The time required for the rain falling at the most distant point in the drainage area (i.e., on the fringe of the catchment ) to reach the concentration point is called the concentration time.
This is a very significant variable since only such storms of duration greater than the time of concentration will be able to produce runoff from the entire catchment area and cause high intensity floods.
The characteristics of the drainage net may be physically described by:
The number of streams
The length of streams
Stream density
Drainage density
The country’s annual renewable fresh water resources amount to some 122 BCM/yr in the twelve river basins.
However, only 3% remains in the country.
The rest, 97% is lost in runoff to the lowlands of neighboring countries.
2. Flow velocity and discharge measurement
• Discharge is the volumetric rate at which water passes
a given location /open channel/
• It is measures in m3/s or l/s
• Key reasons for measuring discharge are
– Studies of occurrence of floods
– Studies of occurrence of low flows
– Water availability study (water balance studies)
4. Flow velocity and discharge measurement
Measurement of stream flow
1. Velocity- Area methods
2. Based on Stage – discharge relationship
3. Discharge measuring Structures
a. Weirs
b. Flumes etc
4. Dilution method
5. 1. Velocity – Area methods
• Velocity area method estimated discharge by using
the following relationship
• where Q is total stream discharge, Vav is the average flow
velocity and A is the cross sectional area of flow
A
V
Q av
6. 1. Velocity – Area methods
• for natural channels, flow velocity varies with position
in the channel
– flow varies vertically, with maximum velocities
occurring at the surface and minimum velocities at the
channel bed
– flow also varies laterally - flow is generally greatest at
the deepest part of the channel
• Vertical as well as lateral variation of velocity
– a suitable control section must be selected
– channel control - a long straight uniform section of
channel is required
7. Velocity – Area methods
• To account for lateral variation, the cross-section is
divided into suitable cross-sections of a1, a2, a3, …..
• The vertical velocity variation is accounted by taking
velocity at different depths
– One point - at 0.6h
– Two point method - at 0.2h and 0.8h
– Three point method – at 0.2h, 0.6h and 0.8h
– Depth - Integrated velocity measurement device lowered and
raised at a constant rate through a vertical section integrates velocity and
provides a means of measuring the average velocity of the section
8. 1. Velocity – Area methods
To account for variations in flow velocity the channel is divided into
vertical sections.
Within each section, average flow velocity has been found equal to the
average of flows at 0.2 and 0.8 of the stream depth measured from the
surface.
For shallow sections, 0.6 of the depth may be used as an approximation
of average flow.
10. Velocity Measurement: FM
1. Measuring velocity using floats
A simple but crude method of measuring velocity.
floats are inexpensive and several measurements can be made in a short space of time.
Steps to be followed are:
1. Select relatively uniform section
2. Locate two points on the selected uniform section, x distance a part
3. Measure the distance between the selected points. E.g. 3m, 4m or 5m.
4. Set watch (000 or known time)
5. Throw the float at first point and set the watch to start simultaneously while throwing the
float.
6. Stop the watch when the float crosses the second point and record the time elapsed.
7. Calculate velocity by dividing the distance with time elapsed.
V =Distance/time
8. Measure the cross-sectional parameters, bed width and flow depth, then calculate
cross sectional area.
9. Determine flow or discharge by multiplying velocity with cross – sectional area.
11. Velocity Measurement: SM
2. Slope Method
• An indirect measure of the flow in a channel can be made
using semi-empirical equations such as the chezy equation
or the manning equation.
Where V is the velocity, R is the hydraulic radius, S is hydraulic gradient
and n is Manning’s roughness coefficient.
• The problem with this method is estimating the roughness
coefficients a 30% error either way is possible.
S
R
n
V 2
1
3
2
1
12. Velocity Measurement: SM measuring in the field
1. The reach should have a uniform cross-section, be free
from obstructions and from back water effects,
2. The length of the reach should be at least 75 times the
mean depth, at least 5 times the mean width and
preferable at least 300 meter.
3. The water surface fall in the reach should be greater than
the velocity head and be at least 0.15 m,
4. The entire reach should have either subcritical flow or
supercritical flow. The water level profile should not cross
the critical depth line in a hydraulic drop or a hydraulic
jump.
