Hydraulic and Hydrologic Design Concept

Chapter Four
Chapter Four
Hydraulic and Hydrologic Design Concept
By Yimam Alemu
December 16, 2020
By Yimam Alemu Chapter Four December 16, 2020 1 / 48

Chapter Four
Outline
1 Introduction
2 The Hydrologic Cycle
3 Measurement of run-oﬀ
4 Hydrologic analysis of hydropower
5 Energy and Power Analysis
6 Electrical terms associated with hydropower

Chapter Four
Learning Objectives
At the end of this chapter the students should be able to:
Understand the definition of hydrology and hydrologic cycle.
Understand methods for measurement of run off and factors affecting run off.
Understand hydrologic analysis of hydropower.
Understand electrical terms associated with hydropower.

Chapter Four
Introduction
Introduction
Hydrology is the study of the occurrence, movement and distribution of water on,
above and within the earth’s surface.
It is the science which deals with the depletion and replenishment of water resources.
It deals with the surface water as well as the ground water.
It is also concerned with the transportation of water from one place to another, and
from one form to another.
It helps us in determining the occurrence and availability of water.

Chapter Four
Introduction
Cont....
Earlier it was pointed out that the principal parameters necessary in making hydro
power studies are water discharge and hydraulic head.
The measurement and analysis of these parameters is hydrological problem.
Part of the hydrology problem is to identify the vertical distance between the level of
water in the forebay (headwater) of the hydro plant and in the tailrace where the water
issues from the draft tube at the outlet to the turbine.
Determination of head is a surveying problem that identiﬁes elevations of water surface.

Chapter Four
Introduction
Cont....
Figure: Hydrostatic distance

Chapter Four
Introduction
Cont....
Figure: Sample photo taken during head measurement

Chapter Four
The Hydrologic Cycle
Most of the earth’s water sources, such as rivers, lakes, oceans and underground sources
etc. get their supply from rains (by Precipitation), while the rain water itself is the
evaporation from these sources.
Water is lost to the atmosphere as vapor from earth which is then precipitated back in
the form of rain, snow, hail, dew, sleet or frost, etc. this evaporation and precipitation
continues forever, and thereby a balance is maintained between the two. This process
is known as hydrologic cycle.
Precipitation: It includes all the water that falls from atmosphere to earth surface.
Precipitation is of two types
Liquid precipitation(rainfall)
Solid precipitation(snow, hail)

Chapter Four
Cont....
Evaporation: transfer of water from liquid to vapour state is called evaporation
Transpiration: the process by which water is released to the atmosphere by the plants
is called transpiration.
Run-off: includes all the water flowing in the stream channel at any given section.
Surface run-off: includes only the water that reaches the stream channel without
first percolating down to the water table.
Hydrologic equation is expressed as follows:
P = R + E
Where P-Precipitation R-Run-Off
E-Evaporation

Chapter Four
Figure: Hydrological Cycle

Chapter Four
Cont....
The best way to study the rainfall pattern is with the help of graphical plots.
The hyetographs are the rainfall intensity time curves which indicate the variation of
the rate of rainfall with respect to time.
The cumulative value of rainfall plotted against time represents the mass curve of
rainfall.
Precipitation absorbed by the soil seeps or percolates into the ground, forming bodies
of water called the water table or ground storage.
It is also called inﬁltration which is a process by which water enters the surface strata
of the soil and makes its way downwards to the water table.
The amount of seepage or inﬁltration depends on the geological character of the surface
and subsoil.

Chapter Four
Measurement of run-off
Measurement of run off
Run-off can be measured by the following methods:
1. From rainfall records: the run off is estimated by multiplying rainfall with run off
coefficient which depends on the drainage area.
runoff = rainfall × coefficient
Table: Run off coefficients
Drainage area Run off coefficient
(a)Commercial and industrial 0.9
(b)Asphalt or concrete pavement 0.85
(c)Forests 0.05 to 0.30
(d)Parks 0.05 to 0.3

Chapter Four
Cont....
2. Empirical formula: is only acceptable to a particular site for determining the run
oﬀ. But no generalized equation can be developed.
a)Khosla’s formula
R = P − 4.811T
Where R-Annual run-oﬀ in mm
P-Annual rainfall in mm
T-mean temperature in C◦
b)Inglis formula for areas of Maharashtra.
For ghat region: R = 0.88P − 304.8
For plain region: R = (P − 177.8)xP/2540

Chapter Four
Cont....
c) Lacey’s formula
R = P/(1 + 3084F/PS)
Where R = monsoon run-oﬀ in mm
S = catchment area factor
F = monsoon duration factor
P = monsoon rainfall in mm
⇒ Value of S is minimum for ﬂat places (0.25) and is maximum for very hilly places (3.45).
F = 0.5 for very short monsoon.
F = 1.0 for standard length monsoon.
F = 1.5 for very long monsoon

Chapter Four
Cont....
3. Run-off curves and tables
Each region has its own catchment area and rainfall characteristics and for the same
region the characteristics mostly remain unchanged. Based on this fact the run-off
coefficients are derived once for all. Then a graph is plotted in which one axis represents
rainfall and the other run-off. The curves obtained are called run-off curves.
4. Actual measurement: stream gauge is used to measure the stream flow,i.e. run-off.

