Feasibility Study

2010

Faculty of Engineering

University of Bristol

10/28/2010

Feasibility study of Irfon Valley

Dam Scheme

Group 19

14501

15052

18134

19329

29213

i Feasibility study of Irfon Valley Dam Scheme

Contents

Executive Summary

1. Introduction 1

1.1 Context 1

1.2 Aim 1

1.3 Objectives 1

2. Initial Site Choices 2

2.1 Site 1 2

2.2 Site 2 3

2.3 Site 3 4

2.4 Site 4 5

2.5 Conclusion 6

3. Final Site Choice 7

4. Hydrological Analysis 9

5. Flood Routing 12

5.1 Introduction 12

5.2 Unit hydrograph 13

5.3 Design Storm 13

5.4 Flood routing 15

5.5 Key Data 16

6. Geological Analysis 16

6.1 Introduction 16

6.2 General Overview 16

6.3 Geology at specific site 16

6.4 Foundation compressive strength 17

6.5 Valley 17

6.6 Materials 18

7. Scheme Design 20

7.1 Dam design 20

7.2 Spillway design 25

7.3 Diversion works 27

7.4 Aqueduct design 28

7.5 Diverted services and dam maintenance 32

8. Environmental Assessment 33

8.1 Introduction 33

8.2 Considerations 34

8.3 Summary 36

9. Health and Safety 37

9.1 Introduction 37

ii Feasibility study of Irfon Valley Dam Scheme

9.2 Site Practice 37

9.3 Site access 38

10. Costing 38

10.1 Preconstruction costs 38

10.2 Scheme costs 40

10.3 Other costs 42

10.4 Total costs 42

11. Conclusion 43

iii Feasibility study of Irfon Valley Dam Scheme

Executive Summary

The following report details a feasibility study into the viability of the construction of a

water supply reservoir in the Irfon Valley, Powys, Wales. The reservoir will supply water to a

town of 200,000 people approximately 10km to the east of the valley.

Several sites were chosen along the length of the River Irfon and were compared with

respect the shape of the valley, the environmental concerns, and the availability of building

materials and the ease of construction.

We decided to further analyse Site 2. The overall decision for choosing Site 2 was the result

of a combination of reasons. One of the overriding criteria was the amount of properties

and land that was going to be flooded, and consequently how much compensation would

have to be paid. The dam site at Site 2 creates the smallest flooded area, floods the fewest

properties, has good site access and lies in close proximity to local building materials.

We decided that the site would suit a rockfill embankment dam, due to the wideness of the

valley and the shallowness of its slopes. This provides good access for the large machinery

that will be necessary for this type of structure and there is a good selection of rock and clay

nearby.

Hydrological analysis of the site has confirmed that the site is suitable for dam construction.

The water supply far exceeds the expected demand. The dam requires a height of 40m and

will have a useful storage of 14.81 million cubic metres.

The chosen site overlies an area of mudrocks, overlain by a thin layer of alluvium, a weak,

loose soil. The underlying mudrock will provide solid foundations, but the alluvium layer

should be removed.

The dam will be constructed of a clay core, sourced from within the flooded area, and will

be stabilised using a rockfill arrangement using locally available mudstones as the fill. The

slope sides will be at 30˚, at a height of 40m, crest width of 400m and a length of 138m. The

core is a tapered design and is 5m wide at the base. The crest of the dam will have a 3 m

wide access road for use during construction and for maintenance.

The spillway of the dam has been designed for a 1 in 10,000 year flood event. The spillway is

16m wide, 460m long to cope with a peak outflow of 158.41 cubic metres.

While the dam construction is ongoing, the current river channel will have to be diverted.

Due to the shape of the valley, the best solution is a cofferdam and culvert design. The two

pipes, encased in concrete, are 2.25m in diameter and are 183m long. The cofferdam will be

7m tall. The combined cost of the culvert and cofferdams will be £454,900.

iv Feasibility study of Irfon Valley Dam Scheme

The aqueduct will carry from the dam site to a service reservoir near to the planned town.

The planned route will be 14.5km long and the pipe will be 1m in diameter. The total cost of

the aqueduct scheme will be £1,386,000. This is the cheapest of the five options considered

and will require the least tunnelling and will not require pumping.

All existing services that run along the valley will be relocated to a new roadway to the west

of the site. This road will use an existing logging track as its base and will run along the top

of the ridge to the town of Abergwesyn. The new roadway will be 12km long and is

projected to cost around £1m. A service road will also connect the new road to the crest of

the dam.

An environmental assessment of the scheme highlighted some of the threatened species in

the valley. Animals such as the otter and Atlantic salmon are rare due to loss of habitat and

plants such as the globeflower only grow in the soil similar to what is provided in the valley.

The reservoir created as a result of building the dam will cover a site of special scientific

importance (SSSI) containing these plants. In addition, standing stones, erected during the

late Bronze Age will also be lost. Large swathes of woodland will be covered and will most

likely have to be cut down prior to the creation of the reservoir.

The overall cost of the dam is expected to be £7.55 million. The total cost for the scheme is

expected to be £22.49 million including a contingency for costs of 15%.

In conclusion, the study has shown that a scheme such as this in the location we have

provided is feasible. Further study must be taken into the exact ground conditions in the

immediate site area and to the reaction of local people and the authorities.

1 Feasibility Study of Irfon Valley Dam Scheme

1.0 Introduction

1.1 Context

A new town of Hywelfynydd is being considered for construction in near Ty-mawr on the

upper reaches of the River Wye in Mid-Wales. It is expected to have a population of

between 150,000 and 200,000, with an anticipated water usage of approximately 300 litres

of treated water per person per day; in addition to currently unknown industrial usage. A

feasibility study is required to determine whether the required water supply can be

obtained from the River Irfon catchment above Llanwrtyd Wells, and how it may be done.

1.2 Aim

The aim of this report is to provide a feasibility study into the creation of a dam across the

Irfon valley to provide water to the new town. The design, cost and environmental impacts

must all be considered and the analysis leading to the recommendations outlined. As such

we have come up with a number of objectives for the report.

1.3 Objectives

1. Find a suitable location for a direct supply reservoir In the Irfon Valley to provide the

town of Hywelfynydd with water.

2. Determine whether this location is suitable using hydrological analysis.

3. Determine whether the local geology can support a dam in this location.

4. Chose a dam type based on research and valley profile.

5. Complete a scheme design for the proposed works.

6. Carry out an environmental impact assessment.

7. Produce initial costing for the scheme and assess cost effectiveness of options provided.


2.0 Initial Site Choices

An investigation into a series of factors was carried out in order to locate possible dam sites.

These features cover the topographical characteristics, environmental concerns and dam

specific aspects.

2.1 Site 1

2.1.1 Topography

This site has a large flat

area at the base of the

valley approximately 80m

across. The river runs

through the centre of the

site and is lined by trees.

Approximately 20m from

the proposed dam site

there is a tributary to the main river. There is a church, graveyard and a bridge downstream

of the dam. There is also a phone line and two minor roads that would need to be relocated.

Site 1 also lies on a fault line, which although dormant may be an indicator of disturbed rock

and subterranean features that we may not be aware of. This will require further and more

detailed site exploration which will be costly.

2.1.2 Environmental Concerns

This site is in close proximity to a church and

graveyard. While we need to pay close attention to it,

as our dam is sufficiently upstream so that we do not

need to relocate these buildings. The site is used as

grazing land for farm animals, which would have to be

relocated. There could also be an issue of noise for

the surrounding residents.

2.1.3 Materials

Due to the profile of the valley at this point and its proximity to the quarry the site lends

itself to a rockfill or earthfill dam. We would procure the clay core from one of the areas of

glacial till shown on the map in appendix 2, however as it is a rocky clay we would need to

provide a wider base for the core. As this is some distance we need to consider the

transportation of the material.

Fig.1 Cross section of valley at site 1

Fig.2 Showing plan of valley at site 1


2.1.4 Dam Construction

Due to this site’s locality to the base of the valley it has good access for large plant meaning

a rockfill or earthfill dam could be constructed quickly and efficiently, with minimum

disruption to the surrounding roads and logging industry. The spillway will be constructed

on the eastern side to avoid relocating the church and graveyard.

2.2 Site 2

2.2.1 Topography

The site has a large flat area at

the base of the valley

approximately 50m across.

There is no evidence of slips on

the surrounding hillsides;

however the river has

previously taken a different

route as shown by a cutting

scar in the bank. There are two minor roads, a power line and a phone line that will all need

to be moved to allow the construction of the dam.


This site is used as grazing land for farm animals,

which will need to be relocated. In addition we will

also need to clear some of the forest. There will be a

substantial amount of disruption to the residents of

the valley who do not need to move and we need to

be aware of the possibility of vibration damage to

people’s properties. There will also need to be a

number of structures moved due the flooded area.

2.2.3 Materials

The profile of the valley at this point again lends itself to a rockfill or earthfill dam. This site

is situated on mudrock with some silt laminations but should be a good source of material

for a rockfill dam. The clay core can be sourced from the borrow pit, or one of the other

glacial till locations.


Fig.4 Showing plan view of valley at site 2



Site 2 has good access to the dam location, the valley is still relatively wide at this point and

we do not envisage any problems in accessing it with construction equipment, as indicated

by the number of logging vehicles passing through the area. We would place the spillway on

the Eastern bank of the river to avoid disturbance to the houses situated downstream.

2.3 Site 3

2.3.1 Topography

The site is situated in a steep sided

valley, with the base measuring

approximately 20 – 30m across. The

valley sides are densely vegetated

with many large trees and

shrubbery. The river channel runs

through the centre of the valley with

a minor road located to the west.

There is also a telephone line which

follows the road, crossing at various

points, which would need to be relocated. There is a property about 100m downstream

which would need consideration during the dam construction phase. There is a picnic area

and Forestry Commission area just upstream which would be affected by the flooded area

and would need consideration.


There is an abundance of large trees in the area, some of

which are used for the logging industry. These could be

removed and sold in preparation for the construction of

the dam. Several upstream buildings, including the

Abergwesyn community, lie within the projected dam

storage area. The hillsides are currently uninhabited by

livestock, however there is Site of Special Scientific Interest

located 200m downstream from the proposed dam site.




2.3.3 Materials

A concrete dam can be considered due to the narrow nature of the valley. The foundations

should be strong enough to support a concrete gravity dam, although further investigation

will be needed. Aggregate can be sourced from the local quarry reducing transportation of

materials.


The dam would preferably be made of concrete due to the valley’s topography. The spillway

would be incorporated within the dam structure, saving space. The riverside road would be

useful for delivering resources to the site. Deforestation would be needed in order to

facilitate the structure, although this may weaken the immediate strata.

2.4 Site 4

2.4.1 Topography

Site 4 has a much narrower base and

steeper sides than at sites 1 and 2. The

Eastern bank is much steeper than the

west, and rocky outcrops can be seen.

These have been subject to much

weathering and as such may be

unstable. The topography of this site

lends itself to a rockfill or earthfill dam.

2.4.2 Environmental concerns

The flood area associated with site 4 extends to

Abergwesyn which would involve not only the

relocation of many residents but also the road junction,

and hence three roads. There is also an area used for

forestry, and a Site of Specific Scientific Interest that

will be affected by the dam as well as a power line, a

telephone line and a bridal path.

2.4.3 Materials

Site 4 is located in close proximity to one of the smaller glacial till deposits, which would be

ideal for the core of a rockfill or earthfill dam. Once again it is mainly situated on mudstone,

which would provide adequate foundation for the aforementioned dam types.


Fig.7 Showing section view of valley at site 4



At Site 4, the valley starts to narrow and access for large construction materials may

become more difficult. There is also a large area that would need to be deforested, possibly

weakening the surrounding soil.

Fig. 9 Table comparing displaced infrastructure for flooded areas.

2.5 Conclusion

Following our investigation of the valley we have

decided that site 2 will be most suitable site to dam the

river. We have made this decision based on a number of

criteria such as the effect on infrastructure, access and

proximity to construction materials. Site 2 will require

the relocation of the least number of structures as it has

the smallest flooded footprint. This has the added

benefit of requiring less land to be purchased. In

addition, it has good site access, which is necessary for

the construction of a rockfill dam to allow plant on

site. The site is close to a quarry, in addition to

sources of glacial till for the core. Finally, this site requires much less deforestation than

upstream sites, therefore lowering the environmental impact further.

