Theta Earth Dam

Athi Water Services Board

Gatundu South Water and Sanitation Company

Design of Theta Weir

Eng. P. Njurumba

September, 2010

ATHI WATER SERVICES BOARD

THETA WEIR IN GATUNDU SOUTH DISTRICT

GATUNDU SOUTH WATER AND

SANITATION COMPANY

DESIGN REPORT AND DRAWINGS

Consultant: Client:

Eng. Peter Njurumba Chief Executive Officer, P.O. Box 212 00206, Athi Water Services Board, KISERIAN Africa Re Centre, Hospital Road, September, 2010 NAIROBI




Eng. P. Njurumba

September, 2010

1 EXECUTIVE SUMMARY

In mid eighties and early nineties, parts of Kiganjo, Kiamwangi and Ngenda used to be supplied

with water by Thiririka Water Project which was constructed by the Director Labour Section of

the then Ministry of Water Development. Other water supply schemes that were constructed by

the Ministry within the vicinity of this area were Ndarugu, Karimenu and Juja. The projects

served the communities for a while, but as populations exploded with time and water demand

parameters rose, the water demand outstripped the supply thereby necessitating water rationing

wihin the supply areas. This situation called for quick intervention measures which included

tapping water from the pipeline destined for Nairobi. Some of the major centers served by

pipeline were Gatundu Town and Ichaweri and the communities along the corridors of the

pipeline. At the same time, the water demand in the city kept on increasing against decreasing

water supply parameters caused mainly by fluctuating water levels in the river courses caused

primarily by prolonged dry spells and climate change. This called for quick disconnection of the

areas that were being served by the pipeline which in return aggravated the water supply

situation.

Against the above background and in order to meet the water demand, Gatundu South Water and

Sanitation Company which is the local Water Service Provider (WSP) appointed by the Athi

Water Services Board conceived the idea increasing storage to cater for the rural population and

thus achieve its objective. This policy was to be achieved by way of constructing Theta Dam that

would regulate the flows of the river and thus enhance water supply to the demand areas. In this

connection, the Client has procured a consultancy service which is aimed at carrying out the

necessary studies in terms of feasibility study, preliminary and final designs and preparation of

tender documents. The client was supposed to carry out an environmental impact assessments of

the project with a view to determining its technical, economic and environmental viability.

Athi Water Services Board through its Water Service Provider namely Gatundu South Water and

Sanitation Co. Ltd commissioned a Consultant to carry out the design of Theta Dam in Gatundu




Eng. P. Njurumba

September, 2010

South District which is one of the areas within the jurisdiction of the board. The other areas

under the mandate of the board are Nairobi, Thika, Gatundu North, Lari, and Limuru.

As a first step towards the design f the project, the consultant visited the site together with

officers from Gatundu South Water and Sanitation Office and identified a suitable dam site

within the Kikuyu Escarpment Forest just about 11 km from Mundoro Town Center.

Engineering considerations have shown that the dam is 17m high and it can command a storage

capacity of about 1,170,000m3.

Water Resources Assessment

The reservoir is located in an area that receives about 1015mm and 1270mm annually, with a

mean value of 1180mm per annum. From the flow duration curve, the 50% flow is about

0.23m3/sec which translates to 19,872m

3/day. From the envelop curve, the flow for the same

return period is equal to Kikuyu Escarpment Forest. The Q(80) and Q(95) flows are 17,194 m3/day

and 6,739 m3/day respectively. The raw water looks very palatable and clear and therefore

requires very little treatment. During the initial stages, chlorination may be the only form of

treatment required while a conventional water treatment plant for a capacity of 8,000m3 per day

may be constructed in future.

Geotechnical and Geological Study

Theta Dam is located in the thicket of the Kikuyu Escarpment Forest where there are good soils

for the construction of an earthfill dam. However, in view of the height of the dam which is 15m,

the dam enters into the category of large dam according to the classification of dams by the

World Commission on Dams. In this regard, the design work has to be stringent especially in the

area of eradication of seepage under the foundation and embankment wall. In order to minimize

seepage through the foundation, pressure grouting with cement and bentonite is essential while




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September, 2010

seepage through the embankment wall will be controlled through good workmanship in terms of

quality control during construction.

Estimated Project Costs

The estimated cost of the project is Ksh. which covers the construction of the

river diversion system, embankment wall with the filters and riprap material, open channel

spillway together with the energy dissipater, intake tower and the draw off system.

Conclusion

The study shows that the construction of the proposed dam is technically and economically

feasible since it will alleviate the water scarcity situation in the supply areas and hence enhance

water and sanitation services as well as social economic development and, therefore, improve the

living standards of the local communities.




Eng. P. Njurumba

September, 2010

TABLE OF CONTENTS

Design of Theta Weir .......................................................................................................... 2 1. Background Information .............................................. Error! Bookmark not defined.

2. Location ......................................................................................................................... 7 3. Sediment Loads ............................................................ Error! Bookmark not defined. 4. Engineering Design and Construction of Theta Weir .. Error! Bookmark not defined. 5. Scope of works ............................................................. Error! Bookmark not defined.

5.1 STAGE I ................................................................ Error! Bookmark not defined.

5.1.1 Desk Study ................................................... Error! Bookmark not defined. 5.2 STAGE II ............................................................... Error! Bookmark not defined.

5.2.1 Field trip to the Selected Site. ...................... Error! Bookmark not defined.

5.2.2 STAGE III .......................................................... Error! Bookmark not defined. 6. Considerations made during the Studies ............ Error! Bookmark not defined.

i. Geology of the area .................................................... Error! Bookmark not defined.

ii. Topography ............................................................ Error! Bookmark not defined. iii. Reservoir area ........................................................ Error! Bookmark not defined. iv. Location of weir with respect to the Consumers ... Error! Bookmark not defined.

7. Results of the Studies ........................................... Error! Bookmark not defined. 8. Site Visits ............................................................. Error! Bookmark not defined.

9. Selected Site ......................................................... Error! Bookmark not defined.

10. Design of the Weir ............................................... Error! Bookmark not defined.

11. Diversion of Flows ............................................... Error! Bookmark not defined. 12. Design of the Weir ............................................... Error! Bookmark not defined.

13. Background Information ...................................... Error! Bookmark not defined. 14. Location ................................................................ Error! Bookmark not defined. 15. Catchment area ..................................................... Error! Bookmark not defined.

16. Geology ................................................................ Error! Bookmark not defined.

17. Foundation ............................................................ Error! Bookmark not defined. 18. Reservoir Characteristics ..................................... Error! Bookmark not defined. 19. Reservoir characteristics ...................................... Error! Bookmark not defined. 20. Flood Estimation .................................................................................................. 19

21. Proposed Weir ...................................................... Error! Bookmark not defined. 22. Design of the weir ................................................ Error! Bookmark not defined. 23. Spillway ................................................................ Error! Bookmark not defined.

24. Determination of the Base Width of the Dam ...... Error! Bookmark not defined. 25.0 Stability Analysis .............................................. Error! Bookmark not defined. 26. Coordinates of the O-Gee Concrete Spillway ...... Error! Bookmark not defined. 26.1 Hydraulic Jump ................................................ Error! Bookmark not defined. 27. Invert Radius ........................................................ Error! Bookmark not defined.

28. Stability calculations ............................................ Error! Bookmark not defined. 29. River Diversion .................................................... Error! Bookmark not defined. 30. Priced Bill of Quantities ....................................... Error! Bookmark not defined.




