Mass and Flow duration curves

S.VIGNESHBTG-12-037

ERG 356 Power Plant Engineering (2+1)

In Presentation

1. Introduction

2. Flow duration curve

3. Mass curves

4. Software's for duration curves

5. Conclusion

6. Reference

Introduction• Hydrologic data generally consist of a sequence of observations of some phase

of the hydrologic cycle made at a particular site. The data may be a record of the

discharge of a stream at a particular place, or it may be a record of the amount

of rainfall caught in a particular rain gage.

• Although for most hydrologic purposes a long record is preferred to a short one,

the user should recognize that the longer the record the greater the chance that

there has been a change in the physical conditions of the basin or in the

methods of data collection.

• If these are appreciable, the composite record would represent only a

nonexistent condition and not one that existed either before or after the

change. Such a record is inconsistent.

• Flow duration curve is a plot of discharge versus percentage of time for which the

discharge is available. It is obtained from hydrograph data. The flow or discharge can be

expressed as cubic meters per second, per week or other unit of time. If the head at

which the flow is available is known, the discharge can be calculated in terms of the

kilowatts power (P) using following equation,

P=(0.736/75)*Qphη kW

• The flow duration curve becomes the load duration curve for hydroelectric plant and thus

it is possible to know the total power available at the site. The maximum and minimum

conditions of flow can also be obtained by the flow duration curve where minimum flow

condition decides the maximum capacity of plant that can be improved by increasing the

storage capacity. Figure shows that flow duration curves are of no use where the time

sequence of the flow is of importance such as in the study of floods.

Flow duration curves

• The flow-duration curve is a cumulative frequency curve that show

the percent of time specified discharges were equaled or exceeded

during a given period.

• It combines in one curve the flow characteristics of a stream

throughout the range of discharge, without regard to the sequence

of occurrence.

• If the period upon which the curve is based represents the long-term

flow of a stream, the curve may be used to predict the distribution

of future flows for waterpower, water-supply, and pollution studies.

Cond….

Flow Duration curves – Effect of observation period

Mass Curve

• It is a plot of cumulative volume of water that can be stored from a stream

flow versus time in days, weeks or months. It shows a mass curve,

Maximum intercept between line AB and mass curve is known as reservoir

capacity

• The unit used for storage is either cubic metre or day-second-metre. A day-

second-metre is the flow at the rate of 1m3/sec for one day and equal to

60*60*24=86400 m3.

• The capacity of plant is based on the storage capacity, which can be

modified by storage for the same mass curve, The water stored in dams is

called pondage and water stored in upstream reservoir is called storage.

• A small storage is used to meet the fluctuating demand for small

period (one day). When powerhouse is away from the main

storage, a small pondage is provided near the powerhouse.

• Sometimes surge tank is used. The capacity of reservoir, made

for a period of deficiency to make available the flow of water at a

required rate, is studied by mass curve.

CONTD….,

Mass curve

Mass curves of water utilization & monthly inflow

Software for Duration curves calculations

• A tool from the older generation that allows calculating and rendering of

duration curves. For example, these curves are often used in the

assessment of hydrological drought.

• Duration curves can be created from the entire imported time series or

from defined parts, such as annual or monthly segments.

• The FDC 2.1 tool was created for the analysis of flow duration curves.

Into the module can be imported daily or weekly types of time series. In

addition to discharge time series can be for the calculation and analysis

used variable time series of a continuous measurements, which are used

in hydrology and hydrogeology.

Stream flow Analysis and Assessment Software (SAAS)

• The aim of that software is to increase our understanding of flow regimes,

linkages to other physical processes, the combined influence on ecological

function, and the result of altering these processes on the integrity of river

systems.

• We are interested in methods to identify and quantify ecologically-important

components of flow regimes, assess their alteration through time with respect

to a reference condition, and relate hydrological alteration to changes in

ecological condition.

• We are also interested in using this information to characterize and classify both

natural and altered flow regimes. Thus the analyses and related methods in

SAAS reflect a focus on environmental flow assessments.

Conclusion

• The present study introduces a variety of nonparametric

quantile-estimation procedures useful for estimating

and interpreting FDCs & MCs. The traditional period-of-

record FDC & MCs can be interpreted as representing

the magnitude and frequency of daily stream flow

during the period of record.

References

• USGS SEARCHY – Flow duration curves

• Hydrology Priciples,Analysis,Design – H.M.Ragunath

• Adamowski, K. (1985). "Nonparametric kernel estimation

of flood frequencies.

• www.hydroengineeringblogs.in

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