Fig. 10 Flooded area map for site 2


3.0 Final Site Choice

Fig.11 Map overlaying all four potential sites

Our final site choice is site number two. After investigating many possible locations up and

down the valley, we decided that this site struck a perfect balance between hydrology,

geology, social and environmental impacts as well as having some advantages from a

construction standpoint.

Fig.12 Map showing the reservoir (green) and maximum flood (red)

All through the site selection process we were keen not to flood the town of Abergwesyn,

and as soon as we had the storage volumes required for each site from our hydrological

analysis we began to optimise our site location. Site two sits just far enough upstream from

a fault line as to not suffer from excessive leakage and is downstream enough that even in a

maximum flood situation where the water would rise from 35m to 38m above the base of

the valley, while the reservoir will be very close, it will not flood the small town.

As well as being upstream of a fault line, it is also just upstream of a church with a graveyard

which would have presented a major ethical challenge to the project. Obviously we were

not keen to interfere with the church of graveyard in any way so we are pleased to be able

to have the dam just upstream of its location.


Fig.13 Cross section of the valley at site 2

The width of the valley lends itself to either an earth fill or rock fill dam, and having a very

wide and flat valley bottom means that we can get large machinery into the valley to place

material and compact it. This should have the positive effect of reducing construction time

even though more material is being placed. In turn this will also reduce our overheads and

should mean that the cost of the project is lowered.

The downstream location of our site is also useful for access. The large machinery required

to build our dam would likely not have been able to be transported up the small valley road

if our site was any further upstream without building separate access roads which would in

turn increase the cost of the build.

As with all the sites there are some negative implications of damming the river at this

location. We will flood the smaller of two sites of special scientific interest (SSSIs) at this

location. While this is unfortunate, we feel that going further upstream would cause more

problems as we would flood the small town of Abergwesyn, we feel this is more significant

than flooding a 0.3 acre SSSI.

There are also some standing stones that will be flooded. These ancient monuments require

special permission to be removed before work would start which we would obviously have

to apply for, but as above we feel that this is the best option to take as the alternative

would be to go further upstream and flood the graveyard in the town of Abergwesyn. Given

that this is a rural community, it is likely that the descendants of those buried in that

graveyard still live in the area, and would be more upset by the graveyard being flooded

than the standing stones.

After considering each site from many standpoints, we believe that site two provides the

best location to create a dam. We feel that the implications of damming further upstream or

downstream of this location would cause more problems for the local people and the

environment.


4.0 Hydrological Analysis

4.1 Introduction

Hydrological analysis is the first step in the determination of whether the dam site is

suitable for supplying the town of Hywelfynydd. Through the process of hydrological

analysis we are able to work out the storage volume and height of the dam.

4.2 Demand

The first step in hydrological analysis is calculating the demand that the dam needs to

satisfy. This is important as it tells us the rate of extraction that the dam has to deal with.

For this project we assumed that the population of Hywelfynydd will be 200000 people, this

was combined with a water consumption of 300 litres/day/person, a compensation flow of

0.15 m3/s and a safety factor of 1.2. This then allowed us to calculate the demand to be

1.013 m3/s using equation 1 in appendix 1. The average flow rate of any catchment must be

at least greater than this in order to satisfy the demand and make a reservoir feasible.

4.3 Flow Rate

There is flow data present at the nearby Cilmery station. There are some pieces of missing

data, which can be reproduced using the Thiessen Polygon Method. The application of this

method is described in more detail in appendix 1. Once the missing flow data has been

reconstructed; then by measuring the catchment area for each of our four sites we were

able to work out the actual flow rate that would enter the dam at each site by using

Equation 3 in appendix 1. The average flow rate for each site can be calculated allowing us

to check the initial feasibility of each dam site. Due to initial calculations that we performed

with regard to the catchment made sure that all of our sites satisfied the initial demand as

shown below.

0

2

4

6

8

0 50 100 150 200 250 300

Flo

w m

3/s

Month

Site one Flow data


Fig. 14 Flow data for the four sites.

4.4 Reservoir Storage Estimation

The design criteria required by the client for this dam is the ability to continue to supply the

town in the event of a one in one hundred year drought. The method used in this project is

the synthetic minimum flow. This involves using the existing flow records available selecting

the lowest monthly runoff for each year and ranking them, which in turn allows a probability

to be assigned to them. This data can then be extrapolated to allow the estimation of the

0

2

4

6

8

0 50 100 150 200 250 300

Flo

w m

3/s

Month

Site Two Flow Data

0

2

4

6

0 50 100 150 200 250 300

Flo

w m

3/s

Month

Site Three Flow Data

0

2

4

6

8

0 50 100 150 200 250 300

Flo

w m

3/s

Month

Site Four Flow Data


one hundred year return period drought or 1% chance drought. This is process is repeated

for consecutive month droughts from two months two eleven months. This is done by

selecting the lowest flow rate in each year for that number of consecutive months. This is

then plotted in figure four in appendix 1. This data is then used to plot the minimum runoff

diagram for the one hundred year return period.

The minimum runoff diagram for site two is

shown in figure 2. This indicates that the

minimum storage required from the Site

Two reservoir, to meet the demand during

droughts, is 14.81 Million m3. This

information is then used in conjunction

with the terrain modelling software which

allows us to model the storage elevation

data for each particular site. This in turn

allowed us to look at the flooded area

present for each site, which allowed us to

look at the impact that placing the dam in each site would have. The flooded area diagrams

are in appendix 5.

4.5 Dam Height

Once the storage elevation curve could be constructed the height required by the storage

can be calculated. This is important information that is used within the selection of the final

dam site.

Dam Height Volume (less

dead storage) Flooded Area

Site # m Million m3 km

2

Site One 28 14.7 1.06

Site Two 36 14.81 1.00

Site Three 25 15.2 1.41

Site Four 32.5 15.1 1.46

Fig. 16 Summary of key hydrological data for the four sites.

4.6 Hydrological Analysis Conclusion

Hydrological analysis has an important role in this project as the relative storage capacities

of each site are an important factor in the final site choice. In addition they are also vital in

Fig. 15 Minimum runoff diagram for Site 2.


the smooth running and fulfilment of purpose of the dam. However it is important to

remember the design criteria that have been applied to the design as they are key to

understanding the limitations of the design. The first that needs to be considered is that we

are estimating the one hundred year return period for the dam from twenty year data and

while the probability analysis underpinning the design are correct and following a design

guide, there is the possibility that this data set will fail to reflect the actual weather

conditions that the dam will experience during operation. In addition the design criteria

imply that a drought event of lower than 1% chance could cause the dam dry. These are

important caveats when considering the design and performance during the use of the dam.

Key information

Design Criteria

Population 200000 people

Water Consumption 300 litres/day/person

Compensation Flow 0.15 m3/s

Safety Factor 1.2

Return Period 100 years

Dimensions for Dam at Site 2

Demand 1.013 m3/s

Average Flow 1.93 m3/s

Storage 14.81 x106 m

3/s

Dam Height (without

freeboard) 36 m

Fig. 17 Hydrological analysis conclusions

5.0 Flood Routing

5.1 Introduction

Flood routing is the technique by which we determined the flow going flow our spillway as a

result of the design storm applied to our catchment. There are a number of design criteria

that have been applied to this stage in the process. They are detailed below.

In this project we will be designing the hydrograph to withstand a one in ten thousand year

chance or 0.01% percent chance storm.


In this project we have been following a simplified version of the design process of the Flood

Studies Report (NERC 1975).

We will also neglect base flow in the calculation of the flow rates for the design storm, this is

due its relative size to the flow generated and as such the base flow is of relatively minor

importance in flood estimation.

5.2 Unit Hydrograph

Statistical techniques are used to determine the shape of our unit hydrograph; these are

described in detail in part 5 in appendix 1. They rely on four key constants (figure 18)

derived from the terrain and region of the dam site. These result in unit hydrograph in figure

20. This hydrograph shows the response of the flow rate from site two’s catchment for each

input of ten millimetre of rain.

5.3 Design Storm

In order to determine the rainfall depth of the design storm first the length of our storm

must be determined using equation 6 in appendix. Duration of our storm is 9 hours.

s1085 23.94812 m/km SAAR 1713.067 mm

Urban 0 MSL 14.17 km

Tp(T) 3.52 hrs TB 8.88 hrs Qp 62.41 m3/s/10mm

Fig. 18 Key topographical constants for hydrograph

0

5

10

15

20

25

30

35

0 2 4 6 8 10Flo

w Q

cu

bic

me

tre

s/se

con

d/1

0m

m

Time Hours

Unit Hydrograph

Fig. 20 Unit

hydrograph

Fig.19 Key characteristics of the hydrograph


In order to estimate the depth of the design storm a number of ratios are applied to

measured storm data to grow the measured data (a two day long storm with a five year

design criteria). These are taken from a number of different tables and charts with the key

figures presented in figure 21 and the equations to apply them are in entry 7 Appendix 1.

The percentage runoff is then calculated to determine the proportion of rain that will

become runoff. This process is detailed in appendix 8. Rainfall over catchment, P, can then

be calculated.

In order to apply the storm to the unit hydrograph we need know how the storm how the

rainfall is distributed during the storm. For this project we have used a summer storm as the

model for the rainfall distribution. An assumption within this model is that there is no snow

melt is included in the flow rates. The storm profiles assume a symmetrical shape with the

peak intensities in the middle of the storm. The percentage distributions are calculated from

the summer storm profile from Flood Studies Report (NERC 1975 vol. 2 pg 44). The

percentage distributions for each hour of the storm are in entry 9 appendix 1. The storm

rainfall is distributed to each hour of the storm; it is now convoluted with the hydrograph to

give flow rates resulting from the rainfall over the duration of the storm. This data is present

in entry 10 appendix 1. Figure 22 shows the flow input for each hour with hour one

representing the inflow for the first hour of the storm.

Hour 0 1 2 3 4 5 6 7 8

Qin m3/s 0 0.76 2.78 7.84 23.43 106.17 196.55 278.55 297.95

Hour 9 10 11 12 13 14 15 16 17

Qin m3/s 249.52 189.54 126.90 65.18 14.56 4.77 1.67 0.44 0.00

r 22 X 42

ARF 0.95 Growth Factor 4.86

M5-2 93 mm ARF = 0.95

M5-DHr 39 mm

M10000-DHr 189 mm

P 180 mm

Fig. 21 Key data relating to storm profile

Fig. 22


5.4 Flood Routing

The general purpose of a flood routing model is to determine the downstream flows from

the input flow into the reservoir. This is important to note if we are looking to protect an

area against flooding. The essential theory behind flood routing is the continuity equation.

Change in reservoir volume = inflow – outflow. This can also be written in terms of discrete

time intervals as shown in entry 11 appendix 1. Both these equations rely on the evaluation

of the reservoir surface, which we assume is level at all times for the purpose of these

calculations.

The reservoir level will be taken from the storage elevation curve for the reservoir location

we have chosen. This is then processed using a flood routing model applying the general

principles outlined above. In addition it uses a constant C derived from the laboratory

experiment of 1.95. Details of the laboratory experiment can be seen in entry 12 appendix

1. A process of trial and error is then used to determine the spillway width that keeps the

flow height of the water below the design criteria which in this project is 3m.This gives a

flood routing graph displayed in figure 23.

Fig.23 Showing the flood routing graph for our reservoir

This gives a number of clues about the information with regard to our reservoir the first is

that while there is a substantial difference between the upstream flow and the downstream

flow it is not as large as in other locations. This is because as the water level rises in our dam

the storage does not increase as quickly as in other locations. This is because the sides of

the valley at site are steeper, the closer the slope is to vertical the closer the increase in

volume will be to linear, where as if the slopes are less steep then the relative increase in

volume will be greater as the flooded area will increase. This ties in with the previous data

0

50

100

150

200

250

300

350

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Flo

w m

3/s

Time (h)

Flood Routing Graph

Inflow

Outflow


about our site in that it has the smallest flooded area of the four trialled, which implies it

has steepest valley sides.

5.6 Key Data

Peak outflow 158.41 m3/s

Peak height (Hd) 2.95 m

Peak Inflow 298.0 m3/s

Peak difference 139.54 m3/s

Spillway Width 16 m

6.0 Geological Analysis

6.1 Introduction

Geological analysis is crucial to ensure a safe and efficient design for the dam.

