
is overall efficiency of generating equipment.
11.5 Daily Pondage
A run-of-the river scheme or a small hydro project can be run as a peaking
station if daily pondage capacity is provided at the diversion site. It is generally
provided by installing high head gates.
When a scheme is designed for a discharge more than Q90, all the
machines provided will generate when during monsoon the design discharge is
available and during non monsoon period when discharges are low most of the
machines will be idle. If daily pondage capacity is provided say for 20 hours in a
day, the power plant can be run at full capacity for four hours during peak
demand. The pondage requirement estimation is as illustrated below:Design discharge is 5 cumec, the minimum flow is 1 cumec. Let the hours
for which the ponding of minimum flow is required to run the power plant for
peaking are X
X = L = (24 - X ) (5 1)
X = 19.2 hours.
7/28/2019 1.4 Manual on Project Hydrology (1)
15/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 12
The power plant will run at design discharge of 5 cumec for 4.8 hours in a
day. The pondage capacity between FRL and MDDL required will be 1 x 3600 x
19.2 = 69120 cum.
12.0 Estimation of Flood Discharge
Estimation of flood discharge is essential for the safety of the diversion
structure of the SHP which is generally a weir or barrage or small dam without
large storage capacity. In such a case moderation of flood peak is not possible
and so the waterway to pass flood should be of adequate capacity. The flood
discharge for which the structure is designed is called design flood. It is fixed
after due consideration of economic, hydrologic factor and safety of life and
property in the downstream. On the basis of the guidelines set out in IS Code
11223 1985 the design flood for the diversion structure shall be a discharge of
100 year return period.
12.1 Computation of Design Flood
12.1.1 Flood Frequency Method
For estimating the design flood one of the standard flood frequency
methods may be used. Large number of flood frequency methods are available
for which the reader can refer any standard book on Hydrology. For the use of
any method of flood frequency analysis long term record (about 30 years) of
observed flood peak discharges is required. Gumbels method is generally
recommended for small hydro projects.
(a) According to Gumbels method of moment the flood frequency equation is
XT = x + s (0.78 Y 0.45)
Where,
XT is flood peak of return period T.
x is average value of annual flood peaks.
s is standard deviation of flood peak series.
Y is called reduced variable and is a function of T and its values are as
below:
7/28/2019 1.4 Manual on Project Hydrology (1)
16/28
7/28/2019 1.4 Manual on Project Hydrology (1)
17/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 14
year return period. This extrapolation can also be done by developing the best fit
line using method of least square to determine the constants in the following
equations:
XT = A + B log10 T
12.1.2 When Peak Flood Data at Site is not available
Generally, long record of flood peaks at SHP sites is not available. In that
case one of the following approaches may be adopted depending on availability
of data.
(a) If long term record of flood peaks of some other site on the same stream
or a site in adjoining hydrometereologically similar catchment is available, the
flood frequency analysis can be carried out, by methods given above, to
determine the peak flood of 100-year return period and the same can be
transposed to the ungauged site of the SHP in proportion to area by using
following equation:
4/3
=
u
g
u
g
A
A
Q
Q
Where,
Qg is flood peak of 100 year return period of site of which record is
available.Ag is catchment area of site of which record is available.
Qu is flood peak of 100 year return period of ungauged site.
Au is catchment area of ungauged site.
(b) In planning SHP, records of long term flood discharges are seldom
available. In such cases an alternative approach would be to estimate
storm rainfall of suitable duration with desired return period say 100 years.
This information can be obtained from IMD. A suitable unit hydrograph (a
characteristic of hydrological the catchment) is derived for the catchment.
It is defined as a flood hydrograph of surface runoff resulting from a unit
depth (1 cm) of excess rainfall distributed uniformly over the basin area at
a uniform rate during unit period (say 3 hr, 3 hr, 4 hr, 6 hr etc.). It can be
7/28/2019 1.4 Manual on Project Hydrology (1)
18/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 15
derived from the observed flood hydrographs of a few years if available at
the site and the storm rainfall record causing these flood hydrographs.
