FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA MINISTRY OF WATER RESOURCES
ERER DAM 8c IRRIGATION DEVELOPMENT PROJECT
Final Detail Design Report
Annex B: Dam Appurtenant Structures
CONCUR T ENGINEERING . IND CONSULTING ENTERPRISE P.L.C. (CECE) ENGINEERS WATER RESOURCES A AGRICULTURAL
DEVELOPMENT PLANNERS
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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 Analysis1
J
Table of Contents
1. Introduction..................................................................................................................... 1
2. Intake................................................................................................................................ 2
2.1 Performance and Capacity   Requirement................................................................... 2
2.1.1 Irrigation Demand................................................................................................ 2
2.1.2 Supply for Downstream Users...........................................................................   3
2.1.3 Design Discharge................................................................................................. 3
2.1.4 Depletion of Reservoir and Diversion..................................................................4
2.2 Selection of Site and Type of Outlet Works.................................................................5
2.2.1 General................................................................................................................. 5
2.2.2 Choice of Alternatives......................................................................................... 5
2.2.3 Selection of Site and Layout................................................................................. 6
2.2.4 Inlet Sill Level...................................................................................................... 6
2.3 Components of Outlet Work....................................................................................... 7
2.3.1 General................................................................................................................. 7
2.3.2 Entrance Device................................................................................................... 7
2.3.3 Conveyance Structure.......................................................................................... 8
2.3.4 Exit Device..........................................................................................................  8
2.4 Geometry and Size of Outlet Works........................................................................... 8
2.4.1 Entrance Structure................................................................................................ 8
2.4.2 Conveyance Structure.......................................................................................... 9
2.4.3 Terminal Structure............................................................................................... 9
2.5 Outlet Works Control.................................................................................................. 9
2.5.1 Entrance Control.................................................................................................. 9
2.5.2 Exit Control........................................................................................................ 10
2.5.3 Control Device Operation.................................................................................. 10
2.6 Reservoir Evacuation Time.......................................................................................10
3. Spillway Structure........................................................................................................ 12
3.1 Location and Suitability............................................................................................ 12
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1
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3.2 Design Criteria.......................................................................................................... 12
3.2.1 Selection of type of Spillway............................................................................ 13
3.3 Components of Spillway...........................................................................................13
3.3.1 General...............................................................................................................13
3.3.2 Control Section.................................................................................................. 13
3.3.3 Conveyance Section.......................................................................................... 13
3.3   4 Energy Dissipation Section..............................................................................14
3.4 Geometry and Size of Spillway................................................................................ 15
3.4 1 Spillway Crest Structure................................................................................... 15
3.4.2 Spillway Chute Structure................................................................................... 15
3.4 3 Spillway Energy Dissipation Structure.............................................................  16
3.4 4 Stilling Basin Freeboard.....................................................................................16
3.5 River Protection and Training at Exit Channel......................................................... 17
3.6 Tail Water Curve Analysis and Flood Submergence................................................ 17
4. Bill of Quantities and Cost Estimates.......................................................................... 18
4.1 Dam Appurtenance Works........................................................................................ 18
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JErer Draft Detail Design Report/ CECE & CES
1. Introduction
This section  deals only  with  the hydraulic design  of the Erer dam appurtenance structures The reader is thus advised to consult the relevant section of the report for a complete treatment of the structural design aspects of theses and other structures.
The appurtenance structures constitute a spillway and an outlet work. The design of both structures is built on the Feasibility Study of the Erer Dam and Irrigation Project that was completed by the Ministry of Water Resources in September 2007 Although the level of design detail accorded during the Feasibility Study to the spillway and the irrigation outlet works was significantly different, the available documents had generally provided valuable background  information  on  the dam appurtenance structures design  not withstanding the need for more field data to complete the designs to an acceptable standard. Incidentally the field geotechnical investigations have not been completed yet.
Annex B : Dam Appurtenance Structure /Final Detail Design Report /2009
1Erer Draft Detail Design Report/ CECE & CES
2. Intake
The Erer dam outlet works pass through  the embankment at foundation  level.  It is provided  in  order to  regulate and  release water impounded  by  the Erer dam at rates dictated by downstream needs and less by evacuation considerations For this the outlet work is designed to satisfy certain performance and capacity requirements.
2.1 Performance and Capacity Requirement
For Erer dam, the minimum irrigation outlet size is determined at the most critical draft from the reservoir that is when the reservoir storage is at its Minimum Drawdown Level (MDL) and daily irrigation demands are at their peak In other words the capacity of the outlet must be sufficient to  release downstream water needs,  both  irrigation  and environmental, with the available reservoir storage at its MDL. Additional performance and  capacity  requirements ought to  be met by  positioning  intakes properly,  gates, operating controls and terminal structures, sizing components, providing adequate means for maintenance, replacement and inspection of all components
2.1.1 Irrigation Demand
The Erer dam outlet works are provided for the sole purpose of releasing irrigation water demand and environmental flows Concurrently the water impounded in the dam, less evaporation and seepage losses, is intended for meeting water supply demand of Harar town, and the livestock and domestic requirements of local villagers But other than a single irrigation outlet, there is no separate outlet provided for abstraction of all other needs mentioned earlier.
Water  demand  for  4000  hectares  irrigation  development  in  the  Erer  project  area  is estimated at 3m3/sec.
Owing to the many competing needs pointed out above, future expansion of the irrigation area would obviously be constrained by limited availability of water resources in the
Annex B : Dam Appurtenance Structure /Final Detail Design Report /2009
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project area.  Yet concerted  effort in  managing  irrigation  water demand  might create scope for some degree of expansion of the irrigation area. In this regard, selection of crop varieties and  optimization  of cropping  patterns to  help  achieve as much  as possible reduced peak irrigation demands should be seriously considered.
2.1.2 Supply for Downstream Users
Mandatory river releases are earmarked for ecological requirements as well as to meet water demand  of wildlife habitat downstream of Erer dam.  A continuous flow of O.O5m3/sec is earmarked for such releases.
Arrangements for continuous releases to meet the downstream requirements are proposed by providing a 10 cm diameter steel pipe in the sidewall of the irrigation outlet. The inlet of the pipe will open up upstream of the service gate in the intake tower and end up at the glacis at the exit end of the outlet conduit.
2.1.3 Design Discharge
Selection of the size of the irrigation outlet has been governed by meeting the peak irrigation water requirements which are estimated at 3m /sec at the most critical period. The most critical period for irrigation development being when the available reservoir storage is low but daily irrigation demands are at their peak
Combining irrigation and environmental demands, the outlet has been designed for a discharge of 3 O5m /sec at the minimum operating head of 1.0 meters. In other words the outlet discharge capacity  would  guarantee release of adequate water to  meet water requirements downstream, both irrigation and environmental, under varying upstream head and even if the available reservoir storage is at its Minimum Drawdown Level (MDL set at El. 1371m).
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2.1.4 Depletion of Reservoir and Diversion
Sometimes diversion  during  construction  or reservoir evacuation  requirements may govern the size and elevation of an outlet conduit. In the case of Erer, its size and elevation  is primarily  governed  by  meeting  peak  irrigation  water demand  Thus the conduit at its start, with its sill level set at El 1366m, is placed at a higher level than the original river bed (El. 1349.50m) at the dam axis to guarantee gravity flow into the primary canal (El 1363.9m) instead of the Erer River bed In addition to releasing the impounded water to meet demand, as and when needed, the irrigation outlet work is envisaged to deplete the reservoir up to the lowest entry port inlet sill level located at El
1370m to permit inspection, to make needed repairs or to maintain the upstream face of the dam or the spillway. Complete evacuation of the reservoir is however not possible owing to the provision of a high level entrance intake structure and in the absence of a bottom outlet at Erer dam.