6.2 General overview

The Irfon valley is mainly underlain by rocks typical of the Ordovician period; rocks typical of

this era are approximately 440 million years old. Geological strata tend to run in a North –

Easterly to South – Westerly direction, with an overall dip of between 200 and 40⁰ to the

North West. Geological maps show that the region mainly consists of turbiditite mudrocks

and some shales. (1) The valley was originally thought to have been shaped by structural

folding, and was made steeper due to glacial activity in the Quaternary period. (2)

6.3 Geology at specific site

The Geology at our chosen site, site 2 was considered suitable for construction of a dam at

this point. The bedrock was mostly comprised of mudrock, which dipped in a North West

direction at approximately 30⁰.

There is an overlying layer of alluvium and weathered rock which would need to be

removed before construction of the dam could begin. Although, in some cases dams can be

built over the material, the thinness of the layer means that it would be preferable to

excavate this material prior to construction to improve stability.

Fig. 24 Flood routing key data


Fig.25 Some overlying alluvium and weathered rock would need to be removed prior to

construction.

6.4 Foundation compressive strength

Foundation material is one of the most important considerations in dam design, and can be

seen to be a common cause of dam failure. An example where this can be seen is in the

collapse of the Teton dam in 1976, where inadequate geological investigation and lack of

proper sealing of the joints in the rocks are both sited as contributing causes to the failure of

the dam.(3)

Fig.26 Teton dam failure 1976

Therefore it is vital to ensure the foundations are suitable for the design.

The mudrock underlying the site has an estimated bearing capacity of 5Mpa (4) and an

estimated compressive strength of 18,700Kpa to 56,400Kpa (5)

6.5 Valley

6.5.1 Topography and stability

The Valley profile at the chosen dam location is relatively shallow and ‘U- shaped’. There is

a flat bottom with relatively gentle sides. The relatively shallow angle of dip of the mudrock


means that we would not anticipate landslides to be a problem. However, the stability of

the valley should be monitored during excavation and Tensar reinforcing anchors may need

to be considered to stabilise the valley sides. The valley topography chart is included in

Appendix 4.

6.6.2 Seepage and Permeability

Whilst the mudrock itself can be considered to be relatively impermeable, the folded nature

of the valley may mean that there could be a high rate of secondary permeability.

Secondary permeability occurs when cracks allow water to enter the rock layer and hence

pass through it. It is therefore necessary to consider the angle and direction of strata dip,

and whether it will be necessary to grout underneath the dam. As the strata dips

northwest, the majority of the seepage should occur into the reservoir area and should not

affect the stability of the system. There is a nearby valley to the west of the area, and

seepage from the reservoir into this valley is a potential concern. As such borehole tests

should be carried out to monitor pore water pressures and any potential seepage.

The presence of faults needs to be considered when assessing the suitability of the site.

There are no faults at the specific site 2, however there are two fairly major faults, one

upstream (the Abergwesyn fault) and one downstream (the Llanwrtyd fault) (1). The upper

of these faults will be submerged by the reservoir area, however it is not anticipated that

the fault will affect the stability of the valley slopes.

6.6 Materials

Selecting the appropriate materials for dam construction is very important. Inadequate clay

core material has also been cited as one of the causes of failure in the Teton dam (3).

6.6.1 Mudrock

The river runs along the mudrock itself, suggesting the material is both strong enough and

impermeable enough to be used as rockfill for the dam. The strength of the mudrock was

also suitable for use in the construction of the dam at Lynn Brianne, a further indication of

its suitability for the rockfill of an embankment dam. Using local materials will then also

reduce transportation costs associated with outsourcing materials.

The Mudrock was tested on site and would not be hard enough to be used as aggregate for

concrete, so alternative sources of aggregate will need to be found.

6.6.2 Quarry

There is a nearby quarry situated to the south east of the site, approximately 1.25 km away

from the proposed dam location. The quarry contains mudrock and Kilsby Tuff, a hard

igneous rock which is much stronger than mudstone and could be used for aggregate. The

band of Kilsby Tuff in the quarry area is approximately 2km long, 100m wide and 40m deep.


Fig. 27 Showing exposed Kilsby Tuff in the nearby quarry. Some weathered rock can be

seen on the top right which would need to be removed before investigation.

The access to the quarry is along the bottom of the

valley, although the access is currently limited and a

new road and conveyor system would need to be put

in place. The environmental impact assessments

associated with the reopening of the quarry will need

to be very detailed as there are a number of

stakeholders and authorities to consider. The time

and associated cost of these would mean that it may

be more economical to source materials from Builth

Wells, especially if constructing an embankment dam

which will not require much concrete.

Fig. 28 Weathered rock (scree) can

be seen at the base of the slope.


6.6.3. Boulder Clay

There are also several areas of glacial till. The largest of these was used as the borrow pit

for the construction of the dam at Lynn Brianne. The more easterly of the two smaller areas

shown in appendix 2 will be flooded by the reservoir area, and although further

geographical investigation will need to be carried out, geological maps suggest there should

be enough boulder clay here for the impermeable core of an embankment dam. Sources of

materials can be seen Appendix 2.

6.6.4 Alluvium

As has already been mentioned, there is a thin layer of alluvium overlying the rock. This will

not be suitable for the clay core of the dam, and so should be removed before construction

of the dam begins.

7.0 Scheme Design

7.1 Dam design

7.1.1 Dam choice

In designing the dam, we first need to determine the most suitable dam type for the

location. There are several different types of dam, which have been divided into two

categories, embankment dams and concrete structures.

7.1.1.1 Embankment dams

Embankment dams are normally constructed of earth or rock materials that can be sourced

close to the site location. An embankment dam would normally be used where the

foundations and abutment conditions were unsuitable for a concrete dam and where

suitable materials for the embankment were present at or close to the site (6) Most

embankment dams are non-homogenous and contain ‘zones’ of different materials. Usual

practice is to place a low-permeability ‘core’ layer at the centre of the dam, with areas of

other suitable material at either side. The upstream face needs to be protected from

erosion by wave action, either by the placing of concrete or rip-rap. Some internal drainage

is also needed to reduce problems associated with the build up of internal pore pressures

and seepage.


7.1.1.2 Concrete structures

Concrete gravity structures consist of large volumes of concrete, and use self weight of the

block to hold back the water. An advantage of concrete gravity dams is that the spillway can

be built into the crest of the dam, so the cost of an additional spillway is avoided. Concrete

dams require a large amount of aggregate and good quality water, which would need to be

sourced prior to construction. The construction period for a concrete dam is longer, more

labour intensive and is also affected by weather conditions.

There are also other types of concrete dams, such as arch or buttress but these require

ground strengths in excess of those found at our site and as such have not been considered

for this project.

7.1.1.3 Final choice of dam type

The flat nature of our valley means that an embankment dam would be most suitable for

the dam location. The valley is fairly wide at its base (approximately 240 m) at this point

and as such a large volume of material is required. The high cost of transporting large

amounts of aggregate to the site means that a concrete dam would not be suitable for our

chosen dam location.

The availability of local materials has meant that we have chosen a rockfilled embankment

dam for our site. As previously mentioned in the ‘Geological analysis’ section of this report,

the local mudrock strata are suitable for use for the rockfill of the dam, whilst the glacial till

could be used to construct the impermeable clay core. Both of the materials have been

successfully used in the construction of the embankment dam at Lynn Brianne, shown

below.

7.1.2 Dam structure

The initial design of the dam has been based on the geological maps and site visit data, as

well as previous examples of dam construction. Detailed geological investigation such as

boreholes as well as further shear strength and bearing capacity tests will need to be carried

out in the area.

The dimensions of the dam can be seen on the cross-section plan shown overleaf.


Fig. 29 Dam through section.


7.1.2.1 General Dimensions

The dam was found to be required to be 40m in height, including the 1m of freeboard.

Details of this height calculation can be seen in Appendix 2. The dam is 400m across the

crest and 138m deep. A 3m wide asphalt road is to be constructed along the crest of the

dam which will be used for maintenance and construction vehicles. The construction

sequence for the dam is included in Appendix 12.

7.1.2.2 Clay Core

The dam incorporates a sloping vertical clay core to provide stability and reduce seepage

through the dam. The clay core will be constructed from the local boulder clay as previously

discussed earlier in this section. The clay core is 3m at the crest and 8m at the base of the

dam. Calculations for these dimensions can be seen in Appendix 2. Construction of the clay

core should be performed in layers, with the clay being placed and compacted slightly wet

of optimum to produce lowest possible permeability. (7) Clay should avoid being placed in

the wetter months to avoid ‘shrinkage cracking’ associated with over-wetted clay (7).

7.1.2.3. Rockfill

The main purpose of the rockfill is to provide stability to the dam. The material being used

as rock fill is predominately mudrock with some siltstone laminations. The mudrock has a

typical ф value of 37° (8). Further laboratory tests should be carried out to check that this is

true for our strata. A Coulomb ‘two- part wedge analysis’ was carried out to ensure stability

and design the optimum slope sides. The optimum slope angle was found to be 30° and the

results for this analysis can be seen in Appendix 11. By changing the location and inclination

of the wedges, it is possible to determine the Factor of Safety at the most critical location.

This was found to be 1.68. As the minimum requirement was 1.5, the dam design can be

considered safe.

7.1.2.4 Foundations

It is necessary to assess whether the foundations were adequate for the embankment dam.

By considering a 1m by 1m cross section block, we can determine the force applied to the

foundations by the dam. A 1m x1m x40m block with a unit weight of 22000N/m3 exerts a

force of 880 Kpa on the ground. As the foundations can withstand a bearing pressure of.5

Mpa this is greater than the force exerted and so the foundations are adequate.

7.1.2.5 Concrete facing

There is a 0.3m thick concrete layer on the upstream face to prevent the rock from being

eroded by wave action. (9)


7.1.2.6 Transition zone

This is a layer of graded rock which is designed to protect the clay core on the upstream

face. Further investigation will need to be carried out on the exact properties of the boulder

clay in order to determine the size and grading of material needed.

7.1.2.7 Filter

The purpose of the filter is to protect the integrity of the clay core itself. The filter allows

water to seep freely from the core, whilst preventing any soil particles from passing (10).

The filter should contain finer material close to the core, and progressively coarser material

towards the rockfill zone. Further investigation will again need to be carried out on the clay

material, as the design of the filter is based on the size of the clay particles.

7.1.2.8 Chimney and blanket zones

The Chimney and blanket drains work in conjunction with the filter to reduce potential pore

pressure build up on the downstream face of the dam. Seepage through the downstream

face of the dam can damage the dams’ structural integrity and can cause failure. It is

therefore important to provide adequate drainage. An example of dam failure due to

inadequate drainage can be seen in the collapse of the Stava Tailings Dam, Italy in 1985. (11)

Fig. 30 The destruction in the valley caused by the failure of the Stava Tailings dam in 1985

The chimney and blanket drains allow water that has passed through the filter to safely run

out through the downstream toe of the dam. The chimney and blanket drains were

designed as 3m thick, in accordance with construction requirements (12) although the

integrity will need to be verified by carrying out further seepage modelling.

7.1.2.9 Grout cut-off curtain

There is an additional grout cut-off curtain provided under the clay core, to reduce seepage

under the dam. The foundations of the dam will be grouted prior to construction to reduce

the secondary permeability of the mudrock.

25 Feasibility Study of Irfon Valley D

7.2 Spillway Design

7.2.1 Introduction

One of the most common caus

mitigated by the design and co

dam at a quickened rate when

perform its important second f

living further down the valley t

7.2.3 Design Criteria

There are three stages to the

the catchment area will respo

which will need to be applied

mitigate against. These steps h

report. The third step is to ap

spillway; this will be covered in

7.2.4 Spillway Design

Once the peak outflow and wid

in the spillway. The type of spil

7.2.5 Laboratory experiment

In order to determine dimens

calculate dimensions which can

Fig. 31 Dimensions of the og

ley Dam Scheme

auses by which a dam can fail is overtopping. T

d construction of a spillway which will allow wate

hen the storage height is exceeded. The spillway

nd function of flood control, which provides ben

ley that could otherwise be affected by high volu

the design of a spillway for the dam the first is

espond to rainfall. The second is to find the a

lied to it in order to represent the level of risk w

ps have been covered in detail in the Flood rout

o apply the results of this analysis to the phys

d in this section.

width is known the weir, chute and stilling basin

spillway that is being designed is an overflow sp

ensions of our weir a lab experiment is perfo

can then be scaled up using comparison factors

by the lab experiment

deriving these values

detail in entry 12 app

values are Hmd=0.044m

Hmd is then compared

growth factor S of 67

allows the model weir t

life size for our dam. T

the weir are shown in fi

The next aspect of th

designed is the chute. T

simple open channel hy

solved using the equatio

13 appendix 1. For this

e ogee weir.

