FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA MINISTRY OF WATER RESOURCES ERER DAM & IRRIGATION DEVELOPMENT PROJECT Final Detail Design Report Ann- x D: Hydrology May 2009 CONCERT ENGINEERING ANI) CONSULTING ENTERPRISE P.L.C. (CECE) ENGINEERS RATER RESOURCES A AGRICULTURAL l>l-l'ELOI*MEN r PLANNERS 6U64 AIIIMS AllAIIA TEI. 2.41.11 4AJ9244 FAX 2SMI4M9245 I'.-maU raiKag^WiliMnrl.riExecutive Summary Volume I : Dam & Appurtenant Structure Annex A: Dam Design Annex B: Dam Appurtenant Structures Annex C: Geotechnical Study Annex D: Hydrology Volume II: Irrigation & Drainage Annex E: Irrigation & Drainage Design Annex F: Road Infrastructure Annex G: Project Control Centers Annex H: Operation & Maintenance Manual Volume III: Project Worth AnalysisTabic of Contents 1. INTRODUCTION l. 1 Project feature and nature....................................................................................1 1.2 Scope of the report....................................................................................................... 2 2. RUNOFF ANALYSIS................................................................................................................................. 3 2.1 General................................................................................................................................... 3 2.2 Objective of the Study.........................................................................................................3 2.2.1 Runoff across the main and secondary canal................................................................. 4 2.2.11 Methods used for estimation of peak floods................................................................ 4 2.2.1.2 Result of calculation on main and secondary canal canals............................................. 8 1 2.2.2 Peak flood estimation at Decheo River.......................................................................... 11 2.2.2.1 General....................................................................................................................... 11 2.2.2.2 Methodology used for flood estimation.................................................................... 12 2.2.2 3 Result of calculation................................................................................................ 17 3. FLOOD FLOW ESTIMATION AFTER ROUTING FOR PROTECTION WORK..........................18 3.1 100 years Return period flood estimation.................................................................... 18 3.2 Reservoir Routing for 100 year return period flood................................................. 20 3.3 Result of Reservoir routing............................................................................................. 21 3.4 Flood way Determination for i oo year return period peak...................................... 23 3.4.1 Equations for basic profile calculations............................................................................ 23 3.4.2 Cross section Subdivision for conveyance calculations................................................... 23 3.4.3 Data requirement to run the HEC-RAS model................................................................. 25 3.4.3.1 Channel cross sections ofErer River......................................................................... 25 3 4.3 2 Manning’s Roughness coefficient.............................................................................. 25 3 4.3.3. Discharge to be Tested for the River Reaches...........................................................25 3.4.4 Result of floodway determination..................................................................................... 26Erer Dam and Irrigation Project/CECE & CES 1. Introduction 1.1 Project Feature and Nature The Erer Irrigation Project is found in the Wabishebel river basin, and the project has a storage (dam) facilitaty and a irrigation scheme, using the Erer river which is the tributary of Wabesheble river. Erer river is located in the semi arid area of the north-eastern part of the basin. With its tributaries, it originates from Ejersa Goro and Waltaha connected to the Kombolcha mountain range which has a maximum altitude of 2670 m a s. I. in the North-East of Harar. The river flows South-east ward during rainy season and it is intermittent during dry season, with significant subsurface flow The tributaries of Erer River in the Wabeshebele consist of the Abiyo, Yasakoy, Kebenawa, Ilimo and , Decheo stream and some field drainages systems These stream and drainages cross the main and secondary canals of the Erer irrigation project The area of Erer catchment from its head to the dam site is 419km2 Its annual rainfall ranges between about 650mm near the irrigation command area to about 800mm at the upper part of the catchment. The proposed irrigation command area is about 3963ha which lies in Oromia and Harari Regional States. The project is located at about 19km south-east of Harar town and 11km from Babile town The dam site is located at about 6km upstream of Harar-Jijiga road. The irrigation command area starts from some distance downstream of the dam and spreads on the right bank of Erer River. Its location is between coordinates 9°17’40” to 9°07’17” North Latitude and 42°12’ to 42° 16’ East Longitude. The altitude of the command area lies approximately between 1280 to 1365masl. The Erer irrigation project dam was designed to the maximum capacity of about 50.11MCM, with the full reservoir level of 1379.0m that will serve some 3960 hectares. According the feasibility study made by WWDSE, the annual catchmnet yield of the reservoir is about 60.4MCM. Annex D : Hydrology / Final Detail