Eng. P. Njurumba

September, 2010

31. Estimated Construction cost ................................. Error! Bookmark not defined.

DESIGN OF THETA DAM

CHAPTER 1.0: INTRODUCTION




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1.1 GENERAL BACKGROUND

Athi Water Services Board is one of the Eight Water Service Boards established under the Water

Act 2002 and it re[orts to the Ministry of Water and Irrigation. Its principal mandate is efficient

and economical provision of water to the consumers through the Water Service Providers. In the

case of this project, the Water Service Provider is Gatundu South Water and Sanitation Company

Limited with its headquarters in Gatundu Town which is also the headquarters of Gatundu South

District.

1.2 PROJECT BACKGROUND

Gatundu South areas of Mundoro, Kiganjo, Kiamwangi, Ngenda and Gathage have been

experiencing water shortages for quite some especially after the yields of the sole sources of

water started diminishing due to prolonged dry spells while at the same time, the population

went on soaring thereby increasing the water demand. In its pursuit to efficiently provide water

and sanitation services to the communities, Athi Water Services Board, therefore, proposes to

construct on river Theta a regulating reservoir which would regulate the river flows and thus

make water available in sufficient quantities. In cognizance of this fact, the Board procured

consultancy services to design the dam and thus realize its objective.

1.3 SCOPE OF STUDY

In April, 2010, Athi Water Services Board through its Water Service Provider namely Gatundu

South Water and Sanitation Company Limited commissioned the design of a weir on Theta

River. Following the completion of the design, it was established that the storage capacity of the

created reservoir is rather small about 30,000m3 and it would not help achieve the intended

objective of providing water to the consumers in sufficient quantities.

Against the above background and in an effort to increase the storage capacity of the reservoir,

the Board decided to construct a 15m high earth dam across the same axis. However, in both




Eng. P. Njurumba

September, 2010

cases the Consultant was guided by the Terms of Reference that were drawn for the project and

these included but not limited to the following:

i. Exploration of Theta River within 2 km upstream and downstream with a view

to establishing the best dam site that can have maximum impacts in terms water supply to

the needy areas.

ii. Carry out flood routing through the reservoir so as to indicate the efficiency of

the reservoir.

iii. Taking into account the findings in (i) and (ii), identify the best alternative that

can meet the objective of the assignment.

iv. Collection, compilation and analysis of data for use in the design of the dam.

v. Carrying out topographical survey within the dam reservoir and spillway areas

to enable preliminary and final design of the dam.

vi. Design the dam identified under item (iii) above including the intake tower.

vii. Environmental Impact Assessment studies within the project area.

viii. Preparation of Tender Documents which shall contain Invitation to Tender,

Instructions to Tenderers, Bills of Quantities, Drawings, Specifications, Conditions of

Contract, Schedules, Pre-qualification Documents, etc. for the dam.

1. 4 PREVIOUS STUDIES

The project area had not been subjected to many studies except the hydrological assessment

study which was done in 2009, and a report of which is attached as an Appendix.




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September, 2010




Eng. P. Njurumba

September, 2010

CHAPTER 2.0: PROJECT AREA




Eng. P. Njurumba

September, 2010

2.1 PROJECT BACKGROUND AND LOCATION

The proposed Theta Earth Dam is located in Gatundu South District in Kikuyu Escarpment

Forest after the Kenya Wildlife Services Offices near Mundoro Town. The dam is on coordinates

S 00.92506o and E 036.71755 o on elevation 2265masl. It can be accessed from Nairobi

through the main road to Mundoro via Gathage, Ngenda, Kiamwangi and Kiganjo Towns

through a distance of approximately 80km. The dam is about 11km from Mundoro Market

Centre via an earth road which passes through the Kenya Forest Offices on the edge of the forest.

This roads farther ahead joins the flyover near Gwa Kanyua where the road to Njabini Market

Centre branches.




Eng. P. Njurumba

September, 2010

Figure 1: Proposed dam site on Theta River

Figure 1.2 Location of Theta District

2.2 CATCHMENT AREA

The catchment area for Theta Dam is 1.6km2 within the Kikuyu Escarpment Forest on 1:50,000

scale topographic map.




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The area rises from 2220 masl to a maximum of 2360 masl through a length of about 1.6km from

the farthest point to the dam site. The area has been well conserved with a good indigenous

forest cover which reduces the amount of erosion and subsequent generation of sediments. This

goes along way in increasing the economic lifespan of the dam.

The average slope along the catchment area is 2.5%.

2.3 RAINFALL

Theta Dam catchment has a bimodal type of rainfall. The long rains are received in the months

of April/March while the short rains occur between the months of October/November.

A meteorological station in Kieni Forest Station has been used to generate the flood flows which

are used for the design of the spillway of the dam.

2.4 GEOLOGY

The geology of this area comprises of basement systems which are mainly grits, sandstones,

shales and limestones that have been metamorphosed by heat and pressure or by impregnation by

pervading fluids. Other types are derived from lavas and volcanic fragmental rocks. The variety

of rocks is extensive and includes mica and mica hornblende gneisses and schists, pyrexinite,

granulites quartzites and marbles. There is also a considerable development of migmatites.

Detailed investigation or review of the geology of the dam site has not been done for this study

since the design is focusing rehabilitation works i.e. rising the existing embankment with the

existing foundation.

SOILS

The predominant soils are black clays (grumosolic soils) which consist of black cotton and

include the calcareous and non-calcareous variants. The adjacent area has rock outcrops that




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have been subjected to geological and accelerated erosion to an extent that they have lost their

original characteristics.

SOIL SAMPLING AND TESTING

Field investigations around the dam site were carried out in June, 2008 and involved geophysical

sounding, trial pitting and soil sampling.

GEOPHYSICAL EXPLORATION

A core trench was excavated along the dam and spillway axis. Similarly, trial holes were dug in

the borrow areas to determine the suitability of the soil materials for use as construction

materials. The materials looked fairly homogeneous and consequently, only two samples were

submitted to the laboratory for tests to establish the following parameters:




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Particle size distribution

Permeability

Atterberg limits

Linear shrinkage

Triaxial Test

Compaction test OMC (%) and MDD (g/cm3)




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September, 2010

The laboratory test results on the soil samples are shown in the table below:

Testing Summary Sheet (Theta Borrow Area)

Soil Classification Red Soils, Soil Type Silt of high plasticity

Conclusion: the test results indicate that the samples are suitable for the construction of the dam.

These material are available in sufficient quantities and this mean necessitate the construction of

a homogeneous dam with the necessary filters.

Soil tests Trial Pit 1 Trial Pit 2

Particle size:

Gravel %

Sand %

Silt %

Clay %

Permeability Test:

Hydraulic Conductivity, k,

(cm/sec)

Atterberg limits:

Liquid Limit (LL) - %

Plastic Index (PI) - %

Plasticity Index (PI) - %

Linear Shrinkage (LS) - %

Shear Strength Test:

Cohesion C (Kg/cm2)

Friction Angle ( o)

Compaction Test:

OMC - %

MDD g/cm3




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September, 2010




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HYDROLOGY AND ESTIMATION OF FLOOD FLOW




Eng. P. Njurumba

September, 2010

2.7 HYDROLOGY

Within the proposed location of the weir, there are no river gauging stations. However, a

hydrological analysis was done by Kibson Consult and the obtained flows along the river with

respect to the position of the proposed weir are presented in the table below:

Table of Flood Flows for various return periods

Return Period Flood flow m3/sec

50 year return period 0.23



2.8 ESTIMATION OF FLOOD FLOWS

The flood flow from the catchment area is a function of the size of the catchment area, its slope

and degree of catchment conservation which is represented through a run off factor, the values of

which are presented below:

Generalized values of run off factor1

Catchment soil type Run off factors (Kr)

Rocky and impermeable 0.80 to 1.00

Slightly permeable, bare 0.60 to 0.80

Slightly permeable, partly cultivated or covered with vegetation 0.40 to 0.60

Cultivated, absorbent soil 0.30 to 0.40

1Ministry of Water Development. Guidelines for design, construction and rehabilitation of small dams and pans in

Kenya. Kenya Belgium Water Development Programme, June 1992, Nairobi.




