world bank document...2.4.2 inspect the inlet and outlet gates and valves of the structures on tono...

MINISTRY OF FOOD & AGRICULTURE (MOFA)

GHANA COMMERCIAL AGRICULTURE PROJECT

(GCAP)

FINAL REPORT

FOR

CONSULTING SERVICES SAFETY ASSESSMENT OF TONO DAM

BY

December, 2018

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Table of Contents

Table of Contents

List of Tables

List of Figures

Abbreviations and Acronyms

Executive Summary

0.1 Introduction

0.1.1 Background

0.1.2 Purpose and Scope of the Assessment

0.2 Methodology

0.2.1 Geotechnical/Dam Engineering Services

0.2.2 Hydrological Services

0.2.3 Hydraulic Engineering Services

0.2.4 Mechanical Services

0.2.5 Instrumentation

0.2.6 Dam Operator Training

0.3 Observations & Findings







0.4 Conclusions





0.4.5 Record Keeping


0.5 Recommendations








1 Introduction

1.1 Background

1.1.1 The Project and Consultancy Assignment

1.1.2 Irrigation Projects being rehabilitation under GCAP

1.2 Purpose and Scope of the Assessment

1.2.1 Consultancy Assignment Scope

1.3 Specific Tasks of the Consultancy

2 Methodology and Analysis

2.1 Geotechnical/Dam Engineering Services

2.1.1 Detailed Dam Inspection

2.1.2 Assessment of Foundation Stability

2.1.3 Assessment of Embankment Stability

2.2 Hydrological Services

2.2.1 Electrical Resistivity Test at Tono

2.2.2 Hydrologic Features and Failure Modes of the Tono Dam

2.2.3 Hydrologic loads (Flood Risk) of Tono Dam analyzed

2.2.4 Investigation of the Dam’s downstream conditions for emergency situations

2.3 Hydraulic Engineering Services

2.3.1 Design and construction data of Tono Dam reviewed

2.3.2 Integrity of appurtenant structures examined

2.3.3 Tono Dam channel for flow discharge capability assessed

2.3.4 Tono Dam downstream conditions assessed and need for emergency concept

ascertained

2.3.5 Installations and instrumentations for Hydraulic Failure assessed

2.4 Mechanical Services

2.4.1 Desk review of data and information on all mechanical equipment on the Tono

dam

2.4.2 Inspect the inlet and outlet gates and valves of the structures on Tono dam.

2.4.3 Desk review of all operation and maintenance reports of the gates and valves

2.4.4 Desk review of the maintenance plan of all the mechanical components of the

facilities

2.4.5 Desk review of the manufacturer's instructions and compare to current use

and operation of the components

2.4.6 Examine instrumentation readings for adequacy and analyze for trends as well

as any information that can be deduced from them, propose any additional

instrumentation required

2.5 Review all the dam operator training programs and reports if any

3 Major Findings and Implications


3.1.1 Visual Inspection of Dams

3.1.2 Foundation Stability

3.1.3 Stability of Embankment


3.2.1 Visual Inspections

3.2.2 Results and Implications of the Electrical Resistivity Test at Tono

3.2.3 Flood Water Evacuation of Tono Dam

3.2.4 Hydrologic failure mode of the Tono Dam

3.2.5 Tono Dam upstream and downstream conditions


3.3.1 Integrity of appurtenant structures

3.3.2 Hydraulic Failure Modes of the Tono Dam Assessed

3.3.3 Tono Dams downstream conditions


3.4.1 Desk review of data and information on all mechanical equipment

3.4.2 Inspection of the inlet and outlet gates and valves

3.4.3 Operation and Maintenance

3.4.4 Maintenance plan

3.4.5 Manufacturer's Instructions

3.5 Instrumentation

3.6 Dam Operator Training

4 Conclusions


4.1.1 Dam Foundation


4.2 Hydrologist Services

4.2.1 Flood risk of the Tono Dam

4.2.2 Hydrologic Failure Mode of Tono Dams


4.3.1 Integrity of the Spillway and Appurtenant Structures

4.3.2 Hydraulic Failure Modes of the Tono Dam

4.3.3 Downstream conditions of Tono Dam


4.4.1 Valves

4.5 Instrumentation



5 Recommendations


5.1.1 Visual Inspection


5.1.3 Regular Visual Observations




5.4.1 Off Take Tower

5.4.2 Scour Tower

5.4.3 Operation and Maintenance (O & M)

5.5 Record Keeping

5.6 Instrumentation

5.6.1 Piezometers


6 Annexes

6.1 Geotechnical Annexes

6.1.1 Trial Pit Records

6.1.2 Grading Curves

6.1.3 Dynamic Cone Penetration Test (DCPT) Results

6.1.4 Soil Test Results

6.2 Terms of Reference

6.3 Work Plan

6.4 Field visits schedule

6.5 List of Stakeholders met during field visits

6.6 Minutes of Meetings

6.7 Data Collection

6.7.1 List of Data Collected

6.7.2 ICOUR Training Needs

7. REFERENCES

List of Tables

Table 1: Key Features and Inventory of Tono Dam

Table 2: Trial Pits and their GPS Locations

Table 3: Peak Daily Flows of Tono Dam

Table 4: Flood risk analysis of Tono Dam

Table 5: Flood inflows and outflows from the spillway considering Reservoir Attenuation

Table 6: Hydrologic Failure Mode of Vea and Tono Dams

Table 7: Hydraulic Failure Mode of Tono Dam

Table 8: Measurements and Instruments for long term Performance Monitoring

List of Figures Figure 1: Geological Map of Upper East Region

Figure 2: Trial Pit and DCPT Locations

Figure 3: Tono dam showing one profile lines for resistivity measurements

Figure 4: Inspection of the Spillway chute of the Tono Dam (GPS Track in Blue)

Figure 5: Cultivation of Pepper on the Upstream Banks of the Tono Dam

Figure 6: Close up of Joint of replaced section of Spillway Wall indicating lateral movement

Figure 7: Joint of replaced section of Spillway Wall indicating lateral movement

Figure 8: Expansion Joint wider than normal

Figure 9: Change in level at column support due to settlement at hinge support

Figure 10: Spalling of concrete in floors of Spillway Channel

Figure 11: Upstream Slope Protection at Tono Dam

Figure 12: Vegetative cover to downstream slope at Tono

Figure 13: Trapezoidal open paved drain


Figure 15: Cross Section of Tono Dam

Figure 16: Resistivity Image for Profile along Tono Dam

Figure 17: Sketch of an Ogee crested weir spillway

Figure 18: Geometry of the Ogee crest

Figure 19 Graph of discharge coefficient against ratio of crest elevation to upstream head

over crest elevation

Figure 20 Daily Reservoir Water levels of Tono from 2010 to 2018

Figure 21 Historical Annual Rainfall at the Tono Dam

Figure 23 Tono Dam Spillway rating curve

Figure 24: Common failure mechanisms of Dams (Source: Dam Safety Manual of Ghana)

Figure 24: Gullies on the downstream embankment of Tono dam due to Soil Erosion

Figure 24: Outlet Structure

Figure 25: Obstruction of the Spillway at the stilling basin of the Tono Dam from a broken

channel wall.

Figure 26: Scour Pipe Outlet Structure

Figure 27: Scour Pipe Outlet Structure blocked with Clay

Figure 28: Location of the 12 observation wells at the toe of the Tono Dam

Figure 29: Non-functioning observation wells at the Tono Dam

Figure 30: Observation wells at the Tono Dam

Abbreviations and Acronyms

AfDB African Development Bank

CBR California Bearing Ratio

DCPT Dynamic Cone Penetration Tests

EMWA Edward Mensah Wood and Associates

GCAP Ghana Commercial Agriculture Project

GEL Golden Exotics Limited

GIDA Ghana Irrigation Development Authority

GHA Ghana Highways Authority

GWCL Ghana Water Company Limited

Ha Hectares

ICOUR Irrigation Company of Upper Region

IPC International Power Company

KIS Kpong Irrigation Scheme

KLBIP Kpong Left Bank Irrigation Project

M&E Monitoring and Evaluation

PDO Project Development Objective

PPP Public Private Partnership

SADA Savanna Accelerated Development Authority

SOP Standard Operating Procedures

USAID United States Agency for International Development

VRA Volta River Authority

Executive Summary

0.1 Introduction

0.1.1 Background

0.1.1.1 The Project and Consultancy Assignment

The International Power Company has been awarded a contract by the Ghana Commercial

Agriculture Project (GCAP), Ghana’s flagship agricultural project, financed by a credit from the

International Development Association (IDA) and grant from the United States Agency for

International Development (USAID) to undertake Dam Safety Assessment of the Tono and

Vea Dams.

The project, restructured in 2015 with the revised Project Development Objective (PDO) to

improve agricultural productivity and production of both smallholder and nucleus farms in

selected project intervention areas with increased access to reliable water, land, finance, and

agricultural input and output markets consists of the following seven components: (i)

Strengthening investment promotion infrastructure and facilitating secure access to land; (ii)

Securing PPPs and smallholder linkages in the Accra Plains; (iii) Securing PPPs and small-

holder linkages in the SADA Zone; (iv) Project Management including M&E and impact

analysis; (v) Investments in physical rehabilitation and modernization of existing public

irrigation and drainage infrastructure; (vi) restructuring and strengthening of public irrigation

and drainage institutions of the Government of Ghana; and (vii) development of Water Users’

Associations and private scheme management.

0.1.1.2 Irrigation Projects being rehabilitation under GCAP

Under Component 5 of the Project, GCAP will support the design review, rehabilitation and

modernization of the scheme including an assessment of the economic rates of return and

poverty reduction impacts of the under listed Irrigation Schemes:

Tono and Vea Irrigation Projects in the Upper East Region

Kpong Irrigation Scheme (KIS) at Asutsuare in the Eastern Region

Kpong Left Bank Irrigation Project (KLBIP) in the North Tongu District of the Volta

Region

0.1.1.3 Tono

The Tono scheme was established by the Ghana Government to promote the production of

food crops by small scale farmers within an organized and managed irrigation scheme. It is

located at Tono near Navrongo in the Upper East region of Ghana. It has a gross area of

3,860 ha with a potential irrigable area of 2,680 ha of which 2,490 ha has been developed.

The source of water is the Tono River. Construction of the Tono Irrigation scheme started in

1975 and completed in 1985. Some rehabilitation of the gravity scheme was carried out in

2008 by replacing the concrete slabs in the main gravity canal.

The Tono Irrigation Scheme is under the management of Irrigation Company of Upper Region

(ICOUR).

0.1.2 Purpose and Scope of the Assessment

The objective of this assignment is to assess and evaluate the safety of the existing dam at

Tono. In particular, safety inspections will be carried out on all critical sections of the dam that

include but not limited to the following areas:

Dam Crest

Upstream Embankment

Downstream Embankment

Offtake Chamber, Spindle and Valve

Spillway Crest and Channel

Toe Drain

0.2 Methodology


A systematic geotechnical evaluation of the dams were carried out which include: performance

of detailed dam inspections; assessment of current foundation stability; assessment of current

embankment stability; assessment of the slopes of the embankment and assessment of

cracking due to differential movements.


The Hydrological Investigations conducted on the Tono Dam included; electrical resistivity test

(field and laboratory), assessment of hydrologic features and failure modes, analysis of

hydrologic loads and downstream conditions for emergency situations.


The hydraulic investigations conducted a review of the hydraulic components of Tono dams

as designed and built. The hydraulic components reviewed include the Outlet canal, Spillway,

Spillway chute, spillway sections and drains.


The mechanical services were performed as follows: Data and information on all mechanical

equipment reviewed; the inlet and outlet gates and valves inspected; operation and

maintenance reports of the gates and valves reviewed; maintenance plan of all the mechanical

components reviewed; compared manufacturer's instructions to current use of the valves.