Design Report /2009 1Ercr Dam and Irrigation Projecl/CECE & CES *• 1.2 Scope of the report This report focuses on the revision of some hydrological parameters of the irrigation project. This is done for the purpose of handling the present detailed design previous feasilibility level report. It also presents the analysis of the cross-drainage design flood with appropriate methods as summarized below; • Analysis and estimation of the runoff flows from creeks that cross the primary and secondary canals. • Analysis of the reservoir routed flood flow from the dam that passes through the main river channel in order to design the necessary protection work • Review of the feasibility hydrology study (firm yield from catchments) in light of the observed flow data at the dam site. Annex D : Hydrology / Final Detail Design Report /2009 2Erer Dam and Irrigation Project/CECE & CES 2. Runoff Analysis 2.1 General The first step involved in hydrology is the collection of relevant data and analysis of the data by applying the principles of engineering hydrology to solve the practical problems, which require adequate and relevant data. Lack of recorded data is the main problem faced during the determination of design discharge for the proposed project. The absence of any gauging station in the catchment and flow records on the river, made impossible the availability of both rainfall and runoff data on reliable bassis As a result looking for methods applicable for ungauged catchment was a must for the project under consideration. The same time in order to enhance the data base a temporary' hydrometry station was established of the dam site to measure the flow during the rainy season. 2.2 Objective and methodology of the Study One of the main purpose of the current hydrological study is to estimate the Peak flood for drainages design across the main canal and secondary canals of the Erer irrigation project The investigation will focus on identification of flush-flood water in the seasonal and intermittent stream channels located in the project area. This will be followed by the flow synthesis for the respective project area and estimation of probable maximum flow which could occur at canal cross drainage site for the design and layout of the structures. The meteorological data from the near by stations have been utilized, while at the same time previous hydrological analysis made for the project have also been reviewed. Regarding the assessment of the firm yield from the catchment of the reservoir, an intensive data collection was made during the period of (June -September 2008) and this data was used to estimate the annual average yield of the river at the dam site. Annex D : Hydrology / Final Detail Design Report /2009 3Ercr Dam and Irrigation Project/CECE & CES 2.3 Runoff across the main and secondary canal The main canal starting from the dam outlet runs at the right bank of the Erer river and crosses about 18 creeks or drainage gullies on the elevation contour of 1360m amsl. The catchment areas of the creeks range between 0 15km to 6.5km 2.4. Methods used for estimation of peak floods Drainages Design flood The flood used for design against failure is termed as the project design flood. It can usually be determined by estimating the run off that result from an occurrence of a design storm based on meteorological factors This hydro meteorological approach is necessary because stream records often are not available. In the design of hydraulic and irrigation structures, the peak flow that can be expected with in assigned frequency is of primary importance. Under estimation of peak flow would result inadequate capacity of structures and its consequence will be failure. To safely estimate the magnitude of a peak flood, the following alternative methods may be used, namely Rational method, Empirical formulas, Unit hydrograph technique, flood frequency studies, Snyder’s and SCS method. 2.4.1 Peak Flood estimation 2.4.1.1 SCS Method All methods if flood estimation are ultimately dependent upon recorded measurement of rainfall in the actual catchment (watershed). Rainfall records are often available at locations close to the proposed project sites, which could be utilized for frequency analysis and computation of flood flows For our proposed project area the available data of Harar station which is of 10 years data is used, for frequency analysis and computation of flood flows. We used the rainfall data of Harar station with its elevation of 2060m above sea level. So it is assumed that the climatic condition of the above mentioned station is also representative for the project area. Here SCS Dimensionless 2 Hydrograph method is considered for catchment areas greater than 0.5 km . Annex D : Hydrology / Final Detail Design Report /2009 4Ercr Dam and Irrigation Project/CECE & CES Data are generally analysed by one of the two probability lows, the extreme value law or log probability law. In our case we used Pearson type III distribution for annual daily highest rainfall analysis to determine the return period of T years. Pt = P.ve + Spx Kt Where: P»ve = Average of all values of annual daily maximum rainfall (mm), Sp = Standard deviation of annual daily maximum rainfall (mm) and K = Frequency factor of return period for T years. G) t Finally, after the analysis of unit hydrograph synthesis, composite hydrographs were prepared and the peak flood for each projects were calculated The results are presented under table 1 and the detailed analysis is presented in Annex A resulted in the following tables See the detail analysis in Annex A. 2.4.1.2 Rational Method The rational method is the most widely used method for the