FEDERAL  DEMOCRATIC  REPUBLIC  OF  ETHIOPIA MINISTRY OF WATER RESOURCES
ERER DAM & IRRIGATION DEVELOPMENT PROJECT
Final Detail Design Report
A lmi*. D-iisiqr. May. IExecutive 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 AnalysisTable of content
1.0           Introduction.................................................................................................................................................... 1
1.1            General.......................................................................................................................................................... 1
1.2           Location and access to dam site................................................................................................................... 1
1.3             General description of major work components..................................................................................... 1
2.0            Type of Dam......................................................................................................................................................2
3.0            Seismicity.......................................................................................................................................................... 3
4.0            Diversion Arrangements................................................................................................................................ 3
4.1             General........................................................................................................................................................... 4
4.2             Design Data.................................................................................................................................................... 4
4.3             Design Flood for Diversion..........................................................................................................................4
4.4             Diversion Channel Design...........................................................................................................................  5
4.5             Closure of Gap.............................................................................................................................................. 6
5.0
Dam Heights Reservoir Levels...................................................................................................................... 8
5.1
General........................................................................................................................................................... 8
5.2
Dead storage Level....................................................................................................................................... 9
5.3
Minimum Draw Down Level (Mddl)................................................................................................................ 9
5.4
Full Reservoir Level..................................................................................................................................... 9
5.5
Maximum Reservoir Level............................................................................................................................. 13
5.5.1
Elevation-Storage-Outflow Data....................................................................................................................... 13
5.5.2
Inflow Hydrograph........................................................................................................................................... 13
5.5.3
Results of Reservoir Routing.................................................................................................................... 14
5.6
Top of Dam Embankment (Dam Crest Level)............................................................................................... 15
5.7
Freeboard...................................................................................................................................................... 15
5.7.1
General............................................................................................................................................................ 15
5.7.2
Calculation of Normal Free board:...................................................................................................................16
5.7.3
Settlement due to strong Ground Motion......................................................................................................... 20
5.7.4
Dam Crest Level or Top Bund Level (TBL):..................................................................................................... 20
5.8
Height of the Dam......................................................................................................................................... 20
5.9              Significant Levels......................................................................................................................................... 20
6.0           Various components of Embankment dam.................................................................................................... 21
6.1             Crest Width.................................................................................................................................................. 21
6.2             Camber/ settlement allowance................................................................................................................... 22
6.3              Parapet wall................................................................................................................................................. 22
6 4            Guard stones.................................................................................................................................................. 22
65             Berms................................................................................................................................................................22
6.6              Impervious Core............................................................................................................................................ 23
6.7              Shell Zones....................................................................................................................................................23
6.8             Cutoff.............................................................................................................................................................24
6.9              Rock Toe........................................................................................................................................................ 24
6.10           Toe Drain........................................................................................................................................................ 25
6.11            Upstream Slope Protection.........................................................................................................................25
6.12           Down Stream Slope Protection.................................................................................................................. 27
7.0            Stability Analysis of dam............................................................................................................................... 28
7.1              General...........................................................................................................................................................28
7.2            Material Properties Considered for Analysis............................................................................................28
7.2.1         Material in Impervious Core................................................................................................................................28
7.2.2         Material in Shell zones............................................................................. .......................................................... 29
7.3              Loading Conditions.......................................................................................................................................31
7.4               Minimum Factor of Safety...........................................................................................................................31
7.5              Stability Analysis..........................................................................................................................................32
8.0. Geology along dam axis.............................................................................■.............................................................. 34
9.0            Foundation treatment.................................................................................................................................... 35
9.1              Surface Treatment........................................................................................................................................ 35
9.2              Subsurface treatment.................................................................................................................................. 36
10.0          Internal Seepage Control Measures........................................................................................................... 3810.1           Transition Filter I Inclineo filter........................... .-............................................... -........... -..................... 38
10.2           Horizontal Filter-................................................................................................................................... . 39
10.3          Special Seepage Control Measures at Contact with Steep Abutments................................................. ..........40
11.0          Dam Instrumentation.............................................. ................................................................. —40
11.1          General.'.......................................................................................................................................................... 40
11.2          Instrumentation Scheme.................................................................................................................... .......—42
11.2.1      General Scheme..............................................................................................................................................-.42
11.2.2     Measurement of Pore Water Pressures-............................................................................................................ 42
11.2.3     Measurement of Internal Movement-.................................................................................................................. 42
11.2.4     Measurement of Surface Movements-.............................................................................................................  43
12.0       Bill of Quantities, Cost Estimates and Tentative Construction Schedule.............................................. 43
12.1        Dam Appurtenance Works........................................................................................................ —43!
iErcr Dam & Irrigation Project /CECE & CES
1.0 Introduction
1.1      General
Erer Dam is one of the major components of Erer Irrigation  Development project.  This is basically  a storage dam across river Erer to  utilize the annual flows of the river for the purpose of irrigation  development in  its command  area.  Three alternative dam sites (dam axes),  including  the one selected  during  pre-feasibility  study,  were studied  in  detail to decide most suitable site from techno-economic considerations.  Option  III has unfavorable geological set-up  and  has a longer length.  Option  II has comparatively  weaker foundation for locating  spillway  and  spill channel and  without any  specific attractive features Thus option I was found to be best suited for locating the dam and chute spillway structure.
The orientation  of dam axis has been  fixed  taking  into  account the topographical configuration  of ridge on  left and  right abutments.  The selected  axis is aligned  exactly  in East-West direction; the coordinates of reference point AR lying  on  the dam axis are 1028361.961 North, 195963.404 East.
The  total  length  of  dam  is  about  1460m,  excluding  30m  crest  length  of  spillway  The maximum height of dam above deepest river bed level is 36.6m.
1.2 Location and access to dam site
The dam site is located  at about 6km upstream of the bridge across Erer river on Harar to  Jijiga road  The site is about 25  km from Harar and  17km from Babile towns respectively
Location plan of the dam site is furnished below vide Fig 1.1
1.3     General description of major work components
•     A zoned earthfill dam 36.6m high above deepest river bed.
•     An irrigation outlet with intake tower through right abutment