. The risk of this is

water to leave the

ay also lets the dam

benefit to people

volumes of rain.

st is to work out how

e amount of rainfall

isk we are looking to

routing section of the

hysical design of the

asin can be designed

w spillway.

erformed in order to

ctors also determined

ent. The process of

ues is described in

appendix 1. The key

44m and Cd=0.625m.

red to Hd to get a

f 67.045. This then

eir to be scaled up to

. The dimensions of

in figure 31.

f the spillway to be

te. This is a relatively

el hydraulics problem

uations listed in entry

this project it was felt


that the most economical option with regard to the terrain and the dam design is to build a

chute that roughly shadows the slope of the underlying terrain. However as the flow in the

chute must remain supercritical, we had to calculate the critical slope to ensure this flow

regime. The key data is provided in figure 32. A plan of spillway is include in the appendix.

yc 2.15 m

Sc 0.00150319 m/m

Terrain slope 0.2 m/m

Height of Chute Water 0.46 m

Height of Wall 1.25 m

Fig. 32 Key data of the spillway chute

The final aspect of the spillway that needs to be designed is the stilling basin. The stilling

basin is a hydraulic structure which serves the purpose of dissipating the energy that is

generated from the spillway and as such prevents erosion of the river bed. The design of the

stilling basin uses a number of equations which are detailed in entry 14 appendix 1. The

downstream depth is checked using the manning equation. The wall should be taller than

the downstream but not be much if this is the case then wall and stilling basin is sunk into

the ground. In our case this is not required.

Wall Height 1.67m

Stilling Basin Length 24.54m

Height of Water after Basin 0.171m

Fig. 33 Key Stilling basin data


7.3 Diversion Works

7.3.1 Introduction

One of the key parts of the scheme is the diversion works. The works will dictate how the

remaining parts of the project will be timetabled and how they must be completed. The two

options for diverting the water while dam construction goes ahead is as follows: Tunnelling

in the valley wall or by building a culvert under the dam site. Due to site topography, mainly

the mild slopes, a culverted solution was sought.

7.3.2 Diversion Works Design

The culvert will consist of a pair of pipes encased in concrete. The design flood for the

temporary works is highlighted as a 1 in 5 year flood event. This is equivalent to an inflow of

61.79 cumecs. As a cost saving measure the culvert will be paired with a cofferdam system

that will eventually be included in the final dam structure.

The height of the cofferdam has been optimised with respect to the cost of the culvert. The

technical specification of the diversion works are highlighted in the table below.

Diameter of Pipes: 2.25m

Height of Cofferdam: 7m

Length of Culvert: 183m

Cost of Culvert: £44,800

Cost of Cofferdam: £410,100

Total Cost of Diversion Works: £454,900

Fig 34. Technical specification of diversion work.

A graph showing the relationship between the total cost, height of cofferdam and size of

culvert is included in Appendix 9. The costing of the culvert has been made on the

assumption that the concrete is ready mixed and is supplied from the Cemex Ltd. Works in

Builth Wells. The supplier is located around 25km (15.7 miles) from site.


7.4 Aqueduct Design

7.4.1 Introduction

The aqueduct will carry the water from the dam site to the service reservoir at Hywelfynydd.

The straight-line distance between the two is 10.64km, although as the region has many

undulations the route must be carefully considered. The route has been chosen as a result

of careful consideration of the economic, social and construction factors.

The aqueduct will be formed of a single pressurised pipe, capable of carrying the required

amount of water to the service reservoir every day.

7.4.2 Water Demand

No. of Residents = 200,000

Demand per Resident = 300litres/day

Demand = 60,000 m3/day

= 0.69 cumecs

7.4.3 Intake Tower

The intake tower is being designed by an outside company and has been costed at £50,000.

The tower will be connected to one of the culvert pipes used in the temporary work in order

to start the aqueduct route. This is the most cost effective option to start the scheme.

7.4.4 Basic Costs

Operation Cost

Tunnelling £320/m

Trenching £80/m

River Crossing £32,000/crossing

Intake Tower £50,000

Fig. 35 Details of the cost data of the aqueduct

29 Feasibility Study of Irfon Valley D

7.4.5 Aqueduct Routes

Route 1:

Route 3:

Fig. 36 Routes considered for a

The initial route is the most dir

local topography, the route req

The second route bypassed the

route 1 and there was an impro

Unfortunately, the extra distan

South of the site there is a train

shallowest contours in the area

that. Route 3 shows an enormo

On further investigation we dis

least 50m away from the train

further north from the track. A

contours of the land and tried t

a small hill near to the dam site

outside the valley, the tunnellin

ley Dam Scheme

Route 2:

Route 4:

for aqueduct.

t direct between the sites. Due to the mountaino

required a lot of tunnelling and as a result was

the worst of the mountainous areas that were e

provement in the amount of tunnelling that wa

stance made the new route even more expensiv

train line. We believed that the train line would

area and that in order to get the best route we s

rmous saving over previous routes by taking this

discovered that for safety reasons the pipe sho

ain line embankment. Therefore, for route 4 we

k. Additionally, for this route we paid special att

ied to avoid steep slopes. We also tested runnin

site. Although there was a saving on the route o

elling that was required to by-pass the hill was n

ainous nature of the

as very expensive.

ere encountered in

t was required.

nsive.

uld follow the

we should follow

this approach.

should remain at

we moved the line

l attention to the

ning the pipe around

te once the pipe was

as not cost-effective.

The final choice of site, shown

mindful of the contours as well

of the dam.

Route 5:

Fig. 37 Final aqueduct route.

7.4.6 Terrain (Route 5)

Fig. 38 Terrain profile and hyd

0

50

100

150

200

250

0 2000 40

Me

tre

s a

bo

ve

Ord

ina

nce

Da

tum

Terrai

Feasibility Study of Irfon Va

wn below, takes the best from routes 3 and 4. W

well as choosing the best route through the valle

hydraulic gradient of the chosen route.

4000 6000 8000 10000 12000

Horizontal Distance (m)

rrain vs. Hydraulic Gradient

30 n Valley Dam Scheme

4. We have been

valley downstream

14000 16000


7.4.7 Route Costs

Route Basic Route

Cost

Cost of Intake

Structure

No. River

Crossings

Cost of River

Crossings Total Cost

Route 1 £2,810,151.74 £50,000.00 8 £256,000.00 £3,116,151.74

Route 2 £3,184,916.63 £50,000.00 9 £288,000.00 £3,522,916.63

Route 3 £1,462,640.00 £50,000.00 6 £192,000.00 £1,704,640.00

Route 4 £1,979,009.17 £50,000.00 6 £192,000.00 £2,221,009.17

Route 5 £1,175,940.13 £50,000.00 5 £160,000.00 £1,385,940.13

Fig. 39 Cost comparison of aqueduct options.

7.4.8 Diameter of Aqueduct Pipe

The next consideration is to look at how large the service pipe to the reservoir is. The pipe

diameter directly affects how much water can be delivered and how much energy is lost on

the way.

Using the formula ��

�� the head loss was found for the length of pipe in the solution

chosen. As stated above the pipe flow is expected to be 0.69 cumecs and route 5 is 14.5km

long. The available head for this system is 25m. The spreadsheet that calculates the head

loss is included in Appendix 8. The results are given below:

Pipe Diameter (m) Head Loss (m) Available Pipe Length (for 25m

head loss (km))

0.75 48.1 7.5

0.85 25.7 14.1

1 11.4 31.8

1.5 1.5 241.3

Fig. 40 Pipe dimensions and associated head losses

7.4.9 Conclusion

The route chosen for the final design will be route 5. The figure 40 highlights the financial

differences between the different options and the maps show the different routes. The

chosen route is the cheapest option of the five potential pipelines. It is one of the longest at

14.5km but it requires the least tunnelling of any option as well has having the fewest river

crossings. Additionally, the contact the pipeline has to local residents is minimised by

diverting the pipeline mainly through the countryside.


We believe that this is the best and most cost effective route, which will not include the

need for pumping. With the above considerations taken into account, the route should also

be the quickest to construct. Additionally, using the culvert as the outlet for the dam will

save on cost and time for the construction team. The pipe will have a 1m diameter. A

connection detail will be added where it connects to the culvert to facilitate the changing of

the pipe diameter.

7.5 Diverted Services and Dam Maintenance

The existing road which connects Abergwesyn to Llanwrtyd Wells has to be relocated due to

the extent of flooding caused by the new reservoir. The proposed alignment follows an

existing forest track to the west of the flooded area. Utilising an existing route reduces the

need for new infrastructure and minimises the budget. However, the road will have to be

suitably improved in order to accommodate heavy goods vehicles, logging trucks and plant.

Excessive vertical grades will be avoided by meandering up steep hillsides. The proposed

alignment will tie into the existing roads to the west of Abergwesyn and north of Llanwrtyd

Wells.

Other services such as telephone lines and power lines will be repositioned to follow the

alignment of the new road. The total length of the proposed road is 12km with projected

costs of £1,037,000. Details of these costs are in appendix.

Fig. 41 Planned diversion works


There will also be a service road (figure 42) which will connect to a 3m wide asphalt road

running along the 400m wide dam crest. This will provide access to the dam for

maintenance and service inspections. We will implement annual checks to ensure that there

is no damage to the dam which would ultimately cause a catastrophic failure.

Fig. 42 Plan view of the service road

8. Environmental Impact Assessment

8.1 Introduction

It is inevitable that the proposed dam and reservoir will affect the local environment due to

the scale and nature of the project. Therefore it is necessary to consider an Environmental

Impact Assessment to investigate the short-term and permanent changes within the valley.

The undertaking of an environmental assessment is necessary if the structure comes under

schedule 1 in the environmental impact assessment guidelines. The building of a dam comes

under schedule 2 ‘ 10(f) a dam or other installation designed to hold water or store it on a

long term basis’, which states that an environmental assessment only need be carried out if

there is significant reason to do so.(13)

The valley of the River Irfon is home to many protected and rare species and flora and is

location of many Sites of Special Scientific Interest (SSSI). In addition, the site is in a very

rural area. It is clear that the scheme is larger than something that would only benefit the

immediate population. It is therefore imperative that an Environmental Assessment is

carried out. The Assessment guidelines state that the impacts of the new development are

to be fully understood and all possible options to be considered. It is also a way to identify

possible issues early on in order to reduce delays at later stages of the project. Our company

values the need for environmental considerations and engineering design to interact

throughout the project.

Figure 43 shows the sensitive features in the local environment which require special

consideration.


Fig. 43 Sensitive features in the local environment that require special attention

8.2 Considerations

The first thing we considered was how the construction of the dam would affect the current

workings of the valley. Currently the valley is used for a combination of agriculture and

logging. The base of the valley contains fields for grazing animals and the hillsides are

heavily wooded. The construction of the dam would mean that neither of these industries

could occur in the vicinity of the construction site or in the flooded area.

In addition, the construction would require large amounts of deforestation in the area. The

current woodland is sustainably farmed by the loggers and represents a significant

investment on their part. Compensation would have to be given to the loggers, although the

money gained from selling the timber from the deforested area could go some way to

offsetting this cost.

The construction of the dam will require local materials and a large amount of fill is required

from the surrounding area. Much of the high quality fill and core required will not be

available from the flooded area and will have to be sourced from elsewhere. This will leave

scars on the landscape.

Moving quantities of materials will inevitably require the use of large plant. Moving these

through a small valley such as this one will create problems, especially with the logging

industry that is in operation. Tension can be reduced with the fellow valley users and

residents by the creation of slipways, alternative routes and limiting the use of heavy plant

to certain times of the day.


Using any vehicle on site can lead to an increase in dust pollution and the construction team

should go to the correct lengths to reduce this. Dust suppression techniques such as wetting

the surfaces where vehicles will drive are a good way to do this, especially with the constant

availability of water in the area. Furthermore, a rock fill dam may require the use of an on-

site rock crusher and the general site works will incur vibrations in the valley. The

construction team must be aware that some areas, such as near to animals, are sensitive to

these vibrations and should be avoided, where possible. It is also worth considering that all

work that creates vibration and noise must be kept within social hours to avoid conflict with

other valley users.