analysis of run off response from small catchments. It has particular application for storm drainage calculation, where it is used to calculate peak run off rates for the design of storm sewers and small drainage structures. Here the rational formula is considered for catchment areas less than 0.5km2 The peak discharge is the product of, 1) Run off Coefficient, 2) Rainfall intensity and 3) Catchment area with all processes being lumped in to these three parameters. The rational method is based on the following formula: Qp = 0.27 C I A where = Q in m /s, I in mm/hr and A in Km . P Annex D : Hydrology / Final Detail Design Report /2009 3 2 5Erer Dam and Irrigation Projcct/CECE &- CES C= runoff coefficient representing a ratio of runoff to rainfall 2.4.1.3 Rainfall Intensity The rainfall intensity (I) is the average rainfall rate in mm/hr for duration equal to the time of concentration for a selected return period. Once a particular return period has been selected for design and a time of concentration calculated for the catchment area, the rainfall intensity can be determined from Rainfall-Intensity-Duration curves (Figure 2). Rainfall-Intensity-Duration curves for use in Ethiopia are given in Figures 2 at the end of this chapter (ERA, 2002). 2.4.1.4 Runoff Coefficient The runoff coefficient (C) is the variable of the Rational Method least susceptible to precise determination and requires judgment and understanding on the part of the designer A typical coefficient represents the integrated effects of many drainage basin parameters. The following discussion considers the effects of soil groups, land use, and average land slope Three methods for determining the runoff coefficient are presented based on soil groups and land slope, land use, and a composite coefficient for complex catchment areas (ERA, 2002). The recommended runoff coefficient (C) for pervious surfaces by selected hydrologic soil groupings and slope ranges From this the C values for the project areas such as forest land, agricultural land, and open space can be determined as in the range 0.12 to 0.17. 2.4.1.5 Hydrological Soil Groups for Ethiopia Soil properties influence the relationship between runoff and rainfall since soils have differing rates of infiltration Permeability and infiltration are the principal data required to classify soils into Hydrologic Soils Groups (HSG) Based on infiltration rates, the Soil Conservation Service (SCS) has divided soils into four hydrologic soil groups as follows: Group_A: Sand, loamy sand or sandy loam. Soils having a low runoff potential due to high infiltration rates. These soils primarily consist of deep, well-drained sands and gravels. Annex D Hydrology / Final Detail Design Report /2009 6Erer Dam and Irrigation Projcct/CECE & CES Group B: Silt loam, or loam. Soils having -a moderately low runoff potential due to moderate infiltration rates These soils primarily consist of moderately deep to deep, moderately well to well drained soils with moderately fine to moderately coarse textures. Group C: Sandy clay loam. Soils having a moderately high runoff potential due to slow infiltration rates. These soils primarily consist of soils in which a layer exists near the surface that impedes the downward movement of water or soils with moderately fine to fine texture. Group D: Clay loam, silty clay loam, sandy clay, silty clay or clay. Soils having a high runoff potential due to very slow infiltration rates. These soils primarily consist of clays with high swelling potential, soils with permanently-high water tables, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious parent material Data from direct field measurements on soil permeability and infiltration rates for Ethiopian soils are very limited Data is generally available only for soil types located near major irrigation projects and agricultural research stations. The hydrological soils groups presented in Table 2 are based on limited field measurements and from profile morphology and physical characteristics, and are subject to further review and refinement. 2.4.1.6 Rainfall Intensity- Duration- Frequency (IDF) Analysis IDF relationships are useful to determine the depth of storm rainfall of different return periods and durations. The values are used to determine design parameters of field drainage works of irrigation schemes. IDF curve has been developed for all stations where there are automatic recorders in Ethiopia (Ethiopian Road Authority, 2002) and surroundings. In order to estimate the Intensity values of specified duration and return period, .the regional IDF curve is also developed According to WMO’s recommendation, at site IDF equations are used to estimate the intensity values of any area within a radius of 25 km from the station. While regional IDF equations can be used to estimate the Intensity of an area located far from 25km radius range from any closer station. Annex D : Hydrology / Final Detail Design Report /2009 7Ercr Dam and Irrigation Projcct/CECE & CES The IDF-equation of different duration has been computed for different return periods and the coefficients A, B and C estimated. (3) Where; 1= rainfall intensity (mm/hr) D= duration of rainfall (minutes) A= coefficient with units of mm/hr B= time constant in minutes C= an exponent usually less than one In this particular study, the project area fall under IDF curve of region A4 and the parameters of this region are used to generate the magnitude and intensity of the different duration rainfall for the Erer drainage irrigation project The typical regional curve for region A4 is given in Figure 2. 