•     A chute spillway with ogee crest located on the left bank
•    River diversion work.
Annex A : Dam Final Detail Design Report/ 2009
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2.0 Type of Dam
Based on the bore holes drilled at the dam site, the thickness of the overburden at the river bed  portion  is found  to  be around  19m followed  by  crystalline gneissic rock.  The overburden  is mainly  characterized  by  about 7m deep  silty  clay,  sand  mixed  with  gravel spreading all along the axis of the dam. But in the river bed portion it is underlain by about 11 m deep coarse gravel and cobble mixed with sand.
The length of the crest of the dam is about 1.46 km and the site falls in high seismic prone area In  high  seismic prone area,  the earthfill dam or earth  and  rockfill dam is always preferred  due its flexibility  than  to  have a rigid  structure like concrete dam.  Now considering  huge overburden,  large length of the dam and seismicity of the area, the site is found not fit for construction of concrete dam
The site appears ideal for construction  of either Earth  dam or Earth  and  Rockfill dam. Since adequate quantity  of suitable earthfill materials are available from different borrow areas at site, it is decided to go for a zoned Earth dam.
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l<Erer Dam & Irrigation Project ZCECE & CES
3.0 Seismicity
The dam site is considered to be falling in zone IV of seismic zoning map of Ethiopia. It’s the zone of major damage in  which  seismic ground  shaking  is said  to  be producing intensities VIII and above.
For not having  any  site specific ground  motion  record  and  information,  seismic co efficient of the area is compared with the zone IV of seismic map of India, for which the basic horizontal seismic coefficient (do) is 0.05
In seismic coefficient method as per Indian standard the design value of horizontal seismic coefficient cth = P I ao
Where
p = a coefficient depending upon the soil foundation system. The value of P for all types of dam is taken as 1.0.
I = a factor depending upon the importance of the structure. For all types of dam, the value of 1 is taken as 3.0 
do = basic horizontal seismic coefficient
The horizontal seismic coefficient (Oh) thus comes to  0.15  and  vertical seismic coefficient (a ) = l/2cth  i.e.  0  075.  These coefficients are adopted  for dynamic analysis of slope
v
stability.
As regards using  of seismic co-efficient value of zone IV of seismic zoning  map  of Ethiopia,  it may  be mentioned  that for some other similar projects,  almost same seismic co-efficient value has been adopted
In  the  light  of  the  fact  stated  above,  it  appears  to  be  quite  justified  to  use  seismic  co efficient value as per zone IV of seismic map of Ethiopia
Annex A : Dam Final Detail Design Report/ 2009
3Erer Dam & Irrigation Project ZCECE & CES
4.0 Diversion Arrangements
4.1 General
The design  for a dam that is to  be constructed  across a river must consider diversion  of stream flows around  or through  the dam site during  construction  period.  Selection  of the most appropriate scheme for handling  the flow of the stream during  construction  is important to  secure economy  in  the cost of the dam.  The design  flood  for river diversion arrangement during  construction  is influenced  mostly  by  the risk  and  damage factors involved  in  the diversion  arrangements In  the case of an  earth  fill dam the risk  of over topping  of embankment during  construction  may  result in  serious damage to  works partially  completed.  This consideration  may  not be so  for a concrete dam where flood water may  overtop  the partially  constructed  dam with  little adverse effect Thus a proper diversion  plan  is to  be evolved  that will minimize serious flood  damages to  the work  in progress at a minimum expense
4.2        Design Data
Design  data like flood  peaks for flood  of different return  periods has been  taken  from the draft report on  studies carried  out by  the Hydrology  Group.  The levels of river bed  at dam site are based on longitudinal profiles parallel to dam axis
4.3        Design Flood for Diversion
For large earth  fill dams,  river diversion  design  flood  of 100-year return  period  is considered  desirable as per BIS 14815: 2000  code.  However in  case of Erer dam,  the valley at the proposed.dam site is very wide and flat. The maximum height of the dam is
36.6  m.  Therefore,  the  river  diversion  works  for  construction  of  Erer  Dam  are  planned  to be  designed  for  a  flood  of  50  years  return  period  or  to  maximum  observed  flood  at  Erer site  whichever  is  higher.  The  estimated  peak  floods  for  various  return  periods,  based  on GEV model are reported to be as below.
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4Erer Dam & Irrigation Project ZCECE & CES Table 4.3: Estimated Flood Flows
Return Period
(year)
Estimate Flood
Peak (m /s)
3
5
58
10
82
20
120
50
151
100
179
The  observed  flood  at the  Erer dam site is not available and in the absence of which the computed flood for 50 year return period has been adopted for design of diversion works
4.4      Diversion Channel Design
As already  indicated  earlier,  the dam site is located  in  a wide valley  with  the foundation comprising of deep overburden both in the river bed and the banks. The diversion of flood by  constructing  a tunnel and  coffer dams is to  be thus ruled  out.  It is planned  to  diven flood  flows through  the body  of the dam by  constructing  a temporary  diversion  channel during  construction  period  The deepest riverbed  at Erer dam site is reported  to  be at El 1348m Accordingly,  it is planned  to  set the bed  level of temporary  diversion  channel at
the  same  elevation.  A  trapezoidal  section  of  the  channel  is  considered  with  the  following parameters:
• Design Flood 151 m3/sec
•    Bed width              30tn
•    Depth of flow
1.5 m
•     Side slope
1.5H:1V
•     Bed slope
1:164
Annex A . Dam Final Detail Design Report/ 2009
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hErcr Dam & Irrigation Project ZCECE <&: CES
• Free Board 0.5m
Area =               1.5 x (30+2x2 25)/2 =             48 75 m2
P =
30+2x 1.5^(1.5)2+(1)2
=
35.4 m A 48.75
R
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P  35.4
R%
=
(1.3777)^
2            IZ2
= 1.24 m
Discharge Q =AXV =AX l/nXR * XS
= 48.75X (1/0 03) X1.24X (1/164)1/2
=157.25 m3/s> 151 m3/s
Hence the assumed diversion channel section is ok.
4.5       Methodology and sequence of diversion of river flow:
Construction of concrete chute spillway of crest level 1379 m should be started in the first
season and completed in one season.
It is planned  to  divert the diversion  design  flood  through  a temporary  channel involving  a gap  of about 30m through  the body  of the dam in  the deepest river channel while the remainder of the embankment is being  constructed.  The side slope of the embankment across the opening  shall be not be steeper than  4:1  to  facilitate filling  of the gap  at the end of construction  period  and  to  decrease the danger of cracking  of the embankment due to differential settlement.  The flat slope also  provides a good  bonding  surface between previously constructed embankment and the material to be placed for closure of the gap
So far as the methodology and sequence of diverting the river water is concerned, this is to state that before the stream is diverted, the foundation treatment i.e. refilling of cut-off trench, construction of diaphragm wall and grouting as will be required should be completed in the area where the temporary opening will be left through the embankment. Embankment section on either side of the diversion channel forming part of fill placement
Annex A : Dam Final Detail Design Report/ 2009
6Erer Dam & Irrigation Project /CECE & CES__________________________________________ of dam with flatter slope 4:1 should be formed duly compacted in layers as per requirement of optimum moisture density  of the material.  The river flow may  then  be channelised through  this area The foundation  treatment followed  by  embankment placement in  laryers on  either side of the channel should  proceed  simultaneously  to  achieve the completion  of dam embankment to top.
During  embankment placement,  river water continues to  flow through  the diversion channel till sufficient progress is made in  completion  of embankment.  Closure of the gap in  the embankment can  be made thereafter by  making  a dyke/ bund  to  facilitate working area dry during lean season flow. The average rale of embankment placement must be such that gap can be filled faster than the water rises in the reservoir and the work requires to be competed in one lean season i e. middle of October to middle of March. It is seen that to fill the gap  up  to  the crest level of dam,  the quantitites of earth  filling  required  is about 420000m3.  Even  for a reasonably  efficient contractor to  achieve the progress of 375  m3/h target is not difficult. At this rate the total time requirement is about 1120hrs. Working for two  shifts (8hrs/ shift) a day,  it can  be completed  in  only  70  days.  However care must be exercised during filling of the gap so that quality of work is not sacrificed due to exigency of the situation. This is of great importance because the diversion gap is in the area where the dam height is maximum. Extreme care must be taken to obtain required densities so as to  avoid  excessive settlement of the embankment.  Proper attention  to  bonding  of new and old earth fill shall be specially attended.
Drawing   (Figure   4.1)   showing   the   layout   and   cross   section   of   diversion   channel   is presented
4.6      Closure of Gap
The portion  of the embankment to  either side of diversion  channel shall be taken  - up  for construction.  The side slope of the embankment across the opening  shall be not be steeper than  1H to  3V to  facilitate filling  of the gap  at the end  of construction  period  and  to decrease the danger of cracking  of the embankment due to  differential settlement.  The flat slope also  provides a good  bonding  surface between  previously  constructed  embankment and the material to be placed for closure of the gap.
Annex A . Dam Final Detail Design Report/ 2009
7E196600
30 CM THICK PITCHING CHANNEL SECTION
NOTES-
1 SIDE Slope  and  bed  slope  required  to  pass  the DIVERSION FLOOD ARE 1 SH IV AND 1 154 RESPECTIVELY
2-SIDE SIOPE OF THE EMBANKMENT ACCROSS THE OPENING SHALL SOT BE STEEPER THAN4H IV
FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA
MINISTRY OF WATER RESOURCES
NCERT ENGINEERING ANS CONSULTING TRTFRPRKr
IN ASSOCIATION WITH
CONSULTING ENGINEERING SERVICt ( INDIA ) PVT. LTD
ERER IRRIGATION PROJECT DETAIL DESIGN
B
A_________Erer Dam & Irrigation Project /CECE & CES_______________________________________________
The diversion  is carried  through  the opening  in  dam until sufficient progress is made in 
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completion  of embankment and  appurtenant works so  that flood  can  be carried  safety through  the completed  or partially  completed  spillway.  Closure of the gap  in  the embankment then  can  be made The closure of the gap  shall be started  by  middle of October and  completed  by  end  of February  to  Middle of March.  The average rate of embankment placement must be such  that the gap  can  be filled  faster than  the water rises in the reservoir.
Care must be exercised  during  filling  of the gap  so  that quality  of work  is not sacrificed due to  exigency  of the situation  This is of great importance because the diversion  gap  is in the area where dam height is maximum.  Extreme care must be taken  to  obtain  required densities so  as to  avoid  excessive settlement of complete embankment Proper attention  to bonding of new and old earth fill shall be specially attended.
Annex A : Dam Final Detail Design Report/ 2009
8IErer Dam & Irrigation Project ZCECE & CES
5.0 Dam Height & Reservoir Levels
5.1     General
The height of the dam is based  on  functional requirement of adequate storage in conjunction  with  operational and  demand  parameters set by  downstream demands.  The topographical features of the valley  may  also  limit the height of a dam Siltation  in  the reservoir over period  of time,  seepage losses through  reservoir and  evaporation  losses are to be accounted for in fixing the height of the dam.
5.2     Dead Storage Level
The sediment inflow into  the reservoir occupies the considerable part of reservoir storage. Studies have been  carried  out for sediment volume accumulated  in  the reservoir considering  average annual silt rate According  to  the feasibility  lavel study  (WWDSE/ SHNERICS) the dead  storage level (DSL) of 1369  85  has been  fixed  based  on  the sedimentation  studies for New Zero  Elevation  after 50  years of reservoir sedimentation (see figure 5.1).
5.3     Minimum Draw Down Level (Mddl)