Chapter Four
Factors affecting run off
Factors affecting run off
The following factors affect run off:
Amount of rainfall
Nature of catchment area: if the catchment area is impervious, large amount of
water is collected.
Condition of the soil: rocky areas will give more run off.
Meteorology: Evaporation varies with temperature, wind velocity and relative hu-
midity. Runoff increases with low temperature, low wind velocity and high relative
humidity and vice versa.
Vegetation: forest consumes some rainfall and reduces run off.
Shape and size of catchment area: large area provides more run off.

Chapter Four
Hydrologic analysis of hydropower
Hydrograph
Hydrograph
Hydrograph
It is defined as a graph showing discharge(run-off) of flowing water with respect to
time for a specified time.
The time period for discharge hydrograph may be hour,day,week or month.
It indicates the power available from the stream at different times of day, week, month
or year.

Chapter Four
Hydrograph
Cont....
Figure: Hydrograph

Chapter Four
Hydrograph
Cont....
A Hydrograph is used to know:
Rate of flow at any instant during the specific recorded period.
Total volume of flow in a given period, as the area under the hydrograph represents
the volume of water in a given duration.
The mean annual run-off for any of the recorded period.
The maximum and minimum run-off for any selected period.
The maximum rate of run during the floods and duration of frequency of floods(peak
of the curve indicates the flood).

Chapter Four
Flow Duration Curve
Flow Duration Curve
This curve is plotted between flow available during period versus the fraction of time.
If the magnitude on the ordinate is the potential power contained in the stream flow,
then the curve is known as power duration curve.
The flow duration curve is drawn with the help of a hydrograph from the available
run-off data and here it is necessary to find out the length of time duration which
certain flow are available.
The area under the flow duration curve gives the total quantity of run-off during that
period as the flow duration curve is representation of graph with its flows arranged in
order of descending magnitude.

Chapter Four
Flow Duration Curve
Cont....
Figure: Flow Duration CurveBy Yimam Alemu Chapter Four December 16, 2020 21 / 48

Chapter Four
Flow Duration Curve
Cont...
Use of flow duration curve
1 A flow duration curve allows the evaluation of low level flows.
2 For hydropower studies, the flow duration curve serves to determine the potential for
firm power generation.
The firm power is also known as the primary power.
Secondary power is the power generated at the plant utilizing water other than that used
for the generation of firm power.
3 The flow duration curve also finds use in the design of drainage systems and in flood
control studies.

Chapter Four
Flow Duration Curve
Cont...
Shortcomings of flow duration curve
It does not present the flows in natural source of occurrence.
It is also not possible to tell from flow duration curve whether the lowest flow occurred
in consecutive periods or were scattered throughout the considered period.

Chapter Four
Mass Curve
Mass Curve
A mass curve is the graph of the cumulative values of water quantity(run-off) against
time.
It is an integral curve of the hydrograph which expresses the area under the hydrograph
from one time to another.
It is a convenient device to determine storage requirement that is needed to produce a
certain dependable flow from fluctuating discharge of a river by a reservoir.
It is used to solve the reserve problem of determining the maximum demand rate that
can be maintained by a given storage volume.

Chapter Four
Mass Curve
Cont....
Figure: Mass Curve

Chapter Four
Example
Example
At a particular site the mean monthly discharge is as follows. Draw the following
a) Hydrograph
b) Flow duration curve
c) Mass curve
Time(month) Discharge(m3
/s) Time(month) Discharge(m3
/s)
January 100 July 1000
February 225 August 1200
March 300 September 900
April 600 October 600
May 750 November 400
June 800 December 200

Chapter Four
Example
a) Hydrograph
Figure: Hydrograph

Chapter Four
Example
b) Flow duration curve
Discharge(m3
/s) Percentage time
1200 8.3
1000 16.7
900 25
800 33.3
750 41.7
600 58.3
600 58.3
400 66.7
300 75
225 83.3
200 91.7
100 100

Chapter Four
Example
Cont....
Figure: Flow duration curve

Chapter Four
Example
c) Mass curve
Month Discharge (m3/s) Total volume( (106m3)
January 100 267.84 267.84
February 225 544.32 812.16
March 300 803.52 1615.68
April 600 1555.2 3170.88
May 750 2008.8 5179.68
June 800 2073.6 7253. 28
July 1000 2678.4 9931.68
August 1200 3214.08 13145.76
September 900 2332.8 15478.56
October 600 1607.04 17085.6
November 400 1036.8 18122.4
December 200 535.68 18658.08

Chapter Four
Example
Cont....
Figure: Mass curve

Chapter Four
Energy and Power Analysis
Energy and power analysis using a flow duration approach
If the head of discharge is known, the possible power developed from water in kw can
be determined.
P(kW) = gHQηo
Where g = gravitational constant (m/s2)
H = head available(m) Q = discharge rate m3/s
ηo = overall efficiency of the plant
Thus the flow duration curve can be converted to a power duration curve.
When a run of river type of power study is done and a flow duration analysis is used,
the capacity of hydropower unit determines the maximum amount of water that will
go through the unit or units.

