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 The availability of reliable data for any engineering

design is vital. Hence before any design is started design is vital. Hence before any design is started any available data need to be collected. ``any available data need to be collected. ``

For most project in raya valley area the available For most project in raya valley area the available metrological data from proximate station is selected metrological data from proximate station is selected Such as highlands of the valley (from Maichew, Such as highlands of the valley (from Maichew, korom and chercher) metereological stations. korom and chercher) metereological stations.

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1.1 Rainfall1.1 Rainfall The rainfall of the project area is bimodal type The rainfall of the project area is bimodal type

where small rains occur during the month of feb-where small rains occur during the month of feb-April, representing the beleg season and big April, representing the beleg season and big rains occur during the month of july to rains occur during the month of july to September, farming the main rainy season September, farming the main rainy season known as Keremt.known as Keremt.

continuedcontinued1.2 stream flow records1.2 stream flow records In these projects the actual record of river runoff is In these projects the actual record of river runoff is

not available therefore the estimation of runoff not available therefore the estimation of runoff volume has been done for each month based on volume has been done for each month based on the available rainfall record from the nearby the available rainfall record from the nearby metreological stationmetreological station

In case of absence of the recorded data ,the In case of absence of the recorded data ,the estimation of peak river flows makes more difficult estimation of peak river flows makes more difficult and challenging. in order to adopt the best and challenging. in order to adopt the best estimate for the design of hydraulic structures estimate for the design of hydraulic structures different empirical formula, which correlate different empirical formula, which correlate catchments size and land cover, slope, soil texture catchments size and land cover, slope, soil texture etc. and rainfall parameters are used. etc. and rainfall parameters are used.

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Even though formulae, which maximize Even though formulae, which maximize measured input parameters, are believed measured input parameters, are believed to simulate the local situations, in the to simulate the local situations, in the absence of such measured data formulae absence of such measured data formulae developed for local situations shall be developed for local situations shall be given priority. Therefore, using the given priority. Therefore, using the following methods carries out design flood following methods carries out design flood estimation for this project.estimation for this project.

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I ) Flood estimation by I ) Flood estimation by Tekeze basin formulaeTekeze basin formulae from from the empirical formulae developed for Tekeze the empirical formulae developed for Tekeze basin basin

q = 33.33 A-0.609q = 33.33 A-0.609

Where q = discharge intensity in m3/sec/km2Where q = discharge intensity in m3/sec/km2

A = Catchment area in km2A = Catchment area in km2

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2 ) 2 ) Flood estimation by Flood estimation by Dr. AdmasuDr. Admasu method method

based on the empirical data developed by Dr. based on the empirical data developed by Dr. Admasu for the Ethiopian highlands.Admasu for the Ethiopian highlands.

Q = Cf QmdQ = Cf Qmd

Where Qmd = 0.87*A 0.7 Where Qmd = 0.87*A 0.7

Cf = 1 + 5*A-0.2Cf = 1 + 5*A-0.2

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iii ) The composite hydrograph method (which is iii ) The composite hydrograph method (which is developed by the US corps of Engineers).developed by the US corps of Engineers).((DesignExperiencesTigray\SCS method)DesignExperiencesTigray\SCS method)

employs the rainfall pattern of the employs the rainfall pattern of the catchment’s area as well as the catchment’s catchment’s area as well as the catchment’s characteristics as its major input parameters characteristics as its major input parameters to predict the design discharge for recurrence to predict the design discharge for recurrence interval of 50 years. interval of 50 years.

iv ) iv ) Historical Maximum FloodHistorical Maximum Flood The Maximum historical flood occurred at the The Maximum historical flood occurred at the

site is determined from flood marks and site is determined from flood marks and information obtained from the near by people information obtained from the near by people and there is a clear flood mark at the bank of and there is a clear flood mark at the bank of the river. In order to estimate the historical the river. In order to estimate the historical maximum flood ,a cross-sectional survey of the maximum flood ,a cross-sectional survey of the weir axis’s was made. From the cross-weir axis’s was made. From the cross-section ,total area of flow was determined section ,total area of flow was determined Applying the well-known slope area method of Applying the well-known slope area method of estimation, the maximum flow estimation, the maximum flow

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The Manning equation is usually used to The Manning equation is usually used to compute discharges from water level, cross compute discharges from water level, cross section(s), the water surface slope, (often section(s), the water surface slope, (often assumed to be the same as the bed slope) assumed to be the same as the bed slope) and an estimated Manning roughness and an estimated Manning roughness coefficient, which for wadis with coarse bed coefficient, which for wadis with coarse bed material is often taken as 0.035 or 0.04.material is often taken as 0.035 or 0.04.

