6.13
Design experiences in Tigray:
Determination of design
discharge (hydrology) and fixing
some parts of weir structure for
spate irrigation
(Presented by Demisew Abate)
1. Hydrology
 The availability of reliable data for any engineering
design is vital. Hence before any design is started
any available data need to be collected. ``
 For most project in raya valley area the available
metrological data from proximate station is selected
Such as highlands of the valley (from Maichew,
korom and chercher) metereological stations.
continued
1.1 Rainfall
 The rainfall of the project area is bimodal type
where small rains occur during the month of febApril, representing the beleg season and big
rains occur during the month of july to
September, farming the main rainy season
known as Keremt.
continued
1.2 stream flow records
 In these projects the actual record of river runoff is
not available therefore the estimation of runoff
volume has been done for each month based on
the available rainfall record from the nearby
metreological station
 In case of absence of the recorded data ,the
estimation of peak river flows makes more difficult
and challenging. in order to adopt the best
estimate for the design of hydraulic structures
different empirical formula, which correlate
catchments size and land cover, slope, soil texture
etc. and rainfall parameters are used.
continued

Even though formulae, which maximize
measured input parameters, are believed
to simulate the local situations, in the
absence of such measured data formulae
developed for local situations shall be
given priority. Therefore, using the
following methods carries out design flood
estimation for this project.
continued
I ) Flood estimation by Tekeze basin formulae from
the empirical formulae developed for Tekeze
basin
q = 33.33 A-0.609
Where q = discharge intensity in m3/sec/km2
A = Catchment area in km2
continued
2)
Flood estimation by Dr. Admasu method
based on the empirical data developed by
Dr. Admasu for the Ethiopian highlands.
Q = Cf Qmd
Where Qmd = 0.87*A 0.7
Cf = 1 + 5*A-0.2
continued
iii ) The composite hydrograph method (which is
developed by the US corps of
Engineers).(DesignExperiencesTigray\SCS
method)
 employs the rainfall pattern of the
catchment’s area as well as the catchment’s
characteristics as its major input parameters
to predict the design discharge for recurrence
interval of 50 years.
iv ) Historical Maximum Flood
 The Maximum historical flood occurred at the
site is determined from flood marks and
information obtained from the near by people
and there is a clear flood mark at the bank of
the river. In order to estimate the historical
maximum flood ,a cross-sectional survey of
the weir axis’s was made. From the crosssection ,total area of flow was determined
Applying the well-known slope area method
of estimation, the maximum flow
continued
 The Manning equation is usually used to
compute discharges from water level, cross
section(s), the water surface slope, (often
assumed to be the same as the bed slope)
and an estimated Manning roughness
coefficient, which for wadis with coarse bed
material is often taken as 0.035 or 0.04.
 Calculations are carried out for a reasonably
uniform and straight wadi reach, located close
to the actual or proposed intake
continued
 Sites should be selected using the following
criteria:
 Local information is used to make a reliable
estimate of the water levels observed during
a historic flood at the site.
 The length of reach should be greater than or
equal to 75 times the mean depth of flow.
 The fall of the water surface should exceed
0.15 m from one end the reach to the other.
continued
 The flow should be confined to one channel
at the flood level with no flow bypassing the
reach as over-bank flow.
 Application of the flow resistance equation
requires that the bed should be largely free of
vegetation and that the banks should not be
covered by a major growth of trees and
bushes
 Sites with bedrock outcrops should also be
avoided.
continued
 It is difficult to satisfy all the above criteria and
some compromise is usually necessary
 The maximum flood water level is levelled to
the same datum used for the cross sections
surveys.
 Calculations can be carried out using the
Manning equation:
Q = (1/n) x A x R0.67 x S0.5
continued
where
 Q = discharge, m3/s
 A = Cross sectional area of the flow, m2
 R = Hydraulic radius, A/P, where P is the wetted




perimeter of the cross section, m
S = the slope of the channel
n = Manning roughness coefficient
Mannings coefficient is tabulated for a range of
channel conditions in most hydraulic text books
For Wadis with coarse bed materials it is often taken
as 0.035 or 0.04.