Theoretically use of the outlet conduit for river diversion during construction can not be ruled out. The Erer dam site being a rather wide valley, it may be economical to divert stream flows via a temporary channel involving a gap in the middle portion of the embankment dam while the remainder of the embankment is being constructed.
The diversion conduit must be designed to accommodate the expected flow during the closing of the gap. Use of the irrigation outlet conduit for diversion during construction presupposes that flows in the river during the closing of the gap would not exceed its discharge capacity. But when it is used for river diversion, large amounts of abrasive sediment could be damaging to the completed outlet works conduit If provision could be made for repair of damage suffered during diversion the outlet conduit can be used for river diversion during construction
Annex B Dam Appurtenance Structure /Final Detail Design Report /2009
4Erer Draft Detail Design Report/ CECE & CES
2.2 Selection of Site and Type of Outlet Works
2.2.1 General
Alternatives have been looked into in the course of design where results of the review on the advantages and disadvantages of the structures proposed during the feasibility stage suggested the need for amendments.
2.2.2 Choice of Alternatives
Cognizant of the severity of soil erosion in the Erer catchment and its impact on rapid sedimentation of the reservoir, provision of a dry versus a wet intake tower and a high versus a low level entrance intake has been analyzed.
By  definition  a dry  intake tower will have no  water inside it if its gates are closed, whereas the wet intake tower will be full of water even if its gates are closed In a wet intake tower, the water enters into the conduit through separate gate controlled openings and in a dry intake tower water is directly drawn into the withdrawal conduit through the gated entry ports.
A dry intake tower with multilevel entry ports is adopted here, as it allows water to be withdrawn  from any  selected  level of the reservoir by  opening  the port at the level. Multilevel entry ports are usually provided for water quality control of reservoir releases, for Erer project it is particularly  adopted to  enhance service life with  rising  sediment accumulation in the reservoir and also to deter the transfer of high silt loads into the irrigation area as it gives flexibility in controlling the gates
Unlike high level intake, a low level intake is susceptible to clogging as it gets buried with rising sediment accumulation in the reservoir. A high level intake is selected for Erer project. The lowest entry port of the high level intake is placed sufficiently high for it ought to function free from interference of sediment deposits and sufficiently low to prevent vortex formation when the stored water is drawn with the reservoir at Minimum
Annex B : Dam Appurtenance Structure /Final Detail Design Report /2009
5Erer Draft Detail Design Report/ CECE & CES
Drawdown Level which is at the Dead Storage Level. Dead storage volume is allowed for sedimentation in reservoir capacity
2.2.3  Selection of Site and Layout
Generally  the alignment of an  irrigation  outlet is decided  based  on  topographic and geological considerations.  Under the given  topography,  the centerline of the outlet is aligned in a straight line at chainage 1266 m along the dam axis. The outlet is located at the foundation  level on  the right abutment of the dam where it is thought that serviceability will not be affected by settlement. However these assumptions have yet to be verified after the completion of the field geotechnical investigation that are currently in progress.
Moreover, the layout is selected in plan and elevation so that it can be more conveniently connected directly to the Primary Canal and also to allow unimpeded flow of water by gravity to the Primary Canal (whose start has been set at El. 1363.90m) that in turn feeds by gravity some 3961 hectares irrigation canal network, also located entirely on the right bank of the Erer River.
The inlet and outlet works (Figure 1) are located as close as possible to the upstream and downstream extremities of the Erer dam profile,  leaving  just sufficient space to accommodate the intake and exit works and required working space between terminal structure and dam toe. Hence, the length of the conveyance structure is shortened by virtue of the selected location.
2.2.4 Inlet Sill Level
The intake tower is provided with three multilevel entry ports. From the hydrology study, the reservoir Dead  Storage Level is set at El 1371m which  corresponds to  sediment accumulation volume during 50 years of reservoir life. While it is necessary that the level of the lowest entry port be placed sufficiently high for it ought to function free from interference of sediment deposits it'should also be positioned sufficiently low to prevent vortex formation when the stored water is drawn with the reservoir at Minimum
Annex B : Dam Appurtenance Structure/Final Detail Design Report/2009
6LONGITUDINAL SECTION
Scale 1:500
NOTES
1.  All dimension are in millimeters and elevations are
in meters unless specified otherwise.
2.  For RCC conduits and intake well M-25 grade concrete
shall be used
3 Loose soil below conduit shall be removed and
backfilling shall be done with the same material used
for shell and core respectively at OMC.
4.  Foundation preparation and compacting around conduit must be equivalent to foundation preparation for the dam and to the compaction of core.
5.  The ends of the first placed conduit section shall be painted with sealing compound to break the bond between sections.
6.   Reinforced concrete cut-off collars to be provided @5000 c/c over the contraction pre molded bituminous joint filler placed
between duct and the cut-off collar in core portion only
FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA MINISTRY OF WATER RESOURCES
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N0_EJVOA^nErer Draft Detail Design Report/ CECE & CES
Drawdown Level which is at the Dead Storage Level For this reason, its sill level is set at El. 1370.00m which provides water seal of one meter above the inlet sill level.
2.3 Components of Outlet Work
2.3.1General
The irrigation outlet work consists of an intake device, a control, a conveyance structure and a terminal device
2.3.2 Entrance Device
A high level entrance intake structure with proper provision of trash racks is provided at the upstream end  of a conduit passing  through  the dam embankment foundation  The intake consists of a tower rising from the base of the outlet conduit to the dam crest level. An access bridge connects the dam crest with the top of the tower
Three entry ports each fitted with trash racks and slide gates to regulate flows through the tower are provided at different levels one each on the three sides of the intake tower in addition to an emergency gate provided on its fourth side. The tower structure houses the upstream control gates whose hoists are mounted on an operating deck that is linked to the access bridge. The entry ports are submerged at all levels to minimize any problems of clogging by debris when the reservoir is at Normal Water Level The sill level of the lowest entry port is kept at El 1370 m which is sufficiently high above the reservoir bed so that sediment is not drawn into it. Arrangement of the intake tower entry ports would permit some selection of the quality of water to be withdrawn from any selected level of the reservoir by opening the entry port at that level Although as stated the arrangement of the entry ports allows selective withdrawal, its ultimate goal is to contribute to the longevity of reservoir service life with rising sediment accumulation, cognizant of the prevailing severe soil erosion in the Erer catchment area
Annex B : Dam Appurtenance Structure /Final Detail Design Report /2009
7Erer Draft Detail Design Report/ CECE & CES
2.3.3 Conveyance Structure
The intake and  outlet works are located  as close as possible to  the upstream and downstream extremities of the Erer dam profile such that the length of the conveyance structure is minimized, leaving bare minimum space to accommodate the intake, outlet works and required working space between the terminal structure and dam toe.
The conduit is designed to prevent piping along the contact between the outside surface of the conduit and  the embankment.  Projecting  collars are provided  on  the conduit according to the common rule that the collars increase the seepage length by at least 25 percent to reduce seepage along the outside of the conduit. Besides, structural joints are inserted in order to allow possible differential settlement, without leakages.
2.3.4 Exit Device
The irrigation outlet is designed to release water at a safe velocity in the primary canal so as to protect it from erosion. In other words appropriate hydraulic setting that permits dissipation of excess energy prior to the start of the canal reach is provided.
2.4 Geometry and Size of Outlet Works
The capacity of the irrigation outlet is determined from reservoir operation studies based on a consideration of a critical period when reservoir storage is low and daily irrigation demands are at their peak.
2.4.1 Entrance Structure
A high level entrance intake with three multilevel entry pons each 2.9m wide by 3.82m fitted with slide gates and trash bars mounted at 10cm apart vertically and horizontally are provided at the upstream end of a conduit passing through the dam foundation. The intake consists of a tower rising from the base of the outlet conduit to an access bridge connecting the dam crest with the top of the tower also rising up to the dam crest level.
Annex 13 : Dam Appurtenance Structure /Final Detail Design Report /2009
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2.4.2 Conveyance Structure
A high  level entrance intake structure is connected  to  a 123  meters long  horizontal reinforced concrete 1.1m by 1.1 m size conduit. The conduit is provided with 75 cm high and 45 cm thick reinforced concrete cut-off collars laid over 15cm thick lean concrete spaced @ 5m center- to- center and 2 cm thick bituminous joint fillers placed between conduit and cutoff collars in the dam core portion Besides, structural joints are inserted every 10 m in order to allow possible differential settlement, without leakages.
2.4.3 Terminal Structure
USBR stilling basin Type III is employed for dissipation of energy by a hydraulic jump at the end of the spillway chute The 8.6m long stilling basin with its apron elevation set at El 1361.1m is fitted with ancillary devices. Installation of ancillary devices such as chute blocks, baffle blocks and end sill is expected to produce a stabilizing effect on the jump, thereby  permit shortening  the basin  length  and  guard  against sweep  out caused  by inadequate tail water depth.  Without the baffle blocks the length would have risen to about 15 meters, which is about 5 times the conjugate depth.
2.5 Outlet Works Control
2.5.1 Entrance Control
An intake tower located at the beginning of the conduit is provided to house four control gates, one emergency and three service gates one each located on four sides of the intake tower, which are to be operated through an access bridge linking the intake tower with the dam crest. The tower structure houses the upstream control gates whose hoists are mounted on the operating deck which is linked to the access bridge. Access to the conduit is thus through the gate well to be provided with proper size rungs when the emergency gate is dropped and service gate is left open. The arrangement enables to centralize all control features i.e. service gates, emergency gate, trash rack all combined in a single intake structure, thus allow operation of outlet conduit at one point
Annex B : Dam Appurtenance Structure/Final Detail Design Report /2009