2.5 Result of Calculation on main and Secondary Canals There exists defined drainage pattern in the Ercr Irrigation project according to the existing flood routes that discharge in the plain area. The present task pertains to the determination of peak floods that cross the main canal. The computed peak floods are as follows: Table 1 The summary of peak flood analysis S/N Rivers/D rainages Area (km ) T e 2 (minutes) Runoff coefficient, C.Cr Intensity in, mm/hr Qrs (m /s) 3 =0.27CIA Drainages on the main canal 1. Drainage #1 2.1 50 0 15 70 5.9 2. Drainage #2 0.5 20 0.15 120 2.4 3. Drainage #3 0.25 35 0.15 90 0.9 4 Drainage #4 0.6 40 0.15 80 2.0 5. Drainage #5 6.5 56 - - 15.8 Annex D Hydrology / Final Detail Design Report /2009 8Erer Dam and Irrigation Projccl/CECE & CES I I J I J J I 1 S/N Rivers/Drainages Area (km ) T c 2 (minutes) RunofT coefficient, C.Cf Intensity in, mm/hr Qis (m /s) 3 =0.27CIA 6. Drainage #6 0.75 45 0.15 70 2.1 7. Drainage #7 0.4 25 0.15 100 1.6 8. Drainage #8 1.8 50 0.15 70 5.1 9 Drainage #9 0.2 30 0.15 92 0.75 10 Drainage #10 0.25 35 0.15 90 0.9 11 Drainage #11 0.2 30 0.15 92 0.75 12 Drainage #12 0.15 30 0.15 92 0.75 13 Drainage #13 0.3 35 0.15 90 0.9 14 Drainage #14 0.35 36 0.15 90 0.9 15 Drainage #15 05 20 0.15 120 2.4 16 Drainage #16 0.3 35 0.15 90 0.9 17 Drainage #17 0.5 20 0.15 120 2.4 18 Drainage #18 0.6 40 0.15 80 2.0 Drainages on the First upper secondary canal 1 Drainage #1 0.3 35 0.17 90 1.2 2 Drainage #2 0.2 30 0.17 92 0.84 3 Drainage #3 0.7 35 0.17 90 2.8 4 Drainage #4 0.7 35 0.17 90 2.8 5 Drainage #5(Abiyo stream) 10.5 42.5 - - 23.8 6 Drainage #6 1 50 0.17 70 3.2 7 Drainage #7 0.9 48 0.17 70 2.8 8 Drainage #8 4 37.3 - 3.9 9 Drainage #9( Yasakoy stream )7.8 51 - - 18.4 10 Drainage #10 0.8 45 0.17 70 2.6 11 Drainage #11 (Kebanawa stream) 40 74.3 87.8 12 Drainage #12 3.5 40 - - 6.0 13 Drainage #13 (Uimo stream) 17 55 - 38.3 Annex D Hydrology / Final Detail Design Report /2009 9Ercr Dam and Irrigation Project/CECE & CES S/N Rivcrs/D rainages Area (km ) T e 2 (minutes) Runoff coefficient, C.C r Intensity in, mm/hr Q (rn /s) 25 3 =0.27CIA Drainages on the second lower secondary canal 1 Drainage #1 (Abiyo stream) 20.5 65 - - 46.1 2 Drainage #2( Yasakoy stream) 10 64 - - 22.7 3 Drainage #3 2 50 0.17 70 6.4 4 Drainage #4 (Kebanawa stream) 42 88 92.2 5 Drainage #5(IIimo stream) 25 69 - - 56.1 6 Drainage #6 (end of the canal) 7.2 46 - - 17.0 Annex D : Hydrology / Final Detail Design Report /2009 10Erer Dam and Irrigation Project/CECE & CES 3.0 Peak flood estimation at Decheo River 3.1 General The project under consideration was planned on a stream which does not have river flow data. Thus for such case, it is unavoidable to use the rainfall and other meteorological data from other near by stations provided that both stations are in the same agro climatic conditions. The nearest meteorological stations to the project area (Decheo stream centeral catchment) are Harar meteorological stations (Air distance of about 12 Km, elevation of 2060m am.si. and 10 years of maximum daily rainfall data), and Bisdimo meteorological station (Air distance of 10 Km, elevation 1420m a m.s 1 and 9 years of rainfall data). The proposed catchment has elevation ranges from 1360m a. m. s.l. at the main canal alignment site to around 2670m a. m. s.l. up in the Harar area. The maximum daily rainfall data from Harar meteorological station was analyzed for the determination of design storm of Decheo River peak rate of runoff. 3.2 Catchment Characteristics of the river The Erer River originates from Ejersa Goro and Waltaha sadden around Kombolcha ridge mountain range at an altitude of 2670 m a s 1. in the North-East of Harar. It flows South-east ward during rainy season & intermittent during dry season. Afterward it debauches into the wabe Shebele basin plain. The tributaries of Erer River in the Wabeshebele consist of the Abiyo stream, Yasakoy stream, Kebenawa stream, Ilimo stream, Decheo Rivers and some field drainages systems. These streams and drainages cross the main and secondary canals of the Erer irrigation project. The catchment area of the Decheo River with its tributaries at the crossing point of the two secondary canals ofErrer Irrigation project is estimated to be about 165 and 161 Km . Out of the total catchment area of Decheo river approximately 20% is covered with grass land, 60% cultivated land and the rest 20% with bush trees. 2 Annex D : Hydrology / Final Detail Design Report /2009 11Ercr Dam and Irrigation Project/CECE & CES_______________________ _________________________________________________ •• 3.3 Methodology used for flood estimation 3.3.1 Drainages Design flood The flood used for design against failure is termed as the project design flood. It can usually be determined by estimating the run off that result from an occurrence of a design storm based on meteorological factors. This hydro meteorological approach is necessary because stream records often are not available. In the design of hydraulic and irrigation structures, the peak flow that can be expected with in assigned frequency is of primary importance. 