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Sandy bare soil 0.20 to 0.30

Heavy forest 0.10 to 0.20

The Theta catchment can be classified as well conserved with good forest cover and a run off

factor of 0.1 can be assumed. The catchment area is given as 1.6 km2.

The Mean Annual Precipitation (MAP) for Theta Dam is 1280mm per annum.

Based on a safe yield of 1,200 mm per annum, the catchment area of 1.6 km2 and the run off

factor of 0.1, the expected annual inflow into the reservoir will be 192,000m3. However, the fact

that Theta river is perennial gives an assurance that there will always be an inflow into the dam

equal to a Q(50) of 0.23m3/sec or 19,872m

3/day obtained as per the hydrological analysis.

The evaporation rates for Theta area are neglected due to the fact that the dam is in a fairly thick

forest where no significant winds are experienced.

Considering the above, replenishment of the reservoir is quite feasible and the Water Service

Provider will therefore realize huge water sales that will ensure the sustainability of the project.

RICHARDS METHOD

In estimating the expected flood flow for the purpose of designing the spillway, Richards

method and the rational formula were used.

The method is based on an empirical formula to calculate the time of concentration (Tc) of the

catchment. Richards method takes account of the rainfall pattern and intensity and the

catchment characteristics determining its run-off, size, shape and slope as well as soil and

vegetation type (the latter two collated into a run-off factor Kr).

Time of concentration Tc:




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afSRKLCTT rcc .../.1/

23

Where: Tc = time of concentration in hours

L = the longest path of the catchment in km (14km)

C = a coefficient function of (Kr.R)

Kr = run off factor (table 3.3)

R = rainfall coefficient; FttR ./1

F = total rainfall in mm for the selected storm duration t

t = selected storm duration (12 hours)

s = slope of the catchment (4%)

a = the area of the catchment in km2

f(a) = ratio of the average rainfall intensity (i) to the maximum rainfall intensity (I) over the

catchment area

Once the time of concentration Tc has been derived, the rainfall intensities are calculated as

follows:

)1/( cTRI (mm/hr)

and

)(. afIi (mm/hr)

Finally the rational formula is used to calculate the expected maximum flood flow

6.3/.. aiKQ rp (m3/s)

The rainfall intensities for return periods of 5, 10, 25, 50 and 100 years were obtained from the

Rainfall Frequency Atlas of Kenya. The rainfall intensity for 1000 years return period was

developed using Gumbels Type 1 External Distribution Values.




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September, 2010

The following table shows flood flows for different return periods

Return period Flood (m3/sec)

5 8.5

20 11.3

50 16.6

100 17.0

500 19.5

1000 21.8

Figure 6.1 PMF for Theta Dam

The flood flow return periods for each dam classification are given in the table 6.2 below.

Recommended return periods for the design of spillways

10 25 50 100 500 1000

5

10

15

20

25

Return Period - Years [Log scale]

Flood Discharge

m3/s

Theta Dam Probable Maximum Flood




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September, 2010

Class of Dam Minimum return period for spillway design

A (Low Risk) 1 in 50

B (Medium Risk) 1 in 100

C (High Risk) 1 in 500

However, in order to ensure maximum safety of the dam and also to take care of the

uncertainties that might have arisen due to limited hydrological data that is available for the

catchment area, a return period of 1 in 1,000 years is adopted.

GENERALIZED TROPICAL FLOOD MODEL

This model combines the merits of the East African Flood Model with the experience gained in

catchment modeling in West Africa and it is applicable in all areas of the tropics where locally

validated alternatives are not existing as is the prevailing situation in the area under study. It

involves determination of model coefficients where three components are established.

These are:

i. Hydrograph base time

ii. Contributing area coefficient

iii. Peak flow factor

After computations, the flood flows for various return periods are shown in the table below:

Return Period Flood flow in m3/sec

5 10.8

20 14.1

50 16.2




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September, 2010

100 17.7

500 21.3

1000 22.8

The flood flow for a 1,000 year return period is 22.8m3/sec but for design purposes, the flood

flow has been rounded to 25m3/sec.




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September, 2010

DAM EMBANKMENT DESIGN




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September, 2010

DESIGN OF DAM AND ANCILLARY WORKS

SELECTION OF DAM TYPE

Dams are classified according to the materials used for construction. Usually, they are made of

earth, rock, concrete and masonry. A combination of dams using earth materials at the abutments

and concrete in the middle to form the spillway section is another form of a dam called

composite structure, but it requires workmanship of a high standard because of the interface

between the soil material and concrete.

The choice of the material depends firstly upon the geology of the proposed dam site and

secondly upon the costs of the various alternatives. Concrete and masonry dams require a hard

rock foundation, but earth dams can be constructed on either rock foundations or on firm clays

and other sound strata not as hard as rock. Whatever the type of the dam used, the foundation

material below the dam must be either watertight, or capable of being made watertight by such

means as grouting. The dam, foundation and abutments must also be stable under all static and

dynamic loading conditions.

EARTH AND ROCK FILL EMBANKMENT DAMS

Classification of dams into either earthfill or rockfill category is determined by the construction

materials.

EARTH DAMS

Earth dams are composed of suitable material from borrow areas or required excavation and

compacted in layers by mechanical means and can either be homogeneous or zoned. Compaction

is done by means of tamping rollers, sheep foot rollers, heavy pneumatic rollers, vibratory

rollers, tractors or earth hauling equipment.




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ROCKFILL DAMS

Rock fill dams are composed largely of fragmented rock with an impervious core. The rock fill

layers are compacted in layers of approximately 250 mm thick by rubber-tyred rollers or steel

wheel vibratory rollers. Free- draining, well-compacted rock fill can be placed with steep slopes

if the dam is on rock foundation. In both cases, a core trench should be constructed for purposes

of minimizing seepage losses under the foundation.

Based on the topography of the dam site and the preliminary geophysical and geological study

results, it is recommended to construct an earth dam with a central impervious clay core since

this is the most economical and technically suitable type of dam considering the availability of

materials within the vicinity of the dam site.

DESIGN CRITERIA

The design of a structure should ensure a safe and economical section which in the case of an

earth dam is realized through optimization of the slopes.

The foundation, abutments and the embankment should be stable for all conditions of

construction and operations.

Other factors to be considered are as follows:

i. Seepage through the embankment, foundation and abutment should not result in

excess forces.

ii. Use of non dispersive to reduce piping

iii. The gross freeboard dam must be sufficient to prevent overtopping and to allow

for settlement of embankment and foundation.




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iv. The spillway and outlet works should be adequately sized to prevent

overtopping the dam crest.

v. The side and longitudinal slopes of the spillway must be stable.

vi. Embankment slopes should be stable under all conditions

The construction materials for the embankment wall will be excavated from the borrow areas

within the reservoir area and this will increase the storage capacity of the reservoir and overly

increase the suitability coefficient of the site.