9Erer Draft Detail Design Report/ CECE & CES
2.5.2 Exit Control
Exit control is avoided due to the inherent dangers of a possible rupture of a conduit subject to the full reservoir head while passing through an earth embankment. Thus all controls are located  upstream However the 10cm diameter steel pipe provided  for environmental releases is controlled by a valve situated at the exit end of the conduit.
2.5.3 Control Device Operation
Three entry ports each fitted with trash racks and slide gates to regulate flows through the tower are provided at different levels one each on the three sides of an intake tower. The intake tower houses all the control gates. When desired, an emergency gate enables complete closure of the conduit.
The upstream gates will be operated from a bridge deck which spans between the dam crest and the intake tower. The gate hoists are mounted on the deck located at the top of the intake tower. Regulation is to be effected by means of the gates which are capable of operating at part gate openings. Storage in the reservoir is drawn down to its minimum level by adjusting gate openings according to varying downstream requirements. Thus regulated discharge will be released through the irrigation outlet under varying upstream head. When desired, arrangement of the entry ports permits some selection of the quality of water to be withdrawn from any selected level of the reservoir by opening the entry port at that level.
2.6 Reservoir Evacuation Time
The minimum irrigation outlet conduit size is determined at the most critical draft from the reservoir that is when the reservoir storage is at its Minimum Drawdown Level (MDL) and daily irrigation demands are at their peak.
Although the lm by lm conduit provided at Erer dam may seem not particularly ideal with respect to reservoir evacuation time, it is suitable when it comes to serving as an 
Annex B : Dam Appurtenance Structure/Final Detail Design Report /2009
10Erer Draft Detail Design Report/ CECE & CES
irrigation  outlet.  The decision  to  adopt a bigger size conduit with  reduced  reservoir evacuation time and a smaller size conduit with longer reservoir evacuation time is thus a matter of economics and judgment. For instance, reservoir evacuation time for the lm by lm conduit provided  as irrigation  outlet for Erer dam turns out to  be about 1  month; while provision of 2 5m by 2.5m conduit reduces it to about 5 days. Yet, in both cases complete evacuation of the reservoir is not possible at Erer dam, given the provision of a high level entrance intake structure and no bottom outlet, lm by lm conduit is adopted for Erer project considering that one cannot afford to waste the limited water resources once stored and also cognizant of the fact that with stringent quality control during the dam’s construction the need for evacuation of water could be avoided
Annex B : Dam Appurtenance Structure /Final Detail Design Report /2009
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<*
3. Spillway Structure
3.1  Location and Suitability
Site conditions generally influence the location, type and components of a spillway. The Erer spillway is located on the left abutment of the dam where elevation of the natural ground  is around  1380  masl and  hydraulic head  at the interface between  the concrete structures and the embankment is rather minimal.
Apparently, the Erer spillway (figure 2) has been located on the left bank of the river as the foundation situation are more favorable owing to the information available from the Feasibility  Study  documents and  the current field  assessment as well as result of core drilling  along  the chute line.  The spillway  works are not intersected  by  the proposed irrigation outlet works which is located on the right bank of the Erer River. It should however be pointed out that there is a 3 metre depth proposed curtain grouting below the foundation of the spillway along the dam axis in order to reduce the permeability of the foundation.
3.2  Design Criteria
Spillway  selection  criteria  fall  into  functional  considerations  and  safety  considerations Generally the spillway is designed to fulfill the following criteria:-
•    adequate capacity to prevent dam overtopping which is detrimental to an embankment dam
•    adequate overflow capacity to limit infringement on dam freeboard during emergency
•    appropriate hydraulic setting to permit excess energy dissipation
•    minimize hydraulic head losses using proper transitions
•    hydraulic and structural safety
•    erosion resistant construction
•    avoid erosion or undermining of the downstream toe of the dam
•    ensure safe discharge capacity of the downstream river channel to avoid over bank flooding
Annex B Dam Appurtenance Structure /Final Detail Design Report /2009
1220000
20000
20000
445000
l
33400 
lErer Draft Detail Design Report/ CECE & CES
3.2.1 Selection of type of Spillway
Both functional and safety considerations justified selection of an un-gated free overflow spillway  crest.  Unlike gated  spillways,  the selected  type gives reliability  due to  its simplicity, freedom from operating mechanisms, and not requiring operating personnel especially  in  remote locations like the Erer project area where flash  floods must be released before endangering dam safety
3.3 Components of Spillway
3.3.1 General
The spillway consists of an approach channel, a control weir, a conveyance chute, an energy dissipator and an exit channel as its major components.
As there is no bridge over the spillway, access along the dam crest for pedestrians and vehicles alike is via a right bank road that links with the 9 meter wide dam crest, serving as a road leading to the control weir.
3.3.2  Control Section
The selected spillway is a free overflow weir with ogee crested profile. Operation and maintenance simplicity have justified selection of an un-gated spillway not to mention other advantages.
3.3.3 Conveyance Section
The spillway could not draw water directly from the reservoir and hence an approach channel is provided upstream of the ogee crested control structure to draw water from the Erer dam reservoir and convey it to the weir. The channel starts with relatively larger bed width at its inlet and ends with a bottom width of about 30m at the entrance to the control weir.
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Since a straight approach channel could not smoothly connect with the ogee crested weir, the connection  has been  made possible by  a smooth  transitional curve subtending  an angle of 60 88 degrees at a radius of 70 meters to suit the topographic feature that is found to be the foot of a range of rising ground with relatively gentle slope on its left side. Finally, the approach channel is given a straight alignment as it reaches the spillway axis for perpendicular connection
Water surface profile along the inclined chute terminating in a stilling basin is computed and  sidewall freeboard  is added  above the profile.  The energy  slope is fixed  from Manning’s formula Velocity  in  the chute is maintained  to  be supercritical,  thus throughout the chute the Froude number is greater than one. The bed slope and elevations were taken  from the most economical longitudinal profile determined  on  the basis of actual site topographic map at 1:2500 scale and 0.5 meter contour interval. The chute floor bed slope is kept steeper than the critical slope. The height of the chute sidewalls is designed to contain the spillway design discharge
3.3.4 Energy Dissipation Section
Two basic parameters, namely apron elevation and length, are determined as they are essential for design of a stilling basin that employs the hydraulic jump for energy dissipation. After the dissipation of energy in the stilling basin, the flows over the spillway will be conveyed back to the river through an exit channel joining the Erer River.
It is necessary that the hydraulic jump curve defining the required post- jump depth superimpose on the tail water curve for the full range of discharge and sufficient length be provided to confine the entire jump. On the other hand the criteria for choice of an energy dissipation type will be governed by the tail water depth and the characteristics of the hydraulic jump itself while its design will be affected by factors like the nature of foundation, magnitude of the flow and its recurrence, velocity of flow, orientation of flow, and depth-discharge relationship of the watercourse at the site of the structure.
Annex B : Dam Appurtenance Structure /Final Detail Design Report /2009
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3.4 Geometry and Size of Spillway
3.4.1 Spillway Crest Structure
The selected spillway is a vertical upstream faced, straight aligned, un-gated overflow weir with ogee crested profile. Upstream and downstream crest profile coordinates (See page 365, Design of Small Dams, United States Bureau of Reclamation, Third Edition, 1987) are defined by the following equations:-
For downstream crest co-ordinates
x’75 =1.869H °”y
d
Hd= is the design hydraulic head For upstream crest co-ordinates
The upstream crest co-ordinates curve extends (x = -0.270 H<i) upstream and (y = -0.126 Hd) downstream from the crest point
Alternatively  it is possible to  define the upstream and  downstream crest profile coordinates using Waterways Experimental Station standard curves as published by the USBR.
3.4.2  Spillway Chute Structure
Beyond the weir axis, a rectangular chute connects the spillway crest curve with the terminal structure. Initially the chute continues in a straight alignment with 30 meters width for a distance of about 57 meters before it gradually converges to 20 meters and maintains the same width as a curve subtending an angle of 56 degrees at a radius of
178 50 meters initially, followed by a straight alignment for the rest of its length. Smooth transitions are provided between straight and curved reaches. Water surface profile along
Annex B : Dam Appurtenance Structure /Final Detail Design Report /2009
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the 590 m long inclined chute terminating in a stilling basin is computed and sidewall freeboard  is added  above the profile Velocity  in  the chute is maintained  to  be supercritical, thus throughout the chute the Froude number is greater than one. The chute floor bed slope is also steeper than the critical slope The height of the chute sidewalls is designed to contain the spillway design discharge.
3.4.3 Spillway Energy Dissipation Structure
USBR stilling basin Type III is employed for dissipation of energy by a hydraulic jump at the end of the spillway chute. The 21m long stilling basin with its apron elevation set at El 1340.7m is fitted with ancillary devices Installation of ancillary devices such as chute blocks, baffle blocks and end sill is expected to produce a stabilizing effect on the jump, thereby  permit shortening  the basin  length  and  guard  against sweep  out caused  by inadequate tail water depth. Without the baffle blocks the length would have risen to more than 40 meters, which is about 5 times the conjugate depth.
3.4.4 Stilling Basin Freeboard
The height of the stilling  basin  sidewalls should  be sufficiently  high  to  contain  the spillway design discharge Hence conjugate depth is calculated not only for the design discharge but also for discharge other than the design
Since tail water fluctuations very much affect jump height at very high Froude number, required tail water depth is taken as 5% greater than the computed conjugate depth for the design discharge. Assuming a value of n equal to 0 008, conjugate depth is also checked approximately for a lower pre- jump depth resulting in a higher post- jump depth. The greater of the two has been adopted as the conjugate depth for design and dimensioning of the stilling basin