Chapter Four
Cont....
A ﬂow duration curve is used to explain discharge capacity, Qc as labeled in the ﬁgure
below.
Figure: Flow duration curve showing discharge capacity value

Chapter Four
Cont....
This Qc is the discharge at full gate opening of the runner under design head.
Even though to the left of that point on the duration curve the stream discharge is
greater, it is not possible to pass the higher discharge through the plant.
If the reservoir or pondage is full, water must be bypassed by a spillway.
To the right of the runner discharge capacity point, it should be noted that all the water
that can go through the turbine is the amount ﬂowing in the stream at the particular
percent of time point.
If hydraulic head and the expected loss in the penstock are known, it is possible to
generate a power duration curve from the ﬂow duration curve.

Chapter Four
Cont....
The Pc value is the full gate discharge value of power and comes from multiplying Qc,
discharge capacity value by the simultaneous value of head, estimated turbine eﬃciency
and appropriate conversion constants.
Energy production for a year or a time period is the product of the power ordinate
and time, thus the area under the power duration curve multiplied by appropriate
conversion constants give the energy produced.
It should be noted that to the left of the power capacity point the power tends to
decrease.
This is due to the fact that net head available is decreasing due to the rising of tail
water caused by the higher ﬂows that are occurring during that time interval.

Chapter Four
Cont....
Figure: Power duration curve

Chapter Four
Computational Table for Power Capacity

Chapter Four
Electrical terms associated with hydropower
Connected load: The sum of continuous ratings of all the equipments which are
connected to the supply system is called connected load.
Maximum Demand: It is defined as the largest demand of load on the generating
station during a specified period.
Demand factor: It is defined as the ratio of maximum demand to its total rated
connected load.it is always less than unity.
Demand factor =
Maximum demand
Connected load

Chapter Four
Average load or demand: It is deﬁned as the average of the loads occurring on the
power system or generating station in a given period. The period may be day or month
or year.
Daily average load =
kWh supplied in a day
24 hrs.
Monthly average load =
kWh supplied in a month
720 hrs.
Yearly average load =
kWh supplied in a year
8760 hrs.

Chapter Four
Load factor: It is deﬁned as the ratio of average load to the maximum demand during
a given period.
Load factor =
Average Demand
Maximum Demand
Load factor =
Units generated in a given perion
Maximum Demand × (number of hrs of operation)
Diversity factor: It is thus deﬁned as the ratio of sum of individual maximum de-
mands to the maximum demand on power station.
The reciprocal of the diversity factor is called the coincidence factor.
Diversity factor =
Sum of individual maximum demands
Maximum demand on power station

Chapter Four
Capacity factor or plant factor: It is deﬁned as the ratio of average load to the
rated capacity of the power plant.
Capacity factor =
Average Demand
Rated capacity of power plant
Plant capacity factor indicates the reserve capacity of the plant.
A power station should have some reserve capacity for increased load demand in the
future. Hence the installed capacity of the plant is greater than the maximum demand.
Reserve Capacity = Plant Capacity − Maximum demand

Chapter Four
Cont....
Plant capacity factor
Plant capacity factor may also deﬁned as the actual energy generated divided by the max-
imum possible energy that the plant might have generated during a given period.
Capacity factor =
Units generated in a given period
Rated capacity of power plant (kW) × (no. of hrs)
Capacity factor =
maximum demand
rated capacity
× Load factor

Chapter Four
Cont....
Plant use factor: It is deﬁned as the ratio of units generated (kWh) to the product
of plant capacity and the number of hours for which the plant was in operation.
Plant use factor =
Station output in kWh
plant capacity × hrs of use
This factor is an indication of best possible and eﬀective utilization of generating sta-
tion.

Chapter Four
Cont....
Utilization factor: it is the ratio of maximum demand to plant capacity.
Utilization factor =
Maximum Demand
Plant capacity
Load curve:
A load curve is a chart illustrating the variation in demand or electrical load over a speciﬁc
period of time.
Generation companies use this information to plan how much power they will need to
generate at any given time.
At certain period of the demand reaches the highest value termed as peak load.

Chapter Four
Load duration curve
When the load elements of a load curve are arranged in the order of descending mag-
nitudes, the curve thus obtained is called a load duration curve.
Figure: Load curve and load duration curve

Chapter Four
Cont....
Important Points
Installed capacity: The total of station capacities available to supply the system load
is called the installed capacity.
Firm power (primary power): This is the amount of power that is the minimum pro-
duced by a hydro power plant during a certain period of time. Or It is the power
always available from the stream even at times of lowest flow.
Secondary power: The excess power available over the firm power during the off peak
hours. There is no guarantee for the secondary power.

Chapter Four
Assignment
Reading Assignment
Discuss the effect of variable load on power station.
How are load factor, capacity factor and diversity factor interrelated?
Discuss the significance of load factor and diversity factor on power stat

Chapter Four
Questions
Thank You!

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