Calculations are carried out for a reasonably Calculations are carried out for a reasonably uniform and straight wadi reach, located close uniform and straight wadi reach, located close to the actual or proposed intaketo the actual or proposed intake

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Sites should be selected using the following Sites should be selected using the following criteria:criteria:

Local information is used to make a reliable Local information is used to make a reliable estimate of the water levels observed during estimate of the water levels observed during a historic flood at the site.a historic flood at the site.

The length of reach should be greater than or The length of reach should be greater than or equal to 75 times the mean depth of flow.equal to 75 times the mean depth of flow.

The fall of the water surface should exceed The fall of the water surface should exceed 0.15 m from one end the reach to the other.0.15 m from one end the reach to the other.

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The flow should be confined to one channel The flow should be confined to one channel at the flood level with no flow bypassing the at the flood level with no flow bypassing the reach as over-bank flow.reach as over-bank flow.

Application of the flow resistance equation Application of the flow resistance equation requires that the bed should be largely free of requires that the bed should be largely free of vegetation and that the banks should not be vegetation and that the banks should not be covered by a major growth of trees and covered by a major growth of trees and bushesbushes

Sites with bedrock outcrops should also be Sites with bedrock outcrops should also be avoided.avoided.

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It is difficult to satisfy all the above criteria and It is difficult to satisfy all the above criteria and some compromise is usually necessarysome compromise is usually necessary

The maximum flood water level is levelled to The maximum flood water level is levelled to the same datum used for the cross sections the same datum used for the cross sections surveys.surveys.

Calculations can be carried out using the Calculations can be carried out using the Manning equation:Manning equation:

Q = (1/n) x A x RQ = (1/n) x A x R0.670.67 x S x S0.50.5

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wherewhere Q = discharge, mQ = discharge, m33/s/s A = Cross sectional area of the flow, mA = Cross sectional area of the flow, m22

R = Hydraulic radius, A/P, where P is the wetted R = Hydraulic radius, A/P, where P is the wetted perimeter perimeter of the cross section, mof the cross section, m

S = the slope of the channelS = the slope of the channel n = Manning roughness coefficientn = Manning roughness coefficient Mannings coefficient is tabulated for a range of Mannings coefficient is tabulated for a range of

channel conditions in most hydraulic text bookschannel conditions in most hydraulic text books For Wadis with coarse bed materials it is often taken For Wadis with coarse bed materials it is often taken

as 0.035 or 0.04as 0.035 or 0.04..

ContinuedContinued In spate irrigation system, the amount of runoff that has to In spate irrigation system, the amount of runoff that has to

be diverted should be consistent with the scheme size to be diverted should be consistent with the scheme size to be developed.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 Therefore, the appropriate estimate of the flood can be handled with optimum cost of the project . This can be handled with optimum cost of the project . This can be described in terms of the ratio of diverted discharge to the described in terms of the ratio of diverted discharge to the total river flood.total river flood.

ContinuedContinued The runoff diversion ratio designated by η The runoff diversion ratio designated by η

assumed to be determined by referring to the assumed to be determined by referring to the already established value of other countries by already established value of other countries by considering the actual condition of valley into considering the actual condition of valley into account. account.

- According to the local condition of the valley and - According to the local condition of the valley and efficiency of our users the size of the most spate efficiency of our users the size of the most spate irrigations scheme determined for the rainy irrigations scheme determined for the rainy season is based on the runoff diversion ratio of season is based on the runoff diversion ratio of η = 0.55.η = 0.55.

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Therefore, the using the diversion ratio and Therefore, the using the diversion ratio and the monthly flows computed command area the monthly flows computed command area to be supplemented is fixed.to be supplemented is fixed.

And accordingly the canal sizing is also done And accordingly the canal sizing is also done using this floodusing this flood.

2.Canal discharge In the previous section the total runoff volume In the previous section the total runoff volume

needed to be diverted is determined on the basis needed to be diverted is determined on the basis of runoff diversion ratio. Then this volume of of runoff diversion ratio. Then this volume of runoff with in the month should be changed to runoff with in the month should be changed to discharge per unit time. and this discharge is the discharge per unit time. and this discharge is the design flow that is used for main and secondary design flow that is used for main and secondary canals. canals.