Stilling basin freeboard, which is taken as 10% of the summation of conjugate depth (=8  85m) and  incoming  velocity  of flow (=18  llm/s),  is added  to  the calculated conjugate depth described above to determine the height of the sidewalls. Accordingly 
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computation of freeboard for the sidewalls for the current configuration resulted in 2.7 meters The sidewall height and  freeboard  can  however be reduced  by  widening  the stilling basin width.
3.5 River Protection and Training at Exit Channel
A wider exit channel with a slope of 10H: IV upward sloping run out transitions from the basin apron to the Erer river channel Protection of the exit channel bed and side slopes is provided  for in  the design  to  prevent channel scour and  potential undermining  of the stilling basin due to flows exiting from the stilling basin.
3.6 Tail Water Curve Analysis and Flood Submergence
Since tail water fluctuations affect the jump  height very  much  at very  high  Froude number, required tail water depth should be 5% more greater than computed conjugate depth y2 = 8.85m.).
Theoretically, the hydraulic jump curve defining the required post- jump depth should superimpose on the tail water curve for the full range of discharge. However only in extremely rare circumstances would site and hydraulic conditions coexist that result in the jump  curve superimposing  on  the tail water curve.  Therefore it is expected  that installation of ancillary devices in the stilling basin will produce a stabilizing effect on the jump.
The stilling basin is located at the end of the chute where a flood plain starts. It is evident that unless land  close to  the structure is graded  in  order to  drain  floods it could submergence the stilling basin during heavy rains. An earthen bund is provided around the stilling basin sidewalls to ward off encroachment of drainage water, from the plain in the low laying areas adjacent to the basin, directly into the stilling basin.
Annex B : Dam Appurtenance Structure /Final Detail Design Report /2009
17Erer Draft Detail Design Report/ CECE & CES
4. Bill of Quantities and Cost Estimates
4.1 Dam Appurtenance Works
The quantities of the various items of works related to the Erer spillway and irrigation outlet have been  defined  with  reasonable accuracy  compatible with  the present knowledge of the technical aspects of the project. The prices adopted for this report are current prices (December 2008) that are based on experience gained from similar projects being undertaken in the country. The bill of quantitites and cost estimate shall be used for the purpose of project worth analysis and to have an estimate of the project cost. The tentative construction  period  is estimated  to  be 12  calendar months as shown  in  the annexure.
Annex B : Dam Appurtenance Structure /Final Detail Design Report /2009
18Erer Draft Detail Design Report/ CECE & CES
Item
no.
Description
Unit
Quantity
Unit Rate (Birr/Unit)
Amount
(Birr)
1
Irrigation Outlet Works
1.1
Structure excavation of common or unsuitable material
m3
3376
1603
54,117
1.2
Supply & place concrete Class C-15
m3
70
570
39,900
1.3
Supply & place concrete Class C-30
m3
900
1105.68
995,112
1.4
Furnish & install PVC water stop width 225 mm
m
110
78.75
8,663
1.5
Supply & install formwork
m2
2420
111.75
270,435
1.6
Supply, backfill & compact granular material on the side structure
m3
539
49
26,411
1.7
Supply and place 20mm thick bituminous joint filler in joints
LM
77
112.2
8,640
1.8
Furnish & install trash racks
m2
34
5780
196,520
1.9
Supply, install & test one slide gate with necessary fittings (4 units in total)
m2
36
30,000
1,080,000
1.10
Supply & install steel bridge width lm to connect the dam crest
PS
50,000
1.11
Sub total
2,729,798
2
Spillway
2.1
Structure excavation of common or unsuitable material
 3 
mo
71520
16.03
1,146,466
2.2
Supply & install stone masonry wall for spillway abutments with 1:3 cement
mortar
3
m
2800
397.69
1,113,532
Annex B Dam Appurtenance Structure /Final Detail Design Report /2009
19Erer Draft Detail Design Report/ CECE & CES
#*
Item
no.
Description
Unit
Quantity
Unit Rate (Birr/Unit)
Amount
(Birr)
2.3
Supply, backfill & compact granular material on the back of side retaining 
walls
m3
28,160
49
1,379,840
2.4
Supply & place concrete Class C-15
m3
2400
570
1,368,000
2.5
Supply & place concrete Class C-30
m3
23524
1105 68
26,010,016
2.6
Furnish and install PVC water stop width 300mm as indicated in the drawings
m
1800
78.75
141,750
2.7
Supply & install formwork
m2
4590
111.75
512,933
2.8
Supply and place 20mm thick bituminous joint filler in joints m3
LM
2000
112.2
224,400
2.9
Channel protection of dumped riprap
J
m
1500
94.21
141,315
Sub total
32,038,253
Annex B : Dam Appurtenance Structure /Final Detail Design Report /2009
2011
1
Design of Stilling Basin
Reservoir Normal Water Level
NWL
=
1379
masl
. <|GIVEN
Canal full supply level
FSL
=
1365
mast
1—
Head
H
=
14
m
1Jr
Velocity
V1
=
16.57
m/s
--
Depth of flow
d1
=
0.18
m
Froude Number
Fr
12 33
1
Conjugate Depth
d2
=
3 12
m
Conjugate Depth d”2=1.05xd2
=
3.3
m
I
Stilling Basin for Froude numbers above 4.5 where incoming velocity, V1 < 18.3m/s
t
USBR Stilling Basin Type III
L/d2
=
2.75
'1
1—
Length of jump
L
=
1
Apron elevation
Elv
=
8.58
m
1361.09
m
1—
TW Depth/*
=
16.50
’ll
rrjl
Minimum tail water depth
rw Deptr
=
3.04
m
=
1
1--------
Chute blocks
Height = Width = Spacing
h1=d1
=
=
0.18
m
Spacing from each of the sidewa!ls=0.5y1
0.09
m
1
—
1--------
Baffle blocks
h3/d1
=
2.7
4.
1
Height of baffle blocks
h3
=
0.50
1
Top thickness=0.2*h3
=
0.10
m
1
Width & spacing of dents and teelh=0.75h3
=
0.37
m
1
Downstream slope 1:1
=
Spacing from each of the sidewalls=O 375h3
=
0.19
m
1
Distance of baffle blocks from chute blocks=0.8y2
X
2.50
m
J
End sill
h4/d1
=
1.75
Height
h4
=
0.32
m
1>
Upstream slope
=
2:1
-
Downstream slope
Vertical
1
Sidewalls of stilling basin
Incoming velocity to the basin in m/s
V1
=
16.57
m/s
1
1—
Conjugate depth in m
dM2
3.28
m
Freeboard for the sidewalls of stilling basin=0.10“(v1+d"2)
=
1.981
m
Height of stilling basin sidewalls
H
=
5.26
m
Elevation of stilling basin sidewalls
El.
=
1366.35
m
L1
□zzERER DAM and Irrigation Development Project SPILLWAY Hydraulic Design Calculations 
Design Data
Manning's "n" in approach channel
Length of approach channel
Assumed side slope of excavated approach channel
Spillway crest level
Assumed coefficient of discharge for free flow Crest length
n       =               0.018
Ogee Crest. Coordinates of Upstream Profile
L=
300 m        Ogee Crest. Coordlna x = 0 27Hd = 0 27*3.44 = -0 93 X                y=(x1.75)/4. x/Hd         ik         ’y/Hd         y
=
Cd Z L=
1379 masl
00
000
0
Design discharge for crest width of 30m as determined from floor Q           =
2.18                    0.25     -0
30 m                 0.5  -0.1
441 m3/s          0.75  -0.1
Head over ogee weir
ForC=2.18,
Height of spillway weir above U/s floor
U/s water level
Design of Approach Channel
Area, assuming a trapeziodal section for approach channel Approach velocity
Velocity head
Wetted penmeter Hydraulic radius (=A'P)
H       s        3.56468633 m                    1   -0.2
P/H =
0.7                    1.25  -0.3
P       =        2.49528043 m                 1.5  -0.4
z        1382 56469 masl           1.75  -0.6
2  -0.7
A s 218.5222 m2 2.5 -1.1
va - 2.0181016 m/s Ha s 0.20758074 m/s P = 47.1401743
3  -1.4 3.5  •1.9
4   -2.4
R      =        4 63558319 m               4 08  -2.5
Head loss due to friction up to spillway crest= (n2v21)/R4/3              hl      8        0.05124307 m
-0.02  -0 069        0.009           -0.03096
-0.04  -0.138        0.018          -0.06192
-0 06  -0 206        0.027           -0.09288
-0.08  -0.275        0.036           -0.12384
-0.1   -0.344        0.045             -0.1548
-0.12  -0.413        0.054           -018576
-0.14  -0.482        0.063           -0.21672
-0.16     -0.55        0.072           -0.24768
-0.18   -0619         0081           -0.27864
-0.2  -0.688          0.09             -0.3096
-0.22   -0.757        0 099           -0.34056
-0.24   -0.826        0 108           -0.37152
-0.26  -0 894        0.117           -0.40248
-0.27   -0 929        0.126           -0.43344
Level of u/s TEL
z        1382.72102 masl
Hydraulic head over crest (including approach velocity head)            He    =        3.72102399 m
Design head
Ratio of Hydraulic head and design head
Ratio of height of weir and Hydraulic head Coefficient of discharge considering He/Hd & P/He i) Correction due to height of weir
P/He=0.67, and He/Hd=1.06
Hd     z         3 51344325 m He/H z        1.05908185 P/He =       0.67058972
z
z
C/Cd *
0.98
-0.93       2.5
0.93     2.07
0.999  -0.031
0.136 -0.062
0.206 -0.093
0.275 -0.124
0.344 -0.155
0.413 -0.186
0.482 -0.217
0.55 -0.246
0.619 -0 279
0.688 -0.31
0.757 -0.341
I.826 -0.372
1.894  -0.402
1.929    -0.09 00
025    -0 02
0.5    -0.06
0 75    -0.13
1    -0.21
1.25    -0.31
1.5    -0 43
ii) Correction due to u/s slope
z
1.75
-0 56
no correction for a vertical u/s slope
z
s
1
2
-0.71
25
-1.05
iii) Assuming that the downstream apron elevation is maintained
such that it does not affect Cd
=
3
-1.45
Corrected value of coefficient of discharge
Cd
=
2.156
3.5
-1.9
Abutment contraction coefficient
Ka
=
0.1
4
-2.4
Effective length
Le
=
29.2557952 m
408
-2.48
Corrected He
He
3.65164793 m
Hence corrected Hd
Hd
3.44406719 m
Design will proceed for a design head of 3.44m
Q=CdLeHe1 5 = 450 43 m3/s - 441m3/s
Design of Crest Profile
Ha/He
005684577
P/He
0.68332996
Downstream crest profileSince Ha/He lies between 0 and 0.08 and P/He-0.68, the equation x1 .75=1.869 Hd0.75y will be used for the downstream profile of the low ogee weir i.e. P/Hd<1.33 Hence the d/s profile xl .75=1.869 (3.44)0.75y
y=(x1.75)74.72
Upstream crest profile
It extends 0.270Hd upstream A 0.126Hd downstream from the crest point x = 0.27Hd = 0.27*3.44 = -0.93
y = 0.126Hd = 0.126*3.44 = 0.43
Position of downstream apron level
=
Water Surface Profile Calculation for Erer Dam Spillway Chute
If downstream apron elevation is maintained such that it does not
affect Coefficient of discharge
=
Q=441m3/s.
hd ♦ d i 1.7
n = 0.014,
He
Width of chute =30m, and 20m
hd ♦ d =1.7xHe
=
6.20780148 m
Length of chute > 500m,
hd ♦ d = 6.21
Initial water depth at the toe of the ogee weir = 1.54m.
Chute bed slope (So)= 0 035 for the first 50 m strech, 0.06 slope from 50 to 450m, and after 450m at 0.25
slope for the rest of its lengthV
Max. apron level with no effect on Coefficient of discharge
Hence the toe of the spillway is provided al El. 1376.5m
The d/s profile of the ogee weir Is designed between El. 1379.0m
& 1376.51m
Maximum ordinate of y
y
2 49=(x1.7S)/4.72, — x1.75»11.755,               —        x«4.08m
1376 51322 masl
Distance Leng Bed Di Bed Level Assumi Velocity Velocity Head Specific Energ TEL A P 28.57H:1V
R
t=    2.48677749 m
0       0                    1376    1.535    957655     4.674325019   6.209325019    1382.2   46 05       33.07   1.3925
25 25 0.875 1375.13 1.44 10.2083 5.311420461 6.751420461 1381.9 36 27.88 1.2912
Design of Discharge Chute
c
Specific discharge d/s of weir, for 30m wide chute stretch
q
s
14.7 m3/s/r
50     25 0.875
1374.25    1.371    10.7221    5.859502675   7230502675     1381.5   27.42
22.742   1.2057
Critical velocity
Vc
s
5.24940124 m/s
16.66H:1V
Specific energy Q base of ogee weir (start of chute section)
E
c
6 20780148 m
75     25      1.5
1372.75    1.971    11.1872   6.378887399    8.349887399   1381 1    39.42
23.942   1 6465
Specific energy Q base of ogee weir (stort of chute section).
hd*d «1.7 He » 6.21
s
6.20932502 m
100     25
1.5
1371.25    1.795    12.2828    7.689400922   9.484600922    1380.7     35.9
23.5904     1.522