Instrumentation readings were examined for adequacy and analyzed for trends as well as any

information that can be deduced from them and additional instrumentation proposed required.


ICOUR provided two sheets (see Section 6.7.2) which listed the training courses with names

of staff to undertake them as well as dates for 2016 and 2017.These were studied for the

review.

0.3 Observations & Findings

Below are the observations and findings:


Some of the defects identified at this dam site include:

Widened expansion joint resulting from differential settlement of supports on the scour

tower bridge.

The trial pits revealed the geological successions at this site to consist of three distinct

layers: topsoil of Sandy Silt, underlain by lateritic gravel grading to hardpan and residual

soil of micaceous Silty Clay.

The dynamic cone penetration tests (DCPT) results gave very high penetration resistance

values (N-values) that were increasing with increasing depth.

The upstream slope protection is ensured by providing riprap.

The downstream slope protection is ensured by vegetative cover.

Surface drainage. provided on the downstream slope, had residual silt and seemed non-

functional at the time of the safety assessment.

As per the design, the core of the embankment fill is made of clay, capped with sand and

gravel filters in succession. These filters are then capped with the riprap.


The laboratory electrical resistivity results of the saturated soil from the Tono dam was

20Ωm.

Low resistivity areas (3.5-30 Ωm) were observed, signifying saturation zones within the

earth dam.

The geophysical investigations revealed that seepage exist in the dam, stretching along

40% of the Tono Dam. Coincidentally, the locations of the seepage areas fall in line with

the areas of the wetlands about 100m downstream of the dam.

Physical investigations on the dam did not observe any cracks or pipes and most

especially at the locations of the low resistivity.

The flood routing of the dam, estimated to determine their impact on the embankment of

the dam, shows the maximum level of flooding on the dam as an inflow of 603.9m3/s and

its likely impact as an outflow of 302m3/s with an assumed attenuation of 50%.

Gullies due to soil erosion were observed on downstream face of the Tono dam.

Shrub was observed in both faces of the dam, with trees growing on the downstream

faces of the dam. The Toe drain of the dam is completely silted.

Surface drains were observed to be non-functional.

No rodent activity observed.

Recession agriculture is practiced seriously along the banks upstream of the reservoir.


Sections of the Spillway Chute Wall are tilted due lateral earth pressure on the spillway

channel walls.

The tilting of the wall panels have created a gaps where seepage can take place between

the wall joints and therefore has the tendency to affect the channel embankment

An old broken wall section is lying on the baffle blocks within the stilling basin acting as a

blockage to the water flow.

The stilling basin is sound.

No signs of spillway chute wall overtopping were observed.

The spillway basin sweepout is located about 550m downstream of the chute and has

free standing baffle blocks about 20m before the end sill, made up of boulder packs.

A Schmidt Hammer test conducted on the spillway structure revealed a concrete strength

greater than 52.5kN/m2, which is adequate because it is greater than the allowable

strength of 35KN/m2. The result shows adequate strength and it is consistent with the age

of the structure.

The spillway drain was found to be in good shape.

Spalling of concrete within floors of spillway channel.


There are two sets of valves, one in the Off-take Tower and other in the Scour Tower.

The Off take Tower is in the middle of the dam wall and houses two valves in series, a

900 mm Gate valve and a 900 mm Butterfly valve.

The Gate valve leaks from the valve body into the Off-take Tower well, housing the valves

The Off-take Tower walls leak into the well.

The valves in the Off-take Tower are constantly under water.

The second set of valves is in the Scour Tower, thirty meters into the reservoir.

The Scour tower houses four valves which are a 1200 mm scour gate valve and three 400

mm water supply gate valves taking water at different elevations into a 400 mm water

supply pipe.

The Scour Tower walls leak into the well.

The Scour pipe is blocked at the discharge end.

The water supply system is unutilised.


The team counted twelve observation wells at the Toe of the dam, seven of them had

been filled with sand and the remaining had no piezometers installed.

The team counted three Piezometers[1] which are installed on the downstream face of the

dam, there are no data collections available on the piezometer readings and they are

currently not functioning.

There is a staff gauge on the Scour Tower Wall.

0.3.5.1 Record Keeping

The general observation is that record keeping at Tono Dam is non-existent. The information

is not gathered in the first place to be kept.


There is no “in-house” Training for the staff at Tono.

There was no technical training for the operating staff for 2016.

A planned external training for selected ICOUR staff for 2017 (to be funded by GCAP) will

not train the operating personnel in the relevant fields.

0.4 Conclusions


The Tono Dam in its current state is safe and sound

The foundation of the dam is very compact and stable.

The slope protective measures at both upstream and downstream faces are adequate

Both longitudinal and transverse cracking which are generally caused by differential

settlements or deformations in the foundation, abutments or adjacent materials within the

embankment, are absent.


The Tono dam is safe for flood events of return period up to 1 in a 1000-year event.

The spillway capacity is adequate.

There is seepage in the dam which is manageable and does not require structural

remedies.

There is lack of maintenance on the dam faces.

The massive flood recession agriculture could enhance sediment transport into the

reservoir.


The integrity of the spillway chute, walls, stilling basin and the channel of the Tono

dam is currently sound.

Stagnation pressure is likely to occur, specifically at the dam chute wall that has tilted.

The concrete strength of the dam is adequate.


0.4.4.1 Valves

There is a safety issue of leakage from the gate valve body and the tower walls, leaving

the valves submerged continuously. Apart from the inconvenience of having to work in

water, there is the danger of drowning.

Flushing of silt through the Scour pipe cannot be done as a result of the discharge end of

the pipe from the Scour Tower being blocked.


The general observation is that record keeping at Tono Dam is nonexistent. The information

is not gathered in the first place to be kept.


The dam operators require technical training as well as training in the importance of record

acquisition and management.

0.5 Recommendations


All structural and non-structural defects identified during the visual inspection and

indicated in this report by our team at this dam site should, as a matter of urgency, be

rectified.

The non-functional system of open paved drains (chutes) along the sloping surface for

surface drainage of downstream slope must be re-instated or re-constructed.

Regular visual observations, an essential aspect of a program for monitoring long-term

performance, should be undertaken.


Data collection on spillage and all water uses should be prioritized.

The seepage rates of the dam should be monitored by providing instrumentation.

There is the need to prepare flood inundation maps for various flows in case of emergency

situations such as dam breach. The maps will be useful for emergency preparedness

planning.


The tilted chute wall at Tono spillway should be given special attention.

The broken wall must be removed from the spillway channel.

Walls need weep holes to relieve them of pressure during surcharge.


0.5.4.1 Off Take Tower

The area around the valves must be dewatered and kept dry at all times.

The 900 mm gate valve with body leakage must be replaced.

The leakage from the walls of the Off-Take Tower must be stopped.

0.5.4.2 Scour Tower

The well should be dewatered and kept dry at all times.

The water supply valves and pipes should be kept in good condition for future use.

The leakage from thewalls of the scour tower must be stopped

0.5.4.3 Operation and Maintenance (O & M)

A combination of routine and periodic maintenance programs for optimum operation of the

dam and as a means of continuous safety is recommended.


Monitoring and surveillance results should be recorded and the records kept. The

data/information should be analyzed, evaluated and reported.


The sand filled observation wells should be reinstated and piezometers installed in some

of them.

Recommended types of additional instrumentation are:

- Leakage Weirs

- Piezometers

- Liquid level gauges

- Staff Gauges


It is recommended that the Management and Technical Staff are attached to a similar facility

to acquire knowledge in good record acquisition and management. Training should cover the

following:

Structural, mechanical and dam instrumentations and data acquisition.

Inspection, Maintenance and repair works procedures.

Record keeping and data management.

Irrigation water management.

GIS and remote sensing applications to catchment area management including

planning and monitoring.

Negotiation, conflict resolution and stakeholder engagement.

Report writing and communication.

1 Introduction

1.1 Background

1.1.1 The Project and Consultancy Assignment

The International Power Company has been awarded a contract by the Ghana Commercial

Agriculture Project (GCAP) to undertake Dam Safety Assessment of the Tono and Vea Dams.

The Ghana Commercial Agriculture Project (GCAP), financed by a US$100 million credit from

the International Development Association (IDA) and grant of US$45 million from the United

States Agency for International Development (USAID), is Ghana’s flagship agricultural

project.

The project was restructured in 2015 with the following revised Project Development Objective

(PDO): to improve agricultural productivity and production of both smallholder and

nucleus farms in selected project intervention areas with increased access to reliable

water, land, finance, and agricultural input and output markets.

The restructured GCAP consists of the following seven components: (i) Strengthening

investment promotion infrastructure and facilitating secure access to land; (ii) Securing PPPs

and smallholder linkages in the Accra Plains; (iii) Securing PPPs and small-holder linkages in

the SADA Zone; (iv) Project Management including M&E and impact analysis; (v) Investments

in physical rehabilitation and modernization of existing public irrigation and drainage

infrastructure; (vi) restructuring and strengthening of public irrigation and drainage institutions

of the Government of Ghana; and (vii) development of Water Users’ Associations and

private scheme management.

1.1.2 Irrigation Projects being rehabilitation under GCAP

Under Component 5 of the Project, GCAP will support the design review, rehabilitation and

modernization of the scheme including an assessment of the economic rates of return and

poverty reduction impacts of the under listed Irrigation Schemes:

Tono and Vea Irrigation Projects in the Upper East Region

Kpong Irrigation Scheme (KIS) at Asutsuare in the Eastern Region

Kpong Left Bank Irrigation Project (KLBIP) in the North Tongu District of the Volta

Region

1.1.2.1 Tono

The Tono scheme was established by the Ghana Government to promote the production of

food crops by small scale farmers within organized and managed irrigation scheme. It is

located at Tono near Navrongo in the Upper East region of Ghana. It has a gross area of

3,860 ha with a potential irrigable area of 2,680 ha of which 2,490 ha has been developed.

The source of water is the Tono River. Construction of the Tono Irrigation scheme with the

dam started in 1975 and completed in 1985. Some rehabilitation of the gravity scheme was

carried out in 2008 by replacing the concrete slabs in the main gravity canal.

The Tono Irrigation Scheme is under the management of Irrigation Company of Upper Region

(ICOUR).

1.2 Purpose and Scope of the Assessment

There are serious consequences in the event of a dam malfunction or failure. Given the huge

financial investments being made on the Tono project, it is prudent to assess and ascertain

the safety of the dam.

The objective of this assignment is therefore to assess and evaluate the safety of the

Tono dam.

After many years of use and lack of maintenance the dam infrastructure needs rehabilitation

to make them more efficient. In particular, safety inspections will be carried out on all critical

sections of the dam that include but not limited to the following areas:

Dam Crest

Upstream Embankment

Downstream Embankment

Offtake Chamber, Spindle and Valve

Spillway Crest and Channel

Toe Drain

Table 1: Key Features and Inventory of Tono Dam

Item Section Description Unit Quantity Remarks/Condition

1. Dam Earth Dam - -

Reservoir

Capacity

Million

cu.m

92.6

Catchment

Area

Sq. km 650

Top Water

Level

MASL 179.22

Design Flood

Level

MASL 181.69

Maximum

Water Depth

m 15.14

Maximum

Height of

Embankment

m 18.59

Lowest

Ground

MASL 163.98

Level in

River Bed

Top of

Embankment

MASL 182.57

Top of Wave

Wall

MASL 183.2

Reservoir

Surface Area

Ha 1860

2.

Dam crest Level MASL 182.57

Width m 5.7 Good condition.

Camber % 6 Camber not maintained

Length km 3.471

Gravel

Surface

- Good condition.