The next major considerations are the people that will be affected by the creation on the

reservoir. The flooded area created by the dam will cover thirteen properties, of various

purposes. These include residences, farms and farmland and holiday cottages. At least a

kilometre square of grazing farmland will also be covered by the water. Compensation will

have to be paid to the owners of these properties although a compulsory purchase order

may have to be obtained for the entire area in the event of residents being unwilling to

move.

Other utilities that will have to be moved are electricity and phone lines. These are owned

and maintained by Western Power Distribution and BT respectively. Each of these services

will have to be rerouted past the dam to the properties and businesses on the other side of

the lake. This is expected to be rerouted in line with the new road that will be built along the

western side of the reservoir.

The construction and operation of the dam carries a certain amount of risk for the workers

on the site and the people living in the vicinity. A dam collapse would be catastrophic for

everyone and the environment. In order to mitigate this risk, the site team will ensure best

practice is used at all times and that the appropriate checks and tests are carried out when

necessary. It is also the contractor’s responsibility that everyone on their team is fully

trained or appropriately supervised while the work is in operation.

The valley has a rich cultural heritage and has been in use for

at least 800 years. The valley contains some historical

standing stones, which have been set aside as a scheduled

ancient monument (SAM). The stones are estimated to be

from the Bronze Age (2100-750BC) and will require special

permissions to move or affect in any way from the Royal

Commission on the Ancient and Historical Monuments of

Wales. (15) As well as the stones the valley contains two

churches with adjoining graveyards. In order to respect the

wishes of the families of the deceased the two sites should remain unaffected by the dam’s

construction.

The valley is home to a variety of different aquatic species and plant life that thrive in the

unique conditions that the valley offers. The large amount of woodland increases the acidity

of the soil, conditions that the rare globeflower thrives on. There is a site of special scientific

Fig. 44 Globe-flower (14)


interest (SSSI) near to the northern edge of our flooded area that contains a very rare

example of globeflowers in a meadow environment. We have taken special care to avoid

damaging all SSSI’s in the area, including the Nant Irfon national parkland to the north of

Abergwesyn and the Cae Pwll-y-Bo meadow mentioned above.

The river itself has been designated an SSSI by the Countryside

Council for Wales. This is due to its populations of Atlantic salmon

and otter, both of which are threatened species in the UK. It is also

the home to various submerged aquatic plants, such as the Water

Crowfoot, and fish species, such as lamprey and bullhead. Steps

should be taken during construction in order to minimise the

pollution going into the river. The use of cofferdams on both sides

of the site should prevent water contamination but the contractor

needs to ensure that proper waste disposal techniques are used.

The other reason that the river has been graded as an SSSI is

because of its mesotrophic (16) and oligo-mesotrophic types. This

describes the bacterial activity within the water. These particular types are highly suited to

drinking water due to their relatively low nutrients, and therefore low algal content.

Additionally, the water has a high oxygen level, which is good for wildlife.

We plan to reduce the impact on the environment by identifying risks and implementing

preventative measures. For example, we plan to encourage staff to promote recycling in

order to reduce waste from food packaging and consumables. Staff will be trained to use

plant and other machinery efficiently to reduce the consumption of fossil fuels. A detailed

site investigation will be carried out to identify undefined materials and ancient

archaeology. Natural habitats will also be relocated to reduce the environmental impact.

Construction will not be carried out during unsociable hours to reduce the extent of light,

noise and air pollution for the locals. Vibrations will inevitably be created due to the nature

of works, however we plan to spread the onerous works out along the valley and avoid

sensitive areas. (See Appendix 6)

8.3 Summary

In summary, the environmental impact incurred by the project has been investigated and

assessed. Key features have been identified and preventative measures have been put in

place where possible. Particular sensitive areas such as the standing stones have been

focused on. The concluding statement of the assessment is that the dam and reservoir will

have a large impact to the environment, however the scheme is believed to be viable due to

the measures implemented allowing harmony between the design and conservation of the

environment and contained ecosystems.

Fig. 45 Otter (17)


9.0 Health and Safety

9.1 Introduction

Health and safety is an important aspect for every engineering job. Recent schemes such as

‘Zero Harm’ (18) aim to eliminate site accidents altogether and the number of man hours

without incident achieved by different companies can be assessed by potential clients. (18)

Dam construction is considered high risk with concerns to site safety due to the large

volumes of water and construction material required. It is important to highlight as many of

these as possible at the planning stage in order to minimise risk later on.

9.2 Site Practice

Keeping a safe working site will be challenging on such a large project, using a relatively

confined site. The necessity of large plant for the duration of the project will require

stringent safety management systems in order to ensure the safety of everyone on the site

team. To mitigate the risk of collisions and accidents on site, detailed planning of works will

be carried out before the project starts. This should ensure that workers walking around on

site will not come into contact with large machinery.

The site will have closed pedestrian walkways for the workers, which will be separated from

routes the plant uses. Full Personal Protection Equipment (PPE) will also be compulsory in all

parts of the working site, excluding the canteen and inside the site management huts. All

site staff, including visitors will be briefed on site safety and will be refreshed on a six month

basis. All visitors must be supervised by a member of the site team for the duration of their

visit. Groups should be limited to ten per member of supervising staff and no group larger

than twenty should be shown round site at any one time.

In parts of the site where noise and vibration are an issue, additional protective clothing will

be provided. Staff using this equipment will also be briefed on considerate use, to mitigate

the disruption to the local residents.

All staff that will be in charge of heavy plant and machinery must possess the relevant

qualifications and/or licenses in order to use these vehicles. The users of heavy plant must

be briefed on how they can alleviate their environmental impact by driving efficiently and

keeping to predefined routes.

Dust on any site can be problematic and can be a risk to the site staff as well as local valley

users and the environment. Routes used by plant and vehicles will be regularly wetted to

avoid excess dust being kicked up. The water will be sourced from the river due to the

unimportance of water quality.

In the unlikely event of an accident, the site management team will set up checkpoints

around the site which will have basic first aid equipment and a communication link to the

main site office. A fully trained

as well as a central first aider in

In the event of adverse weath

procedures, as set out by the c

event of extreme rainfall event

The site should be well lit, es

workers while moving around

residents and valley users in or

9.3 Site Access

The site will have a large wor

consideration will have to be t

in an efficient and safe manner

It is currently planned to set up

workers can live or leave their

transport the workers. The tow

system via the A483 and has a

Workers should adopt safe wo

traffic laws and respecting the

10.0 Costing

The costing for this scheme ha

These are the preconstruction

length of the project or at spec

10.1 Preconstruction Costs

Fig. 45 C


ined first aider will be on duty in each area of th

er in the site office.

eather conditions, site workers should be awar

he contractor. All compaction works should be

vents and during very cold weather.

, especially during the winter months to avoid

nd site. Hours of acceptable lighting will be dis

n order to minimise impact and disruption.

workforce and the site is small and the acces

be taken on how to transport the workforce to

ner.

t up a compound near to the town of Llanwrtyd

heir vehicles. The site will then offer a shuttle bu

town of Llanwrtyd Wells is well connected to t

s a train station, which has a direct link to Swan

e working practice when travelling to and fro

the local residence while the works are ongoing.

e has been split into three stages over the lifesp

tion costs, the scheme costs and other costs tha

specific times during it. First we will look at preco

osts

Chart of preconstruction costs.

Farmland

Forest

Houses

Service relocation

Site clearing


f the site at all times

ware of the relevant

be suspended in the

void accidents to the

discussed with local

ccess is poor. Special

to and from the site

rtyd Wells where the

le bus into the site to

to the road transport

wansea. (2) (3)

m site, obeying all

ing.

fespan of the project.

that appear over the

reconstruction costs.


10.1.1 Land Acquisition

Before construction of the dam and surrounding infrastructure can begin, the land that is to

be built upon and flooded must be purchased, and cleared of obstacles. Some infrastructure

that is removed will also have to be rerouted elsewhere. With any large engineering project,

even in the countryside these are always going to be large costs. To ensure we had accurate

costs for this vital area of the scheme, we did a lot of research. The land acquisitions were

split into three areas; farmland, forest and housing. After contacting Brigtwells land agents

in the nearby town of Builth Wells we established that in the current market in the Irfon

valley, farmland would sell for between £3,000 and £10,000 an acre. An acre costing £3,000

would likely be very steep land which only a few animals could graze on whereas a £10,000

acre would be the same quality as a paddock. Using our knowledge of the land from the site

visit, we made the conservative estimate that we would budget for each acre of farmland to

cost £7,000. At close to 200 acres of farmland on our site we would pay close to £1.4M to

acquire it. The land agents also informed us that forested land would sell for between

£1,500 and £3,000 per acre depending on the demand for the land. As we saw active logging

while we were in the valley we assumed that demand for forest land would be high and so

budgeted to spend £3,000 an acre on the 13 acres of forest that we would need to flood.

This is by far the lowest cost at just £37,200.

The hardest area for us to budget in terms of land acquisition is domestic housing as once it

becomes public knowledge that the dam could be built, house prices would likely rise

quickly. to assess the market in its current state, we checked the land registry and averaged

the prices of all detached houses sold in the local area in the last 12 months, we then

applied this price to every building that we would need to acquire regardless of whether it

was a house or not to allow us to raise offers on inhabited properties if necessary. Then a

15% contingency was added to the total housing budget in the event that we would have to

follow a compulsory purchase route. The total cost for land acquisition came to £5,600,000.

10.1.2 Land Clearing

The land must then be cleared to make way for the dam and reservoir. There is no need to

demolish the houses as they can have their gas and water cut off and then be flooded. The

forest and farmland however must be cleared as debris could block pipes in the draw off

tower or culvert. Figures from the Spon’s Civil Engineering and Highway Works Price book

2008 suggest that it will cost £239,000 to clear the 200 acres of farmland and 13 acres of

forest that we will be using in this project.

10.1.3 Service Relocation

Relocating services will also be a large cost in the preconstruction phase. Using pricing from

the same book a table was generated to cost the relocation of power lines, telephone lines,

sewerage and water and gas mains.

Service

Electric cable

Sewerage/Drainage

Telephone likes

Water mains

Gas mains

Fig. 46 Breakdown of ser

The total preconstruction cost

10.2 Scheme Costs

Fig. 47 Chart of

10.2.1 Dam

As expected, the dam is the la

largely due to the fact that we

we avoided flooding many prop

dam itself will be 400m wide. T

cubic metres of rock. The che

however the quality of rock in

blast significantly more rock th

We looked at getting higher q

away, but once the transporta

per Km thereafter per cubic m


Price (m) Length to be relocated (m) Co

37 4000 £1

118 4000 £4

28 4000 £1

88 4000 £3

115 4000 £4

£1,

f service relocation costs

ost of this project is expected to be £7,401,000

t of scheme costs.

largest individual cost of the project at just ov

we have a very wide valley to dam. Our choice o

properties and larger areas of farmland but it do

de. To construct the rock fill dam, we require ju

cheapest way to get rock is to blast it from th

k in the quarry is not ideal for our project and w

k than we would use to construct the dam.

er quality rock from a RMC owned quarry in B

ortation costs were factored in (£0.95 for the fi

ic metre) it was more economically viable to

Dam

Road

Diver

Aque


Cost

£148,000

£472,000

£112,000

£352,000

£460,000

£1,544,000

00.

st over £7.5M. This is

ice of site means that

it does mean that the

e just over 1,000,000

the quarry on site,

nd we would have to

in Builth Wells 25Km

e first Km and £0.45

to quarry twice the

am

oad building

iversion works

queduct


amount of rock needed and filter the higher quality rock from the quarry on site. Drilling,

blasting and transporting 2,000,000 cubic metres of rock will cost £5,100,000. Filling and

compaction of the dam materials was the second largest cost of the dam at £2,375,000.

Obviously this is a very labour intensive process so a high cost is to be expected even though

the work in question is not priced as skilled labour.