Depth of water at the toe of spillway (@ Start of Chute Section))
d1
S
1.535 m
Width of chute
e
30 m
150     50
3
1368.25    1.585    13.9082    9.859172763    11.44457276    1379.7   31.71
23.1708   1.3684
Critical depth
de
s
2.80031938 m
Hence d1 <dc, flow In the chute Is thus super-critical
200    50
3
1365.25    1.465    15.0512      11.5463026      13.0113026    1378.3     29.3
22 93   1.2778
Manning s "n” in concrete lined channel accounting air swell,
wave action, etc
N
0.014
Hydraulic radius
R
2.35977736 m
250     50
3
1362 25    1.387    15.8953    12.87775102    14.26495102    1376.5   27.74
22.7744   1.2182
For supercritical flow, slope of the chute should also be more
than Sc (critical slope)
Sc
0.00171964 1/581
300     50
3
1359.25    1.335    16.5131    13.89826175    15 23356175    1374.5   26 71
22.6706     1.178
Spillway chute water surface profile
Design of Stilling Basin Energy Dissipator
350     50
3
1356.25    1 299    16.9746    14.68587666    15.96467686    1372.2   25.98
22.598   1.1497
Hydraulic jump computations
Depth at the end of chute
yi
s
1.2175 m
400     50
3
1353.25    1.273    17.3213    15.29189746    16.56489746    1369.8   25.46
22.546   1.1292
Velocity at the end of chute
VI
E
18.11 m/s
Froude No.lat the end of chute
Fl
3
5.24
445     45
27
1350.55    1.256    17.5557    15.70865153     1696465153    1367.5   25.12
22.512   1.1158
Computed conjugate depth
y*
3
8.43402174 m
4H:1V
V1 > 15m/s, hence USBR basin type III Is adopted
e
450       5
1 25
1349.3    1.218    18.1109    16.71784309    17.93534309    1367 2   24.35
22.435   1.0854Since tail water fluctuations affect jump very much at very high
Froude number, required tail water depth should be 5% more
greater than computed conjugate depth y2), say 8 8m
New  3
8 85572283 m
7
Assuming a value of N-0.008 for designing the the stilling basin,
the stilling basin is checked approximately for a lower yl =0.60m,
velocity^ 14.7/0.6=24.5m/s,
Froude No.
F
10.0984787
Conjugate depth for a lower yl
y2-
8.27409333 m
(New y2) is greater than y2" , Hence (New y2) is adopted in
calculations to follow
Required floor level of jump basin
ELV.
1340.69428 masl
Hence stilling basin apron elevation is
ELV. =
1340.69428 masl
Freeboard for the sidewalls of stilling basin
Incoming velocity to the basin in m/s
V1     s
18.11 m/s
Conjugate depth m m
y2     =
885572283 m
Freeboard for the sidewalls of stilling basin«0.10‘(v1 *y2)
B
2 69657228 m
Height of stilling basin sidewalls
H=
11.5522951 m
Elevation of stilling basin sidewalls
El.     =
1352.24657 m
Overflow Spillway Crest Upper Nappe Profiles without Piers 
Ratio of Hydraulic head and design head. Hd=3.44m
He/H S
1.06
He/Hd is approximated to land hence the coordinates for upper
nappe are as follows:•
X/Hd
S
Y/HdX
Y
-1
-0.0   -3
-3.20952
-0.8
-0.9  -3
-3.1476
-0.6
•0.9   -2
-3.07192
-0.4
-0.9  -1
-2.9756
-0.2
■0.8  -1
-282424
0
-0.8    0
-2.5972
0.2
-0.7   1
-2.34264
0.4
-0.6   1
•2.01584
0.6
-0.5    2
-1.5996
0.8
-0.3    3
-1.1008
1
-0.1    3
-0.4988
1.2
0.1    4
0.1892
1.4
0.3    5
1.01136
1.6
0.6    6
1.93672
1.8
09    6
294808
Stilling basin characteristics for Froude numbers abovo 4.5 v/here Incoming velocity , V1 si 8.3m/s 
Froude No. 1 at the end of chute Length of jump (stilling basin) Chute blocks
Height = Width = Spacing
F1      =              5.24
L » 21.2 m
y1    3
1.2175 mSpacing from each of the sidewalls=0.5y1 Baffle blocks
Height of baffle blocks 
h3
0.60875
1.83
Top lhickness=0.2*h3
Width & spacing of dents and teelh=0.75h3
Downstream slope 1:1
Spacing from each of the sidewalls=0.375h3
Distance of baffle blocks from chute blocks=0 8y2
0.366
1.3725
0.68625
6.728
End sill
Height
Upstream slope
Downstream slope is vertical
Exrt Channel Design
Width
w=wb*1.5 m on each side *23m
or
• wb*(0.15)d2 on each side =22 658m,
whichever is greater
Length
Slope
top of end sill
Elevation set at
channel elevation adjacent to the basin end sill set at 20 cm below top of end sill
L=10<natural channel elevation -1342.20)
Erosion Protection
q at end sill
depth= tailwater depth - apron elevation
depth=0.8d2=0.8 x 6 86
Velocity over end sill=q/depth
d50 (min)
Assuming specific stone weight (weight gs) w50 (min)
W100 range = 2W50 to 5W50
i.e. 454 kg to 1134 kg
D100 range =(6W/irgs)1/3
D100 range =0 70m to 0.955m
thickness, t=2d50 or 1.5d100. whichever is greater
h4
1.586 m
w
23 m
1V:10H
1342.4 m
L q d V
43
22 05 m/s/m
7.1 m
3.1056338 m/s 55 cm
2483 kg/m3 226.8 kg
v        2d50=2x 0.55= 1.10m
1.5d 100= 1.5x0.955= 1.43m Adopt riprap thickness 
t
15mR4/3                 St
1.553273506 0.01157247
Av.Sf
LSf=hf              Actual TEL Froude No Freeboard for Chute sidewalls height
1382.21
1 40489201 0 014538608 0.013055539 0.326388464 1381.882937
1.282471131 0.017569857 0.016054232 0.40135581 1381 481581
1.940974087 0 012638056 0.015103957 0.377598915 1381.103982
1.748243875 0.01691397 0.014776013 0.369400328 1380.734582
1.517689753 0.024981157 0.020947564 1.047378184 1379.687203
1 385468137 0.03204804 0.028514598 1.425729914 1378 261473
1 300200818
1 243437482
1.203807264
1.175463728
1.156952125
1.115095199
0.038087693
0.042982494
0.046913518
0.050027318
0.052212993
0.057653194
0.035067866
0.040535093
0.044948006
0.048470418
0 051120156
0.054933094
1.753393303
2.026754654
2.247400277
2.423520884
2.300407002
0.274665469
1376.50808
1374 481325
1372.233925 1369.810404 1367.509997 1367 235332
2467858537 2.716058332 2.923658538 2.544158439
2.926879418 3.526678006 3.970248159
4.308890621
4.562530877
4.755107762
4 901527963
5.001377443 5.24047478
1 051817625
1.071051611
1.086401745
1.170935087
1.207001361
1.258598544
1 29368317
2.586817625 2.511051611 2.457401745 3.141935087
3.002201361 2843998544
2.75868317
1.319025441  2.706225441
1.337289793
1 350783467
1.360841105
1 367604413
2.672589793
2.649783467
2.633841105
2623604413
2.697088296  Stilling BasinErcr Dam and Irrigation Development Project
Dam Appurtenance Structures Construction Schedule
Months
No
Activity Description
1
2
3
4
5
6
r
3
10
11
12
13
14
15
16
1   23        4
rII
I
1
2
3
4
1
2
3
4
1
2
?3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
Irrigation Outlet
11
Excavation L selected backfill around structures
I BBB
I
I
U
Concrete complete wrth formworks
1J
Miscellaneous metal works
II
■
B
14
Installation, testing & commissioning of gales
B
■
B
2
Spillway
21
Excavation A selected backfill around slruclures
2.2
Stone masonry works
2.3
Concrete complete with formworks
24
Channel protection works
II
-
B
B
B
B
B
B
_
Note
a)   The Umo schodulo assumes that advance worts am complolod poor to octual construction of (ho dam appurtonanco worts. b)  N a I so assumes lhal all nocessary consUudion equipment are sufficiently available lor execution of the works
c.) Only majev act/viUes of wort am shown in the schodulo
d) Rainy months have not boon takon Into account although construcbon durations am affoctod by disruptions caused by rainsErer Dam and Irrigation Development Project
Irrigation Outlet (10) Hydraulic Design Calculations
Design data
Dam crest level
=
1384.6
masl
Maximum water level of the reservoir
=
1382.6
masl
Normal water level of the reservoir
=
1379
masl
L
Lowest inlet sill level of high-level front entrance intake
=
1370
masl
—
Outlet  discharge  capacity  required  at  minimum  reservoir  level  (  at 1.0 meter head)
=
3.05
m3/s
I
Minimum reservoir drawdown level (MDDL)
=
1371
masl
—
Ground level at Irrigation Outlet Intake tower
=
1366
masl
—
Dam crest width
=
9
m
Dam upstream slope
=
3:1 & 3.5:1
—
Dam downstream slope
=
2.5:1
Width of two berms @ Elv 1369, one each on upstream and downstream slopes of dam
=
5
m, each
Berms at Elevation both upstream & downstream slopes of dam
=
1369
m
=
Fixation of Outlet Conduit Levels
Sizing of conduit
Minumum driving head
h
=
1
m
Ve)ocity= (2gh)0 5
V
=
4 43
m/s
I
Area of conduit for the outlet discharge= Q/V
A
=
0.69
m2
—1
Minimum Reservoir drawdown level
=
1371
masl
For a 1m by 1m rectangular horizontal conduit
A
=
1
m2
Level of centerline of outlet conduit
Elv.
=
1366.5
masl
Head Losses
According to Manning’s formula
Hydraulic radius
R
0.33
Manning's n
n
0.013
max = 0 008 to
Velocity through rectangular conduit
V
=
3.05
m/s
Friction slope
S
=
0.0068
Length of conduit
L
=
122.8
m
Loss of head due to pipe friction
=
0.83
m
Assuming 50% clogging of trashrack, and 10cm spaced bars
limiting  velocity  @  trashrack  ,  Ag=11.08m2  i.e.  2.9m  wide  by  3.82m high
—
0.55
m/s
Sill level of the middle entry port
=
1373.82
I
m
Sill level of top entry port
=
1377.64
m
1
Trash rack loss coefficient, Kt=1.45 - 0.45*(An/Ag) - (An/Ag)2
Kt
=
0.977
loss of head due to trashracks
ht
=
0.463
m
Entrance loss coefficient
Ke
=
0.16
loss of head at the entrance of conduit
he
=
0.07
m
Gates groove losses coefficient
Kg
=
0.25
m
L
loss of head due to gates groove
-hg
=
0.24
m
Exit loss coefficient
=
Total losses
=
1.61
m
The sill/invert level of conduit at the exit end
=
1364.39
m
Crown level of conduit at exit end
=
1365.39
m
r GIVEN
Canal full supply level
=
1365
m
GIVEN
Canal bed level
=
1363.9
m
—
Checking if the flow is free or not at conduit exit
Free flow
1-------DESIGN REPORT FOR ERERIRRIGA TION DAM STRUCTURAL FACILITIES_______________ ENGINEERING AND consulting ENTERPRISE
CONCERT
Structural Facilities Design for Erer Dam
1. General Criteria
1.1  General
AU design work has generally been undertaken to international standards or Ethiopian design standards where these are considered more appropnate to local condition.
All design have been carried out in SI units
The structures have been assesed using 3D finite element analysis softv/are (SAP 2000, version 11)
1.2  Densities of Engineering Material
The densities of common engineering materials used for the structural design are given in Table-1 below.
Table -1 : Densities of Engineering Materials
Material
Density (KN/m )
3
Water
9.81
Mass Concrete
23.00
Reinforced Concrete
25.00
Mild Steel
78.50
Insitu Soil
18 90
Source "> EBCS-1 [EBCS.1995]
1.3  Engineering Properties of Soils
Soil properties to be used in the design are given in Table-2
Table-2 Engineering Properties of Soils
Property_______________________________________________________ Value Foundations
In situ density
In situ Saturated Density
In situ submerged density
Internal Angle of Friction
Allowable net bearing pressure of saturated soil for intake tower Allowable net bearing pressure of saturated soil for Spillway
16 0 KN/m
17 5 K/V/n?
8.7 KN/m 6 30°
250KN/m ' 150KN/m 3
1
Al»C/i.2OO9DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES ENGINEERING and CONSULTING ENTERPRISE
Table-2 Engineering Properties of Sdils (Cont.)
concert
e
*
Property
Value
Backfill
Compacted Density
Compacted Saturated Density
Compacted submerged density
Effective friction angle when used as backfill
Sliding friction or interface friction angle between concrete & soil Active earth pressure coeffiecent, k a
At rest earth pressure coeffiecent, ko Passive earth pressure coeffiecent, kp
17        .5 KN/m3 18 15 KN/m
8.34 KN/m5 30°
20° 0.30 0.50 3.35
1.3 Concrete
Reinforced concrte shall be used to construct all dam structures. A 75mm thick blinding layer
shall also generally be used for all reinforced concre structures bedding material.
Concrete of Grade C30 shall be used for structural componenets of the Intake Tower,
Drawoff Conduit, Spillway Walls & Slabs, and downstream works ( Stilling Basins).
Grade C20 concrete shall be used for mass concrete and C15 for blinding concrete.
Reinforced concrete specification and design shall be based on British Standard BS8OO7, BS8110, & BS 5328, which are widely used in international projects.
The following properties of concrete have been used
- The modulus of elasticity of concrte ( for calculator defelection)
- Coefficient of linear expansion
1.4  Reinforcement
Reinforcement bar shall be hot rolled high yield steel to Ethiopian Standard ES 547-2:2000 (Equivalent to ISO 6935-2:1991).