Height m 12

3.

Upstream

embankment

Slope 1:3 Maintained

Rip-rap slope

protection

- Good condition. Tree stumps

observed but controlled and

prevented from growing tall.

4. Downstream

embankment

Slope - 1:2.5 Maintained

Slope

protection

- Gullies observed on the slope. Need

to be filled with earth material and

grassed.

Also, open paved drains along the

sloping surface filled partially with

earth and seem non-functional.

5. Offtake Invert MASL 171.85

Chamber No. 1 Good but filled with water at the

time of visit. Need to drain the

water.

Valve No. 2 One gate and one butterfly.

Spindle No. 1 In good condition.

Outlet No. 1 In good condition.

6. Spillway Crest height m 3 Uncontrolled Ogee type. In good

condition.

Width m 60 Verified.

Crest Level MASL 179.22

Design Flow cu. m/s 496

Channel - - Good. However, some side walls

have tilted thereby creating gaps.

Walls need weep holes to relieve

them of pressure during surcharge.

7. Scour Tower

Valve

Chamber Chamber filled with water. May be

due to leakage from the walls and

valves.

Access

bridge

No. 1 In good condition but signs of

settlement affected hinge support,

noting wider than normal expansion

joint of Scour Tower walkway.

Valve No. 1 Submerged continuously. Not

operated for over 25 years.

Spindle No. 1 In good condition.

Outlet No. 1 Blocked with earth.

8. Provision for

Domestic

Water

Valve No 3 Water is abstracted at three

different elevations into a 400mm

pipe for domestic use. Not yet

utilized.

1.2.1 Consultancy Assignment Scope

The scope of the Consultancy Assignment is:

a) Assessment of the condition of the structure based on visual observations, review and

analysis of data on the design, hydrology, construction, operation, maintenance and

performance of the structure;

b) General assessment of hydrologic and hydraulic conditions including review of design

floods;

c) General assessment of seismic safety of dam based on site specific seismic

parameters;

d) Evaluation of operation, maintenance and inspection procedures;

e) Evaluation of the downstream conditions and the need for emergency concept

f) Evaluation of any other conditions which constitute or could constitute a hazard to the

integrity of the structure; and

g) Evaluation of monitoring instrumentation associated with providing data that aids in

evaluating the safety and operation of the dam (i.e. hydrometric stations, climate

stations, seepage weirs, piezometers, survey points, reference points, inclinometers,

extensometers, foundation baseplates, crack monitoring devices, etc.).

h) Make recommendations for necessary monitoring instrumentation for dams

1.3 Specific Tasks of the Consultancy

The specific tasks to be performed in the dam safety assessment are the following:

Review of existing reports and relevant documents on dam safety of Tono Dam, if any.

Inspection of each dam and appurtenant structures as well as assessment of

conditions including reservoir.

Experience based assessment of flood risk, seismic risk and structural stability

Evaluation of safety condition of the dam.

Recommendation of remedial measures with cost estimation including prioritization.

Proposition of monitoring system (including instrumentation needed) for the dam,

related to dam safety and operation.

2 Methodology and Analysis


In order to accomplish a comprehensive geotechnical assessment of the Safety of these

Dams, a systematic geotechnical evaluation of the dams was carried out which include:

a) performance of detailed dam inspections

b) assessment of current foundation stability

c) assessment of current embankment stability

d) assessment of the safe slopes of the embankment

e) assessment of cracking due to differential movements

2.1.1 Detailed Dam Inspection

Visual inspection was undertaken by the geotechnical team on 27th February, 2017 on the

Tono Dam as part of the reconnaissance survey. This was immediately followed by a desk

study of all available design drawings that were made available to the IPC Team.

The visual inspection involved the examination of:

a. embankment (construction and performance)

b. spillway crest and channel (design and adequacy)

c. geological outcrops in the vicinity relating to foundation design, characteristics and

performance

2.1.2 Assessment of Foundation Stability

The assessment of the dams’ foundation stability was carried out through a review / study of

the general geology of the region, seismicity of the region, as well as relevant fieldwork.

2.1.2.1 General Geology of the Region

The Upper East Region of Ghana is generally underlain by rocks of Sedimentary-basin

Granitoids (also known as Cape Coast type Granites). The lithology of the Sedimentary-Basin

Granitoids comprises granite, biotite and muscovite granite, granodiorite, pegmatite, aplite

with biotite schist pendants. These rocks are of middle Precambrian and are at times well

foliated.

The granites are characterized by the presence of many enclaves of schist and gneisses.

Figure 1: Geological Map of Upper East Region (Bright K Amegashie, Charles Quansah, Wilson A Agyare and Paul L. G. Vle March 2011)

2.1.2.2 Seismicity of the Region

Micro-seismic studies have indicated that Ghana’s seismicity is associated with active faulting,

particularly along the Akwapim fault zone (in the Akwapim range) which trends approximately

NE – SW, 20km to the west of Accra, or along the Coastal Boundary faults which lies some

3.0km off-shore and runs almost parallel to the coastline of Accra. Indeed seismic activities in

the country are concentrated and most felt at areas where these two (2) causative faults

intersect close to Nyanyanu near Kasoa. (Geological Survey Department)

The Upper East Region (project site) is some 845km away from the two causative fault zones.

Also, no major or splinter geological discontinuities such as faults have been mapped within

the region. Therefore, the region can be described as traditionally non-seismic.

2.1.2.3 Fieldwork

The fieldwork involved the sinking of trial pits, sampling, performance of DCPT, and laboratory

testing of soil samples.

2.1.2.3.1 Trial Pits

In order to determine the condition and stratification of the subsurface soil and to facilitate

sampling, trial pits were sunk manually at carefully selected positions at the dam site.

Table 2 below gives the GPS locations of each trial pit and the depth explored.

Table 2: Trial Pits and their GPS Locations

Trial Pit No. Depth

explored (m)

GPS Location

W/N

TCP1 & TP1 1.5 0701582/1201184

TCP2 & TP2 1.3 0701469/1201261

TCP3 & TP3 1.5 0702951/1203289

The difficulty in manual excavation within highly weathered rock material formed the basis of

termination of trial pits. All trial pits were backfilled. The soil profile in each trial pit was carefully

logged.

Soil sampling was done at a frequency which enabled an accurate description and

characterization of the ground to be made and provide enough representative samples for

laboratory testing.

All Samples were sealed in airtight plastics, labelled immediately after taking from the trial pit

and protected from excessive heat and temperature variations.

2.1.2.3.2 In-situ Test

The dynamic cone penetration test (DCPT), an in-situ test that gives a measure of the relative

densities and also determines if there is soft compressible layer at depth, was performed at

some selected points of the dam site. The DCPT points are shown in Figure 2.

DCPT3 & TP3

DCPT2 & TP2

DCPT1 & TP1

Figure 2: Trial Pit and DCPT Locations

The locations of the Trial Pits and DCPT points were chosen bearing in mind that such

activities had the potential of destabilizing the dam; a concern expressed by GCAP/GIDA at

the Contract Negotiation Meeting, (See Annex Error! Reference source not found.). The

locations shown in

Figure 2 were therefore selected.

The DCPT employs various forms of rod with cone which are driven down into the soil by

blows of a drop hammer. The number of blows for a given distance of 0.1m penetration is

recorded.

The results of the DCPT given as penetration resistance, “r” (No. of blows/10cm of penetration)

were recorded for plotting and analyses.

2.1.2.3.3 Laboratory Testing

The samples were sent to the Central Materials Laboratory of Ghana Highway Authority, Accra

for testing. Relevant laboratory tests were performed on 19th April, 2017 on representative soil

samples recovered from the trial pits to determine the physical and engineering characteristics

of the subsoil material.

2.1.3 Assessment of Embankment Stability

The stability of an embankment depends basically on the characteristics of foundation and fill

materials as well as the geometry of the embankment section.

Consequently, the assessment of the embankment stability was undertaken via:

a. the consideration of the upstream and downstream slope under static loading as well

as full supply level, maximum flood level and rapid drawdown cases

b. the type and nature of slope protection

c. type and method of surface drainage

d. determination of the engineering characteristics of the embankment fill materials

e. growth of trees close to or near the embankment


The Hydrological Investigations conducted on the Tono Dam included; electrical resistivity test

(field and laboratory), assessment of hydrologic features and failure modes, analysis of

hydrologic loads and downstream conditions for emergency situations. The details of how

these activities were carried out are described in the following sections.

2.2.1 Electrical Resistivity Test at Tono

The main reason for carrying out the electrical resistivity test was to help identify the flow paths

occurring within the dam. The resistivity test provides an image of the portions of the dam

where there are likely to be some Geophysical weaknesses or areas of water movement for

consideration. The resistivity results are then compared to ground observations such as ponds

of water, intake points and areas of high water levels to draw meaningful conclusions.

2.2.1.1 Field Electrical Resistivity Profiling

The electrical resistivity measurements commenced on the 21st of March 2017 and ended on

the 25th of March 2017. Resistivity profiling measurements was conducted along

predetermined profile lines running along the Tono dam. The measurement layout showing

the profile lines for the resistivity profiling is shown in Figure 3.

Figure 3: Tono dam showing one profile lines for resistivity measurements

The resistivity measurements were conducted using multi-electrode cables and the

SuperSting R1/IP resistivity equipment, utilizing twenty eight electrodes with a spacing of 5

meters. To cover the whole length of profile line along the dam crest, the roll along technique

was used. The dipole-dipole configuration was utilized and the measurement protocol was

programmed for the equipment to take the measurements automatically.

2.2.1.2 Laboratory Electrical Resistivity Tests

Soil samples from the two dam sites were taken to the KNUST Civil Engineering, Geotechnical

Laboratory to determine their electrical resistivity. The samples were saturated with water

samples obtained from the respective reservoirs at the dam sites. The saturated samples were

placed in the soil box and the resistivity determined.

2.2.1.3 Electrical Resistivity Tomography

The electrical resistivity data collected at each of the four profile lines at the two earth

dams was processed using the EarthImager 2-D software to obtain an image of the

subsurface on each of the profile lines. The measured resistivities were inverted using

Quasi Newton method to obtain an inverted or true resistivity section of the subsurface.

2.2.2 Hydrologic Features and Failure Modes of the Tono Dam

The assessment of the hydrologic features of the Tono Dam first commenced with the review

of design information provided by SMEC. The following were carried out on the inspection of

hydrologic features;

Assessment of the spillway capacity of the Dam

Assessment of the structural integrity of the spillway

Investigation of spillway channel to ascertain its current state and presence of

cavitation damage.

Inspection of the Spillway Channel to about 550 m downstream of the Tono spillway

crest (Figure 4)

Inspection of the upstream section of the Dam was also undertaken to help identify,

the activities ongoing upstream of the reservoir and their implications on inflows and

sedimentation of the reservoir (Figure 5).

Assessment of piezometers and observation wells.

Physical investigations to identify possible signs of cracks, seepage and piping,

activities of rodents and reptiles on the dam.

Assessment of the effects of tree stumps and gully erosions on the dam.

Figure 4: Inspection of the Spillway chute of the Tono Dam (GPS Track in Blue)

Figure 5: Cultivation of Pepper on the Upstream Banks of the Tono Dam

2.2.3 Hydrologic loads (Flood Risk) of Tono Dam analyzed

The assessment of the hydrologic loads of the Tono Dam first commenced with the collection

of hydrologic design data, dam water levels, spillage, irrigation water use, dam volume

elevation curves, catchment area of dam, rainfall data and evapotranspiration data. Not all the

information could be obtained, notably were the spillage, irrigation water use and dam

hydrologic design data. Daily Time series information provided by ICOUR and could be used

spanned the period 2000-2014.