3.3.2 Peak Flood estimation 3.3.2.1 SCS Method !1 As explained earlier in section 2 4.1.1 this method shall be used for better accuracy and considering the intended purpose The procedure to be applied is as discussed earlier Here also the data are generally analysed by one of the two probability laws, i.e the extreme value law or log -probability law. In our case we used Pearson type III distribution for annual daily highest rainfall analysis of determined return period of T years. Pt ~ P«vc + S x Kt P (1) Where: Pave = Average of all values of annual daily maximum rainfall (mm), Sp = Standard deviation of annual daily maximum rainfall (mm) and Kt = Frequency factor of return period for T years Finally, after the unit hydrograph synthesis, the triangular and composite hydrographs of the peak flood has been prepared as shown in the following tables. (See the detail analysis in Annex A.) 3.3.2.2 Rational Method The rational method is the most widely used method for the analysis of run ofF response from small catchments. It has particular application in storm drainage calculation, where it is used to calculate peak run off rates for the design of storm sewers and small drainage structures. Here the rational formula is considered for catchment areas less than 0.5km2. Annex D : Hydrology / Final Detail Design Report /2009 12Erer Dam and Irrigation Project/CECE & CES 3.3.2.3 Design Storm Determination For design storm determination, the annual series of rainfall data is used. This constitutes the maximum daily rainfall out of the 365 days for every year for which the rainfall record is available The calculation is performed using Harar station (table 2) and found to be the best representation for the rainfall data of the project area. Table 2. Meteo-station: Harar Altitude: 2060m a.s.l. No. Year Annual daily max. Rainfall (mni) 1. 1995 84.4 2. 1996 68 3. 1997 43 4. 1998 33.7 5. 1999 49.5 6. 2000 64 7. 2001 85.2 8. 2002 51.9 9. 2003 51.8 10. 2004 50.1 n Paw = 7 pi = 581,6 =58.16mm. i=1 n 10 n s = p Z (Pi -Pave)2 i_=l__________ = 16.99mm N-l Annex D : Hydrology / Final Detail Design Report /2009 13Ercr Dam and Irrigation Project/CECE & CES P|00 Pave +S .Kioo (Kioo is p frequency factor for 100 year return period from Pearson type III distribution table, K SUBRAMANYA,2006) Pioo=58.16+16 99*2 686=103.8 mm P25=58.16+16.99*1.91=90.mm As the slope of the drainage basin increases, the selected runoff coefficient C should also increase. This is caused by the fact that as the slope of the catchment area increases, the velocity of overland and channel flow will increase allowing less opportunity for water to infiltrate the ground surface. Thus, more of the rainfall will become runoff from the catchment area. Most of the soil in our project area is dominated by Chromic Cambisols, Eutric Cambisols, Vertic Luvisols that are the soil around the main canal alignment is sand loamy clay and the soils around the secondary canals are clay loamy in general. Annex D : Hydrology / Final Detail Design Report /2009 14Erer Dam and Irrigation Project/CECE & CES Annex D Hydrology / Final Detail Design Report /2009 15Intensity, mm/hr Ercr Dam and Irrigation Projccl/CECE <fc CES Intensity-Duration-Frequency Regions A1 & A4 Figure 5-9 — - - —2 Year — —5 Year 10 Year 25 year 50 Year 100 Year Figure 2: Regional IDF curve (ERA, 2002) Annex D : Hydrology / Final Detail Design Report /2009 )6CECE <fc CES 3.4. Result or calculation The Drainage pattern in the Erer Irrigation project defines runoff routes and according to the existing flood routes the plain area is divided into 3 parts for determination of peak floods The peak flood expected to occur in 100 years return period at out let of Decheo River including tributaries and Others drainage channels is estimated to be about 274.0m /sec and 2d7.Jm /sec at upper secondary canal and lower secondary canal respectively. The detail analysis is attached in the annex. 3 3 Annex D Hydrology / Final Detail Design Report /2009 174. Flood flow Estimation After Routing for Protection work 4.1 100 years Return period flood estimation For estimation of 100 years return period flood the hydro-meteorological approach has been adopted Since the rainfall can have a minimum limit of zero value with no upper limit, it generally follows the extreme value distribution such as Gumbels’ distribution and thus it has been adopted The daily annual maximum rainfall at stations of Harar, Jijiga, Babile, Bisdimo, Dire Dawa and Alemaya was used to estimate the design storm By considering the areal design storm value of the six stations the 100 year return period rainfall for Erer dam has been estimated as 93.05mm. The U S. Soil Conservation Service (SCS) dimensionless hydrograph method has been adopted to compute the peak flood of the catchment for the specified return period Considering cover type as brush-weed-grass mixture with brush the major element with poor hydrologic condition and soil type as C category (sandy clay loam), curve number (CN) is estimated as 77 and values of S for wet condition as 75.2mm Table 3. Erer Catchment characteristics and values of SCS method based on Snyder’s parameters Peak runoff calculation of Erer stream No. Designation/Formula Symobl Unit Value 1 Area of the catchment measured from delineated map of the project area (1:50,000) A Km2 419 2 Lenqth of main water course up to drainage site (outlet) L m 36000 3 Level of head of main water course H, m 2663 4 Level of bed of stream at proposed drainage site(outlet) h2 m 1353 5 Slope of main water course S= (Ht-H;) /L s m/m 0.036 6 Time of concentration Tc=1/3OOO*(L/S0,5)0'77 Tc hr 3.8 j7 Rainfall excess duration D~ Tc/6 D hr 1.000 J0J Time to peak L—r----------------------- Annex D : Hydrology / Final Detail Design Report /2009 18CECE & CES tp = 0.5D + 0.6 Tc tp hr 2.809 9 Time of base of Hydrograph Tb = 2.67 tp Tb hr 7.500 10 Lag time ti= 0.6Tc ti hr 2.309 11 Peak rate of discharge created by 1mm rainfall excess on whole of the catchment q D =0.21* A*Q/tp ; (assume Q=1mm) L m3/sec/mm 31.33 The ordinates of flood hydrograph of the cathment have been shown in the table 4. and Fig 3. below Base flow is added to direct runoff hydrograph to obtain hydrograph. Base flow of 25 cum has been adopted in this case. Table 4. 