26.75x103/n
10.8x10 S/°C
The grade of reinforcement to be used is Grade RB400(as defined in ES 547-2:2000) for all works. The yield strength is 400N/mm 2
Minimum cover to reinforcenent shall be 50mm for all structures in contact with water or earth, which is the minimum provided by BS 8007.
Laps in Bars shall be 47xbar diameter for all structures
The minimum steel reinforcement has been set in accordance with the guidelines given in
2
Maicli.2009DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES--------------------------- engineering AND CONSULTING enterprise
CONCERT
BS 8007, namely:
Where:
|Zr]
= minimum steel ratio for crack distribution in imature concrete
= direct tensile strength of imature concrete ( 1.3N/mm2 for Class C30 [Anchor, 1992J)
= Characterstics strength of reinforcement ( 400N/mm  )
2
On this base the minimum reinforcement is set as 0.325%XA , where A is the applicable area as defined in BS 8007. The figure below shows the applicable surface zone
Figure-2-1: Applicable Area & Reinforcement ratio requirement for crack control at early age of concrete
2 Reinforced Concrete Design Philosophy for singly reinforced rectangular section 2.1 Ultimate Limit State Design for Flexural Strength
3
March. 2009DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACIt ITIFSCONCERT
ENGINEERING ANQ CONSULTING ENTERPRISE
Figure-2-5: Singly reinforced section with rectangular stress blocks subjected to bending
Bending of the section will induce a resultant tensile force Fst in the reinforcing steel, and a resultant compressive force in the concrete Fee which acts through the centroid of the effective area of concrete in compression , as shown if figure 2.5.
For equilibrium, the ultimate design moment, M, must be balanced by the moment of resistance of the section so that
Therefore.
Hence,
2.2 Ultimate Limit State Design for Shear Resistance
The shear resistance for reinforced concrete is rendered by the shear resistance offered by the
4
March. 2009DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES------------------------- ENGINEERING AND CONSULTING ENTERPRISE
concrete (VRdl)as welt as the shear resistance of the vertical stirrupsfV^). , The total shear resistance of a section (VRd3) is the sum of the sear resistance of the reinforcement, and concrete shear resistance.
CONCERT
=[r A(I.2 + 40
w                         Pl)^
Where:
Basic Design Shear strength =           0.035/a2'5(JV/mm2)
|A- = (1.6 - d'fjiyorj
Pt = A,i
1bd
-=
Where  more  than  50%  of  tension  reinforcement is curtailed
Area of Longituidnal tension reinforcement extending not less than a full anchorage length one effective depth beyond the section consiae minimum width of section over area considered effective depth of section
Large shearing forces are also liable to cause crushing of concrete along the direction the principal compressive stresses and therefore sections close to supports should be checked to
determine the maximum allowable shear force. VRtf2f any shear reinforcement, given by
which can be sustained irrespective
Where
= is an efficiency factor =
4 /200(>0.5)
Where.
0
Ultimate limit srare load sustained by the vertica stirrups
The cross sectional area of the two legs of stim The stirrup spacing
II
15 greater than VRdl shear reinforcement is required & is calculated from
A,. I 28(K„-J^,)
s
df.t
5DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES------------------------ fngineering and consulting enterprise.
concert '
Where:      Asw
is the cross sectional area of the legs of the stirrups (for single stirrups)
(2*^ /4)
2
0             is the spacing of the stirrups
3 Analysis & Design of Structural Componenets
3.1  Drawoff Conduit Design
(RCC. Rectangular Conduit from Intake tower to downstreeam outlet) 3.1.1  Dimensions of the drawoff conduit cross section
6
-j
March. 2009DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES ENGINEERING AND CONSULTING ENTERPRISE
220 Q
r &O0 J00Q_ &O0'
CONCERT
Figure-1: Reinforced Concrete drawoff conduit typical cross section
3.1.2 Loading
Overburden load from the dam body
Multiplying Factor for ultimate limit state loading :
for Dead Load = 1.3!
for live load = 1.50^
Source: (2)
(EuroCode 2)
Load Type
Service Load
Ultimate
Dam Height over the conduit
18.61 m
Overburden load
337.77 KN/m2
455.99
Uplift Load at conduit bottom
381.41 KN/m2
514 90
Lateral Preassure Load
- Coduit Top
0 KN/m2
0.00
- Coduit Bottom
11.98 KN/m2
16.17
Live Load (Assumed)
5 KN/m2
7.50
3.1.3 Design Bending Momnet, Shear force Axial Force Diagrams(BMD,SFD &AFD) ( As per SAP-2000 Version-11 output)
1
1.6
3O4KN/m2
»
1
i 1
1
l_
*I i
i ! I I i
A
(0
1 OKN/m2
-1                  T
1OKN/m2
343KN/m2
Figure-2-1: Drawoff Conduit idealized frame & loading at ultimate limit state loading
7
March.2009DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIESCONCERT.
engineering AND CONSULTING ENTERPRISE
Figure-2-4: AFD for Drawoff Conduit at ultimate limit state loading
3 1 A Design Calculation for Flexure Strength
Description
M(Design Moment) Concrete Copressive Strength (Fck) Steel Tensile Strength (Fst) Sectionwidth (b)
Section Hight (h) Clear Cover ( c ) Assume Re-Bar Dia. effective depth(d)
K z As
Value            Unit
68  KNm 30  N/mm2
400  N/mm2
1000  mm
600  mm
50  mm
14  mm
543  mm 0.007688
539.2936  mm 362.3301  mm2
3.1.5    Design Calculation for Shear Strength
Description
Value
Unit
V (0esign Shear)
5d
263
KNm
Concrete Copressive Strength (Fck)
30
N/mm2
Steel Tensile Strength (Fst)
400
N/mm2
Sectionwidth (b)
1000
mm
Section Hight (h)
600
mm
Clear Cover ( c )
50
mm
8
March.2009DESVI REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES ENGINEERING AND CONSULTING ENTERPRISE
14  mm 543  mm
N/mm2
0.34
1.057
0 000667
237.917
3.1.6 tesign Calculation for Axial Force Resistance
Description
NPesign Axial Compression Force) MPesign Flexural Moment) Concrete Copressive Strength (Fck) Steel Tensile Strength (Fst) Sectionwidth (b)
Section Hight (h)
tear Cover (d‘)
□Th
y
2
bh fck
jW'/a-
Read from Interaction
Diagram
Value Unit
277 KN
35.88  KNm 30  N/mm2
400  N/mm2
1000  mm
600 mm
50  mm 0.08
0.015389 0.003322
0.00
.inimum Reinforcement Required to prevent crack for imature concrete
Description
Value
Unit
Peri
Section Hight (h) Section Width (w) Applicable Area(A)
As
0.325%
500 mm
1000 mm
250000 mm 812.50
This steel contily will govern from previous strength design calculation
Therefore, provide Dia. 14mm @ 180mm c/c  , both direction wich gives  steel are
equal to
854.78 mm/m
9
March,2009DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES--------------------------- ENGINEERING AND CONSULTING ENTERPRISE
CONCERT
3.1.7   Check for crack width control ( Assumed crack width-0.2mm)
Tr.c- limit erasing i< ratisfied by insuring that th® maximum cafcu’ated sudace wid^h of cracks is nor greater lha i the specified v . ue. depending on the me degree of exposure
of the member for ihe case unaer consideration equal to 0.2mm.
To check the surface crack width, the following procedure is necessary:
(a) Calulate the service bending moment.
(b) Calculate the depth of the the newtral axis, lever arm and steel stress by elastic theory.
( c) Calculate the surface strain allowing for the stiffening effect of the concrete
(d) Calculate the crack width
The maximum service bending momnet is calculated using characterstics loads with load fator=1.
• As # j
T
fs/(Modular ratio
Section
Strains
Stress Blocks
I
■
Cfepnr Jr erre                              ri.-jzsivurc icvrrg-.nt;-
_ .impu^iT^-rcr
= mod ular- ret io =                              = i S^for - normal- concretA
J^c
1
10
March, 2009
<12
IIDESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES------------ engineering AND CONSULTING ENTERPRISE
CONCERT
/ =^i
Jci zbx I
For the crack width formula to be valid, the compressive stress in the concrete and the steel under service conditions must be less than the limiting values as follows:
Concrete        A, < 0.45 /,
f .< 0.8/,
The average strain at the level surface                  kJ 0. This is the adjusted to take in to accounHn
een crack^f?)
is assessed by calculating the apparent strain e stiffening effect of the concrete
The stiffening effect of the concrete may be assesed by deducting from the apparent strain a value obtained from the approximate equation below.
Fora limiting design surface crack width of 0.2mm:
/>,(/?-x)(q'-x)
3^/1, (</-x)
The value a' in this formula is the distance from the compression face of the section to
the point at which the crack width is being calculated. the overall depth h.
Restriction to lever arm z <0.95d& z>0.5d
In the case of a slab , a‘ is equal to
Tt lue ofQ
(u: • modular ratio theory)
represents the calculated average elastic tensile strain in the concrete
=
—xA
d-x E,
obtatined from   the formula,
is the distance b/n the point considered and the surface the nearest longituidnal bar
The minimum cover over the tension steel
11
March. 2009DESIGN REPORT FOR ERERIRRIGA TION DAM_STRUCTURAL FACILITIES.------------- engineering and consulting enterprise
CONCERT
2M,
]cb zbx
For the crack width formula to be valid, the compressive stress in the concrete and the steel under service conditions must be less than the limiting values as follows:
Concrete        A,, < 0.45 /,
/ ,<          0 -8 f y
The average strain at the level surface
R  I . This is the adjusted to take in to accounlt' Thiie stiffening effect of the concrete reen crack^]
The stiffening effect of the concrete may be assesed by deducting from the apparent strain a value obtained from the approximate equation below.
For a limiting design surface crack width of 0.2mm:
6, (/»
The value a’ in this formula is the dislance from the compression face of the section to
is assessed by calculating the apparent strain 0
the point at which the crack width is being calculated. the overall depth h.
Restriction to lever arm z <0.95d& z>0.5d
In the case of a slab , a‘ is equal to
Th a
(us too’ular ratio theory)
represents the calculated average elastic tensile strain in the concrete
d-x Es
The design sunace srack width w is obtalined
from the formula,
e
M-2 ( c.
I - -V J
a      - <=.....  'I
ar
C
is the distance b/n the point considered and the surface the
nearest longituidnal bar
t:
Cfnin
The minimum cover over the tension steel
11
March,2009DESIGN REPORT FOR ERER IRRIGATION QAM STRUCTURAL FAQHJTI£S----------------- ENGINEERING AND CONSULTING ENTERPRISE
CONCERT
Description
Ms(Service Bending Moment)) Section Hight (h)
Section Width (w) Concrete Cover(c J
rn
effective depth(d) As
a.
Check Stress Levels
Value Unit
45.33 KNm
600 mm
1000 mm
50 mm
542 mm
1004.80 mmZ
0.001853875
15
0.21
113.63 mm
504.12 mm
514.S >Z OK!
89.50 N/mm2 1.58 N/mm2 13.5 >1.5 ok!
240 >89.5
ok!
=
I’ ~ *
v
f.
d-x' E.
Assume crack widh
0.2mm
(/?    —    a     —    a     ) 3 XT, xl, (<7 — a )
0.000508067
0.000915969
f, <
-is-n ega t iire-.'. -No- Crack-
4. S /ay
4.1 « metrical DimensionsDESIGN REPORT FOR ERER IRRIGATION QAM STRUCTURAL FACILITIES.------------------------ ENGINEERING AND consulting enterprise
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4.2   Loading
-kiplying Factor for ultimate limit state loading :                                                         for Dead Load = 1.3f for live load = 1.50(2)
>ur.(2)
(EuroCode 2)
_______ -oad Type_________________________________________________ Service Load____________ (J11 i m a t<
Live Load (Assumed) Lateral Preassure Load
- Chute Side Wall - Stilling Basin
10 KN/m2
24.78 KN/m2
65.6175 KN/m2
15.00
33.90
89.03
For the retaining wall analysis & design, assumed too legth which is equal to 50% of retaining wall height have been considered , Accordingly the modeled retaining walls are shown here below.
450
n
13
March. 2009Chute Retaining Wall
(a) Stability Analysis
Earth Preasure = 0.3*18.15*(4 0+1.0)