The weir and spillway require a large amount of concrete, and we looked into a few options

to minimise the cost of over 3000 cubic metres of concrete that need to be poured. The

easiest way to get concrete is to have it delivered premixed and ready to pour into the

formwork, however we investigated other avenues to find a cheaper alternative. Using

ratios provided to us, a spreadsheet was generated to cost the large amounts of concrete

we would require. We priced sand, graded rock and cement as being delivered from the

RMC quarry 25km away, and after investigating different ways of getting suitable water (19)

to mix the concrete (including adding agents to reduce the acidity of the river water and

hiring in a purification system) we found that taking water from the mains at an industrial

rate was significantly cheaper at under £1.25 per cubic metre. After including the costs of

crushing, batching and placing the concrete in formwork on site, the total cost of concrete

came to £173,000. This is a saving or approximately £25,000 over having ready mixed

concrete delivered to the site, at which point it would then have to be placed at extra cost

(20).

10.2.2 Aqueduct

Part of the scheme is to construct an aqueduct to pipe water to the storage reservoir for the

new town. We analysed five potential routes trying to find a balance between the length of

the aqueduct and the amount of tunnelling required to prevent the water from travelling

above the line of hydraulic looses over the run of the aqueduct. The route that we decided

upon requires almost no tunnelling and with a 1.5m diameter tube will cost £1,386,000. This

price includes £50,000 for a draw off tower in the reservoir.

10.2.3 Diversion Works

We plan to use a one stage diversion process in order to construct our dam. This will involve

constructing a cofferdam upstream of our eventual dam site with a culvert running through

it. The culvert will contain two 2.5m diameter pipes that will divert water away from the

dam site while construction is in progress. As the elevation change over the dam site is not

that large, we will construct a second cofferdam downstream to prevent water from flowing

back up and inundating the construction site. Both of these cofferdams will eventually form

part of the dam itself.

There is a cost efficiency process to determine the lowest price for the diversion works. As

the height of the cofferdam increases, its cost of construction also goes up however the

culvert doesn’t need to be as large as the cofferdam can handle a larger surge were a large

rainfall event to happen. A height of cofferdam vs. Cost of diversion works graph was

plotted which showed that the ideal height for the cofferdam is 7m with the total diversion

works totalling £450,000.

10.2.4 Road Building

Given that we will be flooding

devised. In an effort to reduce

road begin and end as close as

road. For this reason it was dec

Due to the elevation changes t

the 4Km that the existing road

points so we think this is the

concrete road has been budget

We will also budget for Km of t

site. This road will use the lowe

already budgeted for in the sc

using similar rock that is not o

enough to be used in roads. Ho

in the budget and look at this s

a cost to be incurred.

10.3 Other Costs

Other costs that do not apply w

onsite staff who aren’t doing t

to the Spon book referenced e

costs and consultancy fees am

at £869,000 and 521,000 respe

10.4 Total Costs

Fig. 48 Graph of all project cos

Preconstruction costs

Scheme costs

Other costs


ing a road to create the reservoir, an alternativ

uce the impact on local people, it was decided

e as possible to the beginning and end of the f

decided to build a road over the hill on the wes

es the new road will be approximately 12Km lo

road travels. This is still the shortest way of co

the best solution for the local residents. The

dgeted at £1,038,000.

of temporary roads to transport materials from

lower grade blasted rock that cannot be used as

e scheme costs. It is possible that the other roa

ot of high enough quality to be used in the d

. However we will retain the cost of purchasing

his situation as a potential saving that could be

ply within the two sections covered so far includ

ng the work budgeted for so far and consultanc

d earlier, on site staff should cost approximatel

amount to 3% of capital expenditure. This wou

spectively.

costs

0 5,000,000 10,000,000 15,000,0

sts

sts

sts


ative route had to be

ded to make the new

he flooded section of

west side of the dam.

m long as opposed to

f connecting the two

The cost of a 12Km

rom the quarry to the

d as rock fill and so is

r roads could be built

e dam yet still good

ing higher grade rock

be made rather than

clude a provision for

tancy fees. According

ately 5% of all capital

would place the costs

0,000


The total cost of the project comes to £22,490,000. This includes a 15% contingency to cater

for unforeseen circumstances that may arise during the construction process. In addition to

this figure, the continual running costs of the site should be considered when assessing the

long term viability of the scheme.

A full breakdown of costing can be found in Appendix 7.

11.0 Conclusion

At the beginning of the project we identified a number of objectives for the successful

completion of the feasibility study about the construction of a reservoir in the Irfon Valley.

These were:-

1. Find a suitable location for a direct supply reservoir in the Irfon valley to provide the

town of Hywelfynydd with water.

2. Determine whether this location is suitable using hydrological analysis.

3. Determine whether the local geology can support a dam in this location.

4. Choose a dam type based on research and valley profile.

5. Complete a scheme design for the proposed works

6. Carry out an environmental impact assessment.

7. Produce initial costing for the scheme and assess cost effectiveness of the options

provided.

The location we have chosen for the dam site is suitable for a number of reasons. The first is

that, as outlined earlier in the report, a dam in this site displaces the fewest number of

houses and requires the relocation of the second least length of services. This was an

important consideration for us as we identified early on the project that large numbers of

displaced population and services would add a substantial amount of compensation to the

project. In addition, we avoid flooding the largest SSSI in the area and avoid flooding any

sites that would cause serious problems of relocation, such as a graveyard.

Through the use of hydrological analysis we have shown that our dam site is more than

capable of servicing the demand placed upon it. This is evidenced by an average flow into

the dam of 1.93 Cumecs compared to the demand of 1.013 Cumecs. Additionally, through

the use of linear regression we have been able to design our dam for a one hundred year

return period or 1% chance drought. Our storage required to weather such a drought is

14.81Mm3. This storage also does not result in a flooded area that displaces large numbers

of people or services.

Through the studying of the various geological maps, assessment of the sites during our visit

and through looking at previous dam projects we have determined that there is readily

available material (mudstone from the valley sides for the embankment and glacial till for

the core) which we can use to construct the different parts of our dam. We know that the

foundations will support the pressure exerted by our dam and that it is not resting on any


major fault lines. However, before a scheme could go much further a site investigation will

need to be carried out to determine the exact nature of the geology and the site conditions.

We chose the rockfill embankment dam type as the valley profile was wide and flat. We felt

that this type of dam suited our site geology, site topography and the local availability for

materials as we were unsure of the viability of reopening the quarry to provide concrete

aggregate. This has been discussed in more detail in the geology section (6.6) of the report.

Our scheme design has taken into consideration the quality of local materials through the

widening of the core and has used wedge analysis to check the slope stability. The width of

138m includes allowances for a wider core to account for lower quality clay. The final height

of the dam of 40m, including a 1m freeboard, is sufficient to account for the lifetime and

purpose of the dam. However there is scope for additional height to be added at a later

date to allow for changing weather conditions as the foundations are only currently being

used at around 20% capacity. In addition, the spillway has been designed to cope with a

10,000 year flood event, which is in accordance with the design guidance on flood

protection.

Our environmental impact assessment has made us aware of the diverse local wildlife, the

industrial importance of the valley, the number of locations of significant historical

importance and the local residents who will be affected by our scheme. We have designed

our scheme to minimise the impact on these stakeholders and have made a number of

recommendations to mitigate any further affect our scheme will have. The most notable is

our focus on minimising the displacement caused by the flooded area of our dam and

reducing the additional works that need to be completed. We have done this by making use

of currently available facilities such as the logging track through the woods, which we plan

to widen and use as our road diversion. This is further evidence of the ethos that we have

tried to apply throughout the project of presenting a holistic solution that is acceptable to

all stakeholders.

The total scheme cost of £22,490,000 and the dam construction cost of £7,550,000 are the

result of a detailed cost analysis where we have assessed each part of the dams design and

construction for cost effectiveness and fitness for purpose. An example of this process is the

choice of aqueduct route. Through detailed analysis of the surrounding terrain and careful

avoidance of tunnelling and pumping we have been able to choose a route that gives a final

aqueduct cost of £1.386 Million, a greater than 50% reduction in cost over all other routes

considered.

The project has succeeded in fulfilling the objectives that were set out at the beginning of

the project. The completion of these objectives is evidence of the comprehensive nature of

the solution and for the feasibility of the project as a whole. The conclusion of this report is

that a water supply scheme at the aforementioned location is feasible and should be

considered for construction.


Appendix 1 - Hydrological analysis

1. Water demand = safety x (abstraction rate +Compensation flow)

Abstraction Rate = population x water consumption

2. Thiessen Polygon Method

Using the rain data from the different stations a weighted average was created for each year. This was

created using the relative areas found by constructing a thiessen polygon. This was then plotted against

the flow data for those months and from that we could work out the flow data for the months where it

was missing using the rainfall during the time period for the flow that was missing. The linear Regression

graph is shown below.

3. Flowrateat damsitei = Cilmery flowrate XDamsitei catchment area

Cilmerycatchment area

where i = 1,2,3 Cilmery catchment =240km2

y = 0.0779x + 2.8651

0

5

10

15

20

25

0 50 100 150 200 250 300

Flo

w D

ata

m3/

s

Rain Fall mm


4.

5. Unit hydrograph

Tp(0)=283.0*S1085-0.33

*(1+URBAN)-2.2

*SAAR-0.54

*MSL0.23

(hours)

S1085 is the stream slope between 10% and 85% of its length in m/km

URBAN is the proportion of built up area in the catchment (=0 here)

SAAR is the standard average annual rainfall

MSL is the main stream length in km

(needs diagram to show the representation on hydrograph.)

6. D = (1+SAAR/1000).Tp then rounded to the nearest odd number.

7. r = M5-60/M5-2 day Rainfalls, X = M5-D/M5-2 day percentage where D is design storm duration.

8. PRRural = SPR + DPRCWI +DPRRAIN

SPR = 10*S1 +30*S2 + 37*S3 + 47*S4 + 53*S5

DPRCWI = 0.25*(CWI-125)

DPRRAIN = 0.45*(P-40)0.7

for p greater than 40 for p less than 40 =0

P= ARFx(MT-D hour) mm

9.

y = 6.0098x-0.767

y = 20.761x-0.805

y = 39.461x-0.748

y = 61.568x-0.656

y = 84.563x-0.585

y = 110.09x-0.51

y = 143.66x-0.429

y = 176.92x-0.35

y = 211.36x-0.292y = 259.82x-0.272

y = 312x-0.269

0.1

1

10

100

1000

1 10 100 1000

Ru

no

ff m

etr

es

Cu

be

d 1

0^

6

Return Period

S1 0

S2 0.05

S3 0

S4 0

S5 0.95

CWI 128

hour 1 2 3 4 5 6 7 8 9

47 Feasibility Study of Irfon Valley

10.

11. S2 − S1 = (I2 − I1

2)δt − (

O2 −O1

2)δ

12.

Measure the discharge using the fl

Measure the head over the top of

Assess whether pressure over the

pressure is negative bubbles will be

Measure the depth of flow over th

Create a hydraulic jump by lowerin

(after jump).

Calculate theoretical values of y1 a

13. V = R2

3 S0 / n and yc

3

2 =Q

b. g the

14.

φ is usually taken as 0.9

depth before the jump

the sequent d

the wall at the

percent 0.75 1.25

V1 = φ 2gh1

y1 =Qmax

bV1

y2 =y1

2( 1+ 8F 2r1 −1)

Q = 1.705bh1.5

∆z = y2 − h

lb = 5y2

ley Dam Scheme

δt S2 − S1 = (I2 − I1

2)δt − (

O2 −O1

2)δt where dt

he flow gauge attached to the pipe in the inlet ta

of the spillway.

the top of the weir is positive or negative using a

ill be observed.)

r the vee weir notch on the outlet tank.

ering the sluice gate. Measure water depths y1

y1 and y2 using the formula �2 � 0.5�1 �1

he second equation utilising the fact that fr =1

s 0.9 to 0.95

nt depth

t the end is treated as a weir hence we ignore en

3 11 68 11

e dt is the time interval.

et tank.

ing a vertical tube. (If

s y1 (before jump) and y2

8� � � 1� .

=1 for critical flow.

e energy loss.