After knowing  the dead  storage level and  fixing  the invert level of Irrigation  Outlet (10), the MDDL is fixed in such a way that the depth of water above the centre line of 10 is at least equal to  the height of opening.  This will avoid  the formation  of vortex  Accordingly MDDL has been fixed at EL. 1371.
5.4     Full Reservoir Level
Simulation   studies   have   been   carried   out  with   known   MDDL  and   assumed   FRL  and outflow  from  outlet,  inflow  into  the  reservoir,  evaporation  and  seepage  losses  The  studies have  been  done  using  35 years data and checking the success rate. The FRL has thus been fixed at EL 1379.
Annex A : Dam Final Detail Design Report/ 2009
9Erer Dam & Irrigation Project ZCECE & CES
Erer Irrigation Project
Elevation Area Capacity’ Curves for 25 years of Sedimentation (FRL=1379 m)
____________ Area (Ha)
« -Revised Area
Original Area
Revised Capacity —Original Capacity
5.4.1 Simulation for Fixing of FRL keeping demands constant
In this case, FRL was varied from 1378m to 1386m keeping irrigation area as 4000 ha and d/s release as 0.1  cumec along  with  demands for domestic and  livestock  water supply indicated above. Results of simulation runs are indicated below in Table 5.1
5. 1: Erer Irrigation Project- Monthly Simulation Results
No.    of    Periodwise    failures    (excluding    first year)
s.
No.
FRL
(m)
MDDL (tn)
Area
(ha)
D/S
release (MCM)
Total
no. of Periods
Failure permissible
As per simulation
Irr
(75%)
WS
(95%
Irr
Critical
WS
1
1378.0
1371.0
4000
0.263
456
114
22
64
50
23
2
1379.0
1371.0
4000
0.263
456
114
22
52
37
20
3
1381.0
1371.0
4000
0.263
456
114
22
33
30
17
4
1382.0
1371.0
4000
0.263
456
114
22
31
30
13
Annex A Dam Final Detail Design Report/ 2009
10Erer Dam & Irrigation Project ZCECE & CES
5            1383.0     1371.0    4000
0.263
456
114
22
28
25
10
6
1384.0
1371.0
4000
0.263
456
114
22
26
24
9
7
1385.0
1371.0
4000
0.263
456
114
22
23
21
8
8
1386.0
1371.0
4000
0.263
456
114
22
22
18
7
Note:
•             For irrigation, supply in deficit years has been curtailed to 90% so as to supply water to 
more no of periods.
•             In case irrigation deficit is more than 25% of the demand, then that period/year has been categorized as critical failure
•             WS includes all demands except irrigation. Failure for WS means failure for irrigation 
also
•             Performance for irrigation and WS are judged for 75% and 95% success respectively.
•             Revised Elevation-Area-Capacity values for 25 years of sedimentation have been 
considered. Dead storage level is as per 50 years of sedimentation.
•             Upper, middle and lower rule levels are same as Full Reservoir Level (FRL), Minimum Draw Down level (MDDL) and MDDL respectively.
From the above table,  it is seen  that for FRL  value of 1379.0  m no  of failures are within the desired  limit of 22,  therefore FRL  of 1379.0  m has been  finally  adopted  to  meet the irrigation  water requirement for 4000  ha,  d/s release of 0.1  cumec (0.263  MCM) and demands for domestic and livestock water supply.
5.4.2   Simulation for Increased Downstream Release with Area Varying
In  this simulation  FRL  was kept as 1379.0  m,  as finalized  above,  d/s release was increased from 0.1  cumec (0.263  MCM) to  as 0.5  cumec (1.315  MCM) and  irrigation  area was varied  till failures are within  the desired  limit.  Upper rule level was kept equal to  FRL (1379.0m),  and  middle and  lower rule levels were kept equal to  MDDL  (1371.0m).  Results of simulations for are given below in Table 5.2
Annex A : Dam Final Detail Design Report/ 2009
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Table 5. 2: Erer Irrigation Project- Monthly Simulation Results
(FRL=1379.0m, D/S Release=0.5 cumec or 1.315 MCM, Area =Varying)
No.  of  Periodwise  failures  (excluding  first year)
s.
No.
FRL
(m)
MDDL
(m)
Middl
e Rule
Curve
Area
(ha)
D/S Release (MCM)
Total
no. of Periods
Failure permissible
As per
simulation
Irr
(75%)
WS
(95%
Irr
Criti
cal
WS
1
1379.0
1371.0
1371.0
4000
1.315
456
114
22
113
93
41
2
1379.0
1371.0
1371.0
3800
1.315
456
114
22
109
86
40
3
1379 0
1371.0
1371.0
3600
1.315
456
114
22
86
70
34
4
5
1379.0
1371.0
1371.0
3400
1.315
456
114
22
64
47
27
1379.0
1371.0
1371.0
3200
1.315
456
114
22
37
30
15
Note: Upper and lower rule levels arc same as Full Reservoir Level (FRL) and Minimum Draw Down level (MDDL) respectively
It is seen  from the above table that when  d/s release is increased  from 0.1  cumec to  0.5 cumec,  irrigation  area will have to  be reduced  from 4000  ha to  3200  ha to  keep  the WS failure within permissible limits with adopted FRL of 1379.0 m.
5.4.3. Conclusions
From the simulation results given above for different alternatives, following conclusions can be drawn:
•     FRL of 1379.0m is sufficient to meet the full estimated annual demand of 50 11 MCM to keep the failures for WS and irrigation within desired limit.
•     With adopted FRL of 1379.0m, irrigation demands for only 3200 ha can be met with increased d/s release from 0 1 cumec to 0 5 cumec
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• ' For d/s release of 0.5 cumec and area of 4000 ha, even FRL of 1386.0m is not enough 
to keep the number of failures within desired limit. This is due to the fact that numbers
of consecutive years are dry to very dry years.
•   Based   on   above   simulation   studies,   FRL   of   1379.0   m   has   been   adopted   to   meet estimated   planned   water   requirement   of   50.11   MCM   for   planning,   design   and   other studies
5.5    Maximum Reservoir Level
The design  inflow flood  of as recommended  by  hydrology  group  has been  adopted  for flood  routing  considering  different length  of spillway.  Finally  30m length  of spillway  has been considered
5.5.1 Elevation-Storage-Outflow Data
Elevation storage data for Erer reservoir for FRL as 1379 0m for 25 years of sedimentation given in Table 3.14 has been used. Outflow from spillway has been worked out using the following equation:
Q = C  L Hl 3
d
where,  Q (cumec) is the outflow discharge,  Cd  is the coefficient of discharge (1.18),  L  (m) is the length  of spillway  (m) and  H (m) is the depth  of flow above the spillway  crest level of 1379.0  m.  Outflow from spillway  has been  worked  out considering  L  as 30m,  40  m and 50 m.
5.5.2 Inflow Hydrograph
Design  flood  hydrograph  (figure  5.1)  as  given  below  with  a  peak  value  of  642  cumec  at  20 hours has been adopted for routing.
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LErer Dam & Irrigation Project ZCECE & CES_________________________________ Initial reservoir level for reservoir routing has been considered as FRL (1379.0m). Figure 5.1: Design Flood Hydrograph
5.5.3  Results of Reservoir Routing
Reservoir  routing  was  carried  out  using  above  data  as  input.  Summary  of  results  of routing are given in Table 5.3
Table 5.3: Summary of Reservoir Routing Results by Modified Puls Method
Peak Inflow
Peak outflow
Case
no.
L
(m)
Time
(hrs.)
Ordinate
(cumec)
Time
(hrs.)
Ordinate
(cumec)
Max.
Reservoir
level
(m)
1
30
20
641.9
26
440.5
1382.55
II
40
20
641.9
25
491.72
1382 16
III
50
20
641.9
25
526.3
1381.85
It is seen  from the above Table 5.3  that maximum reservoir level attained  after reservoir routing  varies from 1381.85m for spillway  length  of 50m to  1382.55m for spillway  length of 30m. Based on the above results, maximum reservoir water level (MWL) and other
Annex A : Dam Final Detail Design Report/ 2009
14R
-
■
■
■
V
R
R
R
II
R
RErcr Dam & Irrigation Project /CECE & CES_______________________________________________ physical, technical and economical factors, spillway length of 30m with MWL of
* <
1382.55m has been adopted for further planning and design. Fig. 5.2 shows plot of inflow, outflow and reservoir elevation for reservoir routing for spillway length as 30 m.
Figure 5.2: Reservoir routing (L=30m)
Erer Irrigation Project-Reservoir Routing (L=30 m)
1383.0
1382.5
1382.0
1381 5
1381.0
1380.5
1380.0
1379.5
1379.0
1378 5
Time (hrs.)
-—-Inflow ——Outflow Elevation
5.6     Top of Dam Embankment (Dam Crest Level)
3  To  arrive  at  Top  of  Bund  level,  free  board  above  FRL  and  above  MWL  are  worked  out. The  free  board  above  FRL  is  known  as  normal  free  board  and  that  above  MWL  is  known as minimum free board. T Seville's method has been used for working out free board
5.7    Free Board
5.7.1 General
Adequate freeboard  over FRL and  MWL  is required  to  be provided  to  guard  against the overtopping  of dam embankment under design  wave conditions.  For this,  the requirement of normal free board at FRL and minimum free board above MWL are calculated. The free
Annex A : Dam Final Detail Design Report/ 2009
15ft
ft
■
V
■
B
II B
B
B
B
B
B
BErcr Dam & Irrigation Project /CECE & CES__________________________________________ board that gives the highest requirement of TBL (Top Bund Level) needs to be finally 
* <
adopted.
Indian  standard  IS 10635  “Guidelines for Freeboard  Requirement in  Embankment Dams” requires that the normal free board at FRL be calculated by adopting the full wind velocity and by considering design wave height (Ho) as 1 67 times the significant wave height (Hs), subject to  the condition  that normal free board  should  not be less than  2.0  m In calculating the minimum free board at MWL, wind velocity is taken as half to two third of
full  wind  velocity  while  the  design  wave  height
(H )  is  taken  as  1.27  times  the  significant
o
wave height (Hs). The minimum free board is subjected to a minimum of 1.5 m.
5.7.2  Calculation of Normal Free board:
Significant  wave  height  is  a  function  of  wind  velocity  and  effective  fetch.  Effective  fetch (Fe) of a reservoir is given by the following relation:
Fe = Zx;«Cos6*Cos0 / ECosO
Where X, = the length of any radial at an angle 0 from the central radial. The values of X, arc read from the map of Erer reservoir after plotting radials at an interval of 6 degrees up to 42 degrees on both sides of central radial.
Table 5.4.2 Computation of Effective Fetch For FRL
0
Cos 0
X,
Xi Cos 8 Cos 8
42°””
.743
1147.02
633.211
36°
.809
4839.701
3167.492
30°
.866
3853.77
2890.157
24°
.914
2816.53
2352.917
18°
.951
2311.344
2090.381
12°
.978
2073.285
1983.063
6°
.995
1875.826
I
1839.294
0°
1.00
1901.514
1901.5141
6°
.995
1412.733
1398.641
7?
.978
1473.26
1409.149
’18°
.951
1550.607
1402.371
24°
.914
1078.272
900.784
Annex A : Dam Final Detail Design Report? 2009Erer Dam &
/CECE & CES
30°
.866
1140.666
855.449
’36°
.809
1117.68
731.50
42°
.743
905.925
500.115
I = 13.512
21289.66
24056.07
= 24056.07/13.502
= 1.78km
Data on  basic hourly  wind  speed  in  Erer region  is inadequate to  arrive at 50year return period  wind  velocity.  USBR recommends for using  wind  velocity  of the order of 120Km/hr to  160  Km/hr.  Accordingly  the basic wind  speed  on  land  for 50  years return 
period  (U) has been  considered  as 143.7  km/hour or 33.33  m/sec.  Wind  velocity  on  water surface (V) is obtained  by  multiplying  wind  velocity  on  land  (U) by  co-efficient Q for given effective fetch.
As  per  IS:  10635,  the  coefficient  Q  for  an  effective  fetch  of  1.78  km  equals  to  1.15 Hence, design wind velocity on water surface (V) shall be 42.3 m/sec or 160.94 km/hr.
Significant   wave   height   (Hs)   is   calculated   using   relationship   given   in   graphical   diagram with related factors i.e. for a given wind velocity and effective fetch
For v = 38.33 m/s and fe = 1.78km
Hs   =   1   2   m   Wave   period   (Ts)   is   also   computed   using   relationships   given   in   graphical diagram
With related factors i.e. wind velocity and effective fetch So Ts = 3.4/ Seconds
Wave Length (Ls) is computed by the relationship: Ls = 1.56 Ts2
= 1.56 x 3.42 = 18 03m
Design wave height (Ho): Ho = 1.67 Hs
= 1 67 x 1.2 = 2.004m
Annex A : Dam Final Detail Design Report/ 2009
17Ercr Dam & Irrigation Project /CECE & CES______________________________________________ Steepness ratio, Ho/Ls = 011
Embankment slope = 1 in 3 i.e. 0.33
For embankment slope of 0.29 and Ho/Ls = 0.11,
R/Ho = 1.3
Where R denotes wave run up on smooth surface
Hence R = 1.3 x 2.004 = 2.605m
The wave run up on rough surface (Ra) is computed by multiplying surface roughness coefficient to wave run up on smooth surface
Value of surface roughness co-efficient for dumped riprap = 0.50
Hence Ra =0.5x2.605
= 1.303m
IS Code provides that if the wave run up on rough surface (Ra) is less than designed wave height (Ho), Ra shall be taken as Ho
Hence Ra = 2..004m
Average water depth (D) in the reservoir has been considered as 31 m. Wind set up (S) is calculated by the formula
S = V . F/ (62000 x D/2)
Where, F = maximum Fetch in km
= 1.9Km
V = wind velocity in km/h = 138
= 138  x 1 9/(62000 x 27/2)
= 0 12m
Normal Free board required above FRL
= Ra + S
= 2.004 + 0.12
= 2 12 m
The effective fetch length is calculated as presented below in table 5.5 Annex A : Dam Final Detail Design Report/ 2009
2
2
18Ercr Dam & Irrigation Project ZCECE & CES______________________________________________