Despite the lack of essential data, the team used the water balance approach to estimate the

Inflows into the dam using the Water Evaluation and Planning Model (WEAP).

2.2.4 Investigation of the Dam’s downstream conditions for emergency situations

The downstream conditions for the Tono Dam were also investigated with respect to

emergency situations. The investigation covered the following:

Inspection of areas around the spillway channel and the drains for possible flooding.

Interaction with farmers cultivating along the spillway drains to ascertain flooding

incidents that they have encountered.

Sand winning along the drains which could increase the spread of flooding was also

investigated.


2.3.1 Design and construction data of Tono Dam reviewed

The hydraulic investigations conducted a review of the hydraulic components of Tono dam as

designed. The hydraulic components reviewed include the Spillway, Spillway chute, spillway

sections and drains.

2.3.2 Integrity of appurtenant structures examined

The assignment investigated issues of erosion around and along the spillway chute,

overtopping of the spillway, erosion or washout at the downstream of the spillway, obstructions

to spillway and other outlets, conditions of control structures, defects in spillway structures,

design capacities of spillways for extreme events and defective drainage systems. These

investigations included site observations, field measurements, physical inspection and

capacity analysis.

2.3.3 Tono Dam channel for flow discharge capability assessed

In order to preserve the integrity of the Dam against overtopping, spillways are provided to

take care of high flows which are likely to harm the dam structurally. The ability to evacuate

the flows depends on the spillway capacity. Since the spillway is already installed, the study

tried to assess the capacity of the spillway against various flows and its ability to safely

discharge the flows without harm to the dam and neighboring surroundings.

2.3.4 Tono Dam downstream conditions assessed and need for emergency concept

ascertained

The downstream conditions for the Tono Dam were also investigated with respect to

emergency situations for the hydraulics of flood evacuation. The spillway condition and

possibilities of blockage in the spillway were investigated. Furthermore, the extent of flooding,

downstream of the dam for various flood scenarios need to be investigated for the downstream

emergency conditions to be assessed. This however, is recommended for future studies as it

requires resources beyond the capacity of this study.

A physical investigation of the downstream end of the project area up to about 1km was

conducted to ascertain the closeness of infrastructure to the stream. It was observed that

majority of the areas are farmlands which are part of the irrigation scheme. The closest

infrastructure to the stream is within a distance of about 70m which is safe for most flooding

scenarios. However further investigation which requires the acquisition of High Resolution

Digital Elevation Models will be able to help provide flooding extents for various emergency

situations needed for an emergency preparedness plan which happens to be beyond the

scope of this assignment.

2.3.5 Installations and instrumentations for Hydraulic Failure assessed

The study also conducted investigations on the existing installations and instrumentations for

regular assessment of potential hydraulic failures in the dam. A physical review was conducted

on the current state of existing installations and an inventory was made on them. Following

that, literature review of dam installations and instrumentations was conducted to help advise

on the best installation and instrumentations for the dam.


The Mechanical Services were performed as follows:

2.4.1 Desk review of data and information on all mechanical equipment on the Tono

dam

Two (2) drawings, Valve Tower and Offtake Arrangements and General Arrangement, Scour

and Water Supply Valve Tower, on Tono Dam were reviewed.

2.4.2 Inspect the inlet and outlet gates and valves of the structures on Tono dam.

All valves were submerged in water so no detailed inspection could be carried out. However,

the visible parts were observed.

2.4.3 Desk review of all operation and maintenance reports of the gates and valves

No written Operation and Maintenance Reports of the gates and valves were available to be

reviewed. Discussions were held with the staff on the maintenance history of the valves.

2.4.4 Desk review of the maintenance plan of all the mechanical components of the

facilities

There is no maintenance plan of mechanical components. Discussions were held with the

operator of the valves who said the stem of the valves, above water, are lubricated before

operation at the beginning of the irrigation season.

2.4.5 Desk review of the manufacturer's instructions and compare to current use and

operation of the components

There were no manufacturers’ instructions on the valves to be reviewed. Information was

obtained from manufacturers of valves such as Cameron, Asahi, AIL, Smith and Engineering

360 and information pertinent to what is available on Tono was obtained.

2.4.6 Examine instrumentation readings for adequacy and analyze for trends as well

as any information that can be deduced from them, propose any additional

instrumentation required

No instrumentation readings were available to be reviewed.

2.5 Review all the dam operator training programs and reports if any

There were no training programmes and reports. There was a list of “Training Needs” which

listed external courses on offer.

3 Major Findings and Implications


3.1.1 Visual Inspection of Dams

Some of the defects identified at this dam site include:

a. Lateral movement of the spillway wall panel resulting from lateral earth pressure on

the spillway channel walls as depicted in Figure 6 and Figure 7 below

Figure 6: Close up of Joint of replaced section of Spillway Wall indicating lateral movement

Figure 7: Joint of replaced section of Spillway Wall indicating lateral movement

b. Widened expansion joint resulting from differential settlement of supports on the scour

tower bridge as depicted in Figure 8and Figure 9 below.

Figure 8: Expansion Joint wider than normal

Figure 9: Change in level at column support due to settlement at hinge support

c. Spalling of concrete within floors of spillway channel

Figure 10: Spalling of concrete in floors of Spillway Channel

3.1.2 Foundation Stability

3.1.2.1 Subsoil Condition

Within the depths explored, the geological successions at this site as revealed by the trial pits

consist of three distinct layers: topsoil of Sandy Silt, underlain by laterised gravel grading to

hardpan and residual soil of micaceous Silty Clay.

The depths of the trial pits ranged from 1.3m – 1.5m. The trial pit logs are presented as figures.

Geo 1 – Geo 3 in the Annex 6.1.1Trial Pit Records

Typical grading curves of the subsoil materials have been plotted and given in Annex 6.1.4.Soil

Test Results

3.1.2.2 Strength Characteristics

The dynamic cone penetration tests (DCPT) results have been plotted and presented in Annex

6.1.3. These tests performed gave very high penetration resistance values (N-values) that

were increasing with increasing depth.

Beneath 0.5m depth below existing ground level, penetration resistance “r” values ranging

from 21 to 70 blows/10cm were recorded upstream whilst beneath 0.4m depth below existing

ground level, penetration resistance “r” values ranging from 17 to 88 blows/10cm were

recorded downstream.

3.1.2.3 Laboratory Results

Tests performed include determination of natural moisture content, Atterberg’s limits, particle

size distribution, California Bearing Ratio (CBR) and compaction test.

All tests were performed in accordance with BS 1377-1990. Summary of laboratory test results

is as presented in Annex 6.1.4 Soil Test Results.


The stability of the existing embankments has been assessed by the following criteria:

3.1.3.1 Slope Protection

3.1.3.1.1 Upstream

Upstream slopes are exposed to wave action and thereby require extensive treatment for

protection. At the Tono dam site, upstream slope protection is ensured by providing riprap

since it appears riprap is the preferred type of upstream slope protection.

Figure 11: Upstream Slope Protection at Tono Dam

A minimum of 300mm thick riprap was measured to have been provided up to the top of dam.

3.1.3.1.2 Downstream

The downstream slope protection at Tono dam site is ensured by vegetative cover as depicted

in Figure 12 below.

Figure 12: Vegetative cover to downstream slope at Tono

3.1.3.2 Surface Drainage

For surface drainage of downstream slope, a system of open paved drains (chutes) along the

sloping surface terminating in the longitudinal collecting drains at the junction of slope is

provided at approximately 50m centre to centre to drain rain water at Tono dam site, as

depicted in Figure 13 and Figure 14 below.



The section of drain is trapezoidal having depth of about 30cm. The open paved drains

(chutes) should ideally terminate in the downstream rock toe or toe drain; however, this was

not exactly so at this dam site. They were freely terminated, allowing water to exit freely and

find its own way and thereby has the tendency to cause erosion at the toe.

These drains seemed non-functional at the time of the safety assessment.

3.1.3.3 Engineering Characteristics of Embankment Fill

The engineering characteristics of the embankment fill materials were obtained by testing of

samples at Ghana Highways Authority (GHA) Materials laboratory. The CBR and compaction

characteristics of the embankment fill materials were tested for analysis. The results of these

tests are presented in Section 6.1.4.

3.1.3.4 Geometry of Embankment Section

The designed geometry of the embankment sections made available to the team assessing

the safety of the dam as indicated below.

Figure 15: Cross Section of Tono Dam

As per the design, the core of the embankment fill is made of clay, capped with sand and

gravel filters in succession. These filters are then capped with the riprap.


This section provides the major findings from the hydrological assessment conducted on the

Tono dam. The hydrological assessment provides findings from the electrical resistivity test,

flood risk analysis, hydrological failure modes, and downstream conditions for emergency

situations and instrumentation for dam safety monitoring.

3.2.1 Visual Inspections

The downstream face of the dam wall and its immediate surroundings showed no sign of

wetness or saturation areas. However, a key observation was that a wetland (covering an area

of about 1,250m2) exists about 100m downstream of the dam wall.

3.2.2 Results and Implications of the Electrical Resistivity Test at Tono

The laboratory electrical resistivity result indicated the electrical resistivity of the saturated soil

from the Tono dam was almost the same with resistivity of about 20Ωm. This information was

used in the interpretation of the results obtained from the field electrical resistivity

measurement.

The continuous resistivity profiling was adopted at the Tono dam and the image showing the

about 1km electrical resistivity section is provided in Figure 16. The lower resistivity sections

observed from the beginning of the profile line to about 500 m can be considered to be possible

saturation or seepage zones. The higher resistivities may be possible weathered rock material.

Figure 16: Resistivity Image for Profile along Tono Dam

Low resistivity areas (3.5-30 Ωm) were observed in Tono, signifying points of saturation or

seepage within the earth dam. Physical inspection revealed the creation of wetlands

downstream of the dam which have direct link with the observed points of saturation or

seepage from the resistivity test results.

The rate of seepage in the Tono dam is evident because of the wetland it has created.

The total area of the wetland which can be considered as being a result of seepage is

about 1250m2. Because of the fact that the wetlands are stable with no flows it can be

assumed that the seepage rate is almost equivalent to the rate of evapotranspiration from

the wetlands created by the seepage. From the Ghana Water Resource management

study (1997), the mean annual Potential Evapotranspiration of the Tono area is 1950mm.

With the wetland having an area of about 1250m2, this translates into an average seepage

rate of about 6.68m3/day. This seepage rate is manageable and does not pose any serious

threat to the dam water storage. The seepage observed in Tono doesn’t require any

structural remedies however there is the need for instrumentation to monitor the seepage.

3.2.2.1 Conclusion

The visual inspection and field resistivity measurements have been able to delineate possible

seepage zones which were observed as low resistivity (2-50 Ωm) zones. It can therefore be

concluded based on the resistivity images that the dam is experiencing some amount of

seepage. This confirms the existence of flow paths which are aiding the observed seepage.

3.2.3 Flood Water Evacuation of Tono Dam

The flood risk of the dam was assessed by first estimating the flood water evacuation capacity

of the spillway. The flows of various flood events were compared to the spillway capacity to

determine the risk of the dam to flooding.

3.2.3.1 Spillway design discharge assessment

During large rainfall events, runoff from the catchment areas increases, leading to a large amount of water flows into the reservoir. The potential is that the reservoir level may rise above the dam crest. Therefore, a spillway provides avenues to evacuate the high flows to avoid overtopping of the dam. Floodwater evacuation through the spillway creates an emergency situation in which the discharge could exceed the design discharge. The analysis of the design spillway capacity is shown as follows:

Figure 17: Sketch of an Ogee crested weir spillway

Figure 18: Geometry of the Ogee crest

The discharge of flow is a function of the spillway design parameters Total Head Line (THL).