3 1.25 0.75


Height of wall

Appendix 2 - Geological analysis

Fig. ... Geological map showing the locations of glacial till deposits and the Kilsby tuff quarry

Appendix 3 – Dam Design

Regular storage in dam = 36m

Maximum flood water level= 39m (from flood routing#0

Freeboard calculations

� � ��

Where:

d- freeboard

e- water level rise near dam due to wind= 0.3m

ha- wave climbing height= 0.2m

A- Extra height = 0.5m

(Values taken from water resources project hand out)

Glaci

al Till

Quarry


Total freeboard= 1m

Clay core dimensions

Width of clay core at base= 20% of flood water level (1)

40*0.2 = 8m

Width of clay core at crest should not be less than 3m

Appendix 4 - Site 2 Valley Profile

Appendix 5 – Flooded area Diagrams

Site 1 Flooded Area Site 2 Flooded area

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Va

lle

y H

eig

ht

Valley Width

Site 2 Valley Profile


Site 3 Flooded Area Site 4 Flooded area

Appendix 6 – EA Checklist

Welfare faci l i tiesWaste from leftovers , food

packaging and consumables .5 6 30

Promote recycl ing to reduce food

packagi ng. Compost heap for disposable

Changes in des ignIncrease i n environmental

impact from construction.3 3 9

Ensure des igns have long been in place

before construction commences .

Discovery of

undefined materials

Gas/as bestos/ancient

archaeology.3 7 21

Ensure a s i te investigation is carried out

and extra time is al located in the project

Noise generationNoise dis ruption to loca l

communities /fauna.5 5 25 Avoid working unsociable hours .

Dust generationAir pol lution for loca l

res idents /fauna.5 6 30 Empl oy dust suppress i on techni ques .

Vi bration generationVibrati ons to surrounding

environment.5 6 30

Spread the vibration works throughout the

va l ley. Avoid works near s ens i ti ve areas .

Discharge to water

(river/groundwater)

Potential contamination of clean

water.4 6 24

The flow of water should be l imited

through the cons truction s i te due to use of

Use of l ightingLight pol lution to s urroundi ng

community/environment.2 3 6 Avoid working unsociable hours .

Fuel use for pl ant

operation.

Air pol lution, us e of non-

renewabl e resources .5 6 30 Train s ta ff to operate plant efficiently.

Adverse weather

conditions .

Eros ion of materia l s tockpi les

caus ing contamination.5 6 30

Poor weather conditions are expected in

Wales , so avoid s tockpi le of materials

Waste generationPotential pol lution al ong

transport routes .3 5 15

Provide tra ining on waste management,

promote recycl ing.

Habi tat damageLos s of ecosystems from s i te

work, impact to loca l flora.5 6 30

Thorough s i te investigation and ensure

ti me for habitat relocation.

Ris

k L

ev

el

(Lo

w,

Me

diu

m,

Hig

h)

Preventative MeasuresDetails of Hazard Risk/Impact

Lik

eli

ho

od

of

Occ

urr

en

ce (

1-5

)

Se

ve

rity

of

Ris

k/I

mp

act

(1

-10

)

Ris

k S

core

(1

-50

)

Environmental Impact

No

ise

& v

ibra

tio

n

Vis

ua

l im

pa

ct/l

igh

t e

mis

sio

ns

Wa

ste

(so

lid

&li

qu

id)

Em

issi

on

s to

wa

ter

Em

issi

on

s to

air

(d

ust

/od

ou

r)

Em

issi

on

s to

la

nd

Flo

ra a

nd

fa

un

a

No

ise

& v

ibra

tio

n

Re

sou

rce

use

(e

ne

rgy

/ma

teri

als

)


Appendix 7 - Costings

Preconstruction costs

Land/Property

Type Quantity Cost Total

Farmland 197.7 acres £1,384,000

Forest 12.4 acres £38,000

Houses 13 x 1.15 £4,196,000

Total cost of acquisition £5,618,000

Service

relocation

Service Cost/m Metres Total

Electric cable 37 4000 £148,000

Sewerge/Drainage 118 4000 £472,000

Telephone likes 28 4000 £112,000

Water mains 88 4000 £352,000

Gas mains 115 4000 £460,000

Site Clearing

Total cost of service relocation £1,544,000

Type of land Cost/acre Acres Total

Farmland 197.7 1010 200000

Forest 12.4 3112 39000

Total cost of site clearing £239,000

Total preconstruction costs £7,401,000

Excavation, Transportation & Placement

Material Volume Transport cost Placement Material cost

Sand Filter 8200 m3 £0 5000 £40,353

Topsoil 13800 m3 £0 £33,948

Stiff Clay 104000 m3 £426,400 107000 £312,000

Broken Rock 27600 m3 £0 1340000 £193,200

Drilling & Blasting 2000000 m3 £1,900,000 £3,200,000

Total cost of Excavation, Transportation & Placement £7,558,000

Road building

Type of Road length of road Cost

Permanent 12 Km £1,050,768

Service 2 Km £145,128

Total cost of road building £1,196,000


Concrete

Concrete required 3400 m3 distance from pit Delivery cost Material cost

Materials

Sand 1478.105 m3 26 £5,055 £13,820

Graded Rock 2458.625 m3 26 £7,081 £20,898

Cement 374.000 m3 26 £1,077 £26,341

Water 448.800 m3 £555

Production

Crushing 2458.625 m3 £5,901

Batching 3400.000 m3 £12,189

Placing 3400.000 m3 £81,260

Total cost of concrete £175,000

Appendix 8 – Head Loss in Pipe

D = 1 m 0.75 m 1.5 m 0.85 m

V = 0.878535 Q/A 1.561841 Q/A 0.39046 Q/A 1.215966 Q/A

Q = 0.69 m^3/s

A = 0.785398 m^2 0.441786 m^3 1.767146 m^4 0.56745 m^5

hf = 11.40821 48.07409 1.502315 25.71123

Available

L = 31775.37 m 7540.444 m 241294.2 m 14098.9 m

= 31.77537 km 7.540444 km 241.2942 km 14.0989 km

Appendix 9 – Diversion Works

£0.00

£500,000.00

£1,000,000.00

£1,500,000.00

£2,000,000.00

£2,500,000.00

£3,000,000.00

0 5 10 15 20 25 30

Height of Cofferdam

Cost(culvert) v Hc

Cost(coffdam) v Hc

Total Cost v Hc


Appendix 10 – Technical Drawings

55 Feasibility Study of Irfon Valley

Appendix 11 – Coulomb Wedge A

X θ1 θ2 δ

60 70 10 20

60 60 20 20

55 70 10 20

55 60 10 20

50 60 20 20

40 45 20 20

60 55 25 20

25 50 5 20

60 50 5 20

60 40 20 20

50 40 20 20

50 50 20 20

ley Dam Scheme

ge Analysis

FOS using β= 30

0

20 3.32

20 1.86

20 4.27

20 2.64

20 2.89

20 2.17

20 1.76

20 3.61

20 2.65

20 1.7

20 1.68

20 1.9

56

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References

1. University of Bristol, Geology map.

2. Nash, D, University of Bristol 2010

3. Bureau of Reclamation, "The Failiure of Teton Dam".

http://www.usbr.gov/pn/about/Teton.html. Retrieved 2010-10-25

4. Hawkins, A.B, ‘Weathering and rocks’ lectures notes 2008

5. Alberto Foyo, ‘A proposal for a Secondary Permeablity index obtained from water pressure tests

in dam foundations, s.I : Engineering Geology 77, 2005

6. Craig, R.F, ‘Craig’s soil mechanics’ 7th edition, page 364

7. ‘Soil Compaction’ notes, Martin Fahey, University of western Australia (slide 25)

8. Wilson, G.Ward et al, ‘The Influence of Shear Strength Properties on the Stability of Rock Piles’

University of Bristish Columbia geoinfo.nmt.edu/staff/mclemore/documents/gsa_wilson.doc .

Retrieved 2010-10-25

9. University of Bristol, Water resources project handbook 2010

10. Fahey, M, ‘Embankment dams: some geotechnical aspects of embankment dams’ lecture notes.

University of Western Australia. 2009

11. ‘Foundation Stava 1985’ http://www.stava1985.it/ Retrieved 2010-10-16

12. Fahey, M, ‘Embankment dams: some geotechnical aspects of embankment dams’ lecture notes.

University of Western Australia. 2009

13. The landscape institute, ‘Environmental impact assessment: a guide to procedures, Part 1: 2000

Guidelines for Landscape and Visual Assessment Guidelines,’ 1995, ‘http://www.environment-

agency.gov.uk/static/documents/Research/eia.pdf


14. Picture of Globeflower, wikiImages – BernrdH, Retreived 2010-10-21

15. Royal commission on the ancient and historical monuments of Wales, ‘Cwm Irfon

StandingStones’

http://www.coflein.gov.uk/en/site/102/details/CWM+IRFON+STANDING+STONES/


16. Michigan Water Research Centre, Accessed online at:

http://mwrc.bio.cmich.edu/glossary.htm, Retrieved 2010-10-27

17. Picture of Otter, http://www.mtuk.org/, Retrieived 2010-10-21

18. Balfour Beatty Civil Engineering Ltd., http://www.bbcel.co.uk/zeroharm, Retrieved 2010-10-

25

19. Charges for Industrial Water Use, Welsh Water,

http://www.dwrcymru.co.uk/English/library/publications/Scheme%20Of%20Charges/Englis

h.pdf, Retrieved 2010-10-21

20. Cemex Ltd., Local Concrete Supplier, www.cemex.co.uk, Retrieved 2010-10-21


Bibliography

1. Blyth, FGH& de Freitas, MH, 1984, ‘A geology for engineers’ (7th edition)

2. Chadwick, A et al,1998, ‘Hydraulics in Civil and Environmental Engineering’ (3rd edition)

3. DTLR/ National assembly for Wales, 2000. Environmental Impact Assessment: a guide to the procedures.

4. http://maps.google.co.uk

5. Maps of SSSI sites, Countryside Council for Wales. Accessed online at:

• Country countryside for Wales interactive maps, ‘Cae-pwll-y-bo’

http://www.ccw.gov.uk/interactive-maps/official-maps-search/official-

maps.aspx?sitetype=SSSI&sitecode=0272, Retrieved 2010-10-27

• Country countryside for Wales interactive maps, ‘Afon Irfon’

http://www.ccw.gov.uk/interactive-maps/official-maps-search/official-

maps.aspx?sitetype=SSSI&sitecode=0777, Retrieved 2010-10-27

6. Shaw, E. 1993 ‘Hydrology in practice’ (3rd edition)

7. http://www.thetrainline.com/buytickets/

8. United States Department of the Interior, Bureau of Reclamation, 1987. ‘Design of Small Dams’ (3rd edition).

Accessed online at www.usbr.gov/pmts/hydraulics_lab/pubs/manuals/SmallDams.pdf


Previously Submitted Documents

Spillway design Lab report

1. Objective

To calculate the Cd value of the spillway profile in the lab at the design discharge. By obtaining the Cd

value of the spillway profile in the Lab, we can then scale up the model in order to design the spillway

for our chosen dam site. We need to obtain the Cd value at Hd, that is the total head over the spillway

which correlates to the ‘design discharge’, which we are taking to be the ‘maximum probable flood’.

This is necessary to make sure that the dam can cope with the maximum probable flood, and will not

cause damage downstream.

2. Procedure

� Measure the discharge using the flow gauge attached to the pipe in the inlet tank.

� Measure the head over the top of the spillway.

� Assess whether pressure over the top of the weir is positive or negative using a vertical tube. (If

pressure is negative bubbles will be observed.)

� Measure the depth of flow over the vee weir notch on the outlet tank.

� Create a hydraulic jump by lowering the sluice gate. Measure water depths y1 (before jump) and y2

(after jump).

� Calculate theoretical values of y1 and y2 using the formula �2 � 0.5�1 ��1 8� � � 1�

3. Experimental apparatus

Fig.1. Plan view of experimental apparatus

Fig.2 Cross section of experimental apparatus

Inlet

tank Outlet

tank

Stillin

g

statio

Spillway Stilling


4, Results

Qin

(m3/h) Qin(m3/s)

H(spillway)

(m) H(weir)(m) Q(vee)(m3/s) Pressure Cd

8 0.002222222 0.026 0.0753 0.002131611 p 0.554872

10 0.002777778 0.031 0.0869 0.003049791 p 0.532747

12 0.003333333 0.034 0.0932 0.003632958 p 0.556578

15 0.004166667 0.0375 0.1013 0.004474497 p 0.600631

17 0.004722222 0.04 0.1061 0.00502353 p 0.617906

19 0.005277778 0.0425 0.1108 0.005598478 p 0.63057

20 0.005555556 0.0445 0.1137 0.005972027 p 0.619517

22 0.006111111 0.046 0.118 0.00655278 n 0.648409

25 0.006944444 0.051 0.1243 0.007462741 n 0.631172

Fig. 3 Table showing experimental results

4. Analysis of results

It can be seen that there is an approximate 5% difference between the flow measurement from the

inlet gauge and that calculated from the head over the vee weir notch. This can be attributed to a

number of factors.