'• Table 5.5.3: Computation Of Effective Fetch For MWL
9
Cos 9
Xi
Xi Cos 9 Cos 9
42°
.743
2257.08
1105.969
^36°“
.809
5338.795
2978.019
3o°'
.866
3970.925
3317..295
24°
.914
2867.568
2338 332
18°
.951
2585.504
2395.555
12°
.978
2238.329
2140.926
.995
1972.056
1952.384
0°
1.00
1979.128
1979.128
6°
.995
1979.128
1949.516
12°~
.978
1502.94
1437..54
18°
.951
1613.86
1534.788
24° ”
.914
1248.5
1042.992
30°
.866
1220.559
936.329
36°
.809
1179.176
771.748
42°
.733
1116.366
599.811
2 = 13.512
I =26477.648
Fe = Sxi cos O.cos 9/ E cos 9
Fe=_26477.648 / 13.512 = 1.96 km
Calculations for minimum free board at maximum water level (MWL) are presented in Table-5.6.
MWL 
Fetch Length (FRL, MWL) -
-             1382.6 m for 1/2PMF 1.96 km
Slope of Embankment -               1 V in 3.0 H up to berm and thereafter 1 in3 5 Table : Computation of Free Board
SI
No
Computed Item
Calculation for    Normal Free board
Calculation 
for Minimum
board
1
Effective fetch (Fe) in km
1.78
1.96
2
Wind velocity over land (U) in km/h
120
80
3
Wind coefficient (Q)
1.15
1.15
4
elocity over water surface
(V) in km/h
138
92
5
Significant wave height (Hs) in m
1.2
0.85
6
Wave period (Ts) in seconds
3.4
3.0
7
Wave length (Ls) in m
18.03
14.04
8
Design wave height (Ho) (m)
2.004
1.419
9
Wave steepness Ho/Ls
0.11
0 101
Annex A : Dam Final Detail Design Report/ 2009Ercr Dam &
&
10
Relative Run-up R/Ho
1.303
1.25
11
Run-up (R) in m
2.605
1.845
12
Run-up(Ra) considering on dumped rip rap in m
1.303
0.922
13
If Ra < Ho, design Ra=Ho
2.004
1.419
14
Average depth of reservoir (D) in m
31
33.6
15
Wind set-up in m
0.12
0.008
16
Free board required
2.12
1.426
17
Permissible freeboard (m)
2..0
1.5
5.7.3   Settlement due to strong Ground Motion
In fixing the freeboard for an earth or rock fill dam, allowance also needs to be made to the settlement of embankment under the shaking  action  of earthquake vibrations/seiches Hence allowance has been  provided  for these aspects in  fixing  the free board  An allowance of 0.5m is considered  in  computing  the free board  for settlement due to earthquake-induced shock
Total freeboard required above FRL= 2.12 + 0.5 = 2.62 m
Total freeboard required above MWL= 1.5 + 0.5 = 2. m
5.7.4   Dam Crest Level or Top Bund Level (TBL):
TBL on consideration of normal freeboard above FRL=1379 + 2.62= 1381.62 m
TBL on consideration of minimum freeboard above MWL = 1382.6 + 2. = 1384.6m
It is seen that the criteria of minimum freeboard above MWL is the governing condition for fixing the TBL and that the TBL is required to be at or above EL 1384.6 m. In the selected scheme, the dam crest is at El 1384.6 m
A 1.0 m high parapet wall shall be constructed above dam crest a|ong the upstream face of the dam
5.8 Height of the Dam
The general river bed level at dam site is 1348m. Thus, the height of Erer Dam, as per selected scheme would be about 36.6m above riverbed level.
Annex A : Dam Final Detail Design Report/ 2009
20I II
r
=•
a
kI R
Erer Dam & Irrigation  Project ZCECE  & CES
*              5.9 Significant Levels
I
I
r
i
i
i
i
i
I
Significant data of Erer dam and reservoir are provided below,in table 5.7 Table 5.7.5 significant data of dam
Dam
Top of parapet guard wall
El. 1385.6
Embankment crest
El. 1384.6
General Riverbed
El. 1348.0
Height above riverbed
36.6 m
Reservoir
Maximum waler level
El. 1382.6
Full Reservoir level
El. 1379.0
Minimum Drawdown Level
El 1371.0
Gross storage
5067.5 ham
Live storage
2303.4 ha m
|
6.0 Various components of Embankment dam
6.1 
Crest Width
_ 
Largely precedent, keeping in view the possible roadway requirements, has determined
the crest width of Erer dam. Following considerations have guided the selection of crest
-
width:
£ 
• The Indian standard: 8826, ‘Guidelines for Design of Large Earth and Rock fill
Dams’ lays down that the crest width should be fixed according to the working
I
space required at the top and that the crest width should not be less than 6m.
• Design of small Dams by USBR suggests following formula for determining crest (width for small earth fill dams:
Annex A : Dam Final Detail Design Report/ 2009
21
I
C".IErer Dam & Irrigation Project /CECE & CES
w = z/5+ 10
Where w = width of crest in feet and z = height of dam in feet above the riverbed. The
height of Erer Dam above river bed being 33.5m
(109.88 feet), width of dam crest = 32 feet or 9.75m
• ‘Design of Large Dams’ by Thomas refers to Japanese Code 1957 that specifies crest
width W in terms of height of dam (H) as:
W = 3.6 x V77" - 3(meler )
For Erer Dam, W = 3 6 x 3/-33.S-3
W=11.6-3          = 8.6 m
Based on above, a crest width of 9 m has been selected for Erer dam.
6.2      Camber/ settlement allowance
Appropriate camber shall be provided  along  the crest of dam to  ensure that the free board is not diminished  by  foundation  settlement or embankment consolidation.  It is expected that the major portion  of the embankment consolidation  will take place during  the construction  and  thus the camber along  the axis of dam is required  to  take care of the expected foundation settlement
IS 8826 stipulates that settlement allowance of 1 percent of the maximum height ofthe dam shall be provided for settlement of embankment dam on unyielding foundation. Accordingly, a longitudinal camber of 36.6 cm has been provided on dam crest along the axis of Erer dam. The camber varies from zero height at the abutments to 35.6cm at the central section in the valley.
6.3    Parapet wall
A parapet wall of lm height on the upstream side of crest of dam is provided for an additional safety. This is over and above the required height of the designed f free board.
Annex A : Dam Final Detail Design Report/ 2009
22Erer Dam & Irrigation Project ZCECE & CES
6.4     Guard stones
Guard  stones  of  size  300x  300x750mm  have  been  provided  in  cement  concrete  blocks  1:3:6 at 3m c/c on the downstream edge of top of dam.
6.5     Berms
Berms  have  been  provided  along  upstream  and  downstream  slopes  for  serving  the  following purpose:
•     For providing level surface for construction and maintenance of the dam
section.
•     For reducing the surface erosion in case of downstream slope and breaking the continuity of the slope.
•     To protect the lower edge of the rip rap and for preventing it from undermining 
in case of the upstream slope
Five meter wide berms have been  provided  at vertical elevation  of 1369m both  on upstream and  downstream slopes.  A 6.0  m wide berm at top  elevation  of rock  toe has also been  provided  on  the downstream slope of dam.  To  prevent rain  water from flowing  over the outer edge of berm, the surface of berm is given a cross slope of 1 50 towards the inner edge. A gutter is provided at the inner edge to cany rainwater to one side of valley where a vertical gutter is provided to lead the water to river channel downstream
6.6     Impervious Core
A centrally located core has been selected for Erer Dam as it has the advantage of providing higher pressure at the contact between the core and the foundation, thus, reducing the possibility of leakage and piping The top level of the core is generally fixed at 0.5 to 1 meter above MWL to prevent seepage by capillary siphoning. In case of Erer dam, the MWL being 1382.6m, the top level of the core is fixed at EL 1383 60m
Annex A : Dam Final Detail Design Report/ 2009
23Ercr Dam & Irrigation Project ZCECE & CES______________________________________________ The minimum top  width  of the core as specified  by  Indian  standards is 3  m.  Since availability  of suitable impervious material is not a constraint,  a width  of 5m has been adopted  at top  of the core with  upstream and  downstream slopes of 1H 1V.  The impervious soils to  be placed  in  impervious core should  not be highly  compressible and  the liquid  limit should  not be very  high  as such  soils are prone to  swelling  and  formation  of cracks.  Soils having  organic content are also  not suitable.  Borrow area investigations carried  out for sourcing  different construction  materials have identified  the clayey  sand,  sandy  clays of medium plasticity which are suitable for impervious core.
6.7      Shell Zones
It is proposed  to  use sandy  gravel material in  shell zones upstream and  downstream of clay core.  It is desirable that material should  be well graded  so  that it may  be compacted  to high  dry  unit weight of the order of 19.0  kN/m3.  The compacted  soil in  this zone should have an  average relative density  of 75% but the soil compacted  to  RD less than  70% in  any layer shall not be acceptable as the soil with  RD lower than  70% stands a high  risk  of liquefaction during a strong earthquake
The material in  upstream shell zone,  above the minimum draw down  level,  is required  to be fairly  free draining  (k  > 5  x  10  3) so  as to  allow rapid  dissipation  of pore water pressures.  This requirement arises to  ensure stability  under seismic loading.  It is, therefore,  being  provided  in  the scheme of zoning  that the material on  upstream shell above MDDL  (zone 2) should  not have silt content more than  6%.  For the material in upstream shell below MDDL  and  downstream shell,  the limitation  of silt to  6% shall not be binding.
The field  investigations of borrow areas indicate that it should  be possible to  get sandy gravel material with  silt less than  6% by  selective borrowing.  In  fact,  the gradation  tests on  the two  samples of sandy  gravel soil collected  from the selected  borrow areas show very low percentage of silt fraction.
The material in  upstream shell zone,  above the minimum draw down  level,  is required  to be fairly  free draining” (k  > 5  x  10  "3) so  as to  allow rapid  dissipation  of pore water pressures.  This requirement arises to  ensure stability  under seismic loading  It is, therefore, being provided in the scheme of zoning that the material on upstream shell
Annex A : Dam Final Detail Design Report/ 2009
24I
LErcr Dam & Irrigation Project ZCECE & CES______________________________________________ above MDDL (zone 2) should not have silt content more than 6%. For the material in 
upstream shell below MDDL, the limitation of silt may go upto 10% to 12%.
6.8      Cutoff
In the abutments the fresh rock is met with at a depth of 7m or so Due to presence of rock 
at relatively shallow depth, it is proposed to construct a positive cutoff trench in the
abutments from RD. 0.0m to RD 468.0m and from RD. 895.0m to RD. 1482m. As
discussed in the foundation treatment chapter , in the river bed portion I,e from RD
468.0m to RD. 1482m where the overburden is about 18m to 19m, a cutoff trench of 4m
deep and 6m wide at its base is suggested to facilitate the construction of plastic concrete
diaphragm wall and to keep the top of diaphragm wall within the trench as already 
explained
6.9  Rock Toe
The principal function  of rock  toe is to  facilitate drainage of seepage water Besides providing  drainage,  a rock  toe improves the stability  of the downstream slope.  It also protects the lower part of the downstream slope from tail water erosion.
Indian  standard  recommends the height of rock  toe to  be about 20  percent of hydraulic head  at that section  with  maximum and  minimum limits of 6m and  lm respectively. Accordingly  the rock  toe of height H/5  (6m) above the deepest riverbed  level of 1348  has been  provided  to  protect it against tail water erosion.  A 6m wide berm has been  provided  at the top  level of rock  toe (El 1354).  The rock  toe consists of stones of size usually  varying from 150mm to  200mm with  maximum stone size of 0.5m3 A filter is to  be provided between  the rock  toe and  the soil mass in  the dam as well as in  the foundation Accordingly  200mm thick  sand  layer overlain  by  200mm thick  gravel/ metal layer at the foundation level and between d/s shell has been provided
6.10    Toe Drain
An open toe drain is provided at the downstream toe of the dam in continuation'of
slope The bed  width  of toe drain  is kept as 0.6-lm and  depth  as 0.6m.  Thcsdcain  is paved 
_______________________________ '__________________ ____________ /< ^
'      \ V
Annex A : Dam Final Detail Design Report/ 2009
I
svm.r> ««-Erer Dam & Irrigation Project /CECE & CES____________________________________________ with stones with suitable gravel and sand layers of 0.2m thickness each so that sloughing of d/s slope is prevented.
6.11 Upstream Slope Protection
Erer dam being  an  earth  fill dam,  its upstream slope needs to  be protected  against destructive wave action As described in the in the design criteria the Dumped rock riprap is most effective and  durable Accordingly,  it is proposed  to  provide dumped  rock  riprap for upstream slope protection of Erer Dam
It is required that the rock for riprap should be hard, dense, and durable, and should be able
to resist long exposure to weathering In case of Erer dam, stone/rock fragment of good 
quality granite gneisses rock are available close to dam site The individual pieces of
dumped riprap need to be of sufficient weight to resist displacement by wave action 