THE PLATE BELOW SHOWS THE GORGE ACROSS WHICH THE DAM WILL BE CONSTRUCTED

SIZING THE DAM




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The estimated storage capacity of the reservoir is based on the topographic survey map done in

July, 2010 by Geosite Systems of Nairobi, using scale of 1:10,000.

SLOPES

The United States Bureau of Reclamation (USBR) as well as the Design Manual for Water

Supply Systems in Kenya 1986 Edition both recommend upstream and downstream slopes of

1:3.0 and 1:2.5 respectively during the preliminary design stages. However, these slopes have

already been optimized during the through slope stability analysis and they are stable.

CREST WIDTH, LENGTH AND DAM HEIGHT

According to the Ministry of Water Irrigation Design Manual, the minimum crest width of an

earth dam should be 5.0m especially to allow for access road and manipulation of machinery and

equipment during construction. The US Bureau of Reclamation recommends a minimum crest

width of 3m. The crest should also be sloped to promote drainage and minimize surface

infiltrations. It should also be cambered so that the design freeboard is maintained after post

construction settlement takes place.

The proposed Theta Dam is a homogeneous earthfill embankment wall with a crest width and

length of 10 m and 60 m respectively. According to the contour survey, the crest of the dam is on

2251 masl while the bed is on 2234 masl thereby giving a 17m high embankment wall.




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SPILLWAY DESIGN




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DAM FREE BOARD

Spillways are provided to facilitate release of surplus water or floodwater that cannot be

contained in the available storage space. The importance of safe spillway cannot be over-

emphasized; many failures of dams have been caused by spillways of insufficient capacity.

Reservoir spillways are therefore major hydraulic structures during dam construction and later in

the operation and regulation of the dam. Spillways are designed to discharge the largest possible

flood during the design life of the dam for a selected return period.

Guidelines for spillway design floods in Kenya are presented in three design manuals produced

by the Ministry of Water and Irrigation (MoWI).

i. Guidelines for the Design, Construction and Rehabilitation of Small Dams and

Pans are used for dams of heights not more than 10m.

ii. Practice Manual for Water Supply Services in Kenya

iii. Water Resources Management Authority (WRMA) Guidelines

The first two manuals are applicable for small dams. For larger dams, Water Resources

Management Authority (WRMA), has published guidelines which are based on the perceived

risks associated with dam failure. The table below shows the categorisation of dams based on

storage depth, storage volumes and runoff catchments.

The minimum required freeboard (above F.W.L) of the dam was evaluated from wind set up,

significant wave height and wave run up. The value of wind set up is a function of the fetch of

the reservoir and the velocity of wind in the direction of predominant winds. In the case of Theta

dam, the fetch of the reservoir is is approximately 450m considering the left arm of the reservoir.

The spillway for this dam will be located on the right hand side of the dam. This location of the

spillway will avoid construction of a bridge on piers for purposes of moving to the right hand




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side of the reservoir. This arrangement will also reduce the construction cost of the bridge and

the necessary piers.

Categorization of dams based on storage depth, storage and runoff catchments

Class of

dam

Maximum Depth

of Water at NWL

(m)

Storage (m3) Catchment Area

(km2)

Minimum Return

Period for

spillway design

A (low

risk)

0 4.99 1,000,000 >1000 1 in 100 Years

Design of Small Dams by the United States Bureau of Reclamation (USBR), 1987, recommends

use of probable maximum flood (PMF) for spillway inflow design. PMF hydrograph represents

the maximum runoff condition resulting from the most severe combination of hydrologic and

meteorological conditions considered reasonably possible for the drainage basin under study.

The PMF is used by design and construction organizations as a basis for design in those cases

where the failure of the dam from overtopping would cause loss of life or widespread




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destruction. In estimating PMF, use is made of Maximum Probable Precipitation which requires

isolation of rainfall storm in the catchment of interest.

This information on particular storm and criteria for developing PMF and PMP is not developed

currently. In view of this, the consultant has used the 1 in 1,000 year frequency flood (25m3/s)

for spillway design.

SELECTION AND SIZING OF THE SPILLWAY

Preliminary hydraulic sizing of the spilling is undertaken based on the peak flow analysis. An

inflow flood of 25m3/s (1:1,000 year return period) is used to size the spillway.

The critical depth is determined using the following equation:

hc = 3q2/g

Where,

hc is critical water depth on top of the spillway sill. The critical depth is two

third the approach depth (ha): hc = 2/3ha.

ha =3/2*3q2/g,

Specific discharge (q) = 25/15 = 1.67 m2/s

Critical depth hc = 3q2/g

A table for selecting the best width of the spillway front through the critical depth equation is

presented below:

Q B q q^2 q^2/g (q^2/g)^0.33 ha

25 5 5 25 2.54842 1.361662353 2.042494

25 10 2.5 6.25 0.637105 0.861766551 1.29265

25 15 1.666667 2.777778 0.283158 0.659431065 0.989147

25 20 1.25 1.5625 0.159276 0.545393347 0.81809




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September, 2010

25 25 1 1 0.101937 0.470705451 0.706058

From the above table, an open channel spillway with a front width of 15m is selected. The

critical depth and the depth of water in the reservoir are as follows:

Description Depth of water (m)

hc 0.659

ha 0.989

The open channel spillway is selected since it fulfills the fundamental requirements of low cost,

less demand for workmanship and availability of local labour. Secondly, the excavated materials

will be used as random fill depending on their suitability.

WIND SET UP

Wind set up (the tilting of the reservoir surface caused by the movement of the surface water

towards the leeward shore under the action of wind) was estimated from the following formula:

Zs = (Vw .F)/ (63,200d) metres

With: Zs = rise above the still water level

Vw = speed of the wind in (km/hr)

Vw = The speed of the wind is assumed to be 54km/hr (15m/s)

F = fetch of the reservoir (0.45 km)

d = average depth of the reservoir along the fetch (10.0 m)

Hence:

Zs = ((54^2) x0.45)/ (63,200x10.0) =0.0021m




Eng. P. Njurumba

September, 2010

Significant wave height: The average height in metres (Zw) of the highest one third of the waves

was estimated as follows:

Zw =0.005Vw (^1.06) F (^0.47) (m)

Hence: Zw =0.005 x 54(^1.06) x4.0 (^0.47) =0.197m

Wave run up: the height to which a wave will run up a slope depends on the surface. The ratio

[wave run up / significant wave height Zr/Zw] depends on the ratio of wave height to wave

length

The wave period tw can be expressed as follows:

tw =0.32Vw 0.44 F0.28 (m)

Hence: tw =0.32 x 54 0.44 x 0.45 0.28 =1.48

The wave length may be computed from:

= 1.56tw2 =1.56x2.19 = 3.42 m

Ratio Zw/ =0.197/3.42 = 0.057

For embankments lined with riprap and an upstream slope of 1/3, the ratio Zr/Zw should not

exceed 0.7

Zr=0.7x 0.057 =0.04m

Depth of Water above the Spillway Sill




Eng. P. Njurumba

September, 2010

Required Dam freeboard: the required dam freeboard was then obtained as the sum of wind set

up, significant wave height and wave run up.

Required dam free board (above F.W.L.) = 0.0021+0.197+0.04+0.987 =1.23m

Allowing a safe board of 0.6m, then a gross freeboard 1.83m which is rounded to 2.0m and

which is considered to be adequate for the dam.