Horizontraf Force on 1m length of wall= 0.5*27.225*5
Vertical Loads
wall = 1/2(0.45+1)*4*25
Base = 1.0’4*25
Earth = (1 + 1+0.55)/2*4*18 15
Total
27.225 68.0625
72.5
100
92.57 265.065
For  stability  calculations  a  partial  factor  of  safety  of  1.5  is  used  for  the lateral loadings, while 1.35 will be used for strength calculations.
(i) Sliding
Frictional Resisting Force = Tan(20°)* 1*265.07
Sliding Force = 1.5*68.0625
Eventhough, the sliding force seems to exceed the frictional resistance, the retaining wall will not slide because of the resistance offered by the one meter base slab.
96.424472
102.09375
(ii) Overturning
Taking  momnents  at  the  adge  of  the  toe,  at  the  ultimate  limit  state
Overturning moment = 1.5*68.0625*5/3
Resisting Moment = 1*(72.5*(4-1.5)+100*4/2+92.57*3.25) Thus, the criteria for overturning is satisfied.
(b) Bearing Preasure at Service Bearing Pressures are given by:
170.15625 682.1025
6M
D2
14
March.2003DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES
CONCERT
FNG INEE RING AND CONSULTING ENTERPRISE
where, M is the moment about the base centerline. Therefore M = 68.0625*5/3+72.5*(1.5-2)+92.57*(0.75-2)
Therefore, maximum bearing pressure = 265.07/3.5+6*(-38.53)/42
*
-38.525 51.81875
which is less than the allowable (c) Bending Reinforcement
(i) wall
= 150KN/mm2
Horizontal Force = 1.35*0 5*0.3*18.15*42 Max. Moment = 58.81*4/3
Reinforcement Calculation Table
58 806 78.413333
Description
M(Design Moment) Concrete Coprcssivc Strength (Fck) Steel Tensile Strength (Fsl) Seclionwidth (b)
Section Hight (h) Clear Cover ( c ) Assume Re-Bar Dia. effective depth(d)
K
z
As
No. Bars /m Spacing
(i) Base
Value           Unit
78.41  KNm
30  KN/m2 400  KN/m2
1000  mm
1000  mm
50  mm
16  mm
942  mm 0.0029  <0.167 939.55 mm 239.81  mm2
1.19334  No. 837.9843    mm
ok! Singly Rein.
The bearing pressure al the ultimate limit state are obtained as: M = 1.35*68.0625*5/3+72.5*(1.5-2)+92.57*(0 75-2)
and
N#d
1*72.5+1*100+1*92.57
1 18
265 07
Therefore; p1 = 265.07/4+6* 1.18/42
= 66.27 + 0.44
p2 = 53.5-31 7
p3 = 65.83+(66 71-65 83)*2/4
Too; taking momontc about the stom conturlmo for tho vertical loads & tha bearing proccuros;
= 1*100*0.5*2/4-66.71*2*1.5+(06.71-66.27)*0.5*2*(2/3*2+.5)
66 71
65 83
66 27
-174 3233
15
MafChJOOODESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES_______________ ENGINEERING AND CONSULTING enterprise
Heel: Taking moment abobt the stem cenrer line M  =-1*100*1.5*1/4-1*54.57*1.25+66.27’1*1.5
CONCERT
$d
-5.9325
Reinforcement Calculation Table
Description
Value Unit
M(Design Moment)
174.32 KNm
Concrete Copressive Strength (Fck)
30 KN/m2
Steel Tensile Strength (Fst)
400 KN/m2
Sectionwidth (b)
1000 mm
Section Hight (h)
1000 mm
Clear Cover ( c )
50 mm
Assume Re-Bar Dia
12 mm
effective deplh(d)
944 mm
K
z
As
0.0065 <0.167
ok!
Singly Rein.
938.54 mm
533.72 mm2
No. Bars /m
4.721531 No.
Spacing
211 7957 mm
3.1 Minimum Reinforcement Required to prevent crack for imature concrete (a) Retaining Wall
Description p cri
Value Unit
0.325%
"action Hight (h) 1000 action Width (w) 1000
jplicable Area(A) s 
250000
812.50
mm
mm
mm
This steel conlity will govern from previous strength design calculation
Therefore, provide Dia. 14mm @ 180mm c/c . both direction wich gives steel are
equal to
854.78 mm2/m
(b) Base Slab
Description
Value Unit
P cti
0.325%
Section Hight (h)
1000 mm
16
March. 2009DESIGN REPORT FOR ERER IRRIGA TION DAM $TRUCTU_RAL FACIUTI^S_____ engineering and consulting enterprise
CONCERT
Section Width (w) .
Applicable Area(At) for Top Rein Applicable AreafAb) for Bott. Rein. Asffor Top Rein.)
As(for Bott. Rein.)
1000 mm 250000 mm
100000 812.50 325.00
From Strength as well as imature concrete crack control requirement, Fortop reinforcement of the base slab provide Dia. 14mm @ 180mm c/c . both direction, wich gives steel are of
For bottom reinforcement there are two cases.
854.78 mm2/m
(i) Near to the retaining wall strength requirement governs, and therefore.
For bottom reinforcement of the base slab provide Dia. 12mm @ 210mm
both direction, wich gives steel are of
538.29 mm2/m
(ii) Far from the retaining wall crack of imature concrte governs, and therefore,
For bottom reinforcement of the base slab provide Dia. 10mm @ 210mm
both direction, wich gives steel are
of
373.81 mm2/m
4.3.2   Check for crack width control (Assumed crack width-
0.2mm)
Description__________________
Value Unit
Ms(Service Bending Moment))
121.72 KNm
Section Hight (h)
1000 mm
Section Width (w)
1000 mm
Concrete Cover(c )
min
50 mm
effective depth(d)
942 mm
As
1004.80 mn/
0.001067
15
0.16
154.11 mm
890.63 mm
; 95d
894.9 >Z
OK'
136.01 N/mm2
2A-/,
zbx
1.77 N/mm2
0.45f^
0 8f
y
13.5
>
136.01 ok!
240
>
1.77 ok!
Assume crack widh 0.2mm b t (h — ■»)(« — *) I
Ha “
a) I
0.00073 0.001506
17
March,2009DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES----------------------------- ENGINEERING AND CONSULTING enterprise
CONCERT
g| <g: =>e;„ -is—negative-: -No-Crack-
4 4       Stilling Basin Retaining Wall
(a) Stability Analysis
Earth Preasure = 0.3*18.15*(11.5+1.5)
Horizontal Force on 1m length of wall= 0.5*70.758*13 Vertical Loads 
wall Base Earth
= 1/2(0.45+1.9)*11.5*25
= 1.5*8.75*25
= (1.5+1.50+0.8)/2*11.5*18.15
Total
70.785
460.1025
337.8125
328.125 396 58
1062.515
For  stability  calculations  a  partial  factor  of  safety  of  1.5  is  used  for  the lateral loadings, while 1.35 will be used for strength calculations.
(i) Sliding
Frictional Resisting Force = Tan(20°)*1*1062.52
Sliding Force = 1.5*460.1025
Eventhough, the sliding force seems to exceed the frictional resistance, the retaining wall will not slide because of the resistance offered by the one meter base slab
386.51091
690 15375
(ii) Overturning
Taking  momnents  at  the  adge  of  the  toe,  at  the  ultimate  limit  state
Overturning moment = 1.5*460.10*13/3
2990 65
Resisting   Moment   =   1*(337   81*(8   75-2   3)+328   13*8.75/2+396   6687   9383 Thus, the criteria for overturning is satisfied.
(b) Bearing Preasure at Service
M, the moment about the base centerline,
M =460.1*13/3+337.81*(2.3-8.75/2)+396.58*(1-8.75/2) Therefore, maximum bearing pressure = 1062.52/8.75+6*45.38/8.752
-45.37658
124.98717
which is less than the allowable (c) Bending Reinforcement
(i) wall
= 150KN/mm2
Horizontal Force = 1.35*0.5*0.3*18.15*11.52 Max. Moment = 486.0683*11.5/3
18 March. 2009
486.06834 1863.262DESIGN REPORT FOR ERER
IRRIGATION DAM STRUCTURAL FACILITIES---------------------- engineering and consulting enterprise-
CONCERT
Reinforcement Calculation Table • Description
M(Design Moment) Concrete Copressive Strength (Fck) Steel Tensile Strength (Fst) Sectionwidth (b)
Section Hight (h) Clear Cover ( c ) Assume Re-Bar Dia. effective depth(d)
K
z
As
No. Bars /m Spacing
(i) Base
Value Unit
1863.262 KNm
30 KN/m2 400 KN/m2
WOO mm
1900 mm
50 mm
24 mm
1838 mm
0.0184 <0.167 1807.70 mm 2961.88 mm2
6.550519 No. 152.6597 mm
ok! Singly Rein.
The bearing pressure at the ultimate limit state are obtained as:
Msd =1.35-460.1M3/3+337.81 *(2 3-8.75/2)+396.58’(1-8.75/2) 652.17175
and
Nsd
Therefore;         p1 = 1062.52/8 75+6-652.17/8.752 = 121 43 + 51.11
p2 =121 43-51.11
p3 = 70.32+3.4/8.75(172.54-70.32)
1062 52
172 54 70.32
110.04
Toe: taking moments about the stem centerline for the vertical loads & the bearing pressures;
'r, = 1'328.125*3.47'5.35/8.75-(110.04)‘5.35’(5.35/2+0.95)-(172.54-110.04)‘5. -2193.05
d
leel: ; aking moment about the stem cenrer line M  = 110.04*2.5*2.5/2-328.125-1.25*2.25/8.75
5d
238.40625
3.99
Reinforcement Calculation Table Description
^(Design Moment)
Concrete Copressive Strength (Fck)
Value Unit
2193.05 KNm
30 KN/m2
19
March. 2009DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES                    — ENGINEERING and CONSULTING ENTERPRL
CONCERT
Steel Tensile Strength (Fst)
400 KN/m2 .
Sectionwidth (b)
1000 mm
Section Hight (h)
1500 mm
Clear Cover ( c )
50 mm
Assume Re-Bar Dia
32 mm
eficcnve depth(d)
1434 mm
tZ
0.0355 <0.167
ok!
Singly Rein
z
1387.54 mm
As
4541.75 mm2
No. Bars /m
5.65007 No.
Spacing
176.989 mm
4.2.2 Minimum Reinforcement Required to prevent crack for imature concrete
Description
Value
Unit
Peri
0.325%
Section Hight (h)
1000
mm
Section Width (w)
1000
mm
Applicable Area(A)
250000 m
m
As
812.50
This steel contity will govern from previous strength design calculation
Therefore, provide Dia 16mm @ 200mm c/c , both direction wich gives steel are
equal to
1004.80 mm/m
4.2.3    Check for crack width control ( Assumed crack width=0.2mm)
Description
Ms(Service Bending Moment)) Section Hight (h)
Section Width (w) Concrete Cover(cmin) effective depth(d)
I
Value______ Unit
121.72 KNm
1000 mm
1000 mm
50 mm
942 mm 1004.80 mm*
0.001067 15
0.16
x
0.95d
154.11 mm
890.63 mm
894.9 >Z
OK!
136.01 N/mrn’
1.77 N/mm2
0.45fcu
13.5
>
136.01 ok!
0.8 f
y
240
>
1.77 ok'
20 Ma/cn.2009DESIGN REPORT FOR ERER IRRIGATION QAM STRUCTURAL FACILITIES
CONCERT
engineering and consulting enterprise.
,, .'*ZixA
' d-x Et
Assume crack widh
0.2mm
0.00073
0.001506
_
Cl ~
— A‘X>y’ — >0
3 £-,X, <</--»>
< r.
—is—negatine-.'.—No-Crack-
5. Intake Tower Design
5.1   Dimensions of the Intake Tower
>
21
Match. 2009DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES------------------------- ENGINEERING AND CONSULTING ENTERPRISE
CONCERT
5.2  Actions on the Intake Tower
The actions on the intake towers are the imposed load from the foot bridge, the self weight
of the intake tower, active and passive lateral earth preasures, and the lateral force due to earth quake. Wind load & water pressure forces are not considered due to the insignificant effect compared to other actions.
The ground acceleration for design has been taken from Etiopian Building Code of Standard (EBCS-8)Sesmic zone category for the reference return period of 100 years. Accordingly,Erer is 
grouped in Zone-1 with ground acceleration ratio correspond to 0.03
The base shear & the lateral induced forces due to earth quake effect are computed based on the criteria state in EBCS-8. The figure below shows the important actions used for anlysis and design of structural componts of the intake tower.
Figure Actions on The Intake Tower
22
Mor ch.2003DESIGN REPORT FOR ERER IRRIGATION DAM STRUCJVRAL£ACIUTIES
ENGINEERING AND CONSULTING ENTERPRISE.
CONCERT
5.3   Foundation Design for the Intake Tower
Load Description
Service Loads
Ultimate Loads
Vertical Load
(N)
9508 KN
12835.8 KN
Moment Induced:
al the base
(M)
22496.508 KNm
30370.2858 KNm
Bearing Pressures are given by:
Pmax =
3924 KN/m2
Very   Greater   than   the   allowable   i.e.   250KN/m2 so go for pile foundation design
4.3.1  Drilled pier or Caisson foundation
This type of pile foundation will be more appropriate to the best applicability of our country current level of construction technology The pier construction will eliminate pile driving operation which would be the difficult construction articulation for especially heavy pile foundations. The drilled pier is constructed by drilling a cylinderical hole of the required depth and subsequently filling it with concrete. The shaft at the base is enlarged by 
uderreaming.
5.3.2   Pile Design
Design Axial load for the single pile= 15090.76 KN
Shaft diameter determination
Allowable Concrete Stress = 0.25fc = 0.25*30                 = 7.5Mpas.
0 7854D2fc         = 15090 76
D=
SQRT(15.09/(0.7854*7)
1 6567 = 1.7m
. akinn Bearing capasity at firm strata = 10000Kpa
= 0.7654B2               ( Reese(1978) recommendations. Alpa(p) =2
= 10000x0.7854’Ba2/(2B) = 3927B
B = Qd/Qp = 1509/3927 = Enlarge the base to a width/diameter of 3.40m
N
3. 0m
8
0.0106965
23
March.2009DESIGN REPORT FOR ERER IRRIGATION DAM STRUCTURAL FACILITIES--------------------- 1-------- FNGINEERING AND CONSULTING enterprise
concert
2
M
™ fck
bhfck
As
Use Dia. No. Bars
24 Bar
0.50617143 0.1
7500
452.16 16.58704883
0.157
Provide 20No. Dia. 24mm Re-Bars
Stirrups Dia. 12mm @ 150mm c/c
24 March, 2009