If observed flood hydrographs are not available, a synthetic unit
hydrograph based on catchment characteristics can be developed. The
procedure for computations of flood hydrograph using the unit hydrograph
and the storm rainfall is given in CWC Publication Estimation of Design
flood Recommended Procedures 1972, Readers can refer to any
standard text book on hydrology for unit hydrograph method of working
out flood hydrograph.
(c) Use of Regional Unit Hydrograph
When no discharge data at site is available, the parameters of unit
hydrograph are evaluated by a regional approach using data of adjoining
basins. CWC has made extensive studies of small catchments of river
basins dividing them in 27 sub zones and has developed regional unit
hydrograph parameters to estimate peak flood discharges resulting from
50 and 100 year return period storm rainfalls. For the derivation of regional
unit hydrograph and its application to compute flood hydrograph,
reference may be made to relevant CWC study report of the sub zone to
which the catchment of proposed SHP belongs.
(d) Regional Flood Frequency Analysis:
Flood frequency analysis by Gumbels method should be carried out and
curve prepared for as many gauging stations in the region as possible
whose flood records of atleast 10-15-years are available. A homogeneity
check should be carried out.
A set of flood ratios (ratio of flood to mean annual flood) is computed for
each station over a range of arbitrarily selected return periods with the
help of frequency curves.
For each of selected return periods the mean of the ratios of all the
stations is computed. The resulting means are the flood ratios for the
regional frequency curve. These are plotted on extreme value probability
7/28/2019 1.4 Manual on Project Hydrology (1)
19/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 16
paper and best fit line by method of least square can be determined. This
line is the required regional frequency curve.
This curve can be used to determine flood of a specific return period only
when mean annual flood of ungauged catchment is known. For this
purpose the mean annual floods of all the gauged stations used for
developing regional frequency curve are correlated with their catchment
areas either by plotting the data on a logarithmic paper or through a
suitable regression model. By using this relation the mean annual flood of
ungauged catchment corresponding to its area is worked out. This mean
annual flood multiplied with flood ratio corresponding to the desired return
period obtained from regional frequency curve will give the flood of desired
return period of ungauged catchment.
(e) When no data of discharges and rainfall are available, the following two
methods can be used to assess the peak flood at site.
(1) Based on Field Information:
A study of physical features near the stream at site shall be made to find
the signs of high flood mark which shall be confirmed from local enquiry
from old persons living near the stream and the records of local revenue
officials and department of Bridge and Roads. After ascertaining the high
flood level, the flood discharge corresponding to this level can be
computed by using Mannings equation. Area of river cross section (A) at
site and river slope (S) shall be obtained by conducting the surveys. The
coefficient of Manning n shall be assumed on the basis of physical
features of river at site for working out the peak flow from the following
equations:
2/13/21SR
nV =
Where, ,P
AR = P is the perimeter below high flood mark.
7/28/2019 1.4 Manual on Project Hydrology (1)
20/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 17
(2) Use of Empirical Formulae:
There are a large number of empirical relations to estimate peak flood
flows on the basis of catchment area. The most commonly used formulae
is of Dickens.
Q = CA3/4
Where, C is the coefficient which varies from region to region. On the
basis of discharge observations of long periods at various locations the
country is divided into regions and each region is assigned a value of C for
the use in Dickens formulae.
12.2 Spillway Design Flood and Construction Floods
As already stated above, a SHP is a run-of-river project where there is no
significant storage at river diversion site. Hence, peak flood cannot be
moderated. Therefore, the water way for the barrage / weir or spill way of
low height diversion dams shall be provided for the design flood. The
design flood is decided on consideration of economic and hydrological
factors as well as the safety in the downstream. Normally the design flood
for barrage / weir is taken as flood of 100 year return period and SPF for
the spillway of a diversion dam.