14. Velocity Measurement: SM
Procedure:
– Select a channel reach where the flow is approximately uniform, using
above mentioned criteria
– Measure the cross-sectional profiles in the sections 1, 2 and 3.
a. calculate the areas and wetted perimeters.
b. calculate the mean values A, P and R.
– Determine the water level slope S from high water marks, from
water level gauges or by direct leveling.
– Estimate the Manning coefficient by comparing the actual
roughness (bed material and vegetation) with a known
roughness from literature ( ‘Open Channel Hydraulics’ tables).
– Calculate the flow velocity
16. Velocity Measurement: CM
3. Current Meter method
• A current meter is a propeller or impeller that has been
accurately calibrated for velocity.
• With care, flow measurements can be made within 2 to 5%
error.
• The device is held in the water and the number of turns of
the blades in a given period of time can be converted to
the velocity and calculate the flow rate by multiplying the
cross-sectional area with the average velocity.
• Depending on the depth of the channel section the
average velocity is determined after taking velocity
measurement at different depths as indicated below.
17. Velocity Measurement: CM
• Current Meters
– Cups
– Propellers
V = a + bN
where V = flow velocity; a = starting velocity to
overcome mechanical friction; b = equipment
calibration constant; N = revolutions/sec.
28. Stage – Discharge relationship
• For a stable channel, there is a relationship between
stream discharge and stage.
this relation is called the rating curve
Stage is the water level above a reference elevation
• Reference elevation is often taken as the channel
bed.
• But not necessarily the channel bed , as the bed
elevation in alluvial channels may change.
29. Rating curve
• For natural channels, stage or gauge height (X-
axis) is plotted against flow (Y-axis)
– The curve is a parabola or other
– Curve fitting
– It is essential that the stage measurement be tied into
some fixed stable benchmark
• Changes in bed elevation due to scour or
aggradation are reflected in a curve shift.
30. Rating curves
• A best estimate of the relationship between stage
and discharge at a given place in a river.
• The relationship should be on the form
Q=C(h-h0)n
or a segmented version of that. Q=discharge, h=stage
• For a known zero-stage, the rating curve can be
written as y=b+nx,
– where y=log(Q), x=log(h-h0) and b=log(C).
• How do we estimate the rating curve parameters of
C, b and ho?
31. Establishing a rating curve
• Least square solution method
– trial and error approach
– Three point dicharge appraoch
• Graphical methods
32. Establishing a rating curve
1. LSS method
• Assume Ho and estimate the coefficients C and n that
gives the best r^2 and minimum standard error
• The LSS equations are given next slide
– Log Q=log C+ nlog (H-Ho)
– Y = b + nX
• Change the value of Ho and evaluate the Se and r^2
• Continue until you have global minimum Se and best
r^2
34. Establishing a rating curve
2. Three point discharge method
• Three discharge values are selected from H-Q curve and in
such a way that Q1/Q2 = Q2/Q3
• Ho is determined from this relation as
2
3
1
2
2
1
2H
H
H
H
H
H
Ho
35. Code River Station Period a b Ho r2
1002 Gilgel_Abbay Merawi Mar 59/Jun 62 33.49 1.979 0.25 0.989
Dec 62/Jul 84 34.58 2.069 0.25 0.994
Aug 84/Feb 94 35.61 2.04 0.2 0.997
1003 Koga Merawi 1959/1981 13.41 2.029 0.5 0.964
except Sep/Dec 69 0.3 0.964
1982/1995 19.98 2.239 0.15 0.99
1005 Ribb Addis Zemen Aug 59/Sep 71 7.23 1.908 0.65 0.946
Nov 71/Dec 75 3.48 2.046 0.75 0.961
1976/1977 3.04 2.194 0.8 0.981
1978/1979 3.5 2.102 1.15 0.995
1980/1981 2.48 2.288 1.15 0.976
1981/1983 3.85 2.069 1.55 0.974
1984/1985 3.49 1.754 1.8 0.953
1986/1988 4.39 1.976 2 0.977
1988/1990 1.29 2.6 2.1 0.899
1990/1994 0.54 3.305 2.2 0.974
1006 Gumara Bahir Dar 59/66 14.9 1.683 0.55 0.956
67/68 5.8 2.373 0.1 0.965
69/95 8.7 1.929 -0.15 0.947
Examples of rating curve equation
36. Shift in rating Curves
The stage discharge relation can vary with time, in response
to degradation, aggradation, or a change in channel shape at
the control section; deposition of sediment causing increased
approach velocity in a weir pond and vegetation growth.