Continued
 In spate irrigation system, the amount of runoff that has to
be diverted should be consistent with the scheme size to
be developed.
- Moreover, the irrigation infrastructure may not be used
fully in all months; the reason is that there is a variation in
the amount of diverted flood.
 Therefore, the appropriate estimate of the flood can be
handled with optimum cost of the project . This can be
described in terms of the ratio of diverted discharge to the
total river flood.
Continued
 The runoff diversion ratio designated by η
assumed to be determined by referring to the
already established value of other countries by
considering the actual condition of valley into
account.
- According to the local condition of the valley and
efficiency of our users the size of the most spate
irrigations scheme determined for the rainy
season is based on the runoff diversion ratio of
η = 0.55.
continued
 Therefore, the using the diversion ratio and
the monthly flows computed command area
to be supplemented is fixed.
 And accordingly the canal sizing is also done
using this flood.
2.Canal discharge
 In the previous section the total runoff volume
needed to be diverted is determined on the basis
of runoff diversion ratio. Then this volume of
runoff with in the month should be changed to
discharge per unit time. and this discharge is the
design flow that is used for main and secondary
canals.
 Since in most of our projects the actual record of
river runoff is not available and the diversion
discharge or the design discharge estimated with
the following empirical formula.
continued
Q=Qk. η (A/(B- η)) …………**
Where Q = design discharge in m3/sec
Qk = 10 year return period peak flood in
m3/sec. (mean annual)
A, B = constants
η = runoff diversion ratio
Continued
 In the area where the flood record data is available the
constant A and B can be determined by curve fitting
method. But in areas with no record data the value of
A and B estimated from empirical value given which
depends on the flow characteristics of the river runoff.
In general, for rivers with much uneven runoff where
the flow pattern shows quick rise and fall of short
duration the value of A ranges between 2.1 and 2.4
and B equals 1.05.
 Similarly for rivers with relatively even flow pattern
value of A ranges from 2.5 to 2.9 and B equals 1.0.
continued
Now calculate the value of Qk
This is done by the studies conducted by Dr.
Admasu on flood frequency analysis of
required nature yielded the following formula
for stream and rivers situated in Ethiopia
Qmax = 0.87 * A0.7
Continued
Where
Qmax = mean annual flood discharge in m3
A = drainage area in Km2
 The required design flood with specified return
period will be as follows
QT = Qmax * Qt
where QT = T-year return period
Qt = Flood growth factor ( 2.0 for 10 year
return period)
Continued
 But Practically there is low frequency and
duration of flood and experiences have shown
us that this discharge is too small to compute
for canal sizing therefore based on the this fact
we simply provide some allowances on the
calculated discharge for (sizing structure in
consultation with the farmer )
3. under sluice
 The purpose of the provision of under sluice is to
provide silt free water to the intakes and to flash
the silts through them before every first rain
splash of the season.
 However the farmers are not willing to loose a
drop of water even for flashing and their interest
is to have no under sluice gates.
 The under sluice is designed to pass 10 – 20%
of the peak flood.
 Mostly we provide bays of each 0.90 x 1.0m
opening of sluice gate. This is made for
simplicity of operation.
4. Off takes
 Previously we
provided large
diameter pipes off
takes which are gated
and with diversion
angles of 900 but this
pipes were silted up
and could not serve
for more than a year.
Off takes…
 The farmers are very
well experienced and
as the discussions
held with them the off
takes are now open
channels and the off
take angles are now
120 – 1500 according
to the interest of the
farmers and the river
nature.
Design of weir (HW) for Spate Irr.
 Sample report (Hw, Infrastructure,
Specification and working drawing )
Design ExperiencesTigray\Diversion,
DesignExperiencesTigray\MAI AKINO HEAD
WORK DR.dwg
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