DESIGN OF THE INFLOW AND OUTFLOW SECTIONS OF THE SPILLWAY

INFLOW SECTION

With a 2.0m gross freeboard, the normal water line will be on level 2249 since the crest is on

level 2251m. The inflow section is 22m long and it will have an invert slope of 1% towards the

reservoir. The level at the entrance point to the spillway is, therefore, 2248.78m. About 10m of

the inflow section before the spillway sill will be rip-rapped or stone pitched to prevent any

erosion since this is the area where transition of flows will take place just before the sill. The

spillway width tapers from 15m at the entrance point to 5m at the exit point.

OUTFLOW SECTIONS OF THE SPILLWAY

The outflow section of the spillway is 70m long. The longitudinal profile of the spillway has

been selected in such a manner as to reduce the volume of earthworks and hence the construction

cost. The depth of water above the spillway sill is 2249m where the depth of water will be equal

to the critical depth (0.659m) and the exit point is on level 2234m. The two levels at the entrance

and exit points of the spillway through the length of 70m give a slope of 0.214m.




Eng. P. Njurumba

September, 2010

NORMAL DEPTH OF WATER AT THE TERMINAL POINT OF THE SPILLWAY

The width of the spillway channel at the terminal point is 5m. The normal depth of water at the

terminal point of the spillway is calculated using the uniform flow equation. And the calculations

are presented in the table below.

214.0

014.0

1

arg

,

6/1

hechannelalslopeoftlongitudini

ofconcreteoefficientroughnesscn

Rn

C

P

AR

adiusHydraulicrR

meterwettedperiP

ficientChezyscoefC

AreaA

eDischQ

RiACQ

Using the equation and the data, the depth of water is calculated using trial and error method.

h B A P R R^(1/6) C (Ri)^0.5 Q

0.25 5 1.25 5.5 0.22727273 0.78119218 55.7994414 0.22053654 15.3822694

0.28 5 1.4 5.56 0.25179856 0.79464936 56.7606689 0.2321312 18.4462907

0.3 5 1.5 5.6 0.26785714 0.80287981 57.3485581 0.23941894 20.5954964

0.32 5 1.6 5.64 0.28368794 0.8106004 57.9000285 0.24639241 22.8258041

0.33 5 1.65 5.66 0.29151943 0.81428778 58.1634131 0.24977021 23.9703553

0.338 5 1.69 5.676 0.29774489 0.81716054 58.3686099 0.25242307 24.8997565

0.339 5 1.695 5.678 0.29852061 0.81751498 58.3939271 0.25275168 25.0167812




Eng. P. Njurumba

September, 2010

0.34 5 1.7 5.68 0.29929577 0.8178684 58.4191717 0.25307962 25.1339934

0.35 5 1.75 5.7 0.30701754 0.82134799 58.6677136 0.25632353 26.3163525

0.38 5 1.9 5.76 0.32986111 0.83123123 59.3736595 0.26568831 29.9722857

0.4 5 2 5.8 0.34482759 0.83740137 59.8143836 0.27164886 32.4970183

From the above calculations, the normal depth of water as a function of height h is equal to

0.339m which is rounded to 0.34m.

FLOW PROFILES

As seen from the calculations the depth of water will be 0.659m above the spillway sill. Due to

the tapering nature of the spillway, this depth will reduce to 0.168m within the first 10m length

of the spillway. Thereafter, the depth of water will keep on increasing and it will attain a level of

0.339m(0.34m) at the terminal point of the spillway.

DEPTH OF THE RETAINING WALL

The purpose of the retaining wall is to confine the flood flows within the spillway channel. The

height of the wall at the sill is 2m which then reduces to 0.85m within the first length of 7m. The

other heights of the retaining wall are established using Mannings equation and they are

established at 10m intervals and presented in the following table.