Shifting in rating curves are best detected from regular
gauging and become evident when several gauging deviate
from the established curve.
It may be possible to shift the rating curve up or down by the
change in the mean bed level.
In rivers with gentle slopes, discharge for a given stage when
the river is rising may exceed discharge for the same stage
when the river is falling.
In such cases, adjustment factors must be applied in
calculating discharge for the rising and falling stages.
37. Requirement of a good H- Q site
• The river reach must be stable and fairly straight on both u/s and
d/s for a length of 0.75 to 1.0km.
• H-Q relation should always be uniform – site is subjected to shift
control
• Easily accessible during all the time of the year
• The site should be sensitive to all H-Q
• Back water and tidal effects should be minimum
• Site should be away from bridge
• When a tributary joins, the site should be located 0.8 km u/s and
d/s of their confluence
• Disturbance due to animals should be minimum
• Site should have stable and high banks to contain floods
• Rock outcrops and vegetal growth at the reach should be
minimum
38. Requirement of a good H- Q site
• Islands should not be present at the gauging station
• Cross section of the entire reach of the river should be
fairly inform
• Cross currents, vortex and eddies formation, reverse
slope in parts of the channel bed should be abscent at
the site
• Velocity at all points are parallel to one another and at
straight angle to the x-section of the stream
• The velocity is greater than 0.15m/s
• The depth of the flow is greater than 0.3m
• No aquatic growth
• Straight and stable reach
39. Network design
• There is no definite relation or rule to the number of stations to
establish
• A basin should have two types of stations
i – the base or permanent stations:
• For which long term data for the important tributaries
and the main rivers are always collected
ii – auxiliary or secondary station:
• To collect short term data and form network.
• The stations should be located at existing or potential
dam site, flood forecasting purpose.
• Data from this station are correlated with data from
primary stations to extend the record.
41. Q using measuring Structures
• Structures control the rate of flow.
• Structures may also be used for measurement of
water flow.
• Each type of structure will produce different
types of flow depending upon size and flow rate
passing through it.
42. Q using measuring Structures
In general flow measurement structures can be used in:
–natural streams
–irrigation and drainage canals
–water purification plants and industries
–hydraulic laboratories.
• Modern flow measurement stations can be equipped
with micro-processors which convert the measured
heads directly into digital records of discharges.
43. Q using measuring Structures
• Discharge measurement structures are
classified according to the shape of the
crest in the flow direction.
• They can further be subdivided according
to the different cross sections:
–Weirs
–Flumes
44. Q using measuring Structures
Weirs are elevated structures in open
channels that are used to measure flow
and/or control outflow elevations from
basins and channels.
• There are two types of weirs in common
use:
– Broad-crested weirs
– Sharp-crested weirs
45. Q using measuring Structures
Broad-crested weirs
• The length of the crest should be sufficient to
allow straight and parallel streamlines at least
along a short distance above the crest.
• The best-known broad-crested weirs are the
following :
– the round-nose horizontal broad-crested weir
– the rectangular broad-crested weir
– the trapezoidal profile weir
– the V-shaped broad-crested weir
46. Q using measuring Structures
Sharp-crested weirs
• Classified under the term ‘sharp-crested’ or ‘thin-plate’
weirs are those overflow structures whose length of crest
in the direction of flow is equal to or less than 2
millimeters.
The main types of sharp-crested weirs are:
– Rectangular weir,
– V-notches weir, and
– The Cipolletti(Trapezoidal) weir.
48. Measuring Q for small channels
• Weirs and flumes
• a weir is an engineered structure that is built into
a channel to control the stage - discharge
relationship
• used in small channels where the measurement
error in relation to the total flow would be
unacceptable, and/or
• where suitable natural control does not exist
• discharge is related to head over the weir crest
using a simple mathematical formula
51. Discharge from using weir formula
For instance, a sharp crested V-notch weir (a type commonly
used in research on small creeks) has the following
formula:
Where: q is the angle of the notch, h is the head over the weir
crest and Cw” is a weir coefficient (approximately 0.32 for
SI units) that varies slightly with head and notch angle.