100 year return period Flood hydrograph for Erer Dam Time(hr) Direct Runoff(m3/se) Base flow(m3/se) Total Runoff (m3/se) 0 0 20 20.0 1 23.8 20 43.8 2 47.5 20 67.5 2.8 66.6 20 86.6 3 74.1 20 94.1 3.8 88.4 20 108.4 4.00 94.5 20 114.5 4.8 165.5 20 185.5 5.00 183.3 20 203.3 5.80 287.3 20 307.3 6.80 346.6 20 366.6 7.50 323.3 20 343.3 7.80 317.1 20 337.1 8.50 257.3 20 277.3 9.50 170.8 20 190.8 10.50 84.4 20 104.4 11.50 24.7 20 44.7 12.50 0 20 20.0 Annex D : Hydrology / Final Detail Design Report /2009 19CECE & CES *« Fig 3. 100 year return period Flood hydrograph for Erer Dam 4.2 Reservoir Routing for 100 year return period flood There are several methods for reservoir routing of a flood wave through a reservoir The most widely used method of flood reservoir routing which is the Modified Puls method is adopted in this case. The basic law used in the Modified Puls method states: The inflow minus outflow is equal to the rate of change in storage. /-C = A5 Where,I = —— — —= (1) 2 A5 = S2 -S2> continuity equation is written as: y,+/,)y-(a+O.)y = ^-i,....................................................................................................................... (2) ■Where, suffixes 1 and 2 denote the beginning and the ond of time interval Ar and Q may incorporate controlled discharge as well as uncontrolled discharge. Separating the known quantities from the unknown ones and rearranging: (/, + A) + (25) / Az - Q,) = (25, / Az + Q )......................................................................................... 2 (3) Annex D : Hydrology / Final Detail Design Report /2009 20CECE&CES Here, the known quantities are Ii (inflow at time I), 12 (inflow at time 2), Q( (outflow at time 1) and Sj (storage in the reservoir at time 1), and the unknown quantities are S2 and Q2 Since one equation with two unknowns can not be solved, there should be another relation that relates between storage, S, and outflow, Q. As the outflow from the reservoir takes place through the spillway , the discharge passing through the spillway can be conveniently related with reservoir elevation, which, in tum, can be related to the reservoir storage. Outflow from the spillway has been worked out using the following equation: Q = C LH'5 (4) d Where, Q(cumec) is the outflow discharge, Cd is the coefficient of discharge (1 18), L (m) is the length of spillway determined previously on the hydrology report of the project that is 30m and H(m) is the depth flow above the spillway crest level of 1379.0m Elevation-Storage-Outflow for the spillway length of 30m is given in Table 5. Table 5. Elevation-Storage-Outflow data for Erer Reservoir Elevation (m) Capacity(MCM) Head(m) Spillway discharge, Q(m /se) J 1379.00(FRL) 35.56 0.00 0.00 1380.00 40.72 1.00 65.40 1381.00 44.37 2.00 184.98 1382.00 48.03 3.00 339.83 1383.00 53.16 4.00 523.20 1384.00 58.28 5.00 731.19 1385 00 63.96 6.00 961.18 1386.00 69.63 7.00 1211.22 The inflow hydrograph have been given in Table 4. of section 3.1 with a peak value of 366.6 cumec at 7 hour has been adopted for routing. Initial reservoir level for reservoir routing has been considered as FRL (1379.0m). 4.3 Result of Reservoir Routing Reservoir routing was carried out using above data as input. Summary of results of routing are given in Table 6 Annex D : Hydrology / Final Detail Design Report /2009 21CECE & CES Table 6. Summary of Reservoir Routing Result by Modified Puls method Time (hour) Inflow (m3/se) Outflow (m3/sec) 1 Elevation(m) Storage (MCM) 0 20 0 1379 35.56 1 43.8 2.1 1379.1 35.57 2 67.5 4.1 1379.2 35.87 3 87.1 8.2 1379.3 36 14 4 114.5 8.3 1379.5 36.47 5 203.3 16.4 1379.6 37.02 6 307.3 30.7 1379.8 37.88 7 366.6 54.95 1379.9 38.98 7.5 343.3 66.23 1379.98 40.04 8.5 277.3 96.6 1380.1 40.94 9.5 190 8 150 24 1380.4 41.5 10.5 104.4 250.5 1380.6 41.58 11.5 44 7 127.4 1380.5 41.5 12.5 20 86 6 1380.4 41.24 13.5 20 66.7 1379.8 36.86 14.5 20 60 6 1379.6 36.4 15.5 20 50.9 1379.44 36.04 16.5 20 43.8 1379.4 35.6 17.5 20 42 1379.25 35.57 18.5 20 40 1379.3 35.56 As it is seen from the above Table 6 the maximum discharge of the reservoir after routing is 250.5 cumec at 10.5 hour Fig 4. Shows plot of inflow and outflow hydrograph for reservoir routing of spillway which has a crest length of 30m. &er Irrigation project-Rsservoir Routing (L=30m) i i i I Time (hrs) Fig. 4 Reservoir Routing (L=30m) Annex D Hydrology / Final Detail Design Report /2009 Inflow (m3/se) ----------- Outflow (m3?sec) 224.4 Flood way Determination for 100 year return period peak- For Floodway determination the basic hydraulic system of computation has been used and here the model called HEC-RAS model has been adopted. HEC-RAS is currently capable of performing one dimensional water profile calculations for steady gradually varied flow and unsteady flow in natural and constructed channels. 4.4.1 Equations for basic profile calculations. Water surface profiles arc computed from one cross section to the next by solving the energy equation with an iterative procedure called the standard step method. The energy equation is written as follows = y,+z, +^--+h. Where, Y|, Y2 =depth of water at cross sections Z|, Z2 = elevations of main channel inverts Vi, V2 = Average velocities (total discharge/Total flow area) a ,a = velocity weighting coefficients x2 g = gravitational acceleration He = energy head loss 4.4.2 Cross section Subdivision for conveyance calculations The determination of total conveyance and velocity coefficients for a cross section requires that flow be subdivided in to units for which the velocity is uniformly distributed The approach used in HEC-RAS is to subdivide flow in the overbank areas using the input cross section n-value break points (locations where n-values change). Conveyance