USBR recommends that the maximum size of stones, for a reservoir fetch 2.5km or less
should be around 1150 kg. The thickness of the riprap needs to be sufficient so as to accommodate the weight and size of stone necessary to resist wave action. USBR
recommends a 1.0 m thickness of dumped riprap for major dams.
The design criteria recommended by Taylor relates the selection of rock size and thickness of the riprap  layer directly  to  the design  wave height.  Recommended  riprap  thickness for different ranges of wave height are as per Table 6 1
Table 6.1 Recommended Riprap Design Criteria
Maximum wave
Height (m)
Average rock 
size
D  (cm)
Maximum rock 
Size (kg)
Layer thickness 
(cm)
60
0-0.31
20
45
31
0.31-0.61
25
91
38
0.61-1.22
31
227
46
1.22-1.83
38
680
61
1,83-2.44
46
1134
76
2.44-3.05
61
1814
91
Annex A : Dam Final Detail Design Report/ 2009
26Erer Dam & Irrigation Project /CECE & CES
3
The criteria of minimum average rock  size imply  that the riprap  should  be composed  of rocks,  half of which  are larger than  the recommended  D30 size for given  wave height The rock  should  be well-graded,  from a maximum size of above 1.5  times the average size varying  down  2.5  cm spalls to  fill voids between  rocks and  to  provide a reasonable degree of protection to the underlying filter layer.
The  normal  size  of  rock  (D)  is  determined  assuming  the  rock  fragments  to  have  a volume between that of a sphere and a cube, or
%. D  = w/y
Where, y = Unit weight of stone in kg/m  and
w - weight of stone in kg
Considering y = 2lOOkg/m3
D = 0.08w1/3
Based  on  the  considerations  narrated  above,  the  thickness  and  gradation  of  dumped rock riprap for Erer dam, proposed to be adopted are as below:
3
•     Thickness of dumped rip rap 1.0 m
•     Maximum size of stones                2000 kg or 1000 mm
•     40 to 50 percent grater than           1000 kg or 800 mm
•     50 to 60%          45 kg- 1000 kgor 300 to 800mm
•     Oto 10% less than 45 kg or 300 mm
•     Sand and rock dust shall be less than 5 % by weight, of the total rip rap 
material
The  shape  of  the  rock  used  for  dumped  riprap  influences  the  ability  of  the  riprap  to  resist displacement by wave action At Erer dam, the riprap will mostly comprise of angplar
Annex A : Dam Final Detail Design Report/ 2009
27I
I
i
LErer Dam & Irrigation Project ZCECE & CES______________________________________________ fragments of quarried  granite rock,  which  provide better resistance to  displacement under wave action.
It is required  that the riprap  should  extend  from the crest of dam to  about 2.0  meter below the minimum draw down  level to  take care of the wave run  - down  with  the reservoir at MDDL.  Accordingly,  the riprap  for protection  of upstream slope has been  provided  upto EL 1369m from dam crest.
Since the shell material in  Erer dam will be sandy  gravel,  it is necessary  to  provide a blanket of graded  gravel underneath  the riprap.  Such  a blanket is necessary  for Erer dam to guard  against the danger of fines getting  washed  out through  the voids in  the riprap  by wave action.  The design  requirement for the filter blanket is that 15  percent of filter material should  be coarser than  50  mm for a graded  riprap.  Accordingly,  a 50  cm thick blanket of crushed  rock  or natural gravel graded  from 4.75  mm to  80  mm is being  provided below the dumped rock rip on upstream slope.
6.12 Down Stream Slope Protection
Downstream shell zone of Erer Dam being  composed  of sandy  gravel material,  the downstream slope needs to  be protected  against erosion  by  wind  and  rainfall runoff.  Such  protection  of downstream slope is generally  provided  either by  vegetative cover or by  a layer of rock  or cobbles.
Erer dam being  located  in  semi-arid  region,  reliance can  not be put on  protection  of downstream slope by  grass turfing.  Hence,  the downstream slope is proposed  to  be protected by a 30cm thick stone pitching.
Annex A : Dam Final Detail Design Report/ 2009
28Erer Dam & Irrigation Project ZCECE & CES
7.0 Stability Analysis of dam
7.1    General
The basic approach  in  design  of dam embankment has been  to  select a dam section  based on  the precedence of outer slopes in  dams adopted  for constructed  dams The selected section  is then  analyzed  by  the approach  specified  in  IS: 7894  “Code of practice for Stability  Analysis of Earth  Dams”.  The factors of safety  computed  for critical combination of external forces are compared  with  the minimum factor of safety  stipulated  in  the code. The outer slopes of the embankment are optimized  such  that the computed  factors of safety are higher than,  but close to,  the minimum desired  values under various loading  conditions Thus,  the pre-requisites for stability  analysis are selection  of test dam section  and  the engineering characteristics of materials to be used in various zones.
7.2      Material Properties Considered for Analysis
7.2.1 Material in Impervious Core
As described  under paragraph  7.1  above,  the material in  impervious core shall be clayey sand/sandy  clay.  Samples of such  soils collected  from identified  borrow areas have been 
tested at WWDSE laboratory. The test results are indicated in the Table 7.1:
Table 7.1: Properties of Clay Borrow Areas:
CLAY
BORROW
AREAS
Laboratory
Tests
EB-1
EB-2
EIP-11/06
EIP-
12/06
EIP-
13/06
EIP-14-16/06
Grain size
analysis
Clay •/.
5.00-35.00
17.00-29.90
16.50
27 00
25.00
9.00-41.00
Silt •/.
13.63-71.29
16.41-24.30
27.44
40.36
32.20
7.5-41.48
Sand %
7.71-81.37
40.10-63.59
56.06
32.64
42.80
17.52-83.5
Atterbere
limit
LL
24.27-50.80
33.00-48.90
39.37
47.69
49.67
24.26-46.35
PL
13.37-2626
14.19-24 29
16.19
26.59
26.27
12.63-23.77
PI
15.29-28.79
14.28-29.18
23.18
21.10
23.40
11.63-22.58
Procter test
MDD (Km/cc)
1.567-1.89
1.76S-1.823
i ■
OMC %
12.20-23.70
14.S0-16.80
Shrinkage
1.00-14.00
11.00-16.00
9.79
14.20
14.98
4.91-14 95
Annex A : Dam Final Detail Design Report/ 2009
29Erer Dam & Irrigation Project ZCECE & CES
limit
Free swell
35-75
40.00-60.00
13.50
14.00
13.00
11.00-14.90
Specific
gravity
2.52-2.70
2.61-2.65
Consolidation
0.083-0.134
Direct shear
C (Kpa)
43.67-92.67
48.67-83.30
*(□)
20.56-34.22
27.47-28.81
Permeability (cm/scc)
0.749-
3.97*10’7
Triaxial test
(UU)
C (Kpa)
16.01-26.67
14.79-32.42
0>(Q)
6.00-16.00
7.00-13.00
7.2.2   Material in Shell zones
The important engineering  properties relevant to  the stability  of dam slope are specific gravity,  dry  density  of compacted  soil,  permeability  and  effective shear parameters.  Theses parameters are presented below (table 7.2) for the materials under consideration
Table 7.2: Properties of Shell Material
Tests Conducted
ESHQ1 (%)
ESHQ2 (%)
Gravel (%)
58.91 -64.28
42.67-47.12
Sieve Analysis
Sand(%)
32.62-39 3
50.16-54 24
fines (%)
1 75-3.25
2.72-3.01
Direct Shear Test
Cohesion (kpa)
N.A
9-23.33
(J)(degree)
NA
37.05-37.25
Permeability
K (cm/s)
NA
0.16-2 35* IO'3
Since  the  sandy  gravel  soil  placed  in  shell  zone  is  cohesionless,  it  is  very  important  that  the material is compacted to fairly high relative density
The soil in  the shell zones is to  be compacted  to  achieve an  average relative density(RD) of 75%.  For the purpose of stability  analysis the compacted  unit weight of the soil has been taken  corresponding  to  70% relative density.  The values of dry  unit weight in  densest state C/dnux) and  dry  unit weight in  loosest state (yumin) have been  considered  as 16.4  and  20.2 kN/m  respectively. The corresponding value of dry weight for 70% relative density works
3
Annex A : Dam Final Detail Design Report/ 2009
30Erer Dam & Irrigation Project /CECE & CES______________ ____________________________________ out to  18.9  kN/m3.  The dry  unit weight of 18  kN/m3 has been  considered  for the purpose of stability analysis.
The value of shear parameters adopted  for sandy  gravel material in  shell zones are guided by  the laboratory  tests on  material in  selected  borrow areas which  are yet awaited.  The assumed  value of <U’ is taken  as 35° for stability  analysis.  Value of O’ for down  stream
shell   zone   material   is   assumed   lower   than   the   corresponding   value   for   u/s   shell   zone   2 
material  Thus,  for  the  purpose  of  stability  analysis  following  two  sets  of  values  of  O’  have been considered:
Set 1 
2 
Zone 2 35° 33°
Zone 2A 32° 30°
The permeability of shell material is generally found to vary from 10J to
102 cm/s. Accordingly the following of permeability have been assumed for stability analy sis
Zone 2 =            5x10“° cm/s
Zone2A= 10'4 cm/s
Based   on   above   considerations,   the   material   characteristics   as   per   table   7.3   have   been considered for stability analysis.
Table 7.3: Material Characteristics
Zone
Sp. Gravity
Unit Weight
3
(kN/m )
Permeability 
(cm/s)
C’
(kpa)
0
Degree
1 Core
2.64
17 9
3.9 x 10!
20
29
2 Casing
2.64
18.5
5x103
0
35/32
2a Casing
2.64
18.5
2.5x10'*
0
33/30
4 Filter
2.64
18.0
10’2
0
28
Sfoundation
2.65
1.6
io-5
25
20
Annex A : Dam Final Detail Design Report/ 2009
31LErcr Dam & Irrigation Project /CECE & CES
7.3      Loading Conditions
IS:  7894  states  that  stability  of  upstream  &  downstream  slopes  be  checked  for  following conditions which are usually found to be critical for the stability of an earth dam
Construction condition with or without partial pool (for U/s and D/s slopes)
•    Sudden drawn down (for U/S slope)
•     Steady seepage (for D/S slope)
•    Earthquake condition (for U/S and D/S slopes).
7.4      Minimum Factor of Safety
Minimum  desired  values  of  factor  of  safety  under  various  loading  conditions,  specified  by Indian Standard are as per Table 7.4
Table  7.4:  Minimum  Desired  Values  Of  Factors  Of  Safety  For  Various
loading Conditions
Case
No.
Loading condition
Critical
Slope
Minimum Factor of
Safety
I
Construction   condition   with   or without partial pool
U/S & D/S
1.0
II
Sudden   draw   downs   with   tail water at maximum
U/S
1.3
III
Steady    seepage    of    Dam    with reservoir full
D/S
1.3
IV
Steady     seepage     with     seismic loading
D/S
U/S
1.0
7.5      Stability Analysis
Stability'analysis and analysis for seepage through the dam body was said to be carried out at the feasibility  stage by  using  soft ware SLOPE/W and  SEEP/W developed  by  Geoslope, Canada*  The software uses the general limit equilibrium (GLE) formulation  for estimating the factor of safety  in  stability  of upstream and  downstream slopes for various loading conditions.
Annex A : Dam Final Detail Design Report/ 2009
321 JErer Dam & Irrigation Project /CECE & CES
The GLE  formulation  is based  on  two  factors of safety  equations and  allows for a range of inter slice shear normal force assumptions.  One equation  gives the factor of safety  with respect to  moment equilibrium (Fm),  while the other equation  gives the factor of safety with  respect to  horizontal force equilibrium (Ff).  The GLE  method  satisfies both  moment
and   force   equilibrium   by   finding   the   cross-over   point   of   the   F
m
and   Ff.   The   software
analyses    large    number    of    slip    circles    based    on    Bishop,    Janbu,    Morgenstem-Price    & Spencer methods and exhibits the minimum factor of safety for the critical failure surface.
Stability  analysis of Erer dam for steady  seepage state was carried  out by  considering  head pond  level at FRL  and  no  water at tail of the dam The stability  of downstream slope under steady  state was checked  with  and  without consideration  of seismic loading  The stability of upstream slope,  under steady  seepage state,  was checked  only  with  seismic force acting from right to left
For the stability  of upstream slope under sudden  draw down  condition  the derived  factor of safety  is dependent upon  the rate of reservoir draw down  and  the rate of dissipation  of pore water pressures.  In  case of Erer Dam,  the draw down  was considered  to  be taking  place from FRL to  MDDL,  by  flow passing  through  irrigation  outlet.  The discharging  capacity of irrigation  outlet being  limited,  the draw down  below spillway  crest level of 1379m to MDDL of 1371 m will be slow
The  construction  condition  represents  a  situation  when  the  dam  is  just  constructed  and  the pore    pressures    developed    as    a    result    of    dam    material    compression    are    only    partly dissipated.   The   residual   pore   water   pressures   depend   on   the   moisture   content   and   the compaction  efforts  deployed  during  construction  as  well  as  on  the  rate  of  rise  of  the  dam The  residual  pore  water  pressures  under  this  condition  are  considered  as  based  on  the  pore water.<pressure  ratio  R^,  a  coefficient  that  relates  the  pore  water  pressure  to  the  overburden stress. The pore water pressure u is given by:
u             =         Ru Yt H
s
s
Where, yi             =         total unit weight
H         =         the height of the soil      column Annex A : Dam Final Detail Design Report/ 2009