Q C h
w
4 28
2
5 2
. tan
" q
52. 52
Measuring Q for small channels
• Have a more definite relationship between stage and flow.
• Higher accuracy than velocity X-sections.
• Only can be used for smaller streams
53. 53
Measuring Q for small channels
Sharp-Crested rectangular Notch Weirs
Q = Cd LH3/2
Where
Q = discharge(cfs)
Cd = coefficient
L = width of notch(feet)
H = depth of flow(feet)
56. Measuring Q for small channels
Flumes
•Flumes consist of a narrowed canal section with a particular, well-
defined shape.
•The advantage of flumes over weirs is the small drop in water level
(head loss), and so flumes can be used in relatively shallow canals
with flat grades.
The drop in water level is only one quarter of the drop needed to be
able to use a weir, for the same discharge under similar conditions.
• Because of this, smaller flumes can easily be used as transportable
measuring devices.
• A disadvantage of flumes is that they are relatively expensive and
they cannot easily be combined with other structures, whereas that
is possible with weirs.
60. Measuring Q for small channels
Selecting Suitable flow measurement structure is
on the basis of:
–channel size and shape
– minimum and maximum flow rate
– mounting considerations
– debris in the flow stream
– upstream conditions
– maintenance
– cost
62. 62
Dilution Gauging
• A solution of a stable tracer is injected into the
stream at either constant rate or all at once
• Useful for small streams and streams with lots
of boulders, wood, or other roughness
elements.
• Some limitations on the size of the stream to be
measured.
• Is suitable for mountainous streams
63. Dilution Gauging
• This method involves introducing a chemical ‘tracer’
substance such as a salt or dye into the stream and then
monitoring changes in its concentration at some point
downstream.
• It is useful in highly turbulent streams which provide
rapid mixing of tracers and at the same time the normal
flow measurement with current meter is difficult.
• The two most frequently used methods of dilution
gauging are slug-injection and constant rate injection.
64. Dilution Gauging
• Computation requires:
– The rate of injection should be known(q)
– The concentration of the tracer in the injected
solution should be known(C1)
– The background concentration of stream(Co)
– The concentration of the tracer in the stream after it
has been mixed should be known(C2)
70. Dilution Gauging
1. Single Injection Method
In this method a solution of known volume and concentration is added to the
stream in one slug or gulp.
Concentration of the tracer is monitored as the wave of marked fluid passes
by, then the discharge is calculated from integration (calculating the area
under the curve) of the concentration hydrograph. The equation for computing
discharge is (Shaw, 1988)
Where V=known volume of tracer, Ct =concentration of tracer in introduced
solution, Co = background concentration of stream, C = changing
concentration of tracer measured downstream, Q = discharge (m3/s) and t1
and t2 are the initial and final times of measurements (seconds). The units of
the concentration should consistent.
71. 71
Dilution Gauging
2. Constant Injection
• In constant-injection method a solution of known concentration
is injected into the stream at a constant rate.
• The discharge can be given as from the equation (Shaw, 1988)
Where C1 is the final, constant concentration of the tracer in the
stream, Q is in m3/s and Qt is the injection rate of the tracer in
liters per second (L/s)
73. Dilution Gauging
Mixing length:
The mixing length is defined as the distance between the injection
point and the sampling (measuring) point, in which the solution
has been mixed completely over the cross-section.
Rimmar(1978)
Where
Bs = average width at the water surface in the measuring reach (m)
d = average depth in the same reach (m)
C = Chézy coefficient for the reach, and 15 < C < 50 (m/s)
g = acceleration due to gravity (m/s2)
74. Dilution Gauging
• Day, 1977, show that mixing lengths in mountain streams
can be quickly estimated from a single geometric
parameter, the mean flow width, using a simple equation:
Xmin = 25 B
Where B is the mean width of the channel
75. Example
A common salt dilution of concentration 20 mgl was added to a stream at a
constant rate of 0.2 cm3/s. The river has background discharge of
0.12ppm.The concentration of this salt in the stream water was measured as
0.5ppm. The average width of the river was 0.8 m.
a. Estimate the mixing length of the salt using dye 1977.
b. Estimate the stream discharge at a point where it starts to complete
mixing.