Summery

Erer Dam Project Summary

Erer Dam & Irrigation Development Project Summary

Project Overview

The Erer Dam and Irrigation Development Project is a major infrastructure initiative in Ethiopia designed to provide water for irrigation and other uses. The final design report covers dam appurtenant structures including the spillway and outlet works.

Key Project Details:

Location: Ethiopia
Client: Ministry of Water Resources, Federal Democratic Republic of Ethiopia
Consultant: Concert Engineering and Consulting Enterprise (CECE) in association with Consulting Engineering Service
Report Date: May 2009
Design Standards: International standards and Ethiopian design standards where appropriate

Main Structures

1. Irrigation Outlet Works

The outlet works are designed to regulate and release water for irrigation and environmental flows.

Component	Description
Design Capacity	3.05 m³/sec (3.0 for irrigation + 0.05 for environmental flows)
Intake Structure	High-level dry intake tower with three multilevel entry ports (sill levels at 1370m, 1373.82m, 1377.64m)
Conduit	123m long, 1.1m × 1.1m reinforced concrete rectangular conduit
Control System	Four control gates (three service, one emergency) operated from intake tower
Terminal Structure	USBR stilling basin Type III (8.6m long) for energy dissipation

2. Spillway Structure

Located on the left abutment, designed to safely pass flood flows.

Component	Description
Type	Ungated free overflow ogee crested spillway
Design Discharge	441 m³/sec
Crest Length	30 meters
Chute	590m long with varying width (30m to 20m) and slope
Energy Dissipator	USBR stilling basin Type III (21m long) with chute blocks, baffle blocks, and end sill

Key Design Features

Hydraulic Design

Spillway crest profile designed using USBR standard equations
Supercritical flow maintained throughout spillway chute
Hydraulic jump analysis for energy dissipation
Tailwater analysis to ensure proper jump formation
Riprap protection for exit channel (1.5m thickness)

Structural Design

Reinforced concrete structures using Grade C30 concrete
RB400 grade reinforcement (400N/mm² yield strength)
50mm minimum cover for water/earth contact structures
Finite element analysis using SAP 2000 software
Pile foundation for intake tower (3.4m diameter base)

Construction Quantities and Costs

Item	Quantity	Unit	Cost (Birr)
Irrigation Outlet Works
Excavation	3,376	m³	54,117
Concrete (C30)	900	m³	995,112
Gates (4 units)	36	m²	1,080,000
Subtotal	2,729,798 Birr
Spillway Works
Excavation	71,520	m³	1,146,466
Concrete (C30)	23,524	m³	26,010,016
Stone Masonry	2,800	m³	1,113,532
Subtotal	32,038,253 Birr

Construction Schedule

The estimated construction period is 12 calendar months, with major activities including:

Excavation and backfill
Concrete works with formwork
Installation of gates and metal works
Channel protection works

Key Considerations

        High sediment load in catchment area influenced intake design (high-level, multi-level entry ports)
No bottom outlet provided - complete reservoir evacuation not possible
Spillway designed for reliability in remote location (ungated, simple operation)
Structural designs account for seismic activity (Zone 1 with 0.03g ground acceleration)
Emphasis on crack control in concrete structures

    