33Erer Dam & Irrigation Project /CECE & CES__________________________________________________ The value of the Ru considered in the analysis is 0.35.
The  stability  analysis  carried  out  on  adopted  section  was  found  to  be  safe  since  the  factor of safety values under all loading conditions were found to be within the permissible limit.
The stability  analysis of the dam section  as adopted  at feasibility  stage is also  carried  out with  the help  of ‘SLIDE’,  a software meant for slope stability  analysis of soil and  rock developed by Rocscience Inc, Canada The section is found to be safe.
The section of the dam finally arrived at is enclosed vide Fig. 7.1
The stability analysis results as carried out by SLIDE are attached vide Annexure-1.
Annex A : Dam Final Detail Design Report/ 20091
o
o
1
CD
J>
4
Axis of Dam
9C3O
▼ 1384.60
1348 0
3
:
—
40(\) --------------
Acceptable foundation grade
Silty Clay Graval
plastic
Concrete diaphragm wall
1)   Impervious Soil - SC.CL,
2)   Pervious to semipervious - SW, GP.GW.SP
2A) Semipervious - SM.GM
Chimney filter
Horizontal filter
Rock Toe
Dumped Rock Riprap
Blanket of crushed rock or gravel below dumped Riprap Hand placed Riprap
Key trench Toe drain
NOTES
1)
All dimensions are in millimeters
and levels in metres unless otherwise specified.
2
2)  For layout plan of dam refer relevant drawing
3) For detail of Crest,Riprap & Rock Toe refer relevant Drawing.
FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA MINISTRY OF WATER RESOURCES
PROJECT:
ERER IRRIGATION PROJECT DETAIL DESIGN■CT-*-Erer Dam & Irrigation Project ZCECE & CES
8.0. Geology along dam axis
Five boreholes have been drilled along dam axis. The exploration revealed overburden zone of about 19m below deepest river bed in the valley portion Out of this 18m, the top 
7m comprises of silty clay and sand mixed with gravel underlain by coarse gravel and cobble mixed with sand resting on crystalline gneisic rock.
In the abutments, the depth of overburden comprises of mainly silty clay, sand mixed with gravel and varies from 6m to 7m in general.
The geological profile along dam axis based on sub-soil exploration result is shown vide Fig. 8.1
Annex A : Dani Final Detail Design Report/ 2009
35IB
IB
lidaK
■
■-
:•
M
M
I
M
k
H
H
HELEVATION IN METER
elevation in meter
EAST LEFT BANK
VEST RIGHT BANK
1390 -----------
1385 _
1380
1375
1370
1365
1360
1355
1350
1345
1340
1335
1330
1325
132Q.
GROUNO LEVEL
(M) CHAINAGE
(M)
SILTY CLAY, SAND. MIXED WITH GRAVEL
FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA
MINISTRY OF WATER RESOURCES
WEATHERED GRANITIC GNEISS
COURSE GRAVEL. COBBLE MIXED WITH SAND
FRESH GRANITIC GNEISS
JOINTS SCHEMATICALLY SHOWN
SURVEYED
DESIGN
BY:
DATE
MagrJOW
DRAWN
KUAx? M.ttol
SCALE
K» - 1 . » vra - 1 • 9B
BY. P DATTE
REVISION nr
DETAIL
APHlOVtU
DY AMHWaSTE
DRAWING ML
3-1Ercr Dam & Irrigation Project ZCECE & CES
9.0 Foundation treatment
9.1      Surface Treatment
After the cutoff trench of the required depth is excavated and stripping of the entire seat of the dam is completed, all loose or objectionable material should be removed by handwork, barring, picking, brooming, water jetting, or air jetting. Accumulated water from washing operation  must be removed.  Loose or unsuitable material occurring  in  cavities ,  shear zones, cracks, or seams should be removed according to the following criteria.
•    For openings narrower than 50 mm, cleaning is to be done to a depth of three times the width of the opening.
•    For openings wider than 50 mm and narrower than 1.5 m cleaning of the opening is to be done to a depth of three times the width of the opening or to a depth where the opening is 12 mm wide or less, but not to a depth exceeding 1.5 m
Dental concrete is to be used to fill potholes and grooves created by bedding planes and other irregularities such  as previously  cleaned  out shear zones,  large joints,  or buried channels.
Comparatively fine cracks are to be treated with slush grout. Slush grout is a neat cement grout or sand - cement slurry used to fill narrow surface cracks But it should not be used to cover exposed areas of the foundation. The maximum particle size in the slush grout mixture should be no greater than one third the crack width. The consistency of the slush grout mix may vary from a very thin mix to mortar as required to penetrate the cracks.
Before the placement of earthfill starts, the entire seat of the dam shall be compacted to increase density of the foundation soil as far as practicable. Earthfill placement procedures should ensure that specified earth materials are forced into intimate contact with the foundation surface and that adequate density is obtained Hand tamping compactors may 
, i
be  used  to  compact earthfill  in  or against irregular surfaces or abutments, potholes and depressions not accessible by heavy compaction equipment
Annex A : Dani Final Detail Design Report/ 2009
36!Erer Dam & Irrigation  Project ZCECE & CES The abutments shall have to be suitably shaped and prepared in order to get good contact between  the impervious core of the embankment and  foundation.  Overhangs should  be removed.  Flattening  of the slopes is very  much  necessary  because of the fact that differential settlement of the embankment may lead to tension along the upper portion of the dam and  possible cracking  along  the longitudinal axis in  the vicinity  of the steep abutment slopes Further,  differential settlement along  the dam axis due to  different compressibility characteristics of compacted fill material with the adjacent foundation soils may  result into  transverse cracks in  the embankment which  can  lead  to  undesirable seepage condition.
9.2     Subsurface treatment
Subsurface permeability is found to be ranging from 2.16 x 10'5 cm/sec to 3.26 x IO"*’ cm I sec in top 7m stratum of silty clay, sand mixed with gravel indicating that seepage loss is not going to be much But no permeability test could be conducted in course gravel and cobble zone below above said  stratum in  the river bed  portion  where the permeability appears to  be quite high.  The partial cutoff with  upstream clay  blanket can  reduce the seepage but cannot stop  it completely  and  that may  result into  piping  It is therefore decided to go for positive cut- off
It may be mentioned here that high saturated nature of groundwater exist in the foundation very close to the surface in the river bed portion and due to that it did not become possible to conduct permeability test in most of the bore holes
In the light of the fact stated above and excessive depth of overburden in the river valley portion,  excavation  of positive cutoff trench  in  the deepest riverbed  portion  is found  not feasible due to  likely  construction  difficulties viz.  heavy  dewatering  requirements and instability of sides of excavation etc. 0.6m thick plastic concrete diaphragm wall is found to be ideal type of positive cutoff in this particular reach of river valley portion since it is flexible and capable to deform under stresses in the surrounding soil. The plastic concrete diaphragm wall will undergo  deformations compatible with  those in  the surrounding  soil without development of cracks. But since the dam falls in high seismic prone area, the top portion of the diaphragm wall will be more vulnerable to damage during earthquqkc It is therefore decided to provide a partial cutoff trench 4m deep below impervious core so as to
Annex A : Dam Final Detail Design Report/ 2009
37Ercr Dam & Irrigation Project ZCECE & CES_______ ______________________________________ keep the top of the diaphragm wall within impervious backfill of partial cutoff trench 2m below the ground level This will facilitate river diversion as planned through a temporary channel taken  through  river bed.  A cover of plastic clay  shall however to  be provided around the top of diaphragm wall to safeguard against any possible damage to the wall or to impervious core. This cover of clay shall consist of relatively high plasticity with its liquid limit more than 50 The bottom of diaphragm wall should go at least 1.0m into rock.
In the abutments, where the overburden zone is only about 7m, positive cutoff trench is provided The positive cutoff trench should be taken lm below fresh rock and overlap the plastic concrete diaphragm wall by at least 3m on its both ends.
Depending on the water head of 34m at deepest river bed portion and overall depth of diaphragm  wall  being  19m  or  so,  no  further  grouting  of  foundation  rock  is  virtually required to be taken up in the fresh rock below diaphragm wall. A nominal 3m depth of curtain grouting is however provided in this reach also in continuation to that required in both the abutments. Details are shown vide Fig. 9.2
Annex A : Dam Final Detail Design Report/ 2009
38ELEVATION IN METER
NOTES
1. All dimensions are in mm.
2. Ground profile os based on geotechnical investigation.
3. All loose soil, rock fragment debris and plants/trees/shrubs shall
be removed. The depth of stripping is tentatively indicated to be 1m.
However, the depth of stripping shall be decided by the Engineer
incharge at the time of construction
4. The bottom level of cut-off trench is tentative based on giological profile.
The depth of cutting at any particular location may vary depending on strata
5. The center line of cut-off is aligned 4.5 m U/S of dam axis.
6. Curtain grouting below cutt-off trench shall be extended not less than half hydraulic head.
FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA ministry of WATER RESOURCES
CONCERT ENGINEERING ANO CONSULTING ENTERPRISE ASSOCIATION WITH
CONSULTlNO ENGINEERING SERVICE ( INDIA ) PVT. LTD
BROJECT;-
ERER IRRIGATION project detail design
TITVEs-
stcnoN along o** WOW,MC rouNoxnoH treatment
SURVEYED
RY:
DATE
DRAWN
SCALE
*'*» - i. *
__—______
D LSI GN
AITROVED
BY- FOATlt
BY- .Mt'-®11
HLSTDON NR
DHAWING NR
DETAIL
>1r '-j-.irTT r.fflUHErcr Dam &. Irrigation Project /CECE & CES
10.0 Internal Seepage Control Measures
10.1 Transition Filter I Inclined filter
The dam being zoned earth fill dam with central clay core , the transition filters need to be provided both in the upstream and downstream of clay core since the specified gradation criteria is not satisfied between the adjacent zones i,e shell and core. Transition filters help to  minimize failure by  internal piping,  cracking  etc that may  develop  in  the core or migration  of fines from core materials Wider transition  zone/ filters are recommended where cracks are likely to develop in the core due to differential settlement, earthquakes etc. and at the contacts with steep abutments. Considering the fact that the site falls in highly seismic prone area, 2m wide transition filter is suggested. The transition zone/ filter should be provided upto the top of core.
The  filter  material  to  be  used  for  drainage  system  shall  have  to  satisfy  the  following criteria.
Filter materials shall be more pervious than base material
• Filter materials shall be of such gradation that particles of base material do 
not totally migrate through to clog the voids in filter material and;
Filter material should help in formation of natural graded layers in zone of base soil adjacent to the filter by readjustment of particles.
I he following limits are recommended as per USBR criteria to satisfy filter stability criteria and to provide ample increase in permeability between bases and filter (at least 10 cm/sec). These criteria are satisfactory for use with filters of either natural sand and gravel or crushed rock
1- D \^°f ,he Fll,er
2) (j of the base material
5 to 40 provided that the filter does not
contain  more  than  5  percent  of  matenal  finer than 0.074mm.
Annex A : Dani Final Detail Design Report/ 2009
39Erer Dam & Irrigation Project ZCECE & CES
2.             D lhe FlI,er _ 5 or less
D ^Ie ^Jase materiai
The grain size curve of the filter should be roughly parallel to that of the base material.
In  context  to  the  design  criteria  of  filter,  it  may  be  mentioned  that  considerable experimentation has been performed by the U.S Army of Corps of Engineers and the
Bureau of Reclamation and others.
Somewhat  different  sets  of  criteria  are  given  by  each  of  these  authorities  Merits  and demerits of those are somehow inconclusive.
The criteria given  above is recommended  not only  for the reason  that it satisfy  filter stability criteria and provide ample increase in permeability between base and filler but also it is very simple to use for design of filters.
In context to design and construction of transition I inclined filter, one more thing is felt necessary to be added and that is about segregation. In spite of taking sufficient care in placing  of filter material,  segregation  sometimes cannot be avoided.  To  minimize segregation, filters should have relatively uniform gradation Filters with D^(F) less than about 20mm, do not generally segregate. Indian standard in this regard have worked out
certain relation between D10(F) and 
to avoid segregation as furnished in Table-9