0m

h B A P R R^(1/6) C (Ri)^0.5 Q

0.05 15 0.75 15.1 0.04966887 0.60622976 43.302126 0.10309772 3.34826272

0.12 15 1.8 15.24 0.11811024 0.70040677 50.0290547 0.15898299 14.3167835

0.15 15 2.25 15.3 0.14705882 0.72647515 51.8910823 0.17739952 20.712269

0.168 15 2.52 15.336 0.16431925 0.74004012 52.8600084 0.18752152 24.9792201

0.17 15 2.55 15.34 0.16623207 0.74146928 52.9620912 0.18860982 25.4723845

0.18 15 2.7 15.36 0.17578125 0.74840542 53.45753 0.19395151 27.9940551

0.19 15 2.85 15.38 0.18530559 0.7550175 53.9298213 0.19913663 30.6072979

h=0.168




Eng. P. Njurumba

September, 2010

10m

h B A P R R^(1/6) C (Ri)^0.5 Q

0.05 13.6 0.68 13.7 0.04963504 0.6061609 43.297207 0.10306259 3.03437921

0.12 13.6 1.632 13.84 0.11791908 0.70021767 50.0155476 0.15885428 12.9665399

0.15 13.6 2.04 13.9 0.14676259 0.726231 51.8736427 0.17722075 18.7538952

0.16 13.6 2.176 13.92 0.15632184 0.73391046 52.4221756 0.18290127 20.8636672

0.179 13.6 2.4344 13.958 0.17440894 0.74742825 53.3877323 0.19319294 25.1087257

0.18 13.6 2.448 13.96 0.17535817 0.74810484 53.4360599 0.19371796 25.3405318

0.19 13.6 2.584 13.98 0.18483548 0.75469786 53.9069897 0.19888387 27.7036598

h=0.179m

20m

h B A P R R^(1/6) C (Ri)^0.5 Q

0.05 12.2 0.61 12.3 0.0495935 0.6060763 43.2911643 0.10301946 2.72049761

0.1 12.2 1.22 12.4 0.0983871 0.67939569 48.5282637 0.14510286 8.59073937

0.11 12.2 1.342 12.42 0.10805153 0.69009086 49.2922042 0.15206258 10.0589605

0.14 12.2 1.708 12.48 0.13685897 0.71782199 51.272999 0.17113685 14.9871866

0.16 12.2 1.952 12.52 0.15591054 0.73358821 52.3991579 0.18266049 18.6830919

0.191 12.2 2.3302 12.582 0.18520108 0.7549465 53.9247498 0.19908046 25.0155457

0.2 12.2 2.44 12.6 0.19365079 0.76058212 54.3272946 0.20357129 26.9851249

h=0.191

30m

h B A P R R^(1/6) C (Ri)^0.5 Q

0.05 10.8 0.54 10.9 0.04954128 0.60596989 43.2835633 0.10296521 2.40661869

0.1 10.8 1.08 11 0.09818182 0.67915919 48.5113705 0.1449514 7.59433448

0.15 10.8 1.62 11.1 0.14594595 0.72555579 51.8254136 0.176727 14.8374988

0.206 10.8 2.2248 11.212 0.19843025 0.76367968 54.5485485 0.20606813 25.0083481

0.25 10.8 2.7 11.3 0.23893805 0.78769876 56.2641973 0.2261255 34.3514789

0.3 10.8 3.24 11.4 0.28421053 0.81081507 57.9153625 0.24661925 46.2770595

h=0.206




Eng. P. Njurumba

September, 2010

40m

h B A P R R^(1/6) C (Ri)^0.5 Q

0.1 9.2 0.92 9.4 0.09787234 0.67880185 48.4858465 0.14472277 6.45564563

0.15 9.2 1.38 9.5 0.14526316 0.72498884 51.7849168 0.17631312 12.5998971

0.2 9.2 1.84 9.6 0.19166667 0.75927748 54.2341054 0.20252572 20.2101941

0.25 9.2 2.3 9.7 0.2371134 0.78669281 56.1923437 0.22526044 29.1131984

0.228 9.2 2.0976 9.656 0.21723281 0.77529229 55.378021 0.21561035 25.0454997

0.12 9.2 1.104 9.44 0.11694915 0.69925425 49.946732 0.15819962 8.72331544

h=0.228

50m

h B A P R R^(1/6) C (Ri)^0.5 Q

0.1 7.7 0.77 7.9 0.09746835 0.67833397 48.4524266 0.14442378 5.38821555

0.15 7.7 1.155 8 0.144375 0.72424802 51.7320014 0.17577329 10.5025353

0.2 7.7 1.54 8.1 0.19012346 0.75825495 54.1610675 0.20170875 16.8241323

0.255 7.7 1.9635 8.21 0.23915956 0.78782045 56.272889 0.22623029 24.9965964

0.228 7.7 1.7556 8.156 0.21525257 0.77410967 55.2935476 0.21462537 20.8344045

0.12 7.7 0.924 7.94 0.1163728 0.6986786 49.9056141 0.15780931 7.27702717

h=0.255m

60m

h B A P R R^(1/6) C (Ri)^0.5 Q

0.1 6.5 0.65 6.7 0.09701493 0.6778069 48.4147787 0.14408745 4.53437539

0.15 6.5 0.975 6.8 0.14338235 0.72341554 51.6725386 0.17516799 8.82509018

0.2 6.5 1.3 6.9 0.1884058 0.75710866 54.07919 0.20079552 14.1165167

0.255 6.5 1.6575 7.01 0.23644793 0.78632433 56.1660232 0.22494412 20.9412142

0.285 6.5 1.8525 7.07 0.26202263 0.79990254 57.1358961 0.23679705 25.0636056

0.3 6.5 1.95 7.1 0.27464789 0.80620224 57.5858746 0.24243483 27.2236026

h=0.285

70m




Eng. P. Njurumba

September, 2010

h B A P R R^(1/6) C (Ri)^0.5 Q

0.1 5 0.5 5.2 0.09615385 0.6768003 48.3428785 0.14344659 3.46731045

0.15 5 0.75 5.3 0.14150943 0.72183166 51.5594041 0.17402017 6.72928212

0.2 5 1 5.4 0.18518519 0.75493569 53.9239782 0.19907192 10.7347499

0.25 5 1.25 5.5 0.22727273 0.7811536 55.7966857 0.22053654 15.3815097

0.339 5 1.695 5.678 0.29852061 0.81748204 58.391574 0.25275168 25.0157731

0.35 5 1.75 5.7 0.30701754 0.82131566 58.6654044 0.25632353 26.3153167

h=0.339

The flow profile is shown in the following graph.

L m

0 0.168

10 0.179

20 0.191

30 0.206

40 0.228

50 0.255

60 0.285

70 0.339




Eng. P. Njurumba

September, 2010

RESERVOIR CHARACTERISTICS

From a topographic survey which was done in July, 2010 and the selected dam height of 17m,

the area volume height relationship curve of the reservoir was computed and the amount of

storage up to the normal water of the reservoir considering a gross freeboard of 2m is

1,170,000m3 while the submerged are is 117,000m

2.




Eng. P. Njurumba

September, 2010

VOLUME CURVE




Eng. P. Njurumba

September, 2010

AREA CURVE

EMBANKMENT VOLUMES

The embankment volume for Theta Dam is estimated using the topographic survey and it is

equal to 20,482m3.

SUITABILITY COEFFICIENT OF THE SITE

The suitability coefficient (SC) of the site is the ration of the volume of fill material to the

volume of the water stored in the reservoir.

In the case of Theta Dam, this ratio is calculated using as follows:




Eng. P. Njurumba

September, 2010

57

20482

000,170,1

SC

SC

fillvolume

storageSC

This ratio represents a very good site whereby the unit cost of water will be very low.

SEDIMENT LOADS

The sedimentation yield of catchments in Kenya is estimated to range between 500 1500

m3/km

2/year depending on the rates of erosion. For this particular catchment and based on the

above figures, the sediment yield is assumed to be 500m3/km

2/year.

DEAD STORAGE

The Dead Storage of the reservoir is dependent of the size of the catchment area and the degree

of environmental degradation/conservation. The amounts of sediment loads determine the

economic lifespan of the dam.

This dam is in a thick forest with very little sediment loads. In this regard, a sediment load of

500m3/km

2/year is assumed for the entire catchment area of 1.6km

2. The annual sediment yield

is 800m3. Assuming a trap efficiency of 6% the total sediment trapped is 28.8m

3. Assuming a

specific gravity of sediment is 1.85t/m3, then the annual volume of sediments is 54m

3.

Considering a lifespan of 200 years, then the volume of the required dead storage is 10,800 m3.

However, due to uncertainties in the estimation of the degree of environmental degradation, and




Eng. P. Njurumba

September, 2010

a desire to increase the volume of sediment loading, a final dead storage volume of 100,000 m3 is

adopted for this dam. This gives the dead water level to be on 2238.8m.

It is necessary to note that most of the sediments will be deposited in the upper reaches of the

reservoir thereby reducing its fetch. This has a positive attribute in the sense that the reservoir

waters will be very clear which reduces the cost of treatment.

Figure 1.3 Trap Efficiency of Reservoirs2

CENTRAL CLAY CORE

The dam will have a central clay core which will be excavated to a depth of 3m to encounter a

good foundation. The core trench will then be cleaned and be filled up with impervious clay

.




Eng. P. Njurumba

September, 2010

material which will be properly compacted at the optimum moisture content until the maximum

densities will be achieved.

UPSTREAM SLOPE PROTECTION

The dam will have a protection of the upstream slope comprising of graded rip rap material on a

protective granular material or geotextile polyfelt material preferably T600. The embankment toe

of the dam will have rock toe drain to protect the slope from both wild and domestic animals

.

DOWNSTREAM SLOPE PROTECTION

The downstream slope will be protected by grassing on a 25mm thick red soil. Both the slope

and the berm will be grassed down to the rock toe drain which will protect the slope from

destruction by both wild and domestic animals.

FILTERS

The dam will have horizontal filter drains that will connect the rock toe with the impervious clay

core. The filters will lower the phreatic line within the limits of the downstream slope of the

embankment wall and thus maintain most of the materials in a dry condition. This will increase

the stability of the downstream slope. The filters will be 4 in number and they will be

constructed using graded sand and ballast all surrounded in geotextile polyfelt material.

DAM STABILITY

The usual failure of an earth embankment dam consists in the sliding of a large mass of soil

along a curved surface. There are various methods of checking the stability of a fill. In all these,

a failure arc is assumed and the forces acting on the sliding mass are worked out. These forces

are resisted by the shear force developed along the sliding surface. The Factor of Safety (F.O.S.)




Eng. P. Njurumba

September, 2010

is the ratio of the shear force to the forces causing sliding. Several failures arcs are assumed and

the respective F.O.S. is calculated. The minimum F.O.S. is then taken as the Factor of Safety.

The slope stability analysis is done based on the adopted slopes and the soil properties (section

5) using the Bishop Microcomputer Tool and the results are presented in appendix II and

drawing no. DRG. THETA.. The F.O.S on the upstream slope is

.. while the downstream slope is both of which are above the

acceptable F.O.S levels of 1.2.




