Manual Standards Guidelines Ahec Roorkee

8/6/2019 Manual Standards Guidelines Ahec Roorkee

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DRAFT July 03, 2008

STANDARDS / MANUALS / GUIDELINES

FOR SMALL HYDRO DEVELOPMENT

SPONSOR:

MINISTRY OF NEW AND RENEWABLE

ENERGY

GOVERNMENT OF INDIA

GENERAL

MANUAL ON PROJECT HYDROLOGY AND

INSTALLED CAPACITY

LEAD ORGANIZATION:ALTERNATE HYDRO ENERGY CENTREINDIAN INSTITUTE OF TECHNOLOGY, ROORKEE


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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


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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


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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 Jack 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.


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(c) Forest Department

(d) District Revenue Department.

4. Collection 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.


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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:


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(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.


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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.


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(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

75

75

50

50 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


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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 Availability 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


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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:


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Regions States covered

A&B Jammu and Kashmir

C Himachal PradeshD Uttarakhand

E Jharkhand

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.


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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.


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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:


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T Y

2 0.37

5 1.5

10 2.25

25 3.2

50 3.9

100 4.6

(b) Method of least square can also be used. The flood frequency equation is

of the following form:

XT = A + B.Y

Where,

Y = - log log1T

Tand

T =m

N 1+

m is the rank of peak discharge in descending series of flood peaks and N

is the number of flood peaks considered, A & B are constants and can be

worked out by using method of least square.

After determining A and B, XT for any value of T desired can be worked out from

the above equation.

(c) Data Plotting Method

The annual flood peak data shall be plotted on a semi log paper. The

data events are plotted on the ordinate which has the rectangular scale and the

return periods are plotted on the abscissa which has the logarithmic scale. For

example, if 20-years flood data is available and arranged in descending order

then highest flow is assumed to have a return period of 20 years, the second

highest flow a return period of 10-years, the third highest a return period of 6.67

years and so on. Such a plot is shown in Fig. 5. It is seen that peak flows of

watersheds generally produce linear or near linear curves when plotted on semi

log paper. The extrapolation of these linear plots can give the peak flood of 100


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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:

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=

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


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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


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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.


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(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


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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


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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


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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.


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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 Jack, J. Fritz., McGraw Hill,

1982.


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Table 1: Regional Flow Estimates for Various levels of Dependability

Region Regional Values for (Q/Qmean)D For Various Dependability 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 of the Regional Models for Mean Flow

Sl.No.

Region State covered m C Coefficient ofcorrelation

(R)1. A Jammu & Kashmir

(Except Leh & Kargil)

0.06046 3.8189 0.0808

2. B Jammu & 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/Jharkhand 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.


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Manual Standards Guidelines Ahec Roorkee

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