The spillway of Tono dam is Ogee type [15]. Per design flow conditions, the flow rate per unit

width is given as,

Where

qdes is the flow rate per unit width in m2/s,

Cdes is the design discharge coefficient in m1/2/s and a function of the Ogee-crest shape

Hdes is the upstream design head, in meters, and

is the crest height, in metres.

To estimate the flow through the spillway, equation (2) provides determination of the design

discharge coefficient by assessing the ratio of crest height to the upstream design head over

the crest height. The design discharge coefficient, Cdes of Ogee crest is presented graphically

in Figure 19

width is given as,

Where

qdes is the flow rate per unit width in m2/s,

Cdes is the design discharge coefficient in m1/2/s and a function of the Ogee-crest shape

Hdes is the upstream design head, in meters, and

is the crest height, in metres.

To estimate the flow through the spillway, equation (2) provides determination of the design

discharge coefficient by assessing the ratio of crest height to the upstream design head over

the crest height. The design discharge coefficient, Cdes of Ogee crest is presented graphically

in Figure 19.

Figure 19 Graph of discharge coefficient against ratio of crest elevation to upstream head over crest

elevation

Now the Design flood level = 181.69 m

Top water level = 179.22 m

Spillway crest height = 3 m

Therefore, spillway ground level/elevation = (179.22 – 3) m = 176.22

Spillway Design Head, Hdes = Design flood level – Spillway ground level

= (181.69 – 176.22) m

= 5.47 m

Therefore,

From Figure 19, 1.2145 corresponds to a design discharge coefficient, Cdes of 2.146 m1/2/s.

Applying equation (2), the design discharge per unit width is given by,

qdes = 2.146 x (5.47 – 3)3/2,

= 2.146 x (2.47) 3/2 = 8.292 m2/s

The total design discharge, Qdes (m3/s) through the spillway is obtained by multiplying the

design areal discharge, qdes (m2/s) by the width of the weir crest, 60m. The total design

discharge is given by,

Qdes = qdes x 60m

= 8.292 x 60

= 497.5 m3/s

Therefore, the total design discharge of the Tono spillway is estimated to be 497.5 m3/s. The

Dam owner provided 496 m3/s as design discharge of spillway. The estimated design

discharge of spillway compares to the figure provided, recognizing the margin of 1.5 m3/s.

Henceforth, the estimated discharge will be used in the discussions following.

3.2.3.2 Historical Flood Analysis

ICOUR provided historical water levels of the reservoir for the years 2010 to 2018 which is

shown in Figure 20. The graph shows that out of the 9 years of record, there has been spillage

5 times with none reaching the design flood level. Meanwhile the flood occurs within one

month to three months. From the chart it is obvious that the 2012 flood event is the most

severe in recent times. It is therefore important to find out on the impact of the 2012 spillage

on the downstream inhabitants of the dam and also to find out on previous and more severe

flooding and their impacts as well. In doing so, the historical rainfall of the area was analysed

to determine the very wet years and interrogate the flooding impact of those periods.

Figure 20 Daily Reservoir Water levels of Tono from 2010 to 2018

Historical rainfall analysis of the area (from 1961-2005), as shown in Figure 21, provides the

major rainfall experienced in the area since the construction of the Dam was in 1991, which

was equivalent to a 50-year return period rainfall. Interacting with management of the dam on

how the 1991 rainfall affected the operations of the dam, they noted that most of them were

not at post at the time and no record to that effect was available. However, on the recent floods

(2009 and 2012), they indicated those events were manageable, noting that those floods did

not create any severe impact on the downstream communities of the dam.

Figure 21 Historical Annual Rainfall at the Tono Dam

Therefore, with the prevailing conditions until today the integrity of the dam is proven, and the

spillway has performed its function as required. It can be noted, however that, the dam is yet

to experience any extreme flood event that threatens the safety of the dam and life and

property of downstream inhabitants. The records of the dam levels show that, the spillage

experienced so far in the dam is not up to the spillway design capacity.

3.2.3.3 Flood Risk Analysis

The flood risk assessment conducted on the dam was able to provide the Peak inflows for the

dam for the 20, 50, 100, 200, 1000 and 10,000 year return periods. The results are shown in

Table 3.

Table 3: Peak Daily Flows of Tono Dam

Return Period (year) Discharge (m3/s)

20 369.2

50 424.7

100 466.4

200 507.8

1000 603.9

10,000 901.4

A graph of the spillway discharge against the elevation of the water level at the spillway (Figure

20) was also plotted to assess the vulnerability of the dam to various flood events. The design

flood discharge of the spillway is about 497.5m3/s which coincides with the design flood level,

at a height of about 2.48 m above the spillway crest. The height of spillway wall above the

crest is about 3.4m, a little below the Top of Embankment of the dam.

Clearly, at a return period of 200 year and beyond, the design flood level will be exceeded and

the spillway capacity will be challenged to pass the floods associated, where reservoir is full.

Figure 23 Tono Dam Spillway rating curve

Beyond the design flood level, any flood wave will pose serious threat to integrity of the dam.

3.2.3.4 Flood Routing

Reservoir flood routing determines the outflow hydrograph from a given inflow hydrograph and

known reservoir characteristics. It is often accomplished by means of hydrologic routing, which

is a method that considers the reservoir as a lumped system and computes the flow as a

function of time at the reservoir outlet. The outflow from the reservoir is computed as a function

of the water level, which is itself a function of the stored volume in the reservoir.

The flood routing hydrograph for the design flood event (1 in 1000 and 1 in 10,000 year) was

routed through the reservoir, based on the standard weir discharge equation and the level-

storage relationship for the reservoir. The reservoir was assumed to be full at the start of the

event.

Table 4: Flood risk analysis of Tono Dam

Reservoir

Peak

Flood (1

in 1000 or

10000)

(m3/s)

Water

level above

spillway

crest (m)

Embankment

Freeboard

from water

level Crest

(m)

Freeboard of

Maximum

flood Level to

Top of Wave

wall (m)

Remarks

Tono

603.9 2.81 0.54 1.17

The design freeboard of

embankment is 0.9m which

will be reduced to about

0.5m at this level of flow. The

spillway channel with height

of about 3.4m is adequate to

pass the flood waters.

Therefore, spillway is

adequate, implying dam is

safe for 1 in 1000 year flood

741.0 3.22 0.13 0.76

The design freeboard of

embankment is 0.9m and

will reduce to about 0.13m

at this level of flow. The

spillway channel with height

of about 3.4m is adequate to

pass the flood waters.

Therefore, flow is expected

not to overtop the spillway,

making it adequate, to pass

the 1 in 10000 year flood.

The flood routing of the dam was estimated to determine its impact on the embankment of the

dam. Table 4 shows the peak inflows and implications for spillway to transport the flood waters

while assessing the likely impacts on the dam. Without considering flood attenuation, it is

observed that even though a 1000-year flood discharge is greater than the design flood

discharge of the spillway, the maximum flood level and wave run-up level in the spillway leaves

a free heard of about 1.17m to the top of wave wall and 0.54m to the embankment. This implies

that, there will be no overflow above the wave wall of the spillway in the case of a 1000-year

flood. Similarly, in the event of 10,000-year flood, the maximum flood level and wave run-up

level without considering flood attenuation leaves a free heard of 0.88m to the top of wave

wall and o.13m to the embankment. This implies that, there will be no overflow above the wave

wall of the spillway in the case of a 10,000-year flood. Therefore, without considering reservoir

flood attenuation in the extreme event of the occurrence of a 10,000- year flood, the Tono dam

is safe.

Table 5: Flood inflows and outflows from the spillway considering Reservoir Attenuation

Reservoir Inflow (m3/s) Outflow (m3/s) Attenuation

Tono 603.9 302 50%

741.2 370.6 50%

In a previous study conducted by Norconsult in association with Mott McDonald Ltd and

Watertech Ltd on the Vea in 2007 a flood attenuation of 50% was used for the reservoir flood

routing of the dam. Applying the flood attenuation to the Tono dam the outflows through the

spillway are reduced by half. The inflow and outflow peaks for the design flood events of 1000-

year and 10,000-year return, together with the degree of attenuation, are shown in the Table

5. Basically, the attenuation is the temporary storage of the inflows before they are released

through the spillway. This storage has the potential to reduce the rate of outflow through the

spillway. Therefore, applying the flood attenuation of 50% to the inflows and comparing to the

spillway characteristics of the makes the Tono dam safe for both a 1000-year and 10-year

extreme flood event, because the spillway is capable of conveying the flows.

3.2.4 Hydrologic failure mode of the Tono Dam

The hydrologic failure modes likely to occur in dams include cavitation damage, boils/piping,

soil/rock erosion, seepage/leakage, spillway chute wall overtopping, stilling basin sweepout,

stagnation pressures, concrete failure, dam slope stability, trees and rodent activity, flooding,

slump and cracks (Figure 24). The existence of the failure modes were determined from the

electrical resistivity test, physical investigations and flood risk analysis conducted. The results

from the flood risk analysis have been discussed in previous section. The study could not rely

on piezometric readings because there were no records.

Figure 24: Common failure mechanisms of Dams (Source: Dam Safety Manual of Ghana)

The major findings on the hydrological failure modes are presented as follows:

3.2.4.1 Seepage/Leakage

The field investigations of the Tono dam observed the presence of wetlands about 100 m

away from the toe of the dam downstream. Further interrogations revealed that, these

wetlands are not as a result of waste water from canals. This is because in the dry season,

one expects a very dry condition in such areas. One of the possible reasons for the wetlands

is the result of seepage or leakage. Further physical investigations did not reveal any seepage

or leakage at the toe of the dam. The electrical resistivity test was conducted to help give

further proof to the presence of seepage in the dams. The geophysical investigations revealed

that seepage exist in the dam, stretching along 40% of the Tono Dam. Coincidentally, the

locations of the seepage areas fall in line with the areas of the wetlands downstream of the

dam. This confirms that there is seepage taking place in the dam. The seepage rate of the

Tono Dam is 6.68m3/day

3.2.4.2 Presence of Pipes/Boils and Cracks

The electrical resistivity revealed portions of low resistivity in the dam, which indicates the

presence of water in those sections of the dam. Physical investigations on the dam did not

observe any cracks or pipes and most especially at the locations of the low resistivity.

The major finding is that there are flow paths in the dam which are contributing to the seepage

observed in the dam.

3.2.4.3 Displacement of Riprap/Soil Erosion

The displacement of the riprap in dams occurs at the upstream end of the dam. This affects

the structural integrity of the dam. The continuous displacement of the riprap at the upstream

face of the dam will expose the clay and laterite materials to the harsh waves of the reservoir

leading to erosion of the dam and subsequent failure. The Tono Dam has no such challenges

and it is in a good condition.

Soil erosion occurs in dams and most especially the downstream face of the embankment

which usually has no riprap. Soil erosion if not taken care of can develop into gullies and

become paths for dam destruction. Gullies due to erosion were observed in the Tono dam

(Figure 24).

Figure 24: Gullies on the downstream embankment of Tono dam due to Soil Erosion

The upstream investigations conducted on the dam indicate that, the Tono reservoir

catchment is highly susceptible to sedimentation due to the heightened recession farming

along the shores of the reservoir. There is little vegetation along the reservoir shore line to

reduce the sediment flows into the dam.

3.2.4.4 Trees and Rodent Activity

The activities of rodents and the penetration of the roots of trees can aid in providing avenues

for piping in dams. Investigations conducted indicate that, trees are controlled on the dam.

Some tree stumps were observed on the dam, but they are prevented from growing tall. Efforts

should be made to remove the tree stumps as the roots can still develop deeper into the dam.

A summary of the findings identified from the investigations conducted on the failure modes is

provided in Table 6.