During construction of a diversion structure the river flow has to be
diverted. The diversion arrangement has to be planned and designed for a
certain discharge which is always associated with some amount of risk of
being exceeded. This again depends on hydrologic, economic factors and
the construction sequence and schedule. When the diversion is to be
done for non-monsoon flow and monsoon flood can be allowed to pass
over in complete barrage / weir, the diversion arrangement can be
planned and designed for the maximum non-monsoon flow in the past 10
years. In case the monsoon flood can not be allowed to pass the
incomplete structure the diversion for a SHP project can be planned for a
flood a return period of 4 to 5 times the construction period of the project.
Suppose the construction will take 3 years to complete, the diversion
design flood shall be of 15 to 20 years return period. However, there is
7/28/2019 1.4 Manual on Project Hydrology (1)
21/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 18
always a risk of its being exceeded. In that case the expected damage
should be assessed and compared with the extra cost involved in
designing diversion arrangement for a flood of higher return period.
13.0 Sedimentation
SHPs generally operate at full installed capacity during monsoon period
like any other run-of-river scheme with a small diversion structure without
storage. During this operation silt laden water of monsoon flows is diverted into
water conductor. It is seen to cause damage and sometimes very serious
damage to under water components of the generating equipment such as
runners, guide vanes etc. resulting in loss of generation and costly repair and
maintenance of equipment. The problem is more severe in projects located on
Himalayan streams which carry lot of sediment during monsoon.
Studies have shown that the rate of sediment erosion may be expressed
as:
W S1, S2, S3, S4, Mr Vx
Where, w is rate of erosion, S1, S2 S3, S4 are the coefficients of sediment
characteristics such as concentration, hardness, size and shape, Mr is
coefficient of erosion resistance of base metal of equipment, and Vx is
relative velocity of flow in turbine with exponent x which depends on typeof turbine (x = 3 for Francis turbine, 2.5 for guide vanes, 2.5 for nozzles of
Pelton wheel and 1.2 for Pelton wheel buckets).
It has been observed that high concentration of even fine angular quartz
particles (hardness 7 on Mohs scale) cause maximum erosion in high
head power plants. A variety of sediment exclusion and extraction
measures are provided to reduce size and concentration of sediment
particles in the flow reaching the generating equipment in order to reduce
the damage due to silt erosion. The planning and design of these
measures depend on the sediment characteristics. Hence, even at the
planning stage the above characteristics of sediment i.e. size, shape,
hardness and concentration, which are site specific should be assessed
with as much accuracy as possible for planning and design of cost
7/28/2019 1.4 Manual on Project Hydrology (1)
22/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 19
effective sediment exclusion and extraction measures. Sediment sampling
at site for concentration, sieve analysis and petrographic analysis (for
mineral composition and shape) is essential at the diversion site. The
knowledge of these sediment characteristics is also important for turbine
manufactures.
The design and dimensions of desilting measures depend on the size of
particles which is to be extracted. There is no universally accepted
criterion to decide the size of particle to be extracted. As a guide line if the
presence of quartz in sediment is not significant, extraction of (+) 0.5 mm
size particles through vortex type desilting measures is adequate for
medium and high head plants. If quartz is predominant desilting basin to
extract + 0.2 mm is generally provided. It may be combined with other
measures such as vortex tube, ejector etc. for greater effectiveness.
14.0 Water Quality
Besides the sediment, the chemical analysis of water is important to have
a knowledge of presence of salts and the nature of water (acidic or alkaline)
which will have the effect on the metal of gates and equipment, and concrete
structure. The parameters generally determined in chemical analysis are:
1. Dissolved solids
2. pH value
3. Suspended solids
4. Total hardness
5. Sulphates, carbonates, bi-carbonates, chlorides
6. iron, calcium, magnesium
7. Electrical conductivity.
8.
15.0 Other Hydrological and Meterelogical Data Required:
15.1 Tail water rating curve:
15.1.1 It is the stage vs discharge curve and is required at the diversion site and
7/28/2019 1.4 Manual on Project Hydrology (1)
23/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 20
at the downstream of the power house for the planning design and
operation of the project. If gauge discharge data for a couple of years is
available the required curve can be obtained by plotting this data. On
logarithmic paper this is generally a straight line. It can be extrapolated for
peak flow.