below This may be followed as far as practicable while designing the range of the filters. Table 10.1: Limits of Dio(F) and D90 (F) for Preventing Segregation
SI No. (1)
DI0(F) Min (mm) (2)
D90(F) Max (mm) (3)
(1)
(2)
20
>)
<0.5
25
i>)
0.5-1.0
30
iii)
1.0-2.0
30
iv)
2.0-5.0
40
v)
5.0-10
50
vi)
10-50
60
Annex A : Dam Final Detail Design Report/ 2009
40Erer Dam & Irrigation Project ZCECE & CES
10.2   Horizontal Filter
It collects the seepage from the transition / vertical filter or from the body of the dam It also  collects seepage from the foundation  and  minimizes possibility  of piping  along  the dam seat. The horizontal filter is to be provided on stripped ground level. Depending on downstream topography, the slope of the horizontal filter towards the toe drain is to be decided.
The horizontal filter must satisfy the following three requirements:
•  Gradation must be such that particles of soil from the foundation are prevented
from entering the filter
•    Capacity of the filter must be such that it adequately handles the seepage flow
from both the foundation and the embankment
•    Permeability must be great enough to provide easy access of seepage water in 
order to reduce seepage uplift forces
The required  thickness of the horizontal filter is quite small theoretically.  But from practical consideration it is desired to provide here 1.0 m thick filter since the filter is to be designed  to  negotiate between  foundation  soil and  rockfill material The criteria for designing the horizontal filter will be the same as transition filter stated above
10.3   Special Seepage Control Measures at Contact with Steep Abutments
On the abutments, the seepage through the dam travel towards the foot of the embankment mainly through the transition/inclined filter At times there is concentration of seepage at the foot of the abutment if the length of such steep abutment is more. To prevent such concentration of seepage at the foot of the abutment, the impervious barrier of lm width with  thickness equal to  that of the horizontal filter may  be put at suitable spacing depending  upon  the length  of steep  abutment/ ground  The necessity  of providing  such barriers and their spacing etc. need to be decided by the site engineer after the final cutoff trench profile is arrived at, abutments are dressed and stripping below the seat of the dam is completed.
Annex A Dam Final Detail Design Report/ 2009
41Erer Dam & Irrigation Project ZCECE & CES
11.0 Dam Instrumentation
11.1 General
A simple scheme has been followed for instrumentation of Erer dam so as to evaluate the performance of dam during its operation and to verify the design assumption. To observe the behavior of dam and its foundation during construction and operation of the reservoir, the instrumentation is done to measure:
•    Settlements and movements
•    Pore pressures
• Seismicity
Internal Movement
The measurement of internal movement primarily  consists of measurement of vertical displacement and relative horizontal movements caused by the low shearing strength and the long term creep of the foundation or embankment material. Various instruments, using different techniques,  for measurement,  like cross arm settlement system,  base plate settlement system,  inclinometer system with  biaxial probe,  digital inclinometer etc,  are available for measuring vertical & lateral settlement of embankment and foundation
External Movement
External movement of the dam crest or the side slopes accompanies internal movement of embankment and/or foundation. External vertical and horizontal movements are measured on the surface of embankments through the use of level and position surveys of reference point Reference points may be surface settlement points installed on the dam crest, slopes or toe of the embankment. Measurement of external movement of these surface settlement points is important for construction control as well as in monitoring the performance of the dam
Annex A : Dam Final Detail Design Report/ 2009
42Erer Dam & Irrigation Project ZCECE & CES______________ _______ _______________________ Seismicity
Surveillance of seismic environment of the project site needs special attention 
Seismographs and accelerographs to record micro seismic activities and strong ground 
motion can be installed at Harar meteorological observatory. No separate instrumentation 
is proposed at dam section to record seismic activities.
Reservoir  and  tail  water  levels,  wave  height,  evaporation  and  rainfall  are  some  of  the  other parameters, which require to be separately monitored.
11.2          Instrumentation Scheme
11.2.1 General Scheme
The general scheme of instrument in  Erer Dam provides for measuring  pore water pressures and  internal movement at three cross-sections of dam embankment The three cross-sections are located  at distances 514m,  834m and  1266m respectively  from left bank
The   installation   of   monitoring   instruments   shall   be   carried   out   with   great   care   under   the supervision /guidance of instrument manufactures.
11.2.2  Measurement of Pore Water Pressures
Pore water pressures in  dam and  foundation  are proposed  to  be measured  by  means of vibrating  wire electrical piezometers.  Vibrating  wire piezometers work  on  measuring  the change in  natural frequency  of a stretched  wire fitted  in  the transducer.  The water pressure existing  at the point of piezometer tip  is transmitted  to  the circular membrane.  The deflection  of membrane changes the tension  of the wire,  which  in  turn  changes the resonant frequency  of the wire The change in  resonant frequency  of wire is read  by  means of a read out unit and is a measure of pressure change.
’It is proposed to install 23 Nos. vibrating wire piezometer^ in the dam foundation. Sixteen piezometers are installed in middle cross-sections at chainage514m and 834m while 7 piezometers are installed at ch. 1366m on the right bank of the river at the irrigation outlet. 
The pore water pressures within dam embankment shall be measured by means of 27 Nos piezometers, 10 each at the middle sections and 7 at the section on the right bank at irrigation outlet
Annex A : Dam Final Detail Design Report/ 2009
43Ercr            Dam            &            Irrigation            Project            ZCECE            &            CES
The   position   of   the   three   instrumented   sections   and   the   location   of   piezometers   at   these sections are shown In the relevant instrumentation drawings.
11.2.3   Measurement of Internal Movement
It is proposed  to  use inclinometer system with  biaxial probe for measurement of vertical and  lateral displacement of dam embankment.  A total of 4  inclinometers are proposed  to  be installed, along the dam axis at ch.514m and 834m.
Internal horizontal movement of the embankment is proposed  to  be monitored  by  means of horizontal movement gauges,  one each  at the two  instrumented  sections in  the river channel portion
The   position   of   inclinometers   and   horizontal   movement   gauges   is   shown   on   the   drawings of dam instrumentation
11.2.4   Measurement of Surface Movements
Surface movement of dam embankment shall be monitored  by  means of a number of surface settlement points,  installed  at dam crest The surface settlement points shall be installed on upstream face of the dam only above MDDL.
A  surface  settlement  point  shall  be  made  of  150  mm  long,  25  mm  dia,  steel  rod having  a  cross  mark  engraved  at  the  top.  The  settlement  points  on  lop  of  the  parapet wall  will  be  made  of  25  mm  dia,  150  mm  long  steel  rod  welded  to  a  225mm  x  75 mm   x   6   mm   steel   plate   The   surface   settlement   points   shall   be   installed   on   the slopes  near  the  crest,  at  the  berm  levels  and  at  the  heel  &  toe  of  the  dam  at  chainage 600m, 800m & 1000m respectively.
The settlement shall be monitored  using  a Total Station  at regular intervals.  The first reading  acts as a reference and  the subsequent readings give the settlement of the surface with respect to the time.
Annex A Dam Final Detail Design Report/ 2009
44wErcr Dam & Irrigation Project ZCECE & CES
12.0 Bill of Quantities, Cost Estimates and Tentative Construction Schedule
12.1 Dam Appurtenance Works
The Quantities of the various items of works related  to  the Erer dam 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  the  quantities  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  36  calendar  months  as  shown  in  the annexure.
Annex A : Dam Final Detail Design Report/ 2009
45IErer Dam & Irrigation Project /CECE & CES
REFERENCE
1.   FDRE, Ministry Water Resource, Design guide line for Small and Medium embankment dams (Part I-E) ,2001
2.   Indian standard: IS 8826; Guide lines for Design of large Earth and Rain fill dams.
3.   William P.Creager Engineering for Dams, 1995
4.   Water works Design and Supervision Enterprise in Association with Synergic Hydro (India) Pvt. Ltd, September 2007, Feasibility Report, Erer Irrigation Project, Volume 6
5.    USBR: Design of small Dams, 1974
Annex A : Dam Final Detail Design Report/ 2009
46Annexure -1
•  Design Parameters
•  Abstract of results for Stability Analysis
•  Stability Analysis Sheets 7 sheets
•  Seepage Analysis - 1 sheetJ
_______________________________________ I
ID [Task Name
M1 ; M2 [M3 LIVM-L^ J
5    M6 I M7 ] M8 I M9 iMIOjMt 1 !M12|Ml3|M14[M15|M16iM17iM1BlM19]M20jM2l|M22iM23lM24|M25
16
5.2 CONSTRUCTION OF OGEE PART
19
5.3 CONSTRUCTION OF SLAB
_______________
20
5.4 CONSTRUCTION OF WALL
21
5.5 BACK FILL
22
6.0 MISCELLANEOUS WORK
Project. ERER DAM & IRRIGATION M1= MONTH ONE
Mllcslono
Summary
External Tasks
External Milestone
Progress
Project Summary
Deadline
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I—Il......... ...Design Parameters
S. No.
Zone
Sp Gr
Unit Weight
(kN/m )
3
Permeability
(cm/s)
C’
O’
Remark
1.
Core
2.64
17.9
3.9 x 10’8
20
29
2.
Casing 2
2.64
18.5
5.0 x 10'3
00
32
3.
Casing 2a
2.64
18.5
2.5 x 10^
00
30
4.
Filter
2.64
18.0
1.0 x 10'2
00
28
5.
Foundation 1
2.65
16.0
1.0 x IO’5
25
20
Depth from
1.0  m  to  6.0 m          below Ground Level
6.
Foundation 2
2.65
16.0
1.0 x 10’5
00
38
Depth  from 6.0  m to  11.0 m below Ground LevelAbstract of results for Stability Analysis
Cases
Condition
Factor of safety
Permissible
Factor of
safety as per
IS 7894 -
1975)
Case I
End                   of Construction
(i) U/S
(ii) D/S
2.075
1.475
2.040
1.467
2 092
1.476
1.0
1.0
Case II
U/S           Steady Seepage
1.788
1.607
1.672
1.5
Case III
Sudden Drawdown (for
U/S)
1.814
1.673
1.746
1.3
Case IV
Steady    Seepage (for D/S)
1.509
1.515
1.512
1.5
CascV
Steady Seepage with Earth Quake
(i) U/S
(ii) D/S
1.009
1.050
—
1.004
1.011
1.051
1.0
1.0
1
Bishop
Simplified
Jnnbu
Simplified
Jnnbu
CorrectedSafety     Factor
Case I. End of Construction US
I
0.000
0.250
0.500
0.750
1.000
1.250
1.500
1.750
2.000
2.250
2.500
2.750
3.000
3.250
3.500
3.750
4.000
4.250
4.500
4.750
5.000
5.250
5.500
5.750
6.000
o____Z___ $0 T_______ i»_________ jg____2____ __________ W
-T"---------- -------------------1------
&ISafety      Factor
0.000
0.250
0.500
0.750
1.000
1.250
1.500
1.750
2.000
2.250
2.500
2.750
3.000
3.250
3.500
3.750
4.000
4.250
4.500
4.750
5.000
5.250
5.500
5.750
6.000
Case I. End of Construction DS
ft100 m
Safety      Factor
Case II. Steady Seepage US
a
8-
0.000
0.250
0.500
0.750
1.000
1.250
1.500
1.750
2.000
2.250
2.500
2.750
3.000
3.250
3.500
3.750
4.000
4.250
4.500
4.750
5.000
5.250
5.500
5.750
6.000Ig-
?-
8^
Safety Factor
0.000
0.250
0.500
0.150
1.000
1.250
1.500
1.750
2.COO 2.250
2.500
2.750
3.000
3.250
3.500
3.750
4.000
4.250
4.500
4.750
5.000
5.250
5.500
5.750
6.000
Case IV. Steady Seepage DS
—r~
------------- r-'
’-r-
T----------------- -   . •  ------------------ r"----------- --------- r~i-------------------------- ■------------- 1----------------- -r ’----------------------- i---------------------- i----------------------------------- -v •  ----------------r-
-100m
P__________ 50100________________________________ 150 200_
250 300
350*oor
Safety     Factor
g
Case III. Sudden Drawdown (for US)
8-
8-
0.000
0.250
0.500
0.750
1.000
1.250
1.500
1.750
2.00C
2.250
2.50C
2.750
3.000
3.250
3.500
3.750
4.000
4.250
4.500
4.750
5.000
5.250
5.500
5.750
6.000
8-
►
—I-----
400 m
’-T"
-50
150
200
.250
300_____________ 350■
■
In
n
IISafety Factor
0.000
0.250
0.500
0.750
1.000
1.250
1.500
1.750
2.000
2.250
2.500
2.750
3.000
3.250
3.500
3.750
4.000
4.250
4.500
4.750
5.000
5.250
5.500
5.750
6.000
I ► 0.15
r*
’    ▼ 0 075
■ r- ------------------- ;
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"I........................... ..... ;----------------------------------------- 1------------ i - ■ -
—r — ------------------------ — —,—f-r
0
50
100
150
200
250
300
.350B
B
B
B
In
B ISafety Factor
0.000
0.250
0.500
0.750
1.000
1.250
1.500
1.750
2.000
2.250
2.500
2.750
3.000
3.250
3.500
3.750
4.COO 4.250
4.500
4.750
5.000
5.250
5.500
5.750
6.000
I « 0 15 I ▼ 0 075
Case V. Steady Seepage with Earth Quake for US
50
100
150
300
350*
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Summery