Since in most of our projects the actual record of Since in most of our projects the actual record of river runoff is not available and the diversion river runoff is not available and the diversion discharge or the design discharge estimated with discharge or the design discharge estimated with the following empirical formula.the following empirical formula.

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Q=Qk. η (A/(B- η)) …………**Q=Qk. η (A/(B- η)) …………**

Where Q = design discharge in m3/secWhere Q = design discharge in m3/sec

Qk = 10 year return period peak flood in Qk = 10 year return period peak flood in m3/sec. (mean annual)m3/sec. (mean annual)

A, B = constants A, B = constants

η = runoff diversion ratioη = runoff diversion ratio

ContinuedContinued In the area where the flood record data is available the In the area where the flood record data is available the

constant A and B can be determined by curve fitting constant A and B can be determined by curve fitting method. But in areas with no record data the value of method. But in areas with no record data the value of A and B estimated from empirical value given which A and B estimated from empirical value given which depends on the flow characteristics of the river runoff. depends on the flow characteristics of the river runoff. In general, for rivers with much uneven runoff where In general, for rivers with much uneven runoff where the flow pattern shows quick rise and fall of short the flow pattern shows quick rise and fall of short duration the value of A ranges between 2.1 and 2.4 duration the value of A ranges between 2.1 and 2.4 and B equals 1.05.and B equals 1.05.

Similarly for rivers with relatively even flow pattern Similarly for rivers with relatively even flow pattern value of A ranges from 2.5 to 2.9 and B equals 1.0.value of A ranges from 2.5 to 2.9 and B equals 1.0.

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Now calculate the value of QkNow calculate the value of Qk

This is done by the studies conducted by Dr. This is done by the studies conducted by Dr. Admasu on flood frequency analysis of Admasu on flood frequency analysis of required nature yielded the following formula required nature yielded the following formula for stream and rivers situated in Ethiopiafor stream and rivers situated in Ethiopia

Qmax = 0.87 * AQmax = 0.87 * A0.70.7

ContinuedContinuedWhere Where

Qmax = mean annual flood discharge in m3Qmax = mean annual flood discharge in m3

A = drainage area in Km2A = drainage area in Km2 The required design flood with specified return The required design flood with specified return

period will be as follows period will be as follows

QT = Qmax * QtQT = Qmax * Qt

where QT = T-year return period where QT = T-year return period

Qt = Flood growth factor ( 2.0 for 10 year Qt = Flood growth factor ( 2.0 for 10 year return period)return period)

ContinuedContinued But Practically there is low frequency and But Practically there is low frequency and

duration of flood and experiences have shown duration of flood and experiences have shown us that this discharge is too small to compute us that this discharge is too small to compute for canal sizing therefore based on the this fact for canal sizing therefore based on the this fact we simply provide some allowances on the we simply provide some allowances on the calculated discharge for (sizing structure in calculated discharge for (sizing structure in consultation with the farmer ) consultation with the farmer )

3. under sluice The purpose of the provision of under sluice is to The purpose of the provision of under sluice is to

provide silt free water to the intakes and to flash provide silt free water to the intakes and to flash the silts through them before every first rain the silts through them before every first rain splash of the season.splash of the season.

However the farmers are not willing to loose a However the farmers are not willing to loose a drop of water even for flashing and their interest drop of water even for flashing and their interest is to have no under sluice gates.is to have no under sluice gates.

The under sluice is designed to pass 10 – 20% The under sluice is designed to pass 10 – 20% of the peak flood. of the peak flood.

Mostly we provide bays of each 0.90 x 1.0m Mostly we provide bays of each 0.90 x 1.0m opening of sluice gate. This is made for opening of sluice gate. This is made for simplicity of operationsimplicity of operation..

4. Off takes

Previously we Previously we provided large provided large diameter pipes off diameter pipes off takes which are gated takes which are gated and with diversion and with diversion angles of 90angles of 900 0 but this but this pipes were silted up pipes were silted up and could not serve and could not serve for more than a year.for more than a year.

Off takes…

The farmers are very The farmers are very well experienced and well experienced and as the discussions as the discussions held with them the off held with them the off takes are now open takes are now open channels and the off channels and the off take angles are now take angles are now 120 – 150120 – 15000 according according to the interest of the to the interest of the farmers and the river farmers and the river nature. nature.

Design of weir (HW) for Spate Irr.

Sample report (Hw, Infrastructure, Sample report (Hw, Infrastructure, Specification and working drawing )Specification and working drawing )

Design ExperiencesTigray\Diversion,

DesignExperiencesTigray\MAI AKINO HEAD WORK DR.dwg

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