Table 6: Hydrologic Failure Mode of Vea and Tono Dams

Failure Mode Observations Remarks

Tono

Flood Risk Analysis

The Tono Dam and spillway capacity is capable of handling 1 in 1000 year flood event without any harm to the Dam.

The current Dam condition is safe for the worst case flood scenario for the dam.

Seepage or leakage

Wetlands observed beyond the Toe of the Dam. Seepage zones in the dam were confirmed from the Electrical Resistivity Test.

Piezometers installed in Tono are not being monitored.

Boils, Piping or Cracks

The seepage zones observed in the Dams give indication of flow paths. No pipes, boils or cracks were observed.

There was no physical observation of pipes, boils or cracks.

Displacement of Riprap/ Soil Erosion

The upstream face is safe with no signs of displacement of riprap

Sections of the downstream face of dam experiencing gully erosion. Some are deep and need immediate attention

There is the need for routine checks for the upstream and downstream slopes and regular maintenance schedule.

Trees and Brush Rodent Activity

Shrub observed in both faces of the dam. No big trees growing on the faces of the dam. No rodent activity observed.

Routine Maintenance schedule should take care of trees and rodent activity.

3.2.5 Tono Dam upstream and downstream conditions

From the field investigations conducted, it was observed that, at Tono recession agriculture is

seriously practiced along the banks of the reservoir which has the tendency to increase the

rate of siltation of the dam. Efforts must be made to improve the protection of the upstream

catchment of the Tono dam.


This section reports on the findings of the hydraulic components and functions of the dam in

relation to the safety of the Tono Dam. An overview of the integrity of the spillway and

appurtenant structures are provided firstly followed by the assessment of hydraulic failure

modes, downstream conditions and the conditions installations and instrumentations.

3.3.1 Integrity of appurtenant structures

3.3.1.1 Outlet Structure

The outlet structure looks properly constructed and no leakages were observed

Figure 24: Outlet Structure

3.3.1.2 Spillway

The integrity of the spillway chute and walls of the Tono dam is very sound. One observation

made was the tilting of a section of the Spillway Chute Wall which was reconstructed. The tilt

is as a result of earth pressure behind the wall panel. This section requires regular observation

in order to determine when to replace it.

The stilling basin of Tono is also sound and has no defects. However, it was observed that, a

broken section of the spillway wall, is comfortably sitting on the stilling basin acting as a

blockage to the water flow which reduces the spillway capacity at the stilling basin (see Figure

25). The broken wall must be removed from the stilling basin to allow full capacity of the

spillway.

Figure 25: Obstruction of the Spillway at the stilling basin of the Tono Dam from a broken channel wall.

3.3.2 Hydraulic Failure Modes of the Tono Dam Assessed

There are key hydraulic factors that can affect the integrity of the dam and cause failure. These

factors include cavitation damage, spillway chute wall overtopping, stilling basin sweepout,

stagnation pressures and concrete failure. The failure modes have been assessed for the dam

and are described as follows. A summary is also presented in Table 7.

3.3.2.1 Cavitation Damage

Cavitation is the result of critical combination of velocity, pressure, vapor pressure, bumps and

unevenness on the concrete surfaces, which causes a deviation in the water flow lines and

decreases the pressure in some areas. When water flows over a dam spillway, the

irregularities on the spillway surface will cause small areas of flow separation in a high velocity

flow, and, in these regions, the pressure will be lowered. If the velocities are high enough the

pressure may fall to below the local vapor pressure of the water and vapor bubbles will form.

When these are carried downstream into high pressure region the bubble collapses giving rise

to high pressures and possible cavitation damage (Nohani et al., 2013).

No cavitation damage was observed at Tono.

3.3.2.2 Spillway Chute Wall Overtopping

If the spillway chute is subjected to discharges larger than the design discharge or air bulking

or cross waves were not incorporated properly into the design, flow depths in the chute will

increase and the walls may overtop. Overtopping flows will likely initiate erosion in the wall

backfill which has the potential to progress to the point of undermining the spillway chute slab

and failing the invert of the spillway. Once this occurs, headcutting can initiate and progress

upstream, ultimately leading to a breach of the reservoir (Bureau of Reclamation, 2015).

No signs of spillway chute wall overtopping were observed for Tono.

3.3.2.3 Stilling Basin Sweepout

Stilling basins are used to dissipate the energy of water exiting the spillway of a dam. Their

purpose is to prevent scouring that occurs when high-velocity water enters the downstream

reach of the dam. This scouring can damage the foundation of the dam, and also causes

severe erosion downstream. The primary method of dissipating energy is to generate a

hydraulic jump to transition flow from supercritical to subcritical.

The stilling basin of the Tono dam is made up of boulder pack with sizes ranging from 450-

900cm. Because of the low head between the Spillway crest and stilling basin, the velocity

generated is not too high and as such, the boulder pack are able to dissipate the energy safely

without any harm to the dam.

The Tono spillway basin sweepout is located about 550m downstream of the chute and has

free standing baffle blocks about 20m before the end sill constructed with boulder packs.

3.3.2.4 Stagnation Pressures

Stagnation pressure related to spillway failures can occur as a result of water flowing into

cracks and joints during spillway releases. If water entering a joint or a crack reaches the

foundation, failure can result from excessive pressure and/or flow into the foundation. If no

drainage exists, or if the drainage is inadequate, and the slab is insufficiently tied down, the

build-up of hydrodynamic pressure under a concrete slab can cause hydraulic jacking. If

drainage paths are available, but are not adequately filtered, erosion of foundation material is

possible and structural collapse may occur (Bureau of Reclamation, 2015).

The investigations revealed that, there is lateral pressure on a section of the Spillway chute

wall that has tilted. The tilting has created a gap where seepage can take place between the

wall joints and therefore has the tendency to affect the foundation of the wall at that section.

This is what led to the failure of the previous wall. Attention must be given to that portion of

the Spillway chute wall and replaced in the near future.

3.3.2.5 Concrete Failure

Concrete failure can result from cracks, corrosion of reinforcement, bulking, sagging of slabs,

erosion/exposure of foundation and displacement of walls. The physical observation made

showed that, the integrity of the concrete was still intact. Using a Schmidt Hammer test to

investigate the concrete strength of the spillway structure reveals a concrete strength of

52.5kN/m2 which is adequate because it is greater than allowable strength of 35kN/m2. The

result shows adequate strength and it’s consistent with the age of the structure.

A summary of the findings on the hydraulic failure modes is provided in Table 7.

Table 7: Hydraulic Failure Mode of Tono Dam

Failure Mode Observations Remarks

Cavitation Damage

There is no cavitation damage at Tono

The dam is safe from cavitation damage

Spillway Chute Wall Overtopping

No observation on the possibility of chute wall overtopping for Tono

The dam is safe from Spillway Chute wall overtopping

Stilling basin sweepout

The Tono dam is adequately designed to take care of the stilling basin sweepout.

The dam is well designed to take care of hydraulic pressures at the stilling basin.

Stagnation pressures

Lateral pressure observed at the section of the chute wall that has tilted.

Attention should be given to openings in the wall of the spillway chute.

Concrete Failure

The concrete structure is sound. Tono has no challenges with concrete strength.

3.3.3 Tono Dams downstream conditions

Assessing the downstream conditions of the spillways of the dam, it was observed that, the

spillway drain is in good shape and as such can channel flows safely from the dam without

obstructions. There is the need to however, prepare flood inundation maps for various flows

in case of emergency situations such as dam break which will be useful for emergency

preparedness plan.


3.4.1 Desk review of data and information on all mechanical equipment

A study of drawings on Tono indicated two sets of valves. They are in the Off-take Tower and

the Scour Tower.

3.4.2 Inspection of the inlet and outlet gates and valves

3.4.2.1 Off Take Tower

The Off take Tower houses two valves in series, a 900 mm Gate valve and a 900 mm Butterfly

valve. The Gate valve is used for opening and shutting the water supply for irrigation. The

Butterfly valve is used for regulation. The Off-take Tower is in the middle of the dam wall, if

viewed in cross section, 2200 mm from the center line of the dam, upstream.

The Gate valve leaks from the valve body into the Off-take Tower well, housing the valves.

The Tower walls also leak into the well, keeping the valves constantly under water.

3.4.2.2 Scour Tower

The second set of valves is in the Scour Tower, thirty meters into the reservoir. It houses four

valves. One is a 1200 mm gate valve, called Scour Valve. The scour valve is used for flushing

silt from the reservoir. Inspection revealed that the scour pipe has been blocked at the

discharge end. This was done after the dam construction when leakage was found from the

scour valve. It is therefore blocked permanently

The tower also houses three (3) 400 mm valves for water supply for domestic purposes. It

takes water at three different elevations into a 400 mm pipe.

The tower leaks through the walls and therefore the valves are submerged in water.

Figure 26: Scour Pipe Outlet Structure

Figure 27: Scour Pipe Outlet Structure blocked with Clay

3.4.3 Operation and Maintenance

No written Operation and Maintenance Reports of the gates and valves were available to be

reviewed. Discussions held with the staff on the maintenance history of the valves revealed

that the glands are not checked for leakage, and they are permanently submerged in water.

3.4.4 Maintenance plan

There is no maintenance plan of mechanical components. Discussions were held with the

operator of the valves who said the stem of the valves, above water, are lubricated before

operation at the beginning of the irrigation season.

3.4.5 Manufacturer's Instructions

There were no manufacturers’ instructions on the valves to be reviewed. Information was

obtained from manufacturers of valves such as Cameron, Asahi, AIL, Smith and Engineering

360 and information pertinent to what is available on Tono was obtained as follows:

3.4.5.1 Gate Valve Function

Gate valves are primarily designed for on-off services. They are best used in systems which

require infrequent use of the valve. The valves are designed for full-area flow to minimize the

pressure drop. Since most of the flow change occurs near the shutoff, the relatively high fluid

velocity causes disk and seat wear and eventual leakage if the valve is used to regulate flow.

3.4.5.2 Butterfly Valve Function

A butterfly valve is from a family of valves called quarter-turn valves. In operation, the valve

is fully open or closed when the disc is rotated a quarter turn. The "butterfly" is a metal disc

mounted on a rod. When the valve is closed, the disc is turned so that it completely blocks

off the passageway. When the valve is fully open, the disc is rotated a quarter turn so that it

allows an almost unrestricted passage of the fluid. The valve may also be opened

incrementally to throttle flow.

3.5 Instrumentation

The study conducted assessed the installations of monitoring devices for dam safety. At Tono,

we found twelve observation wells at the Toe of the dam (Figure 28), however seven of them

had been filled with sand and the remaining five had no piezometers installed (Figure 29 and

Figure 30). During the field investigations we also came across three Piezometers installed

on the downstream face of the dam. Further investigations revealed that, there are no data

collections available on the piezometer readings and they are currently not functioning.

Figure 28: Location of the 12 observation wells at the toe of the Tono Dam

Figure 29: Non-functioning observation wells at the Tono Dam

Figure 30: Observation wells at the Tono Dam

3.5.1.1 Record Keeping

The general observation is that operating records are not gathered and kept.


It was observed that ICOUR did not have an “in-house” training programme. There were no

training reports. Two lists of “Training Needs” which listed external courses on offer for 2016

and 2017 were provided as the training for the staff. (See Section 6.7.2)

A review indicated that there was no technical training for 2016. Technical training listed for

2017 were: Irrigation Water Management for Field Officers and Best Agronomic Practices.

These did not have any institution offering them and there were no dates for them.

4 Conclusions


The Tono Dam in its current state can be described as safe and sound.

4.1.1 Dam Foundation

The trial pits and DCPT results have shown that the foundation of the dam at Tono is very

compact, stable and adequate. In particular, no unsuitable soil material was encountered

within the depths explored and the subsoil materials possess sufficient bearing strengths.