Erer Dam Project Summary

Erer Dam & Irrigation Development Project - Final Design Summary

Project Overview

The Erer Dam is a major storage dam across the Erer River in Ethiopia, designed primarily for irrigation development. The project includes:

A 36.6m high zoned earthfill dam (1460m long)
Irrigation outlet with intake tower
Chute spillway with ogee crest
River diversion works

Key Design Parameters

Parameter	Value
Dam Type	Zoned Earthfill Dam (selected due to seismic conditions and foundation characteristics)
Location	6km upstream of Harar-Jijiga road bridge, 25km from Harar
Seismic Zone	Zone IV (horizontal seismic coefficient = 0.15)
Diversion Design Flood	50-year return period (151 m³/s)

Reservoir Levels

Level	Elevation (m)	Description
Dam Crest	1384.6	Top of embankment with 1m parapet wall
Maximum Water Level (MWL)	1382.6	For PMF condition
Full Reservoir Level (FRL)	1379.0	Normal operating level
Minimum Draw Down Level	1371.0	Below irrigation outlet centerline
Dead Storage Level	1369.85	After 50 years sedimentation

Dam Components

Crest: 9m width with 36.6cm camber
Core: Central clay core (5m top width, slopes 1H:1V)
Shell Zones: Sandy gravel material (upstream and downstream)
Slope Protection:
- Upstream: 1m thick dumped rock riprap with filter blanket
- Downstream: 30cm stone pitching
Filters: Transition and horizontal filters to control seepage
Foundation Treatment: Cutoff trench (4m deep in riverbed), plastic concrete diaphragm wall

Stability Analysis Results

Loading Condition	Factor of Safety	Minimum Required
End of Construction (U/S)	2.075	1.0
End of Construction (D/S)	1.475	1.0
Sudden Drawdown (U/S)	1.814	1.3
Steady Seepage (D/S)	1.509	1.5
Earthquake Condition	1.009-1.051	1.0

Construction Schedule

The project has a tentative construction period of 36 months, with key activities including:

Site preparation and foundation treatment
Embankment construction with diversion channel
Spillway and outlet works construction
Final closure of diversion gap

Instrumentation

The dam will be equipped with monitoring instruments including:

Vibrating wire piezometers (50 total)
Inclinometers (4 nos.)
Surface settlement points