1. The fact that the equation used to calculate Qvee is an empirical relationship and is therefore not always

accurate.

2. The input gauge was quite high and there may have been some human error in reading the level

especially as it was not always at eye height.

3. Frictional losses along the spillway.

The correlation between the coefficient of discharge (Cd) and the head over the spillway (H) can be

seen below:

Fig.2. Graph to show coefficient of discharge against design head (m)

From the graph, we can see that the coefficient of discharge needed to design the spillway, Cd =

0.625, with a corresponding design head of Hd= 0.044m. There was one anomalous data point on the

0

0.01

0.02

0.03

0.04

0.05

0.06

0.45 0.5 0.55 0.6 0.65 0.7

He

ad

ov

er

spil

lwa

y (

m)

Coefficient of discharge

H(spillway) (m)

Linear (H(spillway) (m))

Positive weir pressure

Negative weir pressure

Anomalous

data point


graph, shown above. This corresponded to the flow at 19m3/hr, and although shown on the graph has

been discounted in analysis.

Hydraulic Jump

Y1 Y2 V Fr 8Fr2

y2

(Calc)

0.0145 0.0675 1.57 4.163 138.7 0.07568

From the result shown above it can be seen that there is a difference between the measured height of

the water after the jump and the calculated height of water after the jump. As the Fr number >1, the

flow was supercritical and it was correct to use the formula as outlined in the procedure above.

However the difference could be predominately due to the fact that the edge of the sluice gate was not

sealed properly, creating turbulence, and making it hard to take accurate measurements.

5. Summary

The Cd and Hd values to be used are 0.625 and 0.044m respectively. These will enable us to scale up

the model in order to design the spillway in conjunction with the data obtained from the flood routing.

Site Report Group 19

A site visit was conducted in the Irfon Valley to assess the viability of damming the river. Four sites were identified prior to

the visit and investigated to determine their suitability.

General Geology

The main rock type found in the valley is a mudstone from the Silurian strata. The strata tend to dip to the northwest, with

a dip angle of between 20 – 40 degrees. There are large amounts of alluvium at the base of the valley which will have to be

excavated. The whole valley has been carved out by glacial activity which has several geological implications, such as the

variedly sized, angular rocks. As a result, they have been less well sorted due to their transportation in ice. This has

resulted in an anisotropic soil.

General Environmental Concerns

Special consideration for rare populations of animals such as the otter, atlantic salmon and various other fish must be

taken. In addition the river itself is an SSSI due mesotrophic and oligo-mesotrophic river types which include communities

of submerged water plants. In addition the high acidity level caused by the high proportion of coniferous forestry is a

concern, with our dam looking to avoid destabilising the PH any further.

General Material

The quarry site is in the region of the LLANWRTYD VOLCANIC FORMATION, and is close to all of the potential dam sites.

At the bottom of the quarry there is a slatey mudstone, which is probably indicative of the bedrock in much of the valley.

Whilst it is not soft, it is not hard enough for aggregate but may be good for rockfill. Hard volcanic rock (Kilsbury tuff) can

be used for aggregate. However it may be very difficult to negotiate access to the quarry as there are many different

agencies involved, in addition it may be more economical to source stone from the mine in Builth Wells. If we were to

source our stone/rock from the quarry an environmental impact assessment would have to be produced. If building an

earth or rock filled dam, we will need fine grained, impermeable material for the clay core. Given the local geology of the

area, we have identified 3 main locations of glacial till which although not ideal, would be suitable for the clay core. The

largest of these is the borrow pit, which was used in the constrution of the Lynn Brianne dam located in the neighbouring

valley, although there are two smaller deposits located closer to our dam sites.

Site 1

Topography

This site has a large flat area at the base of the

valley approximately 80m across. The river runs

through the centre of the site and is lined by trees.


Approximately 20m from the proposed dam site there is a tributary to the main river. There is a church and graveyard

downstream of the dam, which should not be affected by the dam structure, consideration needs to be taken for any

vibrations caused during construction. In addition there is also a bridge downstream. There is also a phone line and two

minor roads that would need to be relocated. There were some minor slips present indicated by exposed rock on the

eastern face. Site 1 also lies on a fault line, which although dormant may be an indicator of disturbed rock and subterrain

features we may not be aware of. This will require further and more detailed site exploration which will be costly.

Environmental Concerns

This site is in close proximity to a church and graveyard. While we

need to pay close attention to it, as our dam is sufficiently upstream

we do not need to relocate. The site is used for grazing land for farm

animals, which would have to be relocated. There could be an issue

of noise for the surrounding residents. There are a number of

structures that will need to be moved due to the flooded area.

Materials

Due to the profile of the valley at this point and its proximity to the

quarry the site lends itself to a rockfill or earthfill dam. We would

procure the clay core from the borrow pit, however as it is a rocky clay we would need to provide a wider base for the

core. As this is some distance we need to consider the transportation of the material.

Dam Construction

Due to this sites locality to the base of the valley it has good access for large plant meaning a rockfill or earthfill dam could

be constructed quickly and efficiently, with minimum disruption to the surrounding roads and logging industry. The

spillway will be constructed on the eastern side to avoid relocating the church and graveyard. We will need to source

aggregate for the constuction of the spillway, however more research will need to be undertaken as to the most ideal

loation for extraction. The western side is heavily forested and as such will need to be cleared in order for us to construct

the dam.

Site 2

Topography

The site has a large flat area at the base of the valley

approximately 50m across. There is no evidence of slips on

the surrounding hill sides, however the river has previously

taken a different route as shown by a cutting scar in the

bank. The presence of wetland grasses indicates the large

amount of water flowing into the area making a suitable dam

location. There are houses downstream which although

unaffected will need to considered during construction.

In addition there are two minor roads, a power line and a phone line that will all need to be moved to allow the

construction of the dam.


This site is used as grazing land for farm animals, which will need to be

relocated. In addition we will also need to clear some of the forest.

There will be a substantial amount of disruption to the residents of the

valley who do not need to move and we to be aware of possible

vibration damage to people’s properties. There will also need to be a

number of structures moved due the flooded area.

Materials


The profile of the valley at this point again lends itself to a rockfill or earthfill dam. This site is situated on mudrock with

some silt laminations but should be a good source of material for a rockfill dam. The clay core can be sourced from the

borrow pit, or one of the other glacial till locations.

Dam Construction

Site 2 has good access to the dam location, the valley is still relatively wide at this point and we do not envisage any

problems in access for construction equipment, as indicated by the number of logging vehicles passing through the area.

We would place the spillway on the Eastern bank of the river, so as to avoid disturbance to the houses situated

downstream. Aggregate for the spillway will be sourced from locations as discussed for site 1.

Site 3

Topography

The site is situated in a steep sided valley, with the base

measuring around 20 – 30m across. The valley sides are

densely vegetated with many large trees and shrubbery. The

river channel runs through the centre of the valley with a

minor road located to the west. There is also a telephone line

which follows the road crossing at various points which

would also need to be relocated. There is a property about

100m downstream which would need consideration

during the dam construction phase. There is a picnic area and

Forestry Commission area just upstream which would be

affected by the flooded area and need consideration.


There is an abundance of large trees in the area, some of which are used for the

logging industry. These could be removed and sold in preparation for the

construction of the dam. Several upstream buildings, including the Abergwesyn

community, will be affected by the dam storage area. The hillsides are

currently uninhabited by livestock, however there is Site of Special Scientific

Interest located 200m downstream from the proposed dam site.

Materials

A concrete dam can be considered due to the narrow nature of the valley. The stable

hillsides would provide established abutments for the concrete arch. Aggregate can be sourced from the local quarry

reducing transportation of materials.

Dam Construction

The dam would preferably be made of concrete due to the valley’s topography. The spillway would be incorporated within

the dam structure saving space. The riverside road would be useful for delivering resources to the site. Deforestation

would be needed in order to facilitate the structure, although this may weaken the immediate strata.

Site 4

Topography

Site 4 has a much narrower base and steeper sides

than previously seen at sites 1 and 2. The Eastern bank

is much steeper than the west, and rocky outcrops can

be seen. These have been subject to much weathering

and as such may be unstable. The topography of

this site lends itself to a rockfill or earthfill dam.

Environmental concerns

The flood area associated with site 4 extends to

Abergwesyn which would involve not only the relocation of many

residents but also the road junction, and hence three roads. As

this is considerably more than at the other sites it may not be the


most viable option. There is also an area used for forestry, and a Site of Specific Scientific Interest that will be affected by

the dam as well as a power line, a telephone line and a bridal path.

Materials

Site 4 is located in close proximity to one of the smaller glacial till deposits, which would be ideal for the core of a rockfill

or earthfill dam. Once again it is mainly situated on mudstone, which would provide adequate foundation for a rockfill or

earthfill dam.

Dam Construction

At Site 4, the valley starts to narrow and access for large construction materials may become more difficult. There is also a

large area that would need to be deforested, and may lead to weakening of the surrounding soil. If we were to construct

an earthfill or rockfilled dam, we would place the spillway on the eastern bank of the river due to the valley topography.

Figure 1. Table

comparing displaced

infrastructure for

flooded areas.

Conclusion

Following our investigation of the valley we have decided that site 2 will be most

suitable site to dam the river. We have made this decision based on a number of

criteria such as the effect on infrastructure, access and proximity to

construction materials. Site 2 will require the relocation of the least number of

structures as it has the smallest flooded footprint. This has the added benefit of

requiring less land to be purchased. In addition it has good site access, which is

necessary for the construction of a rock fill dam to allow plant on site. It is also

close to the quarry and sources of glacial till for the core. Finally this site

requires much less deforestation lowering the environmental impact

further.

Figure 2. Flooded area map for site 2

AM

PM

AM

PM

AM

PM

AM

PM

AM

PM

AM

PM

AM

PM

AM

PM

AM

PM

AM

PM

AM

PM

AM

PM

AM

PM

AM

PM

All

Meeting

All

Site Choice

JE, SW

Analyse Sites

All

Identify Potential Sites

GC, FW, DW

Area Geology

GC, FW, DW

Site Geology

GC, FW, DW

Environmental Status & Regs

All

Areas To Be Flooded & Implications

GC, FW, DW

Impact Assessment For Final Site

All

Site R

esearch

General Site Research

All

Prepare For Site V

isit

TBC

Labs

All

Location Of Spillway

FW

Spillway Design

All

Flood Routing

GC

Diversion

All

Aqueduct To Service Reservoir

All

Position Of Draw-off Tower

FW

Cost Of Spillway

All

Finalise Dam Type

GC, DW

Produce Site Plans

GC, FW, DW

Design Dam and Foundations

GC, FW, DW

Calculate Volume Of Materials

GC, FW, DW

Dam Costing

All

Sources/Cost/Transport of Materials

JE, SW

Calculate Product Costs

All

Identify Health And Safety

All

Construction Sequence

All

Individual Writing

All

Compiling

JEForm

at/Proof Reading

GC, FW, DW

Gantt Chart

5pm

JE, SW

Site Data

1pm

All

Site Visit Report & Final Site Choice

5pm

JE, SW

Routed Flood Hydrograph

5pm

FW

Spillway Dimensions

5pm

TBC

Lab Report On Spillway Design

5pm

GC, FW, DW

Spillway Section And Plan

1pm

GC, DW

Plans Of Dam Site

1pm

All

Summary Sheet

5pm

All

Presentation

All

Individual Report

5pm

All

Group Report

5pm

Environmenta

l Analysis

Geological Analysis

Hydrological Analysis

Report

Deliverables (hand in

date

s)

Unknown

Around Lectures

S i t e V i s i t

Research

Responsibility

Design A

nd Construction

Management

Structu

ral and

Foundation D

esign

Hydraulic D

esign

15-Oct

14-Oct

13-Oct

12-Oct

11-Oct

25-Oct

22-Oct

21-Oct

Thr

28-Oct

27-Oct

26-Oct

Mon

Tue

Wed

Thr

Fri

Mon

20-Oct

19-Oct

18-Oct

Tue

Wed

Mon

Tue

Wed

Thr

Fri

5pm

5pm

Feasibility Study

Documents

Transcript of Feasibility Study