Eng. P. Njurumba

September, 2010

RIVER DIVERSION WORKS

AND DRAW OFF SYSTEM




Eng. P. Njurumba

September, 2010

RIVER DIVERSION WORKS AND DRAW OFF SYSTEM

During the construction phase, the flows along the river course will be diverted so as to create a

good working environment. This will be done by way of constructing an upstream coffer dam

and a river diversion channel in form of a box culvert. The draw off pipe will be fixed inside the

box culvert as well as the scour pipe in case it may become necessary to drain the reservoir after

impoundment takes place. During the construction and impoundment phases of the reservoir,

compensation flow equivalent to 85% of the base flow will be released for sustenance of the

ecosystem downstream of the dam. Similarly, compensation flows will be released through the

same pipe even after impoundment and especially when the dam is not spilling. This is made

possible by the fact that the storage capacity of the dam is huge enough.

The river diversion works should be capable of evacuating a peak flood flow 1 in 20 year return

period, the magnitude of which is 14.1m3/sec. In this case, a 8.5m high coffer dam will be

constructed to facilitate release of the diversion flow. The freeboard below the crest of the coffer

dam is 2m which means that a water depth of 5m is considered for purposes of the design of this

component. The coffer dam will ultimately be incorporated into the main embankment wall of

the dam.

The diversion flow is assumed to be 6m3/sec and the size of the pipe is determined as follows:

0.1.........arg

2

tcoefficienedischC

Where

gHCAQ

A = Cross sectional Area

Assuming a freeboard of 2m, the depth of water H, therefore, is equal to 6.5m




Eng. P. Njurumba

September, 2010

224.1

5.681.92

14

2

214

14

mA

xxA

gHC

QA

gHCA

Q

SIZE OF THE DIVERSION PIPE

From the above analysis, the flows will be diverted through a box culvert measuring 1.24m high

and 1.0m wide.




Eng. P. Njurumba

September, 2010

ENERGY DISSIPATION




Eng. P. Njurumba

September, 2010

ENERGY DISSIPATION

The flood flows as they are discharged back to the river bed will cause a hydraulic jump where

excessive energy will be generated. This energy will scour the river bed as well as the bed of the

spillway. It is, therefore, necessary to dissipate the excessive kinetic energy within the limits of

the hydraulic jump and just before the terminal point where the spillway joins the river channel.

This will be accomplished by constructing a flip bucket just near the terminal point of the

spillway whereby the water jet will be deflected to the atmosphere where the water will lose

substantial amount of energy before it falls on the river course.

The profile of the spillway is such that it tapers from 15m wide at the sill to 5m at the terminal

point.

In order to achieve the maximum length to where the water jet will fall from the edge of the flip

bucket, different angles of inclination to the horizontal are assumed and the optimum angle of

inclination to the horizontal is selected.

DETERMINATION OF THE VELOCITY OF FLOW AT THE TERMINAL POINT

From the calculations, the normal depth of water as a function of height h is equal to 0.339m

which is rounded to 0.34m. The velocity of water at the terminal point is expressed using the

following equations:




Eng. P. Njurumba

September, 2010

sec/7.14

7.1

25

7.1

34.05

2

mV

V

A

QV

mA

xA

BhA

AVQ

FLIP BUCKET EQUATION

The equation for the flip bucket is as follows:

)2

{2 2

22

V

gySinSinCos

g

VL

If the angle of inclination is equal to 00, then the equation is as presented below:

Where:

L - Horizontal distance from the heel to the centre of the erosion zone

v - Velocity of water at the terminal point of the spillway = 14.7m/sec.

- Angle of inclination to the horizontal

y - Difference in elevation between the heel and the erosion depth. In this case, y

is assumed to be 2.0m




Eng. P. Njurumba

September, 2010

Angle of inclination () Length L (m)

100 6.87

150 8.06

200 9.20

250 10.21

300 11.03

350 11.62

400 11.92

450 11.94

500 11.63

550 11.01

From the above table, it is evident that the farthest length of possible point of erosion L from the

edge of the flip bucket is achieved when the angle of inclination is 450 when the length is

approximately 11.94m. A corresponding graph for the calculations is presented below.




Eng. P. Njurumba

September, 2010

DETERMINATION OF THE HEIGHT OF THE WALL WITHIN THE HYDRAULIC JUMP SECTION

The height of the wall within the limits of the flip bucket is calculated on the basis of the normal

depth of water which is equal to 0.34m and the congruent depth.

INITIAL DEPTH AT THE TERMINAL POINT OF THE SPILLWAY

The initial depth according to the calculations is 0.34m

THEORETICAL CONGRUENT DEPTH

The congruent depth is determined on the basis of the initial or critical depth of water which is

equal to 1.37m and it also guides on the height of the wall within the limits of the flip bucket.

However, the depth of water at the terminal point of the spillway channel is below the critical

depth and hence supercritical. The initial depth as established earlier is 1.37m




Eng. P. Njurumba

September, 2010

mightofallwithaheConsideraw

mh

xh

h

x

xxxh

gh

qhh

4

87.3

77.22685.0

}122.25

2201{685.0

}137.181.9

551.181{

2

37.1

}18

1{2

2

2

2

32

3

1

2

12

= non uniform distribution of velocity 1.1




Eng. P. Njurumba

September, 2010

GEOLOGICAL INVESTIGATIONS

AND SEISMICITY




Eng. P. Njurumba

September, 2010

GEOLOGICAL INVESTIGATIONS

INTRODUCTION

Some initial investigations were carried out in two stages to determine the feasibility of the dam

site. The first stage desk comprised of the study of topographical and geological maps of the

area. The second stage involved a walk over survey of the selected dam site including the

proposed dam foundation axis and sites of other structures such as spillway and diversion and

outlet works and the area that will be submerged by the reservoir upon its impoundment after the

completion of the dam.

The two stages allowed an assessment of topographical features, suitability of the dam and other

structures foundation in terms of strength, durability, water tightness etc, and how these

conditions are likely to influence the subsequent dam design.

GEOLOGY OF THE AREA

The project area is within the tertiary volcanics of middle and upper tertiary age which are

widespread in Central Kenya. Hey are mainly of alkaline type including basalts phonolites,

nephlenites, trachytes and alkali rhyolites and their pyroclastic equivalents.

.

FAULTS

The probable fault marked in the geological map of Kenya is to the west of the proposed dam

site and it cuts along the Great Rift Valley from Ethiopia to the north to Tanzania to the south.

SEISMIC POTENTIAL

The seismic map of possible intensities in Kenya from Professor L. S. Loupekines report on

Earthquakes in Kenya zones the project site as Zone VI in a scale of V to IX on increasing




Eng. P. Njurumba

September, 2010

intensities. The site is, therefore, in an area of minimal seismic potential. However, appropriate

design seismic parameters will be adopted in the stability calculations.

In accordance with the Code of Practice relating to earthquake design and seismic zoning map of

Kenya the proposed dam site located in Zone VI. This zone (Zone VI) is characterized as having

intensity of Damage is slight; a few instances of fallen plaster or damaged chimneys.

Table 5.1: Earthquake parameters for some dam projects in Kenya

Dam Project Design Basis

Earthquake DBE

Maximum

Credible Earthquake

MCE

Comments

Turkwel amax. = 0.20g

Return period

410 years

amax. = 0.45g

M = 7.5 Occurring at

20km. from dam site.