is calculated within each subdivision from the following form of Manning’s equations. n Annex D : Hydrology / Final Detail Design Report /2009 23CECE &CES Where, K= Conveyance for subdivision n = Manning’s roughness coefficient for subdivision A= flow area for subdivision R= Hydraulic radius for subdivision The programme sums up all the incremental conveyances in the overbanks to obtain a conveyance for the left overbank and the right overbank. The main channel conveyance is normally computed as a single conveyance element The total conveyance for the cross section is obtained by summing the three subdivision conveyances (left, channel, and right). Annex D : Hydrology / Final Detail Design Report /2009 244.4.3 Data requirement to run the HEC-RAS model For determination floodway by HEC-RAS it is important to obtain the basic river cross section parameter and the flow data. The following data are needed at the section: • Channel cross section parameters such as elevations of different points surveyed across the river perpendicular to its longitudinal profile. • The change or distances between the coordinates along the profile. • The estimated Manning’s roughness coefficient of the main natural channel of the river. • The desired discharges to be tested for the river reach at required return period. 4.4.3.1 Channel cross sections of Erer River The actual surveyed channel cross sections of the Erer river at the down stream of the dam have been adopted. There are about nine stream cross sections adopted in this case The first cross section starts at the down stream of dam axis at about 100m. Most of the cross section are about 200m away from each other along the river reach. 4.4.3.2 Manning’s Roughness coefficient Flow in the main channel is not subdivided, except when the roughness coefficient is changed within the channel area. There fore, the Manning’s roughness coefficient is estimated for Erer natural stream, which ranges from 0.02 to 0.04 and it is found to be appropriate. 4.4.3.3. Discharge to be handled by the River Reaches The required discharge to be handled by the Erer river reach was computed before as shown in Table 6 of Section 4.3 The inflow discharge has a peak flow of 366 cumecs and , while it is routed to get reduced to 250.5 cumecs. The routed peak flow of 250.5 cunecs has been used to check of the stream cross section is sufficient to handle it, as shown below In addition to this it was tried to testnhe discharge data before routing with peak discharge of 366.7 cumec.CECE & CES 4.4.4 Result of floodway determination Floodway determination was carried out by using the above data as input. Summary of results of floodway determination model output from HEC-RAS are given in Table 7 and Fig.6 below. Table 7. Summary of floodway determination by HEC-RAS model Rrver Sid Prolie 100TR FLOW 1007R FLOW HOUGT lOOffi FLOW 100/fiI FLOW HOUGT lOtT/R FLOW Q fol^! f Mtn QtEiJCnfw S E.G Slcpef Vrf Ord jFtow Areal Top VVdih Frourfe rt CH ' [mJ (m/mj 367.00; 1348 57 1354 S3: 1354.99 1357 39; 0 019733 250 CO 1348 57 1353 82 1353 82 1355 74 JI 9032 367 oo: 1347 03 1353.75; 1349 44 0 000163 1353 76; GB6 614 047 i*2J 53 53 40 74‘ 773 26 1*1 11.25 1Q64 14473 1 00 i no 0.06 250 00 1347 03 135293 1348.19 1352 94 - 0 000122 038 662 64 138 79 0 06 367 00 1346 47 1353 75 1353 75: 0 000027 Q21 1771 B0 276 90 0 03 ERER 09 ERER ERER ERER . ERER ERER ERER . ERER ERER . ERER ERER OB GG 07___ _ 07 06___ 06 05___ _ D5 D4 04 03__ 03 1U8fR FLDWROUGT 1 DOTH FLOW : 250 00 134647 135293 135293 0 000019 016 1546 22 272.17 0 02 367 00 1346 38 1353 72 1353 74; 0.000115 050 536 44 11208 0 08 250 00 1346 38 135291 135292 0.000080 045 550 43 101 69 006 367.00 1347.68 135369 1353.70 0.000321 042 85864 17097 005 250 00 1347 68 135289 135289 0.000259 034 731 89 16026 0.05 l 367 CO 1347 02 1353.66 135366: 0.000145: 024 1499 60 27520 0.03 250.00 1347.02 135286 1352.96 0X0111 019 128223 268.76 0.03 367.00. 134591 135364; 135364! 0X0113: 019 1939.12 334 25 0.03 I l 250 00 1345.91 135284: 1352 35 0 X0079 0.15 1680 1 7: 31075 0.02 367.00 1348 57 135291! 135200 1353.52 ! 0X4643 345 10624 3451 250 00 1349 57 1352 34; 135276 0X3933 288 86 87 3311 0.63 0.57 ERER erer; 1O0/R FLOW RQUGT 10O/RFLOW lOOr'R FLDWROUGT OOffi FLDW 10CMR FLDWROUGT FLOW PCTfR FLDWROUGT OOfRFLDW 10CJ/R FLOWRQUGT 100VR FLOW 100VR FLDWROUGT 367 00; 1348.60 1350.17; 1350.17 1350 67 05226521“ 11$ 116.64 117.23 1.01 250.00 134860 134392: 1349.92 1350.32 0554199 282 88 67: 10376 i oo It is seen from the above Table 7, that the maximum floodway level attained after reservoir routing is 1353.8m at about 1 4km downstream of the dam axis for 1OO year return period flood. But for the 100 year return period flood before routing, the floodway level was about 1354 9m. But the main canal is running at an elevation of 1360m a.m.s.l. Based on the above results, the main canal can not be affected by flood of 100 years after routed. At this level there is an elevation difference of about 7.0m between the floodway and main canal levels. Fig. 6 shows the plot of water way level and distances along the Erer River channel. Annex D : Hydrology / Final Detail Design Report /2009 26CECE & CES Annex D : Hydrology / Final Detail Design Report /2009 275.0 Comparison of total measured surface Runoff Potential of Erer River with feasibility study In order to get more confirmation and to ascert the analysis made during the feasibility study, the adopted approach is to evaluate the catchment yield of Erer Dam based on the direct measurement of flow data collected at the Dam site by Concert Engineering Laboratory Department. Since this station will best represent the exact location of inlet of the