15.1.2 If the gauge discharge data is not available, it can be developed by using
Mannings equation as described in para 12.1. From the river cross
section, discharges are worked out for different water depths in the section
and the plot gives stage discharge curve.
15.2 Metereological Data
Besides the rainfall data the following metereological data of the proposed
site for SHP are required for proper planning, design and operation of the project.
These can also be obtained from IMD.
(i) Air and water temperature in different days of the year with monthly
and seasonal maxima and minima.
(ii) Wind velocity and direction: Maximum velocity is required for design of
structures.
(iii) Evaporation : It is required to assess loss of water in reservoir, if
storage is provided.
(iv) Annual Climatic Changes: It is needed to plan the construction
sequence and schedule.
(v) Seismicity: It is required for design of structures. Based on
geographical location of project, it can be assessed from IS:1839.
References:
1. Hydrology by Ven-ta-Chaw
2. Hydrology by K.N. Mutraja
3. Hydrology by R.S. Varshney
4. Manual on Planning and Design of SHP, Pub. No. 280 CBI&P.
5. Guidelines for Planning SHP, CEA.
7/28/2019 1.4 Manual on Project Hydrology (1)
24/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 21
6. Report on UNDP- GEF Hilly Hydro Project, Vol. III Regional FDC
Published by MNES, Govt. of India, 2002.
7. Estimation of Design Flood Recommended Procedure CWC, Sept.
1972.
8. Sub-Zone Flood Estimation Reports, CWC.
9. IS : 11223 1983.
10. IS : 1839.
11. Small and Mini Hydropower Systems J ack, J . Fritz., McGraw Hill,
1982.
7/28/2019 1.4 Manual on Project Hydrology (1)
25/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 22
Table 1: Regional Flow Estimates for Various levels of Dependabili ty
Region Regional Values for (Q/Qmean)D For Various Dependabili ty Levels
D = 25% D = 50% D = 60% D = 75% D = 80% D = 90%
A 1.1562 0.6584 0.5428 0.4011 0.3577 0.2686
B 1.2240 0.8434 0.7360 0.5888 0.5396 0.4304
C 1.1797 0.6609 0.5399 0.3917 0.3466 0.2544
D 1.2828 0.8364 0.7078 0.5315 0.4729 0.3447
E 0.7374 0.2711 0.1974 0.1226 0.1031 0.0675
F 1.0942 0.5089 0.3896 0.2551 0.2171 0.1444
G 1.3075 0.8500 0.7148 0.5270 0.4640 0.3257
H 1.1436 0.4909 0.3551 0.2053 0.1646 0.0913
I 1.2451 0.5511 0.3957 0.2198 0.1716 0.0856
Table 2: Values of the parameters o f the Regional Models for Mean Flow
Sl.No.
Region State covered m C Coefficient ofcorrelation
(R)
1. A J ammu & Kashmir
(Except Leh & Kargil)
0.06046 3.8189 0.0808
2. B J ammu & Kashmir
(Leh & Kargil)*
Q/A = (1/2)(Q/A)Leh + (Q/A)Kargil
= 0.05804
3. C Himachal Pradesh 0.86811 0.1200 0.8759
4. D Uttar Pradesh/Uttarakhand 0.89075 0.0463 0.8174
5. E Bihar/J harkhand 0.74795 0.0652 0.7742
6. F West Bengal & Sikkim 0.98920 0.0577 0.8467
7. G North Assam & Arunachal
Pradesh
0.26817 2.2807 0.3706
8. H South Assam & Meghalaya 0.48589 1.4136 0.6820
9. I Manipur, Nagaland,
Mizoram & Tripura
1.22343 0.0151 0.9435
*An average model developed due to lack of data.
7/28/2019 1.4 Manual on Project Hydrology (1)
26/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 23
7/28/2019 1.4 Manual on Project Hydrology (1)
27/28
AHEC/MNRE/SHP Standards/General Manual on Project Hydrology /July 2008 24
7/28/2019 1.4 Manual on Project Hydrology (1)
28/28