Dam completed in

1988 1991

Chemususu amax. = 0.22g

Return period

400years

a max. = 0.50g

M = 7.5

Return period 2540

years

Dam located at the

intersection of Rift

Valley and Kavirondo

Rift

Final design in 1989

Kiambere a max. = 0.12g a max. = 0.25g Dam completed in

1988

Thika a max. = 0.13g a max. = 0.40g Dam completed in

1988

Source: SOGREAH Consultants-Feasibility Study of Rehabilitation of Sasumua Dam




Eng. P. Njurumba

September, 2010

DAM CONSTRUCTION MATERIALS

The most economical dam is one that will utilize the materials found within reasonable distance

from the construction site so as to reduce haulage costs. The following materials were identified

during this initial reconnaissance of the project site:

EARTH FILL MATERIALS

The proposed dam site is located in a thick forest with fairly rich red clay soils of high plasticity.

A core trench dug across the dam axis reveals a gradual change of the soil to lighter colour sandy

murram soils at a depth of about 1.5 meter and presence of a high water table in the upstream

reaches of the reservoir area.

Samples of these soils were tested in the laboratory and the results are attached in Appendix 5. In

summary sample 1, 2 and 3 representing the darkish top soils classify as inorganic clays, silty

clays or sandy clays of high plasticity (CL). They have optimum moisture content of 24% and a

maximum dry density of 1800 kg/m3. Samples number four gives similar results while sample

number five classify as inorganic clays or silts of high plasticity (MH or OH). The light coloured

soil found at a depth of 1 meter in the drainage trench classifies as well graded sand with little or

no fines.

ROCK MATERIALS

The requirements for rock materials for the riprap and toe drain is that the material has to be

hard, durable and able to withstand disintegration from mechanical or chemical weathering and

or from quarrying, loading, haulage and placing operations.

Within the dam site, there are no potential quarries except where such rock materials can be

obtained from excavations during construction. In this regard, the possible quarries from rock

materials can be obtained are in Kiamwangi, Magomano and Ndarugu but the haulage costs are a

bit excessive due to the long distances.




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September, 2010

DAM FOUNDATION

The following materials cover the dam axis defining the centre line of the dam foundation:

Mostly weathered rocks, rock debris and murram on the steeper upper slopes.

Deposits of silt clay and or sandy clays to gravelly silt/sandy clays on the lower gentler slopes.

SANDY GRAVELS ON THE RIVER CHANNEL

From the geology of the area, the above horizons overlie solid rock at relatively shallow depths.

The above top materials will be excavated to rock over the impervious core, filter, drainage

layers areas, and a key or cutoff trench constructed with materials similar to that used for the

impervious core. This will reduce leakage and associated instability on the upper pervious

foundation layers.

In addition, a grout curtain will be constructed into the rock to seal off any joints, cracks,

fissures, and shear zones, which may act as seepage paths. The grout curtain should extend to a

depth equal to the head of water at any particular section.

FILTER MATERIALS

There are no sources of clean sand within the project site. In this regard, the sand for the

construction works as well as filters will be sources from Masinga, Machakos or Kiserian in

Kajiado North District.

FIELD INVESTIGATIONS

RESERVOIR AREA

Thorough investigations to locate potential seepage paths such as sandy / gravel layers,

weathered rock zones, lateritic soil zones, faults and fractured bedrock zones will be carried out




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September, 2010

over the reservoir area during construction. This will be by way of geophysical methods,

geological studies, and limited borings.

Thus:

Electrical resistivity sounding and / or seismic refraction to map the stratification with depth and

electrical resistivity profiling to investigate the lateral variations. A desk study of the areas

geological maps and reports will help identify boundaries of the stratification, including soil

overburden, weathered horizons, fractured bedrock etc.

A few selected borings will be sunk within the reservoir area and identified stratification used to

correlate resistivity sounding and profiling. Samples of the various strata will be taken for

physical properties tests to proof suitability for use as dam material and for strength tests for

slope stability analysis. Borings must be an obsolete minimum and any to be properly resealed to

ensure it does not connect with underlying permeable layers and act as future leakage paths.

DAM AREA

This will be investigated by sinking borings as below:

Along the dam axis at every 100 150 meters intervals and extending beyond the expected dam

height by not less than 50 meters on each bank (to capture the spillway location as well)

A next set of borings at intervals of 200 meters about 50 meters on either side of the dam axis to

map out the lateral extent of stratification.

A set of borings along the spillway axis, intake tunnel, emergency spillway axis at every 50

meter spacing and at locations of intake tower, power house and treatment works.

Disturbed samples for physical properties determination and undisturbed samples for bearing

capacity and settlements tests will be recovered at appropriate strata. Depth to bedrock and




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September, 2010

character of rock are important. In general, borings will be to a depth at least equal to the height

of the dam.

SUMMARY OF GEOLOGICAL INVESTIGATIONS

Based on the findings of the initial reconnaissance, the following conclusions can be made:

That the location of the dam axis is geologically suitable and that a safe economical dam

foundation can be achieved.

That substantial quantities of materials that can be obtained from borrow areas within the

reservoir area have low compacted dry densities (1800 kg/m3) which makes them suitable for

dam construction works. The required quantities are estimated to be about 15,000 m3 which is

obtainable from the nearby borrow areas.

That rocks on hills defining the dam axis and on locations of spillway, diversion and other

structures where excavations will be done appear suitable for use as rock fill and rip rap. The

deficits will be bridged by rock materials from Kiamwangi, Magomano and Ndarugu quarries.

That an earth fill dam with an impervious central clay core may be the most economical and

technically suitable considering the materials available at the site.




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September, 2010

SEISMICITY

According to the seismic zoning map of Kenya (seismic zoning map of Kenya by L. S.

Loupekine, 1973), Theta dam is located in seismic zone V (appendix 2). This demonstrates that

the area is not prone to earthquakes and hence no specific seismic loading design for structural

members is necessary. For this reason no incorporation for seismic loading has been

incorporated or checked for in the dam embankment design.




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September, 2010

INSTRUMENTATION




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September, 2010

INSTRUMENTATION

Appropriate instrumentation enables monitoring of the behaviour of the dam during the

operation phase. This will give an advance indication of the potential effects of any initial

deficiencies or deterioration during operations. Instruments will be required to monitor the

following design and operation parameters.

i. Pore water pressure in the fill

ii. Total pressure in embankment core

iii. Settlement and distortion

iv. Seepage flow

v. Reservoir water level




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September, 2010

AUXILIARY WORKS




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September, 2010

CAMBER

The dam will have a 30cm camber above the design crest level. The camber will take care of any

differential settlement which might arise as a result of the consolidation of the embankment

material. Above the camber will be a thin layer of murram to provide motorability along the

crest.

PARKING BAY

A parking bay will be constructed on the left hand side of the dam to accommodate vehicles for

the people who may wish to visit the dam site either for social or recreational purposes.

FENCING

Fencing of the dam is not of absolute necessity since the dam is in the forest with no noticeable

encroachment. However, this may be done to keep off the wild animals and thus maintain water

of a better quality.

If fencing will be done, then cedar posts will be used with 10 strands of barbed wire G16. This

will be shown in the general specification drawings.

RAMP

Theta dam is located in a forest where there are some animals and even the local people graze

their domestic animals in the forest. In cognizance of the fact that the animals would water from

the dam, it is proposed that ramps be constructed on both sides of the reservoir to facilitate

watering the animals and thus prevent damage to the embankment slopes as the animals look for

water.

Theta Earth Dam

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