inflow from the Erer Catchment it was used directly for the yield evaluation. For this purpose 16 flow samples data were collected by the Laboratory department as shown in Table 8 below. Flow rates varying between 6.58 to 9.73 m3/sec were observed for this river This flow rate observation was made during the main rainy season (June-September) of the area. Table 8. Observed flow rate of Erer at dam site. Observed Flow of Erer at Dam Site Date G H. (m) Mean flow (m /se) 3 Mean flow (MCM) 10- Aug-08 2.09 9.73 102.56 10-Aug-08 2 09 9.73 102.56 10-Aug-08 2.09 9.73 102.56 10-Aug-08 2.09 9.73 102.56 12-Aug-08 2.05 7.51 79.16 12-Aug-08 2.04 7.51 79.16 12-Aug-08 2.04 7.51 79.16 12-Aug-08 2.04 7.51 79.16 15-Aug-08 1.75 6.61 69 67 15-Aug-08 1 75 6.61 69.67 15-Aug-08 1.75 6.61 69.67 15-Aug-08 1.90 6.61 69.67 16-Aug-08 1.91 6.58 69.36 16-Aug-08 1.90 6.58 69.36 16-Aug-08 1.90 6.58 69.36 16-Aug-08 1.90 6.58 69 36 Average 7.61 80.19 The method employed to calculate the annual yield of the catchment was by averaging the observed data as daily mean stream flow and convert it to monthly mean flow of the stream. The mean annual flow of the river is calculated considering the main four rainy seasons of the area Annex D : Hydrology / Final Detail Design Report /2009 28excluding base flow of the stream From these data series, the mean annual yield of the catchment was estimated Based on this approach for assessing catchment yield, it was estimated that the runoff yield of the catchment entering the Erer reservoir will be in the order of 80.2 MCM for the main four rainy seasons only. On the other hand, the total mean annual net flow, which can be impounded in the reservoir from Erer catchment is estimated at the dam site based on the 39 years (1967-2005) of data and was about 60.4 MCM ( WWDSE) But the gross capacity of the reservoir serving at 100% of the total irrigation command of 3963ha area of the project is about 50.11 MCM Therefore, the estimated yield of the catchmnet based on the observed data even for only four months without considering the base flow and other months of the year will be enough to ascert the dependable flow. Annex D Hydrology / Final Detail Design Report /2009 29CECE & CES Annex A Procedure of U.S. Soil conservation Synthetic hydrograph method Peak runoff calculation of Oecho stream No. Designation/Formula Symob Unit Value 1 Area cf the catchment measured from deleneated map of the project area (1.*‘0,000) A Km7 165 2 Lenolh of main water course upto drainage site (cutlet • L m 28500 3 Level of head of main water course H, m 2200 4 Level of bed of stream at proposed drainage site(outleh Hs m 1320 5 Slope of main water course S= (H.-Hz).-L s nvm 0.031 S Time of concentration Tc=1/3000*(L/S*Tn Tc tr 3.4 7 Rainfall excess duration D~ TUG D hr 0.57 8 ’ ime tc peak tp~ 0.50 0 6 Tc 4 U hr 2.340 9 Time of base of Hydrograph Tb = 2.67U T fc hr 6.248 io Lan time h = 0 GTc li if 2.056 11 Peak rate of discharge created by 1mm rainfall excess on whole of the catchment <U =0.21* A*Q/tp ; (assume Q=1mm) m5/sec/m 14.81 Annex D Hydrology / Final Detail Design Report /2009 30 JM" 5U.!>CECE & CES *• 19 20 21 22 23 1 J 25 Descending Order Rearranged Order Rearranged incremental rainfall Cummulative rainfall Time of incremental Me mm mm Time of beginning Time la peak Time Io end 18.2 114 8.2 56 6 4 48 5.6 4.8 10.3 3 1 92 18.2 53 48 2 5 11 4 5.3 19.6 377 49.2 54.5 0 0 57 1.14 1 71 2.28 2.9 2.3 29 3.5 4.1 8.25 6.82 7 39 7.96 4.6 5.2 853 9.10 26 Land use 27 Area ratio 28 Hydrologic 30 Weighted 31 Sum weighted cover condition soil group 29 Curve No. *CN* -err *ctr 4 VC CiV Forest Pasture 0.25 0 25 fair-B fair-6 61 69 15.25 17 25 Wood land Cultivated land 0.25 0.25 fair-6 fair 8 71 75 17.75 18 75 ii in 69.0 83.7 Annex D Hydrology / Final Detail Design Report /2009 31CECE & CES No. Desiqnatlon/Formula Symbol Unit Value 32 S is maximum potential difference between rainfall (P) & direct run off (Q which is S~ 25400/CN - 264 }< S mm 49.6 33 Relation between direct run of f (QJ and rainfall (Pj Q-IEJJW P+0.8xS Q mm 34 Substitute values of P. as mentioned in 0.22. in the above formula and find the corresponding values of Q(C.3- C.22 C.34 Ptmmi Ck'rnni) 4.0 0 6 10.3 0.0 19.6 1.6 37.7 10.0 49.2 54.5 17 3 21.1 Annex D : Hydrology / Final Detail Design Report /2009 32CECE & CES Tirnetbd Ordinate of Hydroqraph(rn3/B) Tcilal 1 2 3 41 51< t0 0.0 0.57 0.1 1,14 0.3 1 71 70 6.1 2.23 0 0.1 ol 0,3 0.00 0.0 o.<1 0.00 5. 0.6 0.00 11. 3 30.4 0.00: 42.3 2.34 48.9 2.85 104.7 2.91 112.7 3.48 188.7 4 05 0.6 0.00 11.9 336 2.79 0.5 0.06 17.0 60.7; 26.45 0.0 0.5 0.00 17.6 64.0 29.23 1.4 0.4 0.00 23.2 94.3 65.68 15.0 03 0 00 28.9 124.7 82.13 20.6 0.2 0.00 165 106.5 108 58 42.2 0.2 0.00 13 1 08.3 92.74 55.8 MN 264.7 4.62 5.19 250.1 6.25 00 165.4 6.82 68 54 6 63.35 40.7 3.383372 36.4 47.51 32.5 119 8 7.39 74.3 7 96 0.0 18.2 31.67 24.4 0.0 15.84 16 3 32 1 8.63 0 8.13 ’ 3.1 9.10 0' 0.0 Composite Hydrograph Tima(hr) Annex D : Hydrology / Final Detail Design Report /2009 33Elevation (m) Elevation (m) CECE & CES Annex-B Cross Sections the Erer River X-section-1 Annex D Hydrology / Final Detail Design Report /2009 34CECE &. CES X-sedtion-3 X-Sectionn-4 Annex D : Hydrology / Final Detail Design Report /2009 35CECE & CES X-SectJon-5 X-Scction-6 Annex D Hydrology / Final Detail Design Report /2009 36CECE CES 37REFERENCES • H.M. Ragunath, Hydrology Principles, Analysis and Design , 1986 • Linsley K, Kohler, Hydrology for Engineers, 1975 • Water works Design and Supervision Enterprise in Association with Synergic Hydro (India) Pvt Ltd, September 2007, Feasibility report, Erer Irrigation Project, Volume3
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