The embankment at this dam site may be described as very stable and safe based on the

following criteria:

The adequacy of the slope protective measures at both upstream and downstream

faces,

Absence of both longitudinal and transverse cracking which are generally caused

by differential settlements or deformations in the foundation, abutments or adjacent

materials within the embankment, and

The adequacy of the embankment section and embankment fill materials.

The ability to safely pass flood flows due to the adequacy of spillways which

precludes excessive pore fluid pressure build up under all conditions of

environment and operations.


4.2.1 Flood risk of the Tono Dam

The hydrological investigation concludes that the Tono dam is safe for flood events up to 1 in

a 1000-year event. The spillway capacity is adequate.

4.2.2 Hydrologic Failure Mode of Tono Dams

The hydrological investigations assessed the various hydrologic failure modes and conclude

as follows on the various failure modes:

4.2.2.1 Seepage/leakage:

There is seepage in the dam. The seepage effects are observed about 100m downstream of

the Toe of the dam where wetlands have been created.

4.2.2.2 Presence of Pipes/Boils and Cracks:

There are no pipes, boils and cracks in the dam although seepage is present, which is normal

with earth dams.

4.2.2.3 Soil/Rock Erosion

There is lack of maintenance on the dam hence the deep gullies present on the downstream

side.

The Tono dam is not experiencing any riprap displacement at the moment.

4.2.2.4 Trees and Rodent Activity

Trees are controlled on both faces of the dam, and prevented from growing tall.

4.2.2.5 Upstream and Downstream conditions of Tono Dam

Siltation is enhanced and will increase in the Tono dam due to the massive flood recession

agriculture taking place at the upstream section of the dam.


4.3.1 Integrity of the Spillway and Appurtenant Structures

The following conclusions are drawn from the findings on the spillway and appurtenant

structures at Tono:

The integrity of the spillway chute and walls of the Tono dam are very sound.

The tilted section of the spillway chute wall panel shows signs of lateral displacement

as a result of earth pressure.

The stilling basin of Tono is sound.

The spillway capacity at the section of the baffle blocks at the stilling basin is reduced

due to the broken section of the spillway chute wall which is sitting comfortably on the

blocks

The outlet structure is sound.

4.3.2 Hydraulic Failure Modes of the Tono Dam

4.3.2.1 Cavitation Damage

There is no cavitation damage.

4.3.2.2 Spillway Chute Wall Overtopping

There are no signs of spillway chute wall overtopping.

4.3.2.3 Stilling Basin Sweepout

The stilling basin of the dam is adequately designed and is sound.

4.3.2.4 Stagnation Pressures

No Stagnation pressure was observed at Tono Dam spillway.

4.3.2.5 Concrete Failure

The concrete strength of the dam is adequate.

4.3.3 Downstream conditions of Tono Dam

The spillway channel is in good shape and can direct flows safely from the dam without

obstructions.


4.4.1 Valves

There are two main types of valves at Tono, gate and butterfly valves.

At Tono, the valves are used appropriately as designed. The safety issue is the leakage of the

gate valve as well as tower walls leaving the valves submerged continuously. Apart from the

inconvenience of having to work in water, there is the danger of drowning in some cases.

An issue at Tono is the end of the pipe from the Scour Tower being blocked.

4.5 Instrumentation

There are twelve observation wells at the Toe of the dam, however seven of them have been

filled with sand and the remaining had no piezometers installed.

Three Piezometers are installed on the downstream face of the dam wall. (These are distinct

from the observation wells). There are no data collections available of the piezometer readings

and they are currently not functioning.


The general observation is that record keeping at the Tono Dam is non-existent. The

information is not gathered in the first place to be kept.

4.6 Dam Operator Training It would be necessary to give the staff technical training as well as training in the importance

of record acquisition and management.

5 Recommendations


The following recommendations have been developed based on the findings of the visual

inspection at the dam sites and results obtained from the field and laboratory tests.

5.1.1 Visual Inspection

All structural and non-structural defects identified during the visual inspection and indicated in

this report by our team at this dam site should, as a matter of urgency, be rectified.


The non-functional system of open paved drains (chutes) along the sloping surface for surface

drainage of downstream slope must be re-instated or re-constructed.

5.1.3 Regular Visual Observations

Regular visual observations are an essential aspect of a program for monitoring long-term

performance. In fact, visual observations by dam staff, trained to look for seeps, boils, shallow

sloughing, cracks or any other signs of distress, to log their observations, and to inform or

contact the responsible engineer when appropriate, is the primary approach to monitoring

long-term performance of the dams. The visual observations should include the spillway and

abutments.

If visual observations indicate a potential problem, it may be necessary to initiate a quantitative

monitoring program to define the problem and assist in selecting a solution.


Data collection on spillage and all water uses should be prioritized

The seepage of the dam should be monitored

There is the need to prepare flood inundation maps for various flows in case of

emergency situations such as dam breach. The maps will be useful for emergency

preparedness planning.

The dam owner/operator should reinstate the drainage system to ensure the smooth

drainage of the dam and ensure the stability of the dam.


The tilted chute wall at Tono spillway should be given special attention and repaired

The old broken wall must be removed from the spillway channel.

The dam operator should keep close monitor the development of the joint and make

necessary treatment when necessary


5.4.1 Off Take Tower

The area around the valves must be dewatered and kept dry at all times.

The 900 mm gate valve with body leakage must be replaced.

The leakage from the walls of the Off-Take Tower should be stopped.

5.4.2 Scour Tower

The well should be dewatered and kept dry at all times

The leakage from the walls of the Scour Tower must be stopped.

The blocked outlet of the Scour pipe should be cleared if found necessary and convenient.

The water supply valves and pipes should be kept in good condition for future use.

5.4.3 Operation and Maintenance (O & M)

We recommend a combination of routine and periodic maintenance programs for optimum

operation of the dams and as a means of continuous safety as follows:

5.4.3.1 Daily Inspections

- Leveling Staff to check markings of scale

- Outlet/control valves to check blockage or restrictions

- Walkway to the Scour Tower

5.4.3.2 Weekly Inspections

- Observation wells to check for seepage or leakage - During filling of the reservoir, downstream slope of the embankment and the

foundation downstream from the embankment should be carefully inspected for

indications of cracks, slides, sloughs, subsidence, impairment of slope protection,

springs seeps, or boggy areas caused by seepage from the reservoir.

- Upstream slope of embankment after periods of sustained high velocity winds and

draw down of reservoir water surface for evidence of cracks, slides, sloughs,

subsidence or damages to the slope protection such as displacement of riprap or other

sighs of serious erosion

- Channel bank or bed erosion and silting

- Condition of riprap areas

- River aggradations or degradations and possible effect on hydraulic operation of

structures involved.

- Abnormal subsidence of backfill of embankment areas.

- Unusual or inadequate operational behaviour.

-

5.4.3.3 Quarterly Inspections

- Dam catchment and reservoir fringes to check river aggradations or degradations.

- Condition of the embankment slopes and the crest.

- Presence and conditions of undergrowth in bottoms and on sides of channels and

estimated effect on tail-water levels

5.4.3.4 Semi-annual inspections

This inspection is expected to be carried out following the rainy season and before onset of

the next rainy season:

- Spillway crest, channel or stilling basin, baffle piers and abutment - Downstream embankment to check for erosion/gullies - Concrete wave wall to check for settlement - Rip Rap for any displacements - During periods of low reservoirs level, the exposed portions of the abutments and the

reservoir floor should be carefully examined for sinks or seepage holes or cracking

5.4.3.5 Maintenance and Operational Schedules (Electrical, Mechanical, Structural

and Dam Instrumentations)

Component Maintenance Activity Frequency

Embankment Vegetation control

Rodent control

Minor earthwork, erosion repair

Erosion protection

Twice per year, minimum

Check once per year, perform as required

Check once per year, perform as required

As required

Spillway Vegetation control

Minor earthwork, erosion repair

Erosion protection

Concrete repair

Twice per year

Check twice per year, perform as required

Check twice per year, perform as required

As required

Intake/Outlet

structures

Trashrack cleaning

Mechanical operation

Internal conduit inspection

Concrete features inspection

After every major storm

Once per year

Once per year

Once per year

Masonry walls Vegetation control

Missing stones

Twice per year

As required

Miscellaneous

Safety

and Access

Features

Vehicle/pedestrian access

route(s) maintenance

Fences, locks, signs inspection

Once per year

Once per year

Valves Routine Maintenance Once per year

Gates Routine Maintenance Once per year

5.5 Record Keeping

Monitoring and surveillance results should be recorded and the records should be kept for

documentation. The data/information should be analyzed, evaluated and reported. The

reports should be also documented. The Dam Operator Training in Section 5.7 below is to

enhance the ability of personnel in record keeping.

5.6 Instrumentation

The filled Tono dam observation wells should be reinstated and piezometers installed

5.6.1 Piezometers

These are used to monitor pore and joint water pressures. Open standpipe piezometers are

generally considered to be more reliable than other types.

A summary of guidance on the selection and use of various types of instrumentation is

presented in Table 8 below.

Table 8: Measurements and Instruments for long term Performance Monitoring

Measurement Type Recommended Instruments

Leakage emerging downstream Leakage Weirs

Performance of relief wells Open standpipe piezometers

Pore Water pressure Open standpipe piezometers

Reservoir Water Levels Staff Gauge

Stage Flow in Spillway Staff

Gauge

5.7 Dam Operator Training It is recommended that the Management and Technical Staff are attached to a similar facility

to acquire knowledge in good record acquisition and management. Training should cover the

following:

Structural, mechanical and dam instrumentations and data acquisition.

Inspection, Maintenance and repair works procedures.

Record keeping and data management.

Irrigation water management.

GIS and remote sensing applications to catchment area management including

planning and monitoring.

Negotiation, conflict resolution and stakeholder engagement.

Report writing and communication.

Monitoring and recording of Dam Seepage

6 Annexes

6.1 Geotechnical Annexes

6.1.1 Trial Pit Records

6.1.2 Grading Curves

6.1.3 Dynamic Cone Penetration Test (DCPT) Results

6.1.4 Soil Test Results

6.2 Terms of Reference

6.3 Work Plan

Schedule for Safety Assessment of Tono and Vea Dams

6.4 Field visits schedule

Schedule of Site Visits

1st Visit 2nd Visit

Designation Date Date Date Date

Mechanical Engineer 26/2/17 29/2/17 27/3/17 30/3/17

Geotechnical Engineer 26/2/17 29/2/17 28/3/17 4/2/17

Hydraulic Engineer 27/2/17 29/2/17 21/3/17 25/3/17

Hydrologist 26/2/17 29/2/17 21/3/17 25/3/17

6.5 List of Stakeholders met during field visits

List of Participants at Meeting Held at ICOUR Headquarters

No, NAME DESIGNATION ORGANIZATION CONTACT Email

1 Sebastian Bagina Ag. Managing

Director

ICOUR Ltd. 0205358328 [email protected]

2 Peter Agao Finance & Admin.

Mgr


3 Agartha Akurugu HR Officer ICOUR Ltd. 0502523292 [email protected]

4 David Atijana Asst, Irrigation Eng. ICOUR Ltd. 0208815494

5 Peter Wilkens T.L. SMEC SMEC 0554450111

6 Zoogah

Augustine T.

Field Extension

Officer


7 Agonnor Ben. Technical Officer ICOUR Ltd. 0209478817

8 Atunya Andrew Field Cashier ICOUR Ltd. 0208504392

Contractor met at Vea Dam undertaking Rehabilitation Work

No, NAME DESIGNATION ORGANIZATION CONTACT

1 David Mbema Site Engineer Proteos Limited 0266464055

6.6 Minutes of Meetings

Minutes of Meetings held at ICOUR HQ, Tono in the

Upper East Region on 27 & 28 February 2017

Agenda of the meeting:

- Introduce the Tono and Vea Dams assessment team to Management of the

facilities and matters arising.

- Discuss the staff strength and training needs.

- Take delivery of documentations to facilitate reporting on the assignment.

- Tour the facilities to ascertain the operations and maintenance of the dams and

appurtenant structures.

Participants at the Meeting:

ICOUR Management Team - Ag. Managing Director: Sebastian Bagina; HR

Officer: Agartha Akurugu; F & A Officer: Peter Agao; Asst. Irrigation Engineer:

David Atujona; Field Cashier: Andrew Atuuya; Field extension Officer:

Augustine Zoogah & Technical Officer (Water Bailiff): Ben Agonnor.

SMEC Team Leader: Robert Wilkens

IPC Team: Stephen Doku, Joseph Suwiir, Maxwell Boateng-Gyimah, Eric

Ofosu Antwi (PhD).

Minutes and key action points noted:

1/ ICOUR Management welcomed the Tono and Vea Dams safety assessment team

to the institution premises, noting that they had received news of the visiting team.

They pledged their support for the process.

2/ Ing. Stephen Doku presented the purpose of the Dams safety assessment for which

reason the team was in the area to undertake reconnaissance survey of the

facilities. He noted further that the preliminary study will be conducted in 2 days and

thanked ICOUR management for the support pledged.

3/ Ag. Managing Director explained the inception of the Tono and Vea Dams

management company, ICOUR. Their mandate was spelt out including (i) operation

and maintenance of the dams; (ii) provision of extension and agronomic services to

farmers; (iii) provision of irrigation water to farmers; (iv) provision of credits and

market facilitation services to farmers; and (v) provision of mechanical services to

farmers.

However, with the GCAP intervention, it seeks to rehabilitate and modernize the 2

schemes in order to (i) improve the infrastructure; (ii) restructure the management

for sustainable development; (iii) Establish Water User Associations (WUAs) and

strengthen their capacities to participate actively in the operation and maintenance

of the schemes and (iii) reduce management operations to bring on board private

sector participation, and thus focus on (a) maintenance of the irrigation schemes;

and (b) provision of water services to farmers.

4/ On staffing and their capacity needs, the Ag. Manager noted a resized outfit from

138 to 42 staff, reducing by about 70%. The organogram is attached annex 1.

However, ICOUR is operating currently at a staffing level of 31. He indicated that

the mode of training of field staff was largely on-the-job training. Others included (i)

soil and irrigation water management (ii) maintenance of irrigation systems. The

team shared concerns of their inability to have the required technical training,

especially for their senior staff and would appreciate if those could be addressed.

In the nutshell, there was the need to formalize the training.

5/ SMEC Rep. noted that he received design drawings from GCAP/GIDA without as-

built drawings. He was candid to share with the dams safety assessment team, but

at a fee.

6/ The safety assessment and ICOUR teams took turns to visit the dams for

inspection, beginning with Tono dam.

- At Tono Dam, the areas inspected include:

o Scour Tower; Offtake structure; Dam Crest and Wall; Dam Toe where

Seepage Wells are located; Dam Embankment; and Spillway.

- At Vea Dam, the areas inspected include:

o Left and Right Intake Structures; Spillway; Crest; Embankment; Toe.

In addition, the team interacted with the contractor in charge of rehabilitation of the

Vea Dam on site to ascertain the progress made with the work.

7/ At a debriefing session, ICOUR Management noted the low morale of staff due to

the slow work pace experienced in recent times. The IPC leader thanked ICOUR

team for their availability and willingness to support the study in whatever capacity

necessary.

ORGANOGRAM FOR THE IRRIGATION COMPANY OF THE UPPER REGION

6.7 Data Collection

6.7.1 List of Data Collected

List of Documents Received

A. List of Drawings Received from GIDA

Drawing No. Drawing Title Rev Date Project Title Format

T1P/304/C19B Dam Sections

Sheet 3

B 2-9-76 Tono Irrigation Project PDF


Sheet 3

B 2-9-76 Tono Irrigation Project TIFF

T1P/304/C28A Plan on Dam A 29-6-76 Tono Irrigation Project PDF

T1P/304/C28A Plan on Dam A 29-6-76 Tono Irrigation Project TIFF

Maximum Section

of Dam; Dam Crest

Details

Ghana Canada Irrigation

Development Project. VEA

PDF

MIN/PT/012 Stability Analysis

for Slope of Dam –

Sheet 12 of 14

July

1965

VEA (YARAGATANGA

RIVER) IRRIGATION

PROJECT

TIFF

B. List of Drawings Received from SMEC

Tono Dam


T1P/304/C78 General

Arrangement

Scour & Water

Supply Valve

Tower Slabs

7/9/76 Tono Irrigation Project JPEG

T1P/304/C3B Centreline of Dam B 15/7/75 Tono Irrigation Project JPEG

T1P/304/C27A Profile Along CL of

Dam (As

Constructed)

Tono Irrigation Project JPEG

T1P/304/C67A Tono Dam, Intake

& Outlet Structures

on Scour Pipe

A 19/2/76 Tono Irrigation Project JPEG

T1P/304/C63 Tono Dam,

Piezometer Layout

19/11/75 Tono Irrigation Project JPEG

T1P/304/C66A Tono Dam, Scour

& Water Supply

Valve Tower 1


T1P/304/C66A General

Arrangement

Scour & Water



Supply Valve

Tower


Sheet 1

B 19/10/77 Tono Irrigation Project JPEG


Sheet 2



Sheet 3



Sheet 6



Sheet 7



Sheet 8



Sheet 9


T1P/304/C28A Plan on Dam A 29/8/76 Tono Irrigation Project JPEG

T1P/304/C66A General

Arrangement

Scour & Water

Supply Valve

Tower


T1P/304/C27A Profile Along CL of

Dam (As

Constructed)

Tono Irrigation Project TIFF

T1P/304/C27C Profile Along CL of

Dam (As

Constructed)

C Tono Irrigation Project TIFF

Vea Dam

Drawing

No.

Drawing Title Rev Date Project Title Format

Maximum Section of

Dam; Dam Crest Details

Ghana Canada

Irrigation

Development

Project. VEA

JPEG

Outlet Works, Plans,

Sections and Details –

Sheet 4 of 12

Oct,

1965

Vea (Yaragatanga

River) Irrigation

Project

JPEG

MIN/PT-

003A

Outlet Works, Sections

and Details of Left

irrigation and Water

Supply Outlets – Sheet 6

of 12

May,

1966

Vea (Yaragatanga

River) Irrigation

Project

JPEG

Revisions to Gatewell and

Access Bridge

4/1/66 Vea (Yaragatanga

River) Irrigation

Project

JPEG

Revisions to Gatewell and

Access Bridge

4/1/66 Vea (Yaragatanga

River) Irrigation

Project

JPEG

General Layout Vea Irrigation

Project

JPEG

General layout sheet 1a Vea Irrigation

Project

JPEG

General layout sheet 1b

(Vea Dam Basin Survey)

Vea Irrigation

Project

JPEG

General layout sheet 1c Vea Irrigation

Project

JPEG

General layout sheet 1d Vea Irrigation

Project

JPEG

MIN/PT-

002

Cross Section along Dam

Axis showing Limits of

Excavation for Cut Off

Trench – Sheet 2 of 12

July,

1965

Vea (Yaragatanga

River) Irrigation

Project

JPEG

MIN/PT-

003

Maximum Section of Dam Sept,

1965

Vea (Yaragatanga

River) Irrigation

Project

JPEG

MIN/PT-

012

Stability Analysis for

Slope of Dam – Sheet 12

of 14

July,

1965

Vea (Yaragatanga

River) Irrigation

Project

P.F.R. A Vea (Yaragatanga

River) Irrigation

Project

Vea Irrigation Project

General Layout – Sheet 1

of 1

Oct,

1965

Vea (Yaragatanga

River) Irrigation

Project

C. Meteorological Data Received from SMEC:

No Title Range

1 Navrongo Monthly WindSpeed (knots) 2010 to 2015

2 Navrongo Monthly SunShine Hrs. 2010 to 2015

3 Navrongo Monthly Min. Temp.(°C) 2010 to 2015

4 Navrongo Monthly Max. Temp.(°C) 2010 to 2015

5 Vea Monthly WindSpeed (knots) 2011 to 2015

6 Vea Monthly SunShine Hrs. 2010 to 2015

7 Vea Monthly Min. Temp.(°C) 2012 to 2015

8 Vea Monthly Max. Temp.(°C) 2010 to 2015

9 Navrongo Monthly Ave. RH (%) 2010 to 2015

10 Element:P.E.T./Unit:mm/Station:Navrongo 2006 to 2013

11 Element:P.E.T./Unit:mm/Station: Vea-Dam. 2006 to 2013

12 Navrongo Daily Rainfall(mm) 1996 to 2015

13 Paga Daily Rainfall(mm) 1996 to 2004

14 Sandema Daily Rainfall(mm) 1996 to 2003

15 Vea Daily Rainfall(mm) 1996 to 2015

16 Zuarungu Daily Rainfall(mm) 1996 to 2015

D. Training Needs Received from ICOUR

a. ICOUR Training Needs 2016

b. ICOUR Training Needs 2017

E. Drawings Received from ICOUR

1. Tono Catchment and laterals

2. Vea Catchment and laterals

3. Reservoir Volume Elevation Curve for Tono,

F. List of Reports Received

a. Vea Dam Engineering Report (for Ghana Water Company Ltd. (GWCL)), by

Nonconsult in Association with Mott MacDonald Ltd. and Watertech Ltd., May

2007. – Received from GCAP

b. Vea Dam Bathymetric Report by Imagen Consulting Ltd, May 2016.-Received

from SMEC

c. Field Visit Report by R Tippins and G. Beavan of Binnie & Partners,1982.-

Received from ICOUR

6.7.2 ICOUR Training Needs

6.7.2.1 ICOUR Training Needs 2016

6.7.2.2 ICOUR Training Needs 2017

7. REFERENCES 1. John Dunnicliff, Gordon E. Green (1968), “Geotechnical Instrumentation for Monitoring

Field Performance”.

2. Junner, N.R, Bates, D.A, Tillotson E., & Deakin, C.S, “The Accra Earthquake of the

22nd June, 1939, Gold Coast Geological Survey” Bulletin No. 13.

3. F.G. Bell – Ground Engineer’s Reference Book, Butterworths and Co. 1987

4. Peck, R.B; Hanson, W.E; and Thornburn T.H.; (1974) - Foundation Engineering, 2nd

edition, Wiley, New York – (1974).

5. Bowles, J.E (1982) – Foundation Analysis and Design, McGraw-Hill International Book

Company – third edition.

6. British Standard (BS. 5930 –1981) – Site Investigations for Civil Engineering Works.

7. Bureau of Reclamation, Stagnation Pressure Failure of Spillway Chutes, Report DSO-

07-07, Technical Service Center, Denver CO, December 2015.

8. Kesse (1985), “Geology Map of Upper East Region – Ghana”.

9. Paulina Ekua Amponsah (2005), “Seismic activity in Ghana: past, present and future”

10. Washington State Department of Ecology (1993), “Dam Safety Guidelines: Part IV,

Dam Design and Construction”.

11. North Carolina Department of Environment and Natural Resources (1985), “Dam

Operation, maintenance, and Inspection Manual”.

12. US Army Corps of engineers (2004), “General Design and Construction

Considerations for Earth and Rock-fill Dams”.

13. A Water Resources Technical Publication (1995), “Safety Evaluation of Existing Dams”

[1] These are not related to the observation wells mentioned above